Phantom spheres

MicrobaUoons have been used for gap filling, where the spheres dampen sound or vibration in the stmcture. In the medical area, microbaUoons have been evaluated as a skin replacement for bum victims and phantom tissue for radiation studies. An important appHcation is in nitroglycerin-based explosives, in which microbaUoons permit a controUed sequential detonation not possible with glass spheres. 

A problem arises, in that the strong r b dependence of My requires that close overlap of spins be prevented. Thus, even though excluded volume interactions have no effect on chain dimensions in the bulk amorphous phase, it is important in the present application to build in an excluded volume effect (simulated with appropriate hard sphere potentials), so that occasional close encounters of the RIS phantom segments do not lead to unrealistically large values of M2. 

These analyses require that the 10) values for normal incidence be modified for irradiation geometries where the field is incident other than perpendicular to the surface of the personal monitor. The methods used in developing these modifications to fl pdO) for nonnormal incidence included extensive Monte Carlo calculations of photon interactions in anthropomorphic phantoms (Xu, 1994) or in PMMA and tissue slabs and the ICRU sphere (Grosswendt, 1991 Grosswendt and Hohlfeld, 1982), and thermoluminescent dosimeter measurements in water cubes (Lakshmanan et al., 1991). Some of these modifications for /fp(lO) are presented in ICRU (1992). 

Xu (1994) explored three photon energies (i.e., 80 keV, 300 keV and 1 MeV) and a number of irradiation geometries, including geometries with a large range of non-normal incidence. In particular, Xu (1994) provided (estimate) values for a variety of point sources at various locations and distances from the body. Xu (1994) computed the/fpdO) values in tissue-equivalent spheres located on the relevant surface of the anthropomorphic phantom. 

The water phantom used by Lakshmanan et al., (1991) was 30 cm thick. Therefore, the front only values for PA irradiation are much lower than observed by the other investigators using phantoms with thicknesses of 15 or 20 cm, but similar to the front only values of ICRU (1988) which used the 30 cm ICRU sphere. 

Figures and faces materialized from the depths, moving effortlessly on unseen currents. Each was unique, each appeared at a different point in my visual sphere, moved toward me, and attained a peak of aesthetic perfection just as it confronted me from the tiniest distance away. Then it dissolved into light as another, equally beautiful, came into being elsewhere. They were a people suspended in time, adrift in a mind-warp through which 1 could not reach but clearly saw, for when I stared into the forest, the visionary faces were superimposed on whatever I looked at, and when I withdrew within by closing my eyes, my brain-space became a theater of many dimensions in which my phantom tribe appeared, peered, and passed in wondrous procession.

Figure 4.12. An illustration of partial volume effect, (a) Cross section of a sphere phantom with six different spheres (b) PET images of the spheres demonstrating partial volume effect. (Reproduced with permission from Rota Kops E, Krause BJ (2000) Partial volume effects/corrections. In Wieler HJ, Coleman RE (eds) PET in Oncology. Springer, Darmstadt)...

Fig. 5.45 Phantom (left) and images obtained from earliest 10% of the photons centre) and all photons (right) of a TCSPC scan. The glass sphere shows up dimly in the image of the early photons. From [34]...

FIGURE 37.10 Measurements of the electrical field induced by the Hesedcoil and the double-cone coil in a phantom brain. (a,b) The electrical field induced in the z-direction (a) and the electrical field in the z-direction relative to the field 1 cm from the coil (b) is plotted as afiinction of distance from the coil. For both coils, the data show the measurements along the line from the point where maximal Ez value is obtained (as described earlier) to the sphere center. 

The 9- to 3-in. sphere ratio became smaller as the distance from tlie bare and steel-shielded assembly was increased, but remained fairly constant behind the concrete shield. The albedo results obtained were within 25% of the expected dose received by the phantoms. 

In Fignre A.9.2, between the times t and f+dt, the change dF of the real volume is represented by blackened smfaces, whereas the change dFf of fictitious volume is represented by the whole of the smfaces ranging between all the concentric spheres. This fictitious volume includes enlargements of phantom nucleus , which would have been bom inside the transformed phase and which, insofar as the reaction of growth is limited by an interfacial step, would always be included inside small islands of the new phase and would not increase the siuface of the interface between the two solid phases. The fraction dF/dFf corresponds to the probabihty of the increase of the nuttiber of nucleus at the expense of the intact matter. If it is assumed that the nuclei are randomly distributed, this probability is equal to the not-yet transformed fraction of the solid at time t, that is, -a. Thus, by taking accoimt of equations [A.9.1] and [A.9.2] we can write ... 

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