摘要:

In nuclear magnetic resonance (NMR) imaging by the zeugmatographic methods, there is a common and unified theoretical description. All forms of two‐dimensional and three‐dimensional imaging involve NMR data which trace various geometric representations, in reciprocal transform space, of the subject's spatially blurred “effective” spin density. The effective density is proportional to the physical density modulated spatially by the several factors of receiver coil (B/I) ratio, rf pulse excitation terms,T1‐relaxation terms, andT2‐relaxation terms. These factors depend upon the rf pulse sequence and field‐gradient modulation sequence, and they may be calculated according to some model or directly measured. From this viewpoint, all different imaging modes appear as variations in data‐collection and image‐reconstruction strategies. The results are used here to describe slice‐oriented polar and Cartesian strategies, three‐dimensional Cartesian and two forms of spherical strategies, and multiecho strategies of the “planar‐echo” type.

ISSN:0094-2405    

DOI:10.1118/1.595470

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For a uniform slab of inhomogeneity in a supervoltage beam, correction factors can be calculated from the Batho equation. In this report, we present a method for calculating the effect of an annular inhomogeneity, concentric about the beam axis, upon the dose at a point on the axis and below the annulus. A derivation of the equation required in the calculation for supervoltage radiation is given. Results from measurements made in60Co beams for polystyrene foam, cedar, and aluminum annuli, all having 3.0×3.0 cm2in cross section but with different inside diameters, are compared with correction values calculated by the method. For situations where the annulus is just submerged in the phantom, measured and calculated values are in good agreement. For a general situation, two calculation types are proposed and the data show that in general the measured scatter perturbation lies between the calculated values of the two types. Application of our technique predicts a sign reversal in the scatter perturbation due to an inhomogeneity. This reversal has previously been observed and reported and is also demonstrated in our measurements.

ISSN:0094-2405    

DOI:10.1118/1.595469

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Thermal dosimetry is one of the most challenging and necessary aspects of therapeutic hyperthermia. Treatment optimization and assessment of efficacy of the therapy can only be achieved by a quantification of the thermal distributions in normal and malignant tissue induced by specific configurations of heat sources. Radial and axial temperature distributions induced in normal porcine tissues by a pair of disk‐shaped direct contact electrodes operating at 0.5 MHz are presented in this communication. Temperature increments induced in the skin, adipose tissue, and skeletal muscle tissue are correlated with the magnitude of the incident power density for various sizes of circular electrodes. The observed thermal distributions very likely relate to the heterogeneous thermal and electrical properties of these tissues.

ISSN:0094-2405    

DOI:10.1118/1.595471

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Theoretically, partial deuteration of body water should allow significantly increased neutron penetration in tissue. To evaluate the possible usefulness of partially deuterated water in neutron capture therapy (NCT), neutron flux density distributions were measured in a 23×16.5 cm (length×diameter) cylinder for incident thermal and epithermal neutron beams, at 20 and 40 at. % deuteration of water. Relative to neutron flux densities in nondeuterated water, flux densities increased significantly with increasing depth and increasing levels of deuteration. For example, at a depth of 6 cm, flux density was increased ∼20% to 50% for 20 to 40 at. % deuteration. In a clinical situation, this would increase tumor dose by ∼30%. Further benefits include the reduced hydrogen neutron capture and the chemical radioprotective effects of partial deuteration for photon radiation

ISSN:0094-2405    

DOI:10.1118/1.595472

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The ratio of coherent to Compton photons scattered by a tissuelike material depends on its effective atomic number. This ratio can, therefore, be used for thein vivocharacterization of tissues. The intrinsic sensitivity of this measurement is defined as the change in the coherent‐to‐Compton ratio for a given change in the atomic number. The effect of the scatter angle on the sensitivity has already been described by us in a paper recently submitted to this journal. In this study, the dependence of the sensitivity on the energy of the incident photons is investigated in two ways. The first approach is quasitheoretical and is based on computations of the cross sections of the coherent and Compton scattering for various energies. The second approach is experimental and it involves the measurement of the scatter ratio from a series of K2HPO4solutions for three primary photon energies: 60, 81, and 140 keV. The combined effect of both the photon energy and the scatter angle on the sensitivity can be described by a single parameter which is the momentum transfer. It is concluded that for the limited range of the atomic numbers which apply to trabecular bone (8≤Z̄≤11) the momentum transfer reflects completely the effect of the scatter angle and photon energy on the sensitivity.

ISSN:0094-2405    

DOI:10.1118/1.595473

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The imaging properties of the phosphor layer in fluorescent screens or image intensifiers are related to its x‐ray absorption characteristics. In this study, we applied Monte Carlo methods for the simulation of x‐ray photon diffusion in a phosphor layer. TheK‐reabsorption factor, absorbed x‐ray energy, quantum absorption efficiency, statistical factor, and noise‐equivalent absorption were determined as a function of the incident energy and angle of the x rays for eight commonly used phosphor layers. These basic physical quantities will be useful for the prediction of the information transfer properties of a phosphor layer.

ISSN:0094-2405    

DOI:10.1118/1.595474

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The feasibility of radiographic image subtraction based on electrostatic imaging techniques is demonstrated. Ionographic latent images are essentially foil electrets with surface charge densities modulated by x‐ray transmission through the object. Latent image charging at one polarity corresponding to the production of the primary image, and latent image discharging with the opposite polarity, are used to create the final latent image representing the region of interest. The technique is simple and could be in its present form applied to industrial radiography. Using high‐pressure xenon ionographic chambers, the technique could be developed into a clinical tool as an inexpensive alternative to conventional digital radiography.

ISSN:0094-2405    

DOI:10.1118/1.595475

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A model is presented for the detection of uniform focal lesions by means of an ideal photon detector. The difference of observed photons between an area overlaying the embedded lesion and an adjacent reference area of equal size constitutes the signal to be detected. Application of ROC analysis reveals that the exact probability distribution of this photon count difference can be approximated well by a Gaussian, on condition that modulation1. Moreover, within these constraints, SNR emerged as the more salient parameter characterizing detection performance. It is shown that in the absence of anatomical variation, lesions of arbitrarily small size may be detected at any prescribed level of confidence, provided one is willing to accept the required high photon exposure. The effect of anatomical variation on detection performance is conveniently demonstrated in a graph of SNR versus exposure. There, two global regions are identified, each characterized by an asymptote, corresponding to either photon‐limited or photon‐saturated imaging. Under the first condition, quantum fluctuations are dominating the noise, and thus, detection performance is influenced by the photon exposure. Under the second, anatomical variations limit the SNR to an upper value, irrespective of exposure magnitude. Data obtained from dental radiographs demonstrate that anatomical variation is amenable to experimental measurement, and that it sets the upper limit for the SNR achievable in the diagnostic task of detecting incipient carious lesions.

ISSN:0094-2405    

DOI:10.1118/1.595476

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For the Clinac 4, open field profiles measured in the principal plane have higher intensity “horns” than those in off‐axis planes. The maximum deviation occurs at 1‐cm depth, where in the worst case of large field sizes and off‐axis distances the deviation can be as high as 16% in the region near the horns. A model is proposed to generate open field beam profiles in off‐axis planes, based on measured profiles in the transverse principal plane and in the largest field size diagonal plane of the machine. Within the central 90% portion of the field, the maximum deviation of the generated profiles from the measured ones at the same off‐axis distance varies from about 3% at 1‐cm depth to about 2% at 13‐cm depth and then increases to less than 5% at 25‐cm depth, even for very large field sizes and off‐axis distances. Very little additional computer time and data storage are required for this procedure. Using these profiles, the Milan and Bentley method can be extended to three‐dimensional treatment planning with good accuracy.

ISSN:0094-2405    

DOI:10.1118/1.595453

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Inner wall surface contamination of an ionization chamber by high‐Zmaterials can produce an over‐response to low‐energy photons which is greatly underestimated by the percentage composition of the contaminant in the entire chamber. Experience with a chamber contaminated in such a manner is presented. Examples are included which illustrate the magnitude of possible clinical effects from low‐energy units which have been calibrated with a chamber contaminated with a thin layer of relatively high‐Zmaterial.

ISSN:0094-2405    

DOI:10.1118/1.595454

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY