Navigating Supply Chain Disruptions of Iodinated Contrast Agent for Neuroimaging and How Business Intelligence Can Help the Decision Process.

A recent coronavirus disease 2019-related shutdown of the main production facility of iohexol in China has led to massive shortages of iodinated contrast material across the globe. This shortage has also jeopardized neuroimaging. In this article, we describe remedies to reduce iodinated contrast material use for stroke imaging, which is its primary use in neuroimaging, that we have implemented in our hospital network.

Time-to-Maximum of the Tissue Residue Function Improves Diagnostic Performance for Detecting Distal Vessel Occlusions on CT Angiography.

BACKGROUND AND PURPOSE: Detecting intracranial distal arterial occlusions on CTA is challenging but increasingly relevant to clinical decision-making. Our purpose was to determine whether the use of CTP-derived time-to-maximum of the tissue residue function maps improves diagnostic performance for detecting these occlusions. MATERIALS AND METHODS: Seventy consecutive patients with a distal arterial occlusion and 70 randomly selected controls who underwent multimodal CT with CTA and CTP for a suspected acute ischemic stroke were included in this retrospective study. Four readers with different levels of experience independently read the CTAs in 2 separate sessions, with and without time-to-maximum of the tissue residue function maps, recording the presence or absence of an occlusion, diagnostic confidence, and interpretation time. Accuracy for detecting distal occlusions was assessed using receiver operating characteristic analysis, and areas under curves were compared to assess whether accuracy improved with use of time-to-maximum of the tissue residue function. Changes in diagnostic confidence and interpretation time were assessed using the Wilcoxon signed rank test. RESULTS: Mean sensitivity for detecting occlusions on CTA increased from 70.7% to 90.4% with use of time-to-maximum of the tissue residue function maps. Diagnostic accuracy improved significantly for the 4 readers (P < .001), with areas under the receiver operating characteristic curves increasing by 0.186, 0.136, 0.114, and 0.121, respectively. Diagnostic confidence and speed also significantly increased. CONCLUSIONS: All assessed metrics of diagnostic performance for detecting distal arterial occlusions improved with the use of time-to-maximum of the tissue residue function maps, encouraging their use to aid in interpretation of CTA by both experienced and inexperienced readers. These findings show the added diagnostic value of including CTP in the acute stroke imaging protocol.

Arterial Spin-Labeling Improves Detection of Intracranial Dural Arteriovenous Fistulas with MRI.

BACKGROUND AND PURPOSE: Intracranial dural arteriovenous fistulas carry a risk of substantial neurologic complications but can be difficult to detect on structural MR imaging and TOF-MRA. The purpose of this study was to assess the accuracy and added value of 3D pseudocontinuous arterial spin-labeling MR imaging for the detection of these lesions. MATERIALS AND METHODS: This retrospective study included 39 patients with a dural arteriovenous fistula and 117 controls who had undergone both DSA and MR imaging with pseudocontinuous arterial spin-labeling. Two neuroradiologists blinded to the DSA results independently assessed MR imaging with and without pseudocontinuous arterial spin-labeling. They recorded specific signs, including venous arterial spin-labeling signal, and the likelihood of a dural arteriovenous fistula using a 5-point Likert scale. Logistic regression and receiver operating characteristic analyses were performed to determine the accuracy of specific signs and the added value of pseudocontinuous arterial spin-labeling. Interobserver agreement was determined by using κ statistics. RESULTS: Identification of the venous arterial spin-labeling signal had a high sensitivity (94%) and specificity (88%) for the presence a dural arteriovenous fistula. Receiver operating characteristic analysis showed significant improvement in diagnostic performance with the addition of pseudocontinuous arterial spin-labeling in comparison with structural MR imaging (Δarea under the receiver operating characteristic curve = 0.179) and a trend toward significant improvement in comparison with structural MR imaging with time-of-flight MRA (Δarea under the receiver operating characteristic curve = 0.043). Interobserver agreement for the presence of a dural arteriovenous fistula improved substantially and was almost perfect with the addition of pseudocontinuous arterial spin-labeling (κ = 0.92). CONCLUSIONS: Venous arterial spin-labeling signal has high sensitivity and specificity for the presence of a dural arteriovenous fistula, and the addition of pseudocontinuous arterial spin-labeling increases confidence in the diagnosis of this entity on MR imaging.

Performance of CT ASPECTS and Collateral Score in Risk Stratification: Can Target Perfusion Profiles Be Predicted without Perfusion Imaging?

BACKGROUND AND PURPOSE: Endovascular trials suggest that revascularization benefits a subset of acute ischemic stroke patients with large-artery occlusion and small-core infarct volumes. The objective of our study was to identify thresholds of noncontrast CT-ASPECTS and collateral scores on CT angiography that best predict ischemic core volume thresholds quantified by CT perfusion among patients with acute ischemic stroke. MATERIALS AND METHODS: Fifty-four patients with acute ischemic stroke (<12 hours) and MCA/intracranial ICA occlusion underwent NCCT/CTP during their initial evaluation. CTP analysis was performed on a user-independent platform (RApid processing of PerfusIon and Diffusion), computing core infarct (defined as CBF of <30% normal). A target mismatch profile consisting of infarction core of ≤50 mL was selected to define candidates with acute ischemic stroke likely to benefit from revascularization. RESULTS: NCCT-ASPECTS of ≥9 with a CTA collateral score of 3 had 100% specificity for identifying patients with a CBF core volume of ≤50 mL. NCCT-ASPECTS of ≤6 had 100% specificity for identifying patients with a CBF core volume of >50 mL. In our cohort, 44 (81%) patients had an NCCT-ASPECTS of ≥9, a CTA collateral score of 3, or an NCCT-ASPECTS of ≤6. CONCLUSIONS: Using an NCCT-ASPECTS of ≥9 or a CTA collateral score of 3 best predicts CBF core volume infarct of ≤50 mL, while an NCCT-ASPECTS of ≤6 best predicts a CBF core volume infarct of >50 mL. Together these thresholds suggest that a specific population of patients with acute ischemic stroke not meeting such profiles may benefit most from CTP imaging to determine candidacy for revascularization.

Performance and Predictive Value of a User-Independent Platform for CT Perfusion Analysis: Threshold-Derived Automated Systems Outperform Examiner-Driven Approaches in Outcome Prediction of Acute Ischemic Stroke.

BACKGROUND AND PURPOSE: Treatment strategies in acute ischemic stroke aim to curtail ischemic progression. Emerging paradigms propose patient subselection using imaging biomarkers derived from CT, CTA, and CT perfusion. We evaluated the performance of a fully-automated computational tool, hypothesizing enhancements compared with qualitative approaches. The correlation between imaging variables and clinical outcomes in a cohort of patients with acute ischemic stroke is reported. MATERIALS AND METHODS: Sixty-two patients with acute ischemic stroke and MCA or ICA occlusion undergoing multidetector CT, CTA, and CTP were retrospectively evaluated. CTP was processed on a fully operator-independent platform (RApid processing of PerfusIon and Diffusion [RAPID]) computing automated core estimates based on relative cerebral blood flow and relative cerebral blood volume and hypoperfused tissue volumes at varying thresholds of time-to-maximum. Qualitative analysis was assigned by 2 independent reviewers for each variable, including CT-ASPECTS, CBV-ASPECTS, CBF-ASPECTS, CTA collateral score, and CTA clot burden score. Performance as predictors of favorable clinical outcome and final infarct volume was established for each variable. RESULTS: Both RAPID core estimates, CT-ASPECTS, CBV-ASPECTS, and clot burden score correlated with favorable clinical outcome (P < .05); CBF-ASPECTS and collateral score were not significantly associated with favorable outcome, while hypoperfusion estimates were variably associated, depending on the selected time-to-maximum thresholds. Receiver operating characteristic analysis demonstrated disparities among tested variables, with RAPID core and hypoperfusion estimates outperforming all qualitative approaches (area under the curve, relative CBV = 0.86, relative CBF = 0.81; P < .001). CONCLUSIONS: Qualitative approaches to acute ischemic stroke imaging are subject to limitations due to their subjective nature and lack of physiologic information. These findings support the benefits of high-speed automated analysis, outperforming conventional methodologies while limiting delays in clinical management.

Diffusion-weighted imaging with dual-echo echo-planar imaging for better sensitivity to acute stroke.

BACKGROUND AND PURPOSE: Parallel imaging facilitates the acquisition of echo-planar images with a reduced TE, enabling the incorporation of an additional image at a later TE. Here we investigated the use of a parallel imaging-enhanced dual-echo EPI sequence to improve lesion conspicuity in diffusion-weighted imaging. MATERIALS AND METHODS: Parallel imaging-enhanced dual-echo DWI data were acquired in 50 consecutive patients suspected of stroke at 1.5T. The dual-echo acquisition included 2 EPI for 1 diffusion-preparation period (echo 1 [TE = 48 ms] and echo 2 [TE = 105 ms]). Three neuroradiologists independently reviewed the 2 echoes by using the routine DWI of our institution as a reference. Images were graded on lesion conspicuity, diagnostic confidence, and image quality. The apparent diffusion coefficient map from echo 1 was used to validate the presence of acute infarction. Relaxivity maps calculated from the 2 echoes were evaluated for potential complementary information. RESULTS: Echo 1 and 2 DWIs were rated as better than the reference DWI. While echo 1 had better image quality overall, echo 2 was unanimously favored over both echo 1 and the reference DWI for its high sensitivity in detecting acute infarcts. CONCLUSIONS: Parallel imaging-enhanced dual-echo diffusion-weighted EPI is a useful method for evaluating lesions with reduced diffusivity. The long TE of echo 2 produced DWIs that exhibited superior lesion conspicuity compared with images acquired at a shorter TE. Echo 1 provided higher SNR ADC maps for specificity to acute infarction. The relaxivity maps may serve to complement information regarding blood products and mineralization.

Improved T2* imaging without increase in scan time: SWI processing of 2D gradient echo.

BACKGROUND AND PURPOSE: 2D gradient-echo imaging is sensitive to T2* lesions (hemorrhages, mineralization, and vascular lesions), and susceptibility-weighted imaging is even more sensitive, but at the cost of additional scan time (SWI: 5-10 minutes; 2D gradient-echo: 2 minutes). The long acquisition time of SWI may pose challenges in motion-prone patients. We hypothesized that 2D SWI/phase unwrapped images processed from 2D gradient-echo imaging could improve T2* lesion detection. MATERIALS AND METHODS: 2D gradient-echo brain images of 50 consecutive pediatric patients (mean age, 8 years) acquired at 3T were retrospectively processed to generate 2D SWI/phase unwrapped images. The 2D gradient-echo and 2D SWI/phase unwrapped images were compared for various imaging parameters and were scored in a blinded fashion. RESULTS: Of 50 patients, 2D gradient-echo imaging detected T2* lesions in 29 patients and had normal findings in 21 patients. 2D SWI was more sensitive than standard 2D gradient-echo imaging in detecting T2* lesions (P < .0001). 2D SWI/phase unwrapped imaging also improved delineation of normal venous structures and nonpathologic calcifications and helped distinguish calcifications from hemorrhage. A few pitfalls of 2D SWI/phase unwrapped imaging were noted, including worsened motion and dental artifacts and challenges in detecting T2* lesions adjacent to calvaria or robust deoxygenated veins. CONCLUSIONS: 2D SWI and associated phase unwrapped images processed from standard 2D gradient-echo images were more sensitive in detecting T2* lesions and delineating normal venous structures and nonpathologic mineralization, and they also helped distinguish calcification at no additional scan time. SWI processing of 2D gradient-echo images may be a useful adjunct in cases in which longer scan times of 3D SWI are difficult to implement.

3D isotropic high-resolution diffusion-weighted MRI of the whole brain with a motion-corrected steady-state free precession sequence.

The main obstacle to high-resolution (<1.5 mm isotropic) 3D diffusion-weighted MRI is the differential motion-induced phase error from shot-to-shot. In this work, the phase error is addressed with a hybrid 3D navigator approach that corrects motion-induced phase in two ways. In the first, rigid-body motion is corrected for every shot. In the second, repeatable nonrigid-body pulsation is corrected for each portion of the cardiac cycle. These phase error corrections were implemented with a 3D diffusion-weighted steady- state free precession pulse sequence and were shown to mitigate signal dropouts caused by shot-to-shot phase inconsistencies compared to a standard gridding reconstruction in healthy volunteers. The proposed approach resulted in diffusion contrast more similar to the contrast observed in the reference echo-planer imaging scans than reconstruction of the same data without correction. Fractional anisotropy and Color fractional anisotropy maps generated with phase-corrected data were also shown to be more similar to echo-planer imaging reference scans than those generated without phase correction.

Clinical assessment of standard and generalized autocalibrating partially parallel acquisition diffusion imaging: effects of reduction factor and spatial resolution.

BACKGROUND AND PURPOSE: PI improves routine EPI-based DWI by enabling higher spatial resolution and reducing geometric distortion, though it remains unclear which of these is most important. We evaluated the relative contribution of these factors and assessed their ability to increase lesion conspicuity and diagnostic confidence by using a GRAPPA technique. MATERIALS AND METHODS: Four separate DWI scans were obtained at 1.5T in 48 patients with independent variation of in-plane spatial resolution (1.88 mm(2) versus 1.25 mm(2)) and/or reduction factor (R = 1 versus R = 3). A neuroradiologist with access to clinical history and additional imaging sequences provided a reference standard diagnosis for each case. Three blinded neuroradiologists assessed scans for abnormalities and also evaluated multiple imaging-quality metrics by using a 5-point ordinal scale. Logistic regression was used to determine the impact of each factor on subjective image quality and confidence. RESULTS: Reference standard diagnoses in the patient cohort were acute ischemic stroke (n = 30), ischemic stroke with hemorrhagic conversion (n = 4), intraparenchymal hemorrhage (n = 9), or no acute lesion (n = 5). While readers preferred both a higher reduction factor and a higher spatial resolution, the largest effect was due to an increased reduction factor (odds ratio, 47 ± 16). Small lesions were more confidently discriminated from artifacts on R = 3 images. The diagnosis changed in 5 of 48 scans, always toward the reference standard reading and exclusively for posterior fossa lesions. CONCLUSIONS: PI improves DWI primarily by reducing geometric distortion rather than by increasing spatial resolution. This outcome leads to a more accurate and confident diagnosis of small lesions.

Model for the correction of motion-induced phase errors in multishot diffusion-weighted-MRI of the head: are cardiac-motion-induced phase errors reproducible from beat-to-beat?

In diffusion-weighted imaging, multishot acquisitions are problematic due to intershot inconsistencies of the phase caused by motion during the diffusion-encoding gradients. A model for the motion-induced phase errors in diffusion-weighted-MRI of the brain is presented, in which rigid-body and nonrigid-body motion are separated. In the model, it is assumed that nonrigid-body motion is due to cardiac pulsation, and that the motion patterns are repeatable from beat-to-beat. To test the validity of this assumption, the repeatability of nonrigid-body motion-induced phase errors is quantified in three healthy volunteers. Nonrigid-body motion-induced phase was found to significantly correlate (P < 0.05) with pulse-oximeter waveforms in ~83% of the pixels tested across all slices and subjects.

Cerebral CT perfusion using an interventional C-arm imaging system: cerebral blood flow measurements.

BACKGROUND AND PURPOSE: CTP imaging in the interventional suite could reduce delays to the start of image-guided interventions and help determine the treatment progress and end point. However, C-arms rotate slower than clinical CT scanners, making CTP challenging. We developed a cerebral CTP protocol for C-arm CBCT and evaluated it in an animal study. MATERIALS AND METHODS: Five anesthetized swine were imaged by using C-arm CBCT and conventional CT. The C-arm rotates in 4.3 seconds plus a 1.25-second turnaround, compared with 0.5 seconds for clinical CT. Each C-arm scan had 6 continuous bidirectional sweeps. Multiple scans each with a different delay to the start of an aortic arch iodinated contrast injection and a novel image reconstruction algorithm were used to increase temporal resolution. Three different scan sets (consisting of 6, 3, or 2 scans) and 3 injection protocols (3-mL/s 100%, 3-mL/s 67%, and 6-mL/s 50% contrast concentration) were studied. CBF maps for each scan set and injection were generated. The concordance and Pearson correlation coefficients (ρ and r) were calculated to determine the injection providing the best match between the following: the left and right hemispheres, and CT and C-arm CBCT. RESULTS: The highest ρ and r values (both 0.92) for the left and right hemispheres were obtained by using the 6-mL 50% iodinated contrast concentration injection. The same injection gave the best match for CT and C-arm CBCT for the 6-scan set (ρ = 0.77, r = 0.89). Some of the 3-scan and 2-scan protocols provided matches similar to those in CT. CONCLUSIONS: This study demonstrated that C-arm CBCT can produce CBF maps that correlate well with those from CTP.

Clinical application of readout-segmented- echo-planar imaging for diffusion-weighted imaging in pediatric brain.

BACKGROUND AND PURPOSE: RS-EPI has been suggested as an alternative approach to EPI for high-resolution DWI with reduced distortions. To determine whether RS-EPI is a useful approach for routine clinical use, we implemented GRAPPA-accelerated RS-EPI DWI at our pediatric hospital and graded the images alongside standard accelerated (ASSET) EPI DWI used routinely for clinical studies. MATERIALS AND METHODS: GRAPPA-accelerated RS-EPI DWIs and ASSET EPI DWIs were acquired on 35 pediatric patients using a 3T system in 35 pediatric patients. The images were graded alongside each other by using a 7-point Likert scale as follows: 1, nondiagnostic; 2, poor; 3, acceptable; 4, standard; 5, above average; 6, good; and 7, outstanding. RESULTS: The following were the average scores for EPI and RS-EPI, respectively: resolution, 3.5/5.2; distortion level, 2.9/6.0; SNR, 3.4/4.1; lesion conspicuity, 3.3/5.9; and diagnostic confidence, 3.2/6.0. Overall, the RS-EPI had significantly improved diagnostic confidence and more reliably defined the extent and structure of several lesions. Although ASSET EPI scans had better SNR per scanning time, the higher spatial resolution as well as reduced blurring and distortions on RS-EPI scans helped to better reveal important anatomic details at the cortical-subcortical levels, brain stem, temporal and inferior frontal lobes, skull base, sinonasal cavity, cranial nerves, and orbits. CONCLUSIONS: This work shows the importance of both resolution and decreased distortions in the clinics, which can be accomplished by a combination of parallel imaging and alternative k-space trajectories such as RS-EPI.

Electrophysiological and diffusion tensor imaging evidence of delayed corollary discharges in patients with schizophrenia.

BACKGROUND: Patients with schizophrenia (SZ) characteristically exhibit supranormal levels of cortical activity to self-induced sensory stimuli, ostensibly because of abnormalities in the neural signals (corollary discharges, CDs) normatively involved in suppressing the sensory consequences of self-generated actions. The nature of these abnormalities is unknown. This study investigated whether SZ patients experience CDs that are abnormally delayed in their arrival at the sensory cortex. METHOD: Twenty-one patients with SZ and 25 matched control participants underwent electroencephalography (EEG). Participants' level of cortical suppression was calculated as the amplitude of the N1 component evoked by a button press-elicited auditory stimulus, subtracted from the N1 amplitude evoked by the same stimulus presented passively. In the three experimental conditions, the auditory stimulus was delivered 0, 50 or 100 ms subsequent to the button-press. Fifteen SZ patients and 17 healthy controls (HCs) also underwent diffusion tensor imaging (DTI), and the fractional anisotropy (FA) of participants' arcuate fasciculus was used to predict their level of cortical suppression in the three conditions. RESULTS: While the SZ patients exhibited subnormal N1 suppression to undelayed, self-generated auditory stimuli, these deficits were eliminated by imposing a 50-ms, but not a 100-ms, delay between the button-press and the evoked stimulus. Furthermore, the extent to which the 50-ms delay normalized a patient's level of N1 suppression was linearly related to the FA of their arcuate fasciculus. CONCLUSIONS: These data suggest that SZ patients experience temporally delayed CDs to self-generated auditory stimuli, putatively because of structural damage to the white-matter (WM) fasciculus connecting the sites of discharge initiation and destination.

Changes in parahippocampal white matter integrity in amnestic mild cognitive impairment: a diffusion tensor imaging study.

In the present study, changes in the parahippocampal white matter (PWM), in the region that includes the perforant path, were investigated, in vivo, in 14 individuals with amnestic mild cognitive impairment (aMCI) compared to 14 elderly controls with no cognitive impairment (NCI). For this purpose, (1) volumetry; (2) diffusion tensor imaging (DTI) derived measures of mean diffusivity (MD) and fractional anisotropy (FA); and (3) tractography were used. In addition, regression models were utilized to examine the association of PWM measurements with memory decline. The results from this study confirm previous findings in our laboratory and others, showing that compared to controls, individuals with aMCI have PWM volume loss. In addition to volume reduction, participants with aMCI demonstrated a significant increase in MD, but no difference in FA, both in the PWM region and in fibers modeled to pass through the PWM region. Further, the DTI metric of MD was associated with declarative memory performance, suggesting it may be a sensitive marker for memory dysfunction. These results indicate that there is general tissue loss and degradation (decreased volume; increased MD) in individuals with aMCI compared to older people with normal cognitive function. However, the microstructural organization of remaining fibers, as determined by measures of anisotropic diffusion, is not significantly different from that of controls.

Perfusion MRI (Tmax and MTT) correlation with xenon CT cerebral blood flow in stroke patients.

BACKGROUND: While stable xenon CT (Xe-CT) cerebral blood flow (CBF) is an accepted standard for quantitative assessment of cerebral hemodynamics, the accuracy of magnetic resonance perfusion-weighted imaging (PWI-MRI) is unclear. The Improved PWI Methodology in Acute Clinical Stroke Study compares PWI findings with Xe-CT CBF values in patients experiencing symptomatic severe cerebral hypoperfusion. METHODS: We compared mean transit time (MTT) and Tmax PWI-MRI with the corresponding Xe-CT CBF values in 25 coregistered regions of interest (ROIs) of multiple sizes and locations in nine subacute stroke patients. Comparisons were performed with Pearson correlation coefficients (R). We performed receiver operating characteristic (ROC) curve analyses to define the threshold of Tmax and absolute MTT that could best predict a Xe-CT CBF <20 mL/100 g/minute. RESULTS: The subjects' mean (SD) age was 50 (15) years, the median (interquartile range [IQR]) NIH Stroke Scale score was 2 (2-6), and the median (IQR) time between MRI and Xe-CT was 12 (-7-19) hours. The total number of ROIs was 225, and the median (IQR) ROI size was 550 (360-960) pixels. Tmax correlation with Xe-CT CBF (R = 0.63, p < 0.001) was stronger than absolute MTT (R = 0.55, p < 0.001), p = 0.049. ROC curve analysis found that Tmax >4 seconds had 68% sensitivity, 80% specificity, and 77% accuracy and MTT >10 seconds had 68% sensitivity, 77% specificity, and 75% accuracy for predicting ROIs with Xe-CT CBF <20 mL/100 g/minute. CONCLUSION: Our results suggest that in subacute ischemic stroke patients, Tmax correlates better than absolute mean transit time (MTT) with xenon CT cerebral blood flow (Xe-CT CBF) and that both Tmax >4 seconds and MTT >10 seconds are strongly associated with Xe-CT CBF <20 mL/100 g/minute. CBF = cerebral blood flow; DBP = diastolic blood pressure; DEFUSE = Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution; DWI = diffusion-weighted imaging; EPITHET = Echoplanar Imaging Thrombolytic Evaluation Trial; FOV = field of view; ICA = internal carotid artery; IQR = interquartile range; MCA = middle cerebral artery; MTT = mean transit time; NIHSS = NIH Stroke Scale; PWI = perfusion-weighted imaging; PWI-MRI = magnetic resonance perfusion-weighted imaging; ROC = receiver operating characteristic; ROI = region of interest; SBP = systolic blood pressure; SVD = singular value decomposition; Xe-CT = xenon CT.

Diffusion MR imaging in multiple sclerosis: technical aspects and challenges.

SUMMARY: Diffusion tensor (DT) MR imaging has frequently been applied in multiple sclerosis (MS) because of its ability to detect and quantify disease-related changes of the tissue microstructure within and outside T2-visible lesions. DT MR imaging data collection places high demands on scanner hardware and, though the acquisition and postprocessing can be relatively straightforward, numerous challenges remain in improving the reproducibility of this technique. Although there are some issues concerning image quality, echo-planar imaging is the most widely used acquisition scheme for diffusion imaging studies. Once the DT is estimated, indexes conveying the size, shape, and orientation of the DT can be calculated and further analyzed by using either histogram- or region-of-interest-based analyses. Because the orientation of the DT reflects the orientation of the axonal fibers of the brain, the pathways of the major white matter tracts can also be visualized. The DT model of diffusion, however, is not sufficient to characterize the diffusion properties of the brain when complex populations of fibers are present in a single voxel, and new ways to address this issue have been proposed. Two developments have enabled considerable improvements in the application of DT MR imaging: high magnetic field strengths and multicoil receiver arrays with parallel imaging. This review critically discusses models, acquisition, and postprocessing approaches that are currently available for DT MR imaging, as well as their limitations and possible improvements, to provide a better understanding of the strengths and weaknesses of this technique and a background for designing diffusion studies in MS.

Diffusion MRI in multiple sclerosis.

Diffusion imaging is a quantitative, MR-based technique potentially useful for the study of multiple sclerosis (MS), due to its increased pathologic specificity over conventional MRI and its ability to assess in vivo the presence of tissue damage occurring outside T2-visible lesions, i.e., in the so-called normal-appearing white and gray matter. The present review aims at critically summarizing the state-of-the-art and providing a background for the planning of future diffusion studies of MS. Several pieces of evidence suggest that diffusion-weighted and diffusion tensor MRI are sensitive to MS damage and able to detect its evolution over relatively short periods of time. Although a significant relationship between diffusion-weighted MRI findings and MS clinical disability was not found in the earliest studies, with improved diffusion imaging technology correlations between diffusion abnormalities and MS clinical aspects are now emerging. However, the best acquisition and postprocessing strategies for MS studies remain a matter of debate and the contribution of newer and more sophisticated techniques to diffusion tensor MRI investigations in MS needs to be further evaluated. Although changes in diffusion MRI indices reflect a net loss of structural organization, at present we can only speculate on their possible pathologic substrates in the MS brain. Postmortem studies correlating diffusion findings with histopathology of patients with MS are, therefore, also warranted.

Clinical importance of microbleeds in patients receiving IV thrombolysis.

BACKGROUND: Cerebral microbleeds (MBs) detected on gradient echo (GRE) imaging may be a risk factor for hemorrhagic complications in patients with stroke treated with IV tissue plasminogen activator (tPA). METHODS: The authors prospectively evaluated patients with acute ischemic stroke treated with IV tPA between 3 and 6 hours of symptom onset. MRI scans, including GRE imaging, were performed prior to tPA treatment, 3 to 6 hours after treatment and at day 30. The authors compared the frequency of hemorrhagic complications after thrombolysis in patients with and without MBs on their baseline GRE imaging. RESULTS: Seventy consecutive patients (mean age, 71 +/- 29 years; 31 men, 39 women) were included. MBs were identified in 11 patients (15.7%) on baseline GRE imaging. There was no significant difference in the frequency of either symptomatic or asymptomatic hemorrhagic complications after thrombolysis between patients with and without MBs at baseline. None of the 11 patients with MBs (0%) at baseline had a symptomatic intracerebral hemorrhage compared with 7 of 59 patients who did not have baseline MBs (11.9%). In addition, no patients with baseline MBs had asymptomatic hemorrhagic transformation observed at the site of any pre-treatment MB. CONCLUSIONS: The presence of cerebral microbleeds on gradient echo imaging does not appear to substantially increase the risk of either symptomatic or asymptomatic brain hemorrhage following IV tissue plasminogen activator administered between 3 and 6 hours after stroke onset.