Sara Brockstedt

Perfusion-related parameters in intravoxel incoherent motion MR imaging compared with CBV and CBF measured by dynamic susceptibility-contrast MR technique

Objective: Perfusion-related parameters obtained by intravoxel incoherent motion (IVIM) MR imaging (MRI) were compared with cerebral blood volume and flow (CBV and CBF), retrieved by dynamic susceptibility-contrast (DSC) MRI. Material and Methods: Twenty-eight volunteers (average age 68.5 years) were investigated. Spin-echo echo-planar imaging with IVIM-encoding gradients was employed (36 different b values, 0-1200 s/mm2). The perfusion fraction and the pseudo-diffusion coefficient were calculated for regions in thalamus gray matter and frontal white matter, using asymptotic and full fitting. In DSC-MRI, a Gd-DTPA-BMA contrast-agent bolus was monitored using simultaneous-dual FLASH. Deconvolution of the measured tissue concentration-versus-time curve with an arterial input function from the carotid artery was applied, and maps of CBV and CBF were calculated. Results: The correlation between the perfusion fraction and CBV was r=0.56 (p<0.0000006) using asymptotic fitting, and r=0.35 (p<0.0004) when full fitting was applied. Average CBF was 41.5 ml/(min 100 g), to be compared with the IVIM-based value of 63.6 ml/(min 100 g), obtained from the median value of the pseudo-diffusion coefficient in combination with assumptions about capillary network structure. Conclusion: The IVIM concept provided results that agreed reasonably with conventional CBV and CBF. The non-linear fitting to noisy signal data was problematic, in accordance with previously presented simulations.

Noninvasive mapping of water diffusional exchange in the human brain using filter-exchange imaging.

We present the first in vivo application of the filter‐exchange imaging protocol for diffusion MRI. The protocol allows noninvasive mapping of the rate of water exchange between microenvironments with different self‐diffusivities, such as the intracellular and extracellular spaces in tissue. Since diffusional water exchange across the cell membrane is a fundamental process in human physiology and pathophysiology, clinically feasible and noninvasive imaging of the water exchange rate would offer new means to diagnose disease and monitor treatment response in conditions such as cancer and edema. The in vivo use of filter‐exchange imaging was demonstrated by studying the brain of five healthy volunteers and one intracranial tumor (meningioma). Apparent exchange rates in white matter range from 0.8 ± 0.08 s−1 in the internal capsule, to 1.6 ± 0.11 s−1 for frontal white matter, indicating that low values are associated with high myelination. Solid tumor displayed values of up to 2.9 ± 0.8 s−1. In white matter, the apparent exchange rate values suggest intra‐axonal exchange times in the order of seconds, confirming the slow exchange assumption in the analysis of diffusion MRI data. We propose that filter‐exchange imaging could be used clinically to map the water exchange rate in pathologies. Filter‐exchange imaging may also be valuable for evaluating novel therapies targeting the function of aquaporins. Magn Reson Med, 2013.

MRI with diffusion tensor imaging post-mortem at 3.0 T in a patient with frontotemporal dementia.

The formalin-fixed brain of a patient with clinically diagnosed frontotemporal dementia (FTD) was examined post-mortem using magnetic resonance imaging (MRI) with diffusion tensor imaging (DTI) at 3.0 T. Frontotemporal atrophy as well as bilateral frontal white matter abnormalities were seen. The white matter changes were slightly more extensive on DTI than on conventional MRI. Correlation with histopathology of the corresponding regions revealed typical frontal lobe degeneration of non-Alzheimer type, with mild frontotemporal degeneration in the outer cortical layers and a moderate frontal white matter gliosis with demyelination. Post-mortem MRI/DTI with histopathologic correlation will enhance our understanding of the basis of white matter changes observed in dementia patients and may improve the in vivo MRI/DTI diagnostic assessment in FTD.

On the effects of a varied diffusion time in vivo: is the diffusion in white matter restricted?

The aim of this work was to study the diffusion-related signal attenuation curves (signal-vs.-b curves) measured perpendicular and parallel to the neuronal fibers of the corticospinal tract in vivo and to determine whether effects of restricted diffusion could be observed when varying the diffusion time (T(D)). A biexponential model and a two-compartment model including exchange according to the Kärger formalism were employed to analyze the signal-vs.-b curves. To validate the two-compartment model, restricted diffusion with exchange was simulated for uniformly sized cylinders, using different diameters and exchange times. The model was shown to retrieve the simulated parameters well, also when the short gradient pulse approximation was not met. The in vivo measurements performed perpendicular to the tracts, using b values up to 28000 s/mm(2) and T(D) values between 64 and 256 ms, did not show the effects of restricted diffusion as expected from previous ex vivo studies. The applied two-compartment model yielded an average axonal diameter of about 4 mum and an intracellular exchange time of about 300 ms, but did not fit statistically well to the data. In conclusion, this study indicates that if the diffusion is modeled as two compartments, of which one is restricted, exchange must be included in the model.

Magnetic resonance imaging artifacts caused by aneurysm clips and shunt valves: Dependence on field strength (1.5 and 3 T) and imaging parameters

To evaluate artifact sizes at 3 T compared to at 1.5 T, and to evaluate the influence of scanning parameters with respect to artifact size on a 3‐T magnetic resonance imaging (MRI) system.

Tumor extension in high-grade gliomas assessed with diffusion magnetic resonance imaging: values and lesion-to-brain ratios of apparent diffusion coefficient and fractional anisotropy.

Purpose: To determine whether the apparent diffusion coefficient (ADC) and fractional anisotropy (FA) can distinguish tumor-infiltrated edema in gliomas from pure edema in meningiomas and metastases. Material and Methods: Thirty patients were studied: 18 WHO grade III or IV gliomas, 7 meningiomas, and 5 metastatic lesions. ADC and FA were determined from ROIs placed in peritumoral areas with T2-signal changes, adjacent normal appearing white matter (NAWM), and corresponding areas in the contralateral healthy brain. Values and lesion-to-brain ratios from gliomas were compared to those from meningiomas and metastases. Results: Values and lesion-to-brain ratios of ADC and FA in peritumoral areas with T2-signal changes did not differ between gliomas, meningiomas, and metastases (P = 0.40, P = 0.40, P = 0.61, P = 0.34). Values of ADC and FA and the lesion-to-brain ratio of FA in the adjacent NAWM did not differ between tumor types (P = 0.74, P = 0.25, and P = 0.31). The lesion-to-brain ratio of ADC in the adjacent NAWM was higher in gliomas than in meningiomas and metastases (P = 0.004), but overlapped between tumor types. Conclusion: Values and lesion-to-brain ratios of ADC and FA in areas with T2-signal changes surrounding intracranial tumors and adjacent NAWM were not helpful for distinguishing pure edema from tumor-infiltrated edema when data from gliomas, meningiomas, and metastases were compared.

Denoising of complex MRI data by wavelet-domain filtering: Application to high-b-value diffusion-weighted imaging.

The Rician distribution of noise in magnitude magnetic resonance (MR) images is particularly problematic in low signal‐to‐noise ratio (SNR) regions. The Rician noise distribution causes a nonzero minimum signal in the image, which is often referred to as the rectified noise floor. True low signal is likely to be concealed in the noise, and quantification is severely hampered in low‐SNR regions. To address this problem we performed noise reduction (or denoising) by Wiener‐like filtering in the wavelet domain. The filtering was applied to complex MRI data before construction of the magnitude image. The noise‐reduction algorithm was applied to simulated and experimental diffusion‐weighted (DW) images. Denoising considerably reduced the signal standard deviation (SD, by up to 87% in simulated images) and decreased the background noise floor (by approximately a factor of 6 in simulated and experimental images). Magn Reson Med, 2006.

Diffusion tensor MRI post mortem demonstrated cerebral white matter pathology.

Sirs: Cerebral white matter changes are a challenge to investigators of dementia, because of their high prevalence and alleged clinical importance in the dementing disease process. Findings of ischemic and other white matter alterations on neuroimaging and in neuropathology are difficult to match adequately. The new MR technique, diffusion tensor imaging (DTI) has the potential to become a powerful tool for the detection of anisotropy-related alterations in tissues, including subtle and diffuse white matter changes in the aged [3] and the demented. A DTI study of ischemic leukoaraiosis in demented patients demonstrated abnormal diffusion pleomorphism, possibly reflecting true morphological tissue damage, i. e. tract disarrangement, in areas on conventional MRI of so-called normalappearing white matter at some distance from white matter lesions [4]. Furthermore, impaired frontal white matter integrity measured by DTI fractional anisotropy (FA) values was reported in individuals with Alzheimer’s disease [1]. These findings match those of other studies attesting to histopathological frontal white matter pathology in cases with Alzheimer’s disease [6]. Data such as these remain to be correlated with DTI, conventional MRI and neuropathological followup in the same group of individuals. The aim of this study was primarily to demonstrate the feasibility of post mortem DTI examination of human cerebral white matter and secondly to correlate the diffusion values obtained with the results from neuropathological assessments. The results were compared with the findings on conventional MRI. The brains from two individuals with dementia and neuroradiologically observed white matter disease during life were selected for post mortem DTI. The brains were fixed in 6 % formaldehyde for 6 weeks prior to the DTI investigation. They were rinsed and placed in a water container for the MR examination. Special care was taken to eliminate air-cavities and bubbles within the ventricles and on the surface of the brain. All scanning was performed at room temperature (approximately 23°C). Imaging was performed in a Siemens Magnetom Allegra 3.0 Tesla scanner. Conventional imaging protocols with T1-weighted and T2-weighted spin echo, T2-weighted fluid attenuation inversion recovery and three dimensional (3D) T1-weighted magnetization prepared rapid gradient echo sequences were used. Full brain coverage was obtained for the 2D sequences by scanning 22 slices with a slice thickness of 6 mm and was also obtained for the 3D sequence. A diffusion imaging protocol based on a segmented EPI pulse sequence with diffusion-encoding performed in six different directions, according to the dual gradient method (xz, -xz, yz, y-z, xy and -xy) [5], and two diffusion sensitivities (b-values 0 and 1000 s/mm2) were used. Slice positioning was similar to that of the conventional protocols, but with the long scan time required for DTI, a limited number of slices were positioned according to the areas of interest identified on the MR images. Five and seven slices were obtained for the two brains, respectively. The total scan time for one repetition of one slice was 1 minute and 20 seconds, using an echo time of 72 ms and a repetition time 1000 ms, respectively. Postprocessing of diffusion images was carried out using a home-developed diffusion-analyse program based on interactive data language (IDL, ©Research Systems, Inc) allowing pixel-wise calculation of the mean apparent diffusion coefficient (ADC) and the fractional anisotropy (FA) index [3]. Evaluation of the calculated ADC and FA maps were performed using regions of interest (ROIs) positioned in areas of suspected pathology and in areas of normal appearance. The brains were re-immersed in formaldehyde solution and were shortly thereafter cut in 1-cm thick whole brain coronal slices, which were paraffin-embedded, sectioned and stained for the detection of pathological alterations, in particular within the white matter, for which Luxol Fast Blue myelin staining with Cresyl violet counterstaining was used. A histopathological analysis of the entire brains was performed, with particular focus on type and degree of white matter changes. In 15 central regions of interest, the white matter was graded as microscopically normal, or as exhibiting mild, moderate or severe histopathology (Fig. 1), according to quantitative scales and qualitative alteration profiles regularly assessed in the department [2, 6]. DTI investigation of the two brains revealed in one case patchy and in the other diffuse-confluent white matter alterations, reflecting lacunar infarcts with perifocal gradient pathology vis-à-vis an adLETTER TO THE EDITORS

Clinical lacunar syndromes as predictors of lacunar infarcts. A comparison of acute clinical lacunar syndromes and findings on diffusion-weighted MRI

Objectives– To evaluate if patients with acute lacunar syndromes have acute lacunar infarcts or other types of cerebral lesions on diffusion‐weighted MRI. Methods– Patients with acute lacunar syndromes underwent echo‐planar diffusion MRI of the brain within 3 days after stroke onset. Localization and size of lesions with hyperintense signal were determined, compared with clinical characteristics and with findings on follow‐up T2‐weighted MRI. Results– Twenty‐three patients participated in the study. Thirteen patients had pure motor stroke, 1 pure sensory stroke, 8 sensorimotor stroke, and 1 ataxic hemiparesis. Twenty‐two patients had at least one lesion with increased signal on diffusion‐weighted MR images. These acute lesions were in the internal capsule/basal ganglia/thalamus in 13 patients, subcortical white matter in 5 patients, brainstem in 2 patients, cortex (multiple small lesions) in 1 patient, and cortex+basal ganglia in 1 patient. The median volume of the lesions was 0.6 ml on the initial examination and on follow‐up, of 17 patients after 1 to 5 months, 0.5 ml. Conclusions– Almost all patients with acute ischemic lacunar syndromes have acute lesions on echo‐planar diffusion‐weighted MRI within 3 days after stroke onset. These lesions are mostly small and subcortical, compatible with lacunar infarcts caused by single penetrating artery occlusion, but in a minor proportion of patients (2 of 23 in our study) a cortical involvement is found.

MRI thermometry in phantoms by use of the proton resonance frequency shift method: application to interstitial laser thermotherapy

In this work the temperature dependence of the proton resonance frequency was assessed in agarose gel with a high melting temperature (95 degrees C) and in porcine liver in vitro at temperatures relevant to thermotherapy (25-80 degrees C). Furthermore, an optically tissue-like agarose gel phantom was developed and evaluated for use in MRI. The phantom was used to visualize temperature distributions from a diffusing laser fibre by means of the proton resonance frequency shift method. An approximately linear relationship (0.0085 ppm degrees C(-1)) between proton resonance frequency shift and temperature change was found for agarose gel, whereas deviations from a linear relationship were observed for porcine liver. The optically tissue-like agarose gel allowed reliable MRI temperature monitoring, and the MR relaxation times (T1 and T2) and the optical properties were found to be independently alterable. Temperature distributions around a diffusing laser fibre, during irradiation and subsequent cooling, were assessed with high spatial resolution (voxel size = 4.3 mm3) and with random uncertainties ranging from 0.3 degrees C to 1.4 degrees C (1 SD) with a 40 s scan time.