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Farahani K Saxton RE Yoon HC De Salles AA Black KL Lufkin RB 《Magnetic resonance imaging》1999,17(10):1297-1494
Focal regions of T1-shortening have been observed in magnetic resonance imaging (MRI)-monitored thermal ablations of perfused tissues. The aims of this study were two-fold: to find evidence for heat-induced conversion of hemoglobin (Hb) to methemoglobin (mHb), and to investigate the effects of heat treatment of in-vitro blood components upon their MR relaxation times. Spectrophotometric studies were performed to confirm the heat-induced formation of methemoglobin. Preparations of whole and fractionated blood, previously submitted to elevated temperatures of 40°C to 80°C, were imaged and the relaxation times were calculated. Optical absorption spectra of samples containing free Hb, heated to 60°C, showed increased light absorption at 630 nm, evident of mHb presence. Short T1 values in whole blood (1.13 s) and packed red blood cell (0.65 s) compartments, heated at 60°C, compared to their baseline values (1.62 s and 0.83 s, respectively), were attributed to mHb formation. In relation to MRI-guided thermal interventions, these results suggest a possible explanation for observation of hyperintense regions on T1-weighted images. 相似文献
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H. Jordan Garber William H. Oldendorf Leon D. Braun Robert B. Lufkin 《Magnetic resonance imaging》1989,7(6):605-610
Following nephrectomy and intravenous injection of tritiated mannitol, adult male rats were exposed to magnetic resonance imaging (MRI) procedures at 1.5 T, 0.5 T, and 0.3 T. Compared to rats similarly handled but not exposed to MRI procedures, brain mannitol concentration, expressed as a percentage of mean body concentration, was significantly increased at 0.3 T and 0.5 T but not at 1.5 T. At 0.3 T, exposure to gradient-field fluctuations used for imaging increased brain mannitol concentration, but exposures to static main field and pulsed radiofrequency energies did not. Increased brain mannitol associated with gradient-field flux may reflect increased blood-brain barrier permeability or blood volume in brain. MRI effects on brain mannitol space are of uncertain clinical significance, but are consistent with prior evidence of an MRI-induced increase of brain capillary endothelial cell transport observed with horseradish peroxidase. Further studies are needed to confirm these findings and to explore the processes underlying changes in mannitol distribution related to MRI. 相似文献
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The purpose of this study was to assess the degree of conspicuity and amount of field distortion caused by a biopsy needle designed specifically for use in MRI studies. Toward this, a number of pulse sequences including spin and field echo were used. Parameters such as field of view, strength of read gradient, direction of read gradient, echo time and slice thickness were varied. The effect of these manipulations on needle visualization was studied. Partial voluming errors with thicker slices decreased needle conspicuity. Smaller field of view improved needle visualization as a result of magnification effect. Shallow read gradient strengths also increased needle conspicuity. Increased image artifacts were noted on field-echo sequences compared to spin echo. This effect increased with longer echo times. This reflects T2* effects on field-echo images. 相似文献
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