Restricted diffusion in spinal cord infarction demonstrated by magnetic resonance line scan diffusion imaging

BACKGROUND AND PURPOSE

We report on the use of line scan diffusion magnetic resonance imaging in the evaluation of spinal cord infarctions.

METHODS

Data on 19 patients with clinical findings consistent with spinal cord infarctions and abnormal findings on line scan diffusion imaging were reviewed. The Apparent Diffusion Coefficient (ADC) measurements for the normal spinal cord and for the areas of abnormality were calculated from trace ADC maps.

RESULTS

Restricted diffusion was found in all 19 patients. Absolute ADC values in the ischemic area ranged between 395.4 and 575.8 × 10(-6) mm(2)/s, with ADC ratios ranging between 39.4% and 57.4%.

CONCLUSIONS

Line scan diffusion imaging is technically feasible and appears to be a reliable method to diagnose spinal cord infarction in the acute setting.

Forensic application of postmortem diffusion-weighted and diffusion tensor MR imaging of the human brain in situ

BACKGROUND AND PURPOSE

DWI and DTI of the brain have proved to be useful in many neurologic disorders and in traumatic brain injury. This prospective study aimed at the evaluation of the influence of the PMI and the cause of death on the ADC and FA for the application of DWI and DTI in forensic radiology.

MATERIALS AND METHODS

DWI and DTI of the brain were performed in situ in 20 deceased subjects with mapping of the ADC and FA. Evaluation was performed in different ROIs, and the influence of PMI and cause of death was assessed.

RESULTS

Postmortem ADC values of the brain were decreased by 49%-72% compared with healthy living controls. With increasing PMI, ADCs were significantly reduced when considering all ROIs together and, particularly, GM regions (all regions, P < .05; GM, P < .01), whereas there was no significant effect in WM. Concerning the cause of death, ADCs were significantly lower in mechanical and hypoxic brain injury than in brains from subjects having died from heart failure (traumatic brain injury, P < .005; hypoxia, P < .001). Postmortem FA was not significantly different from FA in living persons and showed no significant influence of PMI or cause of death.

CONCLUSIONS

Performing postmortem DWI and DTI of the brain in situ can provide valuable information for application in forensic medicine. ADC could be used as an indicator of PMI and could help in the assessment of the cause of death.

Pixel-based comparison of spinal cord MR diffusion anisotropy with axon packing parameters

Water diffusion in nerve fibers is strongly influenced by axon architecture. In this study, fractional diffusion anisotropy and transverse and longitudinal diffusion coefficients were measured in excised human cervical spinal cord with MR line-scan diffusion imaging, at 625 microm in-plane resolution and 3 mm slice thickness. A pixel-based comparison of fractional diffusion anisotropy, transverse diffusion coefficient, and longitudinal diffusion coefficient data with axon packing parameters derived from corresponding stained histological sections was performed for four slices. The axon packing parameters, axon density, axon area-fraction, and average axon size for entire specimen cross-sections were calculated by computerized segmentation of optical microscopy data obtained at 0.53 microm resolution. Salient features could be recognized on fractional diffusion anisotropy, transverse diffusion coefficient, axon density, axon area fraction, and average axon size maps. For white matter regions only, the average correlation coefficients for fractional diffusion anisotropy compared to histology-based parameters axon density and axon area fraction were 0.37 and 0.21, respectively. For transverse diffusion coefficient compared to axon density and axon area fraction, they were -0.40 and -0.36, and for longitudinal diffusion coefficient compared to axon density and axon area fraction, -0.14 and -0.30. All average correlation coefficients for average axon size were low. Correlation coefficients for collectively analyzed white and gray matter regions were significantly higher than correlation coefficients derived from analysis of white matter regions only.

Diffusion imaging of brain tumors

MRI offers a tremendous armamentarium of different methods that can be employed in brain tumor characterization. MR diffusion imaging has become a widely accepted method to probe for the presence of fluid pools and molecular tissue water mobility. For most clinical applications of diffusion imaging, it is assumed that the diffusion signal vs diffusion weighting factor b decays monoexponentially. Within this framework, the measurement of a single diffusion coefficient in brain tumors permits an approximate categorization of tumor type and, for some tumors, definitive diagnosis. In most brain tumors, when compared with normal brain tissue, the diffusion coefficient is elevated. The presence of peritumoral edema, which also exhibits an elevated diffusion coefficient, often precludes the delineation of the tumor on the basis of diffusion information alone. Serially obtained diffusion data are useful to document and even predict the cellular response to drug or radiation therapy. Diffusion measurements in tissues over an extended range of b factors have clearly shown that the monoparametric description of the MR diffusion signal decay is incomplete. Very high diffusion weighting on clinical systems requires substantial compromise in spatial resolution. However, after suitable analysis, superior separation of malignant brain tumors, peritumoral edema and normal brain tissue can be achieved. These findings are also discussed in the light of tissue-specific differences in membrane structure and the restrictions exerted by membranes on diffusion. Finally, measurement of the directional dependence of diffusion permits the assessment of white matter integrity and dislocation. Such information, particularly in conjunction with advanced post-processing, is considered to be immensely useful for therapy planning. Diffusion imaging, which permits monoexponential analysis and provides directional diffusion information, is performed routinely in brain tumor patients. More advanced methods require improvement in acquisition speed and spatial resolution to gain clinical acceptance.

On high b diffusion imaging in the human brain: ruminations and experimental insights

Interest in the manner in which brain tissue signal decays with b factor in diffusion imaging schemes has grown in recent years following the observation that the decay curves depart from purely monoexponential decay behavior. Regardless of the model or fitting function proposed for characterizing sufficiently sampled decay curves (vide infra), the departure from monoexponentiality spells increased tissue characterization potential. The degree to which this potential can be harnessed to improve specificity, sensitivity and spatial localization of diseases in brain, and other tissues, largely remains to be explored. Furthermore, the degree to which currently popular diffusion tensor imaging methods, including visually impressive white matter fiber "tractography" results, have almost completely ignored the nonmonoexponential nature of the basic signal decay with b factor is worthy of communal introspection. Here we limit our attention to a review of the basic experimental features associated with brain water signal diffusion decay curves as measured over extended b-factor ranges, the simple few parameter fitting functions that have been proposed to characterize these decays and the more involved models, e.g.,"ruminations," which have been proposed to account for the nonmonoexponentiality to date.

Convection-enhanced drug delivery of interleukin-4 Pseudomonas exotoxin (PRX321): increased distribution and magnetic resonance monitoring

Convection-enhanced drug delivery (CED) enables achieving a drug concentration within brain tissue and brain tumors that is orders of magnitude higher than by systemic administration. Previous phase I/II clinical trials using intratumoral convection of interleukin-4 Pseudomonas exotoxin (PRX321) have demonstrated an acceptable safety and toxicity profile with promising signs of therapeutic activity. The present study was designed to assess the distribution efficiency and toxicity of this PRX321 using magnetic resonance imaging (MRI) and to test whether reformulation with increased viscosity could enhance drug distribution. Convection of low- [0.02% human serum albumin (HSA)] and high-viscosity (3% HSA) infusates mixed with gadolinium-diethylenetriamine pentaacetic acid and PRX321 were compared with low- and high-viscosity infusates without the drug, in normal rat brains. MRI was used for assessment of drug distribution and detection of early and late toxicity. Representative brain samples were subjected to histological examination. Distribution volumes calculated from the magnetic resonance images showed that the average distribution of 0.02% HSA was larger than that of 0.02% HSA with PRX321 by a factor of 1.98 (p < 0.02). CED of 3.0% HSA, with or without PRX321, tripled the volume of distribution compared with 0.02% HSA with PRX321 (p < 0.015). No drug-related toxicity was detected. These results suggest that the impeded convection of the PRX321 infusate used in previous clinical trials can be reversed by increasing infusate viscosity and lead to tripling of the volume of distribution. This effect was not associated with any detectable toxicity. A similar capability to reverse impeded convection was also demonstrated in a CED model using acetic acid. These results will be implemented in an upcoming phase IIb PRX321 CED trial with a high-viscosity infusate.

MRI-based thermal dosimetry and diffusion-weighted imaging of MRI-guided focused ultrasound thermal ablation of uterine fibroids

PURPOSE

To investigate tissue changes observed in diffusion-weighted imaging (DWI) and its relation to contrast imaging, thermal dosimetry, and changes in the apparent diffusion coefficient (ADC) after MRI-guided focused ultrasound surgery (MRgFUS) of uterine fibroids.

MATERIALS AND METHODS

Imaging data were analyzed from 45 fibroids in 42 women treated with MRgFUS. The areas of the hyperintense regions in DWI and of nonperfused regions in T1-weighted contrast enhanced imaging (both acquired immediately after treatment) were compared with each other and to thermal dosimetry based estimates. Changes in ADC were also calculated.

RESULTS

Hyperintense regions were observed in 35/45 fibroids in DWI. When present, the areas of these regions were comparable on average to the thermal dose estimates and to the nonperfused regions, except for in several large treatments in which the nonperfused region extended beyond the treated area. ADC increased in 19 fibroids and decreased in the others.

CONCLUSION

DWI changes, which includes changes in both in T2 and ADC, may be useful in many cases to delineate the treated region resulting from MRgFUS. However, clear DWI changes were not always observed, and in some large treatments, the extent of the nonperfused region was under estimated. ADC changes immediately after MRgFUS were unpredictable.

Magnetic resonance acoustic radiation force imaging

Acoustic radiation force impulse imaging is an elastography method developed for ultrasound imaging that maps displacements produced by focused ultrasound pulses systematically applied to different locations. The resulting images are "stiffness weighted" and yield information about local mechanical tissue properties. Here, the feasibility of magnetic resonance acoustic radiation force imaging (MR-ARFI) was tested. Quasistatic MR elastography was used to measure focal displacements using a one-dimensional MRI pulse sequence. A 1.63 or 1.5 MHz transducer supplied ultrasound pulses which were triggered by the magnetic resonance imaging hardware to occur before a displacement-encoding gradient. Displacements in and around the focus were mapped in a tissue-mimicking phantom and in an ex vivo bovine kidney. They were readily observed and increased linearly with acoustic power in the phantom (R2=0.99). At higher acoustic power levels, the displacement substantially increased and was associated with irreversible changes in the phantom. At these levels, transverse displacement components could also be detected. Displacements in the kidney were also observed and increased after thermal ablation. While the measurements need validation, the authors have demonstrated the feasibility of detecting small displacements induced by low-power ultrasound pulses using an efficient magnetic resonance imaging pulse sequence that is compatible with tracking of a dynamically steered ultrasound focal spot, and that the displacement increases with acoustic power. MR-ARFI has potential for elastography or to guide ultrasound therapies that use low-power pulsed ultrasound exposures, such as drug delivery.

Biexponential analysis of diffusion-related signal decay in normal human cortical and deep gray matter

Diffusion imaging with high-b factors, high spatial resolution and cerebrospinal fluid signal suppression was performed in order to characterize the biexponential nature of the diffusion-related signal decay with b-factor in normal cortical gray and deep gray matter (GM). Integration of inversion pulses with a line scan diffusion imaging sequence resulted in 91% cerebrospinal fluid signal suppression, permitting accurate measurement of the fast diffusion coefficient in cortical GM (1.142+/-0.106 microm2/ms) and revealing a marked similarity with that found in frontal white matter (WM) (1.155+/-0.046 microm2/ms). The reversal of contrast between GM and WM at low vs high b-factors is shown to be due to a significantly faster slow diffusion coefficient in cortical GM (0.338+/-0.027 microm2/ms) than in frontal WM (0.125+/-0.014 microm2/ms). The same characteristic diffusion differences between GM and WM are observed in other brain tissue structures. The relative component size showed nonsignificant differences among all tissues investigated. Cellular architecture in GM and WM are fundamentally different and may explain the two- to threefold higher slow diffusion coefficient in GM.

Apparent diffusion coefficient of subcutaneous epidermal cysts in the head and neck comparison with intracranial epidermoid cysts

RATIONALE AND OBJECTIVES

Subcutaneous epidermal cysts and intracranial epidermoid cysts are pathologically identical. Although diffusion-weighted imaging (DWI) studies of intracranial epidermoid cysts have been numerously reported, those of subcutaneous epidermal cysts have not been sufficiently investigated. Our hypothesis for this study is that the apparent diffusion coefficient (ADC) values of subcutaneous epidermal cysts and intracranial epidermoid cysts are not different. This study was intended to evaluate the ADC of subcutaneous epidermal cysts of the head and neck in comparison with that of intracranial epidermoid cysts.

MATERIALS AND METHODS

The MR studies were performed in 14 patients with head and neck subcutaneous epidermal cysts and 10 patients with intracranial epidermoid cysts using line scan DWI (LSDWI). The ADC was measured and compared between the two types of cyst.

RESULTS

The ADC values (mean +/- SD) were 0.81 +/- 0.14 x 10(-3) mm(2)/s in subcutaneous epidermal cysts and 1.06 +/- 0.12 x 10(-3) mm(2)/s in intracranial epidermoid cysts. A significant difference was found in ADC values between the two types (P = .0019).

CONCLUSION

Our preliminary study has shown that the ADC provides useful information regarding tissue characterization of subcutaneous epidermal cysts. However, the ADC of subcutaneous epidermal cysts was significantly lower than that of intracranial epidermoid cysts.

Soft-tissue tumors evaluated by line-scan diffusion-weighted imaging: influence of myxoid matrix on the apparent diffusion coefficient

PURPOSE

To compare the apparent diffusion coefficients (ADCs) of myxoid and nonmyxoid soft-tissue tumors using line-scan diffusion-weighted imaging (LSDWI), and to investigate the myxoid matrix influence on ADCs of soft-tissue tumors.

MATERIALS AND METHODS

This study enrolled 44 patients with soft tissue tumors. They were divided into two groups: one with myxoid-containing soft-tissue tumors (N = 23) and the other with nonmyxoid soft-tissue tumors (N = 21). The 44 patients were also classified histologically into 26 with malignant soft-tissue tumors and 18 with benign soft-tissue tumors. LSDWI was performed using b values of 5 and 1000 second/mm(2). The ADCs of the tumors were calculated and compared for myxoid and nonmyxoid tumors and for benign and malignant tumors.

RESULTS

The ADC (mean +/- SD) was 1.92 +/- 0.41 x 10(-3) mm(2)/second in myxoid containing tumors, whereas the ADC was 0.97 +/- 0.33 x 10(-3) mm(2)/second in nonmyxoid tumors. The ADCs of the myxoid and nonmyxoid tumors were significantly different (P < 0.01). The ADCs were 1.45 +/- 0.59 x 10(-3) mm(2)/second in malignant tumors and 1.50 +/- 0.64 x 10(-3) mm(2)/second in benign tumors. The ADCs of benign and malignant soft-tissue tumors were not significantly different.

CONCLUSION

The ADCs of myxoid-containing soft-tissue tumors were significantly higher than those of nonmyxoid soft-tissue tumors. The myxoid matrix influences ADCs of both benign and malignant soft-tissue tumors.

Examination of spinal cord tissue architecture with magnetic resonance diffusion tensor imaging

Magnetic resonance diffusion tensor imaging yields images with detailed information about tissue water diffusion. Diffusion-weighted imaging of the human spinal cord requires dedicated magnetic resonance pulse sequences that minimize the effects of subject motion, distortions, and artifacts from lipids and CSF flow. These problems are accentuated by the anatomic properties of the spinal cord (i.e., a small cross-sectional dimension and a location deep inside the body). The diffusion tensor (a simplified model for complex diffusion in structured tissues) can be estimated for each image pixel by measuring diffusion along a minimum of six independent directions. It can then be used to derive mean diffusivity, diffusion anisotropy, and the dominant orientation of the diffusion process. The observation that diffusion along nerve fibers is much higher than across fibers, allows a noninvasive reconstruction of the spinal cord nerve fiber architecture. This includes not only the primary cranio-caudad running connections, but also secondary, transverse running collateral fibers. With fiber tracking, the pixel-based diffusion information can be integrated to obtain a three-dimensional view of axonal fiber connectivity between the spinal cord and different brain regions. The development and myelination during infancy and early childhood is reflected in a gradual decrease of mean diffusivity and increase in anisotropy. There are several diseases that lead to either local or general changes in spinal cord water diffusion. For therapy research, such changes can be studied noninvasively and repeatedly in animal models.

Quantification of water compartmentation in cell suspensions by diffusion-weighted and T(2)-weighted MRI

When studying water diffusion in biological systems, any specific signal attenuation curve may be reproduced by a broad range of mathematical functions. Our goals were to quantify the diffusion and T(2) relaxation properties of water in a simple biological system and to study the changes that occur in osmotically stressed cells. Human breast cancer cells were incubated in isotonic or hypotonic osmotic buffers. Diffusion-weighted and T(2)-weighted magnetic resonance images were acquired during sedimentation over 12 h. Diffusion-weighted imaging (DWI) data were analyzed with a biexponential fit, the Kärger model for exchange between two freely diffusing populations and the Price-modified Kärger model accounting for restricted diffusion in spherical geometry. We found that only the Price model provided an accurate quantitative description for water diffusion in both cell systems, independent of acquisition parameters, over the entire density range. Model-derived cell radii, intracellular volume fractions and transmembrane water exchange times were in good agreement with results calculated from light microscopy and with model-free exchange times. T(2) data indicated two populations in fast exchange, with volume fractions clearly different from DWI populations. Hypotonic stress led to higher slow apparent diffusion coefficient, longer T(2) and lower membrane permeability. The tortuosity in a hypotonic cell suspension complied with the Wang model for spherical geometry. Quantitative characterization of biological systems is obtainable by DWI, using appropriate modeling, accounting for water restriction and exchange between compartments.

Complimentary aspects of diffusion imaging and fMRI: II. Elucidating contributions to the fMRI signal with diffusion sensitization

Tissue water molecules reside in different biophysical compartments. For example, water molecules in the vasculature reside for variable periods of time within arteries, arterioles, capillaries, venuoles and veins, and may be within blood cells or blood plasma. Water molecules outside of the vasculature, in the extravascular space, reside, for a time, either within cells or within the interstitial space between cells. Within these different compartments, different types of microscopic motion that water molecules may experience have been identified and discussed. These range from Brownian diffusion to more coherent flow over the time scales relevant to functional magnetic resonance imaging (fMRI) experiments, on the order of several 10s of milliseconds. How these different types of motion are reflected in magnetic resonance imaging (MRI) methods developed for "diffusion" imaging studies has been an ongoing and active area of research. Here we briefly review the ideas that have developed regarding these motions within the context of modern "diffusion" imaging techniques and, in particular, how they have been accessed in attempts to further our understanding of the various contributions to the fMRI signal changes sought in studies of human brain activation.

Developmental changes and injury induced disruption of the radial organization of the cortex in the immature rat brain revealed by in vivo diffusion tensor MRI

During brain development, morphological changes modify the cortex from its immature radial organization to its mature laminar appearance. Applying in vivo diffusion tensor imaging (DTI), the microstructural organization of the cortex in the immature rat was analyzed and correlated to neurohistopathology. Significant differences in apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were detected between the external (I-III) and deep (IV-VI) cortical layers in postnatal day 3 (P3) and P6 pups. With cortical maturation, ADC was reduced in both cortical regions, whereas a decrease in FA was only seen in the deep layers. A distinct radial organization of the external cortical layers with the eigenvectors perpendicular to the pial surface was observed at both ages. Histology revealed maturational differences in the cortical architecture with increased neurodendritic density and reduction in the radial glia scaffolding. Early DTI after hypoxia-ischemia at P3 shows reduced ADC and FA in the ipsilateral cortex that persisted at P6. Cortical DTI eigenvector maps reveal microstructural disruption of the radial organization corresponding to regions of neuronal death, radial glial disruption, and astrocytosis. Thus, the combined use of in vivo DTI and histopathology can assist in delineating normal developmental changes and postinjury modifications in the immature rodent brain.

Magnetic resonance imaging-guided, high-intensity focused ultrasound for brain tumor therapy

OBJECTIVE

Magnetic resonance imaging-guided high-intensity focused ultrasound (MRIgFUS) is a novel technique that may have the potential for precise image-guided thermocoagulation of intracranial lesions. The system delivers small volumetric sonications from an ultrasound phased array transmitter that focuses energy selectively to destroy the target with verification by magnetic resonance imaging-generated thermal maps. A Phase I clinical study was initiated to treat patients with recurrent glioma with MRIgFUS.

METHODS

To date, three patients with histologically verified recurrent glioblastoma multiforme have been treated with MRIgFUS. All patients underwent craniectomy 7 to 10 days before therapy to create a bony window for the ultrasound treatment. Sonications were applied to induce thermocoagulation of the enhancing tumor mass. Long-term radiological follow-up and post-treatment tissue specimens were available for all patients.

RESULTS

MRIgFUS treatment resulted in immediate changes in contrast-enhanced T1-, T2-, and diffusion-weighted magnetic resonance imaging scans in the treated regions with subsequent histological evidence of thermocoagulation. In one patient, heating of brain tissue in the sonication path resulted in a secondary focus outside the target causing neurological deficit. New software modifications were developed to address this problem.

CONCLUSION

In this first clinical report, MRIgFUS was demonstrated to be a potentially effective means of destroying tumor tissue by thermocoagulation, although with an associated morbidity and the inherent invasive nature of the procedure requiring creation of a bone window. A modified technology to allow MRIgFUS treatment through a closed cranium is being developed.

Fornix integrity and hippocampal volume in male schizophrenic patients

BACKGROUND

The hippocampus has been shown to be abnormal in schizophrenia. The fornix is one of the main fiber tracts connecting the hippocampus with other brain regions. Few studies have evaluated the fornix in schizophrenia, however. A focus on fornix abnormalities and their association with hippocampal abnormalities might figure importantly in our understanding of the pathophysiology of schizophrenia.

METHODS

Line-scan diffusion tensor imaging (DTI) was used to evaluate diffusion in the fornix in 24 male patients with chronic schizophrenia and 31 male control subjects. Maps of fractional anisotropy (FA) and mean diffusivity (D(m)), which are indices sensitive to white-matter integrity, were generated to quantify diffusion within the fornix. We used high spatial resolution magnetic resonance imaging (MRI) to measure hippocampal volume.

RESULTS

FA and cross-sectional area of the fornix were significantly reduced in patients compared with control subjects. D(m) was significantly increased, whereas hippocampal volume was bilaterally reduced in patients. Reduced hippocampal volume was correlated with increased mean D(m) and reduced cross-sectional area of the fornix for patients. Patients also showed a significant correlation between reduced scores on neuropsychologic measures of declarative-episodic memory and reduced hippocampal volumes.

CONCLUSIONS

These findings demonstrate a disruption in fornix integrity in patients with schizophrenia.

Collateral nerve fibers in human spinal cord: Visualization with magnetic resonance diffusion tensor imaging

Diffusion tensor magnetic resonance imaging provides structural information about nerve fiber tissue. The first eigenvector of the diffusion tensor is aligned with the nerve fibers, i.e., longitudinally in the spinal cord. The underlying hypothesis of this study is that the presence of collateral nerve fibers running orthogonal to the longitudinal fibers results in an orderly arrangement of the second eigenvectors. Magnetic resonance diffusion tensor scans were performed with line scan diffusion imaging on a clinical MR scanner. Axial sections were scanned in a human cervical spinal cord specimen at 625 microm resolution and the cervical spinal cord of four normal volunteers at 1250 microm resolution. The spinal cord specimen was fixed and stained for later light microscopy of the collateral fiber architecture at 0.53 microm resolution. Diffusion measured by MR was found to be anisotropic for both white and gray matter areas of the spinal cord specimen; the average fractional anisotropy (FA) was 0.63 +/- 0.09 (diffusion eigenvalues lambda1 0.38 +/- 0.05 micros/mm2, lambda2 0.14 +/- 0.03 micros/mm2, lambda3 0.10 +/- 0.03 micros/mm2) in white matter and 0.27 +/- 0.04 (lambda1 0.36 +/- 0.04 micros/mm2, lambda2 0.28 +/- 0.03 micros/mm2, lambda3 0.21 +/- 0.04 micros/mm2 in gray matter. The normal-volunteer FA values were similar, i.e., 0.66 +/- 0.04 (lambda1 1.66 +/- 0.14 micros/mm2, lambda2 0.55 +/- 0.02 micros/mm2, lambda3 0.40 +/- 0.01 micros/mm2) in white matter and 0.35 +/- 0.03 (lambda1 1.14 +/- 0.07 micros/mm2, lambda2 0.70 +/- 0.03 micros/mm2, lambda3 0.58 +/- 0.02 micros/mm2) in gray matter. The first eigenvector pointed, as expected, in the longitudinal direction. The second eigenvector directions exhibited a striking arrangement, consistent with the distribution of interconnecting collateral nerve fibers discerned on the histology section. This finding was confirmed for the specimen by quantitative pixel-wise comparison of second eigenvector directions and collateral fiber directions assessed on light microscopy image data. Diffusion tensor MRI can reveal non-invasively and in great detail the intricate fiber architecture of the human spinal cord.

Complementary aspects of diffusion imaging and fMRI; I: structure and function

Studying the intersection of brain structure and function is an important aspect of modern neuroscience. The development of magnetic resonance imaging (MRI) over the last 25 years has provided new and powerful tools for the study of brain structure and function. Two tools in particular, diffusion imaging and functional MRI (fMRI), are playing increasingly important roles in elucidating the complementary aspects of brain structure and function. In this work, we review basic technical features of diffusion imaging and fMRI for studying the integrity of white matter structural components and for determining the location and extent of cortical activation in gray matter, respectively. We then review a growing body of literature in which the complementary aspects of diffusion imaging and fMRI, applied as separate examinations but analyzed in tandem, have been exploited to enhance our knowledge of brain structure and function.

Biexponential characterization of prostate tissue water diffusion decay curves over an extended b-factor range

Detailed measurements of water diffusion within the prostate over an extended b-factor range were performed to assess whether the standard assumption of monoexponential signal decay is appropriate in this organ. From nine men undergoing prostate MR staging examinations at 1.5 T, a single 10-mm-thick axial slice was scanned with a line scan diffusion imaging sequence in which 14 equally spaced b factors from 5 to 3,500 s/mm(2) were sampled along three orthogonal diffusion sensitization directions in 6 min. Due to the combination of long scan time and limited volume coverage associated with the multi-b-factor, multidirectional sampling, the slice was chosen online from the available T2-weighted axial images with the specific goal of enabling the sampling of presumed noncancerous regions of interest (ROIs) within the central gland (CG) and peripheral zone (PZ). Histology from prescan biopsy (n=9) and postsurgical resection (n=4) was subsequently employed to help confirm that the ROIs sampled were noncancerous. The CG ROIs were characterized from the T2-weighted images as primarily mixtures of glandular and stromal benign prostatic hyperplasia, which is prevalent in this population. The water signal decays with b factor from all ROIs were clearly non-monoexponential and better served with bi- vs. monoexponential fits, as tested using chi(2)-based F test analyses. Fits to biexponential decay functions yielded intersubject fast diffusion component fractions in the order of 0.73+/-0.08 for both CG and PZ ROIs, fast diffusion coefficients of 2.68+/-0.39 and 2.52+/-0.38 microm(2)/ms and slow diffusion coefficients of 0.44+/-0.16 and 0.23+/-0.16 um(2)/ms for CG and PZ ROIs, respectively. The difference between the slow diffusion coefficients within CG and PZ was statistically significant as assessed with a Mann-Whitney nonparametric test (P<.05). We conclude that a monoexponential model for water diffusion decay in prostate tissue is inadequate when a large range of b factors is sampled and that biexponential analyses are better suited for characterizing prostate diffusion decay curves.

On the strong field dependence and nonlinear response to gadolinium contrast agent of proton transverse relaxation rates in dairy cream

Dairy cream, as a suspension of lipid droplets in water, is a potentially useful magnetic resonance imaging (MRI) phantom material and an interesting material for studying fundamental relaxation mechanisms. Here we report a strong increase in the transverse relaxation rates with field strength for both the water and lipid protons in dairy cream. Also, studies at 4.7 T reveal a nonlinear response of transverse relaxation rates with increasing concentration of a common gadolinium (Gd)-based contrast agent, including an initial decrease of water relaxation rates as measured with Hahn spin echoes at the lower Gd concentrations. The results are treated within the framework of a model in which the magnetic susceptibility difference between the lipid droplets and the aqueous phase plays the prominent role for transverse relaxation. Second-order polynomial fits of the water proton transverse relaxation rate dependence on field strength and on Gd concentration at 4.7 T provided experimental parameters from which model parameters are extracted and compared with expectations available from the literature.