摘要:

AbstractThe reproducibility ofT1,T2, and proton density, measured in phantoms and the human brain was evaluted by proton imaging techniques. The sequence used to deriveT1and density values was a multiple‐saturation recovery which consists of four pairs of 90° pules, followed by a 180° phase reversal pulse, generting fourT2‐weighted images. The data were analyzed by fitting the pixel intensities to the respective equations by means of nonlinear multi‐parameter least‐squares analysis. Short‐term reproductibility between four consecutive scans was evaluted to b 1–40% depending on location with a covering the entire span of physiologicalT1andT2values. A second phantom containing a series of identiical samples served to study the depeendence of the apparmentT1andT2on position, both radially and axially, with respect to magnet isocneter. Reproducibility across the field of view was found to be better than 7% (T1andT2). This phantom was further used to evaluate effects of long‐term reproductibility, which at each location varied from 5–14% (T1) and 2–10% (T2). Finally, interinstrument reprducibilty, tested by means of the same protcol on three different insturments, all operating at the same magnetic field and using largely indentical hardware for each location, was found to be 1–14% (T1) and 2–10% (T2). The positional dependence of the apparent relaxation times appears to be systematic and may be due to variations in the effective field, caused by magnet and rf inhomogeneity. Finally, brain tissue relaxation and spin‐density data were determined using the same protocol in 37 scans performed on 27 normal volunteers. The tissues analyzed were putamen, thalamus, caudate nucleus, centrum semiovale, internal capsule, and corpus callosum. Excellent accordance was further obtained between left and right hemispheres.

ISSN:0740-3194    

DOI:10.1002/mrm.1910030502

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

年代:1986

数据来源: WILEY

摘要:

AbstractWe describe an improved perfusion system designed for NMR studies with13C‐labeled metabolites. Using this system we have monitored the uptake and metabolism of [13C]acetate and [13]Clglucose in pooled human lymphocytes cast in an agarose gel thread. © 1986 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910030503

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

年代:1986

数据来源: WILEY

摘要:

AbstractThe metabolic consequences of anoxia in the isolated, perfused guinea pig heart were examined by13C NMR spectroscopy of13C‐labeled metabolitesin situ. Upon addition of [3‐13C]pyruvate to the perfusate during normoxic conditions, label is detected in several metabolites, including alanine (C3), glutamate (C2, C3, and C4), and aspartate (C2 and C3), reaching steady state levels 10–15 min after the labeled precursor reaches the heart. During anoxia, the label in glutamate and aspartate decreases and label appears in C2(3) of succinate. This real‐time observation demonstrates that in the isolated intact heart, anaerobic metabolism of the amino acids aspartate and glutamate to succinate occurs. These pathways, which were first noted to occur in skeletal muscle of diving mammals, may provide a mechanism supplemental to glycolysis for the production of nucleoside triphosphates during periods of anoxia. © 1986 Academic Pr

ISSN:0740-3194    

DOI:10.1002/mrm.1910030504

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

年代:1986

数据来源: WILEY

摘要:

AbstractAn improved formalism describing the dependence of the signal strength on relaxation times and spin densities in pulsed nuclear magnetic resonance experiments is presented. It is especially applicable to repetitive experiments such as occur in imaging applications. Essentially, it includes long‐time spin coherence effects not considered in the algorithms usually adopted. Also, some generalizations of classical concepts are made. Validity criteria are studied using numerical simulations. The algorithm assumes a relatively inhomogeneous main magnetic field, but the computer simulations indicate that the inhomogeneity normally present in magnetic resonance imagers is sufficient. The simulations also indicate that the field gradient pulses applied in the spatial encoding procedure do not necessarily interfere seriously with the validity of the formulae, but complicate the choice of optimal formula and may in some circumstances prevent the spin system from reaching a steady state. © 1986 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910030505

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

年代:1986

数据来源: WILEY

摘要:

AbstractProton relaxation enhancement (PRE) values for fatty acid spin labels bound to human serum albumin have been investigated using the inversion‐recovery method at 24 MHz. At 0.1 mM protein concentration and a label‐to‐protein ratio of one‐to‐one, the PRE value for 12‐Doxylstearate‐albumin complex is 7.8 ± 2.3, whereas the PRE values for 5‐Doxylstearate and 16‐Doxylstearate‐albumin complexes are 1.5 ± 0.6 and 1.7 ± 0.7, respectively. Addition of 10‐fold excess of stearic acid reduced the PRE values nearly to 1, indicating that the strong enhancements arise from direct binding of fatty acid spin labels to human serum albumin. PRE values for all three labels exhibit maxima as a function of the label‐to‐protein ratio, suggesting multiple binding sites for fatty acid spin labels with labels in the tightest binding sites not resulting in the most effective relaxation. Based on the rates of reduction of ESR signal amplitudes by sodium ascorbate, the difference in PRE values for the three fatty acid spin labels bound to albumin is attributed to the difference in water accessibility of the nitroxide moieties at various positions along the acyl chain, being greater at the C‐12 position than at C‐5 or C‐16 position. The PRE value of 8 for 12‐Doxylstearate bound to human serum albumin indicates that this complex may be a suitable paramagnetic contrast agent forin vivoNMR

ISSN:0740-3194    

DOI:10.1002/mrm.1910030506

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

年代:1986

数据来源: WILEY

摘要:

AbstractIn 2D Fourier imaging the normal Carr‐Purcell multiple‐echo sequence generally leads to center line and mirror artifacts caused by imperfect rotations by the rf pulses. We describe a method to avoid these distortions using aphasealternating‐phase shift (PHAPS) sequence which also allows multiple‐slice and multiple‐echo imaging at the same time. Measuring phantoms with calibratedT2values, we have shown that the PHAPS imaging sequence leads to an accuracy of quantitativeT2determinations of better than 10%. Contrast‐enhanced images are presented which we calculated from multiple‐echo images and extrapolated to arbitrary echotimes, including negative ones. We believe that these improvements inT2imaging will result in a significant reduction of patient investigation time in magnetic resonance imaging. © 1986 Acade

ISSN:0740-3194    

DOI:10.1002/mrm.1910030507

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

年代:1986

数据来源: WILEY

摘要:

AbstractA method is proposed for measuring on the bench the NMR signal‐to‐noise ratio of rf probes, (over the range 1–100 MHz) and also the power deposited in patients during the imaging experiment. The technique is based on the principle of reciprocity, in that a direct relationship exists between the magnetic field generated (upon transmission) by a matched probe coil and the signal‐to‐noise ration delivered by the same coil when used as a receiver. The construction and use of a calibrated sense coil for measuring the field is described, and the precautions and theory necessary for accurate measurement and understanding are outlined. Finally, the method is verified by comparison with a direct spectral measure of sensitivity obtained from a small doped water sample placed in NMR imaging equipment. © 1986 Academic

ISSN:0740-3194    

DOI:10.1002/mrm.1910030508

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

年代:1986

数据来源: WILEY

摘要:

AbstractSome of the factors involved in the choice of field strength for NMR imaging are examined. The influences of relaxation times and chemical shift upon image quality and signal‐to‐noise ratio are highlighted, and power deposition is introduced as a significant factor which may limit the flexibility and information available at higher fields as long as 180° echo pulses continue to be necessary. Chemical‐shift imaging is examined and found wanting as a means of coping with chemical‐shift artifacts, and the use of multiple echoes (albeit with research) in conjunction with multiple‐slice techniques is advocated as representing an efficient data‐gathering scheme which can improve image signal‐to‐noise ratio. With such use, a medium field strength (0.5–1 T) is presented as representing, for general purpose imaging of head and torso, the best current compromise when imaging time is of major importance, with the important caveat that new techniques may always invalidate this conclusion. © 1986

ISSN:0740-3194    

DOI:10.1002/mrm.1910030509

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

年代:1986

数据来源: WILEY

摘要:

AbstractSmall mice (∼20 g) were anesthetized and placed in a loop‐gap resonator of diameter 25 mm resonating at 1.1 GHz. An oxygen‐permeable capsule containing 2.5 × 10−2Mperdeutero15N TEMPONE (1‐oxyl‐2, 2, 6, 6‐tetramethyl‐4‐piperidone) in light paraffin oil was implanted in the peritoneal cavity. Electron spin resonance (ESR) spectra from the capsule are sensitive to the concentration of dissolved oxygen and calibration curves are given. The oxygen concentration was found to rise from a value close to zero when the animal breathes air to a level of 220 μMwhen the animal breaths pure oxygen. It is speculated that this surprisingly high level is related to the effect of the anesthetic on the cardiovascular system. Encapsulation provides a barrier to spin‐label reductants and to paramagnetic metal ions that might confound the spin‐label oximetric measurements. ©

ISSN:0740-3194    

DOI:10.1002/mrm.1910030510

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

年代:1986

数据来源: WILEY

摘要:

AbstractThe magnetic field dependence of 1/T1over the range 0.01 to 50 MHz proton Larmor frequency (NMRD profile) is reported for water protons in solutions of horse spleen apoferritin, and of ferritin reconstituted at both low and high iron levels. The apoferritin results are in every way typical of diamagnetic spherical proteins of their size (K. Hallenga and S. H. Koenig,Biochemistry15, 4255 (1976)). Titration of up to 24 ferrous ions per protein molecule, with subsequent oxidation to ferric, shows a nonlinear saturating contribution to the NMRD profile which is interpreted as arising from a small number of ferric ions (six to eight) bound close to the outside of each ferritin molecule, and a comparable number of interior sites. The latter become multiply occupied as the core grows and do not contribute measurably to 1/T1in this state. The former sites are never more than singly occupied, and their contribution to the solvent proton relaxation rates is independent of the loading of the core. Measurements of 1/T2at 20 MHz are quite in accord with theoretical expectations for apoferritin and ferritin with up to 24 ferric ions per molecule. However, a marked increase in 1/T2is observed at higher iron loadings that we are unable to account for within the framework of the theory of outer sphere relaxation, even when the effects arising from inhomogeneities in the local magnetic field are included. A sample of human spleen hemosiderin was found to have the same 1/T1NMRD profile as a comparable sample of ferritin. © 1986 Academic Press, Inc

ISSN:0740-3194    

DOI:10.1002/mrm.1910030511

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

年代:1986

数据来源: WILEY