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PubMed2001

Diffusion and perfusion MRI: basic physics.

Diffusion and perfusion MR imaging are now being used increasingly in neuro-vascular clinical applications. While diffusion weighted magnetic resonance imaging exploits the translational mobility of water molecules to obtain information on the microscopic behaviour of the tissues (presence of macromolecules, presence and permeability of membranes, equilibrium intracellular-extracellular water, ellipsis), perfusion weighted imaging makes use of endogenous and exogenous tracers for monitoring their hemodynamic status. The combination of both techniques is extremely promising for the early detection and assessment of stroke, for tumor characterisation and for the evaluation of neurodegenerative diseases. This article provides a brief review of the basic physics principles underlying the methodologies followed.

Cerebrovascular Disorders↗
PubMed2003

Mohr diagram interpretation of anisotropic diffusion indices in MRI.

Descriptions of diffusion anisotropy in MR Diffusion Tensor Imaging are often based on scalar indices such as surface-to-volume ratio, volume ratio, fractional anisotropy and rotational anisotropy. Recently, Mohr diagram was introduced to visualize the anisotropic diffusion information, but in a graphical form. Even though both scalar indices and the Mohr diagram are derived from the elements of the diffusion tensor, so far, the relationships between these measures and the key aspects of the Mohr diagram have not been established. This paper demonstrates these relationships both qualitatively and quantitatively for two microscopically heterogeneous biologic tissues, kidney and spinal cord.

Anisotropy↗
PubMed2002

Diffusion-weighted MRI as a screening tool of stroke in the ED.

This report describes a novel imaging technology for the evaluation of stroke patients. Diffusion-weighted magnetic resonance imaging can visualize hyperacute ischemic stroke which cannot be seen on computed tomography; moreover, it only takes few minutes to scan. We believe that diffusion-weighted magnetic resonance imaging, rather than routine computed tomography, should be considered when the emergency physician evaluates a patient with acute ischemic stroke.

Aged↗
PubMed2002

Radial organization of developing preterm human cerebral cortex revealed by non-invasive water diffusion anisotropy MRI.

Cerebral cortical development involves a complex cascade of events which are difficult to visualize in intact, living subjects. In this study, we apply diffusion tensor imaging (DTI) to the evaluation of cortical development in human infants ranging from 26 to 41 weeks gestational age (GA). Apparent diffusion of water in cortex is maximally anisotropic at 26 weeks GA and anisotropy values approach zero by 36 weeks GA. During this period, the major eigenvector of the diffusion tensor in cerebral cortex is oriented radially across the cortical plate, in accord with a predominately radial deployment of its neuronal constituents. Values for the rotationally averaged water diffusion coefficient increase between 26 and 32 weeks GA, then decrease thereafter. These changes in DTI parameters are specific to cerebral cortex and reflect changes in underlying cortical architecture and formation of neuronal connections. Because of its correlation with tissue microstructure and non-invasive nature, DTI offers unique insight into cortical development in preterm human newborns and, potentially, detection of derangements of its basic cytoarchiteture.

Anisotropy↗
PubMed1999

Myelination and organization of the frontal white matter in children: a diffusion tensor MRI study.

Myelination is critical for the functional development of the brain, but the time course of myelination during childhood is not well known. Diffusion tensor MR imaging (DTI) provides a new method for estimating myelination in vivo. Myelin restricts diffusion of water transverse to the axons, causing diffusion to be anisotropic. By quantifying the anisotropy, the progressive myelination of axons can be studied. Central white matter of the frontal lobe was studied in seven children (mean age 10 years) and five adults (mean age 27 years). Anisotropy in the frontal white matter was significantly lower in children than in adults, suggesting less myelination in children. Measurement of the coherence of white matter revealed that the right frontal lobe had a more regular organization of axons than the left frontal lobe, in both children and adults. The results demonstrate that maturation of the frontal white matter continues into the second decade of life. The time course of prefrontal maturation makes it possible that myelination is a basis for the gradual development of prefrontal functions, such as increased working memory capacity.

Adult↗
PubMed2006

Lesion patterns and etiology of ischemia in the anterior inferior cerebellar artery territory involvement: a clinical - diffusion weighted - MRI study.

The topography and mechanism of stroke in the anterior inferior cerebellar artery (AICA) territory are delineated before, but the detailed clinical spectrum of lesions involving AICA territory was not studied by diffusion weighted imaging (DWI). We reviewed 1350 patients with posterior circulation ischemic stroke in our registry. We included patients if the diagnosis of AICA territory involvement was confirmed, and DWI, and magnetic resonance angiography were obtained in the 3 days of symptoms onset. The potential feeding arteries of the AICA territory were evaluated on magnetic resonance imaging (MRI) using a three-dimensional rotating cineoangiographic method. There were 23 consecutive patients with lesion involving AICA territory, six with isolated lesion in the AICA territory, six with posterior inferior cerebellar artery, 11 with multiple posterior circulation infarcts (MPCIs). The clinical feature of isolated AICA infarct was vertigo, tinnitus, dysmetria, ataxia, facial weakness, facial sensory deficits, lateral gaze palsy, and sensory-motor deficits in patients with pontine involvement. Patients with largest lesion extending to the anterior and inferolateral cerebellum showed mixed symptomatology of the lateral medullary (Wallenberg's syndrome) and AICA territory involvement. Patients with MPCIs presented various clinical pictures with consciousness disturbances and diverse clinical signs because of involvement of different anatomical structures. Large-artery atherosclerotic disease in the vertebrobasilar system was the main cause of stroke in 12 (52%) patients, cardioembolism (CE) in one (4%), and coexisting large-artery disease and a source of CE in four (17%). The main cause of stroke was atheromatous vertebrobasilar artery disease either in the distal vertebral or proximal basilar artery. The outcome was usually good except those with multiple lesions. The new MRI techniques and clinical correlations allow better definition of the diverse topographical and etiological spectrum of AICA territory involvement and associated infarcts which was previously based on pathological and conventional MRI studies.

Adult↗
PubMed2000

Diffusion-weighted MRI and proton MR spectroscopic imaging in the study of secondary neuronal injury after intracerebral hemorrhage.

BACKGROUND AND PURPOSE: Cerebral ischemia has been proposed as contributing mechanism to secondary neuronal injury after intracerebral hemorrhage (ICH). Possible tools for investigating this hypothesis are diffusion-weighted (DWI) and proton magnetic resonance spectroscopic imaging ((1)H-MRSI). However, magnetic field inhomogeneity induced by paramagnetic blood products may prohibit the application of such techniques on perihematoma tissue. We report on the feasibility of DWI and (1)H-MRSI in the study of human ICH and present preliminary data on their contribution to understanding perihematoma tissue functional and metabolic profiles. METHODS: Patients with acute supratentorial ICH were prospectively evaluated using DWI and (1)H-MRSI. Obscuration of perihematoma tissue with both sequences was assessed. Obtainable apparent diffusion coefficient (Dav) and lactate spectra in perihematoma brain tissue were recorded and analyzed. RESULTS: Nine patients with mean age of 63.4 (36 to 87) years were enrolled. Mean time from symptom onset to initial MRI was 3.4 (1 to 9) days; mean hematoma volume was 35.4 (5 to 80) cm(3). Perihematoma diffusion values were attainable in 9 of 9 patients, and (1)H-MRSI measures were obtainable in 5 of 9 cases. Dav in perihematoma regions was 172.5 (120.0 to 302.5)x10(-5) mm(2)/s and 87.6 (76.5 to 102.1)x10(-5) mm(2)/s in contralateral corresponding regions of interest (P=0.002). One patient showed an additional area of reduced Dav with normal T(2) intensity, which suggests ischemia. (1)H-MRSI revealed lactate surrounding the hematoma in 2 patients. CONCLUSIONS: DWI and (1)H-MRSI can be used in the study of ICH patients. Our preliminary data are inconsistent with ischemia as the primary mechanism for perihematoma tissue injury. Further investigation with advanced MRI techniques will give a clearer understanding of the role that ischemia plays in tissue injury after ICH.

Adult↗
PubMed2002

A preliminary study of magnetization transfer and diffusion tensor MRI of multiple sclerosis patients with fatigue.

To investigate whether multiple sclerosis (MS) tissue damage is associated with the presence and severity of fatigue, we obtained magnetization transfer (MT) and diffusion tensor (DT) magnetic resonance imaging from 28 patients with MS (14 with and 14 without fatigue). MT ratio and mean diffusivity did not differ between fatigued and non-fatigued MS patients. No correlation was found between Fatigue Severity Scale scores and any of the MT and DT MRI-derived quantities. This preliminary study suggests that the severity of overall MS pathology in the brain seems not to be a critical factor contributing to the development of fatigue in MS.

Adult↗
PubMed2004

Role of immediate postictal diffusion-weighted MRI in localizing epileptogenic foci of mesial temporal lobe epilepsy and non-lesional neocortical epilepsy.

OBJECTIVE: To determine whether meaningful changes in signal intensity or in the apparent diffusion coefficient of water (ADC) in the ictal onset zone can be detected through immediate postictal and interictal diffusion-weighted magnetic resonance imaging (DWMRI) in patients with localization-related epilepsy. METHOD: In randomly selected 10 medial and lateral temporal lobe epilepsy (TLE) and four extratemporal epilepsy patients, DWMRI was performed immediately after a seizure and during the interictal period. All 14 patients were non-lesional except for hippocampal sclerosis detected on MRI. The mean time interval from seizure onset to postictal DWMRI was 81 min. Regions of interest (ROI) were selected in both the cortex, which was believed to be the ictal onset zone, and the corresponding anatomical region of the contralateral hemisphere in the postictal and interictal DWMRI. The mean ADC measured from all ROIs was compared. Ictal onset zones were determined by ictal electroencephalography (EEG) and seizure semiology. RESULTS: On visual inspection of postictal and interictal DWMRI, signal changes in the ictal onset zone could be identified in only one patient with medial TLE. The mean ADC values from the ictal onset zones were not significantly different from those of the corresponding contralateral regions of the cortices in both postictal and interictal DWMRI. However, the postictal ADC values of the epileptogenic foci of neocortical epilepsy or neocortical temporal portion of TLE without hippocampal sclerosis were decreased compared with interictal ones in whom both interictal and postictal DWMRIs were obtained (P = 0.028). CONCLUSION: Our results demonstrate that water diffusion can change even after a single seizure in non-lesional neocortical epilepsy.

Adolescent↗
PubMedDate not supplied

Decreased signal intensity of the middle cerebellar peduncle on diffusion-weighted MRI in patients with large supratentorial lesions.

PURPOSE: The purpose of this work was to evaluate signal intensity change of the middle cerebellar peduncles on diffusion-weighted imaging in patients with large supratentorial lesions. METHODS: Signal intensity of the middle cerebellar peduncles was measured using a region of interest in 40 normal control subjects and in 28 patients with moyamoya disease on axial diffusion-weighted imaging with the motion-probing gradient perpendicular to the image. An additional six patients without moyamoya but with large supratentorial sequelae of hemorrhage or infarction were also included. RESULTS: There was no difference in signal intensity between both middle cerebellar peduncles in normal subjects, but lower signal intensity was consistently observed in the contralateral middle cerebellar peduncle in 12 moyamoya patients with large supratentorial lesions in the middle cerebral artery distribution. This finding was observed in the six non-moyamoya patients, too. CONCLUSION: Decreased signal intensity of the contralateral middle cerebellar peduncle might be a remote effect caused by a large supratentorial lesion in a crossed cerebellar fashion.

Adolescent↗
PubMedDate not supplied

High b-value diffusion-weighted MRI of normal brain.

PURPOSE: As MR scanner hardware has improved, allowing for increased gradient strengths, we are able to generate higher b values for diffusion-weighted (DW) imaging. Our purpose was to evaluate the appearance of the normal brain on DW MR images as the diffusion gradient strength ("b value") is increased from 1,000 to 3,000 s/mm2. METHOD: Three sets of echo planar images were acquired at 1.5 T in 25 normal subjects (mean age 61 years) using progressively increasing strengths of a diffusion-sensitizing gradient (corresponding to b values of 0, 1,000, and 3,000 s/mm2). All other imaging parameters remained constant. Qualitative assessments of trace images were performed by two neuroradiologists, supplemented by quantitative measures of MR signal and noise in eight different anatomic regions. RESULTS: As gradient strength increased from b = 1,000 to 3,000, both gray and white matter structures diminished in signal as expected based on their relative diffusion coefficients [calculated average apparent diffusion coefficient (ADC) values: gray matter = 8.5 x 10(-4) mm2/s, white matter = 7.5 x 10(-4) mm2/s]. The signal-to-noise ratios for the b = 1,000 images were approximately 2.2 times higher than for the b = 3,000 images (p < 0.0001). As the strength of the diffusion-sensitizing gradient increased, white matter became progressively hyperintense to gray matter. Relative to the thalamus, for example, the average MR signal intensity of white matter structures increased by an average of 27.5%, with the densely packed white matter tracts (e.g., middle cerebellar peduncle, tegmentum, and internal capsule) increasing the most. CONCLUSION: Brain DW images obtained at b = 3,000 appear significantly different from those obtained at b = 1,000, reflecting expected loss of signal from all areas of brain in proportion to their ADC values. Consequently, when all other imaging parameters are held constant, b = 3,000 DW images appear significantly noisier than b = 1,000 images, and white matter tracts are significantly more hyperintense than gray matter structures.

Adult↗
PubMed2000

[Diffusion-weighted MRI in patients with occlusive cerebrovascular disease: the initial DWI findings within 6 hours after onset].

We performed this study to evaluate the accuracy of diffusion-weighted imaging (DWI) in detecting focal ischemia, and to predict the role of DWI in the management of patients with ischemia in the superacute phase. 99 patients with clinically diagnosed acute occlusive cerebrovascular disease were studied with DWI within 6 hours after onset of symptoms. In 88 of 99 patients, early ischemic lesions were identified on initial DWI as hyperintensity areas. The initial DWI findings were classified into 4 types according to the location and extent of the hyperintensity area. The patients with type 1 (no hyperintensity area) were clinically diagnosed as TIA or complete stroke within 2 hours after the onset. 28 patients out of the patients with type 2 (hyperintensity area in the perforator's territory) were diagnosed with lacunar infarction, and the remaining 13 patients were diagnosed as victims of stroke caused by main trunk occlusion. Except for 2 patients with TIA, the patients with type 3 (scattered hyperintensity areas in the cortex) had main trunk occlusion and showed a more extended ischemic area on SPECT than hyperintensity area on DWI. All the patients with type 4 (extended hyperintensity area) had main trunk occlusion and showed severe hypoperfusion in the affected area on SPECT, and the area of hypoperfusion was well matched with the hyperintensity area on DWI. Comparing DWI findings with rCBF on SPECT, a significant difference was noted in rCBF between hyperintensity and non-hyperintensity area. We considered that emergence of hyperintensity on DWI was related to both the time of duration and the degree of hypoperfusion, and the reduced perfusion area where DWI showed no hyperintensity was thought to be the "ischemic penumbra". Our study indicated DWI had high diagnostic accuracy in superacute occlusive cerebrovascular disease and could furnish useful information to reveal the pathologic condition. In addition, DWI is expected to be available for selection as a therapeutic strategy.

Acute Disease↗
PubMed1999

Characterization of white matter damage in ischemic leukoaraiosis with diffusion tensor MRI.

BACKGROUND AND PURPOSE: Information on the neuropathological changes underlying ischemic leukoaraiosis is only available postmortem, and there are limited data on histological appearances early in the disease. Diffusion tensor imaging allows determination of the directionality of diffusion, which is greater in the direction of white matter bundles. Therefore, the technique might be expected to show loss of anisotropy (directional diffusion) in leukoaraiosis. METHODS: Nine patients with ischemic leukoaraiosis (radiological leukoaraiosis and clinical lacunar stroke) and 10 age-matched controls were studied. Diffusion tensor imaging was performed, and maps of diffusion trace and fractional anisotropy were constructed. Mean values of trace and fractional anisotropy were determined in standard regions of the anterior and posterior white matter in both hemispheres. RESULTS: In all patients with ischemic leukoaraiosis, a characteristic abnormal pattern was found, with loss of anisotropy and increased trace in the white matter. For example, in the right anterior white matter mean (SD) trace/3 was 1.12 (0.33) x10(-3) mm2 s-1 in patients and 0.75 (0.11) in controls (P=0.001). In the same region, fractional anisotropy was 0.53 (0.11) in patients and 0.78 (0.09) in controls (P<0.001). Within the white matter regions, there was a strong negative correlation between mean diffusivity and anisotropy (r=-0.92, P<0.0001). CONCLUSIONS: The characteristic pattern found on diffusion tensor imaging in this patient group is consistent with axonal loss and gliosis leading to impairment to and loss of directional diffusion. The "in vivo histological" information obtained may be useful in monitoring disease progression and in investigating the pathogenesis of the cognitive impairment that may be present.

Adult↗
PubMed2000

Combined (1)H MR spectroscopy and diffusion-weighted MRI improves the prediction of stroke outcome.

BACKGROUND: The prognostic value of the biochemical changes seen with proton MR spectroscopy (1H MRS) in ischemic stroke was examined. Acute diffusion-weighted imaging (DWI) was used to identify regions of ischemia for 1H MRS voxel localization. METHODS: Nineteen patients had 36 1H MRS studies, 13 patients acutely (mean, 11.1 hours), 10 subacutely (mean, 3.9 days), and 13 at outcome (mean, 82 days). Single-voxel, long-echo, timepoint-resolved spectroscopy was used to obtain lactate, n-acetylaspartate (NAA), choline, and creatine levels from the infarct core. Outcome measures were final infarct volume and clinical assessment scales (Canadian Neurological Scale, Barthel Index, and Rankin Scale). RESULTS: Acute lactate/choline ratio correlated more strongly with clinical outcome scores (r = 0.76 to 0.83; p < 0.01) and final infarct size (r = 0. 96; p < 0.01) than acute DWI lesion volume or acute NAA/choline ratio. Combination of acute lactate/choline ratio with acute DWI lesion volume improved prediction of all outcome scores (R2 = 0.80 to 0.90). The predictive effect of acute lactate/choline ratio was independent of acute DWI lesion volume (p < 0.001). In subacute and chronic infarction, both lactate/choline and NAA/choline ratios continued to correlate with outcome (p < 0.05). At the chronic stage, persistent lactate/choline ratio elevation strongly correlated with outcome measures (r = 0.71 to 0.87). CONCLUSION: Lactate/choline ratio measured in the acute infarct core by 1H MRS improves the prediction of stroke outcome and provides prognostic information complementary to DWI. Lactate/choline ratio could be used as an additional marker to select patients for acute and chronic therapies.

Aged↗