PEDIATRIC EFFECTIVE DOSES IN DIAGNOSTIC RADIOLOGY Walter Huda1, Nikolaos A Gkanatsios2, Robert J Botash1 and Ann S Botash3 1 Department of Radiology, SUNY Health Science Center at Syracuse, NY, USA 2 Department of Radiology, University of Florida, Gainesville, FL, USA 3 Department of Pediatrics, SUNY Health Science Center at Syracuse, NY, USA ABSTRACT Pediatric effective doses can be obtained for any radiologic examination using the selected radiographic technique factors (kV/mAs), the exposure geometry and the patient mass. The energy imparted ε to the patient may be computed from the exposure area product, x-ray tube voltage, half-value layer and patient thickness. Values of energy imparted may be subsequently converted to an effective dose E using published radiographic projection specific E/ε ratios determined using Monte Carlo techniques applied to anthropomorphic phantoms, with a correction applied for the patient mass. Pediatric effective doses (head, chest, abdomen and extremity) were computed for representative adult patients, as well as for pediatric patients ranging from new born to 15 year old youths. Values of patient effective dose were dependent on body size, selected technique factors as well as the type of radiographic imaging equipment used, with no clear trends for effective dose with patient age. INTRODUCTION The effective dose quantifies the radiation risk to a patient undergoing any diagnostic x-ray examination.1,2 Benefits of the effective dose include the ease of comparing doses associated with diverse types of radiographic examination, as well as the ability to compare patient doses with natural background and regulatory dose limits.3,4 In this study, effective doses to patients ranging from newborns to adults were determined for representative x-ray examinations of major body regions (i.e., head, chest, abdomen and extremities). METHOD Technique factors. For examinations of the head, chest and abdomen, radiographic techniques (i.e., kVp/mAs) were obtained from technique charts for dedicated pediatric room which uses a 600 speed screen-film combination. Pediatric radiographs, ranging from newborns to 15 year olds, are all performed without the use of a scatter removal grid. Corresponding adult technique factors were obtained for conventional radiographic rooms which all use scatter removal grids. Radiographic techniques for the extremity examinations were taken from charts for (non-grid) fine screen and single emulsion film combination, with a nominal speed value of 80. Representative x-ray field sizes for each age and examination were taken from the published literature,5 with an estimate was made of the (water equivalent) patient thickness based on the body part and typical patient thickness. Dosimetry. The entrance skin exposure, and x-ray beam half-value layer (HVL), were theoretically calculated using computed x-ray spectra at each kVp. The predictions of the theoretical model were compared to measurements made on a Philips Classic C-850 three-phase generator with a Eureka ROT-350 x-ray tube, and the total x-ray beam filtration adjusted to ensure a good fit between theory and measurement. Relative and absolute values of x-ray tube output and HVL were generally found to agree within about 5%. Energy imparted, ε, was computed from the exposure-area product, EAP, at the entrance plane of the patient and a conversion factor ω(z) using the expression ε =ω(z)×EAP J (1) where z is the patient thickness.6 For a given x-ray tube voltage, the conversion factor ω(z) is given by ω(z)=α×HVL+β J/R-cm2 (2) were α and β are constants. Values of energy imparted were converted to the corresponding patient effective dose E using 7 the expression E 70.9 E = ε × × mSv (3) ε i M where (E/ε)i is the body/projection specific ratios of effective dose per unit energy imparted for examination i, and M is the patient mass.8 RESULTS Technique factors. Table 1 lists the technique factors used for performing radiographic examinations of the head (AP), chest (PA), abdomen (AP) and extremity (AP view of forearm). As the patient size increases, the kVp generally increases. Also depicted in Table 1 are the corresponding values of x-ray beam cross-sectional area and the estimated patient thickness in terms of water equivalence. As expected, both the x-ray beam cross-section and patient thickness monotonically increase with increasing patient age. Table 1: X-ray technique factors, exposure area and patient thickness for head, chest, abdomen and extremity examinations. Extremity Age Head Chest Abdomen (Forearm) 67 kVp/2.0 mAs 60 kVp/2.0 mAs 66 kVp/2.0 mAs Newborn NA (110 cm2/9.0 cm) (140 cm2/8.0 cm) (200 cm2/10 cm) 72 kVp/2.0 mAs 66 kVp/2.0 mAs 70 kVp/4.0 mAs 56 kVp/5.0 mAs 1-yr-old (160 cm2/12 cm) (250 cm2/9.0 cm) (300 cm2/13 cm) (35 cm2/1.8 cm) 75 kVp/2.0 mAs 70 kVp/2.0 mAs 72 kVp/5.0 mAs 60 kVp/5.0 mAs 5-yr-old (210 cm2/14 cm) (430 cm2/10 cm) (540 cm2/15 cm) (84 cm2/3.3 cm) 77 kVp/2.0 mAs 74 kVp/3.0 mAs 75 kVp/6.0 mAs 62 kVp/6 mAs 10-yr-old (240 cm2/15 cm) (670 cm2/13 cm) (820 cm2/17 cm) (140 cm2/5.0 cm) 79 kVp/2.0 mAs 78 kVp/4.0 mAs 78 kVp/7.0 mAs 65 kVp/6.0 mAs 15-yr-old (270 cm2/16 cm) (780 cm2/14 cm) (900 cm2/20 cm) (200 cm2/6.2 cm) 75 kVp/15 mAs 120 kVp/2.0 mAs 75 kVp/15 mAs 65 kVp/8.0 mAs Adult (320 cm2/20 cm) (1300 cm2/15 cm) (1200 cm2/22 cm) (200 cm2/7.9 cm) Dosimetry. Table 2 summarizes the key dosimetry parameters for the four types of radiographic examination for patients ranging from newborn to the adult. In each cell, the first value is the entrance skin air kerma (free-in-air) in µGy. The second term gives the energy imparted to the patient, expressed in µJ. In parentheses on the second line are the corresponding values of patient effective dose in µSv. As expected, values of the entrance skin exposure and energy imparted for the pediatric examinations all monotonically increase with increasing patient age. Comparison of adult examinations with 15 year old patients is complicated by the differences in radiographic technique, with 15 year old chests are performed at 78 kVp whereas adult chests are performed at 120 kVp. An additional difficulty is the use of grids for the adult head/chest/abdomen examinations, which is expected to have a large impact on the resultant values of entrance skin air kerma and energy imparted. For pediatric head examinations, the effective dose increased as the patient age (size) was reduced, whereas for chest, abdomens and extremity exams, the converse was generally true. Adult head and abdomen examinations have effective doses much greater than pediatric effective doses. Effective doses for adult chest and extremity examinations were comparable to those for 15 year old youths. Overall, there was no clear trend of patient effective doses with patient age; effective doses are a (complex) function of body size, selected technique factors as well as the type of radiographic imaging equipment which is used to perform these studies. Table 2: Values of entrance skin air kerma ( µGy), energy imparted ( µJ) and the corresponding patient effective doses( µSv) for the specified patient ages and type of radiographic examination. Extremity Age Head Chest Abdomen (Forearm) 100 µGy/78.2 µJ 77 µGy/66 µJ 100 µGy/140 µJ Newborn NA (10 µSv) (19 µSv) (62 µSv) 120 µGy/165 µJ 96 µGy/160 µJ 230 µGy/580 µJ 130 µGy/9.5 µJ 1-yr-old (7.3 µSv) (16 µSv) (90 µSv) (0.21 µSv) 140 µGy/260 µJ 110 µGy/340 µJ 320 µGy/1500 µJ 160 µGy/44 µJ 5-yr-old (5.9 µSv) (18 µSv) (120 µSv) (0.50 µSv) 150 µGy/320 µJ 190 µGy/1100 µJ 420 µGy/3300 µJ 200 µGy/130 µJ 10-yr-old (4.3 µSv) (33 µSv) (160 µSv) (0.87 µSv) 150 µGy/400 µJ 280 µGy/2100 µJ 550 µGy/5100 µJ 220 µGy/240 µJ 15-yr-old (3.1 µSv) (36 µSv) (140 µSv) (0.92 µSv) 1100 µGy/3200 µJ 150 µGy/2500 µJ 1100 µGy/13000 µJ 300 µGy/360 µJ Adult (19 µSv) (34 µSv) (290 µSv) (1.1 µSv) CONCLUSION 1. Values of entrance skin exposure and energy imparted to patients generally increased with increasing patient age. 2. Values of patient effective dose were dependent on body size, selected technique factors as well as the type of radiographic imaging equipment used. 3. There was no simple trend when comparing adult effective doses with those of infants and children. 4. There was no simple trend for the variation of pediatric patient effective dose with age. REFERENCES 1 International Commission on Radiological Protection. Publication 60: 1990 Recommendations of the International Commission on Radiological Protection, Annals of the ICRP Vol 21 Nos 1-3. Pergamon Press, Oxford (1991). 2 United Nations Scientific Committee on the Effects of Atomic Radiation. 1993 Report to the General Assembly: Medical Radiation Exposures. United Nations, New York, NY (1993). 3 National Council on Radiation Protection and Measurements. Report No. 100: Exposure of the U.S. Population from Diagnostic Medical Radiation. NCRP, Bethesda, MD (1989). 4 Nuclear Regulatory Commission. 10CFR19: Notices, Instructions, and Reports to Workers: Inspection and Investigations. Nuclear Regulatory Commission, Washington, DC (1995). 5 Hart, D; Jones, D.G.; Wall, B.F. NRPB Report R262: Estimation of Effective Dose in Diagnostic Radiology from Entrance Surface Dose and Dose-Area Product Measurements. National Radiological Protection Board, Didcot, Oxon (1994). 6 Gkanatsios, N.A. Master Thesis: Computation of Energy Imparted in Diagnostic Radiology. University of Florida, Gainesville, FL (1995). 7 Gkanatsios, N. A.; Huda, W. "Energy imparted in diagnostic radiology." Medical Physics 24:571-579 (1997). 8 Huda, W.; Gkanatsios, N. A. "Effective doses and energy imparted in diagnostic radiology." Medical Physics 24:1311- 1316; (1997).
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