High-resolution T1 mapping of the brain at 3T with driven equilibrium single pulse observation of T1 with high-speed incorporation of RF field inhomogeneities (DESPOT1-HIFI)

A novel method for simultaneous 3D B(1) and T(1) mapping: the method of slopes (MoS)

A novel three-dimensional simultaneous B(1) and T(1) mapping method is introduced: the method of slopes (MoS). The linearity of the spoiled gradient recalled echo (SPGR) signal vs flip angle relation is exploited: B(1) mapping is achieved by a two-point extrapolation to signal null with a correction scheme while T(1) mapping uses the slopes of the SPGR signal vs flip angle curves near the origin and near the signal null. This new method improves upon the existing variable flip angle (VFA) T(1)-mapping method in that (i) consistency between B(1) and T(1) maps is ensured (ii) the sampling scheme is T(1)-independent (iii) the noise bias and singularity, associated with using a linear form for the SPGR signal equation, is eliminated by using the full equation. The method is shown to yield accurate and robust results via simulations. Initial estimates of B(1) and T(1) values are obtained from three data points via simple computations and straight line approximations. Initial estimates of B(1) values, for a range of values, are shown to be accurate due to the proposed B(1) correction scheme. The accuracy and robustness of T(1) values is achieved via a non-linear fitting algorithm which includes a fourth data point sampled at high SNR. The MoS was validated by comparing resulting B(1) and T(1) maps with those obtained using other standard methods. Finally, the ability to obtain brain B(1) and T(1) maps using the MoS was demonstrated by in vivo experiments. The MoS is expected to perform well on other motion-free anatomical regions as well.

Investigating white matter development in infancy and early childhood using myelin water faction and relaxation time mapping

The elaboration of the myelinated white matter is essential for normal neurodevelopment, establishing and mediating rapid communication pathways throughout the brain. These pathways facilitate the synchronized communication required for higher order behavioral and cognitive functioning. Altered neural messaging (or ‘disconnectivity’) arising from abnormal white matter and myelin development may underlie a number of neurodevelopmental psychiatric disorders. Despite the vital role myelin plays, few imaging studies have specifically examined its maturation throughout early infancy and childhood. Thus, direct investigations of the relationship(s) between evolving behavioral and cognitive functions and the myelination of the supporting neural systems have been sparse. Further, without knowledge of the ‘normative’ developmental time-course, identification of early abnormalities associated with developmental disorders remains challenging. In this work, we examined the use of longitudinal (T₁) and transverse (T₂) relaxation time mapping, and myelin water fraction (MWF) imaging to investigate white matter and myelin development in 153 healthy male and female children, 3 months through 60 months in age. Optimized age-specific acquisition protocols were developed using the DESPOT and mcDESPOT imaging techniques; and mean T₁, T₂ and MWF trajectories were determined for frontal, temporal, occipital, parietal and cerebellar white matter, and genu, body and splenium of the corpus callosum. MWF results provided a spatio-temporal pattern in-line with prior histological studies of myelination. Comparison of T₁, T₂ and MWF measurements demonstrates dissimilar sensitivity to tissue changes associated with neurodevelopment, with each providing differential but complementary information.

Contrast-enhanced MRI of murine myocardial infarction - part II

Mouse models are increasingly used to study the pathophysiology of myocardial infarction in vivo. In this area, MRI has become the gold standard imaging modality, because it combines high spatial and temporal resolution functional imaging with a large variety of methods to generate soft tissue contrast. In addition, (target-specific) MRI contrast agents can be employed to visualize different processes in the cascade of events following myocardial infarction. Here, the MRI sequence has a decisive role in the detection sensitivity of a contrast agent. However, a straightforward translation of clinically available protocols for human cardiac imaging to mice is not feasible, because of the small size of the mouse heart and its extremely high heart rate. This has stimulated intense research in the development of cardiac MRI protocols specifically tuned to the mouse with regard to timing parameters, acquisition strategies, and ECG- and respiratory-triggering methods to find an optimal trade-off between sensitivity, scan time, and image quality. In this review, a detailed analysis is given of the pros and cons of different mouse cardiac MR imaging methodologies and their application in contrast-enhanced MRI of myocardial infarction.

Individual differences in brain structure underpin empathizing-systemizing cognitive styles in male adults

Individual differences in cognitive style can be characterized along two dimensions: 'systemizing' (S, the drive to analyze or build 'rule-based' systems) and 'empathizing' (E, the drive to identify another's mental state and respond to this with an appropriate emotion). Discrepancies between these two dimensions in one direction (S>E) or the other (E>S) are associated with sex differences in cognition: on average more males show an S>E cognitive style, while on average more females show an E>S profile. The neurobiological basis of these different profiles remains unknown. Since individuals may be typical or atypical for their sex, it is important to move away from the study of sex differences and towards the study of differences in cognitive style. Using structural magnetic resonance imaging we examined how neuroanatomy varies as a function of the discrepancy between E and S in 88 adult males from the general population. Selecting just males allows us to study discrepant E-S profiles in a pure way, unconfounded by other factors related to sex and gender. An increasing S>E profile was associated with increased gray matter volume in cingulate and dorsal medial prefrontal areas which have been implicated in processes related to cognitive control, monitoring, error detection, and probabilistic inference. An increasing E>S profile was associated with larger hypothalamic and ventral basal ganglia regions which have been implicated in neuroendocrine control, motivation and reward. These results suggest an underlying neuroanatomical basis linked to the discrepancy between these two important dimensions of individual differences in cognitive style.

Hydrogen-1 MR spectroscopy for measurement and diagnosis of hepatic steatosis

OBJECTIVE

Hydrogen-1 MR spectroscopy ((1)H-MRS) is gaining acceptance as a noninvasive technique for assessment of hepatic steatosis, and the findings have been found to correlate closely with histopathologic grade. The aims of this study were to validate (1)H-MRS performed with a 3-T MRI system for quantifying hepatic steatosis and to determine threshold values of (1)H-MRS proton density fat fraction corresponding to standard histopathologic grade in patients undergoing diagnostic liver biopsy.

SUBJECTS AND METHODS

We conducted a prospective cross-sectional liver MRS study with 52 subjects undergoing diagnostic liver biopsy. The diagnostic accuracy of (1)H-MRS was evaluated with receiver operating characteristic curves.

RESULTS

The diagnostic accuracy of (1)H-MRS for hepatic steatosis was high with an area under the receiver operating characteristic curve of 0.94 (95% CI, 0.88-1.0). Results were similar for three (1)H-MRS measurements obtained at different locations in the liver, for two independent pathologists, and whether fibrosis was present or absent. One third of participants had elevated transaminase concentrations of unknown cause, and (1)H-MRS estimates of steatosis had perfect agreement with histopathologic grade in this group. Calculated (1)H-MRS proton density fat fraction thresholds for histologic grades were less than 17% for grade 0 or trace steatosis, 17-38.6% for grade 1, and greater than 38.6% for grade 2 or higher.

CONCLUSION

Hydrogen-1 MR spectroscopy is an effective, noninvasive technique that can be used to diagnose and quantify hepatic steatosis. Hydrogen-1 MR spectroscopy thresholds corresponded with histopathologic grades and may be useful in the workup of patients with elevated transaminase concentrations.

Magnetic resonance imaging of the substantia nigra in Parkinson's disease

Until recently, conventional magnetic resonance imaging (MRI) was most often negative in Parkinson's disease or showed nonspecific findings. Recent developments in structural MRI, including relaxometry, magnetization transfer, and neuromelanin imaging, have demonstrated improved contrast and enabled more accurate visualization of deep brain nuclei, in particular, the substantia nigra. Meanwhile, diffusion imaging has provided useful biomarkers of substantia nigra degeneration, showing reduced anisotropy and anatomical connectivity with the striatum and thalamus. These advances in structural imaging are complemented by findings of magnetic resonance spectroscopy on brain metabolism and resting-state functional MRI on functional connectivity. This article presents an overview of these new structural, metabolic, and resting-state functional MRI techniques and their implications for Parkinson's disease. The techniques are reviewed in the context of their potential for better understanding the disease in terms of diagnosis and pathophysiology and as biomarkers of its progression.

Characterization of cerebral white matter properties using quantitative magnetic resonance imaging stains

The image contrast in magnetic resonance imaging (MRI) is highly sensitive to several mechanisms that are modulated by the properties of the tissue environment. The degree and type of contrast weighting may be viewed as image filters that accentuate specific tissue properties. Maps of quantitative measures of these mechanisms, akin to microstructural/environmental-specific tissue stains, may be generated to characterize the MRI and physiological properties of biological tissues. In this article, three quantitative MRI (qMRI) methods for characterizing white matter (WM) microstructural properties are reviewed. All of these measures measure complementary aspects of how water interacts with the tissue environment. Diffusion MRI, including diffusion tensor imaging, characterizes the diffusion of water in the tissues and is sensitive to the microstructural density, spacing, and orientational organization of tissue membranes, including myelin. Magnetization transfer imaging characterizes the amount and degree of magnetization exchange between free water and macromolecules like proteins found in the myelin bilayers. Relaxometry measures the MRI relaxation constants T1 and T2, which in WM have a component associated with the water trapped in the myelin bilayers. The conduction of signals between distant brain regions occurs primarily through myelinated WM tracts; thus, these methods are potential indicators of pathology and structural connectivity in the brain. This article provides an overview of the qMRI stain mechanisms, acquisition and analysis strategies, and applications for these qMRI stains.

B₁ mapping with selective pulses

Knowledge of B₁⁺ distribution is crucial for many applications, such as quantitative MRI. A novel method has been developed to improve the accuracy of the conventionally applied double-angle method for B₁⁺ mapping. It solves the remaining issues raised by the use of selective pulses for slice selection to accelerate the acquisition process. A general approach for reconstructing B₁⁺ maps is presented first. It takes B₁⁺-induced slice profile distortions over off-resonance frequencies into account. It is then shown how the ratio between the prescribed flip angles can be adjusted to reach a compromise between the level of noise propagated onto B₁⁺ maps and the width of the range in which the field can be mapped. Lastly, several solutions are proposed for reducing the B₁⁺-dependent pollution of regions distal to the image slice which participates significantly in the inaccuracy of B₁⁺ mapping. These methods were experimentally tested by comparison with gold standard B₁⁺ maps obtained on a phantom using a non-selective and thus much slower technique. As they are independent and lead to significant improvements, these solutions can be combined to achieve high precision and fast B₁⁺ mapping using spin-echo DAM.

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.

Mapping of low flip angles in magnetic resonance

Errors in the flip angle have to be corrected in many magnetic resonance imaging applications, especially for T1 quantification. However, the existing methods of B1 mapping fail to measure lower values of the flip angle despite the fact that these are extensively used in dynamic acquisition and 3D imaging. In this study, the nonlinearity of the radiofrequency (RF) transmit chain, especially for very low flip angles, is investigated and a simple method is proposed to accurately determine both the gain of the RF transmitter and the B1 field map for low flip angles. The method makes use of the spoiled gradient echo sequence with long repetition time (TR), such as applied in the double-angle method. It uses an image acquired with a flip angle of 90° as a reference image that is robust to B1 inhomogeneity. The ratio of the image at flip angle alpha to the image at a flip angle of 90° enables us to calculate the actual value of alpha. This study was carried out at 1.5 and 4.7 T, showing that the linearity of the RF supply system is highly dependent on the hardware. The method proposed here allows us to measure the flip angle from 1° to 60° with a maximal uncertainty of 10% and to correct T1 maps based on the variable flip angle method.

Relaxation time mapping in multiple sclerosis

Several relaxation mapping techniques have been proposed to quantitatively assess disease-related brain tissue changes in multiple sclerosis. Newer developments also account for the distribution of hydrogen protons in different tissue compartments, and therefore provide markers for myelin and macromolecular content. This article will cover the broad spectrum of the pulse sequences and analysis techniques related to this topic that are currently available. Various technical and practical limitations linked with specific approaches will be discussed. These include acquisition time, accuracy and precision, radiofrequency absorption and limited coverage of the brain. Finally, the application of these techniques in the context of multiple sclerosis will be reviewed.

Fast bound pool fraction mapping using stimulated echoes

Magnetization transfer imaging advanced to an indispensible tool for investigating white matter changes. Quantitative magnetization transfer imaging methods allow the determination of the bound pool fraction (BPF), which is thought to be directly linked to myelin integrity. Long acquisition times and high specific absorption rates are still inhibiting broad in vivo utilization of currently available BPF mapping techniques. Herewith, a stimulated echoes amplitude modulation-based, single-shot echo planar imaging technique for BPF and T(1) quantification is presented at 3T. It allows whole brain mapping in 10-15 min and is low in specific absorption rates. The method was validated with different concentrations of bovine serum albumin (BSA) phantoms. Intra- and inter-subject variability was assessed in vivo. Phantom measurements verified linearity between bovine serum albumin concentrations and measured BPF, which was independent of T(1) variations. T(1) values in the phantoms correlated well with values provided by standard T(1) mapping methods. Intrasubject variability was minimal and mean regional BPFs of 10 volunteers (e.g., left frontal white matter=0.135 ± 0.003, right frontal white matter=0.129 ± 0.006) were in line with previously published data. Assessment of interhemispheric BPF differences revealed significantly higher BPF for the left brain hemisphere. To sum up, these results suggest the proposed method useful for cross-sectional and longitudinal studies of white matter changes in the human brain.

Partial volume correction of multiple inversion time arterial spin labeling MRI data

The accuracy of cerebral blood flow (CBF) estimates from arterial spin labeling (ASL) is affected by the presence of both gray matter (GM) and white matter within any voxel. Recently a partial volume (PV) correction method for ASL has been demonstrated (Asllani et al. Magn Reson Med 2008; 60:1362-1371), where PV estimates were used with a local linear regression to separate the GM and white matter ASL signal. Here a new PV correction method for multi-inversion time ASL is proposed that exploits PV estimates within a spatially regularized kinetic curve model analysis. The proposed method exploits both PV estimates and the different kinetics of the ASL signal arising from GM and white matter. The new correction method is shown, on both simulated and real data, to provide correction of GM CBF comparable to a linear regression approach, whilst preserving greater spatial detail in the CBF image. On real data corrected GM CBF values were found to be largely independent of GM PV, implying that the correction had been successful. Increases of mean GM CBF after correction of 69-80% were observed.

Segmentation of the thalamus in MRI based on T1 and T2

Reliable identification of thalamic nuclei is required to improve positioning of electrodes in Deep Brain Stimulation (DBS), and to allow the role of individual thalamic nuclei in health and disease to be fully investigated. In this work, a previously proposed method for identifying sub-regions within the thalamus based on differences in their T1 and T2 values is explored in detail. The effect on the segmentation of T1 and T2 dependence weighted against priors for spatial position and extent was investigated. When T1 and T2 dependence was highly weighted, good distinction between identified regions was obtained in T1/T2 feature-space, but no contiguous anatomically distinct regions were identified within the thalamus. Incorporating spatial priors was necessary to ensure anatomically distinct regions were defined. Optimal values for segmentation parameters were obtained by assessing performance on a 'synthetic thalamus'. Using these optimum input parameters, within- and between-subjects reproducibility was assessed. Good reproducibility was obtained when six regions were specified to be identified in the thalamus. The six regions identified were similar in the majority of the normal subject group. However, intriguingly these regions were different from those obtained in the same subjects using a well-known connectivity-based segmentation technique. This method shows promise to identify intrathalamic structures on the basis of T1 and T2 signal. A comprehensive characterisation of thalamic nuclei may require a fully multi-modal approach.

Transverse relaxometry with stimulated echo compensation

Presented is a fitting model for transverse relaxometry data acquired with the multiple-refocused spin-echo sequence. The proposed model, requiring no additional data input or pulse sequence modifications, compensates for imperfections in the transmit field and radiofrequency (RF) profiles. Exploiting oscillatory echo behavior to estimate alternate coherence pathways, the model compensates for prolonged signal decay from stimulated echo pathways yielding precise monoexponential T(2) quantification. Verified numerically and experimentally at 4.7 T in phantoms and the human brain, over 95% accuracy is readily attainable in realistic imaging situations without sacrificing multislice capabilities or requiring composite or adiabatic RF pulses. The proposed model allows T(2) quantitation in heterogeneous transmit fields and permits thin refocusing widths for efficient multislice imaging.

Magnetic resonance relaxation and quantitative measurement in the brain

Underlying the exquisite soft tissue contrast provided by magnetic resonance imaging are the inherent biophysical processes of relaxation. Through the intricate relationships between tissue microstructure and biochemistry and the longitudinal and transverse relaxation rates, quantitative measurement of these relaxation parameters is informative of tissue change associated with disease, neural plasticity, and other biological processes. Quantitative imaging studies can further facilitate more detailed characterizations of tissue, providing a more direct link between modern MR imaging and classic histochemical and histological studies. In this chapter, we briefly review the biophysical basis of relaxation, introducing and focusing specifically on the T(1), T(2), and T(2)(*) relaxation times and detail some of the more widely used and clinically feasible techniques for their in vivo measurement. Methods for analyzing relaxation data are covered, and a summary of significant results from reported neuroimaging studies is provided. Finally, the combination of relaxation time data with other quantitative imaging data, including diffusion tensor and magnetization transfer, is examined, with the aim of providing more thorough characterization of brain tissue.

Connectivity-based segmentation of the substantia nigra in human and its implications in Parkinson's disease

The aims of this study were to i) identify substantia nigra subregions i.e. pars reticulata (SNr) and pars compacta (SNc), in human, and ii) to assess volumetric changes in these subregions in the diagnosis of Parkinson's disease. Current MR imaging techniques are unable to distinguish SNr and SNc. Segmentation of these regions may be clinically useful in Parkinson's disease (PD) as substantia nigra is invariably affected in PD. We acquired quantitative T1 as well as diffusion tensor imaging (DTI) data from ten healthy subjects and ten PD patients. For each subject, the left and right SN were manually outlined on T1 images and then classified into two discrete regions based on the characteristics of their connectivity with the rest of the brain using an automated clustering method on the DTI data. We identified two regions in each subjects' SN: an internal region that is likely to correspond with SNc because it was mainly connected with posterior striatum, pallidum, anterior thalamus, and prefrontal cortex; and an external region that corresponds with SNr because it was chiefly connected with posterior thalamus, ventral thalamus, and motor cortex. Volumetric study of these regions in PD patients showed a general atrophy in PD particularly in the right SNr. This pilot study showed that automated DTI-based parcellation of SN subregions may provide a useful tool for in-vivo identification of SNc and SNr and might therefore assist to detect changes that occur in patients with PD.

Correction of main and transmit magnetic field (B0 and B1) inhomogeneity effects in multicomponent-driven equilibrium single-pulse observation of T1 and T2

Multicomponent-driven equilibrium single-component observation of T(1) and T(2) offers a new approach to multiple component relaxation time and myelin water analysis. The method derives two-component relaxation information from spoiled and fully balanced steady-state (SPGR and bSSFP) imaging data acquired over multiple flip angles. Although these steady-state imaging techniques afford rapid acquisition times and high signal-to-noise ratio efficiency, they are also sensitive to main (B(0) ) and transmit (B(1) ) magnetic field inhomogeneities. These effects alter the measured signal from their theoretical values and lead to substantive errors in the derived myelin volume fraction estimates. Here, we incorporate correction techniques to mitigate these effects. DESPOT1-HIFI is used to first calibrate the transmitted flip angles; and B(0) affects are removed through the inclusion of an additional parameter in the multicomponent-driven equilibrium single-component observation of T(1) and T(2) fitting, coupled with the acquisition of multiple phase-cycled bSSFP data. The performance of these correction techniques was evaluated using numerical simulations, demonstrating effective removal of B(0) and B(1)-induced errors in the derived myelin fraction relaxation parameters. The approach was also successfully demonstrated in vivo, with near artifact-free whole-brain, high spatial resolution (1.7 mm × 1.7 mm × 1.7 mm isotropic voxels) myelin water fraction maps acquired in a clinically feasible 16 min.

Template-based B₁ inhomogeneity correction in 3T MRI brain studies

Low noise, high resolution, fast and accurate T₁ maps from MRI images of the brain can be performed using a dual flip angle method. However, B₁ field inhomogeneity, which is particularly problematic at high field strengths (e.g., 3T), limits the ability of the scanner to deliver the prescribed flip angle, introducing errors into the T₁ maps that limit the accuracy of quantitative analyses based on those maps. A dual repetition time method was used for acquiring a B₁ map to correct that inhomogeneity. Additional inaccuracies due to misregistration of the acquired T₁-weighted images were corrected by rigid registration, and the effects of misalignment on the T₁ maps were compared to those of B₁ inhomogeneity in 19 normal subjects. However, since B₁ map acquisition takes up precious scanning time and most retrospective studies do not have B₁ map, we designed a template-based correction strategy. B₁ maps from different subjects were aligned using a twelve-parameter affine registration. Recomputed T₁ maps showed an important improvement with respect to the noncorrected maps: histograms of all corrected maps exhibited two peaks corresponding to white and gray matter tissues, while unimodal histograms were observed in all uncorrected maps because of the inhomogeneity. A method to detect the best nonsubject-specific B₁ correction based on a set of features was designed. The optimum set of weighting factors for those features was computed. The best available B₁ correction was detected in almost all subjects while corrections comparable to the T₁ map corrected using the B₁ map from the same subject were detected in the others.

Unified segmentation based correction of R1 brain maps for RF transmit field inhomogeneities (UNICORT)

Quantitative mapping of the longitudinal relaxation rate (R1 = 1/T1) in the human brain enables the investigation of tissue microstructure and macroscopic morphology which are becoming increasingly important for clinical and neuroimaging applications. R1 maps are now commonly estimated from two fast high-resolution 3D FLASH acquisitions with variable excitation flip angles, because this approach is fast and does not rely on special acquisition techniques. However, these R1 maps need to be corrected for bias due to RF transmit field (B1⁺) inhomogeneities, requiring additional B1⁺ mapping which is usually time consuming and difficult to implement. We propose a technique that simultaneously estimates the B1⁺ inhomogeneities and R1 values from the uncorrected R1 maps in the human brain without need for B1⁺ mapping. It employs a probabilistic framework for unified segmentation based correction of R1 maps for B1⁺ inhomogeneities (UNICORT). The framework incorporates a physically informed generative model of smooth B1⁺ inhomogeneities and their multiplicative effect on R1 estimates. Extensive cross-validation with the established standard using measured B1⁺ maps shows that UNICORT yields accurate B1⁺ and R1 maps with a mean deviation from the standard of less than 4.3% and 5%, respectively. The results of different groups of subjects with a wide age range and different levels of atypical brain anatomy further suggest that the method is robust and generalizes well to wider populations. UNICORT is easy to apply, as it is computationally efficient and its basic framework is implemented as part of the tissue segmentation in SPM8.

Steady-state diffusion-weighted imaging: theory, acquisition and analysis

Steady-state diffusion-weighted imaging (DWI) has long been recognized to offer potential benefits over conventional spin-echo methods. This family of pulse sequences is highly efficient and compatible with three-dimensional acquisitions, which could enable high-resolution, low-distortion images. However, the same properties that lead to its efficiency make steady-state imaging highly susceptible to motion and create a complicated signal with dependence on T(1), T(2) and flip angle. Recent developments in gradient hardware, motion-mitigation techniques and signal analysis offer potential solutions to these problems, reviving interest in steady-state DWI. This review offers a description of steady-state DWI signal formation and provides an overview of the current methods for steady-state DWI acquisition and analysis.

Reproducibility of thalamic segmentation based on probabilistic tractography

Reliable identification of thalamic nuclei is required to improve targeting of electrodes used in Deep Brain Stimulation (DBS), and for exploring the role of thalamus in health and disease. A previously described method using probabilistic tractography to segment the thalamus based on connections to cortical target regions was implemented. Both within- and between-subject reproducibility were quantitatively assessed by the overlap of the resulting segmentations; the effect of two different numbers of target regions (6 and 31) on reproducibility of the segmentation results was also investigated. Very high reproducibility was observed when a single dataset was processed multiple times using different starting conditions. Thalamic segmentation was also very reproducible when multiple datasets from the same subject were processed using six cortical target regions. Within-subject reproducibility was reduced when the number of target regions was increased, particularly in medial and posterior regions of the thalamus. A large degree of overlap in segmentation results from different subjects was obtained, particularly in thalamic regions classified as connecting to frontal, parietal, temporal and pre-central cortical target regions.

Quantitative relaxometry of the brain

The exquisite soft tissue contrast provided by magnetic resonance imaging arises principally from differences in the intrinsic relaxation properties, T1 and T2. Although the intricate relationships that link tissue microstructure and the longitudinal and transverse relaxation times remain to be firmly established, quantitative measurement of these parameters, also referred to as quantitative relaxometry, can be informative of disease-related tissue change, developmental plasticity, and other biological processes. Further, relaxometry studies potentially offer a more detailed characterization of tissue, compared with conventional qualitative or weighted imaging approaches.The purposes of this review were to briefly review the biophysical basis of relaxation, focusing specifically on the T1, T2, and T2* relaxation times, and to detail some of the more widely used and clinically feasible techniques for their in vivo measurement. We will focus on neuroimaging applications, although the methods described are equally well suited to cardiac, abdominal, and musculoskeletal imaging. Potential sources of error, and methods for their correction, are also touched on. Finally, the combination of relaxation time data with other complementary quantitative imaging data, including diffusion tensor imaging, is discussed, with the aim of more thoroughly characterizing brain tissue.

High field (9.4 Tesla) magnetic resonance imaging of cortical grey matter lesions in multiple sclerosis

Multiple sclerosis is an inflammatory, degenerative disease of the central nervous system. The most obvious pathological change in multiple sclerosis is multifocal demyelination of the white matter, but grey matter demyelination may be of equal or even greater importance for its clinical manifestations. In order to assess the pathogenetic role of lesions in the grey and white matter, and to explore the association between demyelinated and non-lesional brain tissue, tools are needed to depict each of these tissue components accurately in vivo. Due to its sensitivity in detecting white matter lesions, T(2)-weighted magnetic resonance imaging at 1.5 T is important in the diagnosis of multiple sclerosis. However, magnetic resonance imaging at 1.5 T largely fails to detect grey matter lesions. In this study, we used T(2)-weighted magnetic resonance imaging at 9.4 T to detect grey matter lesions in fixed post-mortem multiple sclerosis motor cortex. Furthermore, we produced T(1), T(2) and magnetization transfer ratio maps, and correlated these indices with quantitative histology [neuronal density, intensity of immunostaining for myelin basic protein (reflecting myelin content) and phosphorylated neurofilament (reflecting axonal area)] using t-tests and multivariate regression. In 21 tissue samples, 28 cortical grey matter lesions were visible on both T(2)-weighted magnetic resonance imaging and sections immunostained for myelin basic protein, 15/28 being mixed white and grey matter and 11/28 subpial cortical grey matter lesions; 2/28 cortical grey matter lesions involved all layers of the cortex. Compared with non-lesional cortex, cortical grey matter lesions showed reduction of neuronal density (98/mm(2), SD = 34/mm(2;) versus 129/mm(2), SD = 44; P < 0.01), phosphorylated neurofilament (1/transmittance = 1.16; SD = 0.09 versus 1.24; SD = 0.1; P < 0.01) and magnetization transfer ratio (31.1 pu; SD = 11.9 versus 37.5 pu; SD = 8.7; P = 0.01), and an increase of T(2) (25.9; SD = 5 versus 22.6 ms; SD = 4.7; P < 0.01). Associations were detected between phosphorylated neurofilament and myelin basic protein (r = 0.58, P < 0.01), myelin basic protein and T(2) (r = -0.59, P < 0.01), and neuronal density and T(1) (r = -0.57, P < 0.01). All indices correlated with duration of tissue fixation, however, including the latter in the analysis did not fundamentally affect the associations described. Our data show that T(2)-weighted magnetic resonance imaging at 9.4 T enables detection of cortical grey matter lesion in post-mortem multiple sclerosis brain. The quantitative associations suggest that in cortical grey matter T(1) may be a predictor of neuronal density, and T(2) of myelin content (and-secondarily-axons). Successful translation of these results into in vivo studies using high field magnetic resonance imaging (e.g. 3 T and 7 T) will improve the assessment of cortical pathology and thereby have an impact on the diagnosis and natural history studies of patients with multiple sclerosis, as well as clinical trial designs for putative treatments to prevent cortical demyelination and neuronal loss.

Modulated repetition time look-locker (MORTLL): a method for rapid high resolution three-dimensional T1 mapping

PURPOSE

To demonstrate a modification of the Look-Locker (LL) technique that enables rapid high resolution T1 mapping over the physiologic range of intracranial T1 values, ranging from white matter to cerebrospinal fluid (CSF). This is achieved by use of a three-dimensional (3D) balanced steady-state free precession (b-SSFP) acquisition (for high signal-to-noise and resolution) along with variable repetition time to allow effective full recovery of longitudinal magnetization.

MATERIALS AND METHODS

Two modifications to the Look-Locker technique were made to realize high resolution imaging in a clinically reasonable scan time. The 3D b-SSFP acquisition after an initial inversion pulse was followed by a variable repetition time. This technique makes it possible to image a volume of thin contiguous slices with high resolution and accuracy using a simple fitting procedure and is particularly useful for imaging long T1 species such as CSF. The total scan time is directly proportional to the number of slices to be acquired. The scan time was reduced by almost half when the repetition time was modified using a predesigned smooth function. Phantoms and volunteers were imaged at different resolutions on a 3 Tesla scanner. Results were compared with other accepted techniques.

RESULTS

T1 values in the brain corresponded well with full repetition time imaging as well as inversion recovery spin echo imaging. T1 values for white matter, gray matter, and CSF were measured to be 755 +/- 10 ms, 1202 +/- 9 ms, and 4482 +/- 71 ms, respectively. Scan times were reduced by approximately half over full repetition time measurements.

CONCLUSION

High resolution T1 maps can be obtained rapidly and with a relatively simple postprocessing method. The technique is particularly well suited for long T1 species. For example, changes in the composition of proteins in CSF are linked to various pathologies. The T1 values showed excellent agreement with values obtained from inversion recovery spin-echo imaging.

Transverse relaxation time (T2) mapping in the brain with off-resonance correction using phase-cycled steady-state free precession imaging

PURPOSE

To investigate a new approach for more completely accounting for off-resonance affects in the DESPOT2 (driven equilibrium single pulse observation of T(2)) mapping technique.

MATERIALS AND METHODS

The DESPOT2 method derives T(2) information from fully balanced steady-state free precession (bSSFP) images acquired over multiple flip angles. Off-resonance affects, which present as bands of altered signal intensity throughout the bSSFP images, results in erroneous T(2) values in the corresponding calculated maps. Radiofrequency (RF) phase-cycling, in which the phase of the RF pulse is incremented along the pulse train, offers a potential method for eliminating these artifacts. In this work we present a general method, referred to as DESPOT2, with full modeling (DESPOT2-FM), for deriving T(2), as well as off-resonance frequency, from dual flip angle bSSFP data acquired with two RF phase increments.

RESULTS

The method is demonstrated in vivo through the acquisition of whole-brain, 1 mm(3) isotropic T(2) maps at 3T and shown to provide near artifact-free maps, even in areas with steep susceptibility-induced gradients.

CONCLUSION

DESPOT2-FM offers an efficient method for acquiring high spatial resolution, whole-brain T(2) maps at 3T with high precision and free of artifact.

Influence of RF spoiling on the stability and accuracy of T1 mapping based on spoiled FLASH with varying flip angles

There is increasing interest in quantitative T(1) mapping techniques for a variety of applications. Several methods for T(1) quantification have been described. The acquisition of two spoiled gradient-echo data sets with different flip angles allows for the calculation of T(1) maps with a high spatial resolution and a relatively short experimental duration. However, the method requires complete spoiling of transverse magnetization. To achieve this goal, RF spoiling has to be applied. In this work it is investigated whether common RF spoiling techniques are sufficiently effective to allow for accurate T(1) quantification. It is shown that for most phase increments the apparent T(1) can deviate considerably from the true value. Correct results may be achieved with phase increments of 118.2 degrees or 121.8 degrees. However, for these values the method suffers from instabilities. In contrast, stable results are obtained with a phase increment of 50 degrees. An algorithm is presented that allows for the calculation of corrected T(1) maps from the apparent values. The method is tested both in phantom experiments and in vivo by acquiring whole-brain T(1) maps of the human brain.

Gleaning multicomponent T1 and T2 information from steady-state imaging data

The driven-equilibrium single-pulse observation of T(1) (DESPOT1) and T(2) (DESPOT2) are rapid, accurate, and precise methods for voxelwise determination of the longitudinal and transverse relaxation times. A limitation of the methods, however, is the inherent assumption of single-component relaxation. In a variety of biological tissues, in particular human white matter (WM) and gray matter (GM), the relaxation has been shown to be more completely characterized by a summation of two or more relaxation components, or species, each believed to be associated with unique microanatomical domains or water pools. Unfortunately, characterization of these components on a voxelwise, whole-brain basis has traditionally been hindered by impractical acquisition times. In this work we extend the conventional DESPOT1 and DESPOT2 approaches to include multicomponent relaxation analysis. Following numerical analysis of the new technique, renamed multicomponent driven equilibrium single pulse observation of T(1)/T(2) (mcDESPOT), whole-brain multicomponent T(1) and T(2) quantification is demonstrated in vivo with clinically realistic times of between 16 and 30 min. Results obtained from four healthy individuals and two primary progressive multiple sclerosis (MS) patients demonstrate the future potential of the approach for identifying and assessing tissue changes associated with several neurodegenerative conditions, in particular those associated with WM.

Contrast enhanced magnetic resonance imaging of the liver

Dynamic contrast enhanced magnetic resonance imaging (DCE MRI) is a useful tool to characterise and stage a disease. Here we investigate the application of DCE MRI to the liver, an organ subject to large excursions during normal breathing. For DCE MRI it is important to have an estimate of the longitudinal relaxation time parameter T1. We show that by using a T1 mapping approach, which takes into account inaccuracies in transmitted flip angles, we obtain a smoother T1 map, resulting in a more consistent parameter estimation for the subsequent analysis. The dynamic imaging protocol described enables the acquisition of high resolution unblurred images by simulating the normal breathing cycle. The contrast enhanced data is aligned, first rigidly, then non-rigidly, and input to a two-compartment pharmacokinetic model. We observe that rigid registration markedly improves the parameter estimation, but is insufficient in clinically important heterogeneous areas.

High resolution diffusion-weighted imaging in fixed human brain using diffusion-weighted steady state free precession

High resolution diffusion tensor imaging and tractography of ex vivo brain specimens has the potential to reveal detailed fibre architecture not visible on in vivo images. Previous ex vivo diffusion imaging experiments have focused on animal brains or small sections of human tissue since the unfavourable properties of fixed tissue (including short T(2) and low diffusion rates) demand the use of very powerful gradient coils that are too small to accommodate a whole, human brain. This study proposes the use of diffusion-weighted steady-state free precession (DW-SSFP) as a method of extending the benefits of ex vivo DTI and tractography to whole, human, fixed brains on a clinical 3 T scanner. DW-SSFP is a highly efficient pulse sequence; however, its complicated signal dependence precludes the use of standard diffusion tensor analysis and tractography. In this study, a method is presented for modelling anisotropy in the context of DW-SSFP. Markov Chain Monte Carlo sampling is used to estimate the posterior distributions of model parameters and it is shown that it is possible to estimate a tight distribution on the principal axis of diffusion at each voxel using DW-SSFP. Voxel-wise estimates are used to perform tractography in a whole, fixed human brain. A direct comparison between 3D diffusion-weighted spin echo EPI and 3D DW-SSFP-EPI reveals that the orientation of the principal diffusion axis can be inferred on with a higher degree of certainty using a 3D DW-SSFP-EPI even with a 68% shorter acquisition time.

Uncertainty in T(1) mapping using the variable flip angle method with two flip angles

Propagation of errors, in conjunction with the theoretical signal equation for spoiled gradient echo pulse sequences, is used to derive a theoretical expression for uncertainty in quantitative variable flip angle T(1) mapping using two flip angles. This expression is then minimized to derive a rigorous expression for optimal flip angles that elucidates a commonly used empirical result. The theoretical expressions for uncertainty and optimal flip angles are combined to derive a lower bound on the achievable uncertainty for a given set of pulse sequence parameters and signal-to-noise ratio (SNR). These results provide a means of quantitatively determining the effect of changing acquisition parameters on T(1) uncertainty.

Standardized structural magnetic resonance imaging in multicentre studies using quantitative T1 and T2 imaging at 1.5 T

The ability to acquire MRI data with consistent tissue contrast at multiple time points, and/or across different imaging centres has become increasingly important as the number of large longitudinal and multicentre studies has grown. Here, the use of quantitative magnetic resonance relaxation times measurement, or, voxel-wise determination of the intrinsic longitudinal and transverse relaxation times, T1 and T2 respectively, for standardizing the structural imaging component of such studies is reported. To demonstrate the ability to standardize across multiple time-points and imaging centres, T1 and T2 maps of seven healthy volunteers were acquired using the rapid DESPOT1 and DESPOT2 (driven equilibrium single pulse observation of T1 and T2) mapping techniques at three centres across the United Kingdom (each centre utilizing scanners from competing manufacturers and/or with varying gradient performance). An average coefficient of variation of the estimates of T1 and T2 was found to be approximately 6.5% and 8%, respectively, across the three centres and comparable to that achieved between repeated imaging sessions performed at the same centre. With a total combined imaging time of less than 12 min for whole-brain approximately 1.2 mm isotropic voxel T1 and T2 maps, quantitative voxel-wise T1 and T2 mapping represents an attractive and easy-to-implement approach for signal intensity standardization and normalization. Further, as T1 and T2 are related to tissue microstructure and biochemistry, quantitative images provide additional diagnostic information that can be compared between patient and control populations, for example through voxel-based analysis techniques.