Volume 4, Issue 1, January 1977
Index of content:
4(1977); http://dx.doi.org/10.1118/1.594379View Description Hide Description
The star test pattern image contains a large amount of information about the focus OTF which is not presently used. A mathematical analysis of these images for the special case of separable and symmetrical focal spots is presented. Circular blur loci are identified and their changes over the imaging field are described. Harmonic distortion and its application are described. Sources of measurement error are discussed.
4(1977); http://dx.doi.org/10.1118/1.594352View Description Hide Description
In the Siemens star image, exact determination of the first disappearance frequency, which is used to measure the focal spot size, is difficult since the disappearance band has a finite width and the image also has other artifacts. The origin of these artifacts and their appearance was studied by Siemens star image simulation on a digital computer. The simulated images were manipulated by using many different modulation and phase transfer functions. It is shown that the bending of spokes is not related to zero contrast; exact triplet splitting can occur only at the disappearance frequency, and therefore splitting is a valuable indicator of that frequency.
4(1977); http://dx.doi.org/10.1118/1.594378View Description Hide Description
The basic theory of field characteristics is discussed. It is shown that the point spread function and optical transfer function (OTF) of geometric unsharpness at any arbitrary field position can be derived from those at the central beam position. At all locations in the radiation field, the OTF in a direction parallel to the x‐ray tube cathode–target axis does not depend on the field position X, which is perpendicular to the to the tube axis, whereas the OTF in a perpendicular direction does not depend on the field position Y, which is parallel to the tube axis. The theoretical result is used to demonstrate the field characteristic of the uniform, square focal spot.
4(1977); http://dx.doi.org/10.1118/1.594297View Description Hide Description
The effect of asymmetric focal spots in blood vessel imaging is investigated by computer simulation. Phase distortion effects are studied in isolation from MTF effects. Calculations are done for a number of symmetric and asymmetric foci with identical standard deviations. It is concluded that blood vessel image degradation due to focus asymmetry p e r s e is small and that the LSF standard deviation is a useful size parameter for both symmetric and asymmetric foci.
4(1977); http://dx.doi.org/10.1118/1.594298View Description Hide Description
In our previous reports on absorption‐edge fluoroscopy, it was not possible to relate fully the subtleties involved in the selection of spectral parameters. This paper is intended as an overview of this important aspect of the technique. It is shown that, by using the 1‐kVp, 2‐filter technique, it is possible to image certain elements (e.g., iodine and xenon) in the presence of tissue variations of ±2 cm about the thickness at which perfect tissue cancellation takes place. Use of logarithmic signal processing extends this range, but bone thickness variations may not be accommodated because only two x‐ray energies are involved in the imaging process. Use of a 3‐kVp, 3‐filter technique with logarithmic signal processing is shown to solve this general problem. Computer simulations show that 1‐mg/cm2 iodine may be imaged in the presence of 10 cm or more tissue variations and 2000‐mg/cm2 bone variations.
4(1977); http://dx.doi.org/10.1118/1.594299View Description Hide Description
A sensitive and relatively simple method for obtaining an index of the spectral quality of high‐energy x rays is presented. The method is based upon the use of photoactivation threshold detectors. Photonuclear reactions produce measurable amounts of radioactivity for radiation exposures of several thousand roentgens. Since the threshold energies for photonuclear reactions vary for different nuclei, the ratio of radioactivity induced in two appropriately selected foils is very sensitive to the x‐ray spectrum. Photoactivation ratios (PAR) have been measured for 20–35‐MV x rays using Co, Cu, Y, Zr, and Au activation foils. It is shown that the PAR method offers a sensitive and practical means for quality control of x‐ray spectra, comparison of high‐energy accelerators, and the measurement of variations of spectralquality control at different points in an irradiated volume.
4(1977); http://dx.doi.org/10.1118/1.594300View Description Hide Description
A sensitive and relatively simple method for obtaining an index of the spectral quality of x rays in the 4–15‐MV range is presented. The method is based upon the simultaneous production of 115mIn by inelastic scattering of x rays by an indium foil and the production of 116mIn by neutron capture, where the neutrons are obtained from the photodisintegration of deuterium. Since the cross sections for the production of 115mIn and 116mIn have markedly different energy response functions, the ratio of radioactivities is very sensitive to the incident x‐ray spectrum. Using a detector consisting of an indium foil sandwiched between two plastic bottles of D2O, photoactivation ratios (PAR) have been measured for 4‐, 6‐, and 10‐MV x rays. It is shown that the PAR method offers a sensitive and practical means for quality control of x‐ray spectra and comparison of high‐energy accelerators.
4(1977); http://dx.doi.org/10.1118/1.594301View Description Hide Description
The angular dependence of the ESRline shape from polycrystalline samples due to incomplete randomness in the orientation of the crystallites is calculated for centers with axial symmetry. The effect of a preferential axis of orientation is considered and the results are applied to radiation produce centers in bone.
4(1977); http://dx.doi.org/10.1118/1.594383View Description Hide Description
Measurements of dynamic shear compliance versus dead time at room temperatures are reported for a sample of rabbit bone and several samples of human bone, including an extracted wisdom tooth. In all cases an abrupt, second‐order transition is observed in one or both components of the audio‐frequency complex shear compliance, , at about 5 hours after death; this is considered to be an indication of the life‐to‐death transition in bones.
4(1977); http://dx.doi.org/10.1118/1.594302View Description Hide Description
A monoenergetic response correction is described which, along with adequate filtration, may be used to remove the spectral shift artifact encountered in three‐dimensional reconstruction from x rays. Reconstructions were carried out by means of a convolution algorithm for simulated data using this method. These are compared with reconstructions obtained using fixed‐length water‐bath scans as a remedy for the special artifact. These studies suggest that the spectral artifact can be successfully eliminated from computerized cross‐sectional scans without resorting to the use of the water bath while, at the same time, improving quantum statistics and/or permitting operation at a lower tube current.
4(1977); http://dx.doi.org/10.1118/1.594303View Description Hide Description
The use of the stable isotope48Ca as a tracer in biological systems has been studied. Determination of 48Ca concentrations in samples was accomplished by proton activation via the (p,n) reaction with subsequent measurement of γ‐ray spectra from 44‐h 48Sc and 4‐h 44Sc. With protons of about 5.5 MeV and a Ge(Li) γ‐ray detector, and with no chemical processing of the samples, ratios of these two radioisotopes can be determined with sufficient precision to allow 48Ca tracer studies to be performed with sensitivity at least equivalent to that possible with the common 45Ca radioactive tracer.
4(1977); http://dx.doi.org/10.1118/1.594293View Description Hide Description
The relationship between the tissue–phantom ratio and central‐axis percentage depth‐dose data is derived. Relationships between the tissue–phantom ratio, tissue‐maximum ratio, and tissue–air ratio are given. A simple model is developed which suggests that the degree to which the TPR and TMR are independent of distance from the source depends largely on the collimator/flattening‐filter scatter.
4(1977); http://dx.doi.org/10.1118/1.594294View Description Hide Description
In the United States it is common practice to calibrate Cobalt‐60 teletherapy machines ’’in air,’’ despite recommendations by the International Commission on Radiation Units and Measurements (ICRU) and other organizations that calibration be accomplished by measurement at 5‐cm depth in a water phantom. A comparison has been made between the results of ionization measurements in air at 80.5‐cm distance from the source and in water at 80‐cm source–skin distance (SSD) for the determination of absorbed dose at three depth (5, 10, and 15 cm) for each of three field sizes (6×6, 10×10, and 20×20 cm2), for a total of 42 Cobalt‐60 machines. The mean of the ratio, absorbed dose from in‐water measurements to absorbed dose at the same depth calculated from in‐air measurements, ranged from 1.031±0.013 at 15‐cm depth for a 6×6‐cm2field size to 1.009±0.007 at 5‐cm depth for a 20×20‐cm2field size. Reasons for the differences are offered, and compliance with ICRU recommendations is suggested.
4(1977); http://dx.doi.org/10.1118/1.594295View Description Hide Description
Many cobalt and x‐ray therapy machines use mechanical timers to control the given dose. There is no other timer on the machine to verify that the correct time was set or that the timer did not malfunction. This article describes a digital timer that has been installed on the MIR therapy machines as a lapsed‐time indicator.
4(1977); http://dx.doi.org/10.1118/1.594296View Description Hide Description
Radiation parameters for two Varian Clinac‐18 10‐MV x‐ray units were measured and compared with the same parameters for the Toshiba LMR‐13 and the Arco Mevatron XII. Comparison of the percentage depth dose, surface dose, and depth of maximum ionization as a function of field size are presented.
4(1977); http://dx.doi.org/10.1118/1.594380View Description Hide Description
In recent years, the use of tissue‐equivalent materials has become quite common in fast‐neutron dosimetry, with the A‐150 plastic developed by Shonka e t a l. probably the most popular. Information on this specific plastic is scantily reported in the literature and as a consequence a preponderance of authors unknowingly reference an article by Shonka describing an early version of a tissue substitute plastic but having a different elemental composition than the present A‐150 formulation. We have reviewed the results of 21 chemical analyses which have occurred over a time span of four years on a total of 14 samples of A‐150 plastic and based on these data and the formulation of the plastic, have arrived at a suggested composition for A‐150 tissue‐equivalent plastic. The ambiguities of water absorption by nylon, one of the components of of the plastic, and the uncertainty this reflects in the composition of the plastic were evaluated.