Volume 4, Issue 4, July 1977
Index of content:
4(1977); http://dx.doi.org/10.1118/1.594362View Description Hide Description
‘‘The imaging process is fundamentally a sampling process.’’ This philosophy of Otto Schade, utilizing the concepts of sample number and sampling aperture, is applied to a systems analysis of radiographicimaging, including some aspects of vision. It leads to a simple modification of the Rose statistical model; this results in excellent fits to the Blackwell data on the detectability of disks as a function of contrast and size. It gives a straightforward prescription for calculating a signal‐to‐noise ratio, which is applicable to the detection of low‐contrast detail in screen–film imaging, including the effects of magnification. The model lies between the optimistic extreme of the Rose model and the pessimistic extreme of the Morgan model. For high‐contrast detail, the rules for the evaluation of noiseless images are recovered.
4(1977); http://dx.doi.org/10.1118/1.594317View Description Hide Description
Methods for obtaining flattened charged‐particle dose distributions over large areas are described. The system being used at Harvard for protonradiotherapy is discussed in detail. It is an extension of usual multiple‐scattering techniques to include blocking out some of the central peak, followed by rescattering to fill in the profile, resulting in flat distributions up to 30 cm in diameter. The unusually long source‐to‐skin distance (4.6‐m SSD) plus the small lateral spread (2 mm rms) from multiple scattering in the patient results in little divergence of the beam throughout the treatment volume.
4(1977); http://dx.doi.org/10.1118/1.594318View Description Hide Description
A phantom consisting of a 2.5‐cm‐diameter aluminum rod, a balloon filled with castor oil, and a balloon 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. Attenuationimages of a transverse section of a leg of lamb were obtained as well. Opposed transmitting and receiving transducers were scannned 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 velocityimaging should not be as sensitive to reflection and refraction as attenuationimaging, a very narrow ultrasound beam must be employed to obtain reasonable resolution in velocityimaging.
Scattered photons as the cause for the observed d max shift with field size in high‐energy photon beams4(1977); http://dx.doi.org/10.1118/1.594319View Description Hide Description
Measurements on the Sagittaire linear accelerator and Allis–Chalmers betatron at M. D. Anderson Hospital indicate that the observed d max shift with field size is due to the presence of Compton‐scattered photons in the therapy beam, and not electrons as proposed by others. Separating the primary from other radiation components indicates that the secondary fraction represents a percentage contribution to the overall beam that increases as the collimators are opened. This is consistent with what would be expected from Compton scattering and explains the effective softening of the beam as field size increases.
4(1977); http://dx.doi.org/10.1118/1.594320View Description Hide Description
Each section of the Alderson Rando phantom contains a tissue‐equivalent plastic coating layer approximately 2 mm thick, applied to both faces. This compensates for material removed in the sawing process. Conventional use of thermoluminescent dosimeters positions them totally or partially within the coating layer. Analysis shows that, in the lung region, dosimeters placed in this layer received a dose averaging 39% lower than those placed at midsection. Where bony structures interfere, some dosimeters in the coating layer received an 18% higher dose than those at midsection. Therefore, positioning dosimeters at the center of a section is recommended.
Dosimetric measurements in the neutron field produced by a deuterium gas target with a compact medical cyclotron4(1977); http://dx.doi.org/10.1118/1.594321View Description Hide Description
Several dosimetric measurements have been carried out in the neutron field produced by accelerated deuterons incident upon a pressurized deuterium gas target. A significant improvement has been achieved in central‐axis depth dose when compared to the 9Be(3He,n)11C interaction previously studied. Designs for a target with a high‐power dissipation capability are currently underway.
4(1977); http://dx.doi.org/10.1118/1.594316View Description Hide Description