摘要:

AbstractSpin label angiography compares two images by subtraction. The first is obtained after blood in one region is labeled by inversion and flows into a region of interest. Labeling is not used for the second image, so only labeled blood remains in the final angiogram after subtraction. This subtraction is never perfect, but with starting images containing less static tissue signal, the remaining background can be reduced. This can be achieved by observing at the zero crossing following an inversion. Multiple inversions allow one to null the signal from tissues with differing T1 simultaneously. We present equations and sample calculations for inversion times and demonstrate the resistance to subject motions (peristalsis, breathing, speaking) resulting from two inversions. Adequate suppression of static tissue signal allows one to dispense with labeling and subtraction, halving the minimum time needed to acquire an image. © 1991 Academic Press. Inc

ISSN:0740-3194    

DOI:10.1002/mrm.1910180202

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractExcellent water suppression is required to performin vivo1H spectroscopic experiments. However water suppression is difficult due to bothB0and RF inhomogeneities. These inhomogeneities are particularly troublesome in spectroscopic imaging experiments where water suppression is required throughout some large region of interest. In this paper, we propose the use of spectral‐spatial excitation pulses for such experiments. These two‐dimensional pulses are shown to provide water suppression that is insensitive to a range ofB0and RF variations while simultaneously providing spatial localization. Experimental results including images (with voxel volumes ranging from 3.4 to 1.5 cc) of various brain metabolites from both a normal volunteer and a patient with a metastatic lung carcinoma are presented. © 1991 Academic Press.

ISSN:0740-3194    

DOI:10.1002/mrm.1910180203

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractParamagnetic ions complexed to proteins may lose, retain, or enhance solvent paramagnetic relaxation (SPR) relative to free solution. We measuredT1andT2of three mouse cancers, their normal counterparts, and six additional tissues. LongT1of cancers was not caused by necrosis or by different contents of water, fat, or blood. Dissociable (TCA‐extractable) and nondissociable (ashed) Mn, Cu, and Fe were measured by AA. Cancers had less Mn, Cu, and Fe than did normal counterparts. All 12 tissues had inverse correlations betweenT1and dissociable Mn and Cu. For Mn alone to account for reducedT1, the extent to which SPR of the Mn‐protein complexes would be enhanced is by factors of 0.6 to 13, below the maximum observed in Mn‐enzymes. Different amounts of paramagnetic ion‐protein complexes may account for part of the differences inT1of water protons in different tissues, and the longerT1of cancer cell water may be caused in part by reduced amounts Of such complexes. © 1991 Academic Pr

ISSN:0740-3194    

DOI:10.1002/mrm.1910180204

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractComputer simulations of Depth pulse B (Θ(2Θ[±x, ±y])2; acquire) and other Depth pulses, verified by experimental surface coil NMR studies utilizing phantom samples, reveal that the spatial localization performance of Depth pulses degrades when the repetition time is short relative toT1because of differential saturation, i.e.,T1, discrimination effects. Simulations of Depth pulse A (Θ(2Θ[±x, ±y]); acquire) and Depth pulse B indicate that there is no phase‐cycled pulse sequence delivery order which negates the untoward effect ofT1discrimination on spatial localization performance. The results of this study demonstrate the importance of consistent magnetization preparation prior to the delivery of each phase‐cycled multiple pulse sequence in a Depth pulse cycle for obtaining optimal spatial localization performance. The untoward effects of inconsistent magnetization preparation, resulting fromT1discrimination, may be ameliorated by the application of many Depth pulse cycles. © 1991 Academi

ISSN:0740-3194    

DOI:10.1002/mrm.1910180205

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractA volume MR phased array was constructed with two coils placed anteriorly and two coils posteriorly. Data acquired simultaneously from the four coils on a phantom were combined into a single image having a signal‐to‐noise ratio 80% better than that from the body coil. Additional comparisons of the four‐coil phased array with a two‐coil phased array and a Helmholtz pair having the same overall dimensions show how variations of signal amplitude and phase in the individual coils affect the composite SNR. Images of the male and female pelvis demonstrate how the improved SNR can be used to reduce the number of excitations, decrease the field of view, increase the echo time, or reduce the slice thickness. © 1991 Academic Pr

ISSN:0740-3194    

DOI:10.1002/mrm.1910180206

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractA deuterium NMR spectroscopic method to determine relative tumor blood flow (TBF) by measuring the increase in tumor HOD concentration after intravenous injection of 100 μl D2O (0.9% NaCl) is presented. An integration approach analogous to that validated for positron emission tomographic measurement of cerebral blood flow was implemented. Computer simulations indicated that integration from 30 to 120 s minimizes the sensitivity of the uptake integral to the shape of the arterial input function, which cannot be assessed in each mouse, while maintaining both a nearly linear relationship between TBF and the integral and high NMR signal‐to‐noise. A strong positive linear correlation was observed between the uptake integral and TBF measured by D2O clearance in both untreated tumors (n= 19;P<0.001) and tumors after hyperthermia (n= 16;P<0.001). This method can measure relative TBF in tumors with heterogeneous blood flow and is ideally suited to concurrent or interleaved measurement of TBF and metabolism via multinuclear NMR spectroscopy. © 1991 Academic Press

ISSN:0740-3194    

DOI:10.1002/mrm.1910180207

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractA rapid and completely automated method of adjusting the magnetic field (B0) homogeneity forin vivoproton spectroscopy and imaging is described. Bo inhomogeneity maps are generated by a gradient‐recalled echo pulse sequence in which the frequency dispersion is chosen to eliminate the effects of the fat/water chemical shift. Low‐order shim values are derived by magnitude‐weighted least‐squares fits to theB0maps and automatically applied as DC offsets to theX, Y, and Z gradient amplifiers. Imaging with chemical shift selective saturation is used as a measure of the efficacy of the technique. Results indicate that AUTOSHIM improves the overall homogeneity; however, local high‐order field distortions which cannot be corrected by linear gradients are generated by certain air/tissue and bone/tissue morphology. In such cases a “ZOOM SHIM” may be applied over a limited region of interest for local homogeneity improvement at the expense of other regions. It is suggested that such scans are a necessity for recording the homogeneity during clinical MR spectroscopy. © 1991 Acade

ISSN:0740-3194    

DOI:10.1002/mrm.1910180208

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractConcurrent23Na and31P nuclear magnetic resonance spectroscopy has been employed to study the effects of ischemia upon the high‐energy phosphagens and sodium ion concentration within thein vivorat liver. High‐energy phosphates in the form of ATP were depleted within 10 min of the onset of ischemia when measured by NMR. However, similar liver samples subjected to analytical biochemistry retained 27 ± 12% of their ATP after a similar 10‐min ischemic insult. Time‐dependent23Na NMR measurements, obtained in the presence of the shift reagent Dy(TTHA) to distinguish intracellular from extracellular sodium, revealed a rapid rise in the intracellular sodium when the liver was made ischemic. Intracellular and extracellular sodium concentrations approached equilibrium with an exponential time constant of 14.7 ± 7 min. The initial rate of sodium influx was calculated to be 1.50 meq · 1−1· min−1. The results indicate that the ischemic liver has a high passive sodium permeability and that NMR detectable31P signals reflect the actual availability of cytosolic high‐energy phosphates to enzymes, in this instance the membrane‐bound [Na+, K+]‐ATPase. © 19

ISSN:0740-3194    

DOI:10.1002/mrm.1910180209

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractWe have compared various methods ofin vivoNMR spectral parameter estimation, namely a nonlinear fit of the free induction decay signal in the time domain (NLTD), a nonlinear fit of the fast Fourier transform of the FID data in the frequency domain using either a continuous Lorentzian model (NLLM) or a Fourier‐sampled model (NLFM), and a time‐domain linear prediction method using singular value decomposition (LPSVD). Monte Carlo simulations of31P and13Cin vivoexperiments were used to assess the bias and statistical uncertainties of spectral parameters obtained with each method. In the31P case, all methods appear to be equivalent except the LPSVD method that led to significantly biased peak amplitudes (up to 28%). In the13C case, the only methods able to recover the glycogen peak were the NLTD method and its equivalent in the frequency domain (NLFM). In both the31P and the13C cases simulations demonstrated that 256 data points were sufficient. These results demonstrate the feasibility and the robustness of a nonlinear fit of the FID data in the time domain, and we illustrate this on31P and13C data sets obtained in humans. © 1991 Academic Press.

ISSN:0740-3194    

DOI:10.1002/mrm.1910180210

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractAn enhancement to Dixon's technique is described which can provide error‐free decomposition of water and fat proton images even in the presence of off‐resonance conditions which result from susceptibility differences, demagnetization, or shim errors. The method uses three measurements with phase shifts of 0, π and − π between the fat and water resonances. The additional information provided by the third measurement is used to calculate an image of the field inhomogeneity in addition to true water and fat images. The signal‐to‐noise ratio (SNR) in the decomposed images is equivalent to that of a 2.7 NEX acquisition (instead of 3 NEX), yielding an SNR imaging efficiency of 95%. In addition, theB0image which is provided may have diagnostic value in its own right. Examples of head and body scans often portray surprisingly largeB0shifts near interfaces between air or bone and soft tissue. © 1991 Academi

ISSN:0740-3194    

DOI:10.1002/mrm.1910180211

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY