Volume 2, Issue 6, November 1975
Index of content:
2(1975); http://dx.doi.org/10.1118/1.594197View Description Hide Description
New accelerator technology and a gathering of physicists and doctors interested in applications to therapy in the late 1940s made possible the development of early multimegavolt betatron techniques. Dr. Henry Quastler brought experience for an actual x‐ray treatment, and Dr. Lester Skaggs joined the group to extract the electron beam. Some of the collaborating students were G. D. Adams, H. W. Koch, J. S. Laughlin, L. H. Lanzl, and E. F. Lanzl. The physics staff had succeeded in sealing off a vacuum tube for the betatron, and further developments involved field flattening, exposure measurements, collimation, stray electron control, phantom tests, and development of a beam peeler.
2(1975); http://dx.doi.org/10.1118/1.594198View Description Hide Description
The basic theory of x‐ray image formation of blood vessels, which is related to isoplanatism, is discussed. The x‐ray intensity distributions of the blood vessel images are derived first for the actual case, which gives ‘‘correct’’ x‐ray images obtained by the ray‐tracing method. Secondly, as an approximation of the actual case, the image distributions are derived by the convolution method, which corresponds to an isoplanatic case. It is concluded that, under practical conditions, x‐ray images of blood vessels are given approximately by a convolution integral of the object distribution, that is, the input x‐ray pattern of the vessel exposed with a parallel x‐ray beam, with the line spread function of geometric unsharpness. Therefore, this theory provides support for the experimental procedure commonly used in obtaining blood vessel images, and for the validity of applying in angiography the concept of the line spread function and the modulation transfer function of geometric unsharpness.
2(1975); http://dx.doi.org/10.1118/1.594199View Description Hide Description
The analysis and understanding of results of computed tomography(CT) require an understanding of photon attenuation in matter. The high sensitivity and resolution of these devices coupled with the use of a polychromatic photon source require a level and breadth of understanding about photon attenuation not usually required in any particular subspecialty of radiological physics. With this goal in mind, a discussion of narrow‐beam photon attenuation in matter is given and related to those problems currently underway in the field of computed tomography. Measurements and calculations of tissue properties are presented. Calculations of descriptive quantities relevant to polychromatic source attenuation and CT scanning are described and presented.
2(1975); http://dx.doi.org/10.1118/1.594200View Description Hide Description
Continuous bremsstrahlung spectra were calculated for 120 kVp for constant and sinusoidal potentials. Fluorescent radiation for the tungsten target was added to the bremsstrahlung, and the spectra were attenuated through various filter materials. A drawing of an object to be scanned was divided into an array of small squares in which the composition was assumed to be constant. Transmission data for 120 rays at each of 120 angles spanning a range of 180° were calculated. Two algorithms for the reconstruction of attenuation coefficients from projection data, an algebraic reconstruction technique (ART) and the convolution method, were utilized to reconstruct effective coefficients. The effect of spectral filtration on the quality of the reconstruction was evaluated. Lightly filtered x‐ray beams give rise to severe distortions in image quality, with values of the reconstructed coefficients rising toward the periphery of the object. Highly filtered beams give rise to images with less pronounced distortion.
2(1975); http://dx.doi.org/10.1118/1.594201View Description Hide Description
Previous studies on pulse‐height selection for scintillation cameras have been concerned with static imaging or asymmetric window settings around the photo peak. For count densities typical of those encountered in flow studies and for the customary symmetric window it is shown here, both computationally and experimentally, that a window setting in the range of 25–35% provides the maximum lesion detectability rate for lesions of 0.5‐ to 2.0‐cm diameter.
2(1975); http://dx.doi.org/10.1118/1.594202View Description Hide Description
Ionization chambers often exhibit a stem effect, caused by interactions of radiation with air near the chamber end, or with dielectric in the chamber stem or cable. These interactions contribute to the apparent measured exposure. To determine the stem effect for several common ionization chamber systems, exposures were measured with TLD capsules placed at the center of 6 0Co fields of various sizes. These exposure measurements then were repeated with various ionization chamber systems, including two Victoreen R meters (25‐ and 100‐R chambers), a Capintec 192 dosimeter with a Farmer 0.6‐cm3 probe, a PTW transit dose probe, and an EG&G IC‐18 probe with a Keithley 610‐B electrometer. From a comparison of TLD and ionization chamber measurements of the variation in exposure rate with field size, stem corrections for the different systems were determined within 1%.
2(1975); http://dx.doi.org/10.1118/1.594203View Description Hide Description
Analysis of 5575 settings on a computer‐monitored Theratron‐80 60Co unit demonstrates that human error does occur in treating patients with radiation. The errors are due to inaccurate setting of such parameters as field size, gantry angle, collimator rotation, treatment time, etc. The error rate per parameter was found to be about 3%, and more than two‐thirds of the patients monitored with the PDP 11/45 computer had at least one error at some stage during the full course of treatment. Both the dose and the dose distribution may be affected by these errors and have been studied in a few typical cases. The errors in timer setting have the largest effect on the prescribed dose and may change the probability of local control appreciably.
Model for the calculation of output for elongated fields at nonstandard distances for a 25‐MV betatron and for radiocobalt teletherapy units2(1975); http://dx.doi.org/10.1118/1.594204View Description Hide Description
Output‐area factors are usually provided only for square fields at one standard distance. To calculate dose for mantle, extended field, total nodal, subtotal body, and total body irradiation, output‐area factors are required for a continuous range of shapes and distances. Dose rate measurements have been made for different elongation ratios at various distances with an Allis‐Chalmers 25‐MV betatron, an AECL Theratron‐80 6 0Co unit, a Picker C‐9 6 0Co unit, and a Picker C‐10 000 6 0Co unit. A model is presented that permits the extension of dose rates and area factors for square fields at one distance to elongated fields at any distance. An illustrative calculation for one of the units is given.
2(1975); http://dx.doi.org/10.1118/1.594205View Description Hide Description
The relative percent dose reduction by lead of 7‐ to 18‐MeV electrons with a Siemens betatron and of 19‐ to 28‐MeV electrons with a Sagittaire linear accelerator has been measured with a thin‐wall buildup chamber for 6.3×6.3‐ and 10.5×10.5‐cm field sizes at the chamber position for the normal treatment source‐to‐skin distance (SSD) of each machine. The thickness of lead necessary to attenuate the open beam by 95–98% was then determined of 7‐ to 28‐MeV electrons. The required thickness of lead to attenuate 95% of the 7‐ to 28‐MeV electron beam ranged from 2.3 to 18 mm for the 6.3×6.3‐cm field and from 2.4 to 23 mm for the 10.5×10.5‐cm field, respectively. For 98% attenuation, thicknesses from 2.6 to 25.0 mm for the 6.3×6.3‐cm field and from 2.8 to 27.5 mm of lead for the 10.5×10.5‐cm field were necessary.
2(1975); http://dx.doi.org/10.1118/1.594151View Description Hide Description