Measurement of Absorbed Doses in Anatomical Phantoms

Transcription

1 International Journal of Pure and Applied Physics. ISSN Volume 8, Number 1 (2012), pp Research India Publications Measurement of Absorbed Doses in Anatomical Phantoms A.G. Attaelmanan Department of Applied Physics, University of Sharjah, P.O. Box Sharjah, United Arab Emirates aattaelmanan@sharjah.ac.ae Abstract Purpose: Many theoretical models have been designed to predict exposure levels and absorbed doses under various exposure conditions. The aim of this report, however, is to obtain, through a practical method, a quantitative measure of the absorbed doses in patients undergoing routine X-ray examinations to various anatomical sites. Materials and Methods: Anatomical phantoms simulating patients extremities were exposed under routine radiography conditions. Radiation doses were recorded with (D w ) and without the phantoms (D wo ), and the subtraction of D w from D wo gave a quantitative measure of the absorbed dose in the specific anatomy. Results: Calculated absorbed doses are well below the internationally recommended Diagnostic Reference Levels (DRL) of a few mgy, where the measured absorbed dose in the wrist was lowest at 55.3 μgy, in the knee it was 94 μgy, while the highest absorbed dose was for the ankle at μgy. Conclusions: Measured absorbed doses reported here, although calculated assuming no scattering processes, are very low. These absorbed dose values translate into effective doses in the range 0.05 to msv, that are comparable with the ICRP recommended levels of 0.01 msv for routine radiography. Furthermore, it provides a practical quantification process that allows for the estimation of absorbed dose in patients undergoing repeated radiographical examinations. Introduction One of the main concerns in radiological procedures is the reduction in radiation exposure to patients undergoing radiological examinations [ i, ii ]. Millions of people,

2 54 A.G. Attaelmanan around the world undergo routine radiological examinations. Where, radiographs are taken, of some part of their anatomy, in order to diagnose certain medical conditions. According to international regulations the ALARA principle must be observed by radiologists at all times [ iii ], to ensure that patients and hospital staff are not exposed to unnecessary radiation. This means applying the lowest radiation doses possible to obtain the best radiograph, and there are international guidelines setting the limits on absorbed doses for different radiological procedures [ iv, v ]. In some cases repeated radiographs are taken of the same patient, or the same anatomical site, before reaching correct diagnoses. Although, care is taken so as not to expose patients to unnecessary radiation doses, the fact remains, such repeated radiographs exposes patients to higher doses of radiation [ vi ], thus, increasing their risks of developing medical conditions associated with radiation exposure [ vii ]. Moreover, there are no reliable methods, so far, of assessing the amount of absorbed radiation resulting from a single radiograph or repeated radiographs. Different practical [ viii, ix ] as well as theoretical [ x, xi ] dosimetry protocols are proposed in the literature. Absorbed doses are a reliable indicator of the risk to patients from diagnostic radiology; however, a large number of patient dose measurements are done so as to assess only the entrance surface dose [ xii ]. Most diagnostic radiology departments use phantoms in their quality assurance programs in order to simulate patients. Water, plexiglass, aluminium and copper are the most frequently used phantom materials [ xiii ]. The aim of this report, however, is to obtain, through a practical method, a quantitative measure of the absorbed doses in patients undergoing routine X-ray examinations to various anatomical sites. Materials and Methods Research was conducted at the Radiology Department, Hamad General Hospital, Qatar. A standard Siemens X-ray machine for conventional radiography was used for the exposure of phantoms under normal clinical conditions. The X-ray machine was calibrated, according to standard quality assurance procedures, prior to the acquisition of data. The distance between X-ray focus and film (FFD) was fixed at 100 cm. Figure 1: sketch showing the experimental set-up.

3 Measurement of Absorbed Doses in Anatomical Phantoms 55 Three anthropomorphic body section phantoms, representing different parts of the human anatomy (wrist, ankle and knee) were used to represent radiology cases. The phantoms are made from human skeleton limbs embedded in transparent epoxy raisin (with the anatomic and radio-fidelity of PIXY). Radiation doses were measured using a calibrated Victoreen 4000M+ dosimeter. The Victoreen instrument serially measures and displays kvp Maximum, kvp Average, kvp Effective, dose, and time. The Model 4000M+ then automatically resets for the next exposure. A CsI photodiode pair provide the kvp measurements through five user-selectable filter pairs. This ensures optimum accuracy over the entire diagnostic range with minimum filtration dependence. Exposure measurements are made with a parallel plate ionization chamber located above the filter wheel. Exposure time is measured with quartz crystal accuracy. Radiation doses were recorded without phantom (D wo ), and with the presence of the phantom (D w ). To determine appropriate kv and mas settings, a radiograph of each phantom was taken prior to dose measurements and assessed by a senior radiologist on its diagnostic quality. Kodak T-MAT E films (100 NIF / 30x40 cm) were used, and processed with a Kodak X-omat 2000 processor. Settings for the radiographs deemed of best radiological quality were ultimately used for dose measurements. Figure 2: photographs and radiographs of the three anatomical phantoms. For each setting five exposures were taken. For every exposure the Victoreen instrument recorded high voltage in kilo-voltage (kv), time in milli-seconds (ms) and dose in micro-greys. Table 1: Exposure settings for the different anatomical phantoms. Anatomy High Voltage (kv) Current.Time (mas) Wrist Ankle 52 4 Knee 50 4

4 56 A.G. Attaelmanan Results Repeated dose measurements of the same anatomical phantom showed slight variations (table 2). However such slight variations are acceptable within the standard deviations values showed, due to the variations in the number and energy of the electrons emitted from the cathode, as well as, the statistical probabilities governing their interaction processes within the anode, ultimately resulting in variations in X-ray beam intensity. Calculated absorbed doses (Table 3) are well below the internationally recommended Diagnostic Reference Levels (DRL) of a few mgy, where the measured absorbed dose in the wrist was lowest at 55.3 μgy, in the knee it was 94 μgy, while the highest absorbed dose was for the ankle at μgy. Another parameter which would further reduce the calculated absorbed doses values is scattering, which is expected to occur from the phantom s material. Table 2: Measured X-ray doses in μgy for the four anatomical phantoms. Wrist Ankle Knee D wo D w D wo D w D wo D w 1 st Exposure nd Exposure rd Exposure th Exposure th Exposure Average Dose Std.Dev Table 3: Measured absorbed doses in μgy for the four anatomical phantoms. Wrist Ankle Knee 1 st Exposure nd Exposure rd Exposure th Exposure th Exposure Average Dose Std.Dev Discussion The methods used here for the measurement of the absorbed doses didn t follow those recommended by such bodies as the Institution of Physics and Engineering in Medicine and Biology (IPEMB) [ xiv ], who proposed a detailed, but complex, procedure for the determination of absorbed doses for high, medium and low x-ray

5 Measurement of Absorbed Doses in Anatomical Phantoms 57 energies. The IPEMB code depends on many factors including the air kerma. On the other hand, Huda and Gkanatsios [ xv ] argued that The traditional parameter used to specify the amount of radiation received by a patient undergoing a radiographic extremity examination is the entrance skin air kerma (or exposure), which is a poor indicator of the tissue radio-sensitivity, x-ray beam area, penetrating ability of the x- ray beam, or the patient thickness. In addition, the use of entrance skin air kerma for extremity examinations does not permit a meaningful patient dose comparison with other types of radiologic procedures. Therefore, we decided to use a simpler and more direct method of measuring the absorbed doses, while closely simulating real radiological conditions. Moreover, in this report absorbed dose was used rather than the usual radiological quantities, dose equivalent and effective dose equivalent, out of conviction and in agreement with Greening [ xvi ] who argued that dose equivalent and effective dose equivalent are scientifically undesirable, are unstable, are largely unnecessary, are confusing, are potentially wasteful of radiobiological data, attempt to achieve the impossible and are abandoned when of greatest importance. He concluded that absorbed dose should be the primary radiological quantity for radiation protection purposes. Nevertheless, since the weighting factor for medical X-rays is 1 [ xvii ], then absorbed doses measured in this report translate into effective doses in the range 0.05 to msv that are comparable with the ICRP recommended levels for routine radiography [ xviii ]. Measured absorbed doses, reported here, although calculated assuming no scattering processes, are very low. Calculated average absorbed doses are well below the internationally recommended Diagnostic Reference Levels (DRL) of a few mgy. The method proposed in this report for the measurement of absorbed doses compares favorably with other methods and is more direct and easy to utilize. Furthermore, it provides a practical quantification process that allows for the estimation of absorbed dose in patients undergoing repeated radiographical examinations. Acknowledgements My sincere gratitude to the Department of Radiology at Hamad Medical Corporation, Qatar, for their generous assistance and cooperation. References [1] Gregg EC. Effects of ionizing radiation on humans. In Waggener RG and Kereikas JG., editors. Handbook of medical physics, Volume II. Boca Raton, CRC Press Inc., [2] Radiological protection and safety in medicine. ICRP Publication 73. Pergamon [3] ICRP 2008, Radiological protection in medicine. ICRP publication 105, editor J Valentin, Published by Elsevier Ltd.

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