PATIENT EFFECTIVE DOSES IN DIAGNOSTIC RADIOLOGY, NA

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1 Title of Paper: Patient effective doses in diagnostic radiology Authors: N.A. Gkanatsios, and W. Huda * Corresponding Author: Department of Radiology, University of Florida, P.O. Box , Gainesville, FL , USA tel: (352) , Fax: (352) , hudaw@xray.ufl.edu Presenting Mode: Poster Category: Diagnostic PATINT FFCTIV DOSS IN DIAGNOSTIC RADIOLOGY, NA Gkanatsios and W Huda, Department of Radiology, University of Florida, Gainesville, FL , USA The effective dose is the best parameter for describing the amount of radiation received by a patient undergoing any diagnostic x-ray examination. Benefits of the effective dose include the ease of intercomparing doses associated with diverse types of radiologic examination and the ability to compare patient doses with natural background or regulatory dose limits. The major limitation of the effective dose is the requirement of obtaining mean organ doses of the irradiated tissues in the patient. A method is proposed which can be used to generate patient effective doses using the selected radiographic technique factors (i.e., kvp/mas), patient source to skin distance and x-ray beam crosssectional area. An algorithm based on computed x-ray spectra is used to generate the beam output (mr/mas) and half-value layer at the selected tube voltage for a given tube anode angle and waveform ripple. The energy imparted ε to the patient is then obtained from the exposure area product, x-ray beam voltage, half-value layer and patient thickness. nergy imparted is finally converted to an effective dose using published body/projection specific /ε ratios. xamples are provided of how the effective doses may be generated for representative radiographic examinations of the head, chest, abdomen and for the extremities.

2 PATINT FFCTIV DOSS IN DIAGNOSTIC RADIOLOGY Nikolaos A. Gkanatsios and Walter Huda Department of Radiology, University of Florida, Gainesville, FL , USA Computation of the effective dose is difficult and time consuming. A method is proposed which can be used to obtain patient effective doses for any radiologic examinations using the selected radiographic technique factors (i.e., kv/mas), patient source to skin distance and x-ray beam cross-sectional area. The energy imparted ε to the patient is then computed from the exposure area product, x-ray beam voltage, half-value layer and patient thickness. Values of energy imparted are converted to an effective dose using published body/projection specific /ε ratios determined using Monte Carlo techniques applied to anthropomorphic phantoms. INTRODUCTION The effective dose is the best parameter for describing the amount of radiation received by a patient undergoing any diagnostic x-ray examination. Benefits of the effective dose include the ease of intercomparing doses associated with diverse types of radiographic examination, as well as the ability to compare patient doses with natural background and regulatory dose limits. In this study, a method is described which permits the effective dose to be determined for any diagnostic x-ray examination. MTHOD The method used to determine effective doses consists of the following three steps: (i) An algorithm is used to generate the x-ray beam output (mr/mas) and half-value layer at the selected kv for a given tube anode angle, voltage waveform ripple and x-ray tube filtration. (ii) The energy imparted ε to the patient is obtained from the exposure area product, x-ray beam kv, half-value layer and patient thickness. (iii) nergy imparted is converted to an effective dose using published body/projection specific /ε ratios. The /ε ratios for 68 projections have been published by Huda and Gkanatsios 1 and were determined from the Monte Carlo dosimetry data. 2 xposure and Half-Value Layer RSULTS A parametrization model was developed to compute the radiation exposure output and half-value layer of x-ray tubes given the tube voltage and mas, the tube anode angle and filtration, and the voltage waveform ripple. The model was derived from generated tungsten x-ray spectra 3 and accounts for the effects of the anode angle and waveform ripple on the computed exposure output and HVL. An example of the model is illustrated in Fig. 1 where exposures is plotted as a function of tube voltege and in Fig. 2 where the HVL is plotted as a function of tube voltage. The data in Fig. 1 and Fig. 2 were generated for an x-ray tube with a tungsten target (+ 10% rhenium), a 12 anode angle and 3 mm added Al filtration. nergy Imparted nergy imparted is a measure of the total ionizing energy deposited in the patient during a radiologic examination and may be used to quantify the patient dose in diagnostic radiology. nergy imparted, ε, may be computed from the

3 exposure-area product, AP, at the entrance plane of the patient and a conversion factor ω(z) 4 using the expression ε =ω(z) AP J (1) where z is the patient thickness. For a given x-ray tube voltage, the conversion factor ω(z) is given by J were α and β are constants. 5 ω(z)=α HVL+β R cm 2 (2) An example of computed energy imparted for chest examinations is shown in Fig. 3 which shows values of energy imparted computed for a semi-infinite water phantom (15 cm). Also shown are the computed values of energy imparted for an anthropomorphic phantom using Monte Carlo techniques for the same technique factors. The error bars show the energy imparted to a water phantom with ± 2 cm thickness of the assumed 15 cm phantom thickness. There is excellent agreement in Fig. 3 between the values of energy impated to the simulated water phantom data and the corresponding data for an anthropomorphic phantom. Patient ffective Doses in Diagnostic Radiology The effective dose can be obtained from the energy imparted ε using the expression = ε ε i msv (3) where (/ε) i is a ratio that depends on the projection i employed (body region irradiated and x-ray beam orientation). Figure 4 shows /ε for PA chest projections as a function of tube voltage and filtration showning that /ε values have only a minor dependence on the x-ray tube voltage and filtration. Values of (/ε) i were calculated for 68 projections using and ε values obtained from published dosimetry data computed using Monte Carlo techniques on an adult anthropomorphic phantom. 1 Table 1 gives examples of (/ε) i ratios for a variety of common radiologic examinations and projections. Table 2 lists the computed values of energy imparted and effective dose for some common radiologic examinations with specified values of entrance skin exposure (S) and selected technique factors.

4 Table 1: (/ε) i data for common radiologic examinations xamination AP Left LAT Head 6.1 ± ± 0.7 Cervical Spine 24.2 ± ± 0.8 Thoracic Spine 22.3 ± ± 1.1 Chest 22.4 ± ± 0.8 Stomach 40.2 ± ± 0.7 Lumbar Spine 23.2 ± ± 1.3 Abdomen 21.3 ± 1.1 Table 2: nergy imparted and effective doses to patients for common radiologic examinations xamination Projection /kvp S (mr) e (mj) (msv) Head PA/ LAT/ PA/ Chest LAT/ AP Port/ Abdomen AP/ Lumbar Spine AP/ LAT/ Tube Beam xposure (mr/mas at 1m) % Ripple 20% Ripple 50% Ripple 100% Ripple Tube Potential (kv) Figure 1: X-ray tube output exposure as a function of tube voltage. Half-Value Layer (mm Al) % Ripple 20% Ripple 50% Ripple 100% Ripple Tube Potential (kv) Figure 2: X-ray tube half-value layer as a function of tube voltage. nergy Imparted (mj) cm Water Anthrop. Phant. Chest AP View S =10 mr Tube Voltage (kv) Figure 3: nergy imparted as computed using q. 1 versus the anthropomorphic phantom. /e (msv/j) Chest, PA View 5.0 mm Al 3.0 mm Al Tube Voltage (kv) Figure 4: Ratios of /ε for two chest projections as a function of tube voltage and filtration.

5 RFRNCS 1. Huda W, Gkanatsios NA. ffective dose and energy imparted in diagnostic radiology. Med Phys (accepted: 1997). 2. Hart D, Jones DJ, Wall BF. NRPB Software SR262: Normalized Organ Doses for Medical X-Ray xaminations Calculated Using Monte Carlo Techniques, (National Radiological Protection Board, Didcot, Oxon, 1994). 3. Tucker M, Barnes G, Chakraborty D. Semi-empirical model for generating tungsten target x-ray spectra. Med Phys 18, (1991). 4. Gkanatsios NA. Master Thesis: Computation of nergy Imparted in Diagnostic Radiology (University of Florida, Gainesville, FL, 1995). 5. Gkanatsios NA, Huda W. Computation of energy imparted in diagnostic radiology. Med Phys (accepted: 1997).