Volume 7, Issue 1, January 1980
Index of content:
7(1980); http://dx.doi.org/10.1118/1.594766View Description Hide Description
A method for producing microwave images that are improved through the reduction of multipath propagation effects is described. This method employs water immersed antennas and microwave time delay spectroscopy (TDS) at S‐band. It is shown that objects as small as 6 mm in diameter can be detected. Objects 10 mm apart appear as separate responses on the image. It is also shown that is possible to generate a physiologically and anatomically relevant image of an isolated canine renal specimen.
7(1980); http://dx.doi.org/10.1118/1.594661View Description Hide Description
Ten types of lenses were tested for their radiation protection properties in three experimental circumstances: (a) Their good geometry (scatter free) attenuation was determined as a function of HVL in a direct x‐ray beam. (b) Their dose reducing properties were determined under simulated clinical fluoroscopic conditions, using both ion chambers and thermoluminescent dosimeters (TLD’s) while the lenses, mounted in frames, were worn by a head phantom. The head was oriented in four different directions to simulate clinical conditions in a fluoroscopic room. (c) Their dose reducing properties were measured with TLD’s while the glasses were worn by radiologists during clinical fluoroscopic procedures. Although several of the lenses could attenuate a direct x‐ray beam several thousand times, none of the glasses offer more than a factor of 8 reduction in eye exposure when the glasses are worn during fluoroscopy. This is attributed to backscatter from the fluoroscopist’s head and deficient lateral shielding. Some of the glasses may even be causing an increase in eye exposure when the TV monitor is located 90° or more from the patient.
Effect of reduced scatter on radiographic information content and patient exposure: A quantitative demonstration7(1980); http://dx.doi.org/10.1118/1.594773View Description Hide Description
If a normal screen‐film exposure is made using a conventional grid, a reduced exposure may be used with a superior scatter removal device to record the same image information. This theoretical conclusion is not readily demonstrated because of the limited visibility on the underexposed film. In this study radiographs underexposed by a factor of three with a scanning multiple slit assembly (SMSA) were enhanced autoradiographically and found visually to contain more information than properly exposed radiographs obtained under similar conditions using a conventional Bucky grid. This observation was confirmed by contrast scale and noise measurements. Futhermore, the entrance skin exposure of the radiograph using the SMSA was 45% of that required with the grid technique at the same beam quality.
7(1980); http://dx.doi.org/10.1118/1.594654View Description Hide Description
A technique has been developed and implemented by which individualized Ellis compensating filters can be rapidly and accurately produced from Moiré topographic photographs. A novel Moirécamera has been designed such that it has been possible to mount it on a radiotherapy simulator. The camera does not interfere with normal use of the simulator but can be deployed as needed. Depth of missing tissue contours are produced by the camera over the treatment field, accurately registered to the patient’s simulated treatment position. An optical enlargement technique is used to transfer the depth of missing tissue contours in the photograph to a lead sheet compensating filter.
X‐ray computed tomography in the presence of arbitrary symmetrical focal spot intensity distributions7(1980); http://dx.doi.org/10.1118/1.594655View Description Hide Description
A loss of resolution in tomographic images is due to the well‐known bimodal spatial variations in photon emission across the x‐ray focal spot combined with the finite width of the x‐ray detector. Such departures from the ideal infinitely narrow beams which are usually considered can be taken into account by replacing the customary line integrals by suitably chosen integrals over strips. The image degradation resulting from using those strip integrals as ingredients in conventional reconstruction formulas is expressible as a complicated point spread function, which can always be computed in any given situation. Restoring the reconstructed image to provide higher resolution is a nontrivial operation, which however greatly benefits from experience already gained in dealing with optical imagery degraded by the aberrations of astigmatism and curvature of field. Typical point spread functions are shown for a simple bimodal source intensity distribution of primordial importance.
7(1980); http://dx.doi.org/10.1118/1.594656View Description Hide Description
The general requirements for balancing of detectors in a multiple detectorCTscanner are discussed, and the specific requirements for software balancing using data acquired within the body section during the scan are outlned. A particular technique which can be implemented in a scanner employing a simultaneous translation and rotation of the source/detector package is presented. The technique produces an exact balancing of offset and gain factors, to within accuracy limits imposed by the statistical uncertainty due to noise in the measurements. However, the technique admits a redundancy of solutions with allows the statistical sample to be expanded to include virtually all data acquired during the scan, thereby suppressing detector imbalance artifacts to the same level as overall quantum noise in the image.
7(1980); http://dx.doi.org/10.1118/1.594657View Description Hide Description
Using stable materials capable of mimicking any soft tissues of the human body—quantitatively with respect to speed of sound, density, and attenuation coefficient and semiquantitatively with respect to scatter—we have constructed a torso section phantom containing simulated bowel gas, kidney, kidney fat pad, liver, tumor, cyst, bony structures, and resolution fibers. Simple geometries and precise knowledge of ultrasonic properties allow artifacts to be produced and explained with great confidence. Artifacts due to refraction and reflection are particularly dramatic. Phantoms such as this should be useful as stage one phantoms for training ultrasonographers. Such phantoms should also be of use to those who are doing research in ultrasonic image processing. This might involve, for example, recognizing image artifacts and reducing their effect in the formation of the image.
7(1980); http://dx.doi.org/10.1118/1.594658View Description Hide Description
High energy electrons set into motion by photon interactions with matter lose some of their energy by bremsstrahlung. This loss must be evaluated before energy absorption coefficients may be calculated. Recent extensive tables of data published by Plechaty e t a l. contain an appreciable error in this quantity. The error results from two simplifying assumptions and for the case of very high photon energies interacting with high atomic number materials can be as much as a factor of two. This has important implications for the evaluation of quantities used in radiation dosimetry.
7(1980); http://dx.doi.org/10.1118/1.594659View Description Hide Description
The method of aluminum activation to 24Na has been shown feasible as a high‐LET, i n v i v o dosimeter for clinical pion beams at the Clinton P. Anderson Meson Physics Facility in Los Alamos. A 3×3 in. φ NaI (Tl) well detector measures the 24Na activity following exposure by windowing the 2.75 MeV photopeak. Calculations of the 24Na activity agree well with experiment if one assumes a production ratio of 0.075 24Na/stopped π− in aluminum, and an in‐flight cross section of 26 mb. The activity is produced primarily by stopping pions although 15–25% of the activity is the result of neutrons. Thus, the induced activation is a good measure of high‐LET dose. By comparison with high‐LET dose measured by a 7.6 μ silicon detector and a Rossi chamber, the amount of high‐LET dose per activation is found to be 1.35×10−6 rad/(24Na/gm Al). A clinical setup has been installed and a sample patient measurement is compared with high‐LET dose calculated by treatment planning programs.
7(1980); http://dx.doi.org/10.1118/1.594767View Description Hide Description
The precise slice geometry of a CTscanner is an important, albeit tedious to determine, characteristic. A series of computer programs have been developed to analyze the slice thickness insert of the AAPM phantom. Without operator assistance they generate the beam profiles and slice thicknesses at three points in the scan field. A representative analysis is done on an AS&E scanner with slice thickness settings of 2 to 10 mm. The resulting discrepent measured thicknesses, ranging from 3 to 8 mm, indicate the need to perform such slice thickness measurements as part of a regular quality assurance program.
7(1980); http://dx.doi.org/10.1118/1.594652View Description Hide Description
The effects of skin sparing and the gamma/neutron dose ratios in the clinical situations presently in use at the TAMVEC neutron teletherapy facility are not appreciably affected by the presence of filters and/or wedges. It is also shown that if skin sparing is lost due to close proximity of a hydrogenous scattering source, it can be restored by the use of thin lead filters.
7(1980); http://dx.doi.org/10.1118/1.594653View Description Hide Description
A computer interface for the Clinac‐18 linear accelerator has been developed, using a standard CAMAC interface plus buffer amplifiers to isolate the CAMAC from the accelerator electronics. Buffer amplifiers are employed because direct connection of the CAMAC system to the accelerator was found to affect accelerator operation. The total interface accommodates the four gantry position analog signals and thirteen digital signals describing all available treatment options. The interface also allows the computer to inhibit beam operation.
Definition of source for head shielding requirements in linear accelerators may affect room shielding design7(1980); http://dx.doi.org/10.1118/1.594768View Description Hide Description
Siemens Corporation defines the source of the Mevatron 20 linear accelerator as a line source approximately 2 m in length. Calculations for secondary barriers for leakage radiation are generally based on 0.1% of the useful beam 1 m from the target. The difference in source terms can lead to calculating thinner barriers than are required. The placement of a 30×10×10‐cm block of lead behind the head was required to achieve acceptable radiation levels.
7(1980); http://dx.doi.org/10.1118/1.594770View Description Hide Description
A TLD reader incorporating the techniques of photon counting and dc methods has been developed for radiation dose measurements in the range of 10−6 Gy (10−2 Gy=1 rad ) to 100 Gy for varied applications in radiation protection and for dosimetry in medical physics. It utilizes state‐of‐the‐art technology and is provided with reproducible heating programmer, glow curve output, digital dose display, check light source and data processing facilities. The reader has been designed for easy operation and maintenance. The precision of the reader is about ±6% (1 σ) for 10−5 Gy and better than ±2% (1 σ) for doses ⩾0.5 cGy. Further improvements are being effected to measure doses down to 10−7 Gy.
7(1980); http://dx.doi.org/10.1118/1.594769View Description Hide Description
A series of computer programs have been written to analyze a scan of the AAPM CT performance phantom resolution insert with little operator assistance. A representative analysis is done on a scan from an AS&E scanner as an example. Its point spread function, edge response function and modulation transfer function are presented.