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Entrance skin exposure in PA chest radiography.

There are many reasons why a health care facility would want to measure or monitor the amount of radiation produced by its diagnostic imaging equipment. The Joint Commission on Accreditation of Healthcare Organizations[1] requires hospitals to provide risk assessment and performance monitoring data on all medical equipment. For machines that generate ionizing radiation, this means hospitals must have some way of measuring output. In addition, some states[2,3] require health care providers to demonstrate that radiation exposure in typical radiological examinations is below certain maximum limits for body parts of specified thicknesses. In other instances, radiation exposure needs to be determined for medical or legal purposes.

One of several possible methods to measure output from a device that produces ionizing radiation is with a dose-area product (DAP) meter, a large-area ionization chamber attached to the collimator assembly of the radiographic machine. (See Fig. 1.) In a previous article,[4] we compared the DAP meter to thermoluminescent dosimeters in chest radiography and concluded that the DAP meter could be a convenient and accurate method of measuring the entrance skin exposure (ESE) with a known x-ray field size.

[Figure 1 ILLUSTRATION OMITTED]

This article presents further analysis of results on the posteroanterior (PA) chest exams reported in the previous study[4] and investigates whether ESE is related to the thickness of the patient's chest.

Method and Materials

A dose-area product meter (Gammex RMI model 840A, Middleton, Wis.) was attached to the collimator assembly of an x-ray unit used for chest radiography. The product of the dose and the cross-section of the x-ray beam were displayed in units of mGy/[cm.sup.2]. With an appropriate calibration factor (K) and a known x-ray field size at the skin surface ([A.sub.S]), the entrance skin exposure (free in air) could be computed as follows:

exposure ([micro] C/kg) = K DAP/[A.sub.S]

x 258 [micro] C/kg per Roentgen/8.7 mGy per Roentgen

Exposures resulting from PA chest x-ray examinations on 47 adult male patients were studied. The source-to-image distance was 305 cm, and the technique factors for the examinations were 110 kVp and 300 mA with automatic exposure control. The thickness (t) of each thorax was recorded. The length ([X.sub.L]) and width ([Y.sub.L]) of the adjusted light field were measured at the DAP meter. The amounts of misalignment of the light field with respect to the x-ray field were determined ([X.sub.C] and [Y.sub.C]). The corrected area of the x-ray field at the DAP meter became:

[A.sub.d] = ([X.sub.L] + [X.sub.C]) ([Y.sub.L] + [Y.sub.C])

With a known source-to-dose area product meter distance (d), the x-ray field size at the skin entrance could be computed as:

[A.sub.S] = [[SID-t/d].sup.2] [A.sub.d]

The DAP meter was calibrated with a reference ion chamber at 72 cm from the focal spot. This position was far enough away from any surface to avoid significant contribution by scatter radiation. In addition, with the high kilovoltage used in these examinations, the amount of attenuation of radiation by the air between this calibration distance and the skin surface was found to be too small to be measured by the reference ion chamber. The calibration factor (K) was found to be 1.04 for the DAP meter under these circumstances.

If a diaphragm with an aperture or a positive beam-limiting device were used to automatically collimate the x-ray field to a constant area, different DAP values would have been obtained. The area would be the size of the film plus the tolerance permitted by state or federal regulations. However, the resulting entrance skin exposure should be the same from either technique.

To compare DAP values obtained in this study to a hypothetical situation where a constant x-ray field size is used, we multiplied a field size equal to the area of the film plus an extra 3 cm on each border by the entrance skin exposures obtained with the manual technique.

Results and Discussion

We reported in a previous article[4] that the entrance skin exposure in PA chest examinations ranges from 4 [micro] C/kg (16 mR) to 39 [micro] C/kg (150 mR).

An x-ray beam has a broad spectrum of energies and the human chest has very heterogeneous tissue composition. However, in the absence of any pathology or anatomical variation, we expected the entrance skin exposure to increase with the thickness of the thorax when exposure factors were increased to compensate.

The search for a simple mathematical formula to correlate exposure with body part thickness would begin with a model of monoenergetic radiation, homogeneous tissue and a direct proportionality of the chest thickness to tissue thickness. In this model, when automatic exposure control maintains a constant exit dose, the entrance skin exposure would increase exponentially with the thickness of the chest. A plot of the natural logarithm of the measured entrance skin exposure vs the thickness of the thorax shows a general trend of a straight line, an appearance of an exponential function in a semilogarithmic presentation. (See Fig. 2.)

[Figure 2 ILLUSTRATION OMITTED]

Collected data showed the measured thickness of the patients' chests in this study had a mean of 28 cm (3 cm, SD). As Fig. 2 shows, the correlation between entrance skin exposure and chest thickness was weak. For example, at a chest thickness of 28 cm, entrance skin exposures ranged from 7 [micro] C/kg to 16 [micro] C/kg (26 mR to 62 mR).

The distribution of measured dose-area product is presented in Fig. 3A and can be compared to a distribution that would have been obtained with a constant field size or autocollimation, as shown in Fig. 3B. The latter has a narrower distribution, with most cases at the low end. An examination of the measured field sizes showed that the upper end of the distribution in Fig. 3A had field sizes much larger than the films.

[Figure 3 ILLUSTRATION OMITTED]

Conclusion

A test with an anthropomorphic phantom (Humanoid Systems, Carson, Calif.) gave a skin entrance dose of 5 [micro] C/kg (21 mR), which is at the low end of our patient exposure distribution of 4 [micro] C/kg (16 mR) to 39 [micro] C/kg (150 mR). A publication by the American Association of Physicists in Medicine[5] describes the use of an anthropomorphic phantom from the Center for Devices and Radiological Health (CDRH) and one from the American National Standards Institute (ANSI). The CDRH phantom[6] is more realistic and has been demonstrated to be equivalent to a 23 cm-thick chest. The ANSI phantom[7,8] is easier to construct and easier to use, and has been demonstrated to incur 33% more estimated entrance skin exposure.[5] Because most of the patients in our study were thicker than 23 cm, these phantoms do not represent the mean or median of our patient population, nor can they emulate typical patients in all clinics. These test objects are more suitable for comparison of x-ray imaging systems.

When automatic exposure control is used, the resulting patient exposure during a radiographic examination of the chest cannot be predicted with a high degree of accuracy. When it is necessary to accurately measure radiation exposure to a patient, it is preferable to determine the dosimetry for each patient individually. If the control panel displays the resulting technique used (kVp and mAs), calibration results provided by a qualified expert can be used to compute the radiation exposure. If the x-ray machine does not offer this option, a DAP meter can be installed for this purpose.

A comparison of DAP distributions obtained from manual collimation and autocollimation, shown in Figs. 3A and 3B, suggests the following possibility: In the manual collimation technique, an upper limit on the dose-area product can perhaps be used as a quality control criterion to review instances of excessive field sizes.

A risk indicator of radiation is the weighted sum of doses to various parts of the body[9] This effective patient dose increases with entrance skin exposure and the area of the body irradiated.

Our data show that the effective dose and, therefore, the radiation risk, can be reduced by limiting, the radiographic field size.

References

[1.] Comprehensive Accreditation Manual for Hospitals. Oakbrook Terrace, Ill: The Joint Commission on Accreditation of Healthcare Organizations; 1996.

[2.] Oklahoma Administrative Code, 310 ch 280.

[3.] Texas Regulations for Control of Radiation, Part 32, Use of Radiation Machines in the Healing Arts and Veterinary Medicine.

[4.] Parry CK, Chu RYL, Eaton BG, Chen CY. Measurement of skin entrance exposure with a dose-area-product meter in chest radiography. Radiology. 1996;201:574-575.

[5.] Chu RYL, Fisher J, Archer BR, et al. Standardized Methods for Measuring Diagnostic X-ray Exposures. AAPM Report No. 31. College Park, Md: The American Institute of Physics Inc; 1990.

[6.] 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.

[7.] Method for the Sensitometry of Medical X-ray Screen-film Processing Systems. New York, NY: American National Standards Institute; 1982.

[8.] Airmille PA. The Physics of Medical Imaging: Recording System Measurements and Techniques. College Park, Md: The American Institute of Physics Inc; 1979:105-117.

[9.] International Committee on Radiation Protection. 1990 Recommendations of the ICRP. ICRP Publication No. 60. Oxford, England: Pergamon Press; 1991.

Robert Y.L. Chu, Ph.D., is an associate professor in the Department of Radiologic Sciences at the University of Oklahoma Health Sciences Center and a physicist at the Veterans Affairs Medical Center in Oklahoma City, Okla.

Cindy Parry, M.S., is an instructor in the Department of Radiologic Sciences at the University of Oklahoma Health Sciences Center, Oklahoma City.

Bob G. Eaton, M.D,, is chairman of the Department of Radiologic Sciences at the University of Oklahoma Health Sciences Center, Oklahoma City.

This research was supported in part by a grant from Gammex RMI Corporation, Middleton, Wis.

Reprint requests may be sent to the American Society of Radiologic Technologists, Publications Department, 15000 Central Ave. SE, Albuquerque, NM 87123-3917.
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Title Annotation:posteroanterior
Author:Chu, Robert Y.L.; Parry, Cindy; Eaton, Bob G.
Publication:Radiologic Technology
Date:Jan 1, 1998
Words:1689
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