摘要:

AbstractThe artifacts associated with finite sampling in magnetic resonance imaging are shown to be significant when the ratio of half the sampling time,Ts/2, to transverse relaxation time,T2is on the order of unity or greater. For both symmetric and asymmetric sampling, these artifacts include enhanced high spatial frequency image intensity (bright edges), induced systematic noise, loss of resolution, and changes in phase. Simple filtering of the raw data does not remove the problem in tissues with positiondependentT2values. An exact deconvolution method is described to remove these problems associated with finite position‐dependentT2values. © 1987 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910040502

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

年代:1987

数据来源: WILEY

摘要:

AbstractIt has been shown previously that for constant magnetic field gradients, constant velocity flow leads to even‐echo rephasing for all echo delay times. We show that for flow which is not pluglike, even‐echo rephasing also occurs for the pulsed readout gradients used in magnetic resonance imaging if and only if the gradients begin at the time the 90° pulse is applied. We also show for these gradients that even‐echo rephasing for all echo delay times in the case of nonpluglike flow implies a constant flow velocity at the point considered. Furthermore, it suffices to assume the vanishing of the spin‐echo phase for any even echo, since the vanishing of any even echo for all echo delay times implies that all other even echoes also vanish identically. The odd echoes are then all equal to each other and proportional to the flow velocity. If acceleration is present, it may then be seen that for nonpluglike flow, even‐echo rephasing may only be present for some but not all echo delay times.However, for the typicall slice selection gradients used in magnetic resonance imaging or for usual readout gradients starting after the 90° pulse is applied, it is shown that for constant velocity flow the even‐echo phases do not vanish identically. Hence, rephasing cannot always ocur for nonpluglike flow in either of these situations. Furthermore, the spin‐echo phases are proportional to the flow in velocity. The converse is also showh; namely, if the phase of any even echo at given point is proportional to the velocity at the point considered for all echo delay times, then the flow velocity is constant at the point. Furthermore, the phases of all even echoes are also equal to each other and proportional to the flow velocity at that point. In addition, the odd echoes are then also equal to each other and proportional to the flow velocity, but with a proportionally constant different from that of the even echoes. © 1987 Aca

ISSN:0740-3194    

DOI:10.1002/mrm.1910040503

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

年代:1987

数据来源: WILEY

摘要:

AbstractSpatially resolved spectroscopy (SPARS) is a method for obtaining high‐resolution NMR spectra of well‐defined volumes of human tissuesin situ. This method was combined with the Carr‐Purcell‐Meiboom‐Gill multiple‐echo sequence forT2, and with inversion recovery forT1relaxation measurements. Relaxation times obtained by SPARS were compared with standard CPMG and IR relaxation measurements and with imaging methods, using a number of relaxation phantoms. Spectroscopically resolved relaxation data of human bone marrow, muscle, and adipose tissuein situwere obtained.T2measurements of human adipose tissue gave different results when using single‐echo measurements rather than the CPMG method. This difference was interpreted as a J modulation effect, which shows up in fatty acid proton resonances. This J modulation effect influences the intensity of adipose tissue in routine1H NMR images. © 1987 Acade

ISSN:0740-3194    

DOI:10.1002/mrm.1910040504

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

年代:1987

数据来源: WILEY

摘要:

AbstractStudies were performed on 16 cats to evaluate the potential protective effects of Gallopamil on brain ischemia. Brain energy state was determined by31P NMR and lactate concentration was determined by1H NMR. Double‐tuned surface coils (tuned to 35.8 and 88.4, respectively) were placed on the head after skin and muscle were removed from the calvarium. A 2.1‐T, 25‐cm‐bore Oxford magnet interfaced to a Phosphoenergetics 250‐80 spectrometer was used. The cats were bled to 50 mm Hg for 10 min with subsequent application of bilateral carotid occlusion for 10 min to produce ischemia. In all animals, brain energy state as measured byP/PCrand lactate concentrations were determined over 5‐min intervals (before, during, and after the onset of ischemia). While Gallopamil did not prevent decreases in brain energy state or attenuate the rise in lactate concentration seen during ischemia, brain from animals treated with Gallopamil had a more rapid return of pHito baseline during the recovery period. In Gallopamil‐treated cats, higher levels of lactate were necessary to cause a similar decrease in pHiwhen compared to controls. The rate of lactate recovery to baseline levels was similar in both groups (control = −0.38 ± 0.14 mM/min; Gallopamil = −0.44 ± 0.32 mM/min). In conclusion, Gallopamil appears to lessen the acidosis caused by cerebral ischemia. In addition, we have demonstrated that multinuclear NMR spectroscopy is a powerful tool to study the effects of drugs on cerebral metabolism. © 1987

ISSN:0740-3194    

DOI:10.1002/mrm.1910040505

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

年代:1987

数据来源: WILEY

摘要:

AbstractA new echo‐time‐encoded chemical‐shift imaging technique applicable to dual‐peak spectroscopic imaging with large magnetic field inhomogeneity is proposed and studied. The basic concept and its applications to modest field homogeneity (⋍3.0 ppm) as well as to relatively large field inhomogeneity (⋍10.5 ppm) are discussed. Actual pulse sequences are given and some experimental results on human volunteers obtained with a 2.0‐T KAIS NMR system are also presented. © 1987 Acade

ISSN:0740-3194    

DOI:10.1002/mrm.1910040506

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

年代:1987

数据来源: WILEY

摘要:

AbstractSpin‐echo methods have been used to effect solvent suppression during the collection of 400 and 500 MHz1H NMR spectra of urine samples containing urea in the concentration range 0.7–1.2M. There was a marked trough of waterT 2obsvalues in the pH* range 2.6 to 4.1, due to acidcatalyzed proton exchange between urea and the solvent water. Minimum values ofT 2obs(9 ms at 500 MHz) of water were obtained at about pH* 3.2 in urine samples containing 1.2Murea. However, good solvent suppression factors were obtained with endogenous urea concentrations of only 0.7M. There was a strong field dependence on the water,T2over the pH range 2 to 4. Attenuation of the water signal was estimated to be at least an order of magnitude greater at 500 MHz rather than at 400 MHz. Using the Cam‐Purcell‐Meiboom‐Gill spin‐echo pulse sequence, water signals broader than 13 Hz were attenuated by factors of>103, and factors of>104were possible with water linewidths greater than 20 Hz, thus allowing the signals from metabolites in the sample to be efficiently digitized. This approach allowed metabolite signals at or near the water resonance to be observed without serious distortion of their intensities. The utility of this approach to solvent suppression in real systems is illustrated by observation of paracetamol and benzyl penicillin metabolites in human urine samples. © 1987 Aca

ISSN:0740-3194    

DOI:10.1002/mrm.1910040507

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

年代:1987

数据来源: WILEY

摘要:

AbstractPrincipal component analysis (PCA), a common tool from multivariate statistical analysis, has been implemented into the computer display system of a MR imaging device. PCA allows the calculation of images in which the information in a defined region of interest inherent in the basic acquired images is condensed. PCA image calculation has been applied to acquired MR studies of 13 patients with brain lesions. The appearance of the brain lesions on the resultant PCA images was scored in comparison to the acquired images before and after administration of Gd‐DTPA as well as to other calculated images includingT1,T2hydrogen density, and contrast‐optimized images. The conspicuity of a lesion and the number of distinguishable components within a lesion were slightly supenor on PCA than on the acquired images. PCA is an analytical tool for MR imaging that should be helpful in revealing information that is inherent in, but not readily visible on, standard acquired MR images. © 1987 Academic Press,

ISSN:0740-3194    

DOI:10.1002/mrm.1910040508

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

年代:1987

数据来源: WILEY

摘要:

AbstractBed rest is used to simulate the effects of weightlessness on human physiology. A spinecho procedure was used to image the lower leg of 15 normal male volunteers before and after 5 weeks of horizontal bedrest. In addition to noninvasively measuring muscle size changes, accurateT2images were produced to investigate possible relaxation time changes immediately (2–4 h) and 1–2 days after bed rest. Subcutaneous fat showed no change inT2, bone marrow showed a decrease, and muscle showed no change immediately after bed rest but increased 1–2 days following reambulation. © 1987 Academic Pres

ISSN:0740-3194    

DOI:10.1002/mrm.1910040509

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

年代:1987

数据来源: WILEY

摘要:

AbstractAdiabatic 180° inversion rf pulses can be converted to 180° refocusing pulses by inverting the effective field midway during the pulse. Suitable pulses retain insensitivity to large variations in the rf field strength and act over a reasonable spectral width. Such pulses may find application in studies utilizing surface coils. © 1987 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910040510

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

年代:1987

数据来源: WILEY

摘要:

AbstractNatural‐abundance13C NMR imaging of animal tissues is demonstrated using high‐field (4.7‐T) and surface coil techniques. Undecoupled images of an unfertilized egg yolk and an oxtail are presented. Analysis indicates the images map mainly methylene carbon or lipid signal intensity. © 1987 Academic Pres

ISSN:0740-3194    

DOI:10.1002/mrm.1910040511

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

年代:1987

数据来源: WILEY