摘要:

AbstractNet forward adenosine triphosphate (ATP) production rates were calculated from31P nuclear magnetic resonance spectroscopy (MRS) kinetic data collected with 1–8 s time resolution during isometric voluntary contractions of the human gastrocnemius/soleus muscle group. Volume normalized muscle output (Newtons/ml) was then divided by the calculated net ATP use (mM/s) to estimate the metabolic economy (ME) (Newtons · s/μpmol ATP). The ATP production rates from anaerobic glycolysis (An Gly) and creatine kinase (CK) reactions are approximately half of the oxidative phosphorylation ATP production rates (Ox Phos) at the end of 90 s of isometric contractions for a series of force levels. However, ME was independent of the force level in these exercises. The correlation between MVC of healthy, trained subjects, and the maximal cross‐sectional area of the gastrocnemius/soleus shows an average of 21.2 ± 4.6 Newtons/cm2(mean ± SD,N= 15). This was measured using a foot pedal with a transducer measurement point 10.6 cm above the heel. [ADP] versus Ox Phos ATP production rate fits a Michaelis‐Menten kinetic control equation with an offset (underestimation of Ox Phos) of 0.3 m, Ws and aKm, = 27 μM for ADP and aVmax= 1.0 mM/s. This suggests that [ADP] is the controlling factor for mitochondrial function at the end of a 90 to 120 s isometric contraction in normal subjects at any force level. Quantitative measurements of the phosphorus metabolite concentrations were obtained from 20 individuals and these data are als

ISSN:0740-3194    

DOI:10.1002/mrm.1910320102

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe longitudinal relaxivities of seven water‐soluble nitroxide derivatives ot low‐molecular weight have been measured at 5°C and 37°C in water and in serum between 0.01 and 200 MHz. The nuclear magnetic relaxation dispersion (NMRD) profiles show a clear relationship between the relaxivity observed in serum and the relative balance of the hydrophobichydrophilic character of the paramagnetic molecules. From the data analysis, contributions arising from a population of nitroxides characterized by reduced mobility can be extracted. The values of the correlation times are consistent with a system involving nitroxides adsorbed at the surface of albumin and magnetically interacting with the protons of hydrogen bonded water mole

ISSN:0740-3194    

DOI:10.1002/mrm.1910320103

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

年代:1994

数据来源: WILEY

摘要:

AbstractWater proton spin‐lattice relaxation times are commonly used as a guide in establishing the off‐resonance irradiation time as well as the repetition time of the magnetization transfer experiment.T1discrimination effects occur if the motionally restricted spin bath longitudinal magnetization does not reach thermal equilibrium. In this study we developed the formalism necessary for the evaluation ofT1discrimination contributions to proton magnetization transfer arising from the use of a short repetition time relative to the spin‐lattice relaxation time of the motionally restricted spin bath. The results of computer simulation indicate thatT1, discrimination contributions occur when the repetition time is small relative to the spin‐lattice relaxation time of the motionally restricted spin bath, and when the off‐resonance irradiation is weak and far off‐resonance. For somewhat longer repetition times,T1discrimination contributions become important only when the cross relaxation rate is small, and the fractional amount of motionally restricted component large. The occurrence ofT1discrimination effects results in distortion of water proton intensity ratio dispersion curves thereby resulting in the estimation of erroneous magnetization transfer parameters, whereas in magnetization transfer contrast enhanced imaging, such contributions are manifested by a decrease in ima

ISSN:0740-3194    

DOI:10.1002/mrm.1910320104

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

年代:1994

数据来源: WILEY

摘要:

AbstractNew, rapid two‐ and three‐dimensional imaging sequences based on steady‐state gradient echoes and projection‐reconstruction (PR) techniques are proposed. Quantitative studies show that fast PR sequences and classical, fast gradient‐echo Fourier transform sequences lead to identical contrasts. In order to minimize inhomogeneity effects, a particular focus has been placed on echo‐time reduction. The use of a weighting window permits one to acquire severely truncated echoes; partial k‐space scanning may

ISSN:0740-3194    

DOI:10.1002/mrm.1910320105

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

年代:1994

数据来源: WILEY

摘要:

AbstractPhase velocity magnetic resonance imaging (MRI) has shown considerable potential for tracking distinct regions of the myocardium throughout the cardiac cycle. Phase contrast MR imaging produces multiple images, each phase encoded for velocity in a different direction, in which individual pixels depict the local motion of the tissue. In this work we present in detail three algorithms for tracking motion based on these images. Both simulated and phantom data are used to examine some of the problems encountered in practice in tracking points based on velocity maps. Solutions to these problems are offered when possible. The impact of noise and low order phase errors in the data on each of the three tracking approaches is examined. In addition, problems due to tissue expansion and contraction, to 2D versus 3D tracking, and to round off errors from motion which is small relative to pixel size or slice thickness, are considered. An example using data obtainedin vivois included to demonstrate the efficacy of the best of the three tracking algorithms in measuring left ventricular circumferential shortening preinfarct and postinfarct in a canine model.

ISSN:0740-3194    

DOI:10.1002/mrm.1910320106

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

年代:1994

数据来源: WILEY

摘要:

AbstractOff‐resonance spin locking is investigated as a low power method for achieving low field spin‐lattice relaxation contrast using high field clinical MR imaging systems (e.g., 1.5 tesla). Spin‐lattice relaxation times and equilibrium magnetizations in the off‐resonance rotating frame (T 1ρoffβ) were measured for tissue‐mimicking phantom materials as a function of the ratio of the amplitude to the resonance offset of the spin‐locking pulse (f1/Δ). The phantom materials consisted of vegetable oil to simulate fat and two different gels containing 2% and 4% agar to simulate nonfatty tissues with different macromolecular compositions. These measurements were used to verify a signal strength equation for a multislice off‐resonance spin‐locking technique implemented on a clinical MR imaging system operating at 1.5 tesla. Although the oil showed little change in image contrast with increasingf1/Δ, the two gels demonstrated a strong variation which provided improved discrimination compared toT1‐weighted imaging. Off‐resonance spin locking is suggested as a method for improving delineation of breast lesions and a preliminary clinical example from a patie

ISSN:0740-3194    

DOI:10.1002/mrm.1910320107

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

年代:1994

数据来源: WILEY

摘要:

AbstractSaturation transfer contrast (STC) techniques have been employed to decrease the signal intensity of observed water by saturating a broad proton component. The STC pulse, generally spatially nonselective, reduces the signal of brain background tissues as well as inflowing blood in 3D time‐of‐flight (TOF) angiography. We present a novel technique, a slice‐selective off‐resonance sinc pulse (SORS), which is employed to enhance inflowing blood and to suppress the signal of stationary background tissues. The STC effects of the SORS and other spatially nonselective methods influencing inflowing blood is discussed in detail. Angiograms using the SORS method are also presented to demonstrate improved vessel conspicuity. In addition, the SORS method is applicable to imagers with a whole‐body RF transmitt

ISSN:0740-3194    

DOI:10.1002/mrm.1910320108

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

年代:1994

数据来源: WILEY

摘要:

AbstractTo evaluate the sensitivity of the intravoxel incoherent motion (IVIM) technique to changes in cerebral blood flow, we made measurements of IVIM parameters in rat brain under conditions of altered arterial pCO2. The arterial pCO2, was varied over a range which would be expected to change cerebral blood flow from roughly 50 to 500 ml/(100 g·min). The IVIM measurements were made with suppression of extravascular water signal. The parametersf′ (the apparent fraction of spins which have “fast” pseudodiffusion), D 1*(the “fast‐pseudodiffusion” coefficient), and D 2*(the “slow‐pseudodiffusion” coefficient) all showed statistically significant positive linear correlations with arterial pCO2. These results suggest that the IVIM method, when used with suppression of extravascular water signal, is sensitive to

ISSN:0740-3194    

DOI:10.1002/mrm.1910320109

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe image intensity in many contrast agent perfusion studies is designed to be a function of bulk tissueT1, which is, in turn, a function of the compartmental (vascular, interstitial, and cellular)T1s, and the rate of proton exchange between the compartments. The goal of this study was to characterize the compartmental tissue Gd‐DTPA relaxivities and to determine the proton exchange rate between the compartments. Expressing [Gd‐DTPA] as mmol/liter tissue water, the relaxivities at 8.45 T and room temperature were: saline, 3.87 ± 0.06 (mM. s)−1(mean ± SE;n= 29); plasma, 3.98 ± 0.05 (mM·s)−1(n= 6); and control cartilage (primarily an interstitium), 4.08 f 0.08 (mM·s)−1(n= 17), none of which are significantly different. The relexivity of cartilage did not change with compression, trypsinization, or equilibration in plasma, suggesting relaxivity is not influenced by interstitial solid matrix density, charge, or the presence of plasma proteins.T1relaxation studies on isolated perfused hearts demonstrated that the cellular‐interstitial water exchange rate is between 8 and 27 Hz, while the interstitial‐vascular water exchange rate is less than 7 Hz. Thus, for Gd‐DTPA concentrations, which would be used clinicallly, theT1relaxation rate behavior of intact hearts can be modeled as being in the fast exchange regime for cellular‐interstitial exchange but slow exchange for interstitial‐vascular exchange. A measured relaxivity of 3.82 ± 0.05 (mM·s)−1(n = 8) for whole blood (red blood cells and plasma) and 4.16 ± 0.02 (mM·s)−1(n = 3) for frog heart tissue (cells and interstitium) (withT1and Gd‐DTPA concentration defined from the total tissue water volume) supports the conclusion of fast cellular‐extracellular exchange. Knowledge of the Gd‐DTPA relaxivity and maintaining Gd‐DTPA concentration in the range so as to maintain fast cellular‐interstitial exchange allows for calculation of bulk Gd‐DTPA concentration from bulk tissueT1within a ca

ISSN:0740-3194    

DOI:10.1002/mrm.1910320110

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

年代:1994

数据来源: WILEY

摘要:

AbstractPrevious theoretical reports described the dependence of interpretation of the observed phase of the NMR signal on the time origin(s) of moment calculations and position's Taylor series expansion. This work provides experimental confirmation of predictions derived from that theory. For accelerative motion, experimental phase‐encoded velocity measurements give instantaneous values at a time corresponding to the origin used for waveform momemt calculations. For laminar flow, experimental intensity profiles agree well with theoretical simulations; new findings extend amplitude and spatial distributions of oblique flow profiles beyond previous descriptions. Experiments using sequences with controlled position of the time origins for phase and read axes show that displacement and motion artifacts are reduced when they're coincident (pulsed flow, nongated acquisitions), and virtually eliminated when combined with gating. Potentially significant clinical consequences of coincident and noncoincident time origins are demonstrated in human head MIP MRA images. These results have fundamental implications in waveform desig

ISSN:0740-3194    

DOI:10.1002/mrm.1910320111

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

年代:1994

数据来源: WILEY