Monte Carlo studies of x-ray scattering in transmission diagnostic radiology.

Monte Carlo methods have been used to simulate the scattering of x rays in polystyrene and water phantoms. In particular, the ratio of the scattered to total x-ray fluence (scatter fraction) has been calculated for monoenergetic x-ray beams in the energy region relevant to diagnostic radiology and nuclear medicine (30-660 keV). Simulations have been made for representative values of the pertinent geometrical factors; phantom thickness from 5 to 21 cm, x-ray beam diameters of 10 and 25 cm, and scatterer-to-image-plane separations from 0 to 20 cm. As a function of x-ray energy, the scatter fraction was found to vary slowly between 30 and 100 keV, and to decrease between 100 and 660 keV. The present results were generated with a special transport code which included the effects of special geometries and the response of the x-ray detector. With the inclusion of these effects, the results resolved inconsistencies and showed good agreement with previous measured and calculated data.

X-ray attenuation properties of radiographic contrast media.

A systematic study has been carried out to assess the x-ray attenuation properties of various elements when used as radiographic contrast enhancing media. This study examines the effects of solutions of molecules with effective atomic numbers from 40 to 92 on the signal-contrast ratio of radiographic images formed with various input x-ray spectra on suitable phantoms. A variety of x-ray spectra were used including monoenergetic spectra, constant potential x-ray tube spectra, and computed tomography spectra. In addition, a computer model was used to predict the effects studied. In general, the computer model is able to accurately predict the resulting signal-contrast ratio for a given combination of contrast media, input x-ray spectra, and phantom composition. From these data and calculations, it may be possible to design new contrast media which are tailored to specific diagnostic imaging tasks.

X-ray scatter background signals in transmission radiography.

With monoenergetic x-ray beams incident on polystyrene phantoms, the spectra of the tramsmitted x rays were measured with a scintillation spectrometer. The scattered and unscattered components of the transmitted x-ray fluence at a point on the beam axis were determined as a function of (i) the incident x-ray energy (18, 22, 32, 49, 58, 69, and 660 keV), (ii) the phantom thickness (5.3, 10, and 21 cm), (iii) the scatter solid angle determined by the exposed area of the phantom and the separation distance of the image plane (0.090, 0.31, 0.66, 1.8, 3.5 4.3, 4.8, and 5.1 sr), and (iv) the beam diameter at the image plane (25, 17, and 10 cm). The results indicate that, as the incident x-ray energy decreases from 660 to 30 keV, the contribution of the scattered component to the transmitted fluence increases from approximately 50% to 90% for the 21-cm phantom and from 21% to 50% for the 5.3-cm phantom. For typical cases, the data show the effect of the scatter component on the ratio of the image to the background signals. In addition, the examples show that optimum conditions for maximizing this signal ratio may be obtained by a careful selection of the incident x-ray energy for low-, medium, and high-contrast objects.

Image information transfer properties of x-ray fluorescent screens.

The image information transfer efficiency for five x-ray fluorescent screens (calcium tungstate, barium halide, and three rare earth screens) has been experimentally determined with monoenergetic x-ray beams at energies of 18, 22, 32, 49, 51, 58, and 69 keV. The transfer efficiency, which is defined by the ratio of the output signal-to-noise ratios, was determined from measurements of (a) the fraction of incident x rays absorbed in the screen and (b) the statistical distribution of the number of light photons emitted from the screen per absorbed x ray which was determined by light photon counting techniques. Comparisons of the information transfer efficiency, the average number of light photons emitted per absorbed x ray, and the light output energy per Roentgen are given for the above screens and x-ray energies.

New method for the experimental evaluation of x-ray grids.

This work describes a new method for the experimental evaluation of antiscatter x-ray grids in radiography. Five commercial grids are evaluated in terms of two parameters which are determined only by the construction of the grid and the x-ray energy. A comparison of the grid performances was made for the x-ray energies and scatter conditions that usually apply to chest radiography and mammography. The results show that for maximum scatter conditions the grid enhances the subject contrast by factors of approximately 6 and 2 in chest radiography and mammography, respectively, and that the contrast increases as the grid ratio increases. Also, in these examinations, the results show that, with improved grids, it is possible to reduce the patient exposures required for the no-grid case by approximately one-half without loss of the image-information content (signal-to-noise ratio).

Image information transfer properties of x-ray intensifying screens in the energy range from 17 to 320 keV.

The image information transfer properties of a number of x-ray fluorescent screens have been measured for x-ray energies from 17 to 320 keV. The detective quantum efficiency of the screens at each x-ray energy has been determined by separate measurements of the x-ray absorption efficiency and the statistical factor associated with the emission of optical photons upon absorption of an incident x-ray. Data have been recorded for both rare-earth phosphor screens and calcium tungstate screens. The value of the statistical factor for optical photon emission tends toward a constant value as the incident energy increases. Comparisons of the image information transfer properties are presented for several screens, which have been measured over a ten year interval. The utility of the screens for high-energy radiography is discussed.

Intensifying effect of metallic screens on the sensitivity of x-ray films for 400-kV bremsstrahlung photons.

Measurements are carried out to determine the relative photographic effect produced by electrons emanating from metallic screens on a typical radiographic film. The electrons are produced in metal foils of aluminum, copper, and lead by the interaction of photons in a 400-kV bremsstrahlung beam. Intensification factors of up to 1.65, 2.05, and 5.90 aluminum, copper, and lead screens, respectively, are determined as a function of the foil thickness. The equilibrium thickness screens are determined to be 20 mg/cm2 for aluminum and 30 mg/cm2 for copper and lead. These results are compared with the absorbed dose in the film emulsions, calculated by Monte Carlo methods. The results of the present work are compared with those for 60Co and 137Cs photon beams. The dependence of the equilibrium thickness and the intensification factor on photon energy and the atomic number of the screen material is summarized and explained.

Use of storage phosphor imaging plates in portal imaging and high-energy radiography: the intensifying effect of metallic screens on the sensitivity.

The sensitivity of storage phosphor imaging plates (SPIP) at megavolt photon energies (60Co, 6-, 10-, and 18-MV radiotherapy beams) is studied both experimentally and by Monte Carlo radiation transport calculations. In addition, the same techniques are used to investigate the intensifying effect of metal screens on the sensitivity of the SPIP. The results provide evidence that the sensitivity of the SPIPs is proportional to the absorbed energy in the phosphor layer per cGy. The spectral sensitivity is calculated for photon energies between 10 keV and 20 MeV for various SPIP-screen combinations.

X-ray scatter data for diagnostic radiology.

The ratio of the scattered to the total X-ray fluence (scatter fraction) at the centre of the image plane for X-rays transmitted through polystyrene phantoms has been measured for X-ray energies of 32 and 69 keV, X-ray beam diameters from 4 to 40 cm, phantom thicknesses from 5 to 30 cm and phantom-to-image-plane separations from 0.3 to 40 cm. The experimental values for this ratio have less than a 10% variation for these two X-ray energies and the experimental data show good agreement with Monte Carlo calculations and available experimental results for low atomic number materials. Based on these results, simple curves are generated which give estimates (+/- 10%) of the scatter fraction for all combinations of the geometric parameters encountered in diagnostic radiology.

A radiation accident at an industrial accelerator facility.

On 11 December 1991, a radiation overexposure occurred at an industrial radiation facility in Maryland. The radiation source was a 3-MV potential drop accelerator designed to produce high electron beam currents for materials-processing applications. This accelerator is capable of producing a 25 milliampere swept electron beam that is scanned over a width of 112.5 cm and which emerges from the accelerator vacuum system through a titanium double window assembly. During maintenance on the lower window pressure plate, an operator placed his hands, head, and feet in the beam. This was done with the filament voltage of the electron source turned "off," but with the full accelerating potential on the high voltage terminal. The operator's body, especially his extremities and head, were exposed to electron dark current. In an attempt to reconstruct the accident, radiochromic film and alanine measurements were made with the accelerator operated at two beam currents. Measured dose rates ranged from approximately 40 cGy s-1 inside the victim's shoe to 1,300 cGy s-1 at the hand position. Approximately 3 mo after the accident, it was necessary to amputate the four digits of the victim's right hand and most of the four digits of his left hand. Electron paramagnetic resonance spectrometry, which measures the concentration of radiation-induced paramagnetic centers in calcified tissues, was used to estimate the dose to the victim's extremities. A mean dose estimate of 55.0 +/- 3.5 Gy (95% confidence level) averaged over the mass of the bone was obtained for the victim's left middle finger (middle phalanx).

Investigation of applicability of alanine and radiochromic detectors to dosimetry of proton clinical beams.

Cancer therapy studies using proton accelerators are underway in several major medical centers in the U.S., Russia, Japan and elsewhere. To facilitate dosimetry intercomparisons between these laboratories, alanine-based detectors produced at the National Institute of Standards and Technology and commercially available radiochromic films were studied for their possible use as passive transfer dosimeters for clinical proton beams. Evaluation of characteristics of these instruments, including the LET dependence of their response of proton energy, was carried out at the Institute of Theoretical and Experimental Physics. Results of absolute dose measurements were regarded as a preliminary step of dose intercomparison between ITEP and NIST. Measurements made in a number of experiments showed average agreement between the ITEP and NIST dosimetry standards to 2.5%.

Thickening of the nuclear fibrous lamina in injured human semilunar cartilages.

A morphometric study was carried out to assess the thickness of the fibrous lamina in the nuclei of cells adjacent to tears in semilunar cartilages and in cells from relatively normal uninjured parts of semilunar cartilages. Comparisons of mean thickness of the lamina showed a statistically significant thickening of the lamina at the site of injury. This observation supports the growing body of evidence indicating that the fibrous lamina is not a static structure of fixed dimensions for a given cell type but a dynamic structure whose thickness varies as a result of altered physiological or pathological states.

A mechanism of formation of desmosome-like structures between synovial intimal cells.

Desmosomes or desmosome-like structures do not occur between normal synovial cells but such structures do develop between the synovial cells in cases of traumatic arthritis, rheumatoid arthritis and villonodular synovitis. Morphological evidence is presented suggesting that such structures develop as a result of the interaction of fibrin trapped between synovial cells and the plasmamembrane of these cells.