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(Radiology. 2000;215:891-896.)
© RSNA, 2000

Medical Physics

Results of a Nationwide Survey of Chest Radiography: Comparison with Results of a Previous Study1

Richard V. Kaczmarek, MS, Burton J. Conway, MS, Robert O. Slayton, MS and Orhan H. Suleiman, PhD

1 From the U.S. Food and Drug Administration, Center for Devices and Radiological Health, 1350 Piccard Dr, Rockville, MD 20850. Received December 15, 1997; revision requested March 11, 1998; final revision received October 4, 1999; accepted October 26. Address correspondence to R.V.K. (e-mail: rvk@cdrh.fda.gov).


   Abstract
TOP
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To provide public health information by means of measurement of the radiation exposures that patients undergoing chest radiography would receive and to compare the results with those of a similar previous survey.

MATERIALS AND METHODS: Surveyed facilities were randomly selected from each state. Patient exposure was evaluated along with film processing, half-value layer, and image quality. Additional information obtained concerned type of equipment, facility work load, radiographic technique, screen-film system, and grid type.

RESULTS: Mean entrance air kerma in all facilities was 141 µGy (16.1 mR). Mean kilovoltage in all facilities was 101 kV. In 1994, 140 (90%) of 156 hospitals (vs 71% in 1984) and 92 (58%; nearly double the percentage in 1984) of 159 nonhospital sites were using grids. Scoring with the imaging test tool resulted in a mean spatial resolution of 2.3 cycles per millimeter, and a mean low-contrast sensitivity of about 3%. Two hundred fifty-three (80%) of 315 facilities surveyed were processing film at minimum acceptable performance levels.

CONCLUSION: Mean entrance air kerma for all facilities did not substantially change. Although increased grid usage would lead to the expectation of higher measured exposures, this was offset by an increase in the use of faster screen-film combinations.

Index terms: Images, quality, 60.11 • Quality assurance • Radiations, exposure to patients and personnel, 60.11 • Radiations, measurement, 60.11 • Screens and films • Thorax, radiography, 60.11


   Introduction
TOP
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Chest radiography is still among the most frequently performed medical examinations. The most prevalent practice in the hospital setting is the use of conventional screen-film image receptors and high-kilovoltage settings (100–140 kV). This practice has been the norm for more than 2 decades, with modifications occasionally considered (1–3). Recently, there has been considerable investigation into the relative merits of alternative technology; the analysis involves a consideration of patient exposure levels, as well as an appreciation of pathologic findings (4–9).

The Conference of Radiation Control Program Directors (CRCPD), state radiologic health programs, and the U.S. Food and Drug Administration (FDA) Center for Devices and Radiological Health (CDRH) jointly cooperate to carry out the Nationwide Evaluation of X-ray Trends (NEXT) program. This program was begun in the 1970s when the CDRH encouraged diagnostic facilities to quantify radiation exposures to patients during typical diagnostic radiographic examinations. This information was also believed to be useful to state agencies in the evaluation of patterns of usage and in the improvement of specific areas of practice in radiology.

In 1984, attention was turned to chest radiography, and a nationwide survey was performed in hospitals (10). In 1986, a survey of private practice facilities was performed. Although patient exposure levels at chest radiographic examination are considered to be relatively low, consideration was given to the fact that a large proportion of the population undergoes this examination, often repeatedly.

The purpose of this study was to report the findings of the survey conducted by the CDRH, CRCPD, and state agencies in 1994 to determine what changes may have taken place in the practice of chest radiography and to compare the findings with those of a previous survey.


   MATERIALS AND METHODS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
A sample of hospitals was selected from the listing in the American Hospital Association membership guide, which was supplemented with a list of registered hospitals provided by the state radiation control agencies. Each participating state agency provided the FDA with a listing of nonhospital radiographic facilities. Working from these two lists, we randomly selected more than 300 survey sites to provide an adequate sample. The sampling was weighted so that proportionately more surveys would be conducted in states with larger populations. In 1984, only hospitals were surveyed about chest examinations. In 1986, only nonhospital facilities were surveyed. In the 1994 survey, approximately equal numbers of hospitals and nonhospital facilities were surveyed.

A written test protocol was developed by the CRCPD-NEXT committee. Surveyors attend a training session to ensure that all followed the same standard method of data collection. All information collected was entered into a computerized database maintained at the FDA.

Each surveyor used equipment that included an electrometer, a 6-mL ionization chamber (Model 1015; Radcal, Monrovia, Calif), several aluminum sheets (type 1100) for the determination of the half-value layer, the FDA LuCal chest phantom (11), and a 5 x 10-cm imaging test tool. This tool was attached with Velcro to the chest phantom and was imaged when the exposure measurements were obtained. The phantom simulated the attenuation of a 22.5-cm-thick chest radiographed in the posteroanterior projection. The x-ray exit spectra at diagnostic energy levels matched those obtained with an anthropomorphic chest-lung phantom. This phantom was originally developed as part of the pilot testing of the 1984 NEXT survey to address the fact that automatic exposure control was becoming more widely used (12).

Scoring of image spatial resolution and low-contrast sensitivity was performed in 1994. An image test tool, assembled at the FDA, was intended to provide a simple measure of spatial resolution and low-contrast sensitivity (Fig 1). The tool contained a series of eight mesh screens made of copper wire (which had 0.8, 1.2, 1.6, 2.0, 2.4, 3.2, 3.9, and 4.7 wires per millimeter) and a 6.1-mm-thick aluminum disk that had eight 7.94-mm-diameter pockets bored to the following depths: 0.08, 0.11, 0.16, 0.23, 0.32, 0.46, 0.64, and 0.89 mm. The image test tool was positioned in the x-ray beam away from the ionization chamber and its holding bracket to ensure that it was imaged completely. The surveyors recorded the number of spatial resolution mesh patterns and the number of low-contrast holes depicted on the radiographic image.



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  		Figure 1. Diagram depicts the CDRH imaging test tool.

 
To quantify low-contrast sensitivity, we used the results of transmission measurements made in our laboratories. Differences in subject contrast were considered to be characterized by the relative x-ray transmission through each of the eight pockets drilled into the aluminum disk that was part of the test tool. It was not practical to measure the difference in transmission with the available ionization chambers because the test tool was small. Our approach was to duplicate each of the pockets in the test tool by fabricating sheets of aluminum with thicknesses equal to those of the eight holes in the test tool.

The chest phantom was placed in the beam from the overhead tube of a conventional radiographic-fluoroscopic unit. Each sheet was attached to the phantom, in turn, and an ionization chamber was held in place behind the phantom (with an air gap) to record the relative exposure as the beam traversed the phantom and aluminum sheet. Exit exposure measurements were made at 70, 80, 90, 100, 110, and 120 kV. The range of half-value layers was 2.56 (70 kV) to 4.38 mm (120 kV) of aluminum. A monitor chamber was used to compensate for variations in tube output. The relative transmission through both the phantom and each sheet was recorded. These data were used to plot the relative subject contrast against the hole score (linear fit with residual values of 0.862–0.976) obtained at each energy level (Fig 2).



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  		Figure 2. Graph depicts the low-contrast sensitivity (percentage) measured with the NEXT chest phantom versus hole score. For example, at 80 kV, hole scores of 2 and 8 indicate that an observer should be able to distinguish objects that differ in subject contrast by 7% and 1%, respectively. (Values in the key box are kilovolts.)

 
Film processing was evaluated by using the sensitometric technique for the evaluation of processing (STEP) test, a procedure designed to estimate relative processing speed. This test was performed by using a sensitometer that was calibrated against a reference sensitometer maintained at the FDA, a densitometer, and control film (Eastman Kodak, Rochester, NY) from the same emulsion batch that was used to calibrate the sensitometer (13). The surveyor performed the test with the control film that was exposed to the calibrated sensitometer in the darkroom and developed in the same processor that was used to develop the chest radiographs at each facility. The surveyor calculated a relative processing speed from the resultant sensitometric strip and compared it with the value expected from the standard control film when it is developed according to the recommended processing specifications of the film manufacturer.

A processing speed of 100 represented adherence to the recommendations of the manufacturer, a speed of less than 80 indicated underprocessing, and a speed of greater than 120 indicated overprocessing. This ±20% tolerance was used to account for variability in the film emulsion over the useful life of the film, variations attributable to calibration of the densitometer and sensitometer, and normal fluctuations in the day-to-day range of operation with automatic processors (14). Darkroom fog was evaluated by removing an exposed film from its cassette holder and by allowing it to remain in the darkroom work area for 2 minutes with part of the film covered before it was processed. Normally, films are processed immediately after they are removed from the cassette holder.

At each surveyed facility, the type of generator, brand of film and intensifying screen, grid ratio, and names of the equipment and processor manufacturers were recorded. The approximate number of chest examinations performed per week and per unit at each facility were determined. The ionization chamber used to measure x-ray output was positioned 24 cm from the front surface of the phantom so that its measurements would be essentially free of backscatter. X-ray output was measured with the phantom positioned against the chest stand. The phantom was imaged by using clinical techniques, including the use of automatic exposure control, for density setting and film size. Entrance air kerma (free of backscatter) was calculated for the posteroanterior view in a patient with a 22.5-cm-thick chest by using an inverse-square correction from the position of the ionization chamber to the entrance surface of the patient. Beam quality was determined in millimeters of aluminum.


   RESULTS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
The mean weekly workload at hospitals was 195 chest examinations (median, 125 examinations). This number was approximately double that of the 1984 survey, which was 98. At nonhospital facilities, the mean weekly workload was determined to be 21 chest examinations (median, 15 examinations), which was relatively unchanged from the 19 examinations reported in 1986.

The use of older single-phase generators declined. Use of three-phase equipment, including high-frequency generators, increased from 55% of hospitals surveyed in 1984 to 77% (121 of 156) in 1994. More than 96% of nonhospital facilities reported the use of single-phase generators in 1986; this proportion was reduced to 80% (127 of 159) in 1994.

The use of automatic exposure control (phototiming) increased, although more so at hospitals. In 1984, only 54% of hospitals surveyed used automatic exposure control; this percentage increased in 1994 to 82% (128 of 156). Twenty (13%) of the 159 nonhospital facilities surveyed in 1994 used automatic exposure control; this percentage increased from 1986, when only 6% of nonhospital facilities surveyed reported the use of automatic exposure control.

The mean kilovoltage in hospitals was 110 kV (median, 110 kV). The mean kilovoltage in nonhospital facilities was 93 kV (median, 90 kV). The mean kilovoltage in all facilities was 101 kV. The SD was about 15 kV. The distribution is depicted in Figure 3. In both hospitals and nonhospital facilities, the mean values increased by 6 kV, since the mean was 104 kV in hospitals in 1984 and 87 kV in nonhospital facilities in 1986.



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  		Figure 3. Graph depicts percentage of all facilities surveyed versus tube voltage. More than 126 (80%) of 156 hospitals (hosp) operated at greater than 105 kV. The distribution at nonhospital facilities (non-hosp) was more varied; the largest segment (53 [33%] of 159) was in the 75-89-kV range. Overall median tube voltage in all facilities was 110 kV. The mean was 110 kV in hospitals and 93 kV in nonhospital facilities.

 
The mean beam quality, represented by measurements of the half-value layer, was 4.06 mm of aluminum at hospitals (median, 4.16 mm) versus 3.49 mm at nonhospital facilities (median, 3.49 mm). These values were essentially unchanged from those obtained in 1984 and 1986. In 1994, the mean of the product of exposure time and tube current was 5.26 mAs at hospitals (median, 3.21 mAs) and 10.30 mAs at nonhospital facilities (median, 6.44 mAs). These values decreased by 1 mAs for both types of facilities (6 mAs in 1984 at hospitals, 11 mAs in 1986 at nonhospital facilities).

The use of grids to improve image quality has increased. In the recent combined survey, 140 (90%) of 156 hospitals were using grids (median grid ratio, 12:1), representing an increase from 71% in 1984. The percentage of nonhospital facilities using grids (median grid ratio, 10:1) had almost doubled from 33% (1986) to 58% (92 of 159). The mean and median grid ratios in use at hospitals were the same, and these values were the same as those found in 1984. The mean ratio at nonhospital facilities increased from 8:1 to 10:1. The mean entrance air kerma observed was 131 µGy (14.7 mR; median, 105 µGy) at hospitals and 157 µGy (17.6 mR; median, 114 µGy) at nonhospital facilities (Fig 4). Mean entrance air kerma in all facilities was 141 µGy (16.1 mR).



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  		Figure 4. Graph depicts the percentage of all facilities surveyed versus calculated entrance skin exposure. In 114 (74%) of 154 facilities, entrance exposure was below the mean value (15 mR) in hospitals (hosp). In 33 (21%) of 159 nonhospital facilities (non-hosp), entrance exposure was calculated to be greater than 20 mR. The mean entrance exposure was 14.7 mR in hospitals and 17.6 mR in nonhospital facilities.

 
In 1994, the use of faster screen-film combinations was increasing at all facilities. Fifty percent of the hospitals (78 of 156) were using 400-speed systems, as opposed to only 24% in 1984. At nonhospital facilities, 400-speed systems were most commonly used; 60 (38%) of these 159 facilities reported the use of systems with speeds of 400 or greater. When nonhospital facilities were surveyed in 1986, both 200- and 300-speed combinations were common; each was used in just less than 30% of the facilities.

When processing was evaluated, 78 (50%) of 156 hospitals had processing speeds of 80–120, which we considered to be in accordance with the specifications of the film manufacturers. The mean processing speed at hospitals was 115 (median, 115). Fifty (32%) hospitals were overprocessing (speeds greater than 120). Eighty-six (54%) of 159 nonhospital facilities had speeds of 80–120 (mean, 107; median, 107). Thirty-nine (25%) nonhospital facilities were overprocessing (Fig 5). The mean film density (measured in the region of the lungs) at hospitals was 1.61 (median, 1.59), whereas the mean film density measured at nonhospital facilities was 1.74 (median, 1.67).



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  		Figure 5. Graph depicts the percentage of all facilities surveyed versus automatic film processing speed. A speed of 100 indicated that a facility was processing the film optimally according to the recommendations of the manufacturers, as referenced to results of our independent testing. In general, a test speed of 80-120 (for standard processing) is acceptable. Mean speeds were 115 in hospitals (hosp) and 107 in nonhospital facilities (non-hosp).

 
Darkroom fog was evaluated by measuring the difference between the optical density of the covered and uncovered portions of the test film. When this test was performed, the optical density of darkroom fog levels above 0.05 were observed in 70 (55%) of 127 hospitals and in 82 (57%) of 145 nonhospital facilities. The mean measured optical density of the darkroom fog was 0.10 at hospitals (median, 0.07) and 0.12 at nonhospital facilities (median, 0.07).

In 1984–1986, the mean film optical density was virtually the same at both hospitals and nonhospital facilities (1.43 and 1.42). In 1994, the mean film optical density increased by 0.18 at hospitals and 0.32 at nonhospital facilities.

The surveyors counted the number of wire mesh patterns and holes visible on radiographs obtained with the image-quality test tool positioned on the phantom. The median number of mesh screens seen was five of eight (Fig 6). This number corresponded to 2.4 wires per millimeter (60 wires per inch). The median number of holes seen was five of eight (Fig 7).



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  		Figure 6. Graph depicts the percentage of facilities surveyed versus mesh pattern score. Surveyors counted clearly visible patterns on a sample radiograph. Eight patterns were imbedded in the test tool. The mean number of visible patterns was 5.3 in hospitals (hosp) and 5.2 in nonhospital facilities (non-hosp).

 


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  		Figure 7. Graph depicts the percentage of facilities surveyed versus hole score. Surveyors counted clearly visible low-contrast holes on a sample radiograph. Image was made with the phantom in place and included scatter. Eight holes were in one end of the test tool. The mean number of visible holes was 4.8 in hospitals (hosp) and 4.6 in nonhospital facilities (non-hosp).

 
As indicated previously, the relative subject contrast that would be obtained for each energy level was plotted against hole score (linear fit with correlation coefficients of 0.862–0.976) (Fig 2). At 70 kV, the subject contrast ranged from about 10% for a hole score of 1 to just above 1% for a hole score of 8. At 90–120 kV, the measured contrast ranged from about 5% for a score of 1 to 0.5% for a score of 8. By using the mean facility kilovoltage of 101 kV and the mean hole score of 4.8 for hospitals and 4.6 for nonhospital facilities (Fig 7), we were typically able to distinguish objects that differed in subject contrast by 3% (Fig 8).



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  		Figure 8a. Graphs depict the percentage of facilities versus low-contrast sensitivity for kilovolt peaks of <=74, 75-84, and 85-94 kV (top) and 95-104, 105-114, and >=115 kV (bottom). Percent contrast calculations at 70, 80, and 90 kV (top) and 100, 110, and 120 kV (bottom) were obtained from Hole scores for each facility were matched with the selected kilovolt peak used.

 


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  		Figure 8b. Graphs depict the percentage of facilities versus low-contrast sensitivity for kilovolt peaks of <=74, 75-84, and 85-94 kV (top) and 95-104, 105-114, and >=115 kV (bottom). Percent contrast calculations at 70, 80, and 90 kV (top) and 100, 110, and 120 kV (bottom) were obtained from Hole scores for each facility were matched with the selected kilovolt peak used.

 

   DISCUSSION
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
Although the mean selected tube voltage increased in both hospital and nonhospital facilities, the distribution of the kilovolt peak was similar to that found 10 years ago (15). The majority of hospitals used tube voltages of 105 kV or greater, whereas the majority of nonhospital facilities used tube voltages of less than 105 kV. Consistent with findings from the prior survey, the ratio of measured exposure versus tube current (mR/mAs) for hospitals was double the ratio for nonhospital facilities. This finding is probably due to the higher output of three-phase and high-frequency generators, which are more likely to be found in hospitals, as well as the higher mean kilovolt peak used in hospitals.

Overall, the mean entrance skin exposure was essentially unchanged from the exposure from the last NEXT chest survey. The mean slightly decreased by about 9 µGy (1.03 mR) at hospitals and increased by about 18 µGy (2.06 mR) at nonhospital facilities. The mean entrance air kerma (entrance skin exposure) for posteroanterior examinations at both hospitals and nonhospital facilities in 1984–1986 was 140 µGy (16.04 mR). A more complete tabular and graphical summary of all of the survey results will be published by the CRCPD (16).

There were two important technical changes between 1984 and 1986 that affect entrance air kerma, and the effects of the changes tend to offset each other. The increased use of faster screen-film combinations allowed shorter exposure times (lower mAs) and lower patient exposures. However, the use of a grid results in more x-ray beam absorption and will increase patient exposure if no other factor is changed. Therefore, the initial expectation that the mean entrance exposure in patients would decrease was balanced by the finding that more facilities were using antiscatter grids.

When we evaluated film processing, a shift toward faster processing speeds resulted in an overall improvement compared with the findings of prior surveys. As mentioned previously, when processing speed was determined, slightly more than half of both hospitals and nonhospital facilities were within the normal range. The mean STEP value reported by hospitals in 1984 was 97, and 72% of the facilities were in the range of 80–120. The increase in the mean STEP value in hospitals to 107 in the 1994 survey indicated an overall increase in processing speed. This finding is further evidenced by the large increase in the percentage of hospitals that were overprocessing, that is, 32% (50 of 156) in 1994 versus 10% in 1984. The percentage of hospitals that developed film in the 80–120 range decreased apparently because they were now overprocessing; however, this finding is not necessarily undesirable. Underprocessing would be more cause for concern because it might lead facilities to raise patient exposures unnecessarily to compensate for unsatisfactory radiographs. Underprocessing declined from 17% of hospitals surveyed in 1984 to just 4% (six of 156) in 1994.

The film processing results were similar at nonhospital facilities. The mean processing speed increased while still remaining within the target range (speed was 85 in 1986). In contrast to hospitals, however, the number of facilities processing within the normal range increased from 45% to 54% (86 of 159). The percentage of facilities found to be underprocessing decreased substantially from 41% in 1986 to only 14% (23 of 159) in 1994. At the same time, the percentage of nonhospital facilities found to be overprocessing increased from 14% in 1986 to 25% (39 of 159). While the nonhospital facilities were processing at speeds lower that those of hospitals, more nonhospital facilities used speeds in the desired range.

With respect to the scores of the image-quality test tool, use of this aid may prove to be a useful and quick way to evaluate the performance of an imaging facility. The geometry of the tool duplicated that which is clinically used, and imaging was performed under conditions in which scattered radiation was produced by the phantom. Individual facilities that perform conventional screen-film chest radiography may consider the data in Figure 8 as indicators of the low-contrast sensitivity they can detect. The comparative results in the different ranges of the kilovolt peak indicated a slight improvement in the detectable subject contrast as the tube voltage increased. This finding may be the result of a relationship between technique selection and grid use. The data show that the mean kilovolt peat at facilities that use grids was 107 kV, compared with 82 kV for the remainder. Hospitals reported slightly higher scores compared with those of nonhospital facilities, and, for those facilities that did not use grids, their mean of 4.3 was lower than the overall mean.

After the completion of the initial NEXT study, the Swedish National Institute of Radiation Protection used a Lucite and aluminum phantom to perform a NEXT-type survey in that country (17). Despite considerable differences in technique factors, the entrance air kerma values from our study were almost identical in facilities that used scatter suppression. The tube voltage used in Sweden ranged from 100 to 150 kV, and grid usage was almost universal. The 1994 NEXT results showed mean film processing speeds that were slightly lower that those reported in Sweden, although this finding was somewhat offset by slightly higher mean screen-film speeds.


   Acknowledgments
 
The authors gratefully acknowledge the CRCPD for its support of the NEXT program and especially acknowledge the NEXT Committee for its oversight of the survey activities. The authors also acknowledge the state radiation control personnel for their commitment to this work and for the time they donated to make data collection possible.


   Footnotes
 
Abbreviations: CDRH = Center for Devices and Radiological Health, CRCPD = Conference of Radiation Control Program Directors, FDA = U.S. Food and Drug Administration, NEXT = Nationwide Evaluation of X-ray Trends, STEP = sensitometric technique for the evaluation of processing

Author contributions: Guarantors of integrity of entire study, R.V.K., O.H.S.; study concepts, B.J.C., O.H.S.; study design, B.J.C., O.H.S., R.O.S.; definition of intellectual content, B.J.C., O.H.S., R.V.K.; literature research, R.V.K.; data analysis, R.V.K.; statistical analysis, R.V.K.; manuscript preparation, R.V.K.; manuscript editing and review, R.V.K., O.H.S.


   References
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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  7. Dobbins JT, III, Rice JJ, Beam CA, Ravin CE. Threshold perception performance with computed and screen-film radiography: implications for chest radiography. Radiology 1992; 183:179-187.[Abstract/Free Full Text]
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  10. Butler PF, Conway BJ, Suleiman OH, Koustenis GH, Showalter CK. Chest radiography: a survey of techniques and exposure levels currently used. Radiology 1985; 156:533-536.[Abstract/Free Full Text]
  11. Conway BJ, Butler PF, Duff JE, et al. Beam quality independent attenuation phantom for estimating patient exposure from x-ray automatic exposure controlled chest examinations. Med Phys 1984; 11:827-832.[Medline]
  12. Butler PF, Conway BJ, Suleiman OH, Koustenis GH, Showalter CK. Results of a six-state pilot study to collect exposure, technique and processing data in chest radiography. SPIE 1984; 486:21-28.
  13. Suleiman OH, Suleiman OH, Rueter FG, Antonsen RG, Conway BJ, Slayton RJ. The sensitometric technique for the evaluation of processing.. Radiat Protection Dosimetry 1993; 49:105-106.
  14. Suleiman OH, Conway BJ, Rueter FG, Slayton RJ. Automatic film processing: analysis of 9 years of observations. Radiology 1992; 185:25-28.[Abstract/Free Full Text]
  15. Rueter FG, Conway BJ, McCrohan JL, Suleiman OH. Average radiation exposure values for three diagnostic radiographic examinations. Radiology 1990; 177:341-345.[Abstract/Free Full Text]
  16. Kaczmarek RV. Nationwide Evaluation of X-ray Trends (NEXT) tabulation and graphical summary of the 1994 chest radiography survey. Publication 98-2 Frankfort, Ky: Conference of Radiation Control Program Directors, 1998.
  17. Leitz WK, Hedberg-Vikstrom BR, Conway BJ, Showalter CK, Rueter FG. Assessment and comparison of chest radiography techniques in the United States and Sweden. Br J Radiol 1990; 63:33-40.[Abstract/Free Full Text]



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E. Vano, J. M. Fernandez, J. I. Ten, E. Guibelalde, L. Gonzalez, and C. S. A. Pedrosa
Real-Time Measurement and Audit of Radiation Dose to Patients Undergoing Computed Radiography
Radiology, October 1, 2002; 225(1): 283 - 288.
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