摘要:

The current understanding of xenon scintillation physics is summarized and keyed to the use of xenon as a γ‐ray detector in medical radioisotope imaging systems. Liquid xenon has a short scintillation pulse (∼10−8sec) and high γ‐ray absorption and scintillation efficiencies. The fast pulse may facilitate imagingin vivodistributions of hot positron sources and allow recovery of additional spatial information by time‐of‐flight techniques. We begin by describing our own study of the feasibility of making a practical positron scanning system, and consider the problems of scintillation decay time, linearity, efficiency, purity, and electric‐field amplitifcation. The prospects for a practical instrument are considered.

ISSN:0269-3127    

DOI:10.1118/1.594289

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Extensive experimental comparisons of calorimetric and ionometric measurements have been made that cover a broader range of electrons energies and depths in graphite than previously reported. Electron beams of 15, 20, 25, 30, 40, and 50 MeV were used. Calorimetric absorbed‐dose measurements and ionometric specific‐charge measurements in air were compared in graphite at depths from 1 to 51 g/cm2. The medium was irradiated with uncollimated electron beams produced by scattering after passing through a 0.1‐g/cm2aluminium vacuum window, various thicknesses of lead foils, and air. The variation in the quotient of the two measurements was studied as a function of lead‐foil thickness, depth in the medium, beam energy, foil‐to‐detector distance, and off‐axis distance. These studies permitted the measurements to be corrected and compared with theoretical calculations that assume a broad medium irradiated with broad, parallel, monoenergetic electron beams. The overall experimental uncertainty is estimated to be 1%. The results are generally in good agreement with theoretical and experimental results of other investigators. The calorimeter received close to 1 Mrad during preliminary measurements and from 1 to 2 Mrad during the measurements reported. The results showed no detectable heat defect in graphite after prolonged periods of exposing the calorimeter to air at atmospheric pressure.

ISSN:0269-3127    

DOI:10.1118/1.594290

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

At most scientific meetings, the emphasis is on theresultsof research, not on the means by which those results were achieved. However, it was the intent of the Alliance for Engineering in Medicine and Biology in the symposium organized at the 1975 AAPM meeting to describe theinstrumentationnow available to researchers and practicing physicians in the field of ultrasonic diagnostic imaging. We feel that this is an appropriate and useful presentation, especially because it concerns a rapidly changing field. The papers that follow summarize the current state‐of‐the‐art of, first, single‐transducer imaging and, second, two‐dimensional transducer array technology. (A third paper, summarizing the state‐of‐the‐art of ultrasonic tissue signatures, was presented by Dennis H. Le Croissette, Jet Propulsion Laboratory, Pasadena, California 91103, but is not included here because it is to be published this spring in Ultrasonic Tissue Characterization, the proceedings of a seminar held by the National Bureau of Standards 28–30 May 1975.) These papers together offer a very broad picture of where ultrasonic diagnostic imaging instrumentation stands today, and what may be expected in the near future.

ISSN:0269-3127    

DOI:10.1118/1.594245

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper attempts to take note of some recent improvements in ultrasound echo‐ranging technology and make some observations on the single‐ versus multiple‐transducer element approach. It would appear at this time that the problems encountered with visualization systems may not be solved by using multiple transducersper se, and that the unique advantages that accrue to the multiple‐transducer systems have yet to demonstrate that they fulfill an important clinical need.

ISSN:0269-3127    

DOI:10.1118/1.594246

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Attention is given to means of sensing ultrasonic energy distributions over an area. Under the restraints appropriate to real‐time imaging of deep body organs, piezoelectric arrays offer the most promising method. Adaptation of integrated circuit techniques to array assembly permits very large arrays of small elements to be batch fabricated. Further, special semiconductor switching devices specifically designed for addressing the array have been produced and applied. These permit both the passage of weak received signals and the application of voltage and current levels sufficient for transmission without significant disturbance of the basic piezoelectric element properties. Emphasis is placed on the complete data acquisition, processing, and display flexibility that arises from an array capability. Operation becomes possible in any of the presently used A or time motion (TM), in real time B or C scan, or in novel scan patterns adapted to specific organs. Further development with acoustic and electronic focusing, taking advantage of the unique possibilities obtained with the bidirectional array structure, is discussed.

ISSN:0269-3127    

DOI:10.1118/1.594247

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A study was carried out to investigate the errors involved in obtaining volumes by means of ultrasound echograms and a small computer. The work was done on live pigs’ kidneys (130–200 cm3). The errors include those associated with the scanner itself, those involved in the interpretation of the contour boundaries, and those relating to the algorithms used to determine the volumes from the contours. An average deviation of 25.4 cm3from actual volumes was found, while 58% of the measurements were within ±10%. Computer programs allowing for the display of contours and intercepts of corresponding orthogonal scan planes were used to reanalyze the data resulting in an average deviation of 14.7 cm3with 78% of the measurements within ±10%.

ISSN:0269-3127    

DOI:10.1118/1.594248

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Conventional methods for measuring splenic sequestration of labeled cells rely on stationary probe counting over the liver and spleen. A quantitative spleen‐scanning method is proposed as a means of performing a more accurate diagnosis of the degree of splenic sequestration. Improved methods for calibrating a rectilinear scanner employing “constant resolution” collimators, designed for thein vivomeasurement, are reported. Clinical evaluation of this method was performed by scanning several patients scheduled for splenectomy. Good correlation betweenin vivoandin vitromeasurements of splenic radioactivity for 29 patients was obtained. A least‐squares fit to the data yielded a coefficient of determinationr2=0.93.

ISSN:0269-3127    

DOI:10.1118/1.594288

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method is described for measuring the energy spectra of diagnostic x rays using a high‐resolution intrinsic germanium spectrometer. The method is applicable over the wide range of x‐ray exposure rates normally used in radiography. Reduction of the high x‐ray beam intensity to a level acceptable to the germanium spectrometer is achieved by measuring the spectrum of photons scattered through 90° by a lucite disc. From this measured spectrum the primary spectrum is obtained by calculation. Some typical x‐ray spectra are presented.

ISSN:0269-3127    

DOI:10.1118/1.594263

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The transit dosemeter has been used in obtaining inhomogeneity corrections for60Co rotational therapy. A modified approach is presented which features a rapid electronic system for determining the average tissue–air ratio (T̄ĀR̄) for a 360° rotation. Experimental results and early clinical experience show that this system is capable of providing a T̄ĀR̄ value directly, and more rapidly. It can be readily incorporated in a therapy system which already includes a transit dosemeter.

ISSN:0269-3127    

DOI:10.1118/1.594249

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simple, heuristic model of a photographic emulsion is described for the purpose of illustrating the fundamental physical processes and emulsion properties which determine the characteristics of an x‐ray film (viz., the shape of theH−Dcurve, film gamma, and film speed). By means of this model, it is shown that the contrast multiplication afforded by an x‐ray film (i.e., a film gamma greater than unity) is a direct result of the exponential attenuation of the viewing light by the developed film, and that film gamma is proportional to grain size, grain density, and emulsion thickness. The difference in theH−Dcurve that is observed when the same film is exposed to light from an intensifying screen or directly to x rays is also predicted by the model.

ISSN:0269-3127    

DOI:10.1118/1.594264

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY