Volume 3, Issue 3, May 1976
Index of content:
3(1976); http://dx.doi.org/10.1118/1.594217View Description Hide Description
The determination of high‐energy x‐ray spectra has required scintillation spectrometers with massive shielding, neutron time‐of‐flight spectrometers, or the tedious counting of electron tracks in nuclear emulsions. A new approach has been developed which takes advantage of the energy dependence of photoactivation cross sections. Radioactivity is produced in a small packet of C, Cu, Co, Y, Zr, and Au foils by approximately 5000 rad (tissue). Since the amount of radioactivity produced in each foil is given by the integral of the product of photonuclear cross section and differential photon fluence, a numerical method for unfolding the spectrum is required, and the orthonormal expansion has been employed for this purpose. The photoactivation method has been used to determine the x‐ray spectra produced by 30‐MeV electrons incident upon thin and thick tungsten targets, and filtered by equivalent amounts of lead and aluminum. These spectra have been compared to calculated thin‐target spectra as well as to those determined by a neutron time‐of‐flight spectrometer. The central‐axis and off‐axis x‐ray spectra produced by a 33‐MeV betatron have also been determined.
Comparison of voltage‐divider, modified Ardran–Crooks cassette, and Ge(Li) spectrometer methods to determine the peak kilovoltage (kVp) of diagnostic x‐ray units3(1976); http://dx.doi.org/10.1118/1.594280View Description Hide Description
This report describes several different techniques that have been used to determine the peak kilovoltage (kVp) of single‐ and three‐phase diagnostic x‐ray units operating in both the radiographic and fluoroscopic mode, from 60 to 110 kVp, using (1) a voltage divider with oscilloscope display; (2) a voltage divider and special summing amplifier with digital display of the voltage utilizing a multichannel analyzer; (3) a version of the Ardran–Crooks cassette technique; and (4) a Ge(Li) spectrometer method. Each technique presents a distinct advantage: Method (1) enables the waveform to be viewed directly; method (2) is probably the most accurate and reproducible technique; method (3) has the greatest ease of operation; and method (4) provides the ability to visualize the x‐ray spectrum.
Feasibility study: I n v i v o neutron activation for regional measurement of calcium using Californium 2523(1976); http://dx.doi.org/10.1118/1.594218View Description Hide Description
The feasibility of using a collimated 252Cf neutron source to measure regional changes in skeletal calcium was tested because i n v i v o regional activation of diseased bone should offer advantages over the more widely reported total‐body calcium measuring techniques. Regional activation allows examination of discrete regions where the greatest changes in calcium content occur. Additionally, a simpler radiation facility is required for regional studies. Using a 5.5‐μg 252Cf source, thermal neutron flux and absorbed dose were measured in a tissue‐equivalent phantom. Detection efficiency of 49Ca γ rays for conditions simulating regional activation were measured using a 29‐cm‐diameter ×10‐cm‐thickness sodium iodide detector. These i n v i t r o measurements indicate that a collimated 252Cf source can be used for regional neutron activation of the lower spine and legs. Preliminary calculations indicate that a 1–3‐mg source provides adequate count rates for statistical accuracy with a bone marrow dosage acceptable for human patients and normal subjects.
3(1976); http://dx.doi.org/10.1118/1.594281View Description Hide Description
The original scattering and collimation system for the Siemens Mevatron XII linear accelerator used a lead scattering foil and box‐type plastic collimators. This arrangement achieves excellent field flatness by repeated electron scattering. The electrons reaching the patient are widely distributed in energy and direction. This has detrimental effects on the depth‐dose curves: slower falloff and increased surface dose. We have developed an alternative system for this accelerator, designed to minimize electron scatter and improve the safety of patient setup. Primary‐electron scatter occurs in the bending‐magnet exit window. Field uniformity is accomplished with a flattener of thin aluminum discs of different diameters, piled concentrically. An adjustable electron collimator 25 cm from the patient limits beam size, and a final electron collimator, either a cutout from lead sheet or a a custom‐made collimator of Lipowitz’s metal, in contact with the patient, define the area to be treated. This design results in lower surface dose, sharper dose falloff, bremsstrahlung contamination ⩽1%, and a field flatness expressed by a homogeneity index >0.8 for large fields. Since there is no mechanical connection between the machine and the final collimator, the safety aspects of the system are considerably improved.
3(1976); http://dx.doi.org/10.1118/1.594219View Description Hide Description
We have recently reported on a 1‐kVp, two‐filter image subtraction method for visualizing low concentrations of elements like iodine which have K‐shell absorption edges in the diagnostic x‐ray energy range. However, in the application of this technique to human thyroid imaging, superimposed images due to variations in tissue and bone thickness presented serious difficulties. In this paper, a technique is described for implementing a 3‐kVp, three‐filter approach. Using carefully chosen spectra and logarithmic image processing,images are produced which are compatible with our previously described two‐stage storage‐tube subtraction device. Proper manipulation of the resulting difference images results in a final absorption‐edge image in which only the element of interest is visualized, with unwanted background images suppressed. Computer calculations are presented to illustrate the relative sizes of difference signals arising from the element of interest and from tissue and bone backgrounds. Phantom studies using iodine concentrations as small as 1 mg/cm2, with variations of 10 cm of tissue and 2 g/cm2 of bone, suggest that the theory is sound and that, with straightforward apparatus modifications, images of good quality should be possible.
3(1976); http://dx.doi.org/10.1118/1.594220View Description Hide Description
The dose in the build‐up region for four different Cobalt‐60 therapy units was measured. It was found that, for large collimator openings and relatively short SSDs, a new dose peak occurs at a depth very much smaller than 0.5 cm. The dose at this peak is a function of collimator openings and SSDs and, in some extreme case, could be 15% or more higher than the dose at the conventional peak dose of 0.5 cm. The new dose peak is probably due to electrons produced in the source capsule and the part of the collimator close to the source; it can be almost eliminated by a filter placed just below the collimator.
3(1976); http://dx.doi.org/10.1118/1.594221View Description Hide Description
The bremsstrahlung spectrum from an 8‐MeV linear accelerator has been measured using a NaI(T1) spectrometer system. The spectrum shows a low‐energy cutoff at 0.4 MeV and the maximum photon energy to be approximately 6% greater than the nominal energy. The maximum emission of energy fluence was 1.6 and 1.8 MeV for measured and calculated values, respectively. The fast neutron dose in the photon beam was approximately 0.09% of the x‐ray dose. The weighted mean energy was 2.3 MeV, measured value, and 2.4 MeV, calculated value.
3(1976); http://dx.doi.org/10.1118/1.594222View Description Hide Description
Methodology and instrumentation are presented which are potentially capable of presenting fluoroscopically derived transverse axial body sections for use in radiation‐oncology treatment planning and beam monitoring. These combine the methods of Takahashi for generating transverse axial tomograms with electronic radiography, electrofluorotomography, and a contouring program for extracting body and tumor contours in a digital format. The system will also be capable of assuring both initial and day‐to‐day beam alignment.
3(1976); http://dx.doi.org/10.1118/1.594232View Description Hide Description
The influence of bone on dose distributions due to fast neutrons generated at the Naval Research Laboratory (NRL) Cyclotron was investigated. A paired dosimeter consisting of a parallel‐plate, tissue‐equivalent ionization chamber and thermoluminsecent material was used to partition the absorbed dose into neutron and γ‐ray components. Several thicknesses of bone were simulated using bone‐equivalent liquid and plastic. Based on these measurements, the authors conclude that, as a result of the increased absorption of neutrons by bone, lower dose levels are found behind bone that would be predicted from dose distributions in muscle‐equivalent liquid.
Non‐Invasive Brain Imaging: Computed Tomography and Radionuclides by H. J. DeBlanc, Jr. and J. A. Sorenson3(1976); http://dx.doi.org/10.1118/1.594215View Description Hide Description