T2* mapping and B0 orientation-dependence at 7 T reveal cyto- and myeloarchitecture organization of the human cortex.

Ultra-high field MRI (≥ 7 T) has recently shown great sensitivity to depict patterns of tissue microarchitecture. Moreover, recent studies have demonstrated a dependency between T2* and orientation of white matter fibers with respect to the main magnetic field B0. In this study we probed the potential of T2* mapping at 7 T to provide new markers of cortical architecture. We acquired multi-echo measurements at 7 T and mapped T2* over the entire cortex of eight healthy individuals using surface-based analysis. B0 dependence was tested by computing the angle θ(z) between the normal of the surface and the direction of B0, then fitting T2*(θ(z)) using model from the literature. Average T2* in the cortex was 32.20 +/- 1.35 ms. Patterns of lower T2* were detected in the sensorimotor, visual and auditory cortices, likely reflecting higher myelin content. Significantly lower T2* was detected in the left hemisphere of the auditory region (p<0.005), suggesting higher myelin content, in accordance with previous investigations. B0 orientation dependence was detected in some areas of the cortex, the strongest being in the primary motor cortex (∆R2*=4.10 Hz). This study demonstrates that quantitative T2* measures at 7 T MRI can reveal patterns of cytoarchitectural organization of the human cortex in vivo and that B0 orientation dependence can probe the coherency and orientation of gray matter fibers in the cortex, shedding light into the potential use of this type of contrast to characterize cyto-/myeloarchitecture and to understand the pathophysiology of diseases associated with changes in iron and/or myelin concentration.

In vivo evidence of disseminated subpial T2* signal changes in multiple sclerosis at 7 T: a surface-based analysis.

Cortical subpial demyelination is frequent in multiple sclerosis (MS) and is closely associated with disease progression and poor neurological outcome. Although cortical lesions have been difficult to detect using conventional MRI, preliminary data using T2*-weighted imaging at ultra-high field 7T MRI showed improved sensitivity for detecting and categorizing different histological types of cortical MS lesions. In this study we combined high-resolution 7T MRI with a surface-based analysis technique to quantify and map subpial T2*-weighted signal changes in seventeen patients with MS. We applied a robust method to register 7T data with the reconstructed cortical surface of each individual and used a general linear model to assess in vivo an increase in subpial T2*-weighted signal in patients versus age-matched controls, and to investigate the spatial distribution of cortical subpial changes across the cortical ribbon. We also assessed the relationship between subpial T2* signal changes at 7T, Expanded Disability Status Scale (EDSS) score and white matter lesion load (WMLL). Patients with MS showed significant T2*-weighted signal increase in the frontal lobes (parsopercularis, precentral gyrus, middle and superior frontal gyrus, orbitofrontal cortex), anterior cingulate, temporal (superior, middle and inferior temporal gyri), and parietal cortices (superior and inferior parietal cortex, precuneus), but also in occipital regions of the left hemisphere. We found significant correlations between subpial T2*-weighted signal and EDSS score in the precentral gyrus (ρ=0.56, P=0.02) and between T2*-weighted signal and WMLL in the lateral orbitofrontal, superior parietal, cuneus, precentral and superior frontal regions. Our data support the presence of disseminated subpial increases in T2* signal in subjects with MS, which may reflect the diffuse subpial pathology described in neuropathology.

The Causative Classification of Stroke system: an international reliability and optimization study.

BACKGROUND: Valid and reliable ischemic stroke subtype determination is crucial for well-powered multicenter studies. The Causative Classification of Stroke System (CCS, available at http://ccs.mgh.harvard.edu) is a computerized, evidence-based algorithm that provides both causative and phenotypic stroke subtypes in a rule-based manner. We determined whether CCS demonstrates high interrater reliability in order to be useful for international multicenter studies. METHODS: Twenty members of the International Stroke Genetics Consortium from 13 centers in 8 countries, who were not involved in the design and development of the CCS, independently assessed the same 50 consecutive patients with acute ischemic stroke through reviews of abstracted case summaries. Agreement among ratings was measured by kappa statistic. RESULTS: The κ value for causative classification was 0.80 (95% confidence interval [CI] 0.78-0.81) for the 5-subtype, 0.79 (95% CI 0.77-0.80) for the 8-subtype, and 0.70 (95% CI 0.69-0.71) for the 16-subtype CCS. Correction of a software-related factor that generated ambiguity improved agreement: κ = 0.81 (95% CI 0.79-0.82) for the 5-subtype, 0.79 (95% CI 0.77-0.80) for the 8-subtype, and 0.79 (95% CI 0.78-0.80) for the 16-subtype CCS. The κ value for phenotypic classification was 0.79 (95% CI 0.77-0.82) for supra-aortic large artery atherosclerosis, 0.95 (95% CI 0.93-0.98) for cardioembolism, 0.88 (95% CI 0.85-0.91) for small artery occlusion, and 0.79 (0.76-0.82) for other uncommon causes. CONCLUSIONS: CCS allows classification of stroke subtypes by multiple investigators with high reliability, supporting its potential for improving stroke classification in multicenter studies and ensuring accurate means of communication among different researchers, institutions, and eras.

A score to predict early risk of recurrence after ischemic stroke.

BACKGROUND: There is currently no instrument to stratify patients presenting with ischemic stroke according to early risk of recurrent stroke. We sought to develop a comprehensive prognostic score to predict 90-day risk of recurrent stroke. METHODS: We analyzed data on 1,458 consecutive ischemic stroke patients using a Cox regression model with time to recurrent stroke as the response and clinical and imaging features typically available to physician at admission as covariates. The 90-day risk of recurrent stroke was calculated by summing up the number of independent predictors weighted by their corresponding beta-coefficients. The resultant score was called recurrence risk estimator at 90 days or RRE-90 score (available at: http://www.nmr.mgh.harvard.edu/RRE-90/). RESULTS: Sixty recurrent strokes (54 had baseline imaging) occurred during the follow-up period. The risk adjusted for time to follow-up was 6.0%. Predictors of recurrence included admission etiologic stroke subtype, prior history of TIA/stroke, and topography, age, and distribution of brain infarcts. The RRE-90 score demonstrated adequate calibration and good discrimination (area under the ROC curve [AUC] = 0.70-0.80), which was maintained when applied to a separate cohort of 433 patients (AUC = 0.70-0.76). The model's performance was also maintained for predicting early (14-day) risk of recurrence (AUC = 0.80). CONCLUSIONS: The RRE-90 is a Web-based, easy-to-use prognostic score that integrates clinical and imaging information available in the acute setting to quantify early risk of recurrent stroke. The RRE-90 demonstrates good predictive performance, suggesting that, if validated externally, it has promise for use in creating individualized patient management algorithms and improving clinical practice in acute stroke care.

In vivo imaging of cortical pathology in multiple sclerosis using ultra-high field MRI.

OBJECTIVE: We used ultra-high field MRI to visualize cortical lesion types described by neuropathology in 16 patients with multiple sclerosis (MS) compared with 8 age-matched controls; to characterize the contrast properties of cortical lesions including T2*, T2, T1, and phase images; and to investigate the relationship between cortical lesion types and clinical data. METHODS: We collected, on a 7-T scanner, 2-dimensional fast low-angle shot (FLASH)-T2*-weighted spoiled gradient-echo, T2-weighted turbo spin-echo (TSE) images (0.33 x 033 x 1 mm(3)), and a 3-dimensional magnetization-prepared rapid gradient echo. RESULTS: Overall, 199 cortical lesions were detected in patients on both FLASH-T2* and T2-TSE scans. Seven-tesla MRI allowed for characterization of cortical plaques into type I (leukocortical), type II (intracortical), and type III/IV (subpial extending partly or completely through the cortical width) lesions as described histopathologically. Types III and IV were the most frequent type of cortical plaques (50.2%), followed by type I (36.2%) and type II (13.6%) lesions. Each lesion type was more frequent in secondary progressive than in relapsing-remitting MS. This difference, however, was significant only for type III/IV lesions. T2*-weighted images showed the highest, while phase images showed the lowest, contrast-to-noise ratio for all cortical lesion types. In patients, the number of type III/IV lesions was associated with greater disability (p < 0.02 by Spearman test) and older age (p < 0.04 by Spearman test). CONCLUSIONS: Seven-tesla MRI detected different histologic cortical lesion types in our small multiple sclerosis (MS) sample, suggesting, if validated in a larger population, that it may prove a valuable tool to assess the contribution of cortical MS pathology to clinical disability.

Diffusion spectrum magnetic resonance imaging (DSI) tractography of crossing fibers.

MRI tractography is the mapping of neural fiber pathways based on diffusion MRI of tissue diffusion anisotropy. Tractography based on diffusion tensor imaging (DTI) cannot directly image multiple fiber orientations within a single voxel. To address this limitation, diffusion spectrum MRI (DSI) and related methods were developed to image complex distributions of intravoxel fiber orientation. Here we demonstrate that tractography based on DSI has the capacity to image crossing fibers in neural tissue. DSI was performed in formalin-fixed brains of adult macaque and in the brains of healthy human subjects. Fiber tract solutions were constructed by a streamline procedure, following directions of maximum diffusion at every point, and analyzed in an interactive visualization environment (TrackVis). We report that DSI tractography accurately shows the known anatomic fiber crossings in optic chiasm, centrum semiovale, and brainstem; fiber intersections in gray matter, including cerebellar folia and the caudate nucleus; and radial fiber architecture in cerebral cortex. In contrast, none of these examples of fiber crossing and complex structure was identified by DTI analysis of the same data sets. These findings indicate that DSI tractography is able to image crossing fibers in neural tissue, an essential step toward non-invasive imaging of connectional neuroanatomy.

Neuroanatomic correlates of stroke-related myocardial injury.

BACKGROUND: Myocardial injury can occur after ischemic stroke in the absence of primary cardiac causes. The neuroanatomic basis of stroke-related myocardial injury is not well understood. OBJECTIVE: To identify regions of brain infarction associated with myocardial injury using a method free of the bias of an a priori hypothesis as to any specific location. METHODS: Of 738 consecutive patients with acute ischemic stroke, the authors identified 50 patients in whom serum cardiac troponin T (cTnT) elevation occurred in the absence of any apparent cause within 3 days of symptom onset. Fifty randomly selected, age- and sex-matched patients with ischemic stroke without cTnT elevation served as controls. Diffusion-weighted images with outlines of infarction were co-registered to a template, averaged, and then subtracted to find voxels that differed between the two groups. Voxel-wise p values were determined using a nonparametric permutation test to identify specific regions of infarction that were associated with cTnT elevation. RESULTS: The study groups were well balanced with respect to stroke risk factors, history of coronary artery disease, infarction volume, and frequency of right and left middle cerebral artery territory involvement. Brain regions that were a priori associated with cTnT elevation included the right posterior, superior, and medial insula and the right inferior parietal lobule. Among patients with right middle cerebral artery infarction, the insular cluster was involved in 88% of patients with and 33% without cTnT elevation (odds ratio: 15.00; 95% CI: 2.65 to 84.79). CONCLUSIONS: Infarctions in specific brain regions including the right insula are associated with elevated serum cardiac troponin T level indicative of myocardial injury.

Telemedicine in trauma and disasters--from war to earthquake: are we ready?

Every year many disasters cause thousands of injuries, deaths, refugees. Earthquakes and war often cause severe injuries (burns; amputations; Crush-Syndrome; gunshots; landmines; nuclear, biological or chemical warfare / hazardous material; infectious diseases; pediatric specialties). Referring to big earthquakes in the last few years up to 20.000 thousand people were killed (India 2001). 310.000 deaths were caused by war in 2001. The Mass Casualty Incident is characterized by the disbalance between victims and the normal community emergency response. Because of this a lot of different institutions and organizations are involved in coping with the disaster. This produces an extensive demand of qualified Command, Control and Communication (C3). Furthermore a lot of data has to be collected during the treatment and the injuries need special medical treatment. The use of health telematics in disaster response helps to cope with the scenario. Modern technologies provide support for building up medical aid although the normal infrastructure is destroyed. To cope with disaster scenarios there are some telematic tools which can be used: computer based Command and Control System, telemedical support, and data-resources-network/Medical Intelligence. The International Center for Telemedicine at the University of Regensburg Medical Center provides support for Health Care Professionals as a competence center for telemedicine. For the eastern part of Bavaria it develops a telemedical network with many components: The mobile emergency care system NOAH (Notfall-Organisations- und Arbeits-Hilfe) supports the Emergency Medical Service. Local Health Networks and the Clinical Network of Eastern Bavaria connect physicians and hospitals with the Regensburg Medical Center. With an online-education tool participants from all over the country can take part in trainings and courses.

Perfusion-weighted magnetic resonance imaging in patients with carotid artery disease before and after carotid endarterectomy.

OBJECTIVE: The purpose of this study was to investigate the potential of perfusion-weighted magnetic resonance imaging for preoperative and postoperative evaluation of cerebral hemodynamics in patients undergoing carotid endarterectomy for carotid artery stenosis. METHODS: We examined 26 patients with angiographically proven stenoses (60%-99%) of the internal carotid artery preoperatively. Perfusion imaging studies were performed by bolus-tracking of a dosage of 0.2 mmol/kg body weight of gadolinium diethylenetriaminepentaacetic acid on a 1.5-T scanner using a T2*-weighted fast low-angle shot sequence. The observed signal intensities were converted pixel by pixel into concentration-time curves. In each patient, the hemispheres were compared and the difference between the normalized first moments (NFMs) and the percentage changes of the regional cerebral blood volume (CBV) were calculated. Three months postoperatively, perfusion-weighted magnetic resonance imaging was performed in 13 patients. RESULTS: In patients with <80% stenosis (n = 10), there was no significant alteration of NFM and regional CBV compared with the contralateral hemisphere (-0.16 +/- 0.7 s, +5.9 +/- 24.6%). In patients with stenoses >or=80% (n = 16), we found an increase in NFM ipsilateral to the stenosis of 1.2 +/- 0.92 s (P < .001) and an increase of CBV of 16.8 +/- 15.2% (P < .005). Three months postoperatively, perfusion parameters were normal in all 13 patients examined. CONCLUSIONS: Perfusion-weighted magnetic resonance imaging is well suited to evaluate the preoperative and postoperative hemodynamic changes in patients with carotid artery stenosis. This noninvasive, semiquantitative magnetic resonance technique could prove to be a valuable adjunct in identification of patients who might benefit from carotid endarterectomy.

Does the "keyhole" technique improve spatial resolution in MRI perfusion measurements? A study in volunteers.

We examined the potential of the 'keyhole' technique to improve spatial resolution in perfusion-weighted MRI on whole-body imagers with standard gradient hardware. We examined 15 healthy volunteers. We acquired a high-resolution image with 256 phase-encoding steps before a bolus-tracking procedure. For the dynamic series we collected only 34 lines in the center of k-space. Data reconstruction was performed by both zero-filling and keyhole methods. The dynamic datasets, concentration-time curves calculated from user-defined regions and maps of the cerebrovascular parameters using both reconstruction methods were compared. Using keyhole series, anatomical structures could easily be defined which were not seen on the original dynamic series because of blurring due to ringing artefacts. Comparison of signal-time curves in large regions yielded no significant difference in signal loss during bolus passage. In the parameter maps truncation artefacts were significantly reduced using keyhole reconstruction. The keyhole method is appropriate for enhancing image quality in perfusion-weighted imaging on standard imagers without sacrificing time resolution or information about transitory susceptibility changes. However, it should be applied carefully, because the spatial resolution of the dynamic signal change and the cerebrovascular parameters is less than that afforded by the spatial resolution of the reconstructed images.

The HIV/AIDS Prevention Program Archive (HAPPA): a collection of promising prevention programs in a box.

This article describes the methods used and findings obtained in establishing the HIV/AIDS Prevention Program Archive (HAPPA), a collection of effective HIV/AIDS prevention programs in a box. The HAPPA collection builds on a previously established collection of 13 effective HIV/AIDS/sexually transmitted disease (STD) prevention programs for adolescents known as PASHA, the Program Archive on Sexuality, Health, & Adolescence. Together, the HAPPA and PASHA collections provide a rich source of 23 promising programs designed to prevent the spread of HIV/AIDS. The HAPPA and PASHA programs are available for use by communities, schools, family planning clinics, STD clinics, mental health centers, and drug rehabilitation centers throughout the country.

Cerebral MR perfusion imaging: first clinical application of a 1 M gadolinium chelate (Gadovist 1.0) in a double-blinded randomized dose-finding study.

The purpose of this study was to evaluate efficacy and safety of the 1 M gadolinium chelate Gadovist 1.0 for assessment of cerebral hemodynamics with dynamic susceptibility contrast-enhanced magnetic resonance (MR) imaging. Eighty-nine patients with carotid artery stenosis or cerebral infarcts were included in this multicenter, double-blinded study using five dose groups from 0.1 to 0.5 mmol/kg. Imaging was performed with 1-T scanners using a T2*-weighted fast low-angle shot (FLASH) sequence. Dose-dependent changes in quantitative and qualitative parameters describing signal-time curves and relative regional cerebral blood volume maps were investigated. For safety evaluation, vital signs, clinical and laboratory tests, and adverse events were assessed. The quantitative measurements revealed an optimal dose of 0.4 mmol/kg. The qualitative evaluation revealed that the required qualitative assessment for clinical purposes was already reached at a dose of 0. 3 mmol/kg. No significant changes in vital signs and laboratory tests were found. No serious adverse events were observed. The combined results revealed the dose of 0.3 mmol/kg as the diagnostically adequate dose given the gradient-echo sequence and field strength used. Gadovist 1.0 has been shown to be a safe and well-tolerated contrast agent. J. Magn. Reson. Imaging 2000;12:371-380.

Pharmacy database for tracking drug costs and utilization.

A pharmacy database for tracking drug costs and physician prescribing trends is described. Accuracy problems plagued data systems used to make drug-use-policy decisions at a tertiary care teaching hospital because of structural deficiencies within the systems and their nonclinical orientation. To resolve these problems, a programmer analyst, a clinical supervisor, and a clinical pharmacist developed a hierarchical database of drug costs. The database was designed to be valid for tracking drug costs according to patterns of clinical use. Internal controls were created that could identify and correct cost-tabulation errors arising within the ordering, order-entry, and billing processes. The database was able to tabulate drug costs according to the clinical service on which the patient was being treated at the time so that reports could compare aggregate prescribing trends from one time period to another for the same service. Similarly, the database could track and report drug use by disease or financial classification. Flagging elements were introduced to the database for cancer chemotherapy and antimicrobial drug products to enable reporting by these categories and by therapeutic subcategories within the antimicrobial category. Routine monthly reports were distributed to end users. Development of a database for tracking drug costs and utilization allowed a teaching hospital to derive the cost of medications from billing-charge information and to report data to health care professionals on the basis of important factors like clinical services.

Reducing motion artefacts in diffusion-weighted MRI of the brain: efficacy of navigator echo correction and pulse triggering.

Diffusion-weighted MRI (DWI) is extremely sensitive to motion of the object being examined. Pulse triggering and navigator echo correction are methods for reducing motion artefacts which can be combined with conventional DWI sequences. Implementation of these methods in imaging sequences with a readout of one, three, or five echoes is presented and imaging results compared in a study of five healthy volunteers. As an objective measure for motion-induced image artefacts, the "artefacticity" of an image is defined. Pulse triggering and navigator echo correction significantly improve image quality and provide a technique for high-quality DWI on standard imagers without improved gradient hardware.

Simultaneous assessment of cerebral hemodynamics and contrast agent uptake in lesions with disrupted blood-brain-barrier.

The purpose of this study was to develop a method that eliminates the influence of the T1 relaxation time upon the signal-time course in perfusion-weighted imaging of cerebral lesions with blood-brain-barrier (BBB) disruption. On a 1.5 T whole body clinical magnetic resonance (MR) imager, we implemented a dual-echo RF-spoiled FLASH sequence (TE=6/23.6 ms). We developed a postprocessing routine that allowed to calculate a signal-time course representing only the change in T2* and another one representing only the change in T1. Using this method, we examined 7 patients with various brain lesions showing evidence of BBB disruption. In the signal-time-curves obtained from the early echo we found a distinct signal drop due to the T2* effect. These effects could be eliminated by the correction algorithm yielding a 67% higher signal increase. Correction of the signal-time curve of the late echo yielded a more pronounced maximum signal drop and a decrease in postcontrast signal intensity. We found that without this correction the relative regional cerebral blood volume and the first moment of the concentration-time curve were underestimated by 72% and 22%, respectively. The dual echo-sequence combined with the postprocessing algorithm separates T1 and T2* effects and thus allows to assess cerebral hemodynamics and contrast agent kinetics simultaneously. This method may be a useful tool for characterizing, staging, and therapy monitoring of brain tumors.

Comparison of echo-planar sequences for perfusion-weighted MRI: which is best?

We compared gradient-echo (GRE), spin-echo (SE) and stimulated-echo (STE) echo-planar imaging sequences for perfusion-weighted imaging at different field strengths. Focal cerebral ischaemia was induced by endovascular occlusion of the middle cerebral artery in eight rats. MR was performed at 4.7 T or 2.35 T. With each sequence, we acquired data sets before, during and after bolus injection of Gd-DTPA with a time resolution of 1.2 s per image. The perfusion-weighted images were assessed with regard to image quality, artefacts, signal-to-noise ratio (SNR), and signal-at-tenuation-to-noise ratio (delta SNR) of the non-ischaemic tissue. Visual assessment showed GRE-EPI images acquired at 4.7 T to suffer from distortion due to susceptibility artefacts. Artefacts were less marked with the SE and STE series. The GRE-EPI sequence gave the highest SNR and delta SNR. At 2.35 T, the SNR of the STE sequences was less than 3 and therefore did not allow construction of reliable signal-time curves. SE-EPI was best suited for perfusion-weighted imaging at high field strength thanks to its minimal distortion artefacts and high SNR. Using lower field strengths (2.35 T and less), susceptibility artefacts are reduced; GRE-EPI sequences are then best suited, because they have the highest SNR and T2* sensitivity.

Perfusion-weighted MRI using gadobutrol as a contrast agent in a rat stroke model.

The purpose of this study was to examine the new nonionic contrast agent gadobutrol in MR perfusion-weighted imaging, including the influence of different concentrations and dosages of the agent on the sensitivity to perfusion alterations. Sixteen rats were examined within 35 to 105 minutes after endovascular occlusion of the middle cerebral artery. A fast T2*-weighted fast low-angle shot (FLASH) sequence was used to acquire four images before and 16 images after bolus injection of .1, .2, .3, and .4 mmol/kg gadobutrol as .5 molar and 1.0 molar formulation. From user-defined regions, we obtained the maximum signal decrease, the relative regional cerebral blood volume, and the bolus delay. Contrast between ischemic and nonischemic regions during bolus passage increased with dose and concentration of the contrast agent. For low doses (.1 and .2 mmol/kg), the ischemic lesion could not or could barely be discerned. For higher doses (.3 and .4 mmol/kg), administration of the 1 molar contrast agent yielded a better contrast between ischemic and nonischemic tissue. Our results suggest that administration of gadobutrol at higher dosage and higher concentration increases sensitivity to perfusion alterations. These results are potentially useful for perfusion-weighted imaging of the human brain, because the volume of contrast agent will be reduced if a solution with higher concentration is used. When using contrast agents in higher concentrations for human examinations, a significant signal decrease may be achieved also with the low doses (.1-.15 mmol/kg).

Dynamic contrast-enhanced T2*-weighted MRI in patients with cerebrovascular disease.

Our purpose was to investigate the potential of dynamic susceptibility contrast-enhanced MRI in assessing regional haemodynamics in patients with cerebrovascular disease. T2*-weighted FLASH sequences were performed on a control group of 10 healthy subjects, 13 patients with unilateral stenosis or occlusion of the internal carotid artery and 6 patients with acute onset of neurological symptoms, the observed signal intensities being converted into concentration-time curves. A gamma-variate function was fitted to the measured concentration-time curves to eliminate effects of tracer recirculation. In each patient the two cerebral hemispheres were compared and the difference between the mean transit times and the percental change of the regional cerebral blood volume, calculated for each side. Patients with haemodynamically significant unilateral carotid obstruction can be divided into two subgroups: those with good and those with poor collateral supply. Patients with good collateral supply had a slight but not statistically significant increase in mean transit time and cerebral blood volume on the diseased side, whereas those with poor collaterals had a significant increase compared with the control group. In patients with acute onset of neurological symptoms perfusion maps clearly demonstrated the disturbed perfusion at a time when T2-weighted images were still normal. Perfusion imaging is a reliable and noninvasive method of assessing changes in cerebral perfusion in patients with unilateral carotid stenosis. This MR technique permits monitoring of haemodynamic changes during therapy and thus may become an alternative to SPECT and PET scanning. In patients with acute occlusion of a cerebral artery, perfusion imaging reveals the entire, perfusion deficit before conventional MRI and thus allows early intervention.

Accuracy of gamma-variate fits to concentration-time curves from dynamic susceptibility-contrast enhanced MRI: influence of time resolution, maximal signal drop and signal-to-noise.

Concentration-time curves derived from dynamic susceptibility-contrast enhanced magnetic resonance imaging are widely used to calculate cerebrovascular parameters. To exclude effects of recirculation, a non-linear regression method is used to fit a gamma-variate function to the concentration-time course. In previous studies the errors arising from the fitting procedure have not been quantified. In a computer simulation we investigate the uncertainties of parameters calculated from the fitted gamma-variate function, exploring the dependencies on signal-to-noise (SNR), time resolution (delta t), and maximal signal drop (MSD). Our study was performed to give a framework on how to design MR-sequences and choose contrast media and their application in order to yield concentration-time curves which allow a reliable performance of the gamma-variate fitting procedure. We recorded 396 concentration-time curves from regions of interest of 40 patients. The gamma-variate fitting procedure was applied to these curves resulting in 396 parameter sets. Ideal concentration-time curves as gamma-variate functions were generated from these sets with a given delta t, MSD, and SNR. Recirculation effect was simulated. Then the gamma-variate fitting was performed again. From ideal and simulated gamma-variate function the area and the normalized first moment were calculated. The uncertainties of the values calculated from the simulated curve relating to the values of the original one were determined. Increase of SNR decreases the involved errors. With SNR values of 100 and more there is only minor influence of delta t and MSD and the fitted curve approximates the original data very well. Smaller values of SNR lead to a stronger influence of delta t and MSD and a higher number of fitting failures. With increasing delta t the uncertainties also increase. Intermediate values of MSD (30% to 70%) yield the smallest errors while increasing or decreasing MSD yields an increase of uncertainty. To achieve low uncertainties in the calculation of cerebrovascular parameters from gamma-variate fits, delta t of the imaging sequence and MSD must be considered. This is more important the lower SNR is. The shown dependencies should be taken into account when choosing MR sequence parameters and application of contrast media.

[Studies on contrast medium dosage in perfusion-weighted MR imaging]. / Untersuchungen zur Kontrastmitteldosierung bei der perfusionsgewichteten MRT.

PURPOSE: In this study we investigated, whether increasing the dosage of a paramagnetic contrast agent results in a stronger signal decrease in T2*-weighted perfusion sequences and therefore more meaningful parameter maps. MATERIAL AND METHODS: In a prospective study bolus injection of gadolinium-DTPA was performed at dosages of 0.1, 0.2, and 0.3 mmol/kg body weight (BW) in 10 patients each. Before, during and after bolus injection 40 T2*-weighted images of a reference brain slice were acquired within 65.6 seconds on a 1.0 T clinical scanner and perfusion parameters were calculated. RESULTS: Due to the limited signal decrease during bolus passage and the resulting low signal-difference-to-noise ratio (delta S/N) no reliable differentiation of gray and white matter was possible at a contrast agent dosage of 0.1 mmol/kg BW. Only at higher dosages, both, signal decrease and delta S/N were strong enough to allow differentiation of gray and white matter and to yield reliable parameter maps. CONCLUSION: For meaningful MR perfusion imaging at 1.0 T and with the given sequence a contrast agent dosage of at least 0.2 mmol/kg BW is necessary, if a 0.5-molar contrast agent is used.