1.3 Quality control The need for more frequent quality control (QC) evaluations for x-ray, fluoroscopy, mammography and CT equipment could be attributed to the incompetent servicing or lack of qualified and experienced technologists or medical physicists. However, performing QC tests regularly, can maintain the consistent performance and reduce the number of repeated examinations and subsequently reduce the patient exposure to unjustified radiation doses. Implementation of the QC program implies the optimum use of equipment, decreasing image retake and create high-quality medical images. The benefits of using high-quality medical images in diagnosing disease are well known, however, the QC of these images influenced by a number of parameters. The various components of the image are ideally tested independently and can be evaluated through measurements of different physical parameters, such as distance accuracy, brightness, contrast, uniformity, and spatial resolution. QC is part of quality assurance (QA) program which is often handled with medical equipment to ensure that they working properly. Moreover, in the acceptance test, the radiology department tries to answer if they get what they ordered, equipment is correctly placed in the room, function as advertised, and providing acceptable images. These tests should be performed immediately after the installation of a new equipment of diagnostic imaging, and prior to clinical use. Acceptance tests based on valid international standards and should be independent of manufacturers or agent using calibrated instruments.Many facilities in Najran, Saudi Arabia assign the task of acceptance tests of X-ray, fluoroscopy, mammography or CT units to the agent or the manufacturer, leaving no guarantee to ensure compliance with the equipment supply contract. This is can be due to several factors, the most important of which is the paucity of a highly qualified medical physicist to perform acceptance tests. This may be a major cause of not considering the valid international standards, which leads to increase the error in the results of quality control tests as well as increase the radiation doses to the patient undergoing different imaging modality. 1.4 Practical dose quantities The American Association of Physicists in Medicine (AAPM) and the Radiological Society of North America (RSNA) (15) defined the Entrance Surface Dose (ESD) as a measure of the radiation dose absorbed by the skin where the x-ray beam enters the patient. The International Commission on Radiological Protection (ICRP) (16) defined the Effective Dose (ED) as the tissue-weighted sum of the equivalent doses in all specified tissues and organs of the human body. The dose received by patients during different x-ray examinations is commonly estimated by measuring the ESD or ED. ESD can be evaluated either by direct measurements using an ionization chamber or thermoluminescent dosimeters (TLDs). In addition, ESD can also be evaluated using a mathematical model to calculate the patient doses based on data on X-ray tube output and a software such as DoseCal and PCXMC. ED can be obtained by correcting the calculated absorbed organ dose using the radiation weighting factor to give a weighted average of the equivalent dose quantity. Subsequently, the result is correct for the organs using tissue weighting factor. The sum of effective doses to all organs of the body represents the effective dose for the whole body (16). Computed tomography (CT) scans can provide detailed information to diagnose many conditions. Several patients undergoing computed tomography (CT) examinations and received a large radiation dose comparing with other imaging modality. As recommended by the European Guidelines for Computed Tomography (17) the radiation dose in CT commonly evaluated by the computed tomography dose index (CTDI) and dose-length product (DLP). The CTDI was defined in 1981 by Shope et al (18) to evaluate the dose output of CT scanner in conjunction with patient size. CTDI100 defined by Dowsett et al (19) as nonclinical average dose measured over a 100 mm pencil ionization chamber inside a PMMA phantom over the width of 14 CT slices. The weighted computed tomography dose index (CTDIw) is measured with the same chamber placed in the center of a PMMA phantom and in several locations on the periphery in order to derive a weighted value for the complete cross-section that is an approximation to the average dose (20). The X-ray examination of the human breast is known as Mammography (21) and considered as one of the frequent x-ray examinations that help in detecting most of characteristic masses or microcalcifications in the breast tissue before they turn into breast cancer. However, the radiation dose delivered to the breast should not be too high for that purpose and must be optimized for x-ray examinations. There is worldwide interest in developing QC protocols for mammography and in the guidelines set out by the National Radiation Protection Authority (NRPA) (22), the European Communities (EC) (23), Institute of Physics and Engineering in Medicine (IPEM) (24), and International Atomic Energy Agency (IAEA) (25). Mammography dose can be evaluated by calculating the ESD or mean dose absorbed (MGD) by the glandular tissue in the breast. MGD can be determined by measuring the incident air kerma and subsequently corrected and multiplying it by a conversion factor determined either by direct measurements using an ionization chamber or TLDs or through Monte Carlo calculations. MGD depend on the tissue composition and tissue distribution in the breast and this can result in deviation of MGD estimated using a simple breast model. 1.5 Objectives: The overall objective of this study is to investigate radiation dose and image quality evaluation in common X-ray, fluoroscopy, mammography and CT examinations. The hope is to help sharpen the tools for the optimization radiation dose and image quality in these imaging modalities. The specific objectives of the present study are: – Estimating the radiation doses for patients undergoing common X-ray and fluoroscopy examinations for anterior to posterior, posterior to anterior and lateral projections. – Evaluated mammography dose by calculating the mean glandular dose (MGD)- Estimating the weighted computed tomography dose index (CTDIw) using Unfors xi CT detector and standard dosimetry phantoms.- Evaluating the ESD and Effective Dose (ED) for patients undergoing common X-ray and fluoroscopy examinations using DoseCal and CALDose software for AP, PA and LAT projections. – Comparing all ESD, ED and CTDIw result for all imaging modalities with other reference levels to assess the variation of doses.- Evaluating the image quality for all CT scanner. 1.6 Important of objectives As mention above the doses reached patients during the diagnostic examinations might cause stochastic effects that increased with dose and increase the risk of cancer. From the economic point of view, the cost of cancer treatment and hospitalization is high and to reduce the cost it’s important to reduce the dose for patients during diagnostic examinations.
Determination of the ESD, CTDIw, MGD for patients undergoing common examinations in x-ray, CT, and mammography respectively, will encourage further surveys and that will eventually lead to establishing the dose reference levels in Saudi Arabia. On the other hand, evaluating the image quality in the imaging units will decreasing image retake and create high-quality medical images. This will reduce the film cost for some facilities and generally will reduce the radiation dose for patients.