Most modern radiology facilities are all digital, and film is not available for physics QC testing. An important test is
the measurement of the effective focal spot sizes of the x-ray tube which has an impact on overall image system spatial
resolution. Focal spot size measurements typically utilize a star pattern which is magnified and recorded on plain film.
Plain film is ideal because it has a spatial resolution of 50-75 LP/mm. Utilizing digital image receptors to record the
star pattern images does degrade the measurement because of the size of the individual detector elements (DEL). This
article presents a method for the determination of focal spot size using the digital image receptor (DIR) to record the
image of the star pattern. An equation which corrects for the image blur introduced by the finite size of the digital
detector is given. Measurements of effective focal spot sizes are still important to the assessment of radiographic
spatial resolution.
A monochromatic CT for imaging the human head and neck is being developed at the National Synchrotron Light Source. We compared the performance of this system, multiple energy computed tomography (MECT), with that of a conventional CT (CCT) using phantoms. The advantage in image contrast of MECT, with its beam energy tuned just above the K-edge of contrast element, over CCT carried out at 120 kVp, was approximately equal to 3.2-fold for iodine and approximately equal to 2.2 fold for gadolinium. Image noise was compared by simulations because this comparison requires matching the spatial resolutions of the two systems. Simulations at a 3- rad dose and 3-mm slice height on an 18-cm-diameter acrylic phantom, with MECT operating at 60.5 keV, showed that image noise for MECT was 1.4 HU vs. 1.8 HU for CCT. Simulations in the dual-energy quantitative CT mode showed a two-fold advantage for MECT in image noise, as well as its superior quantification. MECT operated in the planar mode revealed fatty tissue in the body of a rat using xenon K-edge subtraction. Our initial pan for clinical application of the system is to image the composition of carotid artery plaques non-invasively, separating the plaques' main constituents: the fatty, fibrous, and calcified tissues.
A contrast-detail phantom was used to evaluate grid performance. The phantom is constructed of 1 cm of plastic. Holes of varying diameter (detail) and varying depth (contest) were drilled into the contrast-detail phantom. The phantom was placed next to the bucky assembly. A 15 cm block of lucite was placed between the x ray tube and the phantom. A set of radiographs were taken of the phantom at different kVps and different phantom thicknesses. This was done both with and without the grid in place. An ion chamber was used so that the bucky factor could be determined. This entire procedure was repeated for the conventional, reciprocating grid. Contrast-detail curves were generated from the data. As would be expected the reciprocating grid had a lower bucky factor. The contrast improvement factor (contrast with grid/contrast without grid) was higher for the reciprocating grid. The contrast improvement factor for the high-strip- density grid was comparable to that for the reciprocating grid at high kVps and also when a thinner block of lucite was used. Grid lines were seen on the radiographs of the high-strip-density grid.
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