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Online Appendix for the following May 2 JACC article TITLE: Radiation Exposure of Computed Tomography and Direct Intracoronary Angiography: Risk Has its Reward AUTHORS: Pat Zanzonico, PhD, Departments of Medical Physics and Radiology (Nuclear Medicine Section), Memorial Sloan Kettering Cancer Center, New York, New York, Lawrence N. Rothenberg, PhD, Departments of Medical Physics and Radiology (Nuclear Medicine Section), Memorial Sloan Kettering Cancer Center, New York, New York, H. William Strauss, MD, Departments of Medical Physics and Radiology (Nuclear Medicine Section), Memorial Sloan Kettering Cancer Center, New York, New York APPENDIX Technical Aspects of Radiation Exposure Except in the very early days of diagnostic radiology, when radiation precautions virtually were nonexistent, radiation safety concerns have focused on stochastic (or statistical) (1,2) effects. The rapidly increasing use of lengthy interventional procedures has led to renewed concern regarding high-dose deterministic effects (3– 8). At the relatively low energies (typically <150 keV) and limited penetrabilities of diagnostic X-rays, radiation doses are highest at the beam entrance point, and the most likely deterministic effects therefore are skin damage, manifesting as erythema or, in severe cases, ulceration. The threshold absorbed doses for deterministic skin damage associated with acute irradiation, i.e., 200 cGy for transient erythema to 2,000 cGy for dermal ulceration (7,8), are at least two orders of magnitude greater than those associated with either CT or noninterventional CCA. Accordingly, as addressed by Coles et al. (9) in this issue of the Journal, the relevant radiogenic risks associated with MSCT and CCA are stochastic risks, namely, carcinogenesis (10–15). Direct measurement of the fluoroscopy skin entrance dose with dosimeters, although the most accurate, has been superseded by an indirect method, the dose-area product (DAP) (5,10,14,15). The standard DAP meter uses an air ionization chamber mounted on the face of the X-ray tube collimator. The ionization chamber measures exposure (i.e., the total electric charge produced by X- and gamma-rays per unit mass of air), which is then converted to absorbed dose (i.e., the energy deposited per unit mass of matter) using the “f factor,” or exposure-to-dose conversion factor (f = 34.9 to 36.8 Gy/C/kg or 0.90 to0.95 rad/R for water or soft tissue) (16). The dose-area product reflects the absorbed dose over the entire X-ray field and is a function of the field size and the exposure at the collimator; it usually is expressed in units of Gray-centimeter squared (Gy-cm2). This value is sometimes recast as the “air-kerma-area product.” “Kerma” is defined as the ratio dEtr dm where dEtr is the sum of the initial kinetic energies of all the charged ionizing particles liberated by uncharged ionizing particles (including photons) in matter and dm is the mass of matter in which the charged ionizing particles were liberated (17–19). For air, this quantity generally is referred to as “air kerma” or “free air kerma.” The measured DAP (or, alternatively, the air kerma-area product) is independent of distance from the focal spot because the X-ray field area varies directly and the exposure rate varies inversely as the square of the distance from the X-ray focal spot to the measurement point. A given DAP reading can result from a high dose over a small field or a low dose over a large field. The stochastic risk can be assumed to be approximately equivalent under these two conditions, and DAP measurements have thus been used to estimate total stochastic risk (6). For selective contrast coronary angiography, DAP varies from 6 to 109 Gy-cm2, with most reported values lying in the 30- to 60-Gy-cm2 range (20). In a novel study, Katritis et al. (21) used thermoluminescent devices (TLDs) mounted on a specially designed catheter advanced to the right or left sinus of Valsalva to directly measure the absorbed dose to the coronary arteries in CCA. A linear relationship between the DAP and the coronary artery dose, DAP = 1,273 cm2·dose (mGy), was thus derived; the constant of proportionality is presumably dependent on the system and the parameter. In another noteworthy study, Kuon et al. (22) reported a 66% reduction in DAP (from 37.1 to 12.9 Gy-cm2) by limiting of cinegraphic runs, systematic use of low-level fluoroscopy, and blind positioning of the region of interest and resulting avoidance of oblique positions. For MSCT, the basic radiation dose parameter is the computed tomography dose index (CTDI), which defined as the integral under the exposure or absorbed dose profile along the patient’s longitudinal (z) axis for a single tomographic image (10,14,15). This parameter is scanner-specific, and its value generally is provided by the manufacturer. The maximum of the radiation dose profile is termed the “peak dose.” The volume CTDI (CTDIvol) is derived from the CTDI and is the average dose delivered to a scan volume (vol) for a specific examination (10,14,15). The 100-mm CTDI (CTDI100) is the integral under the exposure or absorbed dose profile along a 100-mm length of the patient’s longitudinal (z) axis (10,14,15). The weighted 100- mm CTDI (CTDIw) is the weighted average of the CTDI100 measurements at the center and periphery of a dose-measurement phantom (10,14,15): CTDIw [2/3 CTDI100 (p) + 1/3 CTDI100 (c)]·f (1) where CTDI100 (p) = the CTDI100 at the periphery (p) of a cylindrical phantom, CTDI100 (c) = the CTDI100 at the center (c) of a cylindrical phantom, and f = the exposure-to-absorbed dose conversion factor = 34.9 to 36.8 Gy/C/kg, or 0.90 to 0.95 rad/R, for water or soft tissue. The CTDIw thus reflects the average absorbed dose over the transverse (x and y) dimensions of such a phantom and is an approximation of the average radiation dose to the cross section of a patient. Measurements of the CTDI100 (p) and CTDI100 (c) typically are performed using ionization chambers or TLDs positioned in a commercially available soft tissue-equivalent polymethylmethacrylate (i.e., Plexiglas) phantom cylindrical in shape and either 16 or 32 cm in diameter. Ionization chambers actually measure exposure, which is then converted to absorbed dose using the aforementioned f factor. On the other hand, TLDs, yield absorbed the dose directly. Several parameters have been devised to estimate the dose associated specifically with MSCT examinations (10,14,15). The multiple-scan average dose (MSAD) is the average dose of the central scan of a MSCT examination. The MSAD is directly related to the spatial separation of successive scans and, therefore, the “pitch,” or the advance (or feed) of the patient table during a spiral CT examination. Pitch is now rigorously defined as the distance (mm) of patient table advance in the longitudinal (z) direction per gantry rotation divided by the total nominal scan length, and is thus a dimensionless quantity (10,14,15). For MSCT systems, the total nominal scan length includes all simultaneously acquired scans and is the distance in the z direction spanned by all detector rows that are active during a scan. If the table advance during one gantry rotation is less than the total nominal scan width (i.e., pitch <1), scans overlap. Scan overlap, and therefore patient dose, increases as pitch decreases. The dose-length product is defined as the product CTDIvol·total scan length; the total scan length incorporates the number of scans and the length of each scan. Like the MSAD, the dose-length product (mGy-cm) is thus a measure of the integral radiation dose of an entire CT examination. The ED is currently the most widely used and most rigorous measure of stochastic risk. ED provides a single-value estimate of the overall stochastic risk (i.e., the total risk of cancer and genetic defects) of a given irradiation whether received by the whole body, part of the body, or a single or multiple individual organs (17–19): ED wT HT (2) T = wT wR DT,R (3) R T where wT is the weighting factor for tissue or organ T, a dimensionless quantity representing the fraction contributed by tissue or organ T to the total stochastic risk (i.e., the combined total risks of cancer or of demonstrable germ-cell mutagenesis) due to a uniform, total-body irradiation and wR is the weighting factor for radiation R, a dimensionless quantity selected to account for the differences in biological effectiveness of different types of radiation and ranging from 1 for sparsely ionizing X- and gamma-rays (including diagnostic X-rays) to 20 for densely ionizing alpha- rays. The ED is similar in concept to the effective dose equivalent, HE (introduced by the International Commission on Radiological Protection and the NCRP) (1,19), representing a single-value estimate of the net “harm” from any “low-dose” (e.g., diagnostic) exposure. However, the HE is related to the absorbed doses at individual points within organs and thus is problematic to rigorously implement in practice. The ED, in contrast, is based on average organ absorbed doses. The units of ED and HE are the same: the signal intensity unit is the sievert (Sv) and the conventional unit is the rem; 1 Sv = 100 rem and 1 rem equals 1 cSv (or 10 mSv). Neither the ED nor the HE, however, are applicable to “high-dose” (e.g., interventional) exposures (23). For estimation of EDs, organ-absorbed doses generally are either measured using dosimeters (e.g., ionization chambers or TLDs) positioned in tissue-equivalent anthropomorphic phantoms or calculated by Monte Carlo simulations in mathematical anthropomorphic models (5,10,12–15,20). In the report by Coles et al. (9), a standard adult hermaphrodite model, with a weight of 71 kg and height of 174 cm, was mathematically modeled and used in conjunction with Monte Carlo analysis. The average organ absorbed doses thus determined, the corresponding tissue weighting factors (wT) and the radiation weighting factor(s) (wR = 1 for diagnostic X-rays) are then substituted into Equation (3) to yield the ED. Of course, to the extent that the size, shape, and composition of individual patients deviate from those of the phantom (e.g., patients in the study by Coles et al.  typically were heavier and shorter than the foregoing phantom), patients’ EDs will differ from the phantom-derived ED. For MSCT coronary angiography, the scanner is set to record thin slices at high resolution, requiring a high flux of X-rays. Coles et al. (9) found a mean effective dose from MSCT coronary angiography of 14.7 mSv and that from selective coronary angiography of 5.6 mSv. The dose these investigators measured is high compared to publications by other investigators (Table 1), typically of the order of 10 mSv versus 1 mSV for CCA (5,12–15,20,21). For calcium-scoring MSCT, reported EDs range from 2 to 4 mSv (24). REFERENCES 1. International Commission on Radiological Protection. Age-dependent Doses to Members of the Public from Intake of Radionuclides. International Commission on Radiological Protection (ICRP) Publication 56 (Part 1), Vol. 20. Oxford: Pergamon Press, 1990. 2. Zanzonico PB. Internal radionuclide radiation dosimetry: a review of basic concepts and recent developments. J Nucl Med 2000;41:297–308. 3. Koenig TR, Mettler FA, Wagner LK. Skin injuries from fluoroscopically guided procedures: part 2, review of 73 cases and recommendations for minimizing dose delivered to patient. AJR Am J Roentgenol 2001;177:13–20. 4. Koenig TR, Wolff D, Mettler FA, Wagner LK. Skin injuries from fluoroscopically guided procedures: part 1, characteristics of radiation injury. AJR Am J Roentgenol 2001;177:3–11. 5. Mahesh M. Fluroscopy: patient radiation exposure issues. Radiographics 2001;21:1033–45. 6. Mahesh M. AAPM/RSNA physics tutorial for residents: digital mammography: an overview. Radiographics 2004;24:1747–60. 7. Mettler FA Jr., Koenig TR, Wagner LK, Kelsey CA. Radiation injuries after fluoroscopic procedures. Semin Ultrasound CT MR 2002;23:428–42. 8. Vlietstra RE, Wagner LK, Koenig T, Mettler F. Radiation burns as a severe complication of fluoroscopically guided cardiological interventions. J Interv Cardiol 2004;17:131–42. 9. Coles DR, Smail MA, Negus IS, et al. Comparison of radiation doses from multislice computed tomographic coronary angiography and conventional diagnostic angiography. J Am Coll Cardiol 2006;47:1840–5. 10. Gerber TC, Kuzo RS, Morin RL. Techniques and parameters for estimating radiation exposure and dose in cardiac computed tomography. Int J Cardiovasc Imaging 2005;21:165–176. 11. Gerber TC, Stratmann BP, Kuzo RS, Kantor B, Morin RL. Effect of acquisition technique on radiation dose and image quality in multidetector row computed tomography coronary angiography with submillimeter collimation. Invest Radiol 2005;40:556–63. 12. Hong C, Bae KT, Pilgram TK, Suh J, Bradley D. Coronary artery calcium measurement with multi-detector row CT: in vitro assessment of effect of radiation dose. Radiology 2002;225:901–6. 13. Hunold P, Vogt FM, Schmermund A, et al. Radiation exposure during cardiac CT: effective doses at multi-detector row CT and electron-beam CT. Radiology 2003;226:145–52. 14. Morin RL, Gerber TC, McCollough CH. Physics and dosimetry in computed tomography. Cardiol Clin 2003;21:515–20. 16. Johns H, Cunningham J. The Physics of Radiology. Springfield, IL: Charles C Thomas, 1974. 17. International Commission on Radiation Units and Measurements. Radiation Quantities and Units. ICRU Report 33. Bethesda, MD: International Commission on Radiation Units and Measurements (ICRU), 1980. 18. International Commission on Radiation Units and Measurements. Fundamental Quantities and Units for Ionizing Radiation. ICRU Report 60. Bethesda, MD: International Commission on Radiation Units and Measurements (ICRU), 1998. 19. National Council on Radiation Protection and Measurements (NCRP). SI Units in Radiation Protection and Measurements. NCRP Report 82. Bethesda, MD: National Council on Radiation Protection and Measurements (NCRP), 1985. 20. Maeder M, Verdun FR, Stauffer JC, Ammann P, Ricklit H. Radiation exposure and radiation protection in interventional cardiology. Kardiovaskuläre Medizin 2005;8:124–32. 21. Katritsis D, Efstathopoulos E, Betsou S, et al. Radiation exposure of patients and coronary arteries in the stent era: a prospective study. Catheter Cardiovasc Interv 2000;51:259–64. 22. Kuon E, Dorn C, Schmitt M, Dahm JB. Radiation dose reduction in invasive cardiology by restriction to adequate instead of optimized picture quality. Health Phys 2003;84:626–31. 23. Zanzonico P. The fallacy of the chest X-ray as a basis for comparing radiogenic risks (abstr). J Nucl Med 1993;34:133P. 24. Efstathopoulos EP, Makrygiannis SS, Kottou S, et al. Medical personnel and patient dosimetry during coronary angiography and intervention. Phys Med Biol 2003;48:3059–68. Table 1. Reported Radiation Doses From Coronary CT Angiography Author kVp mA Gated Acquisition Pitch ED in mSv (slices) Hoffmanni 120 240 Prospective ECG 0.2 8.6 (16 slice) Morinii 120 300 Prospective ECG 0.375 9.3, 11.3 (4 slice) Lauiii 140 250 Prospective ECG 1.3-1.5 (4 slice) Hackeriv 120 500 Prospective ECG 4.3 (16 slice) Hunoldv 120 400, 0.375 6.7-13 (4 slice) 300, 200 i Hoffmann MH, Shi H, Schmitz BL, Schmid FT, Lieberknecht M, Schulze R, Ludwig B, Kroschel U, Jahnke N, Haerer W, Brambs HJ, Aschoff AJ. Noninvasive coronary angiography with multislice computed tomography. JAMA 2005;293:2471-8. ii Morin RL, Gerber TC, McCollough CH. Radiation dose in computed tomography of the heart. Circulation 2003;107:917-22. iii Lau GT, Ridley LJ, Schieb MC, Brieger DB, Freedman SB, Wong LA, Lo SK, Kritharides L. Coronary artery stenoses: detection with calcium scoring, CT angiography, and both methods combined. Radiology 2005;235:415-22. iv Hacker M, Jakobs T, Matthiesen F, Vollmar C, Nikolaou K, Becker C, Knez A, Pfluger T, Reiser M, Hahn K, Tiling R. Comparison of spiral multidetector CT angiography and myocardial perfusion imaging in the noninvasive detection of functionally relevant coronary artery lesions: first clinical experiences. J Nucl Med 2005;46:1294-300. v Hunold P, Vogt FM, Schmermund A, Debatin JF, Kerkhoff G, Budde T, Erbel R, Ewen K, Barkhausen J. Radiation exposure during cardiac CT: effective doses at multi-detector row CT and electron-beam CT. Radiology 2003;226:145-52.
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