QUALITY CONTROL AND PATIENT DOSES FROM X-RAY EXAMINATIONS IN SOME HOSPITALS IN THAILAND

Transcription

1 QUALITY CONTROL AND PATIENT DOSES FROM X-RAY EXAMINATIONS IN SOME HOSPITALS IN THAILAND P. Plainoi, W. Diswath, N. Manatrakul Ministry of Public Health, Nonthaburi, Thailand XA Abstract Quality control measurements on 203 diagnostic X-ray units were carried out in 126 hospitals in the central region of Thailand during The measurements consisted of tube voltage, half-value layer (HVL), exposure time, radiation output, beam alignment, light beam diaphragm and entrance surface dose (ESD) in four common radiographic procedures namely adult chest PA, adult mass chest PA in mobile bus unit, abdomen AP and mammography (cephalo caudal view). ESD measurements of 320 examinations were performed using parallel plate ionization chamber and Keithley model 35050A Dosimeter on 192 X-ray units (conventional and mobile) and 11 mammography units. The analysis of test results showed that: 1. 92% had X-ray tube voltage within the tolerance limit of 10% and HVL 3.03 mmal ± 0.55 SD at 80 kvp measured 2. 90% had exposure time within tolerance limit of 10% 3. 86% and 98% had acceptable beam alignment and light beam diaphragm 4. 9 had radiation output > 25 p,gy at 1 m. for true 80 KVp 5. It was found that the ESD values were adult chest (PA) varied from 0.18 mgy to 1.17 mgy (mean ± SD). adult mass chest (PA) varied from mgy to 1.03 mgy (mean ± SD). abdomen (AP) varied from mgy to 6.30 mgy (mean ± SD) mammography (cephalo caudal view) varied from 3.49 mgy to mgy (mean SD) Further surveys are necessary and are being done to include measurements of image quality and for propagation of quality assurance activities in Thailand so as to reduce patient doses while maintaining the image quality. 1. Introduction Diagnostic radiology is the main contributor to the man-made exposure of general population. Since Quality Assurance (QA) programmes ensure high quality diagnostic images with the least possible radiation dose to the patient, all countries have been recommended to introduce programmes for their radiological facilities. In Thailand periodic quality control of all X ray equipment in use is required by legislation. For monitoring purposes radiation leakage, total beam filtration, exposure time, tube voltage (kvp), radiation output, accuracy of beam limiting devices and adequacy of room shielding design were measured. Where available, tolerance limits established by FDA, NCRP, AAPM and ACR were used as a reference [1-4]. According to a programme of patient dose measurements was introduced as part of the quality assurance service already provided for X-ray departments throughout many countries. The emphasis that each patient exposure should be as low as reasonably practicable and encouragement to X ray departments to formulate a strategy for dose reduction [5]. We, therefore, initiated to add the patient dose measurement of entrance surface dose in four common radiographic procedures into our routine protocol as a pilot study. The performance characteristics of 203 diagnostic X-ray units were carried out in 126 hospitals during These diagnostic units were located in the central region of Thailand in government and private hospitals; they covered the whole range of commercial equipment in Thailand. They represent 8% of the total diagnostic units of central region and 3% of the country. 99

2 2. Materials and methods The quality control measurements were carried out on 203 X ray units. The measured units consisted of conventional and mammographic X ray units. Performance measurements were focused on tube voltage, beam quality (half - value layer), exposure time, radiation output, beam alignment and accuracy of beam limiting devices, and entrance surface dose (ESD) in four common radiographic procedures namely adult chest PA, adult mass chest PA in mobile bus unit, abdomen AP and mammography (cephalo caudal view). The parameters which were measured are listed in Table 1 and Table 2 for conventional and mammography X ray units respectively, as well as the acceptability criteria [1-4]. The X ray tube potential and exposure time were measured at 60, 70, 80, 90 and 100 kv using Keithley dosimeter/kvp readout model 35050A and kvp divider model 35080A at 56 cm. X ray source to table top. The X ray beam quality, radiation output and radiation output reproducibility were performed at 80 kv actual beam for 20 mas at 40 cm. from the X ray tube focus to chamber using Keithley dosimeter/kvp readout model 35050A, ionization chamber model B and 100 cm. aluminuim sheets (No. 1100) were insert between the tube and ionization chamber in case of beam quality measurement. The linearity of the radiation output (variation of the output as a function of mas) was checked between 5 mas and 64 mas. The correspondance between the light beam and the actual X ray beam, and X ray beam alignment were tested using the collimator and beam alignment test tools by RMI at 100 cm. from the X ray tube focus to the test tool which was placed on the table. The patient doses for conventional X ray units, were calculated measuring the X ray output and using the tabulated exposure parameters of each X ray examination used in the hospital in question. While the surface dose of the mammographic X ray units were measured on RMI acryric phantom as required by the American College of Radiology (ACR) with the Keithley dosimeter/kvp readout model A, ionization chamber model B using the local examination technique in each place. Table 1. Acceptability criteria of the measured parameters for conventional X ray units kv accuracy kv reproducibility Filtration at 80 kv actual beam Tube output (80 kv at 1 m from the focus) Reproducibility of the tube output Linearity of the tube output Exposure time Accuracy Reproducibility Correspondance light beam and actual X ray beam Leakeage radation at lm from the focus in any direction > 2.3 mmal >25 ugy(ma.s) ~ l < 2% of FID <1 u.gy.s 4 100

3 Table 2. Acceptability criteria of the measured parameters for mammography X ray units. Results kv accuracy kv reproducibility Filtration at 30 kv actual beam Reproducibility of the tube output Linearity of the tube output Automatic exposure cells reproducibility Correspondance light beam and actual X ray beam Leakage radiation at 1 m from the focus in any direction ± 2% > 0.3 mmal ^ 1% of FID <1 ugy.s"' The analysis of X ray units test results showed that 92% had acceptable deviation between nominal and measured values of X ray tube voltage with in the tolerance limit of 10% and HVL 3.03 mm.al ± 0.55 SD at 80 kvp actual beam whereas 90% of the X ray units meet the recommended limits of exposure time (deviation < 10%). Of the X ray generators assessed, 84% had tolerable (ugy/mas) linearity. For the light beam and actual X ray beam 2% of the X ray units showed a difference of more than 2% of FID (focus to image distance). Moreover, in 86% of the X ray tubes, the beam was properly aligned. Measurements on the X ray tubes showed that 96% had adequate beam filtration (HVL > 2.3 mm.al, 80 kvp) and 9 had radiation output less than 25 j,gy at 1 m. for true 80 kvp whereas in 98% exposure reproducibility had acceptable variation within the tolerance limit of. The entrance surface doses for three main types of examinations (some variety of chest such as adult chest PA nonbucky, adult chest PA with bucky and adult mass chest PA in mobile bus units, abdomen, and mammogram) which were measured during the years , are presented in Table 3. Figures 1 to 5 show the entrance surface dose of three categories of chest techniques, abdomen and mammogram respectively in term of mode (frequency of data) and median values for entrance surface dose of each technique. Table 3. for three main types of examinations in different years Examination Adult Chest PA (without bucky) Adult chest PA (with bucky) Adult mass chest PA (mobile bus unit) Abdomen Mammogram mean ESD (mgy)

4 10 ' J V 1 1 ZL r (mgy) Figure 1. of adult chest PA nonbucky Figure 2. of chest PA with bucky stand 3 2 -TT ).050.0eD.070.0SD.100.mil0.mi ).lSO.ia) D.lS0.2C0.2C e ).3ID.32D.33) ).3«.33).3E n 0.' Figure 3. of adult mass chest PA in mobile bus units 102

5 15 T * % C Figure 4. of abdomen Figure 5. of MAMMOGRAM 3. Discussion The results presented here confirm that the acceptable level of the quality control in diagnostic X ray equipment have led to the improvement and reduction in patient doses. In addition, the procedure which was adopted has been well received by the radiologists, commercial firms and public services. This encouraged us to extend the inspection to technical parameters such as focus, image quality, image recording system, television monitors, film storage conditions, and film development methods. The implemention of Quality Assurance in Thailand may take time. Staff training and administrative actions of the radiation protection authorities will improve the existing situation for the benefit of patients and staff. References [1] Title 21, Code of Federal Regulations, Part , FDA, U.S. Government Printing Office, Washington, D.C., 1991 and revisions. [2] National Council on Radiation Protection and Measurements: Quality Assurance for Diagnostic Imaging Equipment. NCRP 99, NCRP, Bethesda, Meryland, [3] AAPM report No. 31, Standardized Methods for Measuring Diagnostic X-Ray Exposures, AAPM Diagnostic X-Ray Imaging Committee TG #8 (1991). [4] American College of Radiology, Mammography Quality Control Radiologist's manual, Radiologic Technologist's Manual and Medical Physicist's Manual, [5] Guidance Note PM77 from the Health and Safety Executive. Fitness of Equipment used for Medical Exposure to Ionizing Radiation (1992). 103