Volume 4, Issue 2, March 1977
Index of content:
4(1977); http://dx.doi.org/10.1118/1.594381View Description Hide Description
The immediate goal of clinically based x‐ray‐transmission computed tomography(CT) is to provide a measurement of the x‐ray linear attenuation coefficient in cross section with the ultimate goal of impacting on patient managerment and care. To do this with the accuracy needed for clinical goals requires the careful integration of x‐ray physics, detector technology, and mathematical reconstruction theory. Performance evaluation and quality assurance are necessary adjuncts to a CT scanning program. A number of investigative studies are under way.
Sample mass determination using Compton‐ and total scattered excitation radiation for energy‐dispersive x‐ray fluorescent analysis of trace elements in soft tissue specimens4(1977); http://dx.doi.org/10.1118/1.594388View Description Hide Description
Compton profiles and total scattered intensities have been measured to determine the total sample mass, analyzed by an x‐ray probe energy‐dispersive analyzer. Under photon excitation, fluorescent x rays are emitted from the trace elements in a biological matrix. From incident radiation, the number of photons which are Compton and elastically scattered by low‐atomic‐number elements is directly proportional to the total specimen mass. Tissue specimen masses have been measured from Compton intensities using Zr Kα and Ma Kα excitation x rays and mass calibration standards based upon carbonscattering. This procedure has been extended to include lower‐energy excitation radiation, such as Cu, where the resolution of an energy‐dispersion system requires that the total scattered intensity be used to determine the sample mass. Trace element weight‐fraction concentrations are determined from this scheme with precisions of 1% in 2%, relying only upon information contained in the energy‐dispersive x‐ray spectrum. By adjusting for the difference between tissue and carbonscattering, the accuracy of the elemental weight‐fraction concentrations is brought to within l0% of elemental concentrations measured by atomic absorption spectroscopy, for samples weighing up to 25 mg. In the case of heavier samples, absorption corrections are necessary to achieve this accuracy.
4(1977); http://dx.doi.org/10.1118/1.594307View Description Hide Description
The suitability of the intense K αx rays of terbium emitted in the electron‐capture decay of 159Dy for use in transmission imaging and bone mineral analysis is investigated. It is found that this radionuclide offers all the advantages of radiations from 210Pb and none of the disadvantages inherent in the use of the latter. Yields of the K α and K βx rays of terbium and the 58‐keV γ rays emitted in 159Dy decay are determined using a high‐resolution Si(Li) photon spectrometer. Attenuation coefficients for these photons in gadolinium (critical) absorber are measured in a narrow‐beam geometry. For Tb K βx rays, whose average energy is only about 0.4 keV above the K edge of Gd, our experimental attenuation coefficient is about 10% less than the theoretical value given by Storm and Israel. Transmission images of regular and irregular bones obtained using 159Dy are presented.
4(1977); http://dx.doi.org/10.1118/1.594389View Description Hide Description
The depth of penetration of heavy charged‐particle therapy beams is sensitive to the density of tissues traversed. Maximum depth of dose contours will vary appreciably as the beam passes through bone, muscle, lung, and air or gas. Calculations suggest that beam activation of the short‐lived positron‐emitting isotope 15O i n v i v o will permit localization of proton therapy beams with reasonable detected‐event density and dose. Preliminary tests of this method indicate that the beam can be located at depth with a typical dose of 15 rad, using a large field‐of‐view positron camera on‐line. This technique is also applicable to other heavy charged‐particle beams, negative pions, and heavy ions.
4(1977); http://dx.doi.org/10.1118/1.594382View Description Hide Description
Fast‐neutron beams are being employed in radiotherapy trials and associated radiobiology studies at numerous centers in the U.S., Europe, and Japan. Since collimated beams of various sizes and shapes are employed, it is desirable to know the composition of the scatteredradiation component contributed by the collimator. A simple method is shown for deducing the field composition in terms of a three‐component model, from measurements made with three ionization chambers (tissue‐equivalent, graphite, and magnesium). The dose contributed by the scatteredradiation in the present example was found to be predominantly due to fast neutrons indistinguishable from those in the primary spectrum (from 35‐MeV D+ on Be). This method may prove useful for measurements in phantoms as well.
4(1977); http://dx.doi.org/10.1118/1.594308View Description Hide Description
Multiplicative corrections for percent depth‐dose values were measured for situations with nonmaximal backscatter because of reduced thickness of the irradiated phantom. Data were obtained for common clinical field sizes for a 60Co beam as well as beams from a 2.5‐MV and a 4.0‐MV generator. Functional forms, which summarize the results and include field size effects, depth, and the thickness of the backscatter medium as variables, were obtained by regression analysis.
4(1977); http://dx.doi.org/10.1118/1.594309View Description Hide Description
We have measured the effect of a 10‐kG magnetic field on the dose distribution of electrons in a polystyrene phantom. Isodensity plots and depth‐dose curves are presented for 22‐ and 28‐MeV electron beams with and without the magnetic field applied. The measurements show that magnetic fields as low as 10 kG can produce a substantial modification of the absorbed dose distribution. When compared with the zero‐magnetic‐field distribution of the same energy, the magnetic field significantly improves the D max‐surface dose ratio and increases the fall off in dose past the D max region.
4(1977); http://dx.doi.org/10.1118/1.594390View Description Hide Description
The relative sensitivity of various materials for the measurement of half‐value‐layer thickness has been calculated for bremsstrahlung x‐ray spectra in the energy range 5–40 MeV. It is concluded that low-atomic-number materials such as water are more sensitive to changes in spectral quality of megavoltage x rays than high-atomic-number materials such as lead.
4(1977); http://dx.doi.org/10.1118/1.594391View Description Hide Description
Central‐axis percentage depth doses and tissue–maximum ratios (TMR) for 45‐MV photon beams from a betatron have been measured in water. Also the influence of field size and collimator scatter on the dose in the buildup region have been investigated. The maximum dose for TMR has been shown to occur at a point about 2 cm deeper than the maximum dose for percentage depth dose. This difference is significant in the characterization of the photon beam at this high energy. The measured physical data have been computerized for use in routine treatment planning. Computer‐generated beams have been found to be in close agreement with measured isodose curves. Computer‐generated isodose distributions for typical clinical irradiation techniques have been verified using RP/V film in an Alderson phantom.
4(1977); http://dx.doi.org/10.1118/1.594310View Description Hide Description
The calibration (in rad/nC) of an air‐filled tissue‐equivalent ionization chamber for neutrons of l5‐MeV average energy was determined by measuring absolute fluences and calculating the kerma per unit fluence. The neutroncalibration determined from the γ‐ray calibration and application of the Bragg–Gray relation was found to be 4% higher than that based on the fluence measurement. Additional data were taken to obtain calibration factors with tissue‐equivalent gas; the same difference betwen the two methods was observed.
4(1977); http://dx.doi.org/10.1118/1.594311View Description Hide Description
An overall practical dosimetric study of a 10‐MV photon beam produced by a Varian Clinac‐18 linear accelerator is presented. In particular measurements were made to provide data which could be utilized in computerized dosage calculations using the concept of dividing the radiation beam into primary and scatter components. From the measured percentage depth doses, tissue–phantom ratios are calculated. Special consideration is given to the derivation and measurement of zero‐area tissue–phantom ratios such that scatter–phantom ratios could be obtained.. The computer techniques were tested under a number of specified conditions by comparing the calculated results to the measured data. The variation of dose with field size and distance is considered and attenuation data for shielding, wedge and compensating materials is provided.
Comparison of convolution and ray‐tracing methods for computing small blood vessel images in angiography4(1977); http://dx.doi.org/10.1118/1.594384View Description Hide Description
Two methods for computing x‐ray images of small blood vessels in angiography are presently available, namely, convolution and ray tracing. The convolution method, which is simpler and more powerful than the ray‐tracing method, is based on the assumption that blood vessel imaging is isoplanatic, whereas ray tracing is considered to provide correct images. In this study, the approximation error (difference between two images, normalized by the maximum value) due to nonisoplanatic imaging was determined by computation of blood vessel images according to both methods. The approximation error for geometric conditions normally encountered in angiography was less than 0.01. It is concluded that an approximation error of this magnitude is negligible and that the convolution method can be applied instead of the ray‐tracing method for the computation of images of small blood vessels.
Fast Fourier digital quantum mottle analysis with application to rare earth intensifying screen systems4(1977); http://dx.doi.org/10.1118/1.594304View Description Hide Description
The advent of fast Fourier techniques has greatly facilitated the digital analysis of noise power spectra (Wiener spectra) by circumventing the need for the autocorrelation function. We are now able to Fourier analyze film data at about the same rate the microdensitometer–computer system can collect it (1000 points/sec). The new technique has been applied to the analysis of the quantum mottle of several rare earth intensifying screen systems confirming earlier estimates from our pilot studies that such screens are c a p a b l e of reducing exposure by a factor of about 2 with imaging parameters comparable to those of conventional calcium tungstate systems.
4(1977); http://dx.doi.org/10.1118/1.594305View Description Hide Description
The two‐dimensional modulation transfer function [MTF(ν x ,ν y )] has been determined for the EMI CT head scanner by measuring the point spread function (PSF) in different locations in the field of view. This PSF was obtained by scanning a fine wire supported perpendicular to the tomographic slice. Based on these MTFs, the resolving power of the EMI scanner was found to be 3.1 line pairs/cm. Our results also verify the symmetry of the system response and the uniformity of the system resolution.