How to Reduce the Radiation Risk in CT Scan through Various Methods - Case Study Example

Reflective journal Since I joined the BSc program, my favourites included Medical Imaging Specialisation 428, and computed tomography (CT) in specific. Different imaging modalities like CT, magnetic resonance imaging (MRI), and ultrasound (US) were included in the unit. There was the option for students to select one of the modalities and specialise in it. During the classes, a lot of different interesting techniques in computed tomography (CT), especially in multi slice CT were presented. The unit curriculum included important topics like physics of Multi Slice CT, Multi Slice CT techniques, application protocols, CT angiography, 3D reconstruction as well as radiation reduction techniques. After attending classes at the university, I was given an opportunity of observing and practicing in the CT scan department at Sir Charles Gardian Hospital. After two years of working in general radiography, I was given on job training for 3 months in order to officially join the CT department. It is a well known fact that the CT equipment produces radiation doses that are higher than those of conventional x-ray equipments. Since the CT scan operator directly controls technical factors such as x-ray tube voltage, the tube current, and rotation time, which directly affect the radiation dose, the scan operator plays an important role in the whole process. Despite these facts, most CT technologists are provided on job training just to operate the CT equipment. I believe that the primary responsibility of any radiology technologist is to obtain the best image quality while delivering the smallest radiation dose possible. In fact, my knowledge in some aspects such as the principle of Automatic Exposure Control (AEC), the effect of scan collimation in CT scan, and patient centring was very superficial. Therefore, I realised that I really needed a proper training course to study these factors in detail, especially those that related to radiation dose reduction. Although the classes were able to provide me with information on different ways of radiation dose reduction during CT procedures, all these were summarised and delivered to us within a short period of time. I feel that the CT modules should focus more on patient assessment, radiation physics, radiation protection, examination protocols, and other issues that promote safe, quality patient care. It is a fact that rapid advances are being made every day in diagnostic radiology and CT technology in particular. It is really a challenge, therefore, for CT technologists to cope up and update themselves with newer advances. Hence, I feel it is very important for CT technologists to have proper training programs. I am sure that my learning during the CT scan classes in this semester will help me in the future as a CT scan technologist. The learning outcome will enable me to apply what I have learnt during classes. Introduction Perhaps, the most important advance in diagnostic radiology over the years has been the use of CT. However, when as compared to conventional radiography, CT involves much higher doses of radiation with resultant risks to the patient. The widely prevalent practice of using CT as a screening technique even for minor complaints like headache, has added to the controversy.  Due to all these concerns, there is a pressing concern to incorporate various safety measures and techniques to avoid excess radiation dose from CT scanning. This essay explores the methods to reduce radiation risk from CT scans. Radiation doses from CT scans “Radiation exposure (expressed in coulombs per kilogram) is defined as the total charge produced in dry air when all electrons liberated by photons in a unit mass of air are completely stopped in air” (Lee et al. 2008.) The absorbed dose, effective dose, and CT dose index (CTDI), are some of the measures, which are used to describe the radiation dose delivered by CT scanning. “The absorbed dose is the energy absorbed per unit of mass and is measured in grays (Gy). One gray equals 1 joule of radiation energy absorbed per kilogram” (Brenner and Hall 2007). The level of risk to an organ in the body from radiation is determined mainly by the organ dose (or distribution of dose in the organ). The effective dose is expressed in sieverts (Sv) (Brenner and Hall 2007). The CT dose index is the quantity of CT dose measured for a single slice in standard cylindrical acrylic phantoms (Brenner and Hall 2007.) When compared to conventional radiography, the organ doses from CT scanning are much larger; for example, an abdominal CT scan delivers 50 times more stomach dose than a conventional anterior–posterior abdominal x-ray exam (Brenner and Hall 2007.) In any given CT study, the radiation doses to particular organs depends on various factors, which includes the number of scans, the tube current and scanning time in milliamp-seconds (mAs), the patient size, the axial scan range, the scan pitch, the tube voltage in the kilovolt peaks (kVp), and the specific design of the scanner (Brenner and Hall 2007.) Radiations knock electrons out of their orbits, creating ions, which leads to damage. Damage to DNA leads to point mutations, chromosomal translocations, and gene fusions, which are implicated in the cause cancer (Brenner and Hall 2007.) There are three ways to reduce the radiation risk from CT. One is to reduce the CT-related dose, secondly by replacing CT with other modalities like ultrasound and magnetic resonance imaging (MRI), whenever possible, and thirdly by decreasing the number of CT studies that are prescribed (Brenner and Hall 2007.) Reducing the CT dose Other than noise filtering techniques and various scanner geometry, a very effective method of controlling radiation dose is tube current modulation (Lee et al. 2008.) a. Angular tube current modulation-“angular (x- and y-axis) tube current modulation involves variation of the tube current to equalize the photon flux to the detector as the x-ray tube rotates about the patient” (McCollough, Bruesewitz, and Kofler 2006). The initial value for the tube current–time product is chosen by the CT operator, and also modulates the tube current within one gantry rotation (McCollough, Bruesewitz, and Kofler 2006.) b. Longitudinal tube current modulation-“longitudinal (z-axis) tube current modulation involves variation of the radiation dose among anatomic regions (e.g. shoulders vs. abdomen vs. pelvis) by varying the tube current along the z-axis of the patient” (McCollough, Bruesewitz, and Kofler 2006). While tube current is varied cyclically in relation to the starting tube current value in angular tube current modulation, longitudinal tube current modulation aims to produce uniform noise levels across various regions. For this, the operator uses the following manufacturer-specific methods: the reference noise index (GE Healthcare Technologies, Waukesha, Wis), reference image acquisition (Philips Medical Systems, Best, the Netherlands), reference tube current–time product value (Siemens Medical Solutions, Forchheim, Germany), or reference standard deviation or image quality level (Toshiba Medical Systems, Tokyo, Japan). This gives the operator the required level of image quality for input to the algorithm (McCollough, Bruesewitz, and Kofler 2006.) c. Angular-longitudinal tube current modulation-this involves the simultaneous combination of angular and longitudinal (x-, y-, and z-axis) tube current modulation. The tube current is varied during both gantry rotation and along the z-axis of the patient (i.e., from the anteroposterior direction to the lateral direction, and from the shoulders to the abdomen). As before, the operator uses the manufacturer-specific methods to indicate the desired level of image quality. Since the x-ray dose is adjusted according to the patient-specific attenuation in all three planes, this method is the most comprehensive approach to dose reduction (McCollough, Bruesewitz, and Kofler 2006) d. Automatic exposure control (AEC)-while the operator determines the image quality requirements, the CT system determines the right tube current–time product. However, it may not be that easy to determine the image quality requirements for the various patient age groups and CT examination types. Depending on the diagnostic task, the operator can make a choice between low noise and a low dose to get the required image quality. Based on this, the CT system will make adjustments to the tube current during the gantry rotation, during movement along the z-axis, or during movement in all three dimensions (McCollough, Bruesewitz, and Kofler 2006). “The main purpose of AEC is to adjust radiation dose according to the patient’s attenuation and ultimately to reduce the radiation dose to the patient while sustaining diagnostic image quality” (Lee et al. 2008.) The advantages of AEC includes: maintenance of image quality, better control of patient radiation dose, avoiding photon starvation artifacts, and reduced load on the x-ray tube. However, some concerns include the increase in the image noise, especially in the area next to the contrast material and prosthesis-related artifacts. Types of AEC methods include: patient-size AEC, z-axis AEC, and rotational AEC. A combination of these methods is used by most recent multidetector CT software (Lee et al. 2008). However, since there is no standard dose modulation technique of applying automatic exposure control (AEC), radiologists must be aware about different multidetector CT scanner dose modulation techniques and how to apply these techniques (Lee et al. 2008). The parameters, which are necessary for scanning protocol optimization includes: tube voltage, tube current, section thickness, collimation, pitch, and the image reconstruction kernel (Lee et al. 2008) Another way of reducing radiation dose is by shielding of radiosensitive tissues like the gonads, breast, eyes and thyroid. However, the regular use of shielding techniques during CT scans requires time, resources and trained personnel (Mozumdar 2003.) Conclusion Ever since the advent of CT scanning, it has emerged as one of the most important diagnostic imaging modalities. However, concerns have been raised about the risk of radiation exposure from CT scans. Some methods to reduce radiation dose includes angular tube current modulation, longitudinal tube current modulation, angular-longitudinal tube current modulation, and Automatic exposure control (AEC). Angular tube current modulation involves variation of the tube current as the x-ray tube rotates about the patient. Longitudinal tube current modulation involves varying the tube current along the z-axis of the patient. Angular-longitudinal tube current modulation involves the simultaneous combination of angular and longitudinal tube current modulation. In Automatic exposure control (AEC), the CT system will make adjustments to the tube current, during movement along the z-axis, or during movement in all three dimensions Some practical solutions include replacing CT with other modalities like ultrasound and magnetic resonance imaging (MRI), whenever possible, and by avoiding the routine use of CT studies. References Brenner, DJ and Hall, EJ. Computed Tomography-An Increasing Source of Radiation Exposure. NEJM. (2007). 357:2277-2284. Lee, CH, Goo, JM, Ye, HJ, Ye, SJ, Park, CM, Chun, EJ, Im, JG. Radiation Dose Modulation Techniques in the Multidetector CT Era: From Basics to Practice. RadioGraphics (2008). 28: 1451-1459. McCollough, CH, Bruesewitz, MR, Kofler, Jr. JM. CT Dose Reduction and Dose Management Tools: Overview of Available Options. RadioGraphics (2006). 26: 503-512. Mozumdar, BC. The Control of Radiation Exposure from CT Scans. The Internet Journal of Radiology. 2003 3(1). Read More