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Different Echo Times (different + echo_time)
Selected AbstractsEcho combination to reduce proton resonance frequency (PRF) thermometry errors from fatJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2008Viola Rieke PhD Abstract Purpose To validate echo combination as a means to reduce errors caused by fat in temperature measurements with the proton resonance frequency (PRF) shift method. Materials and Methods Computer simulations were performed to study the behavior of temperature measurement errors introduced by fat as a function of echo time. Error reduction by combining temperature images acquired at different echo times was investigated. For experimental verification, three echoes were acquired in a refocused gradient echo acquisition. Temperature images were reconstructed with the PRF shift method for the three echoes and then combined in a weighted average. Temperature measurement errors in the combined image and the individual echoes were compared for pure water and different fractions of fat in a computer simulation and for a phantom containing a homogenous mixture with 20% fat in an MR experiment. Results In both simulation and MR measurement, the presence of fat caused severe temperature underestimation or overestimation in the individual echoes. The errors were substantially reduced after echo combination. Residual errors were about 0.3°C for 10% fat and 1°C for 20% fat. Conclusion Echo combination substantially reduces temperature measurement errors caused by small fractions of fat. This technique then eliminates the need for fat suppression in tissues such as the liver. J. Magn. Reson. Imaging 2007. © 2007 Wiley-Liss, Inc. [source] Focal liver lesions: Breathhold gradient- and spin-echo T2-weighted imaging for detection and characterization ,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2006Takeshi Yoshikawa MD Abstract Purpose To evaluate breathhold gradient- and spin-echo (GRASE) T2-weighted imaging for the detection and characterization of focal liver lesions. Materials and Methods Two GRASE sequences with different echo times (75 and 90 msec, GRASE75 and GRASE90) were compared with respiratory-triggered fast spin-echo (SE) and breathhold fast SE in 64 patients with 103 malignant and 51 benign lesions. Compared with respiratory-triggered and breathhold fast SE, GRASE reduced scan time by 77% to 82% and 21% to 27%, respectively. Two independent readers evaluated image quality and reviewed 504 liver segments on a segment-by-segment basis. Observer performance was evaluated with receiver operating characteristic (ROC) curve analysis. The signal-to-noise ratio (SNR) of liver and spleen, and lesion-to-liver contrast-to-noise ratio (CNR) were also measured. Results The overall quality of the GRASE images was higher than that of the respiratory-triggered and breathhold fast SE images, although signal inhomogeneities were more frequently observed with GRASE. No significant difference in the values of the area under the ROC curve (Az) for malignant lesion detection was found. The mean SNR and CNR were highest for respiratory-triggered fast SE. Conclusion T2-weighted breathhold GRASE has the potential to provide faster liver imaging. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc. [source] Changes in the proton T2 relaxation times of cerebral water and metabolites during forebrain ischemia in rat at 9.4 TMAGNETIC RESONANCE IN MEDICINE, Issue 6 2003Hao Lei Abstract Proton T2 relaxation times of cerebral water and metabolites were measured before, during, and after transient forebrain ischemia in rat at 9.4 T using localized proton magnetic resonance spectroscopy (1H-MRS) with Hahn echoes formed at different echo times (TEs). It was found that the T2 values of water and N-acetyl aspartate (NAA) methyl, but not total creatine (tCr) methyl, decrease significantly (,10%) during ischemia, and this T2 reduction is reversed by reperfusion. The T2 reduction observed for NAA was most likely caused by the extravascular component of the blood oxygenation level-dependent (BOLD) effect induced by a drastically increased deoxyhemoglobin content during ischemia. The absence of T2 changes for tCr can probably be explained by the fact that the BOLD-related T2 decrease was counterbalanced by the conversion of phosphocreatine (PCr) to creatine (Cr), which has a longer T2 than PCr, during ischemia. The changes in T2 should be taken into account for the quantification of metabolite concentrations during ischemia. Magn Reson Med 49:979,984, 2003. © 2003 Wiley-Liss, Inc. [source] Eliminating spurious lipid sidebands in 1H MRS of breast lesionsMAGNETIC RESONANCE IN MEDICINE, Issue 2 2002Patrick J. Bolan Abstract Detecting metabolites in breast lesions by in vivo 1H MR spectroscopy can be difficult due to the abundance of mobile lipids in the breast which can produce spurious sidebands that interfere with the metabolite signals. Two-dimensional J -resolved spectroscopy has been demonstrated in the brain as a means to eliminate these artifacts from a large water signal; coherent sidebands are resolved at their natural frequencies, leaving the noncoupled metabolite resonances in the zero-frequency trace of the 2D spectrum. This work demonstrates that using the zero-frequency trace,or equivalently the average of spectra acquired with different echo times,can be used to separate noncoupled metabolite signals from the lipid-induced sidebands. This technique is demonstrated with simulations, phantom studies, and in several breast lesions. Compared to the conventional approach using a single echo time, echo time averaging provides increased sensitivity for the study of small and irregularly shaped lesions. Magn Reson Med 48:215,222, 2002. © 2002 Wiley-Liss, Inc. [source] | ||||||||||