摘要:

AbstractA modular, user‐friendly computer simulation of NMR imaging was created for the DEC VAX computer. This simulation is novel in that complex imaging phenomena, such as chemical shifts and flow, may be easily studied. Other features include spin warp and backprojection imaging, movie generation, and simulation of image contrast, composite pulse sequences, motion artifacts, aliasing, selective excitation, field inhomogeneity, noise and signal averaging. The simulation incorporates fast Fourier transform and graphics modules for processing and display of one‐and two‐dimensional images. Examples which illustrate features of the program are shown. The simulation is a useful tool for both the sophisticated and the beginning user of NMR imaging. It serves as a drawing board for studying MRI phenomena and a teaching program for demonstrating basic techniques. © 1986 Academic Pres

ISSN:0740-3194    

DOI:10.1002/mrm.1910030302

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

年代:1986

数据来源: WILEY

摘要:

AbstractWith increasing field strength for whole‐body MR scanners there is a need for rf pulses of higher bandwidth to perform selective excitation or inversion. AM pulses will have to become shorter thus leading to an enormous requirement for the peak power of the power amplifier. To reduce these requirements the use of frequency modulation is investigated. The effect of pulses based on a linear frequency sweep on the spin system is analyzed by calculation and computer simulation. It turns out that the nonlinear phases induced by those pulses may compensate each other, that therefore spin‐echo experiments as usual in MR imaging may be performed, and that the required peak power may be reduced by up to a factor of 50. © 1986 Academic Press,

ISSN:0740-3194    

DOI:10.1002/mrm.1910030303

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

年代:1986

数据来源: WILEY

摘要:

AbstractThis study was to determine if manipulation of magnetic resonance signal intensity by means of an intravenously injected paramagnetic contrast agent is useful for the detection and characterization of periarticular inflammation. Arthritis was induced in 20 rats by means of intradermal injection of Freund's complete adjuvant. MR imaging was performed with a resistive magnet operating at 0.35 T. A double spin‐echo technique withTE's of 28 and 56 ms andTR's of 0.5 and 2.0 s was used. The hindpaws of the adjuvant‐injected rats were imaged on Day 8, Day 11, or Day 15 following injection of the adjuvant. The images were obtained in the transverse plane before and after intravenous injection of gadolinium‐DTPA (0.2 mmol/kg). Because of their longT2relaxation time, inflammatory lesions were characterized by high MR signal intensity on precontrast images obtained with longTRand longTE(T2‐weighted images). On the other hand, because of their longT1relaxation time, the inflammatory lesions were of relatively low intensity and not easily recognized on precontrast images obtained with shortTRand shortTE(T1‐weighted images). PostcontrastT1‐weighted images were also sensitive in detecting periarticular inflammation as a result ofT1shortening by the gadolinium‐DTPA. However, in our particular model, the data did not indicate any greater MR sensitivity for detecting arthritis by means of gadolinium‐DTPA enhancement. © 1986 Aca

ISSN:0740-3194    

DOI:10.1002/mrm.1910030304

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

年代:1986

数据来源: WILEY

摘要:

AbstractIn principle, multiple‐echo magnetic resonance imaging (MRI) can be used to estimate the spin‐spin relaxation time,T2, which can then be used for quantitative tissue characterization. Although multiple echoes can be used to enhance the signal‐to‐noise ratio in an image, by echo addition, rf pulse imperfections modify the echo amplitudes resulting in significant errors in the estimate ofT2. Imperfect 180° pulses do not completely invert the transverse magnetization so that the magnitude of the transverse component is reduced and a longitudinal component is generated. Successive application of such imperfect pulses generates many components that interact in a complex manner. The amplitudes of successive echoes are affected whenever the transverse components refocus, whereas the longitudinal components may be rotated into the transverse plane by subsequent pulses and may often add to the image signal or give rise to an image artifact. These effects have been analyzed theoretically and have been demonstrated for a wide range of rf pulse imperfections using both simple and composite pulses, through computer simulations based on the numerical solution of the Bloch equations. The theoretical and simulation results have been substantiated through experiments performed on a mineral oil phantom using a resistive prototype MR scanner operating at 6.35 MHz. In this paper we report the effects of improper pulse amplitude or duration for nonselective rf pulses on resonance. We separately describe the other types of imperfections caused by off‐resonance effects and the use of selective pulses. © 1986 Academic

ISSN:0740-3194    

DOI:10.1002/mrm.1910030305

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

年代:1986

数据来源: WILEY

摘要:

AbstractParameter‐selective imaging for tissue characterization is performed at low magnetic fields (0.24–0.28 T) employing echo trains with a large number of echoes (≦50), high‐power pulses, and the projection reconstruction method. Preprocessing of the raw echoes is important for the quality of the echo images which are the bases for further evaluation and generation of parameter‐selective images. Two new algorithms are implemented in the image preprocessing routine TOMIKON: Echo shape analysis and image size adjustment. Echo shape analysis allows the detection and the removal of spikes in the echo signals to avoid image artifacts. Image size adjustment is performed by expanding and contracting the time basis of the echoes resulting in a size normalization of the echo images, an indispensable condition for generating diffusion images. © 1986 Academic

ISSN:0740-3194    

DOI:10.1002/mrm.1910030306

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

年代:1986

数据来源: WILEY

摘要:

AbstractThe effect of induced field inhomogeneity (IFI) on transverse NMR relaxation of water protons in tissue has been investigated by examining the field dependence of the effective transverse relaxation rates (1/T2eff) for in vitro canine brain tissue samples. At fields of 0.47, 2.35, 7.05 T (corresponding to 20, 100, and 300 MHz, respectively) the transverse relaxation rates for both white and gray matter samples follow a field dependence of the form 1/T2eff = C0+C1B0, whereB0is the applied field. The linearly dependent term,C1B0, which reflects the IFI contribution, does not contribute much (i. e., less than 20%) at fields less than 2.0 T. However, at greater field strengths the contribution is appreciable, e. g., greater than 60% at 7.0 T. Results from model systems of glass beads are also reported to illustrate IFI effects. For both the model systems and canine brain tissue samples, the effects of restricted diffusion are qualitatively evident in Hahn spin‐echo experiments. © 1986 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910030307

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

年代:1986

数据来源: WILEY

摘要:

AbstractThein vivodog brain31P NMR spectrum has a large peak in the phosphodiester region accounting for more than 35% of the total observable phosphorus metabolites. It is possible to reduce the intensity of this peak by off‐resonance saturation. To characterize the nature of this peak, extracts of dog brain frozenin situwere analyzed by high resolution31P NMR. ATP, phosphocreatine, inorganic phosphate, and phosphomonoesters were recovered in appropriate amounts in the methanol: HCI extract. However, acid soluble phosphodiesters accounted for only 8% of the observable phosphorus. More NMR observable phosphodiesters were selectively recovered following CHCI3:methanol extraction of phospholipids. These results suggest that thein vivophosphodiester resonance has substantial contributions from a fraction of mobile brain phospholipids. © 1986 Academic Press, I

ISSN:0740-3194    

DOI:10.1002/mrm.1910030308

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

年代:1986

数据来源: WILEY

摘要:

AbstractLarge unilamellar liposomes containing the paramagnetic contrast agent manganese chloride were shown to selectively enhance the intensity of the proton NMR signal from the livers of experimental mice. The kinetics of the observed effect indicate that the liposomes are rapidly removed from the circulation and deposited in high concentration within the liver. This process is known to involve the phagocytic activity of macrophages. These results show that liposomes could provide a useful and efficient vehicle for delivering NMR contrast agents to organs like the liver, spleen, and bone marrow. It may also be possible to use liposomes to selectively deliver contrast agents to tissue regions of developing pathology that are rich in macrophage activity. © 1986 Academic Press, Inc

ISSN:0740-3194    

DOI:10.1002/mrm.1910030309

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

年代:1986

数据来源: WILEY

ISSN:0740-3194    

DOI:10.1002/mrm.1910030310

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

年代:1986

数据来源: WILEY

摘要:

AbstractA surface coil on the neck causes adiabatic fast passage in blood as it flows by through a magnetic field gradient. This allows separation of blood and stationary tissue images of the head. Coronal and sagittal images of a volunteer are presented showing the vertebral and common, internal and external carotid arteries in projection views. © 1986 Academic Press, Inc

ISSN:0740-3194    

DOI:10.1002/mrm.1910030311

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

年代:1986

数据来源: WILEY