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Biomedical subjects

T C Zhu

Publications and source records attributed to T C Zhu.

25 records · Page 2Linked to original sources
PubMed1995

The fraction of photons undergoing head scatter in x-ray beams.

The output factor in air for a high-energy x-ray beam varies with the collimator setting. Collimator backscatter and obscuring of the source in the target contribute to this variation, but the main component is photon scatter in structures in the accelerator head. To determine the scatter-to-primary ratio, SPR, between the air kerma from such scattered photons and from those that originate directly from the source, it is essential to know the shape of this function SPR(c) of the square collimator opening c, especially for small c. To determine this, simulated head scatter was generated in lead blocks inside the head. By taking the ratio of the output factors for two such blocks of slightly different thickness, the effects of source obscuring and collimator backscatter were eliminated. Using the measured difference in transmission through the two lead blocks, the limiting value for c-->0 could be determined and hence the scatter-to-primary ratios SPR(c). The results could be fitted well with an error function, which then was applied to data measured for clinical beams of 6 and 25 MV. For c = 20 cm, SPR for one 6 MV beam was determined to be 0.016 without a flattening filter, 0.06 for the open beam, and 0.16 with the built-in wedge. At 25 MV, the SPR was higher, 0.09 and 0.20 for the open and wedged beams, respectively.

Humans↗
PubMed1995

Scattered photons from wedges in high-energy x-ray beams.

The presence of a wedge increases the fraction of "head-scattered" photons in a high-energy x-ray beam. We have compared internal and external wedges for x-ray beam energies between 6 and 25 MV by determining their SPRw, i.e., the ratio of the dose contribution from photons scattered by the wedge and photons either coming directly from the target or scattered by other structures, including the flattening filter. Marked differences were observed. First, SPRw for the thickest external wedge (60 degrees) was 1.8% at a field size of 10 x 10 cm2 and mildly dependent on the photon energy, while SPRw for internal wedges for this field size varied between 4.4% and 5.4% depending mostly on the location and size of the wedge and marginally on the photon energy. Second, the variation of SPRw with the collimator setting c x c was different for the internal and external wedges. SPRw for the internal wedge approached a limiting value at large c and could be fitted with an error function, while SPRw for the external wedge increased quadratically with c. As a result, the difference in SPRw (c) for internal and external wedges is reduced for large fields.

Humans↗
PubMed1995

X-ray source and the output factor.

When the collimator setting of a linear accelerator is made sufficiently small, the output factor in air, R, is greatly reduced because the collimators obstruct the periphery of the x-ray source. This has been utilized to examine the size of the source by varying the width y of a narrow field and determining how R(y) varies. The sources diameters in the two principal directions were clearly influenced by the design of the accelerators. The x-ray sources of two accelerators with bending magnets were found to be noncircular while that of a linear accelerator without a magnet showed circular symmetry. The position of the source relative to the axis of collimator rotation was determined by measuring R for offset narrow fields. For one of the accelerators, the source was initially moving and off the central axis by about 2 mm for the first five monitor units. The results correlated well with sharpness in portal-film images. The technique can serve to evaluate the major source characteristics in acceptance testing and quality control.

Equipment Design↗
PubMed1995

Detection of epidermal cellular DNA content and its clinical significance in psoriasis vulgaris.

Epidermal cellular DNA content was detected by flow cytometry on 14 samples of normal skin and both lesional and nonlesional areas of 36 patients with psoriasis vulgaris. The results showed that the proliferative activity of epidermal cells increased in nonlesions and even more in lesions of patients from the levels in normal skin. These differences were significant (p < 0.05). Clinical features and histopathological changes were observed, and self comparisons of 6 patients before and after treatment were also done. The clinical significance of these changes was discussed.

Adult↗
PubMed1994

The head-scatter factor for small field sizes.

The head-scatter factors H were examined for four different linear accelerators and were found to be similar at field sizes larger than 3 x 3 cm2. Sharply reduced values for small collimator openings were observed for all the accelerators. It is concluded that the head-scatter (or collimator-scatter) factor has two major components. Scatter of photons in various structures in the beam path, especially the flattening filter, causes a slow (about 10%) increase with increased collimator opening. Insertion of a built-in wedge may double this number. When the collimators are closed, they ultimately block photons from the periphery of the source. This may cause a considerable reduction of the primary photon fluence and typically affects fields smaller than 3 x 3 cm2. The effect can be used to estimate the source size, with results that correlate with the design of the bending magnet.

Biophysical Phenomena↗