摘要:

The longitudinal (T1) and transverse (T2) proton (1H) nuclear magnetic resonance (NMR) relaxation times of pathological human and animal tissues in the frequency range 1–100 MHz are archived, reviewed, and analyzed as a function of tissue of origin, NMR frequency, temperature, species, andin vivoversusin vitrostatus.T1data from specific disease states of the bone, brain, breast, kidney, liver, muscle, pancreas, and spleen can be characterized by simple dispersions of the formT1=AνBin the range 1–100 MHz withAandBempirically determined pathology‐dependent constants. Pathological tissueT2values are essentially independent of NMR frequency. Raw relaxation data, best‐fitT1parametersAandB, and the meanT2values, are tabulated along with standard deviations and sample size to establish the normal range of pathological tissue relaxation times applicable to NMR imaging orin vitroNMR examination. Statistical analysis of relaxation data, assumed independent, reveals that most tumor and edematous tissueT1values and some breast, liver, and muscle tumorT2values are significantly elevated (p≥0.95) relative to normal, but do not differ significantly from other tumors and pathologies. Statistically significant abnormalities in theT1values of some brain, breast, and lung tumors, and most pathological tissueT2values could not, however, be demonstrated in the presence of large statistical errors. BothT1andT2in uninvolved tissue from tumor‐bearing animals or organs do not demonstrate statistically significant differences from normal when considered as a group, suggesting no appreciable systemic effects associated with the presence of tumors compared to the statistical uncertainty. Statistical prediction analysis for bothT1andT2indicates that of all the tissues studied, only liver hepatoma can be reliably distinguished from normal liver based on a singleT1measurement (p≥0.95) given the scatter in the current published data. Indeed, data scatter, not easily attributable to temperature, species,in vivoversusin vitrostatus, the inclusion of implanted or chemical induced tumors, or the possible existence of multiple component relaxation, is recognized as the major factor inhibiting the diagnostic utility of quantitative NMR relaxation measurements. Malignancy indexes that combineT1andT2data as a diagnostic indicator suffer similar problems of uncertainty. The literature review reveals a dearth of information on the temperature and frequency dependence of pathological tissue relaxation and the possible existence of multiple relaxation components. The causes of differences in pathological tissue relaxation times are presently ambiguous, although increases in tissue water content, growth rate, Na+and K+concentrations, and reductions in tissue glycogen and protein content have been correlated with elevated tumorT1values.

ISSN:0094-2405    

DOI:10.1118/1.596111

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The design of a pulse sequence for volume localization in magnetic resonance spectroscopy is described in detail. The sequence is based on the volume selective excitation technique (VSE) proposed by Aueet al. [J. Magn. Reson.56, 350 (1984)] and overcomes the high rf power requirements of VSE. The implications of various design stages are demonstrated experimentally and by computer simulations.

ISSN:0094-2405    

DOI:10.1118/1.596092

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Capillary flow or microscopic random directional coherent flow as a model of perfusion is investigated both theoretically and experimentally. In the model, we assumed that molecular motion within a finite resolvable volume element (voxel) is a superposition of flow of randomly oriented small capillaries. In such a case, the observed signal from the capillary flow within a voxel will be attenuated in signal amplitude without any change in phase. Although this attenuation effect is similar to the diffusion phenomenon, it differs basically in the following aspects: since the motion in each capillary segment is coherent, phase cancellation occurs at even echoes due to spin rephasing, while the diffusion phenomenon is a purely random Brownian motion of the thermally agitated molecules, changing both in direction and speed during the measurement period. Because of the random character of diffusion, even‐echo rephasing cannot be observed. Thus capillary flow or perfusionlike microscopic flow can be measured based on the above distinct flow characteristics, i.e., signal restoration at even echoes versus signal amplitude attenuation at odd echoes. By applying a suitable mathematical algorithm, information on the capillary flow alone can be extracted from the two separate distinct measurements, i.e., one with a single echo and the other with a double echo. Both a theoretical calculation of the capillary flow, as well as the experimental results with a human volunteer by a 0.6‐T nuclear magnetic resonance imager, are presented.

ISSN:0094-2405    

DOI:10.1118/1.596093

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Electronic scanning‐slit fluorography involves replacing paired fore and aft slits for scatter rejection with only one beam‐defining tantalum fore aperture. Since the video signal within the projection of the aperture on the image intensifier is much more intense than behind the tantalum, one can discriminate electronically between these two signals and thus eliminate the unwanted x‐ray scatter and veiling glare. The general features of a prototype unit are described along with the rationale for the choice of design factors employed. Imaging time of 1–2 s has been achieved using multiple scanning slits. Small focal‐spot size and large number of pixels are favored for higher dose utilization, shorter imaging time, and lower x‐ray tube loading, as well as for better spatial resolution. Images of a chest phantom show better visibility of low‐contrast details, especially in poorly penetrated areas, when compared with the image obtained and displayed under the same conditions, but using a conventional grid to reject scattered radiation.

ISSN:0094-2405    

DOI:10.1118/1.596094

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technique for correction of motion between images which are obtained in high‐speed digital subtraction or cine angiographic acquisitions is being tested. The method is based on the application of quadratic polynomial equations which transform one image so that it matches a reference image. Images which have been processed in this manner can be summed to improve the signal‐to‐noise ratios over individual images. The technique for motion correction currently being tested uses operator interaction to establish the appropriate polynomial transformation. An operator selects fiducial (reference) points on an image which will be the reference. Then he selects the corresponding fiducial points on the image to be processed. The algorithm calculates the coefficients of a pair of quadratic polynomial equations and applies them to each pixel in the image. Results demonstrate the application of the technique in phantoms and in digitized cine angiograms.

ISSN:0094-2405    

DOI:10.1118/1.596095

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Bone‐mineral measurements using quantitative computed tomography (QCT) are commonly based on comparisons with solutions in water of known concentrations of K2HPO4. In this paper are described theoretical and experimental studies that have led to the conclusion that large systematic errors can arise in these measurements, depending on the soft‐tissue and fat concentrations in the vertebral spongiosa. In the case of single energy scanning, such large errors have been identified to be due to the varying water content (displacement effect) in the calibration samples and the varying fat content in the region of interest (ROI) within the patient. In the case of dual energy scanning, the error arises because when normalized to that of water, the mass attenuation coefficient of fat increases with photon energy while the reverse is true for K2HPO4. Our studies have also revealed that total trabecular bone density (which includes the mineral, soft tissue, and fat) can be much more accurately determined by the dual energy QCT method than bone mineral alone. This finding is especially interesting since there have been several reports in the literature suggesting that bone density rather than bone‐mineral content is a better predictor of the risk of osteoporosis‐related fractures.

ISSN:0094-2405    

DOI:10.1118/1.596096

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In three‐dimensional radiation treatment planning, essentially all fields are irregular and compensated. Consequently, it is important to predict accurately dose for such fields to ensure adequate coverage of the target region and sparing of healthy tissues. Traditional approaches, namely, those involving scatter integration and extended source and those utilizing negatively weighted fields, are inaccurate, especially near the boundaries defined by blocks and collimators. In the method presented in this paper, dose distributions for arbitrarily shaped beams are calculated by two‐dimensional convolution of the relative primary photon fluence distributions and kernels representing the cross‐sectional profiles of a pencil beam at a series of depths. The pencil beam dose distributions are computed, once and for all, with the Monte Carlo method for photon energy spectrum for each treatment machine. The finite size of the source, which is important for cobalt machines, is also taken into account using convolution of the source with the relative primary fluence distribution. Convolutions are performed using fast Fourier transforms on an array processor. Results of calculations are in excellent agreement with measured data. While no data are presented for fields modified by compensators, the method of calculation should apply at least as well for such fields since the variations in fluence distribution for compensated fields are not as sharp as for points near the block boundaries.

ISSN:0094-2405    

DOI:10.1118/1.596097

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An apparatus capable of measuring small fractional changes in ionization current has been used to study the effect of small inhomogeneities on photon dose in water. Small ring‐shaped inhomogeneities were introduced into a water phantom and measurements have been made for 4‐, 6‐, and 18‐MV x‐rays. The results show (1) beyond the range of secondary electrons, the dose perturbation is basically a photon transport phenomenon which becomes less important as the beam energy increases; (2) within the range of secondary electrons, dose perturbation also involves electron transport, which has a strong dependence on atomic number and could result in a substantially large effect on dose deposition.

ISSN:0094-2405    

DOI:10.1118/1.596106

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The extended net fractional depth dose (ENFD) is developed from the net fractional depth dose (NFD) previously described for unit‐density media, basically by scaling the two geometric parameters, the side of the equivalent square field, and the depth along the ray by the relative electron density. Specifically, in the analytical description for the NFD, the geometric depth is replaced by the radiologic depth and, along the ray path, the geometric field side is scaled by the relative electron density. Interface effects on the electron and scattered‐photon fluences are accounted for. In addition, a simple function is developed to correct for the effect of lateral as well as longitudinal electron transport at the central ray. In the present work the inhomogeneities are assumed to be of planar parallel shape and to extend across the entire beam. The treatment of smaller inhomogeneities is outlined but will be treated in detail separately. Calculated results are compared to measured and calculated data from the literature for60Co and 10‐MV x rays, and to 15‐MV data measured at the NCI.

ISSN:0094-2405    

DOI:10.1118/1.596107

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Clinical dosimetry data are presented for the lower‐energy x‐ray beam of a Varian Clinac 1800 linear accelerator. This beam has comparable characteristics to single energy linear accelerators with the same stated 6‐MV x‐ray energy. The nominal beam energy was found to be 5.3±0.3 MV on the central axis. Beam quality expressed in terms of half‐value layer in water was found to vary by less than 10% over the entire field. The surface doses are only slightly, but consistently, larger than those reported in the literature for other 6‐MV linacs. Dosimetric results presented include central axis percentage depth dose (% DD) and tissue–maximum ratio (TMR), surface and buildup doses, output factors, and inverse square law applicability. The flatness and symmetry characteristics are within the manufacturer's specifications for both large and small fields.

ISSN:0094-2405    

DOI:10.1118/1.596113

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY