摘要:

Abstract1H NMR localized spectroscopy (STEAM) can assess unambiguously the presence of free chemically unchanged silicone in animal tissue after injection of silicone oil. Although the signal—to—noise ratio obtained in1H imaging is sufficient to detect the distribution of relatively large amounts of siliconein vivo, the specificity of silicone detection can be improved by using1H localized spectroscopy techniques. The sensitivity of the STEAM experiments is sufficient to detect silicone at a concentration of 0.5% in a voxel of 27 mm3. Preliminary results from rats with silicone gel—filled implants show no detectable amounts of silicone in sites such as lymph nodes, the liver or the spleen, 3 or 6 months after implant

ISSN:0740-3194    

DOI:10.1002/mrm.1910300202

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

年代:1993

数据来源: WILEY

摘要:

AbstractThe time course of proton spectra of meningloma after embolization has been followed using a localized PRESS experiment with 135 ms of echo time. The most conspicuous findings were the observation of transient lactate signal within 24 h after blocking of the capillary bed. After that time large aliphatic signals from necrosis were observed, whereas other metabolite signals vanished. It was demonstrated, that tumor necrosis as shown by proton spectroscopy is complete within 4 days. This finding is significant for the timing of surgery, which can be performed after this period.

ISSN:0740-3194    

DOI:10.1002/mrm.1910300203

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

年代:1993

数据来源: WILEY

摘要:

AbstractImage processing strategies for functional magnetic resonance imaging (FMRI) data sets acquired using a gradient‐recalled echo‐planar imaging sequence are considered. The analysis is carried out using the mathematics of vector spaces. Data sets consisting ofNsequential images of the same slice of brain tissue are analyzed in the time‐domain and also, after Fourier transformation, in the frequency domain. A technique for thresholding is introduced that uses the shape of the response in a pixel compared with the shape of a reference waveform as the decision criterion. A method is presented to eliminate drifts in data that arise from subject movement. The methods are applied to experimental FMRI data from the motor—cortex and compared with more conventional image—subtraction methods. Several finger motion paradigms are considered in the context of the various image processing strategies. The most effective method for image processing involves thresholding by shape as characterized by the correlation coefficient of the data with respect to a reference waveform followed by formation of a cross‐correlation image. Emphasis is placed not only on image formation, but also on the use of signal processing techniques to characterize the temporal response of the brain to t

ISSN:0740-3194    

DOI:10.1002/mrm.1910300204

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

年代:1993

数据来源: WILEY

摘要:

AbstractMagnetic resonance imaging has been used to follow the time course of lesions induced in the rat brain as an animal model for characterization of the volume of the lesion. The dispersion in spin‐spin relaxation has been used to characterize the nature of the brain lesion. Parameter selective estimation of T2quantitative determination of the lesion sire and volume selectivein vivoproton spectroscopy have been employed for the purpose. The work has been carried out on rats which were subject to lesioning by ibotenic acid as a model for excitotoxicity and also on rats which doses of ibotenic acid and subsequent doses of the NMDA antagonist drug MK 801 (dizocilpine). The time course of the progress of the lesions in untreated animals and the effect of neuroprotection by MK 801 was continuously monitored inalltest animals. Further, a relatively new inhalation anesthetic agent, isoflurane, has been employed. A more logical and semiquantitativeT2bandwidth demarkation useful in distinguishing different degrees of lesioning from the onset and up to the ‘edema’ stage through penumbra (mild lesion), medium degree lesion and severe lesion has been pro

ISSN:0740-3194    

DOI:10.1002/mrm.1910300205

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

年代:1993

数据来源: WILEY

摘要:

AbstractThe classical CPMG sequence and its extension as an imaging sequence, fast spin echo (FSE, based on RARE), suffer from signal magnitude variations in the early echoes when the re‐focusing pulses are not set exactly to 180°. It has been suggested that by varying the value of the nutation angle of each refocusing pulse the signal magnitude could be made constant. This article describes an algorithm permitting the generation of sequences of nutation angles yielding series of echsoes with constant signal magnitudes. This result is then usesd to design selective pulses for the FSE imaging techniq

ISSN:0740-3194    

DOI:10.1002/mrm.1910300206

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

年代:1993

数据来源: WILEY

摘要:

AbstractMyocardial tagging is a new noninvasive MRI method that allows the study of myocardial motion with high accuracy. However, with conventional tagging techniques tagging contrast is impaired at later heart phases due to longitudinal relaxation. An improved method, called Complementary SPAtial Modulation of Magnetization (CSPAMM), which separates the component of the magnetization with the tagging information from the relaxed component by subtraction of two measurements with first a positive and then a negative tagging grid, is presented. This technique improves the grid contrast and greatly facilitates the automatic evaluation of the myocardial motion. Thus the motion assessment of the entire heart cycle becomes possible. The improvements are documented by numerical simulations and by experiments on phantoms and on human volunteers.

ISSN:0740-3194    

DOI:10.1002/mrm.1910300207

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

年代:1993

数据来源: WILEY

摘要:

AbstractA diffusion‐weighted fast spin‐echo (FSE) imaging sequence for high‐field MR microscopy was developed and experimentally validated in a phantom and in a live rat. Pulsed diffusion gradients were executed before and after the initial 180° pulse in the FSE pulse train. This produced diffusion‐related reductions in image signal intensity corresponding to gradient (“b”) factors between 1.80 and 1352 s/mm2. The degree of diffusion weighting was demonstrated to be independent of echo train length for experiments using trains up to 16 echoes long. Quantitative measurements on a phantom and on a live rat produced diffusion coefficients consistent with literature values. Importantly, the eight‐ to 16‐fold increase in imaging efficiency with FSE was not accompanied by a significant loss of spatial resolution or contrast. This permits acquisition ofin vivothree‐dimensional data in time periods that are appropriate for evolving biological processes. The combination of accurate diffusion weighting and high spatial resolution provided by FSE makes the technique particularly useful

ISSN:0740-3194    

DOI:10.1002/mrm.1910300208

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

年代:1993

数据来源: WILEY

摘要:

AbstractA technique is presented for collecting the spin velocity distribution as a function of position and time. It uses a multidimensional excitation pulse to select a cylinder, giving localization in two dimensions. Resolution in the third spatial dimension is achieved in the readout. During readout, an oscillating gradient encodes the acquired data in both one spatial (dimension (x) and one veioclty dimension (v). Two acquisitions (42 ms each) are needed to get a complete coverage ofkx—kv, space, which makes this technique real‐time. The data is interpolated from the nonuniformly sampledkx, —kv, space to a Cartesian frame with a gridding scheme to take advantage of the Fast Fourier Transform. The technique was successfully applied to phantoms and normal volunteers, giving reasonable real‐time measurements of v

ISSN:0740-3194    

DOI:10.1002/mrm.1910300209

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

年代:1993

数据来源: WILEY

摘要:

AbstractA gradient technique to measure electron paramagnetic resonance spectra simultaneously at several different locations is described. The technique is based on the use of point probes containing paramagnetic centers. The value of the magnetic field gradient is chosen to be sufficient to separate the EPR signals from the different paramagnetic probes yet at the same time small enough to change only minimally the shape of individual signals. The conditions to apply this technique are considered in detail. When experimental data have a high signal‐to‐noise ratio, the lineshape distortion induced by the gradient can be corrected with the aid of a known distribution function of paramagnetic centers within the probe. The maximum entropy deconvolution algorithm is successfully applied for the correction of significantly distorted lines. The technique is experimentally tested and applied to measure the concentration of oxygen in hypertrophied rat myocardium and normal rat kidneyin vivoby low frequency EPR (L‐band, 1.2 GHz). No types of EPR oxygen‐sensitive probes‐lithium phthalocyanine crystals and synthetic carbohydrate chars

ISSN:0740-3194    

DOI:10.1002/mrm.1910300210

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

年代:1993

数据来源: WILEY

摘要:

AbstractThis paper presents a combination of speculative approaches, some related to earlier work and some apparently novel, which show great promise in providing a new class of MRI machines that would be considerably less expensive. This class would have advantages and disadvantages as compared to existing MRI, over and above that of low cost. The disadvantages include the apparent inability to perform classic spectroscopy, and limited flexibility in the area of selective excitation. The advantages include a fundamental immunity to inhoinogenelty and susceptibility problems, the ability to create a wide class of machines that are designed for specific anatomy‐related applications, the ability to design open machines for physician access, and improved capability for high speed imaging. Generic to all of the methods presented are a pulsed polarizing field and an oscillatory readout bias field. The pulsed field initially polarizes the magnetic moments. Sinat it is not on during the readout operation it has negligible homogeneity requirements since changes in the field amplitude will merely shade the image intensity. During readout a relatively low bias field is used. To enable the use of a relatively inhomogeneous bias field, an osciliatory field is used that has a zero average value. This prevents any long‐term buildup of phase errors due to a frequency error associated with inhomogeneity. Thus the average bias frequency will be determined solely by the frequency rather than the amplitude of the bias field. Three methods are described, all including the above features. The first two involve imaging in the laboratory frame, while the third involves imaging in the rotating frame. The second approach requires no RF excitation and the third approach uses RF bias and gradient signals. Some approaches to slice selection are descri

ISSN:0740-3194    

DOI:10.1002/mrm.1910300211

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

年代:1993

数据来源: WILEY