1 Nationwide Evaluation of X-Ray Trends (NEXT) Survey of Patient Radiation Exposure from Computed Tomographic (CT) Examinations in the United States Stanley H. Stern, Richard V. Kaczmarek, David C. Spelic, and Orhan H. Suleiman U.S. Food and Drug Administration, Rockville, Maryland Presented at the 87th Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, November 25-30, 2001

2 ABSTRACT PURPOSE: To estimate radiation dose that patients in the United States currently receive from x-ray CT examinations and to compare findings historically and internationally. MATERIALS AND METHODS: Inspectors from 39 States were specially trained to obtain metrics of patient workload and CT-scanner irradiation conditions, e.g., kVp, mAs, tomographic section thickness, number of slices comprising exams, table increment, pitch in helical scanning, etc. They surveyed 263 CT facilities across the U.S. in randomly selected samples proportional to State populations. Working with the facility’s most frequently used CT scanner, surveyors measured x-ray exposure (air kerma) centrally and peripherally in a standard head dosimetry phantom as well as free in air on the scanner axis of rotation. For head exams, the measurements can be related to common indices of CT dose—MSAD, CTDI—and for body exams they can also be used to derive effective dose. Facility responses to a separate questionnaire provided complementary data on the frequencies and techniques of various body exams, on the prevalence of use of special techniques for pediatric patients, and on the implementation of CT scanner quality-assurance programs. RESULTS: Findings presented here are preliminary and do not represent the complete sample. For routine examinations of the adult head (brain), the most common protocol uses axial scanning in a single no-contrast phase. 64% of the facilities use one and 36% use two different values of tomographic section thickness in covering this anatomic region. The mean width of the widest tomographic section = 8.2 mm (S.D. = 2.3 mm, n = 200); mean tube voltage = 127 kVp (S.D. = 9 kVp, n = 204); mean tube current = 189 mA (S.D. = 57 mA, n = 193); mean current- time product = 355 mAs (S.D. = 114 mAs, n = 190); and mean multiple scan average dose (MSAD) in the center of a standard head phantom = 50 mGy (S.D. = 19 mGy, n = 203). Effective doses E are derived from air kerma measurements, facility mAs values and other exam techniques. For the most frequent routine examinations of adult patients E is 2 mSv (S.D. = 1 mSv, n = 45) for head exams; 12 mSv (S.D. = 7 mSv, n = 21) for abdomen-pelvis exams; 6 mSv (S.D. = 4 mSv, n = 21) for chest exams; 6 mSv (S.D. = 4 mSv, n = 19) for abdomen exams; 15 mSv (S.D. = 10 mSv, n = 18) for chest-abdomen-pelvis exams; and 6 mSv (S.D. = 4 mSv, n = 15) for pelvis exams. From facility reports of examination workload, we estimate that there were 58 ± 9 million CT examinations and procedures in the U.S. in the survey year CONCLUSION: Our most important observation is that on average for any given examination of the head or body, effective doses are significantly smaller with helical-scanning techniques than with axial-scanning techniques. With respect to historical trends, the following findings apply to adult head examinations done axially: The mean MSAD (50 mGy; S.D. = 19 mGy, n = 203) in the survey is 10% larger than the corresponding MSAD in 1990 (46 mGy; S.D. = 18 mGy, n = 250) despite the much lower mean current-time product (355 mAs) presently compared to the 1990 value (460 mAs; S.D. = 246 mAs, n = 250). On the other hand, from 1990 to 2000 there was a drop in the weekly CT head-exam workload per facility (from 44 ± 2 to 26 ± 5). We hypothesize that the drop-off in CT head exams may be associated with a switch to other imaging modalities for the head and also with a commensurate or even larger upsurge in the number of x-ray CT body exams. The observations suggest that advances in CT scanner technology have been rapidly adopted in clinical practice and have thereby significantly affected collective dose.

3 CT Survey Design and Methods [1] Survey year : May 2000 through April 2001 Overall sample size n = 263 facilities randomly selected in 39 States –State sub-sample proportional to State population Two parts to survey for each facility (1) On-site measurements, interview of CT radiologic technologist by specially trained State radiation-control inspectors (2) Questionnaire on types of CT units, patient workload data, scanning protocols, and quality-assurance practices Air kerma measured in head phantom and free-in-air on axis of rotation  –multiple scan average dose (MSAD) for head exams –CT radiation output (+ calculations)  effective dose (E), head & body exams CT exam workloads are facility estimates, not based on record reviews Caveat: Data in this poster are preliminary—from sub-samples not nationally representative—reflecting a work in progress!

4 States Contributing to the NEXT CT Survey Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Florida, Hawaii, Idaho, Illinois, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nevada, New Hampshire, New Jersey, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota, Texas, Utah, Vermont, Virginia, Washington, Wisconsin

5 State inspectors receiving training at the National Naval Medical Center, Bethesda, Maryland.

6 Head phantom (polymethyl methacrylate). A pencil ionization chamber (held by 3-prong extension clamp) can be inserted into the phantom (for MSAD measurements) or can be held free-in- air on the CT axis of rotation without any phantom present (for measurements of air kerma).

7 Excerpt from Facility Questionnaire: Helical Techniques and Workload

8 CT Survey Year : Techniques and Radiation Output

9 Technological and Practice Adaptation: Excerpted Survey Questions and Answers Q: Helical scanning available? A: 81% (n = 177) of the most frequently used CT units in facilities can do helical scanning. Q: CT fluoroscopy available? A: 5% (n=186) of the most frequently used CT units in facilities can do CT fluoroscopy. Q: For the most frequently used CT unit, does facility use dedicated techniques (e.g., selectable physical filter, kVp, mA, scan time, etc.) with pediatric patients that are different than those used with adult patients? A: 43% yes; 1% no; 56% not responding to this question (n = 157).

10

11

12 Note Measurements of air kerma for single axial scans were made free in air on the axis of rotation. Values reported in the table represent the pooled data from two sets. One set corresponds to CT system kVp, mAs, slice- thickness, and beam-filtration settings for routine head exams. In the second set identical system settings were used except (a) the slice thickness was fixed at 5 mm, and (b) filtration was selected for routine body exams. There were not significant differences between respective mean values of these two sets.

13 CT Survey Year versus 1990 [2, 3] Routine Adult Head Examination

14

15 Note All of the MSAD data in this table refer to a single (no-contrast) phase axial-scanning examination of the adult head. In the survey year , for each facility this examination MSAD was determined from the mean of dose measurements in the center hole of a PMMA head phantom. When a "split" technique set was used, dose measurements associated with the wide slice (typically 10 mm) and the narrow slice (typically 5 mm) were weighted according to their respective fractions of the total number of slices used to comprise a single (no-contrast) phase exam. The mean value of the ratio of the number of narrow slices to total number of slices used in split-technique exams is 0.50 (s.d. = 0.09, n = 17). There are no significant differences amongst sample MSAD means (survey year ) in a comparison of wide- slice values versus narrow-slice values.

16

17

18

19

20 U.S CT Workload Statistics, Population Projection, and Effective Dose All CT Examinations

21

22

CT Effective Dose

24 Notes 1. It is assumed that each hospital enumerated in the American Hospital Association registry (see ref. [4]) has a facility to perform CT examinations and procedures. The total number of CT facilities in the U.S. is estimated from the ratio of the number of registered hospitals to the fraction of CT facilities that hospitals comprise in the survey sample. The uncertainty represents one standard deviation and reflects the number (n = 207) of facilities in the survey sample used in the estimation. 2. The total number of annual CT examinations and procedures in the U.S. is estimated from the product of the annual number of exams and procedures per facility and the total number of CT facilities. The uncertainty represents one standard deviation and reflects the number (n = 68) of facilities in the survey sample used in the estimation. 3. "Routine" exams are those associated with the most typically used protocols covering a particular anatomical region. The data cited for particular types of examinations, percentages of total adult examinations, percentages of axial vs. helical scanning done with each kind of examination, and for effective doses all refer to the most frequently used scanner of the facilities surveyed. 4. The distribution of adult examinations is based on 56 facilities reporting weekly averages of 3165 axial-scanning examinations and 2680 helical-scanning examinations (5845 total per week). Not all facilities report data for each examination category.

25 5. For each facility, the effective dose E is estimated as follows for a complete (i.e., multi-phase) examination of the type indicated: E = Current-time product X (K a /mAs) X (D tissue /K a ) X (NT/I) X (f nc + f c + 2f nc+c ) X E n, where the current-time product (per x-ray tube rotation) is the value reported by the facility; the air kerma K a (free-in-air) per mAs is measured by the surveyors; D tissue /K a (ratio of free-in-air tissue dose to air kerma, equal to the ratio of the respective mass energy absorption coefficients of x-ray photons) is approximately 1.06 for the energy range [5]; NT is the product of the number N of tomograms and tomographic section thickness T, and I is the table increment (so that for helical scanning I/NT is the pitch, reported by facilities); f nc, f c, and f nc+c are the respective fractions (reported by facilities) for which a particular examination has a no-contrast phase exclusively, a contrast phase exclusively, and 2 phases-- without and with contrast; E n is the effective dose normalized to the free-in-air dose to tissue. For each of four different kinds of routine adult exams, E n (mSv effective dose per mGy to ICRU muscle tissue free in air) was determined from NRPB data [6] as the mean [7] of values representing 20 different scanner models and irradiation conditions ( kVp) modeled in Monte Carlo computer simulations of radiation transport: head exam E n = 0.018; chest exam E n = 0.120; abdomen exam E n = 0.091; pelvis exam E n = Normalized effective dose values for the abdomen-pelvis exam (E n = 0.185) and for the chest-abdomen-pelvis exam (E n = 0.305) correspond simply to summations of E n values associated with constituent exam-ranges. 6. The estimates of standard deviation cited in these columns do not include contributions from the distributions of E n values of the 20 different models of scanners and irradiation conditions. The coefficients of variation [7] for the values of E n are as follows: head exam COV = 22%; chest exam COV = 31%; abdomen exam COV = 32%; pelvis exam COV = 30%.

26 Conclusions Preliminary results of an analysis of survey data provide a broad picture and statistical details of CT system irradiation conditions, dose, and patient examination workload in the U.S. Axial scanning comprises approximately 54% and helical scanning 46% of an estimated 58  9 million CT exams annually. Axial predominates over helical scanning for adult head exams, and vice versa for adult body exams. For any particular exam, the effective dose associated with helical scanning is significantly less than that for axial scanning; values are somewhat smaller than those reported from the UK [8]. For adult head exams, the multiple scan average dose (MSAD) of 50 mGy in the U.S. in is approximately 10% larger than it was in 1990 despite a 23% lower average 355 mAs per tube rotation in than in The number of head exams in is 26  5 per week per facility compared to 44  2 in 1990.

27 References [1]. Stanley H. Stern, David C. Spelic, Richard V. Kaczmarek, NEXT 2000 Protocol for Survey of Computed Tomography (CT), December 18, 2000, Conference of Radiation Control Program Directors, Inc., Frankfort, Kentucky, (August 3, 2001.) [2]. Burton J. Conway et al., “Average Radiation Dose in Standard CT Examinations of the Head: Results of the 1990 NEXT Survey,” Radiology Vol. 184, No. 1, pp (July 1992). [3]. Burton J. Conway, Nationwide Evaluation of X-Ray Trends (NEXT) Summary of 1990 Computerized Tomography Survey and 1991 Fluoroscopy Survey, CRCPD Publication 94-2, Conference of Radiation Control Program Directors, Inc., Frankfort, Kentucky (January 1994). [4]. American Hospital Association, “Fast Facts on U.S. Hospitals from Hospital Statistics TM,” (updated January 12, 2001, accessed November 5, 2001). [5]. K.A. Jessen et al., “Dosimetry for optimisation of patient protection in computed tomography,” Applied Radiation and Isotopes Vol. 50, No. 1, pp (January 1999). [6]. D.G. Jones and P.C. Shrimpton, Normalised Organ Doses for X-ray Computed Tomography Calculated Using Monte Carlo Techniques, NRPB-SR250, National Radiological Protection Board, Chilton, UK, [7]. Stanley H. Stern and Jung Ok Yoon, “Development of a Handbook of Patient Tissue Doses for X-Ray Computed Tomographic Examinations,” 2000 World Congress on Medical Physics and Biomedical Engineering, Chicago, July 25, [8]. European Commission, Referral Guidelines for Imaging, Radiation Protection 118, pp (European Commission Directorate-General for the Environment, 2000;

28 Acknowledgment We are grateful to the Conference of Radiation Control Program Directors for administering the program, to the CRCPD NEXT Committee for its diligent oversight, to State radiation control agencies and surveyors for their labor, to the CT facilities and staff for their generous participation, to the National Naval Medical Center and Fairfax Hospital for the use of their facilities for training and prototype measurements, to the American College of Radiology for travel funding of surveyors for training, to the external reviewers of the survey protocol for their careful critiques and pilot surveys, and to our colleagues in the FDA Center for Devices and Radiological Health for technical and logistical support.