A model-based framework for correcting B 1 + inhomogeneity effects in magnetization transfer saturation and inhomogeneous magnetization transfer saturation maps

PURPOSE

In this work, we propose that Δ B 1 + -induced errors in magnetization transfer (MT) saturation (MT_sat ) maps can be corrected with use of an R₁ and B 1 + map and through numerical simulations of the sequence.

THEORY AND METHODS

One healthy subject was scanned at 3.0T using a partial quantitative MT protocol to estimate the relationship between observed R₁ (R_1,obs ) and apparent bound pool size ( M 0 , a p p B ) in the brain. MT_sat values were simulated for a range of B 1 + , R_1,obs , and M 0 , a p p B . An equation was fit to the simulated MT_sat , then a linear relationship between R_1,obs and M 0 , a p p B was generated. These results were used to generate correction factor maps for the MT_sat acquired from single-point data. The proposed correction was compared to an empirical correction factor with different MT-preparation schemes.

RESULTS

M 0 , a p p B was highly correlated with R_1,obs (r > 0.96), permitting the use of R_1,obs to estimate M 0 , a p p B for B 1 + correction. All B 1 + corrected MT_sat maps displayed a decreased correlation with B 1 + compared to uncorrected MT_sat and MT_sat corrected with an empirical factor in the corpus callosum. There was good agreement between the proposed approach and the empirical correction with radiofrequency saturation at 2 kHz, with larger deviations seen when using saturation pulses further off-resonance and in inhomogeneous (ih) MT_sat maps.

CONCLUSION

The proposed correction decreases the dependence of MT_sat on B 1 + inhomogeneities. Furthermore, this flexible framework permits the use of different saturation protocols, making it useful for correcting B 1 + inhomogeneities in ihMT.

Cerebral oxygen extraction fraction: Comparison of dual-gas challenge calibrated BOLD with CBF and challenge-free gradient echo QSM+qBOLD

PURPOSE

To compare cortical gray matter oxygen extraction fraction (OEF) estimated from 2 MRI methods: (1) the quantitative susceptibility mapping (QSM) plus quantitative blood oxygen level dependent imaging (qBOLD) (QSM+qBOLD or QQ), and (2) the dual-gas calibrated-BOLD (DGCB) in healthy subjects; and to investigate the validity of iso-cerebral metabolic rate of oxygen consumption assumption during hypercapnia using QQ.

METHODS

In 10 healthy subjects, 3 tesla MRI including a multi-echo gradient echo sequence at baseline and hypercapnia for QQ, as well as an EPI dual-echo pseudo-continuous arterial spin labeling for DGCB, were performed under a hypercapnic and a hyperoxic condition. OEFs from QQ and DGCB were compared using region of interest analysis and paired t test. For QQ, cerebral metabolic rate of oxygen consumption = cerebral blood flow*OEF*arterial oxygen content was generated for both baseline and hypercapnia, which were compared.

RESULTS

Average OEF in cortical gray matter across 10 subjects from QQ versus DGCB was 35.5 ± 6.7% versus 38.0 ± 9.1% (P = .49) at baseline and 20.7 ± 4.4% versus 28.4 ± 7.6% (P = .02) in hypercapnia: OEF in cortical gray matter was significantly reduced as measured in QQ (P < .01) and in DGCB (P < .01). Cerebral metabolic rate of oxygen consumption (in μmol O₂ /min/100 g) was 168.2 ± 54.1 at baseline from DGCB and was 153.1 ± 33.8 at baseline and 126.4 ± 34.2 (P < .01) in hypercapnia from QQ.

CONCLUSION

The differences in OEF obtained from QQ and DGCB are small and nonsignificant at baseline but are statistically significant during hypercapnia. In addition, QQ shows a cerebral metabolic rate of oxygen consumption decrease (17.4%) during hypercapnia.

Prenatal exposure to maternal cigarette smoking and structural properties of the human corpus callosum

Alterations induced by prenatal exposure to nicotine have been observed in experimental (rodent) studies. While numerous developmental outcomes have been associated with prenatal exposure to maternal cigarette smoking (PEMCS) in humans, the possible relation with brain structure is less clear. Here we sought to elucidate the relation between PEMCS and structural properties of human corpus callosum in adolescence and early adulthood in a total of 1,747 youth. We deployed three community-based cohorts of 446 (age 25-27 years, 46% exposed), 934 (age 12-18 years, 47% exposed) and 367 individuals (age 18-21 years, 9% exposed). A mega-analysis revealed lower mean diffusivity in the callosal segments of exposed males. We speculate that prenatal exposure to maternal cigarette smoking disrupts the early programming of callosal structure and increases the relative portion of small-diameter fibres.

Focused ultrasound resolves persistent radiosurgery related change in a patient with tremor

We report on a patient who underwent magnetic resonance guided focused ultrasound (MRgFUS) thalamotomy to treat tremor 3 years after a stereotactic radiosurgery (SRS) thalamotomy. The SRS produced only limited and transient improvements and was associated with a persistent hyperintensity on T2-FLAIR MR images. The MRgFUS thalamotomy was successful, with tremor improvement at 3 months, no adverse effects, and radiological appearance of the MRgFUS lesion similar to other patients undergoing this therapy. We also observed that the SRS-related T2-FLAIR hyperintensity had increased signal intensity 1 day post-MRgFUS, but appeared completely resolved 3 months post-MRgFUS. In conclusion, the case demonstrates that MRgFUS thalamotomy may effectively control tremor in patients with a history of SRS thalamotomy. We also speculate on the potential mechanisms of the apparent resolution of radiation-related change, and discuss possible applications of MRgFUS to reduce persistent SRS-related inflammation.

Improving recorded volume in mesial temporal lobe by optimizing stereotactic intracranial electrode implantation planning

PURPOSE

Intracranial electrodes are sometimes implanted in patients with refractory epilepsy to identify epileptic foci and propagation. Maximal recording of EEG activity from regions suspected of seizure generation is paramount. However, the location of individual contacts cannot be considered with current manual planning approaches. We propose and validate a procedure for optimizing intracranial electrode implantation planning that maximizes the recording volume, while constraining trajectories to safe paths.

METHODS

Retrospective data from 20 patients with epilepsy that had electrodes implanted in the mesial temporal lobes were studied. Clinical imaging data (CT/A and T1w MRI) were automatically segmented to obtain targets and structures to avoid. These data were used as input to the optimization procedure. Each electrode was modeled to assess risk, while individual contacts were modeled to estimate their recording capability. Ordered lists of trajectories per target were obtained. Global optimization generated the best set of electrodes. The procedure was integrated into a neuronavigation system.

RESULTS

Trajectories planned automatically covered statistically significant larger target volumes than manual plans [Formula: see text]. Median volume coverage was [Formula: see text] for automatic plans versus [Formula: see text] for manual plans. Furthermore, automatic plans remained at statistically significant safer distance to vessels [Formula: see text] and sulci [Formula: see text]. Surgeon's scores of the optimized electrode sets indicated that 95% of the automatic trajectories would be likely considered for use in a clinical setting.

CONCLUSIONS

This study suggests that automatic electrode planning for epilepsy provides safe trajectories and increases the amount of information obtained from the intracranial investigation.

Saguenay Youth Study: a multi-generational approach to studying virtual trajectories of the brain and cardio-metabolic health

This paper provides an overview of the Saguenay Youth Study (SYS) and its parental arm. The overarching goal of this effort is to develop trans-generational models of developmental cascades contributing to the emergence of common chronic disorders, such as depression, addictions, dementia and cardio-metabolic diseases. Over the past 10 years, we have acquired detailed brain and cardio-metabolic phenotypes, and genome-wide genotypes, in 1029 adolescents recruited in a population with a known genetic founder effect. At present, we are extending this dataset to acquire comparable phenotypes and genotypes in the biological parents of these individuals. After providing conceptual background for this work (transactions across time, systems and organs), we describe briefly the tools employed in the adolescent arm of this cohort and highlight some of the initial accomplishments. We then outline in detail the phenotyping protocol used to acquire comparable data in the parents.

Dual-temporal resolution dynamic contrast-enhanced MRI protocol for blood-brain barrier permeability measurement in enhancing multiple sclerosis lesions

PURPOSE

To design a more accurate and reproducible technique for the measurement of blood-brain barrier (BBB) permeability in gadolinium-enhancing multiple sclerosis (MS) lesions.

MATERIALS AND METHODS

Four MS patients were scanned using a new dynamic contrast-enhanced (DCE)-magnetic resonance imaging (MRI) protocol based on an uninterrupted two-part acquisition consisting of an initial part at high temporal and low spatial resolutions and a second part at low temporal and high spatial resolutions. The method preserves both the high spatial resolution needed for the often small size of lesions and the high temporal resolution required during the first minute after injection to sufficiently sample the first-pass bolus. Simulations compared the performance of this new protocol with the conventional one at low temporal and high spatial resolutions throughout.

RESULTS

The BBB permeability estimates changed by up to 33% between the two protocols. The new protocol led to simulated error on K(trans) of 7%-10%, versus 7%-30% with the conventional protocol, and was more robust with respect to offsets between acquisition and injection start times, differences in shape of the first-pass peak, and permeability values.

CONCLUSION

The dual-temporal resolution protocol produces improved BBB permeability estimates and provides a more complete view of active inflammatory MS lesion pathology.

Diffusion tensor imaging, permanent pyramidal tract damage, and outcome in subcortical stroke

BACKGROUND

Studies in chronic stroke patients suggest that diffusion tensor imaging (DTI) parameters of the pyramidal tract (PT) relate to residual motor function. We performed a prospective controlled study to evaluate if the DTI parameters tract volume (TV) and fractional anisotropy (FA) in patients with acute subcortical infarcts are correlated with permanent PT damage and clinical outcome after 6 months.

METHODS

We acquired DTI in 18 stroke patients with subcortical ischemic infarcts either affecting the PT (PT group, n = 12) or not (non-PT group, n = 6) and in 7 age- and risk factor-matched controls at median times of 12 and 180 days. The PT was isolated using tractography and tract volume ratios (R(TV)) and FA ratios (R(FA)) were calculated (affected tract/unaffected tract). Ratios were compared within and between groups at initial and follow-up time points, as well as in tract portions above and below the infarcts, and were correlated to Rivermead Motor Function Test (RMFT) scores.

RESULTS

Mean R(FA) and R(TV) of the PT group were smaller than those of both non-PT and control groups initially and at follow-up (p < 0.01). Tract portions above the infarct had lower R(TV) than below (p < 0.05). There was no significant change in R(FA) and R(TV) over time for the whole tract or tract portions. R(FA) and R(TV) both were highly correlated with initial and follow-up RMFT scores.

CONCLUSIONS

DTI parameters of PT integrity acquired within the first weeks after acute subcortical stroke measure permanent ischemic PT damage and are highly correlated with residual motor function in the acute and chronic stage.

Sexual dimorphism in the adolescent brain: Role of testosterone and androgen receptor in global and local volumes of grey and white matter

Here we examined sex differences in the volumes of grey and white matter, and in grey-matter "density," in a group of typically developing adolescents participating in the Saguenay Youth Study (n=419; 12-18 years). In male adolescents, we also investigated the role of a functional polymorphism in androgen-receptor gene (AR) in moderating the effect of testosterone on volumes of grey and white matter and grey-matter density. Overall, both absolute and relative volumes of white matter were larger in male vs. females adolescents. The relative grey-matter volumes were slightly larger in female than male adolescents and so was the grey-matter density in a large number of cortical regions. In male adolescents, functional polymorphism of AR moderated the effect of testosterone on relative white- and grey-matter volumes. Following a discussion of several methodological and interpretational issues, we outline future directions in investigating brain-behavior relationships vis-à-vis psychopathology.

Information content of SNR/resolution trade-offs in three-dimensional magnetic resonance imaging

In MRI, a trade-off exists between resolution and signal-to-noise ratio, since different fractions of the available scan time can be used to acquire data at higher spatial frequencies and to perform signal averaging. By comparing a wide variety of 3D isotropic MR scans with different combinations of SNR, resolution, and scan duration, the impact of this trade-off on the image information content was assessed. The information content of mouse brain, mouse whole-body, and human brain images was evaluated using a simple numerical approach, which sums the information contribution of each individual k-space data point. Results show that, with a fixed receiver bandwidth and field of view, the information content of trade-off images is always maximized when the SNR is equal to about 16. The optimal imaging resolution is dependent on the scan duration, as well as certain MR system properties, such as field strength and coil sensitivity. These properties are, however, easily accounted for with the acquisition of a single scout MR image, and the optimal imaging resolution can then be calculated using a simple mathematical relationship. If the imaging task is approached with a predetermined resolution requirement, the same scout scan can be used to calculate the scan duration that will provide the maximum possible information. Using these relationships to maximize the image information content is an excellent technique for guiding the initial selection of imaging parameters.

Sex differences in the growth of white matter during adolescence

The purpose of this study was to examine sex differences in the maturation of white matter during adolescence (12 to 18 years of age). We measured lobular volumes of white matter and white-matter "density" throughout the brain using T1-weighted images, and estimated the myelination index using magnetisation-transfer ratio (MTR). In male adolescents, we observed age-related increases in white-matter lobular volumes accompanied by decreases in the lobular values of white-matter MTR. White-matter density in the putative cortico-spinal tract (pCST) decreased with age. In female adolescents, on the other hand, we found only small age-related increase in white-matter volumes and no age-related changes in white-matter MTR, with the exception of the frontal lobe where MTR increased. White-matter density in the pCST also increased with age. These results suggest that sex-specific mechanisms may underlie the growth of white matter during adolescence. We speculate that these mechanisms involve primarily age-related increases in axonal calibre in males and increased myelination in females.

Mathematical methods for diffusion MRI processing

In this article, we review recent mathematical models and computational methods for the processing of diffusion Magnetic Resonance Images, including state-of-the-art reconstruction of diffusion models, cerebral white matter connectivity analysis, and segmentation techniques. We focus on Diffusion Tensor Images (DTI) and Q-Ball Images (QBI).

Human brain myelination from birth to 4.5 years

The myelination of white matter from birth through the first years of life has been studied qualitatively and it is well know the myelination occurs in a orderly and predictable manner, proceeding in a caudocranial direction, from deep to superficial and from posterior to anterior. Even if the myelination is a continuous process, it is useful to characterize myelination evolution in normal brain development in order to better study demyelinating diseases. The quantification of myelination has only been studied for neonates. The original contribution of this study is to develop a method to characterize and visualize the myelination pattern using MRI data from a group of normal subjects from birth to just over 4 years of age. The method includes brain extraction and tissue classification in addition to the analysis of T2 relaxation times to attempt to separate myelinated and unmyelinated white matter. The results agree previously published qualitative observations.

Corpus callosum in adolescent offspring exposed prenatally to maternal cigarette smoking

Teratogens, such as alcohol or anti-epileptic drugs, affect the size of the corpus callosum. Here we report findings obtained in a case-control study that investigated possible effects of teratogens contained in cigarette smoke on the size and structural properties of this structure. We recruited and scanned with magnetic resonance imaging a total of 408 adolescents (12 to 18 years of age); a subsample of 300 adolescents is considered in this report. Cases (n=146) were exposed to maternal cigarette smoking during pregnancy; non-exposed controls (n=154) were matched to cases by maternal education. We measured the size of corpus callosum (CC) and its sections (corrected for brain size), as well as mean values of magnetization-transfer ratio (MTR) in each CC section. Corpus callosum, and especially its posterior part, was smaller in the exposed vs. non-exposed female adolescents; no significant effects were found in males. Exposed and non-exposed subjects did not differ in the MTR-based index of myelination in either gender in any CC section. Given the lack of exposure effect on the myelination index, this finding might reflect a lower number of inter-hemispheric connections in female offspring of mothers who smoked during pregnancy.

Regional variations in normal brain shown by quantitative magnetization transfer imaging

A quantitative magnetization transfer imaging (qMTI) study, based on a two-pool model of magnetization transfer, was performed on seven normal subjects to determine, on a regional basis, normal values for the pool sizes, exchange, and relaxation parameters that characterize the MT phenomenon. Regions were identified on high-resolution anatomical scans using a combination of manual and automatic methods. Only voxels identified as pure tissue at the resolution of the quantitative scans were considered for analysis. While no left/right differences were observed, significant differences were found among white-matter regions and gray-matter regions. These regional differences were compared with existing cytoarchitectural data. In addition, the pattern and magnitude of the regional differences observed in white matter was found to be different from that reported previously for an alternative putative MRI measure of myelination, the 10-50-ms T2 component described as myelin water.

The effect of spatial and temporal information on saccades and neural activity in oculomotor structures

It has been argued that saccade generation is supported by two systems, a'where' system that decides the direction and extent of an impending saccade, and a 'when' system that is involved in the timing of the release of fixation. We evaluated the contributions of these systems to saccade latencies, and used functional MRI to identify the neural substrates of these systems. We found that advance knowledge of the direction and the timing of an impending target movement had both overlapping and discrete effects on saccade latencies and on neural activation. Knowledge of either factor decreased regular saccade latencies. However, knowledge of target direction increased the number of predictive and express saccades while knowledge of target timing did not. The brain activation data showed that advance knowledge of the direction or the timing of the target movement activated primarily overlapping structures. The precentral gyrus, in the region of the frontal eye fields, was more active in conditions in which some aspect of the target movement was predictable than in saccade control and fixation conditions. In the basal ganglia, activation discriminated between advance knowledge of target timing and target direction. The lenticular nuclei were more active when only target timing was known in advance, while the caudate was more active when only target direction was known in advance. These data suggest that the neural structures supporting the 'where' and 'when' systems are highly overlapping, although there is some dissociation sub-cortically. Knowledge of target timing and target direction converge in precentral gyrus, a region where there is strong evidence of context-dependent modulation of neural activity.

Quantitative imaging of magnetization transfer exchange and relaxation properties in vivo using MRI

We describe a novel imaging technique that yields all of the observable properties of the binary spin-bath model for magnetization transfer (MT) and demonstrate this method for in vivo studies of the human head. Based on a new model of the steady-state behavior of the magnetization during a pulsed MT-weighted imaging sequence, this approach yields parametric images of the fractional size of the restricted pool, the magnetization exchange rate, the T(2) of the restricted pool, as well as the relaxation times in the free pool. Validated experimentally on agar gels and samples of uncooked beef, we demonstrate the method's application on two normal subjects and a patient with multiple sclerosis.

Selective activation of the ventrolateral prefrontal cortex in the human brain during active retrieval processing

The present study examined the role of the prefrontal cortex in retrieval processing using functional magnetic resonance imaging in human subjects. Ten healthy subjects were scanned while they performed a task that required retrieval of specific aspects of visual information. In order to examine brain activity specifically associated with retrieval, we designed a task that had retrieval and control conditions that were perfectly matched in terms of depth of encoding, decision making and postretrieval monitoring and differed only in terms of whether retrieval was required. In the retrieval condition, based on an instructional cue, the subjects had to retrieve either the particular stimulus that was previously presented or its location. In the control condition, the cue did not instruct retrieval but shared with the instructional cues the function of alerting the subjects of the impending test phase. The comparison of activity between the retrieval and control conditions demonstrated a significant and selective increase in activity related to retrieval processes within the ventrolateral prefrontal cortical region, more specifically within area 47/12. These activity increases were bilateral but stronger in the right hemisphere. The present study by strictly controlling the level of encoding, postretrieval monitoring, and decision making has demonstrated a specific increase in the ventrolateral prefrontal region that could be clearly related to active retrieval processing, i.e. the active selection of particular stored visual representations.

The cortical deficit in humans with strabismic amblyopia

To further our understanding of the cortical deficit in strabismic amblyopia, we measured, compared and mapped functional magnetic resonance imaging (fMRI) activation between the fixing and fellow amblyopic eyes of ten strabismic amblyopes. Of specific concern was whether the function of any visual area was spared in strabismic amblyopia, as recently suggested by both positron emission tomography (PET) and fMRI studies, and whether there was a close relationship between the fMRI response and known psychophysical deficits. To answer these questions we measured the psychophysical deficit in each subject and used stimuli whose relationship to the psychophysical deficit was known. We observed that stimuli that were well within the amblyopic passband did produce reduced fMRI activation, even in visual area V1. This suggests that V1 is anomalous in amblyopia. A similar level of reduction was observed in V2. In two subjects, we found that stimuli outside the amblyopic passband produced activation in visual area V3A. We did not find a close relationship between the fMRI response reduction in amblyopia and either of the known psychophysical deficits even though the fMRI response reduction in amblyopia did covary with stimulus spatial frequency.

Oxidative metabolism and the detection of neuronal activation via imaging

Recent years have witnessed a great growth of interest in non-invasive imaging methods, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), permitting identification of brain structures that mediate specific cognitive and behavioural tasks in humans. Because these techniques use physiological responses such as increased perfusion or metabolism as surrogate indicators of evoked neuronal electrical activity, understanding the role of these processes in sustaining the information processing function of neurons is vital to the proper interpretation of functional neuroimaging data. An ultimate goal of these non-invasive techniques is to approach the sensitivity and spatial resolution of earlier autoradiographic methods, which have repeatedly demonstrated exquisitely detailed delineations of neuronal response patterns using metabolic glucose uptake as a physiological tag. Although glucose is generally metabolized in conjunction with oxygen, technical challenges in imaging tissue oxygen consumption in vivo have limited the use of this complementary process in the detection of neuronal activation. In this article we review concepts linking cerebral blood flow and metabolism to neuronal activation, and compare functional imaging techniques that exploit these relationships. We also describe recently introduced MRI based methods for measurement of oxygen consumption and assess the relative contributions of different metabolic pathways during neuronal activation. Our calculations suggest that the bulk of the energy demand evoked during stimulation of neurons in visual cortex is met through oxidative metabolism of glucose, supporting the use of oxygen uptake as a marker for increased neuronal electrical activity.

Texture analysis and morphological processing of magnetic resonance imaging assist detection of focal cortical dysplasia in extra-temporal partial epilepsy

In many patients, focal cortical dysplasia (FCD) is characterized by minor structural changes that may go unrecognized by standard radiological analysis. To increase the sensitivity of magnetic resonance imaging (MRI) for the detection of subtle lesions of FCD, we developed voxel-based image postprocessing methods, including first-order texture analysis and morphological processing modeled on known MRI features of FCD. We selected 16 patients with histologically proven FCD. Image processing features were calculated over a neighborhood for each voxel in the three-dimensional T1-weighted MRI. Three feature maps were generated: (1) gray matter thickness map to model cortical thickening, (2) gradient map to model blurring of the gray matter-white matter junction, and (3) relative intensity map to model the hyperintense signal within the lesion. Two observers detected lesions on conventional MRI in 8/16 and on ratio maps in 14/16 patients. Sensitivity was 87.5% (14/16) for the ratio maps compared to 50% (8/16) for MRI (p < 0.003). Specificity was 95% (19/20) for ratio maps and 100% (20/20) for MRIs. Cohen's kappa was 0.53 for MRIs, indicating moderate agreement, and 0.83 for ratio maps, indicating strong agreement beyond chance between the 2 observers. The image-processing methods developed in this study improve visual detection of FCD, even in cases where no lesion is obvious on MRI. These techniques could increase the number of patients with partial epilepsy who could benefit from surgery.

Magnetization transfer ratio is unable to lateralize epileptic foci in patients with temporal lobe epilepsy

BACKGROUND AND PURPOSE

A preliminary report suggested that magnetization transfer ratio (MTR) was useful to lateralize epileptic foci in patients with refractory temporal lobe epilepsy (TLE). We attempted to confirm this finding in a larger group by investigating the relationship between MTR of mesial temporal structures and seizure lateralization in patients with refractory TLE.

METHODS

We compared the MTR of amygdalae and hippocampi of 10 patients with unilateral TLE to values obtained from 10 healthy control participants.

RESULTS

Three of 10 patients with TLE had MTR values that were 2 SD below the normal mean; the MTR abnormality was concordant with electroclinical lateralization in only one of the three.

CONCLUSION

We conclude that MTR measurements of amygdalae and hippocampi are not useful for lateralization of TLE.

An inverse problem approach to the correction of distortion in EPI images

Magnetic resonance imaging using the echo planar imaging (EPI) technique is particularly sensitive to main (B0) field inhomogeneities. The primary effect is geometrical distortion in the phase encoding direction. In this paper, we present a method based on the conjugate gradient algorithm to correct for this geometrical distortion, by solving the EPI imaging equation. Two versions are presented: one that attempts to solve the full four-dimensional (4-D) imaging equation, and one that independently solves for each profile along the blip encoding direction. Results are presented for both phantom and in vivo brain EPI images and compared with other proposed correction methods.

Quantitative interpretation of magnetization transfer in spoiled gradient echo MRI sequences

A method for analyzing general pulsed magnetization transfer (MT) experiments in which off-resonance saturation pulses are interleaved with on-resonance excitation pulses is presented. We apply this method to develop a steady-state signal equation for MT-weighted spoiled gradient echo sequences and consider approximations that facilitate its rapid computation. Using this equation, we assess various experimental designs for quantitatively imaging the fractional size of the restricted pool, cross-relaxation rate, and T(1) and T(2) relaxation times of the two pools in a binary spin bath system. From experiments on agar gel, this method is shown to reliably and accurately estimate the exchange and relaxation properties of a material in an imaging context, suggesting the feasibility of using this technique in vivo.

Multiple sclerosis: magnetization transfer MR imaging of white matter before lesion appearance on T2-weighted images

PURPOSE

To determine the evolution of magnetization transfer (MT) in white matter regions before and after plaque development in patients with multiple sclerosis (MS).

MATERIALS AND METHODS

In a 5-year longitudinal evaluation, 30 patients with MS underwent conventional magnetic resonance (MR) imaging, MT MR imaging, and clinical assessment. Cross-sectional data in 12 healthy subjects were also collected. Semiautomated lesion classification with use of T2-weighted MR images was used to measure the time course of the MT ratio (calculated with MR data acquired without and with MT saturation) in every voxel and to help analyze the relationship with the status of lesions depicted on T2-weighted images.

RESULTS

There was a significant (P <.001) temporal decline in lesion MT ratio after lesion appearance on T2-weighted images. A significant (P <. 001) progressive decline in MT ratio was also present in voxels that later became lesions, prior to initial detection on T2-weighted images. Even 1(1/2) years prior to lesion appearance, the MT ratio (33.3%) in regions destined to become such lesions was significantly (P <.001) lower than that in both white matter in healthy subjects (41.3%) and other normal-appearing white matter in patients with MS (38.1%).

CONCLUSION

The MT ratio reveals progressive focal abnormalities in MS that antedate by up to 2 years the appearance of lesions on T2-weighted MR images.

Correction for B(1) and B(0) variations in quantitative T(2) measurements using MRI

A new method is described for compensating for the bias introduced by variations in radiofrequency (RF) field strength and main magnetic field strength when making quantitative T(2) measurements using MRI. Field measurements made during the MRI study are used in combination with a signal model for off-resonance and imperfect RF pulses to correct the estimated T(2) value at every voxel. Applicable to both multicomponent and conventional single-component T(2) studies, the method has been validated experimentally using paramagnetic salt solutions in a multicompartment phantom. Studies of the human head are used to demonstrate the method in practice.

Relating axonal injury to functional recovery in MS

A patient was followed after the new onset of hemiparesis from relapse of MS with serial MR spectroscopy and functional MRI. The association of clinical improvement with recovery of N-acetylaspartate, a marker of neuronal integrity, and progressive reduction of abnormally large functional MRI cortical activation with movement demonstrates that dynamic reorganization of the motor cortex accompanies remission of MS.

Presurgical motor and somatosensory cortex mapping with functional magnetic resonance imaging and positron emission tomography

OBJECT

Accurate identification of eloquent cortex is important to ensure that resective surgery in the region surrounding the central sulcus is performed with minimum risk of permanent neurological deficit. Functional localization has traditionally been accomplished using intraoperative cortical stimulation (ICS). However, this technique suffers from several disadvantages that make the development and validation of noninvasive methods desirable. Functional localization accomplished by activation studies in which positron emission tomography (PET) scanning and the tracer [15O]H2O have been used has been shown to correlate well with the results of ICS. Another noninvasive method for functional localization is functional magnetic resonance (fMR) imaging. We compared the locations of activation peaks obtained in individual patients using fMR and [15O]H2O PET imaging.

METHODS

Twenty-six combined PET activation-fMR imaging studies were performed in 11 patients who were admitted for evaluation before undergoing surgery in the region surrounding the central sulcus. The PET scans were obtained using bolus injections of the cerebral blood flow tracer [15O]H2O (10 mCi). Multislice T2*-weighted gradient-echo echoplanar images were acquired using a 1.5-tesla MR imaging system. Activation maps were aligned with anatomical MR images and transformed into stereotactic space, after which the locations of activation peaks obtained using both modalities were compared. The average distance between activation peaks obtained using fMR imaging and those obtained using PET imaging was 7.9+/-4.8 mm (p>0.05), with 96% of the peaks being located on either the same or adjacent sulci and gyri. Overlapping of voxels activated by each modality occurred in 92% of the studies. Functional MR imaging failed to activate the primary sensorimotor cortex in one study and produced results that were ambiguous in the clinical setting in three cases.

CONCLUSIONS

Overall, fMR imaging produced activation that correlated well with that obtained using PET scanning. Discrepancies between the sites of activation identified using these two techniques may reflect differences in their physiological bases.

MRI simulation-based evaluation of image-processing and classification methods

With the increased interest in computer-aided image analysis methods, there is a greater need for objective methods of algorithm evaluation. Validation of in vivo MRI studies is complicated by a lack of reference data and the difficulty of constructing anatomically realistic physical phantoms. We present here an extensible MRI simulator that efficiently generates realistic three-dimensional (3-D) brain images using a hybrid Bloch equation and tissue template simulation that accounts for image contrast, partial volume, and noise. This allows image analysis methods to be evaluated with controlled degradations of image data.

Investigation of BOLD signal dependence on cerebral blood flow and oxygen consumption: the deoxyhemoglobin dilution model

The relationship between blood oxygenation level-dependent (BOLD) MRI signals, cerebral blood flow (CBF), and oxygen consumption (CMR(O2)) in the physiological steady state was investigated. A quantitative model, based on flow-dependent dilution of metabolically generated deoxyhemoglobin, was validated by measuring BOLD signals and relative CBF simultaneously in the primary visual cortex (V1) of human subjects (N = 12) during graded hypercapnia at different levels of visual stimulation. BOLD and CBF responses to specific conditions were averaged across subjects and plotted as points in the BOLD-CBF plane, tracing out lines of constant CMR(O2). The quantitative deoxyhemoglobin dilution model could be fit to these measured iso-CMR(O2) contours without significant (P </= 0.05) residual error and yielded MRI-based CMR(O2) measurements that were in agreement with PET results for equivalent stimuli. BOLD and CBF data acquired during graded visual stimulation were then substituted into the model with constant parameters varied over plausible ranges. Relative changes in CBF and CMR(O2) appeared to be coupled in an approximate ratio of approximately 2:1 for all realistic parameter settings. Magn Reson Med 42:849-863, 1999.

Linear coupling between cerebral blood flow and oxygen consumption in activated human cortex

The aim of this study was to test the hypothesis that, within a specific cortical unit, fractional changes in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen consumption (CMR(O(2))) are coupled through an invariant relationship during physiological stimulation. This aim was achieved by simultaneously measuring relative changes in these quantities in human primary visual cortex (V1) during graded stimulation with patterns designed to selectively activate different populations of V1 neurons. Primary visual cortex was delineated individually in each subject by using phase-encoded retinotopic mapping. Flow-sensitive alternating inversion recovery MRI, in conjunction with blood oxygenation-sensitive MRI and hypercapnic calibration, was used to monitor CBF and CMR(O(2)). The stimuli used included (i) diffuse isoluminant chromatic displays; (ii) high spatial-frequency achromatic luminance gratings; and (iii) radial checkerboard patterns containing both color and luminance contrast modulated at different temporal rates. Perfusion responses to each pattern were graded by varying luminance and/or color modulation amplitudes. For all stimulus types, fractional changes in blood flow and oxygen uptake were found to be linearly coupled in a consistent ratio of approximately 2:1. The most potent stimulus produced CBF and CMR(O(2)) increases of 48 +/- 5% and 25 +/- 4%, respectively, with no evidence of a plateau for oxygen consumption. Estimation of aerobic ATP yields from the observed CMR(O(2)) increases and comparison with the maximum possible anaerobic ATP contribution indicate that elevated energy demands during brain activation are met largely through oxidative metabolism.

Stimulus-dependent BOLD and perfusion dynamics in human V1

Blood oxygenation level-dependent (BOLD) fMRI signals often exhibit pronounced over- or undershoot upon changes in stimulation state. Current models postulate that this is due to the delayed onset or decay of perfusion-dependent attenuating responses such as increased cerebral blood volume or oxygen consumption, which are presumed to lag behind the rapid adjustment of blood flow rate to a new steady-state level. If this view is correct, then BOLD overshoot amplitudes in a specific tissue volume should be correlated with steady-state increases in perfusion, independent of stimulus type. To test this prediction, we simultaneously recorded BOLD and relative perfusion signals in primary visual cortex while inducing graded perfusion increases with three types of visual stimulus. Two of these, a diffuse chromatic stimulus with no luminance variation and a very high spatial frequency luminance grating, did not produce detectable BOLD overshoot (or undershoot) when an equal mean luminance baseline was used. Radial checkerboard stimuli, however, caused pronounced over/undershoot of both BOLD and perfusion signals even when temporal mean luminance was held constant and stimulus contrast was adjusted to produce the same steady-state blood flow increases evoked by the other stimuli. Transient amplitudes were relatively invariant in spite of large changes in steady-state response, demonstrating nonlinear BOLD and perfusion step responses in human V1. These findings suggest that, rather than a purely tissue-specific biomechanical or metabolic phenomenon, BOLD overshoot and undershoot represent transient features in the perfusion signal whose effects may be amplified by slowly evolving blood volume changes.

Combined magnetization transfer and proton spectroscopic imaging in the assessment of pathologic brain lesions in multiple sclerosis

BACKGROUND AND PURPOSE

Conventional MR imaging of multiple sclerosis (MS) provides relatively poor pathologic specificity, which has led to the investigation of more sophisticated MR techniques. The purpose of this study was to combine magnetization transfer (MT) imaging and proton MR spectroscopic imaging (MRSI) to evaluate the specific pathologic features of myelination and neuronal integrity in patients with MS and to determine the relationship between these measures within plaques.

METHODS

We acquired conventional MR, MT, and proton MRSI data and evaluated clinical disability in 30 patients with MS, whose conditions were categorized as relapsing-remitting, primary progressive, or secondary progressive. The lesions were classified, using a semiautomated edge-following technique, on T2-weighted MR images, and an analysis of MT and proton MRSI data was conducted for lesion regions as well as for tissue that was categorized as normal.

RESULTS

The MT ratio (MTR) of normal-appearing white matter in the patients with MS was significantly lower than in the healthy participants, whereas gray matter values were unchanged. MS lesions showed a large reduction in MTR, with old lesions exhibiting a lower MTR than new lesions. The average lesion MTR and the MR spectroscopic imaging-measured relative concentration of N-acetylaspartate, a marker of neuronal integrity, was positively correlated in patients with relapsing-remitting MS. This relationship was strengthened in regions containing new lesions.

CONCLUSION

The integrated use of MT and MR spectroscopic imaging provides a more complete description of the pathologic features of MS than does conventional MR imaging alone, and our data suggest that axonal damage occurs in step with new demyelination and is not a late feature of the disease.

Interictal spikes increase cerebral glucose metabolism and blood flow: a PET study

PURPOSE

In patients with reflex epilepsy, it is sometimes possible to evoke interictal spikes predictably, thus providing an uncommon but important experimental paradigm for examining the physiological changes produced by epileptiform discharges.

METHODS

To examine the changes in regional cerebral blood flow (rCBF) and glucose consumption (rCMRglc) produced by interictal spikes, we performed positron emission tomography (PET) scans with the blood-flow tracer [15O]H20 and with [18F]fluorodeoxyglucose in a patient with fixation-off epilepsy. The scans were performed in states of high and low spike frequency produced by eye closure and opening, respectively.

RESULTS

The rCBF study revealed a focal increase in blood flow associated with the state of increased interictal spiking. The focus was in the posterior portion of the left superior parietal lobule (Talairach coordinates: x: -36, y: -71, z: 39; t = 4.5; p<0.05) and corresponded to the site of maximal ictal EEG abnormality recorded with implanted electrodes. In a volume of interest of 10-mm diameter centered on the t statistic peak in the rCBF study, the mean rCMRglc was 39.1 micromol/100 g/min with eyes open and 44.1 micromol/100 g/min (13% increase) with eyes closed. An identical activation paradigm was used in six normal subjects studied with functional magnetic resonance imaging. In the normal subjects, no significant activation was observed in the parieto-occipital region, indicating that the changes observed in the patient were due to interictal spiking rather than to task performance alone.

CONCLUSIONS

Interictal spiking produces focal increases in cerebral blood flow and glucose metabolism.

Perfusion-based functional magnetic resonance imaging with single-shot RARE and GRASE acquisitions

For perfusion-based functional magnetic resonance imaging, the previously introduced flow-sensitive alternating inversion recovery (FAIR) technique is combined with single-shot RARE (rapid acquisition with relaxation enhancement) and GRASE (gradient and spin echo) imaging sequences. The advantages of these sequences compared to commonly used echo-planar imaging (EPI) are an increased signal-to-noise ratio and the absence of distortions and artifacts due to magnetic field inhomogeneities. RARE- and GRASE-FAIR are applied to functional brain mapping studies in humans during visual stimulation. Results demonstrate that the presented techniques allow for perfusion maps with higher spatial resolution compared to EPI-FAIR. Relative regional cerebral blood flow change in the occipital cortex during visual stimulation was measured to be 41+/-4% (n = 5). The comparison of FAIR data obtained with RARE and GRASE techniques shows that RARE yields images with the higher signal-to-noise ratio. However, the GRASE technique features a shorter acquisition time and less RF power deposition and is thus better suited for multi-slice acquisitions.

Quantitative magnetic resonance imaging in the assessment of degenerative disc disease

Understanding degenerative intervertebral disc diseases hinges on the ability to objectively and noninvasively assess the disc matrix composition and integrity. The potential of quantitative magnetic resonance imaging to meet these demands was evaluated. Analyzing the T1 and T2 signal patterns in the nucleus pulposus with increasing Thompson morphological grade revealed a significant reduction related to grade. This significant decrease in T1 and T2 in the nucleus pulposus with grade 4 degeneration and the corresponding low correlation coefficients with respect to the content of individual matrix molecules suggest that matrix integrity plays an important and distinct role in determining T1 and T2 signal. Similarly, the significant increase in magnetization transfer in the nucleus pulposus observed in grade 4 degeneration did not correlate with the changes in molecular content in these highly degenerated discs. Again, this lack of correlation clearly indicates that the tissue integrity and matrix composition independently contribute to the magnetization transfer signal. This study presents the first clear evidence that quantitative magnetic resonance analysis reflects not only the disc matrix composition, but also the structural integrity of the matrix of the disc.

Standing-wave and RF penetration artifacts caused by elliptic geometry: an electrodynamic analysis of MRI

Motivated by the observation that the diagonal pattern of intensity nonuniformity usually associated with linearly polarized radio-frequency (RF) coils is often present in neurological scans using circularly polarized coils, a theoretical analysis has been conducted of the intensity nonuniformity inherent in imaging an elliptically shaped object using 1.5-T magnets and circularly polarized RF coils. This first principle analysis clarifies, for the general case of conducting objects, the relationship between the excitation field and the reception sensitivity of circularly and linearly polarized coils. The results, validated experimentally using a standard spin-echo imaging sequence and an in vivo B1 field mapping technique, are shown to be accurate to within 1%-2% root mean square, suggesting that these electromagnetic interactions with the object account for most of the intensity nonuniformity observed.

Magnetization transfer imaging of multiple sclerosis

While conventional magnetic resonance imaging (MRI) measures signal primarily from the hydrogen nuclei of water, magnetization transfer (MT) MRI indirectly detects macromolecular associated hydrogen nuclei via their magnetic interaction with the observable water. In the normal adult CNS, white matter exhibits the largest MT effect due to the macromolecular content of the highly structured and lipid rich myelin. Pathologies which alter the structural integrity and the relative macromolecular-water composition, such as multiple sclerosis (MS), therefore show abnormal MT. Conventional MRI, which has a high MS lesion detection sensitivity but poor specificity in terms of differentiating the pathological state of a plaque, can thus be supplemented by MT to provide more specific information on the extent of demyelination and axonal loss. In this paper we review the basic concepts of MT imaging and its role in MS lesion characterization.

Density compensation functions for spiral MRI

In interleaved spiral MRI, an object's Fourier transform is sampled along a set of curved trajectories in the spatial frequency domain (k-space). An image of the object is then reconstructed, usually by interpolating the sampled Fourier data onto a Cartesian grid and applying the fast Fourier transform (FFT) algorithm. To obtain accurate results, it is necessary to account for the nonuniform density with which k-space is sampled. An analytic density compensation function (DCF) for spiral MRI, based on the Jacobian determinant for the transformation between Cartesian coordinates and the spiral sampling parameters of time and interleaf rotation angle, is derived in this paper, and the reconstruction accuracy achieved using this function is compared with that obtained using several previously published expressions. Various nonideal conditions, including intersecting trajectories, are considered. The new DCF eliminated intensity cupping that was encountered in images reconstructed with other functions, and significantly reduced the level of artifact observed when unevenly spaced sampling trajectories, such as those achieved with trapezoidal gradient waveforms, were employed. Modified forms of this function were found to provide similar improvements when intersecting trajectories made the spiral-Cartesian transformation noninvertible, and when the shape of the spiral trajectory varied between interleaves.

Magnetization transfer magnetic resonance imaging: a clinical review

Magnetic resonance imaging has traditionally used the T1 and T2 relaxation times and proton density (PD) of tissue water (hydrogen protons) to manipulate contrast. Magnetization transfer (MT) is a new form of tissue contrast based on the physical concept that tissues contain two or more separate populations of hydrogen protons: a highly mobile (free) hydrogen (water) pool, Hr, and an immobile (restricted) hydrogen pool, Hr, the latter being those protons bound to large macromolecular proteins and lipids, such as those found in such cellular membranes as myelin. Direct observation of the Hr magnetization pool is normally not possible because of its extremely short T2 time (< 200 microseconds). But saturation of the restricted pool will have a detectable effect on the mobile (free) proton pool. Saturation of the restricted pool decreases the signal of the free pool by transferring the restricted pool's saturation. Exchange of magnetization between the free and restricted hydrogen protons is a substantial mechanism for spin-lattice (T1) relaxation in tissues and the physical basis of MT. Through an appropriately designed pulse sequence, magnetization transfer contrast (MTC) can be produced. MT contrast is different from T1, T2, and PD, and it likely reflects the structural integrity of the tissue being imaged. A variety of clinically important uses of MT have emerged. In this clinical review of the neuroradiological applications of MT, we briefly review the physics of MT, the appearance of normal brain with MT, and the use of MT as a method of contrast enhancement/background suppression and in tissue characterization, such as evaluation of multiple sclerosis and other white-matter lesions and tumors. The role of MT in small-vessel visualization on three-dimensional time-of-flight magnetic resonance angiography and in head and neck disease and newer applications of MT are also elaborated.

Pulsed magnetization transfer contrast in gradient echo imaging: a two-pool analytic description of signal response

Magnetization transfer (MT) imaging with a rapid gradient-echo sequence and pulsed saturation provides an efficient means of acquiring high resolution three-dimensional data in vivo. This paper presents a derivation of the theoretical steady-state signal equation for this sequence based on the two-site coupled Bloch equations. Numerical simulations are used to validate the derived expression and experiments are performed on an agar gel model and normal brain. Experimental agar data indicate that direct saturation of the liquid component can be a major source of signal attenuation whereas MT normally dominates in brain tissue. The signal equation presented here establishes the necessary theory for sequence design and optimization and provides insight into model parameters and experimental results.

Measure of magnetization transfer in multiple sclerosis demyelinating plaques, white matter ischemic lesions, and edema

PURPOSE

To define the percentage of magnetization transfer of multiple sclerosis (MS) plaques, ischemic white matter lesions, and vasogenic edema to determine whether this measurement can help differentiate these entities.

METHODS

Findings were compared in 25 patients with proved MS, 20 patients with white matter ischemic lesions, and 72 patients with white matter edema (caused by tumors, infections, or acute/subacute infarctions) in the periventricular system, centrum semiovale, and subcortical white matter. Magnetization transfer was performed using an on-resonance binomial pulse. The percentage of magnetization transfer of the normal white matter was also calculated.

RESULTS

Magnetization transfer was significantly higher in white matter ischemic lesions (range, 31% to 38%; mean, 34% +/- 0.6%) than in demyelinating plaques of MS (range, 19% to 28%; mean, 22.5% +/- 1%) and in edema (range, 29% to 37%; mean, 30.2% +/- 0.4%). No statistical difference in percentage of magnetization transfer was found among lesions in the periventricular system (34% +/- 0.6%), centrum semiovale (35% +/- 0.5%), or subcortical white matter (33% +/- 0.6%), or in vasogenic edema associated with tumors, infections, or infarction.

CONCLUSION

Differences in magnetization transfer suggest less change of demyelination in white matter ischemic lesions than in MS plaques and are significantly different in this respect from similar MS plaques. Magnetization transfer of edema was less than that of normal white matter or fell between ischemic abnormalities and MS plaques. Percentages of magnetization transfer below the mid-20% range is highly suggestive of demyelination. Vasogenic edema, our surrogate for increased water content of white matter, caused a decrease in the percentage of magnetization transfer.

Improved detection of enhancing and nonenhancing lesions of multiple sclerosis with magnetization transfer

PURPOSE

To determine whether magnetization transfer imaging can improve visibility of contrast enhancement of multiple sclerosis plaques.

METHODS

Fifty-nine enhancing and 63 nonenhancing lesions in 10 patients with multiple sclerosis were evaluated to calculate contrast-to-noise ratios on conventional T1-weighted and T1-weighted magnetization transfer images. The signal intensity of the lesion and the background (white matter) were measured on precontrast T1-weighted and T1-weighted magnetization transfer images (800/20/1 [repetition time/echo time/excitations]) and on postcontrast T1-weighted and T1-weighted magnetization transfer images. Mean contrast-to-noise ratios was calculated for all lesions.

RESULTS

The contrast-to-noise ratio was significantly higher for enhancing and nonenhancing lesions on T1-weighted magnetization transfer images than on conventional T1-weighted images. For enhancing lesions, the contrast-to-noise ratio was significantly higher on postcontrast T1-weighted magnetization transfer images, 32 +/- 2 compared with 21 +/- 2 on conventional T1-weighted images. Fifty of the 59 enhancing lesions were seen on both the T1-weighted and the T1-weighted magnetization transfer images. Nine enhancing lesions were seen only on the postcontrast T1-weighted magnetization transfer images. In addition, of 63 nonenhancing lesions seen on proton-density, T2-weighted, and T1-weighted magnetization transfer images, 16 were not seen on the conventional T1-weighted images. Seven of the 63 nonenhancing lesions and 7 of the 59 enhancing lesions had high signal intensity on the precontrast T1-weighted magnetization transfer images suggestive of lipid signal, a finding not seen on the conventional precontrast T1-weighted images.

CONCLUSION

Magnetization transfer improves the visibility of enhancing multiple sclerosis lesions, because they have a higher contrast-to-noise ratio than conventional postcontrast T1-weighted images. High signal intensity on both nonenhancing and enhancing lesions noted only on precontrast T1-weighted magnetization transfer suggests a lipid signal was unmasked. If magnetization transfer is used in multiple sclerosis patients, a precontrast magnetization transfer image is necessary.

Central nervous system tumor, infection, and infarction: detection with gadolinium-enhanced magnetization transfer MR imaging

PURPOSE

To quantitatively measure the degree of contrast enhancement of central nervous system (CNS) tumor, infection, and infarction by means of magnetization transfer (MT) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

T1-weighted MR images obtained before and after administration of contrast material with and without MT in 14 patients with CNS tumors were evaluated by means of a contrast-to-noise ratio (C/N). Another 72 patients with a variety of lesions underwent contrast material-enhanced T1-weighted MR imaging prospectively with and without MT; C/N was also evaluated.

RESULTS

All lesions had a higher C/N on T1-weighted postcontrast MT images than on conventional images. C/N was 65 +/- 5 (mean +/- standard error) for MT and 42 +/- 4 for conventional images. C/N improved by a factor of 1.6-2.1 in the three disease categories. In intracranial tumors, the MT technique itself did not contribute significantly (P < .001) to the increase in C/N in the absence of gadopentetate dimeglumine. In fact, the C/N was lower for nonenhanced T1-weighted MT images.

CONCLUSION

Concurrent use of gadopentetate dimeglumine and MT results in a statistically significant (P < .001) increase in C/N in CNS tumor, infection, and infarction.

Three-point phase-contrast velocity measurements with increased velocity-to-noise ratio

We describe a technique to increase the velocity-to-noise ratio (VNR) of phase-contrast magnetic-resonance velocity images based on making three measurements/flow encoding axis rather than the usual two. A phase-aliased high first moment data set and a nonaliased low first moment data set are acquired, and the high-moment data are phase unwrapped using the low-moment data. The VNR of the resulting measurement is given by that of the high-moment measurement and increases linearly with the first moment. A factor of 4 gain in VNR was observed with only a 50% increase in scan time. Thus, this method is a much more efficient way to increase VNR than simple averaging.

Magnetization transfer contrast MRI of musculoskeletal neoplasms

Magnetic resonance imaging (MRI) examinations were performed in 15 patients with musculoskeletal neoplasms to assess the value of magnetization transfer contrast in tumor characterization. Multiplanar gradient-recalled echo sequences (TR 500-600/TE 15-20/flip angle 20-30 degrees) were performed first without and then with magnetization transfer contrast generated by a zero degree binomial pulse (MPGR and MTMPGR). Standard T1-weighted spin echo images (SE; TR 300-400/TE 12-20) and either T2-weighted SE (TR 2000-2900/TE 70-80) or T2-weighted fast spin echo (FSE; TR 4000-5000/TE 100-119 effective) images were also obtained. Signal intensities on MTMPGR scans were compared to those on MPGR scans for both tumors and normal tissues. Signal intensity ratios (SIR) and contrast-to-noise ratios (CNR) were also compared for all sequences. MTMPGR images provided better contrast between pathologic tissues and muscle than did standard MPGR images, increasing both conspicuity of lesions and definition of tumor/muscle interfaces. Benign and malignant tumors, with the exception of lipoma, underwent similar degrees of magnetization transfer and could not be distinguished by this technique.