Showing papers on "Imaging phantom published in 1968"

Attention Research: The Case of the Verbal Phantom

Abstract: To layman and professional alike, the association of attention with learning and schooling is firm and long standing. Those trusted with the development of reading, writing, and 'rithrnetic skills speak of providing for proper attention, and credit undesirable behaviors or inferior achievement to inattention. Academic proficiency is not the sole user of the attention concept, however, for it also occupies a major explanatory role in accounting for a variety of complex interpersonal as well as educational behaviors. Education, however, has not taken primary responsibility for collecting the supportive data or for developing the requisite theories. It is rather, in the vast literature of psychology that one is more apt to find both the breadth and the detail which is needed for a proper appreciation of the concept of attention. For a while, it seemed that attention would remain an historical curiosity, although the recent years have wimessed its rediscovery. Accompanying its Phoenix like resurection has been a parade of symposia, conferences, and specialized monographs. There is reason to believe that this activity will persist. For the topic of attention had once dominated the thinking of education and psychology and a concept with such an impressive lineage does not pass quietly on nor remain denied too long. In its own right it holds the prospect for providing explanatory refinement to large bodies of data. It is not the purpose of this paper to eulogize attention, although a proper historical account must await the skilled talents of an insightful historian. The amateur risks a false or imprecise historical reconstruction; one which is open to biased attempts at eliminating competition, or its counterpart of claiming spurious ancestors. The absence of such an historical appreciation has left a void in establishing the accurate meaning or meanings which are to be understood by the term attention. With the increased frequency with which the word is used, and in the face of the diverse experimental tactics which have been suggested in the name of praising or burying attention-research, a

Flux density distributions in and around a man-sized phantom irradiated with thermal neutrons.

Abstract: Using a broad beam of thermal neutrons from the graphite column of a nuclear reactor, the presence of the phantom is shown experimentally to produce a disturbance of flux density levels over distances as large as 70 to 100 cm from its surface. From the experimentally determined flux density levels inside the phantom, the level of flux density equivalent to 25 mrem hr−1 is derived as 915 n cm−2 sec−1 when gonads and red bone-marrow are considered to be the critical organs. This is greater than the recommended level of 670 n cm−2 sec−1, which may, therefore, be needlessly restrictive. The significance of the results for the calibration of instruments and dosemeters is discussed briefly.

Practical inferences from studies with a "breathing" liver phantom

Abstract: A simple and inexpensive liver phantom capable of simulating respiratory motion was constructed from materials readily available in most radioisotope laboratories and radiology departments. The effects of motion on liver scanning were studied simulating conditions in the adult human as closely as possible in regards to technical and physical factors as well as the type of radionuclide and dosage used. It is felt that these studies have shown: (1) that the indentations (picket fence effect) noted at the margins of the liver are due to the combined effects of respiratory movement of the liver and rectilinear movement of the conventional scanner; (2) that counting rates and thereby the efficiency of the detector are affected by this motion; and (3) that the smallest nodule detectable in a motionless phantom will not be seen at all when a motion similar to human diaphragmatic excursion is added.

Effects of phantom geometry on dose distribution

Abstract: Calculations have been made of the spatial distribution of dose and dose equivalent due to protons of energies 250 MeV and 400 MeV incident on a tissue phantom which has dimensions approximating those of a human torso and has the form of a right elliptical cylinder with a height of 70 cm and with semiaxes of the elliptical base as 20 cm and 10 cm. The results obtained have been compared with the corresponding data previously obtained for a tissue slab having thickness of 30 cm. It is shown that within the limits of accuracy due to statistical fluctuations the distribution of dose and dose equivalent are not substantially different in a cylindrical phantom from the corresponding values in a slab phantom.

On depth-dose distributions for fallout and simulated fallout fields

Abstract: Gamma-ray depth-dose patterns in a phantom exposed to fallout were calculated by the Monte Carlo method. The phantom consisted of a tissue equivalent vertical right cylinder 60 cm in height and 30 cm in diameter. The center of the phantom was 1111.8 cm above a smooth ground surface uniformly contaminated with 235U fission products. The energy and angular distribution of the gamma rays incident upon the phantom were taken from a previous Monte Carlo study. The depth-dose patterns were found to be relatively insensitive to fallout age over the period investigated (1 hr to 9 days). The dose rate at the center of the phantom is approximately 65% of the free-field dose rate, while that at the lateral surface is approximately 80%. Except near the extremities, the dose rate along the vertical axis of the phantom varies at approximately the same rate with height above ground as does the free-field dose rate. Approximately one-half of the dose rate at the center of the phantom is from photons which have suffered previous collisions in the phantom. The depth-dose patterns were also calculated for two arrangements of artificial sources which, although not duplicating the fallout energy spectra, were intended to simulate fallout biological effects. The patterns produced by revolving the phantom on its vertical axis while exposed to a point 60Co source at a horizontal distance of 61 m are similar to those from the fallout, except for internal positions near the bottom of the phantom. A special arrangement of 60Co, 137Cs and 144Ce sources produced substantially the same depth-dose patterns throughout the phantom as did the fallout.

Depth dose distribution in a beagle phantom within a mixed field of neutrons and gamma-rays

Abstract: As a result of the recommendations adopted by the International Commission on Radiological Units and Measurements (ICRU) for Class B irradiations (cases where the irradiation is not uniform because of absorption) a series of investigations for the evaluation of depth dose has been initiated in order to more accurately describe the absorbed dose at all points of interest for different size phantoms. The work to be reported here includes studies with two phantoms the approximate size of a beagle. These include a homogeneous tissue equivalent liquid phantom and an Alderson beagle phantom constructed of tissue equivalent plastic containing a skeleton. The incident radiation field being studied is the mixed field of neutrons plus gamma-rays within a fast neutron exposure volume of the Walter Reed Research Reactor, a homogeneous L-54 type designed by Atomics International. The detectors being used in this study are miniature tissue equivalent condenser ionization chambers to measure the total dose (neutron plus gamma), Li7F (TLD-700) for measuring the gamma dose within the mixed field and the threshold detectors of Pu239, Np237, and U238 (shielded with B10) and S32 to obtain the neutron dose. Dose variations with respect to depth are currently being obtained and evaluated for both phantoms. The linear energy transfer (LET) distribution of dose with respect to depth is also being evaluated using the threshold detector technique of Hurst. Results from these evaluations are being compared with those obtained by use of a modified Rossi tissue equivalent LET proportional counter. The results of the neutron and gamma depth dose variations and distribution of neutron LET will be discussed with respect to the techniques employed and input parameters used.