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PubMed2005

Generalized scalar measures for diffusion MRI using trace, variance, and entropy.

This paper details the derivation of rotationally invariant scalar measures from higher-rank diffusion tensors (DTs) and functions defined on a unit sphere. This was accomplished with the use of an expression that generalizes the evaluation of the trace operator to tensors of arbitrary rank, and even to functions whose domains are the unit sphere. It is shown that the mean diffusivity is invariant to the selection of tensor rank for the model used. However, this rank invariance does not apply to the anisotropy measures. Therefore, a variance-based, general anisotropy measure is proposed. Also an information theoretical parametrization of anisotropy is introduced that is frequently more consistent with the meaning attributed to anisotropy. We accomplished this by associating anisotropy with the amount of orientational information present in the data, regardless of the imaging technique used. Using a simplified model of fibrous tissue, we simulated anisotropy values with varying orientational complexity and tensor models. Simulations suggested that a lower-rank tensor model may produce artificially low anisotropy values in voxels with complex structure. This was confirmed with a spin-echo experiment performed on an excised rat brain.

Animals↗
PubMed2001

Rapid three-dimensional diffusion MRI facilitates the study of acute stroke in mice.

MRI studies using mouse brain models of ischemia are becoming a valuable tool for understanding the mechanism of stroke, since transgenic models are now available. However, the small size of the mouse brain and the surgical complexity of creating ischemia in mice make it technically challenging to obtain high-quality MRI data. Therefore, there are few reports of MRI studies in murine cerebral ischemia. In this project a newly developed rapid 3D diffusion-weighted imaging (DWI) technique was applied to study experimental stroke in a mouse model of reversible middle cerebral artery occlusion (MCAO). Ischemic volumes were successfully delineated using this 3D whole-brain imaging technique with high spatial (0.34 x 0.5 x 1.0 mm(3) before zero-filling) and temporal (7 min) resolution. The 3D observation revealed the characteristic evolution of stroke after transient MCAO. There was a temporarily high diffusion constant in the cortex during early reperfusion, followed by a secondary energy failure in the cortex and caudate-putamen at 6 and 21 h of reperfusion. Magn Reson Med 46:183-188, 2001.

Animals↗
PubMedDate not supplied

Early response of prostate carcinoma xenografts to docetaxel chemotherapy monitored with diffusion MRI.

For many anticancer therapies, it would be desirable to accurately monitor and quantify tumor response early in the treatment regimen. This would allow oncologists to continue effective therapies or discontinue ineffective therapies early in the course of treatment, and hence, reduce morbidity. This is especially true for second-line therapies, which have reduced response rates and increased toxicities. Previous works by others and ourselves have shown that water mobility, measured by diffusion-weighted magnetic resonance imaging (DW-MRI), increases early in tumors destined to respond to therapies. In the current communication, we further characterize the utility of DW-MRI to predict response of prostate cancer xenografts to docetaxel in SCID mice in a preclinical setting. The current data illustrate that tumor volumes and secreted prostate-specific antigen both respond strongly to docetaxel in a dose-responsive manner, and the apparent diffusion coefficient of water (ADC(w)) increases significantly by 2 days even at the lowest doses (10 mg/kg). The ADCw data were parsed by histogram analyses. Our results indicate that DW-MRI can be used for early detection of prostate carcinoma xenograft response to docetaxel chemotherapy.

Animals↗
PubMed2003

Reduction of eddy-current-induced distortion in diffusion MRI using a twice-refocused spin echo.

Image distortion due to field gradient eddy currents can create image artifacts in diffusion-weighted MR images. These images, acquired by measuring the attenuation of NMR signal due to directionally dependent diffusion, have recently been shown to be useful in the diagnosis and assessment of acute stroke and in mapping of tissue structure. This work presents an improvement on the spin-echo (SE) diffusion sequence that displays less distortion and consequently improves image quality. Adding a second refocusing pulse provides better image quality with less distortion at no cost in scanning efficiency or effectiveness, and allows more flexible diffusion gradient timing. By adjusting the timing of the diffusion gradients, eddy currents with a single exponential decay constant can be nulled, and eddy currents with similar decay constants can be greatly reduced. This new sequence is demonstrated in phantom measurements and in diffusion anisotropy images of normal human brain.

Artifacts↗
PubMed2004

Diffusion MRI in the postmortem brain: case report.

Postmortem brain of a ten-month-old child was examined by MR imaging, and diffusion MR imaging at the 12th hour after death in order to disclose the cause of death. There were basal ganglion lesions indicating a mitochondrial disorder. There was a prominent difference between the ADC values of the white matter (0.28+/-0.04 x 10(-3) mm2/s) and cortex (0.42+/-0.04 x 10(-3) mm2/s), and this was statistically significant (p< 0.0001). This difference suggested that in the postmortem brain the conditions in the white matter leading to restriction of movement of water molecules are more severe than that in the cortex.

Acidosis, Lactic↗
PubMed2003

Rapid hyperpolarized 3He diffusion MRI of healthy and emphysematous human lungs using an optimized interleaved-spiral pulse sequence.

PURPOSE: To develop and validate an interleaved-spiral diffusion pulse sequence capable of hyperpolarized (3)He MR imaging of the whole lung in less than 10 seconds. MATERIALS AND METHODS: Hyperpolarized (3)He diffusion measurements were performed in seven healthy volunteers and five patients with emphysema using an interleaved-spiral pulse sequence that provided 11 contiguous 15-mm thick coronal ADC maps, with an in-plane resolution of 3.9 mm, covering the whole lung in 5.5 seconds. The resulting means and SDs of ADC values were compared statistically to those from a gradient-echo pulse sequence with identical resolution and diffusion-weighting gradients that acquired five ADC maps in 10.5 seconds. RESULTS: High-quality diffusion-weighted interleaved-spiral images covering the whole lung were obtained, and showed no significant susceptibility-induced image degradation compared to corresponding gradient-echo images. On a subject-by-subject basis, the means and SDs of ADC values for the interleaved-spiral technique were not statistically different from those for the gradient-echo technique. The mean ADC values from the two techniques were highly correlated on a section-by-section basis (R = 0.99). CONCLUSION: The interleaved-spiral diffusion pulse sequence permits rapid acquisition of contiguous ADC maps covering the whole lung during a short breath-hold period, and provides ADC values that are statistically equivalent to those from standard gradient-echo techniques.

Adult↗
PubMed2006

Distribution of microstructural damage in the brains of professional boxers: a diffusion MRI study.

PURPOSE: To investigate and localize cerebral abnormalities in professional boxers with no history of moderate or severe head trauma. MATERIALS AND METHODS: Diffusion tensor imaging (DTI) was used to determine the apparent diffusion coefficient (ADC) and fractional anisotropy (FA) in the brains of 81 professional male boxers and 12 male control subjects. Voxel-based analysis (VBA) of both the diffusion and anisotropy values was performed using statistical parametric mapping (SPM). From this objective analysis, regions of microstructural abnormalities in the brains of the boxers were located. RESULTS: Increases in the ADC, and decreases in FA were identified in deep white matter (WM), while decreases in ADC were identified in cortical gray matter (GM). Regions of positive correlation between ADC and age were also found in both the boxer and control groups, although the regions and strength of the correlation were not the same in each group. CONCLUSION: Using VBA, we localized previously unreported abnormalities in the brains of professional boxers. These abnormalities are assumed to reflect cumulative (chronic) brain injury resulting from nonsevere head trauma.

Adult↗
PubMed2004

Reversible diffusion MRI abnormalities and transient mutism after liver transplantation.

Transient mutism was observed in a liver transplant patient under immunosuppressant therapy with cyclosporine A and antifungal prophylaxis with amphotericin B. Fluid-attenuated inversion recovery and diffusion-weighted images revealed reversible bilateral symmetric hyperintensity located in the frontal motor cortex and corticospinal tracts. These MRI abnormalities may be caused by acute edema, possibly a combination of cytotoxic and vasogenic edema, which resolved with a prompt change in therapy.

Diffusion Magnetic Resonance Imaging↗
PubMed2002

Simultaneous diffusion MRI measurements from multiple perfused rat hippocampal slices.

Rat brain slices provide a controllable tissue model in which to investigate the biophysical basis of diffusion-weighted magnetic resonance (MR) signal changes observed clinically in nervous tissue after ischemic injury. This study describes a new multislice perfusion chamber that allows for the simultaneous acquisition of diffusion-weighted MR images from multiple perfused rat hippocampal slices (eight slices in the present study). These images had a signal-to-noise ratio (SNR) of 48 +/- 3 at b = 8080 s/mm(2), which was sufficient to analyze the multicomponent diffusion properties of water in rat hippocampal slices. The tissue water diffusion parameters (f(fast) = 0.527 +/- 0.041, D(fast) = 1.268 +/- 0.087 x 10(-3) mm(2)/s, and D(slow) = 0.060 +/- 0.003 x 10(-3) mm(2)/s) were stable for at least 8 hr after slice procurement (ANOVA, P > 0.05), suggesting that it may be possible to study the acute temporal evolution of diffusion changes in multiple brain slices following experimental perturbation.

Analysis of Variance↗
PubMed2002

Diffusion MRI for prediction of response of rectal cancer to chemoradiation.

Prediction of tumour response before onset of treatment could have considerable clinical benefit. Since the apparent diffusion coefficient (ADC) of a tumour's water content can show the extent of necrosis, we looked for a possible correlation of ADC with response to treatment. We measured mean tumour water ADC before and after chemotherapy and chemoradiation in 14 patients with locally advanced rectal cancer, with a quantitative magnetic resonance diffusion imaging sequence. We found a strong negative correlation between mean pretreatment tumour water ADC and percentage size change of tumours after chemotherapy (r=-0.67, p=0.01) and chemoradiation (r=-0.83, p=0.001). Persistence of low ADC in responders after chemotherapy could represent loss of a non-viable fraction of the treated tumour.

Adenocarcinoma↗
PubMed2005

Visual recovery after perinatal stroke evidenced by functional and diffusion MRI: case report.

BACKGROUND: After perinatal brain injury, clinico-anatomic correlations of functional deficits and brain plasticity remain difficult to evaluate clinically in the young infant. Thus, new non-invasive methods capable of early functional diagnosis are needed in young infants. CASE PRESENTATION: The visual system recovery in an infant with perinatal stroke is assessed by combining diffusion tensor imaging (DTI) and event-related functional MRI (ER-fMRI). All experiments were done at 1.5T. A first DTI experiment was performed at 12 months of age. At 20 months of age, a second DTI experiment was performed and combined with an ER-fMRI experiment with visual stimuli (2 Hz visual flash). At 20 months of age, ER-fMRI showed significant negative activation in the visual cortex of the injured left hemisphere that was not previously observed in the same infant. DTI maps suggest recovery of the optic radiation in the vicinity of the lesion. Optic radiations in the injured hemisphere are more prominent in DTI at 20 months of age than in DTI at 12 months of age. CONCLUSION: Our data indicate that functional cortical recovery is supported by structural modifications that concern major pathways of the visual system. These neuroimaging findings might contribute to elaborate a pertinent strategy in terms of diagnosis and rehabilitation.

Asphyxia Neonatorum↗
PubMed1995

High resolution quantitative relaxation and diffusion MRI of three different experimental brain tumors in rat.

The potential of quantitative parameter images of the relaxation times T1 and T2, the proton density rho and the apparent diffusion coefficient (ADC) to characterize three different experimental rat brain tumors (F98 glioma, RN6 Schwannoma, and E376 neuroblastoma) was studied. All parameter values, as determined in histologically confirmed regions of interest (ROI), were higher in edema than in tumor, which in turn were elevated with respect to normal brain. ROI values of ADC and T2 delivered statistically significant (P < 0.01) differentiation between tumor and edema. Multidimensional parameter combinations improved differentiation between different tissues. However, the three tumor types could not be differentiated. All parameter maps allowed the identification of the whole tumor-edema area. On T2 images, edema could be identified best, whereas the tumor itself was hardly visualized. In many cases, tumor presentation using T1 maps corresponded best with histology, nevertheless suffering from a poor tumor-edema differentiation.

Animals↗
PubMed2005

Continuous wave MRI diffusion study of water in bentonite clay.

Three-layered clay minerals such as montmorillonite (bentonite) exhibit very short transverse relaxation times. This is especially true for samples with relatively low water contents in the region of 20% to 30%, which is the water content typically used in environmental technology applications (e.g., as a mineral liner material for landfills). The diffusion of water in samples with such short transverse relaxation times can be measured with NMR by observing the moisture gradients or isotope tracer fronts propagating through appropriately prepared samples by means of continuous wave MRI. The first results from such studies on bentonite clays are presented in this paper.

Aluminum Silicates↗
PubMed2005

Interictal diffusion MRI in partial epilepsies explored with intracerebral electrodes.

Patients with refractory partial seizures may benefit from epilepsy surgery. However, invasive investigations are often needed to define the precise location and limits of the epileptogenic zone (EZ). In this study, we asked whether diffusion tensor imaging (DTI) might provide a non-invasive alternative to locate the EZ or at least provide insights to help place intracerebral electrodes for stereo-electroencephalography (SEEG). Whole brain DTI and voxel-based analysis (SPM99) was used to assess diffusion properties objectively in 16 epilepsy patients investigated with SEEG. Epilepsy was symptomatic in two patients and cryptogenic in the 14 remaining patients. The suspected onset of seizures was temporal in 10 patients, frontal in 2 and occipital in 4. Individual maps of apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were calculated and compared to a database of 40 healthy volunteers. Thirteen of 16 patients exhibited diffusion abnormalities. ADC abnormalities were better correlated with SEEG data than FA abnormalities which were usually located at a distance or in the white matter. A significant increase in ADC (P < 0.01) was found in 11 patients and was located in the regions explored with depth electrodes in 7 of them. Surgery outcome was available in 3 of these 7 patients (2 were seizure free and 1 not). DTI specificity was better in extratemporal lobe epilepsy (83%) than in temporal lobe epilepsy (20%). When abnormalities concurred with the SEEG data, the concordance was optimal between the localization of the diffusion abnormalities and the irritative zone defined by SEEG. These encouraging, preliminary results, suggest that DTI examinations may provide accurate spatial data on the location and extent of the epileptogenic network in extratemporal lobe epilepsies.

Adolescent↗