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Partial Volume Effects (partial + volume_effects)
Selected Abstracts1H spectroscopic imaging of human brain at 3 Tesla: Comparison of fast three-dimensional magnetic resonance spectroscopic imaging techniquesJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2009Matthew L. Zierhut PhD Abstract Purpose To investigate the signal-to-noise-ratio (SNR) and data quality of time-reduced three-dimensional (3D) proton magnetic resonance spectroscopic imaging (1H MRSI) techniques in the human brain at 3 Tesla. Materials and Methods Techniques that were investigated included ellipsoidal k -space sampling, parallel imaging, and echo-planar spectroscopic imaging (EPSI). The SNR values for N-acetyl aspartate, choline, creatine, and lactate or lipid peaks were compared after correcting for effective spatial resolution and acquisition time in a phantom and in the brains of human volunteers. Other factors considered were linewidths, metabolite ratios, partial volume effects, and subcutaneous lipid contamination. Results In volunteers, the median normalized SNR for parallel imaging data decreased by 34,42%, but could be significantly improved using regularization. The normalized signal to noise loss in flyback EPSI data was 11,18%. The effective spatial resolutions of the traditional, ellipsoidal, sensitivity encoding (SENSE) sampling scheme, and EPSI data were 1.02, 2.43, 1.03, and 1.01 cm3, respectively. As expected, lipid contamination was variable between subjects but was highest for the SENSE data. Patient data obtained using the flyback EPSI method were of excellent quality. Conclusion Data from all 1H 3D-MRSI techniques were qualitatively acceptable, based upon SNR, linewidths, and metabolite ratios. The larger field of view obtained with the EPSI methods showed negligible lipid aliasing with acceptable SNR values in less than 9.5 min without compromising the point-spread function. J. Magn. Reson. Imaging 2009;30:473,480. © 2009 Wiley-Liss, Inc. [source] Improved bolus arrival time and arterial input function estimation for tracer kinetic analysis in DCE-MRIJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2009Anup Singh PhD Abstract Purpose To develop a methodology for improved estimation of bolus arrival time (BAT) and arterial input function (AIF) which are prerequisites for tracer kinetic analysis of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) data and to verify the applicability of the same in the case of intracranial lesions (brain tumor and tuberculoma). Materials and Methods A continuous piecewise linear (PL) model (with BAT as one of the free parameters) is proposed for concentration time curve C(t) in T1 -weighted DCE-MRI. The resulting improved procedure suggested for automatic extraction of AIF is compared with earlier methods. The accuracy of BAT and other estimated parameters is tested over simulated as well as experimental data. Results The proposed PL model provides a good approximation of C(t) trends of interest and fit parameters show their significance in a better understanding and classification of different tissues. BAT was correctly estimated. The automatic and robust estimation of AIF obtained using the proposed methodology also corrects for partial volume effects. The accuracy of tracer kinetic analysis is improved and the proposed methodology also reduces the time complexity of the computations. Conclusion The PL model parameters along with AIF measured by the proposed procedure can be used for an improved tracer kinetic analysis of DCE-MRI data. J. Magn. Reson. Imaging 2009;29:166,176. © 2008 Wiley-Liss, Inc. [source] Two-point dixon fat,water separation: Improving reliability and accuracy in phase correction algorithmsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 5 2008Maria A. Schmidt PhD Abstract Purpose To propose an advanced phase-correction region-growing algorithm for two-point fat,water separation suitable for parotid assessment, and to evaluate the general performance of phase-correction algorithms. Materials and Methods Two region-growing algorithms were evaluated in test objects and in head images: the original phase-correction algorithm (OPC) and the advanced phase-correction algorithm with voxel size manipulation (VSM) which includes: 1) starting the region-growing process from images of lower resolution and subsequently stepping toward the original matrix size, and 2) limiting the use of low-pass filters to fat,water interfaces with partial volume effects Results Fundamental problems relate to biological tissue spectrum being poorly approximated by two discrete peaks for fat and water. The VSM algorithm was shown to be less noise-sensitive, faster, and to produce a better approximation for the field inhomogeneity map. In head images (6 volunteers, 10 slices each) 43 errors were found with the OPC algorithm and only 6 errors with the VSM algorithm. Only the OPC algorithm produced errors surrounding the parotids (10 errors). Conclusion The VSM algorithm provides a more accurate and less noise-sensitive fat,water separation. This highly significant performance improvement allows the application of phase-correction algorithms to a wider range of clinical applications. J. Magn. Reson. Imaging 2008. © 2008 Wiley-Liss, Inc. [source] White-marker imaging,Separating magnetic susceptibility effects from partial volume effectsMAGNETIC RESONANCE IN MEDICINE, Issue 3 2007Jan-Henry Seppenwoolde Abstract By applying dephasing gradients, local magnetic field inhomogeneitiescan selectively visualized with positive contrast, such as those created by magnetically labeled cells. This is known as "white-marker imaging." In white-marker imaging, subvoxel signal variations are also visualized as a result of partial volume (PV) effects and may compromise the identification of magnetic structures (e.g., magnetically-labeled cells). This study presents the theory and proof-of-principle experiments of a strategy to eliminate PV effects during white-marker imaging. The strategy employs the asymmetry of the signal response curves for non-PV effects as a function of externally applied gradients. In the case of PV effects, subtraction of the symmetrical signal responses eliminates their contribution. In vitro experimental images were made using a spherical phantom with cylindrical elements. In vivo images of the brain were obtained at a location that included air cavities (susceptibility effects) and the circle of Willis (PV effect). The results show that PV effects were eliminated in the in vitro experiments and were virtually absent under in vivo conditions. Magn Reson Med, 2007. © 2007 Wiley-Liss, Inc. [source] Use of tissue water as a concentration reference for proton spectroscopic imagingMAGNETIC RESONANCE IN MEDICINE, Issue 6 2006Charles Gasparovic Abstract A strategy for using tissue water as a concentration standard in 1H magnetic resonance spectroscopic imaging studies on the brain is presented, and the potential errors that may arise when the method is used are examined. The sensitivity of the method to errors in estimates of the different water compartment relaxation times is shown to be small at short echo times (TEs). Using data from healthy human subjects, it is shown that different image segmentation approaches that are commonly used to account for partial volume effects (SPM2, FSL's FAST, and K-means) lead to different estimates of metabolite levels, particularly in gray matter (GM), owing primarily to variability in the estimates of the cerebrospinal fluid (CSF) fraction. While consistency does not necessarily validate a method, a multispectral segmentation approach using FAST yielded the lowest intersubject variability in the estimates of GM metabolites. The mean GM and white matter (WM) levels of N-acetyl groups (NAc, primarily N-acetylaspartate), choline (Ch), and creatine (Cr) obtained in these subjects using the described method with FAST multispectral segmentation are reported: GM [NAc] = 17.16 ± 1.19 mM; WM [NAc] = 14.26 ± 1.38 mM; GM [Ch] = 3.27 ± 0.47 mM; WM [Ch] = 2.65 ± 0.25 mM; GM [Cr] = 13.98 ± 1.20 mM; and WM [Cr] = 7.10 ± 0.67 mM. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] High-resolution blood flow velocity measurements in the human fingerMAGNETIC RESONANCE IN MEDICINE, Issue 4 2001M. Klarhöfer Abstract MR phase contrast blood flow velocity measurements in the human index finger were performed with triggered, nontriggered, and cine acquisition schemes. A strong (Gmax = 200 mT/m), small bore (inner diameter 12 cm) gradient system inserted in a whole body 3 Tesla MR scanner allowed high-resolution imaging at short echo times, which decreases partial volume effects and flow artifacts. Arterial blood flow velocities ranging from 4.9,19 cm/sec were measured, while venous blood flow was significantly slower at 1.5,7.1 cm/sec. Taking into account the corresponding vessel diameters ranging from 800 ,m to 1.8 mm, blood flow rates of 3.0,26 ml/min in arteries and 1.2,4.8 ml/min in veins are obtained. The results were compared to ultrasound measurements, resulting in comparable blood flow velocities in the same subjects. Magn Reson Med 45:716,719, 2001. © 2001 Wiley-Liss, Inc. [source] Kinetics of PME/Pi in pig kidneys during cold ischemiaNMR IN BIOMEDICINE, Issue 7 2007Dominik von Elverfeldt Abstract Quality assessment of renal grafts via 31P magnetic resonance spectroscopy (MRS) has been investigated since 1986. As ATP concentrations decay rapidly during cold ischemia, the ratio of phosphomonoesters (PME) to inorganic phosphate (PiO) within the organ (PME/PiO) is commonly used as a quality marker and is considered to be the most reliable parameter. MRS did not lead to any delay in the transplantation procedure since it was performed during the time necessary for immunological matching (cross-match). Differences in the time period until transplantation call for extrapolation of the measured ratio to the end of cold ischemia before correlating with graft performance after transplantation. Therefore, quantitative determination of PME/PiO kinetics is essential. As a model for metabolite decay in human renal grafts, pig kidneys obtained from a slaughterhouse were monitored for up to 80,h via 31P MRS at 2,T. By employing chemical shift imaging (CSI) with a spatial resolution of approximately 1,×,1,×,4,cm3, it was possible to reduce partial volume effects significantly. The improved spectral resolution gained through CSI enabled reliable PME/PiO ratios to be determined only from those voxels containing renal tissue. Spectra were fitted automatically using the magnetic resonance user interface (MRUI), with prior knowledge obtained from unlocalized spectra when necessary. A monoexponential time dependence of PME/PiO for histidine,tryptophane,alpha-ketoglutarate (HTK)-perfused kidneys during cold ischemia was observed, and the determined value of the decay constant , was 0.0099,±,0.0012,h,1. In University of Wisconsin solution (UW)-perfused kidneys, an , of 0.0183,±,0.0053,h,1 was determined. Determination of the decay constant enables a usable extrapolation of PME/PiO for quality assessment of UW perfusion and a reliable extrapolation for HTK-perfused human renal grafts. Copyright © 2007 John Wiley & Sons, Ltd. [source] Coronary stent assessability by 64 slice multi-detector computed tomographyCATHETERIZATION AND CARDIOVASCULAR INTERVENTIONS, Issue 7 2007Tej Sheth MD Abstract Background: We evaluated the assessability of contemporary stent platforms by 64-slice multi-detector computed tomography (MDCT). Methods. Patients undergoing coronary stenting were included in a prospective protocol of MDCT imaging within 48 hr of stent implantation. MDCT data were acquired using a "Sensation 64" MDCT scanner (Siemens Medical Solutions, Forchheim, Germany). Stent assessability was assessed by two independent blinded observers and disagreement was resolved by a third observer. Assessability was defined at visualization of the in-stent lumen without influence of partial volume effects, beam hardening, motion, calcification, or contrast to noise limitations. Results: Fifty four stents (Cypher n = 25, Vision/Minivision n = 19, Taxus Express n = 8, Liberte n = 1, Driver n = 1) in 44 patients were included in the study. The two independent observers classified 30 of 54 stents (56%) as assessable. Interobserver reproducibility was good with , = 0.66. Stent size was the most important determinant of assessability. Consistently assessable stents were 3.0 mm or larger (85%), whereas those under 3 mm were mostly nonassessable (26%).Conclusions: Contemporary stent designs evaluated on a 64-slice MDCT scanner showed artifact free assessability only in larger stents. Increase in spatial resolution of MDCT scanners or modifications in stent design will be necessary to noninvasive evaluate stents <3 mm in diameter, where in-stent restenosis is more frequent. © 2007 Wiley-Liss, Inc. [source] | ||||||||||