Study the Quality Assurance of Conventional X-ray Machines Using Non Invasive KV meter

Transcription

1 Study the Quality Assurance of Conventional X-ray Machines Using Non Invasive KV meter T.M.Taha Radiation Protection Department, Nuclear Research Center, Atomic Energy Authority, Cairo.P.O Egypt. ABSTRACT The aim of the present paper is to investigate some factors affecting on quality assurance of conventional x-ray such as reproducibility of tube voltage, dose out put, time, and x-ray tube efficiency, Accuracy of kvp, ma, time, and focal spot size. and half value layer.examinations of these factors are studied using non invasive kv meter NeroMax 8000 which connected with suitable ionization chambers. The quality assurance tests of X-ray diagnostic examination are measured and compared with the international tolerance. Key words. Quality Assurance, NeroMAX8000, X-ray INTRODUCTION The principle goal of quality assurance of x-ray machine is obtain accurate and timely diagnosis. The secondary goals are minimization of radiation exposure and obtain high image quality. This can be assess by performance the x-ray machine by optimum operating parameters such as reproducibility of tube voltage, dose out put, time, x-ray tube efficiency, Accuracy of kvp, ma, time, focal spot size. and half value layer Many authors publish work concerning quality assurance of x-ray machine, AAPM, (1) describes quality assurance protocol for diagnostic x-ray equipment at the radiologic technologist level. Some important parameters in diagnostic x-ray such as film processor monitor, exposure time, mas reciprocity, peak tube potential, and x-ray out put and beam quality, and focal spot size were described. J.Bosnjak et.al 2008 (2) has been systematically performed quality control surveys of diagnostics imaging equipment in Republic of Srpska. Harold Elford Johns 1961 (3) has been expressed the quality of x-ray in terms of the half value layer. The quality assurances of diagnostic x-ray are based on the Basic Safety Standard BSS (4) and International Commission of Radiological Protection, the use of diagnostic reference levels (DRL for patients, ICRP- Report No., (5) The aim of the present paper is to investigate some factors affecting on quality assurance of conventional x-ray such as reproducibility of tube voltage, dose out put, time, x-ray tube efficiency, Accuracy of kvp, ma, time, and check the focal spot size and half value layer. 105

2 Material and Method Reproducibility of dose, time and high voltage setting- parameters were measured with Nero Max 8000, which were placed on the couch inside the selected field size and six exposures were made. The reproducibility Pz was calculated using the formula. Pz = SD /Zav. 100% Where : SD is the estimator of standard deviation of a series of measurements dose[mgy], time [ms] or voltage [KV], Zav is the means value of the parameters measured [dose[mgy], time[ms] or voltage [KV]. Accuracy of tube voltage and time setting were examined for each machine. Three exposures were recorded for KV and time accuracy. The accuracy Rx was calculated using the formula. Rx= X m -X n /X n. % Where: Xm is the measured value of time [ms] or voltage [KV], and Xn is the nominal value of time [ms] or voltage [KV] Extensive measurements c made to assess of changes in KVp, ma, s and mas on reproducibility and linearity of radiation output. It formed over a range of clinical settings. Calibrated ionization chamber used to measure output expressed as ugy per mas, at a set distance, without backscatter. The linearity was checked using the following equation. X1-X2 /X1+X2 <0.1 Where X1 and X2 are two successive readings. KV, ma, Time accuracy were performed by made the operating conditions constant and change these parameters over a range of clinical settings. This work has been done in nuclear medicine department in Alkhair hospital, Mansora City, Aldaqhlia Country. Beam Quality Filtration of x-ray machine was measured at 70 KVp, 32 mas, 200 ma, 0.16 sec and source to detector distance 66 cm. The dose output was measured using Nero max The dose output was measured with different thickness. The thickness which gave half of zero doses is considered as the half value thickness. It is the thickness of some standard material required to reduce the intensity of a beam to one half its original value, Harold Elford Johns (1961) 3 106

3 Focal spot size using determination using STAR resolution pattern Focal spot is defined as the area on the target of the x-ray tube which the electron stream strikes and from which x-rays are emitted. called also focus. The imaging diameter of star resolution pattern was measured via fixation the star resolution pattern in outrance of x-ray beam and record its image on the x-ray film. From the image the focal spot size for x-ray machine under study was calculated using the following equation. f = Π θ D / 180 (M-1) M = Imaging diameter /Original diameter D: the internal diameter between starting and ending the resolution θ = 2 0 F = focal spot size, mm X-ray Beam Alignment The x-ray beam alignment was checked using perpendicularity of reference axis with table/bucky with field size of 20 x 20 cm, 5 mas, 70 KVp and 100 cm source to detector distance. The quality assurance procedures of x-ray equipment were carried out for six machines of the same type and manufacture..neromax 8000 has been used to check the quality assurance of x- ray machine. The NERO Max consists of the control console, detector cable, two filter cards, ma's leads, Excel Add-in, AC adapter, and HVL plates; the compact control console houses the rechargeable battery, supper bright easy to read backlit display, eight control buttons, and the sophisticated electronic necessary for accurate, reproducible measurements. Connectors or power input, RS-232, printer, scope output and the NERO mix detector contains sensors for simultaneously measuring KV, exposure or rate and am or mass. Solid state detectors are used to measure KV, An ion chamber, located in the top of the detector, is used for exposure / rate measurements. The filter cards contain the various filters needed to accurately measure kilo voltage. Each filter card is coded so that the NERO max knows which filter is in use and its position.. Reproducibility RESULTS AND DISCUSSION X-ray tube out put and reproducibility (difference between the measured and nominal value) for high voltage, time,, Dose output for six x-ray machines were carried out six times for machines numbered from M1-M2 and presented as shown in table (1) 107

4 Table 1 the results of Quality Assurance control: efficiency of X-ray tube, reproducibility of dose, time and high voltage. Code FFD (m) Efficiency (µgy/mas) FFD [cm] Reproducibility [%] Dose Time High Voltage M M M M M M Reproducibility of dose output was ranged from 0.1 to 0.7 %, of time was ranged from 0.01 to 3 % and of high voltage was ranged from 0.1% to 0.7% which is lower than the tolerance ;limit The efficiency of the x-ray tubes were ranged from 62% to 80% which were greater than 25 µgy/mas for 1 m FDD, Focus to Detector Distance and at 80 kv tube voltage (6). KV and Time Accuracy KV accuracy for different settings of six x-ay machines voltage was examined by setting the source to detector distance at 66 cm of exposure, time at 0.1 sec for different KV intervals from KV and average of KV accuracy was presented as shown in table 2. In addition Time accuracy for x-ray machines was checked by variation the time interval from second as shown in table 2. Table. 2 KV and time accuracy for different settings of six x-ray machines Machine No. Mean kvp Accuracy % Error Mean time Accuracy % Error M M , Recalibrated and adjusted to M M M M KV accuracy is good at all KVp stations for six machine except one of the examined machines gave accuracy of 20 % which is higher than the tolerance limit.(± 5%). That mean this machine needs calibration. It recalibrated and adjusted to 5% KV accuracy. Time accuracy is good at all time settings stations for all examined machine which is lower than the tolerance limit. (± 10%). (7) 108

5 Beam Quality Half value layers were calculated as shown un table 3. that thickness has an ability to prevent the hazard of soft x-ray, by reducing the surface doses during x-ray imaging. Table. 3. Measurement of the Half value thickness Machine No. Half Value Layer, mm M M M M M M HVL is exceed the minimum value, passed above 2.3 mm Al at 70 KeV. This is within the accepted value of IAEA.. Focal spot size using STAR resolution pattern The results of focal spot size estimation are presented as shown in table.4. The estimated focal spot size was compared with assigned value stated by the manufacture and was within the tolerance level ±0.3 of nominal focal spot (NNL). Table. 4 Focal spot sizes for some conventional x-ray machines Code "Nominal size" mm Acceptable size, mm Estimated size, mm M M M M M M The estimated sizes of focal spot are lower than the acceptable values X-ray Beam Alignment The beam alignment was calculated as shown in table

6 Table 5. X-Ray Beam Allignment % Machine No. Perpendicularity, cm M1 0.7 M2 0.4 M3 0.6 M4 0.3 M5 0.5 M6 0.6 The alignment of x-ray machines was investigated using perpendicularity test tool The alignment was ranged from 0.3 to 0.7 cm which is lower than the tolerance level (± 1 cm at 100 cm at FDD) (7) CONCLUSION The quality assurance tests of x-ray machines under study obtained accurate and timely diagnosis. The measured output doses were within the international reference doses. The quality assurance program should generalize for all hospitals to ensure the quality of the x-ray machines under services. REFERENCES (1) A merican Association of Physicists Medicine " Basic Quality Diagnostic Radiology " Diagnostic Radiology Committee Task Force On Quality Assurance Protocol, AAPM Report No.4, (2) J. Bosnjak, O. Ciraj-Bjelac and Herzegovina (Republic of Srpska) (3) Harold Elford Johns" The Physics of Radiology" second edition. Charles C Thomas, Publisher, Publications number 419, (4) International Commission Of Radiological Protection" Basic Safety Standard for ionizing radiation " Safety Series: BSS (5) International Commission Of Radiological Protection, ICRP-No (6) Polish Health Minster executive order dated 24. December 2002 (in Polish) Dz U Nr 241, poz (7) John R.Cameron" Medical Physics" John Wiley and Sons, Inc. 1978, p