Mapping of cerebral metabolic rate of oxygen using dynamic susceptibility contrast and blood oxygen level dependent MR imaging in acute ischemic stroke.

INTRODUCTION: MR-derived cerebral metabolic rate of oxygen utilization (CMRO(2)) has been suggested to be analogous to PET-derived CMRO(2) and therefore may be used for detection of viable tissue at risk for infarction. The purpose of this study was to evaluate MR-derived CMRO(2) mapping in acute ischemic stroke in relation to established diffusion- and perfusion-weighted imaging. METHODS: In 23 patients (mean age 63 ± 18.7 years, 11 women) with imaging findings for acute ischemic stroke, relative oxygen extraction fraction was calculated from quantitative transverse relaxation times (T2, T2*) and relative cerebral blood volume using a quantitative blood oxygenation level dependent (BOLD) approach in order to detect a local increase of deoxyhemoglobin. Relative CMRO(2) (rCMRO(2)) maps were calculated by multiplying relative oxygen extraction fraction (rOEF) by cerebral blood flow, derived from PWI. After co-registration, rCMRO(2) maps were evaluated in comparison with apparent diffusion coefficient (ADC) and time-to-peak (TTP) maps. Mean rCMRO(2) values in areas with diffusion-restriction or TTP/ADC mismatch were compared with rCMRO(2) values in the contralateral tissue. RESULTS: In tissue with diffusion restriction, mean rCMRO(2) values were significantly decreased compared to perfusion-impaired (17.9 [95 % confidence interval 10.3, 25.0] vs. 58.1 [95 % confidence interval 50.1, 70.3]; P < 0.001) and tissue in the contralateral hemisphere (68.2 [95 % confidence interval 61.4, 75.0]; P < 0.001). rCMRO(2) in perfusion-impaired tissue showed no significant change compared to tissue in the contralateral hemisphere (58.1 [95 % confidence interval 50.1, 70.3] vs. 66.7 [95 % confidence interval 53.4, 73.4]; P = 0.34). CONCLUSION: MR-derived CMRO(2) was decreased within diffusion-restricted tissue and stable within perfusion-impaired tissue, suggesting that this technique may be adequate to reveal different pathophysiological stages in acute stroke.

Technical considerations on the validity of blood oxygenation level-dependent-based MR assessment of vascular deoxygenation.

A blood oxygenation level-dependent (BOLD)-based apparent relative oxygen extraction fraction (rOEF) as a semi-quantitative marker of vascular deoxygenation has recently been introduced in clinical studies of patients with glioma and stroke, yielding promising results. These rOEF measurements are based on independent quantification of the transverse relaxation times T2 and T2* and relative cerebral blood volume (rCBV). Simulations demonstrate that small errors in any of the underlying measures may result in a large deviation of the calculated rOEF. Therefore, we investigated the validity of such measurements. For this, we evaluated the quantitative measurements of T2 and T2* at 3 T in a gel phantom, in healthy subjects and in healthy tissue of patients with brain tumors. We calculated rOEF maps covering large portions of the brain from T2, T2* and rCBV [routinely measured in patients using dynamic susceptibility contrast (DSC)], and obtained rOEF values of 0.63 ± 0.16 and 0.90 ± 0.21 in healthy-appearing gray matter (GM) and white matter (WM), respectively; values of about 0.4 are usually reported. Quantitative T2 mapping using the fast, clinically feasible, multi-echo gradient spin echo (GRASE) approach yields significantly higher values than much slower multiple single spin echo (SE) experiments. Although T2* mapping is reliable in magnetically homogeneous tissues, uncorrectable macroscopic background gradients and other effects (e.g. iron deposition) shorten T2*. Cerebral blood volume (CBV) measurement using DSC and normalization to WM yields robust estimates of rCBV in healthy-appearing brain tissue; absolute quantification of the venous fraction of CBV, however, is difficult to achieve. Our study demonstrates that quantitative measurements of rOEF are currently biased by inherent difficulties in T2 and CBV quantification, but also by inadequacies of the underlying model. We argue, however, that standardized, reproducible measurements of apparent T2, T2* and rCBV may still allow the estimation of a meaningful apparent rOEF, which requires further validation in clinical studies.

MR-based hypoxia measures in human glioma.

Hypoxia plays a central role in tumor stem cell genesis and is related to a more malignant tumor phenotype, therapy resistance (e.g. in anti-angiogenic therapies) and radio-insensitivity. Reliable hypoxia imaging would provide crucial metabolic information in the diagnostic work-up of brain tumors. In this study, we applied a novel BOLD-based MRI method for the measurement of relative oxygen extraction fraction (rOEF) in glioma patients and investigated potential benefits and drawbacks. Forty-five glioma patients were examined preoperatively in a pilot study on a 3T MR scanner. rOEF was calculated from quantitative transverse relaxation rates (T2, T2*) and cerebral blood volume (CBV) using a quantitative BOLD approach. rOEF maps were assessed visually and by means of a volume of interest (VOI) analysis. In six cases, MRI-targeted biopsy samples were analyzed using HIF-1α-immunohistochemistry. rOEF maps could be obtained with a diagnostic quality. Focal spots with high rOEF values were observed in the majority of high-grade tumors but in none of the low-grade tumors. VOI analysis revealed potentially hypoxic tumor regions with high rOEF in contrast-enhancing tumor regions as well as in the non-enhancing infiltration zone. Systematic bias was found as a result of non-BOLD susceptibility effects (T2*) and contrast agent leakage affecting CBV. Histological samples demonstrated reasonable correspondence between MRI characteristics and HIF-1α-staining. The presented method of rOEF imaging is a promising tool for the metabolic characterization of human glioma. For the interpretation of rOEF maps, confounding factors must be considered, with a special focus on CBV measurements in the presence of contrast agent leakage. Further validation involving a bigger cohort and extended immuno-histochemical correlation is required.

[Functional magnetic resonance imaging for cortical mapping in epilepsy]. / Funkcionális mágnesesrezonancia- képalkotáson alapuló térképezési eljárások epilepsziában.

It is not only the total curative resection of pathological tissue or the minimization of symptoms to be considered in epilepsy surgery or other neurosurgical procedures, it is equally desirable to maintain the best possible quality of life. Cortical mapping methods can help achieve this goal by delineating eloquent areas, i.e. brain regions that are vital for providing an acceptable quality of life, albeit not prone to compensatory reorganization. These areas include among others the Broca and Wernicke regions for speech, the primary motor, sensory and visual cortices. Functional MRI gained importance in the last decade as a non-invasive clinical cortical mapping technique. This method is capable of localizing cortical areas selectively activated by a given task condition. Thus, selecting appropriate tasks can help mapping eloquent brain regions. Using functional MRI provides information that is complementary to other mapping methods. Moreover, it can replace invasive methods such as the Wada test. Here, we explain the background of functional MRI, compare it to other clinical mapping methods, explain the intricacies of paradigm selection, and show the limitations of the technique while also pointing out alternative uses.

Analysis of three leakage-correction methods for DSC-based measurement of relative cerebral blood volume with respect to heterogeneity in human gliomas.

PURPOSE: Aim of this study was to investigate the influence of contrast agent leakage on relative cerebral blood volume (rCBV), using clinical dynamic susceptibility contrast (DSC) protocols. Different correction methods were compared, in order to identify a clinically reliable method. MATERIALS AND METHODS: DSC perfusion data from patients with glioma were acquired with a single-shot EPI technique at 3.0T using a pre-dose. Three different post-processing methods for leakage correction were compared, concerning rCBV, the permeability related parameter K2 and the predominant leakage effect in tumor regions (T1 effect: K2>0; T2* effect: K2<0). Additionally, simulations were performed, to investigate the influence of noise and input curve modifications on correction results. RESULTS: Our results indicate several differences between post-processing methods with regard to rCBV values, reflected by the fact that the distribution of detected leakage effects and the correction strength differed between methods. Leakage was heterogeneous within tumorous tissue and between patients, with a general predominance of T2* effects but an increased amount of T1 effects in low grade glioma. Simulations confirmed differential dependencies on signal-to-noise ratios, mean transit times and input curves as possible reasons. CONCLUSION: The impact of leakage is complex, thus adequate correction necessitates care. Standardized input parameters are one important factor for comparability of rCBV and K2 values among patients. The extension of DSC analysis with K2 maps could potentially allow improved differentiation between tumor grades. Further methods need to integrate special advantages of existing approaches to achieve more reliable rCBV estimates within clinically reasonable calculation times.

EVALUATION OF DOSE REDUCTION POTENTIALS OF A NOVEL SCATTER CORRECTION SOFTWARE FOR BEDSIDE CHEST X-RAY IMAGING.

Bedside chest X-rays (CXR) for catheter position control may add up to a considerable radiation dose for patients in the intensive care unit (ICU). In this study, image quality and dose reduction potentials of a novel X-ray scatter correction software (SkyFlow, Philips Healthcare, Hamburg, Germany) were evaluated. CXRs of a 'LUNGMAN' (Kyoto Kagaku Co., LTD, Kyoto, Japan) thoracic phantom with a portacath system, a central venous line and a dialysis catheter were performed in an experimental set-up with multiple tube voltage and tube current settings without and with an antiscatter grid. Images with diagnostic exposure index (EI) 250-500 were evaluated for the difference in applied mAs with and without antiscatter grid. Three radiologists subjectively assessed the diagnostic image quality of grid and non-grid images. Compared with a non-grid image, usage of an antiscatter grid implied twice as high mAs in order to reach diagnostic EI. SkyFlow significantly improved the image quality of images acquired without grid. CXR with grid provided better image contrast than grid-less imaging with scatter correction.

Postoperative ischemic changes following brain metastasis resection as measured by diffusion-weighted magnetic resonance imaging.

OBJECT: Brain metastases occur in 10% to 40% of patients harboring cancer. In cases of neurosurgical metastasis resection, all postoperative neurological deterioration should be avoided. Reasons for postoperative deficits can be direct tissue damage due to resection, hemorrhage, venous congestive infarcts, or arterial ischemic events leading to tissue infarction. The aim of this study was to evaluate whether postoperative ischemic infarctions occur in surgery for brain metastasis and to determine their influence on new postoperative neurological deficits. METHODS: Patients who underwent resection of brain metastases and had preoperative and early postoperative (within 48 hours) MRI scans, including diffusion-weighted imaging sequences and apparent diffusion coefficient maps, between January 2009 and May 2012 were included in this study. Clinical and histopathological data (histopathological results, pre- and postoperative neurological status, and previous tumor-specific therapy) were recorded. RESULTS: One hundred twenty-two patients (56 male, 66 female) who underwent resection of brain metastases were included. The patients' mean age was 60 years (range 21-89 years). The mean time span from initial tumor diagnosis to resection of brain metastasis was 44 months (range 0-338 months). The mean preoperative Karnofsky Performance Status was 80% (exact mean 76% ± 17% [SD]), and the mean postoperative value was 80% (exact mean 78% ± 17%). Twelve (9.8%) of the 122 patients had postoperative permanent worsening of a neurological deficit or a new permanent neurological deficit; 44 (36.1%) of the 122 patients had postoperative ischemic lesions. When comparing patients with and without previous brain irradiation, 53.8% of patients with previous brain irradiation had ischemic lesions on postoperative imaging compared with 31.3% of patients without previous brain irradiation (p = 0.033). There was a significant association between ischemia and postoperative neurological status deterioration (transient or permanent); 13 (29.5%) of 44 patients with ischemic lesions had deterioration of their neurological status compared with 7 (9%) of the 78 patients who did not have ischemic lesions (p = 0.003). CONCLUSIONS: This study demonstrates a high prevalence of vascular incidents in patients undergoing resection for metastatic brain disease. Patients harboring postoperative ischemic lesions detected by MRI have a higher rate of neurological deficits (transient or permanent). Patients who had previous irradiation therapy are at higher risk of developing postoperative ischemic lesions. A large number of postoperative neurological deficits are caused by ischemic incidents.