Tube Current-exposure Time Research Articles

Prospective Evaluation of Image Quality with Use of a Patient Image Gallery for Dose Reduction in Pediatric 16-MDCT

The purposes of this study were to evaluate prospective adjustment of dose settings in pediatric 16-MDCT by use of computer-simulated images in a patient image gallery and to compare the dose reduction achieved with that of standard pediatric protocols. The image gallery consisted of images from weight-dependent sample examinations performed with varied simulated tube current-exposure time settings. The scanning parameters prospectively chosen on the basis of the image quality of the image gallery were used for 30 16-MDCT examinations (chest, n = 15; abdomen, n = 8; pelvis, n = 7) of 22 children (14 boys, eight girls; mean age, 6.8 +/- 5.8 years; mean body weight, 26.7 +/- 19.6 kg). Three blinded radiologists used a 4-point grading scale to rate the overall image quality of the image gallery and the 16-MDCT scans. Objective and subjective image quality was assessed for the simulated and actual CT scans. The concordance correlation coefficient (K) was determined. There was mainly moderate concordance with regard to objective (chest, K = 0.69; abdomen, K = 0.33; pelvis, K = 0.55) and subjective (chest soft-tissue window, kappa coefficient [kappa] = 0.00; chest lung window, kappa = 0.53; abdomen, kappa = 1.00; pelvis, kappa = 0.48) analysis of image gallery compared with actual 16-MDCT examinations. Compared with use of previous weight-adapted pediatric standard protocols, use of an image gallery resulted in further dose reduction for abdominal and pelvic CT but not for thoracic CT. A patient image gallery can be used as a basis for pediatric 16-MDCT examinations. The gallery provides a preexamination overview of expected image quality. Radiation exposure can be optimized with regard to patient weight and the image quality needed to answer the clinical question.

Fractal-Feature Distance Analysis of Radiographic Image

We have conducted a fractal analysis of low-dose digital chest phantom radiographs and evaluated the relationship between the fractal-feature distance and the tube current-exposure time product. Chest phantom radiographs were obtained at various mAs values (0.5-4.0 mAs) and 140 kVp with a computed radiography system, and the reference images were acquired at 13 mAs. The lung field images were converted to binary images after processing them using the rolling-ball technique; a fractal analysis was conducted using the box-counting method for these binary images. The fractal-feature distances between the low-dose and reference images were calculated using the fractal dimension and the complexity. For all binary images of lung fields, the relationship between the length of the square boxes and the number of boxes needed to cover the positive pixels of the binary image was linear on a log-log scale (r > or = 0.99). For mAs > or = 3.0, the fractal-feature distances were almost constant, whereas for mAs < or = 2.5, they increased depending on the reduction in mAs values. We have shown that a binary image of the lung field obtained from a chest phantom radiograph can be analyzed by the box-counting method and that its fractal-feature distance grows as the radiation dose declines.

SU‐FF‐T‐321: Micro‐Computed Tomography: A Tool for the Determination of the Sensitive Volume of Cylindrical and Plane Parallel Ion Chambers

Purpose: To use micro‐computed tomography (micro‐CT) as a tool for non‐destructive imaging of air ionization chambers for independent sensitive volume determination and quality assurance of the chambers. Methods and Materials: A GE Locus Micro‐CT imaging system was used to acquire high‐resolution images of several common small‐volume chambers (Exradin T11 plane‐parallel chamber and Exradin A1SL cylindrical chamber). Initial scans were taken of these chambers with a 4cm field of view, 80kVp energy, 450μA tube current, 399 views, and 400ms exposure time per view. Images were reconstructed with 89μm pixel size. GE MicroView visualization software was used to determine the chambers' air volumes for comparison with the manufacturer's specifications. The ability of micro‐CT to differentiate materials and the effect of high density materials on image quality was assessed. In addition, four Exradin A1SL cylindrical chambers were imaged with an increased number of views (720) and acquisition angles (360°) and reconstructed with 20μm pixel size. Air volumes from all of the chambers were compared to the chamber relative ionization signals generated in a 10×10cm2 reference field from a Varian 2100C/D linac. Results: The air cavity volumes derived from the micro‐CT images agreed with nominal volumes given by the manufacturers within 5% for both the cylindrical and plane parallel chambers. The relative response of the cylindrical chambers agreed with the relative volumes (semi‐automatic method) within 2%. The presence of the connecting cables or pins within the chambers did not affect the ability to accurately visualize the sensitive air volumes. Conclusions: Micro‐CT is a promising tool for the measurement of ion chamber air volume and potentially for determining calibration factors for use in dosimetry. These high resolution images could also prove useful as input to Monte Carlo simulations and the calculation of ion chamber response factors from first principles.

MAPD--an objective way to select mAs for paediatric brain CT.

CT is an advanced imaging modality, but the imaging parameters are normally selected subjectively. For standard head examinations, most of the parameters used are consistent amongst different centres, with the exception of large variations in the selection of the tube current-exposure time product (mAs). As a result, CT images may contain unacceptable levels of noise, or the patient may receive excessive radiation. In this study, the maximum anteroposterior diameter (MAPD) was shown to be a good criterion for mAs selection, and could be measured in a pilot view. 200 paediatric brain CT studies were randomly selected to determine the MAPD at the mid brain level. With knowledge of MAPD distribution, a phantom study was performed to determine the relationship between MAPD and the mAs required for consistent and acceptable image noise. It was found that the required mAs increased linearly with MAPD. Assuming the manufacturer's recommended value is "appropriate" for the average MAPD, the appropriate mAs value could be estimated. Using this method, appropriate mAs values were calculated retrospectively for a group of 240 randomly selected paediatric brain CT studies and compared with the actual mAs subjectively determined by the radiographer. Although their average values were similar, the difference between the calculated and actual values deviated markedly in some cases. When the actual mAs was smaller than the calculated value, higher image noise was observed. However, reduction of image noise was barely observed when the applied mAs was larger than the calculated value. Thus, this method is more objective and appropriate for determination of the mAs value for paediatric brain CT than the traditional subjective method.