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Chest Radiography: Optimization of X-ray Spectrum for Cesium Iodide–Amorphous Silicon Flat-Panel Detector¹

¹ From the Departments of Radiology (J.T.D., E.S., H.G.C., A.H.B., C.E.F., C.E.R.) and Biomedical Engineering (J.T.D., E.S., R.J.W., A.H.B., C.E.F.), Duke University Medical Center, 161F Bryan Research Bldg, Research Drive, DUMC Box 3302, Durham, NC 27710. Received December 11, 2001; revision requested February 18, 2002; revision received April 1; accepted May 23. J.T.D. supported in part by grant R01 CA80490 from the National Cancer Institute. E.S. supported in part by grant R21 CA91806 from the National Cancer Institute. Address correspondence to J.T.D. (e-mail: jtd@scott.mc.duke.edu).

View larger version (108K): [in a new window]   Figure 1. Computer-generated FOM as a function of kilovoltage and filter material (Z = atomic number). FOM is defined as the squared SNR of a 1-mm incremental tissue thickness divided by the incident patient exposure (in milliroentgens [SI unit conversion factor, 0.000258 mC/kg]) at 150 cm from the focal spot. Data in these graphs are for 1 HVL of each filter material; graphs of data for 0.5-HVL filter thickness demonstrated similar trends (not shown). Background tissue thicknesses are the following: A, 8 cm; B, 16 cm; C, 24 cm; and D, 32 cm.

View larger version (78K): [in a new window]   Figure 2. Computer-generated FOM, as defined in Figure 1, as a function of kilovoltage and copper filter thickness (given in units of HVL). Background tissue thicknesses are the following: top left, 8 cm; top right, 16 cm; bottom left, 24 cm; and bottom right, 32 cm.

View larger version (110K): [in a new window]   Figure 3. Computer-generated replacement ratio of contrasts as a function of kilovoltage and filter material (Z = atomic number). Replacement contrast ratio is defined as the ratio of tissue contrast to replacement bone contrast. Data in these graphs are for 1 HVL of each filter material; graphs of data for 0.5-HVL filter thickness demonstrated similar trends (not shown). Background tissue thicknesses are the following: A, 8 cm; B, 16 cm; C, 24 cm; and D, 32 cm.

View larger version (79K): [in a new window]   Figure 4. Computer-generated replacement ratio of contrasts as a function of kilovoltage and copper filter thickness (given in units of HVL). Background tissue thicknesses are the following: upper left; 8 cm; upper right, 16 cm; bottom left, 24 cm; and bottom right, 32 cm.

View larger version (30K): [in a new window]   Figure 5a. (a, b) Full-field experimental results show the replacement ratio of contrasts (Ct/Cbr) and the FOM in the lung and mediastinum regions of the acrylic geometric chest phantom as a function of filter material (Al = aluminum, Cu = copper, Mo = molybdenum, Sn = tin, Pb = iron) (a) at about 1-HVL filtration and 120 kVp and (b) as a function of kilovoltage at 1-HVL copper filtration. (c) Full-field experimental results show the replacement ratio of contrasts (Ct/Cbr) and the FOM in the lung and mediastinum regions of the acrylic geometric chest phantom as a function of copper filter thickness at 120 kVp.

View larger version (30K): [in a new window]   Figure 5b. (a, b) Full-field experimental results show the replacement ratio of contrasts (Ct/Cbr) and the FOM in the lung and mediastinum regions of the acrylic geometric chest phantom as a function of filter material (Al = aluminum, Cu = copper, Mo = molybdenum, Sn = tin, Pb = iron) (a) at about 1-HVL filtration and 120 kVp and (b) as a function of kilovoltage at 1-HVL copper filtration. (c) Full-field experimental results show the replacement ratio of contrasts (Ct/Cbr) and the FOM in the lung and mediastinum regions of the acrylic geometric chest phantom as a function of copper filter thickness at 120 kVp.

View larger version (23K): [in a new window]   Figure 5c. (a, b) Full-field experimental results show the replacement ratio of contrasts (Ct/Cbr) and the FOM in the lung and mediastinum regions of the acrylic geometric chest phantom as a function of filter material (Al = aluminum, Cu = copper, Mo = molybdenum, Sn = tin, Pb = iron) (a) at about 1-HVL filtration and 120 kVp and (b) as a function of kilovoltage at 1-HVL copper filtration. (c) Full-field experimental results show the replacement ratio of contrasts (Ct/Cbr) and the FOM in the lung and mediastinum regions of the acrylic geometric chest phantom as a function of copper filter thickness at 120 kVp.

View larger version (86K): [in a new window]   Figure 6a. (a) Radiograph of anthropomorphic chest phantom shows a region of interest (rectangle). (b-d) Detail views of the region of interest are magnified, and the contrast is enhanced to show the effect of the optimum spectrum on SNR and contrast. Images were acquired with (b) the traditional spectrum (120 kVp, no added filtration), (c) the recommended optimum spectrum (120 kVp, 0.2 mm of added copper filtration), and (d) the traditional spectrum but at 25% less entrance exposure. In b and c, comparable image appearance is suggestive of equivalent SNR, as predicted, even though c was acquired at about 25% less entrance exposure to the phantom; d demonstrates increased relative noise content.

View larger version (113K): [in a new window]   Figure 6b. (a) Radiograph of anthropomorphic chest phantom shows a region of interest (rectangle). (b-d) Detail views of the region of interest are magnified, and the contrast is enhanced to show the effect of the optimum spectrum on SNR and contrast. Images were acquired with (b) the traditional spectrum (120 kVp, no added filtration), (c) the recommended optimum spectrum (120 kVp, 0.2 mm of added copper filtration), and (d) the traditional spectrum but at 25% less entrance exposure. In b and c, comparable image appearance is suggestive of equivalent SNR, as predicted, even though c was acquired at about 25% less entrance exposure to the phantom; d demonstrates increased relative noise content.

View larger version (110K): [in a new window]   Figure 6c. (a) Radiograph of anthropomorphic chest phantom shows a region of interest (rectangle). (b-d) Detail views of the region of interest are magnified, and the contrast is enhanced to show the effect of the optimum spectrum on SNR and contrast. Images were acquired with (b) the traditional spectrum (120 kVp, no added filtration), (c) the recommended optimum spectrum (120 kVp, 0.2 mm of added copper filtration), and (d) the traditional spectrum but at 25% less entrance exposure. In b and c, comparable image appearance is suggestive of equivalent SNR, as predicted, even though c was acquired at about 25% less entrance exposure to the phantom; d demonstrates increased relative noise content.

View larger version (122K): [in a new window]   Figure 6d. (a) Radiograph of anthropomorphic chest phantom shows a region of interest (rectangle). (b-d) Detail views of the region of interest are magnified, and the contrast is enhanced to show the effect of the optimum spectrum on SNR and contrast. Images were acquired with (b) the traditional spectrum (120 kVp, no added filtration), (c) the recommended optimum spectrum (120 kVp, 0.2 mm of added copper filtration), and (d) the traditional spectrum but at 25% less entrance exposure. In b and c, comparable image appearance is suggestive of equivalent SNR, as predicted, even though c was acquired at about 25% less entrance exposure to the phantom; d demonstrates increased relative noise content.