View box luminance measurements and their effect on reader performance

RATIONALE AND OBJECTIVES: The purpose of this study was to investigate the importance of view box luminance and viewing conditions on low-contrast detection by readers. MATERIALS AND METHODS: Radiographs of a mammographic contrast-detail phantom were examined on 632 view box panels. The luminance of these panels was obtained by using a calibrated meter and ranged from 860 to 3,300 nit. Twelve radiologists reported the number of contrast-detail disks for each size (diameter, 0.3-7.0 mm) deemed to be visible on films with optical densities of 1.00-2.60. Radiologist performance in reading low-contrast phantom images was also studied as a function of room illuminance and image masking. RESULTS: Median luminance was 1,700 nit, with 25- and 75-percentile values of 1,450 and 2,150 nit, respectively. Low-contrast visibility generally was independent of view box luminance, regardless of film density or disk diameter. Low-contrast visibility deteriorated when masking around the image was removed and at normal room illuminance. The greatest deterioration in performance occurred at the highest film densities and with the smallest size disks. CONCLUSION: Detection of low-contrast features on radiographs is relatively independent of view box luminance, but it is degraded by the presence of stray light and by increased room illuminance.     

Effect of radiographic techniques (kVp and mAs) on image quality and patient doses in digital subtraction angiography

We investigated how varying the x-ray tube voltage and image receptor input exposure affected image quality and patient radiation doses in interventional neuroradiologic imaging. Digital subtraction angiography (DSA) images were obtained of a phantom with 1 mm diameter vessels containing iodine at concentrations between 4.5 and 50 mg/cc. The detection threshold concentration of iodine was determined by inspecting DSA images obtained at a range of x-ray tube voltages and input exposure levels. Surface doses were obtained from measured x-ray tube output data, and corresponding values of energy imparted were determined using the exposure-area product incident on the phantom. In one series of experiments, the air kerma at the image intensifier (X) was varied between 0.44 microGy per frame and 8.8 microGy per frame at a constant x-ray tube voltage of 70 kVp. In a second series of experiments, the tube voltage was varied between 50 and 100 kVp, and the mAs adjusted to maintain a constant exposure level at the input of the image intensifier. At a constant x-ray tube voltage, the surface dose and energy imparted were directly proportional to the input exposure per frame used to acquire the DSA images. On our DSA system operated below 2.2 microGy per frame, the threshold iodine concentration was found to be proportional to X(-0.57), which is in reasonable agreement with the theoretical prediction for a quantum noise limited imaging system. Above 2.2 microGy per frame, however, the threshold iodine concentration was proportional to X(-0.26), indicating that increasing the input exposure above this value will only achieve modest improvements in image quality. At a constant image intensifier input exposure level, increasing the x-ray tube voltage from 50 kVp to 100 kVp reduced the surface dose by a factor of 6.1, and the energy imparted by a factor of 3.5. The detection threshold iodine concentration was found to be proportional to kVp(n), where n was 2.1 at 1.1 microGy per frame, and 1.6 at 3.9 microGy per frame. For clinical situations that can be modeled by a uniform phantom, reducing the x-ray tube voltage rather than increasing the exposure level would best achieve improvements on our DSA imaging system performance.     

Adult patient doses in interventional neuroradiology

We investigated radiation doses to 149 adult patients who underwent interventional neuroradiologic procedures, consisting of 132 patients who had diagnostic imaging examinations and 17 patients who had therapeutic procedures. The interventional procedures were carried out on a biplane system capable of performing fluoroscopy and digital subtraction angiography (DSA). The x-ray imaging system was interfaced to a patient dosimetry system, which computed surface (skin) doses based on the selected radiographic technique factors in each of the radiographic and fluoroscopic imaging modes. For each patient, an assessment was made of the maximum surface dose received during the procedure, which predicts the possibility of inducing deterministic effects. Knowledge of the surface doses, beam quality and x-ray cross sectional area permitted the computation of the total energy imparted to each patient. Energy imparted values were converted to effective dose, which provides an estimate of the stochastic radiation risk to the patient. The median surface dose for the frontal plane during diagnostic imaging examinations was 1.3 Gy, with a maximum surface dose of 5.1 Gy. The median surface dose for the frontal plane during therapeutic procedures was 2.8 Gy with a maximum surface dose of 5.0 Gy. Ratios of the lateral to frontal median surface doses were 0.47 for diagnostic examinations and 0.68 for interventional procedures. The median energy imparted was 1.8 J during fluoroscopy, and 4.3 J during radiography, showing that on average, 66% of the patient exposure comes from radiographic imaging (DSA). For diagnostic examinations, the median patient effective dose was 33 mSv, with a maximum of 152 mSv. For therapeutic procedures, the median patient effective dose was 74 mSv, with a maximum of 156 mSv. In interventional neuroradiology, surface doses could induce deterministic effects, and the corresponding effective doses are noticeably higher than those normally encountered in diagnostic radiology.