Accurate actual flip angle imaging (AFI) in the presence of fat

PURPOSE

To investigate the effect of incomplete fat spoiling on the accuracy of B1 mapping with actual flip angle imaging (AFI) and to propose a method to minimize the errors using the chemical shift properties of fat.

THEORY AND METHODS

Diffusion-based dephasing is the main spoiling mechanism exploited in AFI. However, a very low diffusion in fat may make the spoiling insufficient, leading to ghosts in the B1 maps. As the errors retain the chemical-shift signature of fat, their impact can be minimized using chemical-shift-based fat signal removal from AFI acquisition modified to include multi-echo readout. The source of the errors and the proposed correction were studied in simulations and phantom and in-vivo imaging experiments.

RESULTS

Our results support that AFI artifacts are caused by the incomplete fat spoiling present in clinically attractive short TR acquisition regimes. The correction eliminated the ghosting and significantly improved the B1 mapping accuracy as well as the accuracy of R1 mapping performed with AFI-derived B1 maps.

CONCLUSIONS

The incomplete fat signal spoiling may be a source of AFI B1 mapping errors, especially in subjects with high fat content. Achieving complete fat spoiling requires longer TR, which is undesirable in clinical applications. The proposed approach based on fat signal removal can reduce errors without significant prolongation of the AFI pulse sequence. We propose that, when attaining complete fat spoiling is not feasible, AFI mapping should be performed in a multi-echo regime with appropriate fat separation or suppression to minimize these errors.

Language Experience during Infancy Predicts White Matter Myelination at Age 2 Years

Parental input is considered a key predictor of language achievement during the first years of life, yet relatively few studies have assessed the effects of parental language input and parent-infant interactions on early brain development. We examined the relationship between measures of parent and child language, obtained from naturalistic home recordings at child ages 6, 10, 14, 18, and 24 months, and estimates of white matter myelination, derived from quantitative MRI at age 2 years (mean = 26.30 months, SD = 1.62, N = 22). Analysis of the white matter focused on dorsal pathways associated with expressive language development and long-term language ability, namely, the left arcuate fasciculus (AF) and superior longitudinal fasciculus (SLF). Frequency of parent-infant conversational turns (CT) uniquely predicted myelin density estimates in both the AF and SLF. Moreover, the effect of CT remained significant while controlling for total adult speech and child speech-related utterances, suggesting a specific role for interactive language experience, rather than simply speech exposure or production. An exploratory analysis of 18 additional tracts, including the right AF and SLF, indicated a high degree of anatomic specificity. Longitudinal analyses of parent and child language variables indicated an effect of CT as early as 6 months of age, as well as an ongoing effect over infancy. Together, these results link parent-infant conversational turns to white matter myelination at age 2 years, and suggest that early, interactive experiences with language uniquely contribute to the development of white matter associated with long-term language ability.SIGNIFICANCE STATEMENT Children's earliest experiences with language are thought to have profound and lasting developmental effects. Recent studies suggest that intervention can increase the quality of parental language input and improve children's learning outcomes. However, important questions remain about the optimal timing of intervention, and the relationship between specific aspects of language experience and brain development. We report that parent-infant turn-taking during home language interactions correlates with myelination of language related white matter pathways through age 2 years. Effects were independent of total speech exposure and infant vocalizations and evident starting at 6 months of age, suggesting that structured language interactions throughout infancy may uniquely support the ongoing development of brain systems critical to long-term language ability.

Language input in late infancy scaffolds emergent literacy skills and predicts reading related white matter development

Longitudinal studies provide the unique opportunity to test whether early language provides a scaffolding for the acquisition of the ability to read. This study tests the hypothesis that parental language input during the first 2 years of life predicts emergent literacy skills at 5 years of age, and that white matter development observed early in the 3rd year (at 26 months) may help to account for these effects. We collected naturalistic recordings of parent and child language at 6, 10, 14, 18, and 24 months using the Language ENvironment Analysis system (LENA) in a group of typically developing infants. We then examined the relationship between language measures during infancy and follow-up measures of reading related skills at age 5 years, in the same group of participants (N = 53). A subset of these children also completed diffusion and quantitative MRI scans at age 2 years (N = 20). Within this subgroup, diffusion tractography was used to identify white matter pathways that are considered critical to language and reading development, namely, the arcuate fasciculus (AF), superior and inferior longitudinal fasciculi, and inferior occipital-frontal fasciculus. Quantitative macromolecular proton fraction (MPF) mapping was used to characterize myelin density within these separately defined regions of interest. The longitudinal data were then used to test correlations between early language input and output, white matter measures at age 2 years, and pre-literacy skills at age 5 years. Parental language input, child speech output, and parent-child conversational turns correlated with pre-literacy skills, as well as myelin density estimates within the left arcuate and superior longitudinal fasciculus. Mediation analyses indicated that the left AF accounted for longitudinal relationships between infant home language measures and 5-year letter identification and letter-sound knowledge, suggesting that the left AF myelination at 2 years may serve as a mechanism by which early language experience supports emergent literacy.

Brain myelination at 7 months of age predicts later language development

Between 6 and 12 months of age there are dramatic changes in infants' processing of language. The neurostructural underpinnings of these changes are virtually unknown. The objectives of this study were to (1) examine changes in brain myelination during this developmental period and (2) examine the relationship between myelination during this period and later language development. Macromolecular proton fraction (MPF) was used as a marker of myelination. Whole-brain MPF maps were obtained with 1.25 mm3 isotropic spatial resolution from typically developing children at 7 and 11 months of age. Effective myelin density was calculated from MPF based on a linear relationship known from the literature. Voxel-based analyses were used to identify longitudinal changes in myelin density and to calculate correlations between myelin density at these ages and later language development. Increases in myelin density were more predominant in white matter than in gray matter. A strong predictive relationship was found between myelin density at 7 months of age, language production at 24 and 30 months of age, and rate of language growth. No relationships were found between myelin density at 11 months, or change in myelin density between 7 and 11 months of age, and later language measures. Our findings suggest that critical changes in brain structure may precede periods of pronounced change in early language skills.

Development of executive function-relevant skills is related to both neural structure and function in infants

The development of skills related to executive function (EF) in infancy, including their emergence, underlying neural mechanisms, and interconnections to other cognitive skills, is an area of increasing research interest. Here, we report on findings from a multidimensional dataset demonstrating that infants' behavioral performance on a flexible learning task improved across development and that the task performance is highly correlated with both neural structure and neural function. The flexible learning task probed infants' ability to learn two different associations, concurrently, over 16 trials, requiring multiple skills relevant to EF. We examined infants' neural structure by measuring myelin density in the brain, using a novel macromolecular proton fraction (MPF) mapping method. We further examined an important neural function of speech processing by characterizing the mismatch response (MMR) to speech contrasts using magnetoencephalography (MEG). All measurements were performed longitudinally in monolingual English-learning infants at 7- and 11-months of age. At the group level, 11-month-olds, but not 7-month-olds, demonstrated evidence of learning both associations in the behavioral task. Myelin density in the prefrontal region at 7 months of age was found to be highly predictive of behavioral task performance at 11 months of age, suggesting that myelination may support the development of these skills. Furthermore, a machine-learning regression analysis revealed that individual differences in the behavioral task are predicted by concurrent neural speech processing at both ages, suggesting that these skills do not develop in isolation. Together, these cross-modality results revealed novel insights into EF-related skills. HIGHLIGHT: Monolingual infants demonstrated flexible learning on a task requiring executive function skills at 11 months, but not at 7 months. Infants' myelin density at 7 months is highly predictive of their behavioral performance in the flexible learning task at 11 months of age. Individual differences in the flexible learning task performance are also correlated with concurrent neural processing of speech at both ages.

Macromolecular Proton Fraction as a Myelin Biomarker: Principles, Validation, and Applications

Macromolecular proton fraction (MPF) is a quantitative MRI parameter describing the magnetization transfer (MT) effect and defined as a relative amount of protons bound to biological macromolecules with restricted molecular motion, which participate in magnetic cross-relaxation with water protons. MPF attracted significant interest during past decade as a biomarker of myelin. The purpose of this mini review is to provide a brief but comprehensive summary of MPF mapping methods, histological validation studies, and MPF applications in neuroscience. Technically, MPF maps can be obtained using a variety of quantitative MT methods. Some of them enable clinically reasonable scan time and resolution. Recent studies demonstrated the feasibility of MPF mapping using standard clinical MRI pulse sequences, thus substantially enhancing the method availability. A number of studies in animal models demonstrated strong correlations between MPF and histological markers of myelin with a minor influence of potential confounders. Histological studies validated the capability of MPF to monitor both demyelination and re-myelination. Clinical applications of MPF have been mainly focused on multiple sclerosis where this method provided new insights into both white and gray matter pathology. Besides, several studies used MPF to investigate myelin role in other neurological and psychiatric conditions. Another promising area of MPF applications is the brain development studies. MPF demonstrated the capabilities to quantitatively characterize the earliest stage of myelination during prenatal brain maturation and protracted myelin development in adolescence. In summary, MPF mapping provides a technically mature and comprehensively validated myelin imaging technology for various preclinical and clinical neuroscience applications.

Data-Driven Retrospective Correction of B1 Field Inhomogeneity in Fast Macromolecular Proton Fraction and R1 Mapping

Correction of B₁ field non-uniformity is critical for many quantitative MRI methods including variable flip angle (VFA) T₁ mapping and single-point macromolecular proton fraction (MPF) mapping. The latter method showed promising results as a fast and robust quantitative myelin imaging approach and involves VFA-based R₁=1/T₁ map reconstruction as an intermediate processing step. The need for B₁ correction restricts applications of the above methods, since B₁ mapping sequences increase the examination time and are not commonly available in clinics. A new algorithm was developed to enable retrospective data-driven simultaneous B₁ correction in VFA R₁ and single-point MPF mapping. The principle of the algorithm is based on different mathematical dependences of B₁ -related errors in R₁ and MPF allowing extraction of a surrogate B₁ field map from uncorrected R₁ and MPF maps. To validate the method, whole-brain R₁ and MPF maps with isotropic 1.25 mm³ resolution were obtained on a 3 T MRI scanner from 11 volunteers. Mean parameter values in segmented brain tissues were compared between three reconstruction options including the absence of correction, actual B₁ correction, and surrogate B₁ correction. Surrogate B₁ maps closely reproduced actual patterns of B₁ inhomogeneity. Without correction, B₁ non-uniformity caused highly significant biases in R₁ and MPF ( ). Surrogate B₁ field correction reduced the biases in both R₁ and MPF to a non-significant level ( 0.1 ≤ P ≤ 0.8 ). The described algorithm obviates the use of dedicated B₁ mapping sequences in fast single-point MPF mapping and provides an alternative solution for correction of B₁ non-uniformities in VFA R₁ mapping.

Long-term monitoring of chronic demyelination and remyelination in a rat ischemic stroke model using macromolecular proton fraction mapping

Remyelination is a key process enabling post-stroke brain tissue recovery and plasticity. This study aimed to explore the feasibility of demyelination and remyelination monitoring in experimental stroke from the acute to chronic stage using an emerging myelin imaging biomarker, macromolecular proton fraction (MPF). After stroke induction by transient middle cerebral artery occlusion, rats underwent repeated MRI examinations during 85 days after surgery with histological endpoints for the animal subgroups on the 7th, 21st, 56th, and 85th days. MPF maps revealed two sub-regions within the infarct characterized by distinct temporal profiles exhibiting either a persistent decrease by 30%-40% or a transient decrease followed by return to nearly normal values after one month of observation. Myelin histology confirmed that these sub-regions had nearly similar extent of demyelination in the sub-acute phase and then demonstrated either chronic demyelination or remyelination. The remyelination zones also exhibited active axonal regrowth, reconstitution of compact fiber bundles, and proliferation of neuronal and oligodendroglial precursors. The demyelination zones showed more extensive astrogliosis from the 21st day endpoint. Both sub-regions had substantially depleted neuronal population over all endpoints. These results histologically validate MPF mapping as a novel approach for quantitative assessment of myelin damage and repair in ischemic stroke.

Myelin development in cerebral gray and white matter during adolescence and late childhood

Myelin development during adolescence is becoming an area of growing interest in view of its potential relationship to cognition, behavior, and learning. While recent investigations suggest that both white matter (WM) and gray matter (GM) undergo protracted myelination during adolescence, quantitative relations between myelin development in WM and GM have not been previously studied. We quantitatively characterized the dependence of cortical GM, WM, and subcortical myelin density across the brain on age, gender, and puberty status during adolescence with the use of a novel macromolecular proton fraction (MPF) mapping method. Whole-brain MPF maps from a cross-sectional sample of 146 adolescents (age range 9–17 years) were collected. Myelin density was calculated from MPF values in GM and WM of all brain lobes, as well as in subcortical structures. In general, myelination of cortical GM was widespread and more significantly correlated with age than that of WM. Myelination of GM in the parietal lobe was found to have a significantly stronger age dependence than that of GM in the frontal, occipital, temporal and insular lobes. Myelination of WM in the temporal lobe had the strongest association with age as compared to WM in other lobes. Myelin density was found to be higher in males as compared to females when averaged across all cortical lobes, as well as in a bilateral subcortical region. Puberty stage was significantly correlated with myelin density in several cortical areas and in the subcortical GM. These findings point to significant differences in the trajectories of myelination of GM and WM across brain regions and suggest that cortical GM myelination plays a dominant role during adolescent development.

Three-dimensional fast single-point macromolecular proton fraction mapping of the human brain at 0.5 Tesla

Fast single-point macromolecular proton fraction (MPF) mapping is a recent magnetic resonance imaging (MRI) method enabling quantitative assessment of myelin content in neural tissues. To date, the reported technical implementations of MPF mapping utilized high-field MRI equipment (1.5 T or higher), while low-field applications might pose challenges due to signal-to-noise ratio (SNR) limitations and short T₁ . This study aimed to evaluate the feasibility of MPF mapping of the human brain at 0.5 T. The three-dimensional MPF mapping protocol was implemented according to the single-point synthetic-reference method, which includes three spoiled gradient-echo sequences providing proton density, T₁ , and magnetization transfer contrast weightings. Whole-brain MPF maps were obtained from three healthy volunteers with spatial resolution of 1.5×1.5×2 mm³ and the total scan time of 19 minutes. MPF values were measured in a series of white and gray matter structures and compared with literature data for 3 T magnetic field. MPF maps enabled high contrast between white and gray matter with notable insensitivity to paramagnetic effects in iron-rich structures, such as globus pallidus, substantia nigra, and dentate nucleus. MPF values at 0.5 T appeared in close agreement with those at 3 T. This study demonstrates the feasibility of fast MPF mapping with low-field MRI equipment and the independence of brain MPF values of magnetic field. The presented results confirm the utility of MPF as an absolute scale for MRI-based myelin content measurements across a wide range of magnetic field strengths and extend the applicability of fast MPF mapping to inexpensive low-field MRI hardware.

Scan-Rescan Repeatability and Impact of B0 and B1 Field Nonuniformity Corrections in Single-Point Whole-Brain Macromolecular Proton Fraction Mapping

BACKGROUND

Single-point macromolecular proton fraction (MPF) mapping is a recent quantitative MRI method for fast assessment of brain myelination. Information about reproducibility and sensitivity of MPF mapping to magnetic field nonuniformity is important for clinical applications.

PURPOSE

To assess scan-rescan repeatability and a value of B₀ and B₁ field inhomogeneity corrections in single-point synthetic-reference MPF mapping.

STUDY TYPE

Prospective.

POPULATION

Eight healthy adult volunteers underwent two scans with 11.5 ± 2.3 months interval.

FIELD STRENGTH/SEQUENCE

3T; whole-brain 3D MPF mapping protocol included three spoiled gradient-echo sequences providing T₁ , proton density, and magnetization transfer contrasts with 1.25 × 1.25 × 1.25 mm³ resolution and B₀ and B₁ mapping sequences.

ASSESSMENT

MPF maps were reconstructed with B₀ and B₁ field nonuniformity correction, B₀ - and B₁ -only corrections, and without corrections. Mean MPF values were measured in automatically segmented white matter (WM) and gray matter (GM).

STATISTICAL TESTS

Within-subject coefficient of variation (CV), intraclass correlation coefficient (ICC), Bland-Altman plots, and paired t-tests to assess scan-rescan repeatability. Repeated-measures analysis of variance (ANOVA) to compare field corrections.

RESULTS

Maximal relative local MPF errors without correction in the areas of largest field nonuniformities were about 5% and 27% for B₀ and B₁ , respectively. The effect of B₀ correction was insignificant for whole-brain WM (P > 0.25) and GM (P > 0.98) MPF. The absence of B₁ correction caused a positive relative bias of 4-5% (P < 0.001) in both tissues. Scan-rescan agreement was similar for all field correction options with ICCs 0.80-0.81 for WM and 0.89-0.92 for GM. CVs were 1.6-1.7% for WM and 0.7-1.0% for GM.

DATA CONCLUSION

The single-point method enables high repeatability of MPF maps obtained with the same equipment. Correction of B₀ inhomogeneity may be disregarded to shorten the examination time. B₁ nonuniformity correction improves accuracy of MPF measurements at 3T. Reliability of whole-brain MPF measurements in WM and GM is not affected by B₀ and B₁ field corrections.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2020;51:1789-1798.

pH-triggered delivery of magnetic nanoparticles depends on tumor volume

Nowadays there is growing recognition of the fact that biological systems have a greater impact on nanoparticle target delivery in tumors than nanoparticle design. Here we investigate the targeted delivery of Fe₃O₄ magnetic nanoparticles conjugated with pH-low-insertion peptide (MNP-pHLIP) on orthotopically induced MDA-MB-231 human breast carcinoma xenografts of varying volumes as a model of cancer progression. Using in vivo magnetic resonance imaging and subsequent determination of iron content in tumor samples by inductively coupled plasma atomic emission spectroscopy we found that MNP-pHLIP accumulation depends on tumor volume. Transmission electron microscopy, histological analysis and immunohistochemical staining of tumor samples suggest that blood vessel distribution is the key factor in determining the success of the accumulation of nanoparticles in tumors.

Plant polyprenols reduce demyelination and recover impaired oligodendrogenesis and neurogenesis in the cuprizone murine model of multiple sclerosis

Recent studies showed hepatoprotective, neuroprotective, and immunomodulatory properties of polyprenols isolated from the green verdure of Picea abies(L.) Karst. This study aimed to investigate effects of polyprenols on oligodendrogenesis, neurogenesis, and myelin content in the cuprizone demyelination model. Demyelination was induced by 0.5% cuprizone in CD‐1 mice during 10 weeks. Nine cuprizone‐treated animals received daily injections of polyprenols intraperitoneally at a dose of 12‐mg/kg body weight during Weeks 6–10. Nine control animals and other nine cuprizone‐treated received sham oil injections. At Week 10, brain sections were stained for myelin basic protein, neuro‐glial antigen‐2, and doublecortin to evaluate demyelination, oligodendrogenesis, and neurogenesis. Cuprizone administration caused a decrease in myelin basic protein in the corpus callosum, cortex, hippocampus, and the caudate putamen compared with the controls. Oligodendrogenesis was increased, and neurogenesis in the subventricular zone and the dentate gyrus of the hippocampus was decreased in the cuprizone‐treated group compared with the controls. Mice treated with cuprizone and polyprenols did not show significant demyelination and differences in oligodendrogenesis and neurogenesis as compared with the controls. Our results suggest that polyprenols can halt demyelination, restore impaired neurogenesis, and mitigate reactive overproduction of oligodendrocytes caused by cuprizone neurotoxicity.

Direct comparison between apparent diffusion coefficient and macromolecular proton fraction as quantitative biomarkers of the human fetal brain maturation

BACKGROUND

Apparent diffusion coefficient (ADC) is known as a quantitative biomarker of prenatal brain maturation. Fast macromolecular proton fraction (MPF) mapping is an emerging method for quantitative assessment of myelination that was recently adapted to fetal MRI.

PURPOSE

To compare the capability of ADC and MPF to quantify the normal fetal brain development.

STUDY TYPE

Prospective.

POPULATION

Forty-two human fetuses in utero (gestational age [GA] = 27.7 ± 6.0, range 20-38 weeks).

FIELD STRENGTH/SEQUENCE

1.5 T; diffusion-weighted single-shot echo-planar spin-echo with five b-values for ADC mapping; spoiled multishot echo-planar gradient-echo with T₁ , proton density, and magnetization transfer contrast weightings for single-point MPF mapping.

ASSESSMENT

Two operators measured ADC and MPF in the medulla, pons, cerebellum, thalamus, and frontal, occipital, and temporal cerebral white matter (WM).

STATISTICAL TESTS

Mixed repeated-measures analysis of variance (ANOVA) with the factors of pregnancy trimester and brain structure; Pearson correlation coefficient (r); Hotelling-Williams test to compare strengths of correlations.

RESULTS

From the 2^nd to 3^rd trimester, ADC significantly decreased in the thalamus and cerebellum (P < 0.005). MPF significantly increased in the medulla, pons, thalamus, and cerebellum (P < 0.005). Cerebral WM had significantly higher ADC and lower MPF compared with the medulla and pons in both trimesters. MPF (r range 0.83, 0.89, P < 0.001) and ADC (r range -0.43, -0.75, P ≤ 0.004) significantly correlated with GA and each other (r range -0.32, -0.60, P ≤ 0.04) in the medulla, pons, thalamus, and cerebellum. No significant correlations or distinctions between regions and trimesters were observed for cerebral WM (P range 0.1-0.75). Correlations with GA were significantly stronger for MPF compared with ADC in the medulla, pons, and cerebellum (Hotelling-Williams test, P < 0.003) and similar in the thalamus. Structure-averaged MPF and ADC values strongly correlated (r = 0.95, P < 0.001).

DATA CONCLUSION

MPF and ADC demonstrated qualitatively similar but quantitatively different spatiotemporal patterns. MPF appeared more sensitive to changes in the brain structures with prenatal onset of myelination.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2019;50:52-61.

Congenital medulloblastoma: Fetal and postnatal longitudinal observation with quantitative MRI

Congenital medulloblastoma is extremely rare. MRI appearance of this tumor in the fetal brain has not been described. A case of congenital medulloblastoma initially observed by antenatal MRI with postnatal follow-up and treatment is presented. A pregnant female underwent fetal MRI on the 31st gestational week for routine indications. Midline cerebellar lesion of ≤2 cm in size with minor T₂ hypointensity and T₁ hyperintensity was identified. Additionally, quantitative MRI including apparent diffusion coefficient (ADC) and fast macromolecular proton fraction (MPF) mapping was performed. The lesion showed a marked ADC decrease and MPF increase. MPF maps depicted the lesion most conspicuously. After term delivery, a male neonate presented with symptoms of increased intracranial pressure. Postnatal MRI identified obstructive hydrocephalus caused by a large posterior fossa mass. The child was treated by cerebrospinal fluid shunt placement. Follow-up quantitative MRI on the fifth month revealed tumor growth and vivid changes of its tissue contrast associated with brain maturation. The tumor appeared nearly isointense on T₁- and T₂-weighted images and slightly hypointense on the ADC map. MPF contrast showed the most remarkable change from hyper- to hypointensity due to brain myelination with stable MPF in the tumor. Subsequently, the child underwent partial tumor resection, and currently continues treatment with chemotherapy. The pathological diagnosis was desmoplastic/nodular medulloblastoma. The described case illustrates evolution of the tumor contrast in the course of fetal and postnatal brain development and highlights the added diagnostic value of MPF mapping in fetal and neonatal neuroimaging.

Quantitative Assessment of Normal Fetal Brain Myelination Using Fast Macromolecular Proton Fraction Mapping

BACKGROUND AND PURPOSE

Fast macromolecular proton fraction mapping is a recently emerged MRI method for quantitative myelin imaging. Our aim was to develop a clinically targeted technique for macromolecular proton fraction mapping of the fetal brain and test its capability to characterize normal prenatal myelination.

MATERIALS AND METHODS

This prospective study included 41 pregnant women (gestational age range, 18-38 weeks) without abnormal findings on fetal brain MR imaging performed for clinical indications. A fast fetal brain macromolecular proton fraction mapping protocol was implemented on a clinical 1.5T MR imaging scanner without software modifications and was performed after a clinical examination with an additional scan time of <5 minutes. 3D macromolecular proton fraction maps were reconstructed from magnetization transfer-weighted, T1-weighted, and proton density-weighted images by the single-point method. Mean macromolecular proton fraction in the brain stem, cerebellum, and thalamus and frontal, temporal, and occipital WM was compared between structures and pregnancy trimesters using analysis of variance. Gestational age dependence of the macromolecular proton fraction was assessed using the Pearson correlation coefficient (r).

RESULTS

The mean macromolecular proton fraction in the fetal brain structures varied between 2.3% and 4.3%, being 5-fold lower than macromolecular proton fraction in adult WM. The macromolecular proton fraction in the third trimester was higher compared with the second trimester in the brain stem, cerebellum, and thalamus. The highest macromolecular proton fraction was observed in the brain stem, followed by the thalamus, cerebellum, and cerebral WM. The macromolecular proton fraction in the brain stem, cerebellum, and thalamus strongly correlated with gestational age (r = 0.88, 0.80, and 0.73; P < .001). No significant correlations were found for cerebral WM regions.

CONCLUSIONS

Myelin is the main factor determining macromolecular proton fraction in brain tissues. Macromolecular proton fraction mapping is sensitive to the earliest stages of the fetal brain myelination and can be implemented in a clinical setting.

Quantitative assessment of demyelination in ischemic stroke in vivo using macromolecular proton fraction mapping

A recent MRI method, fast macromolecular proton fraction (MPF) mapping, was used to quantify demyelination in the transient middle cerebral artery occlusion (MCAO) rat stroke model. MPF and other quantitative MRI parameters (T₁, T₂, proton density, and apparent diffusion coefficient) were compared with histological and immunohistochemical markers of demyelination (Luxol Fast Blue stain, (LFB)), neuronal loss (NeuN immunofluorescence), axonal loss (Bielschowsky stain), and inflammation (Iba1 immunofluorescence) in three animal groups ( n = 5 per group) on the 1st, 3rd, and 10th day after MCAO. MPF and LFB optical density (OD) were significantly reduced in the ischemic lesion on all days after MCAO relative to the symmetrical regions of the contralateral hemisphere. Percentage changes in MPF and LFB OD in the ischemic lesion relative to the contralateral hemisphere significantly differed on the first day only. Percentage changes in LFB OD and MPF were strongly correlated (R = 0.81, P < 0.001) and did not correlate with other MRI parameters. MPF also did not correlate with other histological variables. Addition of T₂ into multivariate regression further improved agreement between MPF and LFB OD (R = 0.89, P < 0.001) due to correction of the edema effect. This study provides histological validation of MPF as an imaging biomarker of demyelination in ischemic stroke.

Iron-Insensitive Quantitative Assessment of Subcortical Gray Matter Demyelination in Multiple Sclerosis Using the Macromolecular Proton Fraction

BACKGROUND AND PURPOSE

Fast macromolecular proton fraction mapping is a recent quantitative MR imaging method for myelin assessment. The objectives of this study were to evaluate the macromolecular proton fraction as a measure of demyelination in subcortical GM structures in multiple sclerosis and assess a potential relationship between demyelination and excess iron deposition using the macromolecular proton fraction and T2* mapping.

MATERIALS AND METHODS

Macromolecular proton fraction and T2* maps were obtained from 12 healthy controls, 18 patients with relapsing-remitting MS, and 12 patients with secondary-progressive MS using 3T MR imaging. Parameter values in the caudate nucleus, globus pallidus, putamen, substantia nigra, and thalamus were compared between groups and correlated to clinical data.

RESULTS

The macromolecular proton fraction in all subcortical structures and T2* in the globus pallidus, putamen, and caudate nucleus demonstrated a significant monotonic decrease from controls to patients with relapsing-remitting MS and from those with relapsing-remitting MS to patients with secondary-progressive MS. The macromolecular proton fraction in all subcortical structures significantly correlated with the Expanded Disability Status Scale and MS Functional Composite scores with absolute Pearson correlation coefficient (r) values in a range of 0.4-0.6. Significant correlations (r = -0.4 to -0.6) were also identified between the macromolecular proton fraction and the 9-Hole Peg Test, indicating a potential relationship with nigrostriatal pathway damage. Among T2* values, weak significant correlations with clinical variables were found only in the putamen. The macromolecular proton fraction did not correlate with T2* in any of the studied anatomic structures.

CONCLUSIONS

The macromolecular proton fraction provides an iron-insensitive measure of demyelination. Myelin loss in subcortical GM structures in MS is unrelated to excess iron deposition. Subcortical GM demyelination is more closely associated with the disease phenotype and disability than iron overload.

Histological validation of fast macromolecular proton fraction mapping as a quantitative myelin imaging method in the cuprizone demyelination model

Cuprizone-induced demyelination in mice is a frequently used model in preclinical multiple sclerosis research. A recent quantitative clinically-targeted MRI method, fast macromolecular proton fraction (MPF) mapping demonstrated a promise as a myelin biomarker in human and animal studies with a particular advantage of sensitivity to both white matter (WM) and gray matter (GM) demyelination. This study aimed to histologically validate the capability of MPF mapping to quantify myelin loss in brain tissues using the cuprizone demyelination model. Whole-brain MPF maps were obtained in vivo on an 11.7T animal MRI scanner from 7 cuprizone-treated and 7 control С57BL/6 mice using the fast single-point synthetic-reference method. Brain sections were histologically stained with Luxol Fast Blue (LFB) for myelin quantification. Significant (p < 0.05) demyelination in cuprizone-treated animals was found according to both LFB staining and MPF in all anatomical structures (corpus callosum, anterior commissure, internal capsule, thalamus, caudoputamen, and cortex). MPF strongly correlated with quantitative histology in all animals (r = 0.95, p < 0.001) as well as in treatment and control groups taken separately (r = 0.96, p = 0.002 and r = 0.93, p = 0.007, respectively). Close agreement between histological myelin staining and MPF suggests that fast MPF mapping enables robust and accurate quantitative assessment of demyelination in both WM and GM.

High-resolution three-dimensional quantitative map of the macromolecular proton fraction distribution in the normal rat brain

The presented dataset provides a normative high-resolution three-dimensional (3D) macromolecular proton fraction (MPF) map of the healthy rat brain in vivo and source images used for its reconstruction. The images were acquired using the protocol described elsewhere (Naumova, et al. High-resolution three-dimensional macromolecular proton fraction mapping for quantitative neuroanatomical imaging of the rodent brain in ultra-high magnetic fields. Neuroimage (2016) doi: 10.1016/j.neuroimage.2016.09.036). The map was reconstructed from three source images with different contrast weightings (proton density, T₁, and magnetization transfer) using the single-point algorithm with a synthetic reference image. Source images were acquired from a living animal on an 11.7 T small animal MRI scanner with isotropic spatial resolution of 170 µm³ and total acquisition time about 1.5 h. The 3D dataset can be used for multiple purposes including interactive viewing of rat brain anatomy, measurements of reference MPF values in various brain structures, and development of image processing techniques for the rodent brain segmentation. It also can serve as a gold standard image for implementation and optimization of rodent brain MRI protocols.

High-resolution three-dimensional macromolecular proton fraction mapping for quantitative neuroanatomical imaging of the rodent brain in ultra-high magnetic fields

A well-known problem in ultra-high-field MRI is generation of high-resolution three-dimensional images for detailed characterization of white and gray matter anatomical structures. T₁-weighted imaging traditionally used for this purpose suffers from the loss of contrast between white and gray matter with an increase of magnetic field strength. Macromolecular proton fraction (MPF) mapping is a new method potentially capable to mitigate this problem due to strong myelin-based contrast and independence of this parameter of field strength. MPF is a key parameter determining the magnetization transfer effect in tissues and defined within the two-pool model as a relative amount of macromolecular protons involved into magnetization exchange with water protons. The objectives of this study were to characterize the two-pool model parameters in brain tissues in ultra-high magnetic fields and introduce fast high-field 3D MPF mapping as both anatomical and quantitative neuroimaging modality for small animal applications. In vivo imaging data were obtained from four adult male rats using an 11.7T animal MRI scanner. Comprehensive comparison of brain tissue contrast was performed for standard R₁ and T₂ maps and reconstructed from Z-spectroscopic images two-pool model parameter maps including MPF, cross-relaxation rate constant, and T₂ of pools. Additionally, high-resolution whole-brain 3D MPF maps were obtained with isotropic 170µm voxel size using the single-point synthetic-reference method. MPF maps showed 3-6-fold increase in contrast between white and gray matter compared to other parameters. MPF measurements by the single-point synthetic reference method were in excellent agreement with the Z-spectroscopic method. MPF values in rat brain structures at 11.7T were similar to those at lower field strengths, thus confirming field independence of MPF. 3D MPF mapping provides a useful tool for neuroimaging in ultra-high magnetic fields enabling both quantitative tissue characterization based on the myelin content and high-resolution neuroanatomical visualization with high contrast between white and gray matter.

Fast macromolecular proton fraction mapping of the human liver in vivo for quantitative assessment of hepatic fibrosis

The macromolecular proton fraction (MPF) is a quantitative MRI parameter determining the magnetization transfer (MT) effect in tissues, and is defined as the relative amount of immobile macromolecular protons involved in magnetization exchange with mobile water protons. MPF has the potential to provide a quantitative assessment of fibrous tissue because of the intrinsically high MPF specific for collagen. The goal of this study was to investigate the relationship between histologically determined fibrosis stage and MPF in the liver parenchyma measured using a recently developed fast single-point clinically targeted MPF mapping method. Optimal saturation parameters for single-point liver MPF measurements were determined from the analysis of liver Z spectra in vivo based on the error propagation model. Sixteen patients with chronic hepatitis C viral infection underwent 3-T MRI using an optimized liver MPF mapping protocol. Fourteen patients had prior liver biopsy with histologically staged fibrosis (METAVIR scores F0-F3) and two patients had clinically diagnosed cirrhosis (score F4 was assigned). The protocol included four breath-hold three-dimensional scans with 2 × 3 × 6-mm(3) resolution and 10 transverse sections: dynamic acquisition of MT-weighted and reference images; dynamic acquisition of three images for variable flip angle T1 mapping; dual-echo B0 map; and actual flip angle imaging B1 map. The average liver MPF was determined as the mode of the MPF histograms. MPF was significantly increased in patients with clinically significant fibrosis (scores F2-F4, n = 6) relative to patients with no or mild fibrosis (scores F0-F1, n = 10): 6.49 ± 0.36% versus 5.94 ± 0.26%, p < 0.01 (Mann-Whitney test). MPF and fibrosis scores were strongly positively correlated, with a Spearman's rank correlation coefficient of 0.80 (p < 0.001). This study demonstrates the feasibility of fast MPF mapping of the human liver in vivo and confirms the hypothesis that MPF is increased in hepatic fibrosis and associated with fibrosis stage. MPF may be useful as a non-invasive imaging biomarker of hepatic fibrosis.

Analysis and correction of biases in cross-relaxation MRI due to biexponential longitudinal relaxation

PURPOSE

Cross-relaxation imaging (CRI) is a family of quantitative magnetization transfer techniques that utilize images obtained with off-resonance saturation and longitudinal relaxation rate (R1) maps reconstructed by the variable flip angle (VFA) method. It was demonstrated recently that a significant bias in an apparent VFA R1 estimation occurs in macromolecule-rich tissues due to magnetization transfer (MT)-induced biexponential behavior of longitudinal relaxation of water protons. The purpose of this article is to characterize theoretically and experimentally the resulting bias in the CRI maps and propose methods to correct it.

THEORY

The modified CRI algorithm is proposed, which corrects for such biases and yields accurate parametric bound pool fraction f, cross-relaxation rate k, and R1 maps. Additionally, an analytical correction procedure is introduced to recalculate previously obtained parameter values.

RESULTS

The systematic errors due to unaccounted MT-induced biexponential relaxation can be characterized as an overestimation of R1, f, and k, with a relative bias comparable with the magnitude of f. The phantom and human in vivo experiments demonstrate that both proposed modified CRI and analytical correction approaches significantly improve the accuracy of the CRI method.

CONCLUSION

The accuracy of the CRI method can be considerably improved by taking into account the contribution of MT-induced biexponential longitudinal relaxation into variable flip angle R1 measurements.

Time-efficient, high-resolution, whole brain three-dimensional macromolecular proton fraction mapping

PURPOSE

Macromolecular proton fraction (MPF) mapping is a quantitative MRI method that reconstructs parametric maps of a relative amount of macromolecular protons causing the magnetization transfer (MT) effect and provides a biomarker of myelination in neural tissues. This study aimed to develop a high-resolution whole brain MPF mapping technique using a minimal number of source images for scan time reduction.

METHODS

The described technique was based on replacement of an actually acquired reference image without MT saturation by a synthetic one reconstructed from R1 and proton density maps, thus requiring only three source images. This approach enabled whole brain three-dimensional MPF mapping with isotropic 1.25 × 1.25 × 1.25 mm(3) voxel size and a scan time of 20 min. The synthetic reference method was validated against standard MPF mapping with acquired reference images based on data from eight healthy subjects.

RESULTS

Mean MPF values in segmented white and gray matter appeared in close agreement with no significant bias and small within-subject coefficients of variation (<2%). High-resolution MPF maps demonstrated sharp white-gray matter contrast and clear visualization of anatomical details, including gray matter structures with high iron content.

CONCLUSIONS

The proposed synthetic reference method improves resolution of MPF mapping and combines accurate MPF measurements with unique neuroanatomical contrast features.

Fast whole-brain three-dimensional macromolecular proton fraction mapping in multiple sclerosis

PURPOSE

To evaluate the clinical utility of fast whole-brain macromolecular proton fraction ( MPF macromolecular proton fraction ) mapping in multiple sclerosis ( MS multiple sclerosis ) and compare MPF macromolecular proton fraction with established quantitative magnetic resonance (MR) imaging measures of tissue damage including magnetization transfer ( MT magnetization transfer ) ratio and relaxation rate (R1).

MATERIALS AND METHODS

In this institutional review board-approved and HIPAA-compliant study, 14 healthy control participants, 18 relapsing-remitting MS multiple sclerosis ( RRMS relaxing-remitting MS ) patients, and 12 secondary progressive MS multiple sclerosis ( SPMS secondary progressive MS ) patients provided written informed consent and underwent 3-T MR imaging. Three-dimensional MPF macromolecular proton fraction maps were reconstructed from MT magnetization transfer -weighted images and R1 maps by the single-point method. Mean MPF macromolecular proton fraction , R1, and MT magnetization transfer ratio in normal-appearing white matter ( WM white matter ), gray matter ( GM gray matter ), and lesions were compared between subject groups by using analysis of variance. Correlations (Pearson r) between imaging data and clinical scores (Expanded Disability Status Scale [EDSS] and MS multiple sclerosis Functional Composite [ MSFC MS functional composite ]) were compared by using Hotelling-Williams test.

RESULTS

RRMS relaxing-remitting MS patients had lower WM white matter and GM gray matter MPF macromolecular proton fraction than controls, with percentage decreases of 6.5% (P < .005) and 5.4% (P < .05). MPF macromolecular proton fraction in SPMS secondary progressive MS was reduced relative to RRMS relaxing-remitting MS in WM white matter , GM gray matter , and lesions by 6.4% (P < .005), 13.4% (P < .005), and 11.7% (P < .05), respectively. EDSS Expanded Disability Status Scale and MSFC MS functional composite demonstrated strongest correlations with MPF macromolecular proton fraction in GM gray matter (r = -0.74 and 0.81; P < .001) followed by WM white matter (r = -0.57 and 0.72; P < .01) and lesions (r = -0.42 and 0.50; P < .05). R1 and MT magnetization transfer ratio in all tissues were significantly less correlated with clinical scores than GM gray matter MPF macromolecular proton fraction (P < .05).

CONCLUSION

MPF macromolecular proton fraction mapping enables quantitative assessment of demyelination in normal-appearing brain tissues and shows primary clinical relevance of GM gray matter damage in MS multiple sclerosis . MPF macromolecular proton fraction outperforms MT magnetization transfer ratio and R1 in detection of MS multiple sclerosis -related tissue changes.

Neuroimaging, behavioral, and psychological sequelae of repetitive combined blast/impact mild traumatic brain injury in Iraq and Afghanistan war veterans

Abstract Whether persisting cognitive complaints and postconcussive symptoms (PCS) reported by Iraq and Afghanistan war veterans with blast- and/or combined blast/impact-related mild traumatic brain injuries (mTBIs) are associated with enduring structural and/or functional brain abnormalities versus comorbid depression or posttraumatic stress disorder (PTSD) remains unclear. We sought to characterize relationships among these variables in a convenience sample of Iraq and Afghanistan-deployed veterans with (n=34) and without (n=18) a history of one or more combined blast/impact-related mTBIs. Participants underwent magnetic resonance imaging of fractional anisotropy (FA) and macromolecular proton fraction (MPF) to assess brain white matter (WM) integrity; [(18)F]-fluorodeoxyglucose positron emission tomography imaging of cerebral glucose metabolism (CMRglu); structured clinical assessments of blast exposure, psychiatric diagnoses, and PTSD symptoms; neurologic evaluations; and self-report scales of PCS, combat exposure, depression, sleep quality, and alcohol use. Veterans with versus without blast/impact-mTBIs exhibited reduced FA in the corpus callosum; reduced MPF values in subgyral, longitudinal, and cortical/subcortical WM tracts and gray matter (GM)/WM border regions (with a possible threshold effect beginning at 20 blast-mTBIs); reduced CMRglu in parietal, somatosensory, and visual cortices; and higher scores on measures of PCS, PTSD, combat exposure, depression, sleep disturbance, and alcohol use. Neuroimaging metrics did not differ between participants with versus without PTSD. Iraq and Afghanistan veterans with one or more blast-related mTBIs exhibit abnormalities of brain WM structural integrity and macromolecular organization and CMRglu that are not related to comorbid PTSD. These findings are congruent with recent neuropathological evidence of chronic brain injury in this cohort of veterans.

Magnetic Resonance Imaging Tracking of Graft Survival in the Infarcted Heart: Iron Oxide Particles Versus Ferritin Overexpression Approach

The main objective of cell therapy is the regeneration of damaged tissues. To distinguish graft from host tissue by magnetic resonance imaging (MRI), a paramagnetic label must be introduced to cells prior to transplantation. The paramagnetic label can be either exogenous iron oxide nanoparticles or a genetic overexpression of ferritin, an endogenous iron storage protein. The purpose of this work was to compare the efficacy of these 2 methods for MRI evaluation of engrafted cell survival in the infarcted mouse heart. Mouse skeletal myoblasts were labeled either by cocultivation with iron oxide particles or by engineering them to overexpress ferritin. Along with live cell transplantation, 2 other groups of mice were injected with dead-labeled cells. Both particle-labeled and ferritin-tagged grafts were detected as areas of MRI signal hypointensity in the left ventricle of the mouse heart using T2*-weighted sequences, although the signal attenuation decreased with ferritin tagging. Importantly, live cells could not be distinguished from dead cells when labeled with iron oxide particles, whereas the ferritin tagging was detected only in live grafts, thereby allowing identification of viable grafts using MRI. Thus, iron oxide particles can provide information about initial cell injection success but cannot assess graft viability. On the other hand, genetically based cell tagging, such as ferritin overexpression, despite having lower signal intensity in comparison with iron oxide particles, is able to identify live transplanted cells.

Quantification of MRI signal of transgenic grafts overexpressing ferritin in murine myocardial infarcts

The noninvasive detection of transplanted cells in damaged organs and the longitudinal follow-up of cell fate and graft size are important for the evaluation of cell therapy. We have shown previously that the overexpression of the natural iron storage protein, ferritin, permits the detection of engrafted cells in mouse heart by MRI, but further imaging optimization is required. Here, we report a systematic evaluation of ferritin-based stem cell imaging in infarcted mouse hearts in vivo using three cardiac-gated pulse sequences in a 3-T scanner: black-blood proton-density-weighted turbo spin echo (PD TSE BB), bright-blood T(2) -weighted gradient echo (GRE) and black-blood T(2) -weighted GRE with improved motion-sensitized-driven equilibrium (iMSDE) preparation. Transgenic C2C12 myoblast grafts overexpressing ferritin did not change MRI contrast in the PD TSE BB images, but showed a 20% reduction in signal intensity ratio in black-blood T(2) -weighted iMSDE (p < 0.05) and a 30% reduction in bright-blood T(2) -weighted GRE (p < 0.0001). Graft size measurements by T(2) iMSDE and T(2) GRE were highly correlated with histological assessments (r = 0.79 and r = 0.89, respectively). Unlabeled wild-type C2C12 cells transplanted to mouse heart did not change the MRI signal intensity, although endogenous hemosiderin was seen in some infarcts. These data support the use of ferritin to track the survival, growth and migration of stem cells transplanted into the injured heart.

Fast macromolecular proton fraction mapping from a single off-resonance magnetization transfer measurement

A new method was developed for fast quantitative mapping of the macromolecular proton fraction defined within the two-pool model of magnetization transfer. The method utilizes a single image with off-resonance saturation, a reference image for data normalization, and T(1), B(0), and B(1) maps with the total acquisition time ~10 min for whole-brain imaging. Macromolecular proton fraction maps are reconstructed by iterative solution of the matrix pulsed magnetization transfer equation with constrained values of other model parameters. Theoretical error model describing the variance due to noise and the bias due to deviations of constrained parameters from their actual values was formulated based on error propagation rules. The method was validated by comparison with the conventional multiparameter multipoint fit of the pulsed magnetization transfer model based on data from two healthy subjects and two multiple sclerosis patients. It was demonstrated theoretically and experimentally that accuracy of the method depends on the offset frequency and flip angle of the saturation pulse, and optimal ranges of these parameters are 4-7 kHz and 600°-900°, respectively. At optimal sampling conditions, the single-point method enables <10% relative macromolecular proton fraction errors. Comparison with the multiparameter fitting method revealed very good agreement with no significant bias and limits of agreement around ± 0.7%.

Simultaneous variable flip angle-actual flip angle imaging method for improved accuracy and precision of three-dimensional T1 and B1 measurements

A new time-efficient and accurate technique for simultaneous mapping of T(1) and B(1) is proposed based on a combination of the actual flip angle (FA) imaging and variable FA methods. Variable FA-actual FA imaging utilizes a single actual FA imaging and one or more spoiled gradient-echo acquisitions with a simultaneous nonlinear fitting procedure to yield accurate T(1)/B(1) maps. The advantage of variable FA-actual FA imaging is high accuracy at either short T(1) times or long repetition times in the actual FA imaging sequence. Simulations show this method is accurate to 0.03% in FA and 0.07% in T(1) for ratios of repetition time to T1 time over the range of 0.01-0.45. We show for the case of brain imaging that it is sufficient to use only one small FA spoiled gradient-echo acquisition, which results in reduced spoiling requirements and a significant scan time reduction compared to the original variable FA method. In vivo validation yielded high-quality 3D T(1) maps and T(1) measurements within 10% of previously published values and within a clinically acceptable scan time. The variable FA-actual FA imaging method will increase the accuracy and clinical feasibility of many quantitative MRI methods requiring T(1)/B(1) mapping such as dynamic contrast enhanced perfusion and quantitative magnetization transfer imaging.

Fast bound pool fraction imaging of the in vivo rat brain: association with myelin content and validation in the C6 glioma model

Cross-relaxation imaging (CRI) is a quantitative magnetic resonance technique that measures the kinetic parameters of magnetization transfer between protons bound to water and protons bound to macromolecules. In this study, in vivo, four-parameter CRI of normal rat brains (N=5) at 3.0 T was first directly compared to histology. The bound pool fraction, f, was strongly associated with myelin density (Pearson's r=0.99, p<0.001). The correlation persisted in separate analyses of gray matter (GM; r=0.89, p=0.046) and white matter (WM; r=0.97, p=0.029). Subsequently, a new time-efficient approach for solely capturing the whole-brain parametric map of f was proposed, validated with histology, and used to estimate myelin density. Since the described approach for the rapid acquisition of f applied constraints to other CRI parameters, a theoretical analysis of error was performed. Estimates of f in normal and pathologic tissue were expected to have <10% error. A comparison of values for f obtained from the traditional four-parameter fit of CRI data versus the proposed rapid acquisition of f was within this expected margin for in vivo rat brain gliomas (N=4; mean±SE; 3.9±0.2% vs. 4.0±0.2%, respectively). In both whole-brain f maps and myelin density maps, replacement of normal GM and WM by proliferating and invading tumor cells could be readily identified. The rapid, whole-brain acquisition of the bound pool fraction may provide a reliable method for detection of glioma invasion in both GM and WM during animal and human imaging.

Efficient flow suppressed MRI improves interscan reproducibility of carotid atherosclerosis plaque burden measurements

PURPOSE

To determine if better flow suppression can meaningfully improve the reproducibility of measurements associated with carotid atherosclerotic disease, particularly for lumen and wall areas.

MATERIALS AND METHODS

Eighteen subjects with carotid artery stenosis identified by duplex ultrasound (11 with 16%-49% stenosis; 7 with 50%-79% stenosis) underwent two carotid magnetic resonance imaging (MRI) examinations on a 3T scanner with a 4-channel phased array coil. High-resolution intermediate-weighted TSE (TR/TE = 4000/8.5 msec, 0.55 mm in-plane resolution, 2 mm slice thickness, 16 slices, 3-minute scan time) with two different flow-suppression techniques (multislice double inversion recovery [mDIR] and motion-sensitized driven-equilibrium [MSDE]) were obtained separately. For each subject, bilateral arteries were reviewed. One radiologist blinded to timepoints, flow suppression techniques, and clinical information measured the arterial lumen area, wall area, and total vessel wall area.

RESULTS

Compared to mDIR, the MSDE technique had a smaller interscan standard deviation (SD) in lumen (SD: 3.6 vs. 5.2 mm(2), P = 0.02), wall area measurements (SD: 4.5 vs. 6.4 mm(2), P = 0.02), and a trend towards smaller SD in total vessel area measurement (SD: 4.4 vs. 4.9 mm(2), P = 0.07).

CONCLUSION

The results from this study demonstrate that vessel wall imaging could quantify atherosclerotic plaque measurements more reliably with an improved blood suppression technique. This relationship between flow-suppression efficiency and reproducibility of plaque measurements is important, as more reliable area measurements will be useful in clinical diagnosis and in serial MRI studies that monitor carotid atherosclerotic lesion progression and regression.

Carotid plaque assessment using fast 3D isotropic resolution black-blood MRI

Black-blood MRI is a promising tool for carotid atherosclerotic plaque burden assessment and compositional analysis. However, current sequences are limited by large slice thickness. Accuracy of measurement can be improved by moving to isotropic imaging but can be challenging for patient compliance due to long scan times. We present a fast isotropic high spatial resolution (0.7×0.7×0.7 mm3) three-dimensional black-blood sequence (3D-MERGE) covering the entire cervical carotid arteries within 2 min thus ensuring patient compliance and diagnostic image quality. The sequence is optimized for vessel wall imaging of the carotid bifurcation based on its signal properties. The optimized sequence is validated on patients with significant carotid plaque. Quantitative plaque morphology measurements and signal-to-noise ratio measures show that 3D-MERGE provides good blood suppression and comparable plaque burden measurements to existing MRI protocols. 3D-MERGE is a promising new tool for fast and accurate plaque burden assessment in patients with atherosclerotic plaque.

Enhanced image quality in black-blood MRI using the improved motion-sensitized driven-equilibrium (iMSDE) sequence

PURPOSE

To propose an improved motion-sensitized driven-equilibrium (iMSDE) pulse sequence to enhance the tissue signal-to-noise ratio (SNR) while maintaining the same flow suppression capability in black-blood carotid artery magnetic resonance imaging (MRI).

MATERIALS AND METHODS

Compared to the traditional MSDE sequence, the iMSDE sequence uses an extra refocusing pulse and two extra gradients to achieve SNR improvement. Computer simulation and phantom studies were used to evaluate both eddy currents and local B(1) inhomogeneity effects on SNR behaviors on both MSDE and iMSDE images. To further assess the SNR improvements brought by iMSDE in vivo, five healthy volunteers were also scanned with both sequences. The paired t-test was used for statistical comparison.

RESULTS

Both simulations and phantom studies demonstrated that eddy currents and local B(1) inhomogeneity will cause image SNR reduction in the MSDE sequence, and that these factors can be partially compensated for with the iMSDE sequence. In vivo comparison showed that the iMSDE sequence significantly improved the tissue-lumen contrast-to-noise ratio (CNR) and static tissue SNR (P < 0.001 for both), while maintaining low lumen SNR in carotid MRI.

CONCLUSION

Compared to the traditional MSDE sequence, the iMSDE sequence can achieve improved soft-tissue SNR and CNR in carotid artery MRI without sacrificing flow suppression capability and time efficiency.

MR carotid plaque imaging and contrast-enhanced MR angiography identifies lesions associated with recent ipsilateral thromboembolic symptoms: an in vivo study at 3T

BACKGROUND AND PURPOSE

Recent research has suggested the importance of plaque composition to identify patients at risk for stroke. This study aims to identify specific plaque features on 3T carotid MR imaging and CE-MRA associated with recent carotid thromboembolic symptoms in patients with mild/moderate versus severe stenosis.

MATERIALS AND METHODS

Ninety-seven consecutive patients (symptomatic, 13; asymptomatic, 84) with 50%-99% stenosis by sonography or CT angiography underwent carotid plaque imaging combined with MRA at 3T. The symptomatic carotid artery or the most stenotic asymptomatic carotid artery was chosen as the index vessel to be analyzed. Plaque features were compared by symptomatic status in patients with mild/moderate (30%-70%) versus severe (70%-99%) stenosis on MRA.

RESULTS

Ninety (92.8%) patients had sufficient image quality for interpretation. In 50 patients with mild/moderate stenosis, there were significant associations between the presence of the following plaque characteristics and symptoms: thin/ruptured fibrous cap (100% versus 36%, P = .006) and lipid-rich necrotic core (100% versus 39%, P = .022), with marginal association with hemorrhage (86% versus 33%, P = .055). In 40 patients with severe stenosis, only the angiographic presence of ulceration (86% versus 36%, P = .039) was associated with symptoms.

CONCLUSIONS

Several plaque components identified on 3T MR imaging are correlated with recent ipsilateral carotid thromboembolic symptoms. These preliminary results also suggest that associations between plaque characteristics and symptom history may vary by degree of stenosis. If confirmed in larger studies, carotid MR imaging may distinguish stable from unstable lesions, particularly in individuals with mild/moderate stenosis in whom the role of surgical intervention is currently unclear.

Arterial remodeling in [corrected] subclinical carotid artery disease

OBJECTIVES

We sought to identify clinical and/or plaque characteristics that affect atherosclerotic disease progression and arterial remodeling in the carotid artery with subclinical stenosis.

BACKGROUND

Increasing severity of stenosis has been associated with a higher risk of stroke. Factors that drive subclinical lesions to become stenotic plaques remain ambiguous. Carotid magnetic resonance imaging (MRI) has been validated with histology to accurately quantify in vivo arterial morphology and plaque composition.

METHODS

A total of 67 asymptomatic participants with 16% to 49% carotid stenosis as demonstrated by duplex ultrasonography were imaged at 1.5-T with a carotid MRI protocol at baseline and at 18-month follow-up. Clinical and/or intra-arterial metrics with a significant association with change in plaque burden during multivariate analysis were evaluated for effects on lumen, wall, and total vessel volume.

RESULTS

From multiple regression analysis, intraplaque hemorrhage (IPH) (p < 0.001) and statin therapy (p = 0.015) were identified as key determinants of change in plaque burden. The group with IPH compared with the group without IPH demonstrated luminal narrowing, with a mean +/- SD decrease in lumen volume (-24.9 +/- 21.1 mm(3)/year vs. -0.5 +/- 26.9 mm(3)/year; p = 0.005), a larger increase in wall volume (44.1 +/- 36.1 mm(3)/year vs. 0.8 +/- 34.5 mm(3)/year; p < 0.001), and no difference in total vessel volume (19.3 +/- 27.4 mm(3)/year vs. 0.4 +/- 42.4 mm(3)/year; p = 0.15). The nonstatin group compared with the statin group demonstrated outward remodeling, with an increase in wall volume (22.4 +/- 35.6 mm(3)/year(3)/year vs. 0.9 +/- 38.0 mm(3)/year; p = 0.026) and total vessel volume (19.2 +/- 36.9 mm(3)/year vs. -4.9 +/- 40.4 mm(3)/year; p = 0.019) and no difference in lumen volume (-5.8 +/- 26.6 mm(3)/year vs. -3.2 +/- 29.5 mm(3)/year; p = 0.72).

CONCLUSIONS

IPH may represent an indication of accelerated plaque growth and impending luminal compromise in the subclinical carotid artery. Statin therapy may stabilize lesions by slowing or halting lesion progression. This phase of plaque stenosis (16% to 49%) may be a critical stage for intrinsic and extrinsic factors to affect the atherosclerotic disease process.

Carotid intraplaque hemorrhage imaging at 3.0-T MR imaging: comparison of the diagnostic performance of three T1-weighted sequences

PURPOSE

To compare the diagnostic performances of three T1-weighted 3.0-T magnetic resonance (MR) sequences at carotid intraplaque hemorrhage (IPH) imaging, with histo logic analysis as the reference standard.

MATERIALS AND METHODS

Institutional review board approval and informed consent were obtained for this HIPAA-compliant study. Twenty patients scheduled for carotid endarterectomy underwent 3.0-T carotid MR imaging, including two-dimensional fast spin-echo, three-dimensional time-of-flight (TOF), and three-dimensional magnetization-prepared rapid acquisition gradient-echo (RAGE) sequences. Two reviewers blinded to the histologic findings assessed the presence, area, and signal intensity of IPH with each sequence. Detection statistics (sensitivity, specificity, and Cohen kappa values) and agreement between area measurements (Pearson correlation coefficient [r] values) were calculated for each sequence.

RESULTS

When all 231 available MR sections were included for analysis, the magnetization-prepared RAGE (kappa = 0.53) and fast spin-echo (kappa = 0.42) sequences yielded moderate agreement between MR and histologic measurements, while the TOF sequence yielded fair agreement (k = 0.33). However, when 47 sections with either small IPHs or heavily calcified IPHs were excluded, sensitivity, specificity, and kappa values, respectively, were 80%, 97%, and 0.80 for magnetization-prepared RAGE imaging; 70%, 92%, and 0.63 for fast spin-echo imaging; and 56%, 96%, and 0.57 for TOF imaging. MR imaging-histologic analysis correlation for IPH area was highest with magnetization-prepared RAGE imaging (r = 0.813), followed by TOF (r = 0.745) and fast spin-echo (r = 0.497) imaging. The capability of these three sequences for IPH detection appeared to be in good agreement with the quantitative contrast of IPH versus background plaque tissue.

CONCLUSION

The magnetization-prepared RAGE sequence, as compared with the fast spin-echo and TOF sequences, demonstrated higher diagnostic capability for the detection and quantification of IPH. Potential limitations of 3.0-T IPH MR imaging are related to hemorrhage size and coexisting calcification.

Improvements in carotid plaque imaging using a new eight-element phased array coil at 3T

PURPOSE

To design and compare an eight-channel phased array (PA) coil for carotid imaging to an established four-channel PA design at 3T.

MATERIALS AND METHODS

An eight-channel PA (8PA) coil was designed specifically for imaging the carotid bifurcation and compared with the existing four-channel (4PA) design using a phantom and by in vivo black-blood magnetic resonance imaging (MRI). The 8PA and 4PA were compared in terms of coverage, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR).

RESULTS

The 8PA showed up to 1.7-fold improvement in SNR at a depth of 3.5 cm and greater longitudinal coverage at a given SNR on a phantom. The 8PA showed improved vessel wall SNR for high spatial resolution (0.63 mm(2)) PD, T1, and T2 (1.7, 1.7, 1.6 times, respectively; P <or= 0.002) and improved CNR (1.7, 1.6, 1.5 times, respectively; P <or= 0.002). Ultrahigh-resolution (0.27 mm(2)) T1-weighted images showed better SNR and CNR (1.4 times, P <or= 0.0001) on 8PA compared to 4PA.

CONCLUSION

Carotid imaging studies may benefit from the improved SNR and larger coverage provided by use of the 8PA.

Predictors of surface disruption with MR imaging in asymptomatic carotid artery stenosis

BACKGROUND AND PURPOSE

Surface disruption, either ulceration or fibrous cap rupture, has been identified as a key feature of the unstable atherosclerotic plaque. In this prospective observational study, we sought to determine the characteristics of the carotid lesion that predict the development of new surface disruption.

MATERIALS AND METHODS

One hundred eight asymptomatic individuals with 50%-79% carotid stenosis underwent carotid MR imaging at baseline and at 3 years. Multicontrast imaging criteria were used to determine the presence or absence of calcification, LRNC, intraplaque hemorrhage, and surface disruption. Volume measurements of plaque morphology and the LRNC and calcification, when present, were collected.

RESULTS

At baseline, 21.3% (23/108) of participants were identified with a surface disruption. After 3 years, 9 (10.6%) of the remaining 85 individuals without disruption at baseline developed a new surface disruption during follow-up. Among all baseline variables associated with new surface disruption during regression analysis, the proportion of wall volume occupied by the LRNC (percentage LRNC volume; OR per 5% increase, 2.6; 95% CI, 1.5-4.6) was the strongest classifier (AUC = 0.95) during ROC analysis. New surface disruption was associated with a significant increase in percentage LRNC volume (1.7 +/- 2.0% per year, P = .035).

CONCLUSIONS

This prospective investigation of asymptomatic individuals with 50%-79% stenosis provides compelling evidence that LRNC size may govern the risk of future surface disruption. Identification of carotid plaques in danger of developing new surface disruption may prove clinically valuable for preventing the transition from stable to unstable atherosclerotic disease.

Direct quantitative comparison between cross-relaxation imaging and diffusion tensor imaging of the human brain at 3.0 T

Cross-relaxation imaging (CRI) describes the magnetization transfer within tissues between mobile water protons and macromolecular protons. Whole-brain parametric maps of the principle kinetic components of magnetization transfer, the fraction of macromolecular protons (f) and the rate constant (k), revealed detailed anatomy of white matter (WM) fiber tracts at 1.5 T. In this study, CRI was first adapted to 3.0 T, and constraints for transverse relaxation times of water and macromolecular protons were identified to enable unbiased f and k estimation. Subsequently, whole-brain CRI and diffusion tensor imaging (DTI) were performed in five healthy subjects. The parameters f and k were compared to DTI indices (fractional anisotropy (FA), apparent diffusion coefficient (ADC), radial diffusivity (RD), and axial diffusivity (AD)) across a range of anatomic regions. In WM, neither f nor k was significantly correlated to FA, RD, and AD. In contrast, both f (r=0.90 and r=-0.80) and k (r=0.92 and r=-0.89) in gray matter (GM) were strongly correlated to FA and RD, respectively. A moderate correlation between ADC and k (r=0.48) was identified in WM, while an inverse correlation was identified in GM (r=-0.72). The lack of association between CRI and FA in WM is consistent with differences in the underlying physical principles between techniques - fiber density vs. directionality, respectively. The association in GM may be attributable to variable axonal density unique to each structure. Our findings suggest that whole-brain CRI provides distinct quantitative information compared to DTI, and CRI parameters may prove constructive as biomarkers in neurological diseases.

Comparison between 2D and 3D high-resolution black-blood techniques for carotid artery wall imaging in clinically significant atherosclerosis

PURPOSE

To compare two- (2D) and three-dimensional (3D) black-blood imaging methods for morphological measurements of the carotid artery wall and atherosclerotic plaque.

MATERIALS AND METHODS

A total of 18 subjects with 50% to 79% carotid stenosis were scanned with 2D (2-mm slice thickness) and 3D (1-mm/0.5-mm actual/interpolated slice thickness) T1-weighted fast spin-echo (FSE) black-blood imaging sequences with double inversion-recovery (DIR) blood suppression. Morphological measurements (lumen area, wall area, vessel area, mean wall thickness, and maximal wall thickness), signal-to-noise ratio (SNR) in the wall and lumen, and wall-lumen contrast-to-noise ratio (CNR) were compared between 2D and 3D images. The effect of improved slice resolution in 3D imaging was evaluated for visualization of small plaque components.

RESULTS

Lumen SNR (P = 0.16), wall SNR (P = 0.65), and CNR (P = 0.94) were comparable between 2D/3D. There was no difference in average lumen area (P = 0.16), average wall area (P = 0.99), average vessel area (P = 0.0.58), mean wall thickness (P = 0.09), and maximum wall thickness (P = 0.06) between 2D/3D. Distributions of small plaque components such as calcification were better characterized by the 3D acquisition. There was a higher sensitivity to motion artifacts with 3D imaging, resulting in three examinations with low image quality.

CONCLUSION

2D and 3D protocols provided comparable morphometric measurements of the carotid artery. The major advantage of 3D imaging is improved small plaque component visualization, while the 2D technique provides higher reliability for image quality.

Carotid plaque morphology and composition: initial comparison between 1.5- and 3.0-T magnetic field strengths

PURPOSE

To prospectively compare the interpretation and quantification of carotid vessel wall morphology and plaque composition at 1.5-T with those at 3.0-T magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Twenty participants (mean age, 69.8 years [standard deviation] +/- 10.5; 75% men) with 16%-79% carotid stenosis at duplex ultrasonography were imaged with 1.5-T and 3.0-T MR imaging units with bilateral four-element phased-array surface coils. This HIPAA-compliant study was approved by the institutional review board, and all participants gave written informed consent. Protocols designed for similar signal-to-noise ratios across platforms were implemented to acquire axial T1-weighted, T2-weighted, intermediate-weighted, time-of-flight, and contrast material-enhanced T1-weighted images. Lumen area, wall area, total vessel area, wall thickness, and presence or absence and area of plaque components were documented. Continuous variables from different field strengths were compared by using the intraclass correlation coefficient (ICC) and repeated measures analysis. The Cohen kappa was used to evaluate agreement between 1.5 T and 3.0 T on compositional dichotomous variables.

RESULTS

There was a strong level of agreement between field strengths for all morphologic variables, with ICCs ranging from 0.88 to 0.96. Agreement in the identification of presence or absence of plaque components was very good for calcification (kappa = 0.72), lipid-rich necrotic core (kappa = 0.73), and hemorrhage (kappa = 0.66). However, the visualization of hemorrhage was greater at 1.5 T than at 3.0 T (14.7% vs 7.8%, P < .001). Calcifications measured significantly (P = .03) larger at 3.0 T, while lipid-rich necrotic cores without hemorrhage were similar between field strengths (P = .9).

CONCLUSION

At higher field strengths, the increased susceptibility of calcification and paramagnetic ferric iron in hemorrhage may alter quantification and/or detection. Nevertheless, imaging criteria at 1.5 T for carotid vessel wall interpretation are applicable at 3.0 T.

Micro magnetic resonance angiography of the finger in systemic sclerosis

OBJECTIVE

To characterize vascular lesions in SSc disease with high-resolution magnetic resonance angiography (Micro-MRA) of the finger.

METHODS

Eight SSc subjects and eight age- and sex-matched healthy controls were recruited for this study. Among the SSc subjects, the mean +/- s.d. age was 54.5 +/- 4.9 yrs, and the mean +/- s.d. duration of disease was 8.3 +/- 8.4 yrs. The numbers of SSc subjects that had telangiectasia, calcinosis and impaired finger flexion were 3, 2 and 3, respectively. The 2D time-of-flight micro-MRA was performed on a 3T clinical MRI scanner using a custom-designed finger coil with an in-plane resolution of 0.16 x 0.21 mm(2) and slice thickness of 1.2 mm. The data for the proper palmar digital artery lumen area, the number of visible dorsal digital veins and a semi-quantitative vascular score, which evaluates the overall integrity of digital vessels, were independently evaluated by two experienced reviewers who were blinded to the status of the subject.

RESULTS

Micro-MRA detected significant differences in the digital vasculature between SSc subjects and healthy volunteers. The SSc subjects had a significantly decreased digital artery lumen area (0.13 +/- 0.06 vs 0.53 +/- 0.26 mm(2), P < 0.001), a reduced number of digital veins (0.63 +/- 1.06 vs 3.13 +/- 0.99, P = 0.001) and a lowered overall vascular score (1.75 +/- 1.04 vs 3.5 +/- 0.53, P = 0.001). The study also found that both the digital artery lumen area (Pearson's; r = -0.72, P = 0.044) and vascular scores (Spearman's; rho = -0.75, P = 0.047) of the SSc subjects were inversely correlated with the duration of the disease.

CONCLUSIONS

Micro-MRA can be used to identify and quantitatively characterize the vascular disease in SSc fingers. The parameters derived from micro-MRA could potentially be used as prospective biomarkers for clinical evaluation.

Differences in carotid arterial morphology and composition between individuals with and without obstructive coronary artery disease: a cardiovascular magnetic resonance study

OBJECTIVE

We sought to determine differences with cardiovascular magnetic resonance (CMR) in the morphology and composition of the carotid arteries between individuals with angiographically-defined obstructive coronary artery disease (CAD, > or = 50% stenosis, cases) and those with angiographically normal coronaries (no lumen irregularities, controls).

METHODS AND RESULTS

191 participants (50.3% female; 50.8% CAD cases) were imaged with a multi-sequence, carotid CMR protocol at 1.5T. For each segment of the carotid, lumen area, wall area, total vessel area (lumen area + wall area), mean wall thickness and the presence or absence of calcification and lipid-rich necrotic core were recorded bilaterally. In male CAD cases compared to male controls, the distal bulb had a significantly smaller lumen area (60.0 +/- 3.1 vs. 79.7 +/- 3.2 mm2, p < 0.001) and total vessel area (99.6 +/- 4.0 vs. 119.8 +/- 4.1 mm2; p < 0.001), and larger mean wall thickness (1.25 +/- 0.03 vs. 1.11 +/- 0.03 mm; p = 0.002). Similarly, the internal carotid had a smaller lumen area (37.5 +/- 1.8 vs. 44.6 +/- 1.8 mm2; p = 0.006) and smaller total vessel area (64.0 +/- 2.3 vs. 70.9 +/- 2.4 mm2; p = 0.04). These metrics were not significantly different between female groups in the distal bulb and internal carotid or for either gender in the common carotid. Male CAD cases had an increased prevalence of lipid-rich necrotic core (49.0% vs. 19.6%; p = 0.003), while calcification was more prevalent in both male (46.9% vs. 17.4%; p = 0.002) and female (33.3% vs. 14.6%; p = 0.031) CAD cases compared to controls.

CONCLUSION

Males with obstructive CAD compared to male controls had carotid bulbs and internal carotid arteries with smaller total vessel and lumen areas, and an increased prevalence of lipid-rich necrotic core. Carotid calcification was related to CAD status in both males and females. Carotid CMR identifies distinct morphological and compositional differences in the carotid arteries between individuals with and without angiographically-defined obstructive CAD.

Improved suppression of plaque-mimicking artifacts in black-blood carotid atherosclerosis imaging using a multislice motion-sensitized driven-equilibrium (MSDE) turbo spin-echo (TSE) sequence

In this study, a turbo spin-echo (TSE) based motion-sensitized driven-equilibrium (MSDE) sequence was used as an alternative black-blood (BB) carotid MRI imaging scheme. The MSDE sequence was first optimized for more efficient residual blood signal suppression in the carotid bulb of healthy volunteers. Effective contrast-to-noise ratio (CNR(eff)) and residual signal-to-noise ratio (SNR) in the lumen measured from MSDE images were then compared to those measured from inflow saturation (IS) and double inversion-recovery (DIR) images. Statistically significant higher CNR(eff) and lower lumen SNR were obtained from MSDE images. To assess MSDE sequence in a clinical carotid protocol, 42 locations from six subjects with 50% to 79% carotid stenosis by duplex ultrasound were scanned with both MSDE and multislice DIR. The comparison showed that MSDE images present significantly higher CNR and lower lumen SNR compared to corresponding multislice DIR images. The vessel wall area and mean wall thickness measurements in MSDE images were slightly but significantly lower than those obtained with other blood suppression techniques. In conclusion, in vivo comparisons demonstrated that MSDE sequence can achieve better blood suppression and provide a more accurate depiction of the lumen boundaries by eliminating plaque mimicking artifacts in carotid artery (CA) imaging.

Reader and platform reproducibility for quantitative assessment of carotid atherosclerotic plaque using 1.5T Siemens, Philips, and General Electric scanners

PURPOSE

To evaluate the platform and reader reproducibility of quantitative carotid plaque measurements.

MATERIALS AND METHODS

A total of 32 individuals with >or=15% carotid stenosis by duplex ultrasound were each imaged once by a 1.5T General Electric (GE) whole body scanner and twice by either a 1.5T Philips scanner or a 1.5T Siemens scanner. A standardized multisequence protocol and identical phased-array carotid coils were used. Expert readers, blinded to subject information, scanner type, and time point, measured the lumen, wall, and total vessel areas and determined the modified American Heart Association lesion type (AHA-LT) on the cross-sectional images.

RESULTS

AHA-LT was consistently identified across the same (kappa = 0.75) and different scan platforms (kappa = 0.75). Furthermore, scan-rescan coefficients of variation (CV) of wall area measurements on Siemens and Philips scanners ranged from 6.3% to 7.5%. However, wall area measurements differed between Philips and GE (P = 0.003) and between Siemens and GE (P = 0.05). In general, intrareader reproducibility was higher than interreader reproducibility for AHA-LT identification as well as for quantitative measurements.

CONCLUSION

All three scanners produced images that allowed AHA-LT to be consistently identified. Reproducibility of quantitative measurements by Siemens and Philips scanners were comparable to previous studies using 1.5T GE scanners. However, bias was introduced with each scanner and the use of different readers substantially increased variability. We therefore recommend using the same platform and the same reader for scans of individual subjects undergoing serial assessment of carotid atherosclerosis.