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.

Radiation dosimetry for extremity radiographs.

The energy imparted, epsilon, to a patient undergoing an extremity x-ray examination may be obtained from the dose-area product incident on the patient. Values of energy imparted can be subsequently converted into the corresponding effective dose, E, using an extremity specific E/epsilon ratio. In this study, an E/epsilon ratio of 3 mSv/J was used to convert values of energy imparted into the corresponding upper limit of adult effective doses for all types of extremity examinations. A modification factor, based on the patient mass, was employed to determine the corresponding extremity effective doses to pediatric patients undergoing extremity examinations. Representative clinical technique factors for six common extremity examinations (hand, forearm, elbow, ankle, tibia/fibula, knee) were used to determine the dose-area product and the corresponding values of energy imparted. For adult extremity x-ray examinations, values of energy imparted ranged from 55 microJ to 920 microJ, with the energy imparted to 1-y-old patients being a factor of about 20 lower. Upper limits of effective doses for adult extremity x-ray examinations ranged from 0.17 to 2.7 microSv, whereas the corresponding doses to 1-y-old patients were about a factor of three lower.

Effective dose and energy imparted in diagnostic radiology.

The patient effective dose, E, is an indicator of the stochastic radiation risk associated with radiographic or fluoroscopic x-ray examinations. Determining effective doses for radiologic examinations by measurement or calculation is generally very difficult. By contrast, the energy imparted, epsilon, to the patient may be obtained from the x-ray exposure-area product incident on the patient. As energy imparted is approximately proportional to the effective dose for any given x-ray radiographic view, the availability of E/epsilon ratios for common radiographic projections provides a convenient way for estimating effective doses. Ratios of E/epsilon were obtained for 68 projections using E and epsilon values obtained from published dosimetry data computed using Monte Carlo techniques on an adult anthropomorphic phantom. The average E/epsilon ratio for the 68 projections in adults was 17.8+/-1.4 mSv/J, whereas uniform whole body irradiation corresponds to 14.1 mSv/J. The major determinant of E/epsilon ratios was the projection employed (the body region irradiated and x-ray beam orientation), whereas the tube potential and beam filtration were of secondary importance. Adult E/epsilon ratios may also be used to obtain effective doses to pediatric patients undergoing x-ray examinations by application of a correction factor based on the patient mass.

Evaluation of an on-line patient exposure meter in neuroradiology.

PURPOSE: To assess the clinical performance and usefulness of an on-line patient exposure meter installed on a neuroradiologic biplane imaging system. MATERIALS AND METHODS: A commercial on-line patient exposure meter was installed on each plane of a biplane neuroradiologic imaging system. The meter computed skin exposures on the basis of selected technique factors (tube potential and current) and information about patient location relative to the x-ray tube. Simulations were performed to measure the system accuracy with an angiographic anthropomorphic head phantom with the skin exposures measured with an ionization chamber. Skin doses were computed for 114 consecutive patients who underwent diagnostic and interventional neuroradiologic procedures. RESULTS: Agreement between measured and computed skin exposures in fluoroscopy, plain radiography, and digital imaging was generally within 5% of the true skin dose. For all fluoroscopic and radiographic procedures, total median skin doses were 1.20 and 0.64 Gy for the frontal and lateral planes, respectively. In both planes, patient skin doses resulted primarily from digital subtraction angiographic acquisitions. In 29 (25%) patients, the skin dose exceeded 2.00 Gy, but no radiation-induced deterministic effects were observed. CONCLUSION: An on-line patient exposure meter can provide accurate radiation skin dose data in patients undergoing diagnostic and therapeutic neuroradiologic procedures.

Computation of energy imparted in diagnostic radiology.

Energy imparted is a measure of the total ionizing energy deposited in the patient during a radiologic examination and may be used to quantify the patient dose in diagnostic radiology. Values of the energy imparted per unit exposure-area product, omega (z), absorbed by a semi-infinite water phantom with a thickness z, were computed for x-ray spectra with peak x-ray tube voltages ranging from 50-140 kV and with added filtration, ranging from 1-6 mm aluminum. For a given phantom thickness and peak x-ray tube voltage, the energy imparted was found to be directly proportional to the x-ray beam half-value layer (HVL) expressed in millimeters of aluminum. Values of omega (z) were generated for constant waveform x-ray tube voltages and an anode angle of 12 degrees, and were fitted to the expression omega (z) = alpha x HVL + beta. Fitted alpha and beta parameters are provided that permit the energy imparted to be determined for any combination of tube voltage, half-value layer, and phantom thickness from the product of the entrance skin exposure (free-in-air) and the corresponding x-ray beam area. The results obtained using our method for calculating energy imparted were compared with values of energy imparted determined using Monte Carlo techniques and anthropomorphic phantoms for a range of diagnostic examinations. At 60, 80, and 120 kV, absolute values of energy imparted obtained using our method differed by 8%, 10%, and 12%, respectively, from the corresponding results of Monte Carlo computations obtained for an anthropomorphic phantom. The method described in this paper permits a simple determination of energy imparted for any type of diagnostic x-ray examination which may be used to compare the radiologic risks from differing types of x-ray examinations, optimize imaging techniques with respect to the patient dose, or estimate the patient effective dose equivalent.

Effect of patient support pads on image quality and dose in fluoroscopy.

An investigation was performed of the changes in image quality and patient dose as a result of increasing filtration for fluoroscopy performed under automatic brightness control. Filtration was added either at the x-ray tube housing (i.e., scatter-free geometry) or adjacent to a tissue equivalent phantom simulating the patient (i.e., with-scatter geometry). Patient doses were expressed in terms of the total energy imparted to patients simulated by either a 10 cm (i.e., pediatric) or 20 cm (i.e., adult) acrylic phantoms. Changes in image quality were determined by measuring the relative visibility of circular disks in a Leeds Test Object 10 contrast-detail phantom. In the scatter-free geometry, the addition of 4 mm Al filtration reduced the energy imparted by 27% (10 cm phantom) and 20% (20 cm phantom). In the with-scatter geometry, the corresponding reductions in energy imparted were 17% and 9% for the 10 and 20 cm phantoms, respectively. The visibility of low contrast disks generally decreased as the thickness of the added aluminum increased but the location of the added Al (i.e., with-scatter or scatter-free geometry) had no significant effect on the resultant image quality. These results demonstrate that the use of patient support pads with a thickness of approximately 4 mm Al will generally have an adverse impact on fluoroscopic image quality and result in modest reductions (approximately 10%) of adult patient doses.