Reference Books and Articles on Diagnostic X Ray and CT

We continue to use handbooks available through the Center for Devices and Radiological Health. Click on site index, click on "R," then go to CDRH organ dose handbooks. The handbook for this question is FDA 89-8031 Handbook of Selected Tissue Doses for Projections Common in Diagnostic Radiology. For your question, the entrance skin exposure to the skin of the back for a chest x ray is about 30 mR (milliroentgen-unit of radiation exposure). We use 14" x 17" fields at 72" and at 120 kVp. We also have our tubes filtered generally at 3.0 mm Al. If I look that up in the table, I find that the dose to the lung is 13 mrad, breast is 2 mrad, and bone marrow is about 3 mrad.

Tables providing gonadal doses for adults for typical medical exams can be found in the following book: J.G. Keriakes and M. Rosenstein; CRC Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-Rays; CRC Press, Inc.; Boca Raton, FL; 1980.

There is a book that should provide the information that you need on x-ray and nuclear medicine doses. It is J.G. Keriakes and M. Rosenstein; CRC Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-Ray, CRC Press, Inc., Boca Raton, FL.

1. Estimate the equivalent size of the patient being scanned in terms of an equivalent cylinder of water based on the CT image data.
    
2. Assume that the scan is being performed on a GE CT/i CT scanner, and use your kV and mA(s) values. In determining the effective kV, mAs, and slice thickness, average the actual techniques in a way that will provide an accurate value of energy imparted. That is, 2 x 5 mm sections that overlap are equivalent to 1 x 10 mm, since the energy imparted remains the same even if the local doses don't. It needs to be kept in mind that the effective dose is a measure of the stochastic risk, which is directly related to energy imparted.
    
3. Determine the mean section dose based on the data presented in Figure 1 and Table 5 by using equation 5 (Huda et al. 1997).
    
4. Determine the energy imparted, and convert this into an effective dose using a patient size dependent effective dose per unit energy imparted coefficient using Equation (5).
    
5. Scale this to your specific CT scanner using the ratio of your CTDI to those of the CT/i CTDI as listed in Table 8.
    

Reference:
Huda W, Atherton JV, Ware DE, Cumming WA. An approach for the estimation of effective radiation dose at CT in pediatric patients. Radiology 203(2):417-22; 1997.

Reference:
Frey GD, Sprawls P, eds. The expanding role of medical physics in diagnostic imaging. Proceedings of the 1997 American Association of Physicists in Medicine Summer School. (AAPM Monograph Series Number 23). American Association of Physicists in Medicine; June 1997.

Following are a number of websites that will give you information on medical x rays:

• Something About X Rays for Everybody 
• The History of X-Rays
• The Trail of Invisible Light: A Century of Medical Imaging

You might also find the Radiation Information Network website useful.

There are a variety of formulae that may be used for calculation of various parameters for diagnostic x-ray imaging units. Typically, leakage radiation limits are fixed by regulation or standard to a value that limits the dose received by the portion of the patient's body outside the useful beam. In the United States, the National Council on Radiation Protection and Measurements recommends that the leakage radiation be limited to 100 mR in one hour at one meter from the target (source of radiation). This limit is specified as an integrated value to account for the workloads allowed on various tubes. NCRP Report Number 102 discusses some of these standards.

A standard reference in the field of medical physics has been The Physics of Radiology by Johns and Cunningham, available from Amazon. Other groups have used the text by E. Christensen et al., An Introduction to the Physics of Diagnostic Radiology.

According to graphs presented in numerous textbooks (Bushberg et al. 1994), x rays can be produced with any potential voltage across the x-ray tube. However, due to the space charge effect, the relationship "between the filament current and the tube current" is not linear below about 40 kVp. The use of compensation circuits with the filament current will produce the desired tube current at these low operating voltages. Although x rays are produced at very low voltages, most of the low-energy x rays are absorbed by the x-ray tube housing, filtration, etc. The useful x-ray production is then demonstrated by a curve starting at zero intensity at some kVp, increasing to a maximum intensity at approximately 1/3 of the maximum x-ray energy and then decreasing to zero intensity at the maximum energy. X-ray tubes that operate at such low kVp are typical of mammography units in diagnostic radiology (Eichholz and Shonka 1993). Shielding for x rays of this low energy usually consists of two sheets of commercially available drywall, with one sheet on each side of the wall. If the source is an industrial unit and is mounted inside a cabinet, the shielding of the cabinet alone may be sufficient.

References used in this answer that may be of interest to you:

• Bushberg JT, Seibert JA, Leidholdt Jr. EM, Boone JM. The essential physics of medical imaging. Lippincott, Williams & Wilkins: NY; Chapters 4 and 9; 1994.
• Eichholz GG, Shonka JJ, eds. Hospital health physics. In: Proceedings of the 1993 Health Physics Society Summer School. Richland, WA: Research Enterprises; 1993: 85-86.