A method for absolute calibration of diagnostic x-ray generators is presented and discussed. The method employs a Ge(Li) detector and relies on the direct observation of the onset of fluorescence radiation from scattering foils placed in the primary beam.

Filtration↗

PubMedDate not supplied

Stem corrections for ionization chambers.

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 efffect for several common ionization chamber systems, exposures were measured with TLD capsules placed at the center of 60Co 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 and 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%.

Cobalt Radioisotopes↗

PubMedDate not supplied

Intensity distribution, modulation transfer function, and the effective dimension of a line-focus x-ray focal spot.

The radiant intensity along the anode-cathode dimension of a line-focus x-ray focal spot tends to peak at the center and decrease gradually to zero at the edges. In recognition of this varying intensity, the ICRU has recommended that the anode-cathode dimension of the focal spot be reported as 0.7 times the dimension measured with a pinhole camera. In this paper more objective correction factors are developed. These correction factors are based upon comparison of calculated MTFs of focal spots with Gaussian and cosine intensity distributions with the MTF for focal spots with a uniform intensity distribution.

Radiography↗

PubMedDate not supplied

Backscatter factors in the mammographic energy range.

Backscatter factors for kVps and half-value layers typical of mammographic beams have been determined as a function of beam diameter and half-value layer. The method utilized thin TLD-100 chips with the scattering medium alternately present and absent. Results indicate that the field-size dependence of the backscatter factor for lower half-value layers is less than that shown in the British Journal of Radiology Supplement 11.

Mammography↗

PubMedDate not supplied

Imaging soft tissue through bone with ultrasound transmission tomography by reconstruction.

A phantom consisting of a 2.5-cm-diameter aluminum rod, a balloon filled with castor oil, and a ballon filled with a phenylated silicone fluid was imaged in water with ultrasound transmission tomography by reconstruction (UTTR). This phantom was chosen for investigation of the feasibility of detecting small changes in the ultrasound velocity and ultrasound attenuation coefficient of soft tissues in planes containing bones. Attenuation images of a transverse section of a leg of lamb were obtained as well. Opposed transmitting and receiving transducers were scanned on either side of the imaged object, just as an x-ray tube and detector are scanned in the pencil beam geometry employed with the first x-ray computerized axial tomographic system. More than adequate signal was transmitted through the aluminum rod and the bone to obtain reconstructions; but, in these crude images, there were numerous artifacts which maintain the uncertainty regarding the future utility of this promising technique. Although velocity imaging should not be as sensitive to reflection and refraction as attenuation imaging, a very narrow ultrasound beam must be employed to obtain reasonable resolution in velocity imaging.

Models, Structural↗

PubMedDate not supplied

Effects of red cell shape and orientation on propagation of sound in blood.

In this paper, the red blood cell (RBC) is assumed to have an oblate spheroidal shape and the same volume (87 micrometer3) and about the same sphericity index (0.7) as a typical human RBC. The acoustic field is assumed to be either parallel or perpendicular to the axis of symmetry of the spheroid, and a wave equation is formulated for a dilute RBC suspension. Because of the small density difference between the RBC and plasma (about 5%), the assumption of spherical shape of the RBC suffices for computation of the velocity and the scattering of sound in blood. For all practical purposes, scattering of sound in blood follows Rayleigh's law of scattering. However, the viscous absorption coefficient at 1 MHz for a spheroidally shaped RBC oscillating broadside and edgewise to an acoustic field is about 40% and 136%, respectively, of that for a spherically shaped RBC. These results illustrate the significant effects of RBC shape and orientation on the viscous loss of sound energy in blood.

Erythrocytes↗

PubMedDate not supplied

Computational analysis and dosimetric evaluation of a commercial irregular-fields computer program.

The proper evaluation of the accuracy of a new computer program for radiation-therapy dosimetry requires consideration of both the mathematical algorithm used in the program and the performance required in clinical applications. As an example, our evaluation of the irregular-fields dosimetry program currently marketed by SHM for their Rad-8 system is described. The evaluation begins with an explanation of the mathematical computation described. The evaluation begins with an explanation of the mathematical computation method, with emphasis on the points where the calculation differs from previous methods. Next, the procedure for setting up the beam data file is discussed. Finally, a step-by-step procedure is described in which calculated doses are compared with measured doses, using a Varian Clinac-4 with lead flattening filter, and the limits within which a +/- 5% accuracy is attainable are estimated. Some sources of error and areas for possible improvements are mentioned.

Computers↗

PubMedDate not supplied

Effective size of the transverse dimension of x-ray-tube focal spots.

Corrections for nonuniform x-ray intensity distributions have been computed for the dimension perpendicular to the anode-cathode axis of x-ray-tube focal spots. These correction factors were developed from comparison of OTFs for focal spots with uniform intensity distributions to those for focal spots with symmetric double-peaked intensity distributions.

Technology, Radiologic↗