摘要:

AbstractThe formation of glycogen in the liver of normal volunteers was followed noninvasively with13C manetic resonance spectroscopy (MRS) under tow different conditions; a) intravenous infusion of [1‐13C] glucose under hyperglycemic and hyperinsullinemic clamp conditions, and b) oral Intake of glucose in the form of a bolus. For the intravenous infusion, [1‐13C]glucose with an enrichment level of 99% was employed. The C1signals of α‐ and β‐glucose could be detected in the human liver already after an infusion period of 8 min. However, an increase in the glycogen signal was observed only after a prolonged infusion of about 60 min. Changes in the glycogen signal correlated well with the time course of insulin and glucagon during the measurement. Experiments showed also that liver glycogen formation in man can be followed noninvasively by13C‐MRS using nonlabeled glucose or [1‐13C]glucose with a low level of enrichment (6.6%). The use of nonlabeled glucose may therefore simplify the quantitation of net liver glycogen synthesis since it can be based directly on changes in the natural abundance13C MRS glycogen signal, avoiding label dilution through the various metabolic pathways of glucose. The glucose uptake, estimated from the increase in the glycogen signal, was consistent with findings from more complex and invasive studies of glucose uptake in the liver. The average liver glycogen concentration in 12 h overnight fasted volunteers (n = 18) without any special dietary preparation was assessed to be 229 ± 34 mM (minimum = 160 mM; max

ISSN:0740-3194    

DOI:10.1002/mrm.1910290502

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

年代:1993

数据来源: WILEY

摘要:

AbstractA method has been developed to minimize baseline roll in chemical shift imaging (CSI). The technique is fully automated and employs knowledge based data processing in the frequency domain. The key feature of the algorithm is the computation of the “trough” and “ripple” components in the CSI data. The baseline roll can be regarded as an artifact that appears as a result of the summation of several sinc functions. Using prior knowledge, a mirror component corresponding to the artifact is created and added to the delayed spectrum. The method compensates for noise and zero‐order phase error when computing the roll artifact. The results obtained on implementing the baseline roll minimization procedure on simulated time‐delayed spectra indicated that the peak heights and areas were between 91% and 97% in magnitude when compared with the same peaks in the nondelayed spectra. The correction procedure was also assessed on clinicalin vivospectra. Nonlocalized31P MR spectra of the liver were obtained with and without an acquisition delay of 2.1 ms, and the time delayed spectra subjected to the baseline minimization routine. Metabolite peak heights and areas in the corrected spectra were approximately 94% in magnitude when compared with the same peaks in the original nondelayed whole volume spectra. Implementation of the baseline minimization procedure onin vivolocalized spectra with varying signal to noise ratios produced good results. It takes approximately 13 s to implement the baseline roll minimization procedure. In this paper, the technique will be referred to as BaseLine Artifact Suppression Technique (BLA

ISSN:0740-3194    

DOI:10.1002/mrm.1910290503

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

年代:1993

数据来源: WILEY

摘要:

AbstractWe have previously described a novel monocrystalline iron oxide nanocompound (MION), a stable colloid that enables target specific MR imaging. In this study, the physicochemical properties of MION are reported using a variety of analytical techniques. High resolution electron microscopy indicates that a MION consists of hexagonal shaped electron‐dense cores of 4.6 ± 1.2 nm in diameter. This iron oxide core has an inverse spinel crystal structure which was confirmed by x‐ray powder diffraction. Chemical analysis showed that each core has 25 ± 6 dextran molecules (10 kD) attached, resulting in a unimodal hydrodynamic radius of 20 nm by laser light scattering. Because of the flexibillity of the dextran layer, the radius is only 8 nm in nonaqueous reverse micelles. At room temperature, MION exhibit superparamagnetic behavior with an induced magnetization of 68 emu/g Fe at 1.5 T. Mössbauer studies show that the saturation internal magnetic field is 505 KOe, and blocking temperature is at 100 K. The R1 relaxivity of MION is 16.5 (mM‐sec)‐1and the R2 relaxivity is 34.8 (mM‐sec)‐1in aqueous solution at 37°C and 0.47 T. In vitro phantom studies show that the detectability of MION in liver tissue is less than 50 nmol Fe/g tissue using gradient echo ima

ISSN:0740-3194    

DOI:10.1002/mrm.1910290504

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

年代:1993

数据来源: WILEY

摘要:

AbstractIntracellular free calcium concentration ([Ca2+]i) was measured in Langendorff‐perfused ferret hearts (30°C, pH 7,4) by loading paced hearts with the19F NMR calcium indicator, the 5,5′‐difluoro derivative of 1,2‐bis(o‐aminophenoxy) ethane‐N,N,N′,N′‐tetraacetic acid (5FBAPTA), to an initial cytosolic concentration of approximately 120 uM. Increasing the pacing frequency raised the end‐diastolic [Ca2+]i, from 299 ± 44 nM (mean ± SEM) at 0.2 Hz to 522 ± 54 nM at 1.0 Hz and 691 ± 166 nM at 2.0 Hz. Raising [Ca]ofrom 1.8 to 7.0 mM at a pacing frequency of 1.0 Hz increased end‐diastolic [Ca2+]i to 625 ± 39 nM. In unpaced hearts perfused with diltiazem (100 uM), [Ca2+]i fell rapidly to a steady‐state value of<100 nM after 60 min. Raising [Ca]ofrom 1.8 to 7.0 mM had no detectable effect on resting [Ca2+]i. The time course of the [Ca2+]i transient was measured in hearts paced at 1.1 Hz and perfused with 1.8 mM [Ca]o. The peak [Ca2+]i was ∼ 2 uM at approximately 150 msec after the pacing pulse, and peak developed LVP occurred at 550 msec compared with 280 msec in control hearts not loaded with 5FBAPTA. Comparisons with data obtained by other techniques, including fluorescent [Ca2+]i indicators, imply that although the end‐diastolic [Ca2+]i values obtained with 5FBAPTA in beating hearts are elevated by the concentrations of intracellular 5FBAPTA required for signal detection, the changes in [Ca2+]i observed in response to experimental interventions are qualitativ

ISSN:0740-3194    

DOI:10.1002/mrm.1910290505

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

年代:1993

数据来源: WILEY

摘要:

AbstractA simple, linear kinetic model has been developed for the noninvasive assessment of capillary permeability to macromolecules in the rat by dynamic magnetic resonance imaging using albumin‐Gd‐DTPA. Data required by the model are signal intensity responses from a target tissue and a venous structure such as inferior vena cava before and after bolus intravenous injection of albumin‐Gd‐DTPA. Additional requirements include an early temporal resolution of approximately one image/min and a blood sample for hematocrit. The model does not require measurement of albumin‐Gd‐DTPA concentration in either arterial or venous blood. Pilot experiments suggest that this technique is adequate for estimation of the fractional leak rate of macromolecules from plasma to interstitial water as well as tissue plasma volume, the product of which yields a measure of the permeability surface area product of the tissue if the extraction fraction is modest (<0.2). The technique may be generally applicable to the study of abnormal capillary permeability in humans as well

ISSN:0740-3194    

DOI:10.1002/mrm.1910290506

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

年代:1993

数据来源: WILEY

摘要:

AbstractWith the increasing use of three‐dimensional MRI techniques it is becoming necessary to explore automated techniques for locating pathology in the volume images. The suitability of a specific technique to locate and identify healthy tissues of the brain was examined as a first step toward eventually identifying pathology in images. This technique, called multispectral image segmentation, is based on the classification of tissue types in an image according to their characteristics in various spectral regions. The spectral regions chosen for this study were the hydrogen spin‐lattice relaxation time T1spin‐spin relaxation time T2, and spin density, ϱ. Single‐echo, spin‐echo magnetic resonance images of axial slices through the brain at the level of the lateral ventricles were recorded on a 1.5 Tesla imager from 20 volunteers ranging in age from 17 to 72 years. These images were used to calculate the T1, T2, and ϱ images used for the classification. Tissue classification was performed by locating clusters of pixels in a threedimensional T1‐1‐T2‐1‐ρ histogram. Gray matter, white matter, cerebrospinal fluid, meninges, muscle, and adipose tissues were readily classified in magnetic resonance images of the volunteers with a single set of T1, T2, and ρ values. Cluster characteristics, such as size, shape, and location, provided information on the imaging procedure and t

ISSN:0740-3194    

DOI:10.1002/mrm.1910290507

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

年代:1993

数据来源: WILEY

摘要:

AbstractA subtractive time‐of‐flight technique for magnetic resonance anglography is described. In this approach, the arterial supply to an organ is inverted in a steady‐state fashion by applying off‐resonance irradiation in the presence of a linear magnetic field gradient. An angiogram is formed by subtracting an image acquired with arterial inversion from a control image acquired with no arterial inversion. A single coil is used to apply both the inversion and observation pulses. Intracranial angiograms obtained from normal volunteers using a two‐dimensional projective implementation of this technique at 1.5 T illustrate excellent small vessel detail and background su

ISSN:0740-3194    

DOI:10.1002/mrm.1910290508

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

年代:1993

数据来源: WILEY

摘要:

AbstractAn ECG‐triggered, two‐sequence MRI technique is proposed for the precise measurement of proton T1relaxation times of the human myocardium at a field strength of 0.5 T. The combination of an inversion recovery (IR) sequence and a spin echo (SE) sequence is not new. It is, however, rarely used in quantitativein vivocardiac studies. Our approach employs a synchronization of the 90° read pulse to the systolic period. In a study of 22 healthy volunteers, the globally measured T1value was estimated to be 714 ± 23 ms. Four of the volunteers also underwent additional imaging scans for the purposes of reproducibility assessment. The T1precision was found to be 3.9 ± 1.1% for the IR/SE combination and 16.9 ± 5.3% for a combination of SE sequences. Total imaging time for the IR and SE sequences was 19.2 ± 3.0 mins. The relative rapidity of this classic technique and the T1precision obtained give this technique an obvious application in the discrimination of normal and diseased myocardium. In the same study, valuable supplementary tissue characterization is provided by T2, calculated from the SE sequence. T2was evaluated to be 5

ISSN:0740-3194    

DOI:10.1002/mrm.1910290509

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

年代:1993

数据来源: WILEY

摘要:

AbstractTraditionally, the method of nonlinear least squares (NLLS) analysis has been used to estimate the parameters obtained from exponential decay data. In this study, we evaluated the use of Bayesian probability theory to analyze such data; specifically, that resulting from intravoxel incoherent motion NMR experiments. Analysis was done both on simulated data to which different amounts of Gaussian noise had been added and on actual data derived from rat brain. On simulated data, Bayesian analysis performed substantially better than NLLS under conditions of relatively low signal‐to‐noise ratio. Bayesian probability theory also offers the advantages of: a) not requiring initial parameter estimates and hence not being susceptible to errors due to incorrect starting values and b) providing a much better representation of the uncertainty in the parameter estimates in the form of the probability density function. Bayesian analysis of rat brain data was used to demonstrate the shape of the probability density function from data sets of different qual

ISSN:0740-3194    

DOI:10.1002/mrm.1910290510

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

年代:1993

数据来源: WILEY

摘要:

AbstractThe feasibility of the determination of the time‐average of pulsatile velocity obtained via a nontriggered magnetic resonance (MR) acquisition is studied. The advantage of this method, in comparison with a triggered acquisition, is a considerable reduction (≈15×) in acquisition time. However, pul‐satility causes image artifacts, known as ghosts, and the Fourier transform technique required for the imaging procedure accomplishes time‐averaging of thecomplexMR signal. Both effects can result in errors in the velocity determined. Calculations show that these errors depend on the velocity time function and the acquisition parameters.In vivocomparison of triggered and nontriggered MR velocity measurements in the femoral artery of volunteers (n = 7) shows larger statistical and systematic errors in the latter, which depend on the excitation angle. Therefore, this nontriggered average velocity measurement is only useful as a fast and rough estimation of the time‐average

ISSN:0740-3194    

DOI:10.1002/mrm.1910290511

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

年代:1993

数据来源: WILEY