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

Introduction 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 will get an opportunity of observing and practicing in CT scan at Perth Metropolitan Hospital. 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. 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 is one of the reasons that I am composing this reflective journal, which would explore the methods to reduce radiation risk from CT scans. Reflective journal 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. 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. 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. 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). The CT dose index is the quantity of CT dose measured for a single slice in standard cylindrical acrylic phantoms. Of these measures, the most important one is effective dose which represents the biological risk of radiation to patients. 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. 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. These parameters can be adjusted according to patient’s size and clinical requirements, so that reduction of radiation dose can be achieved while still acquiring diagnostic images. This is the information that I have learnt most from studying in this unit. During the course of my training, I came to know that 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. I came to know that other than noise filtering techniques and scanner geometry, a very effective method of lowering radiation dose is tube current modulation. This could be angular tube current modulation, longitudinal tube current modulation and Angular-longitudinal tube current modulation. Angular 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. I remember choosing the initial value for the tube current–time product and also having modulated the tube current within one gantry rotation. Longitudinal tube current modulation involves variation of the radiation dose among anatomic regions (e.g. shoulders vs. abdomen vs. pelvis). I have observed that this is done by varying the tube current along the z-axis of the patient. 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, I have observed the operator using 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. Angular-longitudinal tube current modulation involves the simultaneous combination of angular and longitudinal (x-, y-, and z-axis) tube current modulation. I have observed that 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.  I have learnt that in Automatic exposure control (AEC), while the operator determines the image quality requirements, the CT system determines the right tube current–time product. However, I also realized that 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, I learnt that 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. 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. 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. However, since there is no standard dose modulation technique of applying automatic exposure control (AEC), I have come to know that radiologists must be aware about different multidetector CT scanner dose modulation techniques and how to apply these techniques. The parameters, which are necessary for scanning protocol optimization includes: tube voltage, tube current, section thickness, collimation, pitch, and the image reconstruction kernel. These parameters can be changed during individual patient’s scan, especially for young and pediatric patients. My previous experience working in the hospital shows that there is a strong need for optimizing the CT scanning protocols as it seems overexposure occurs quite frequently. Radiographers should work together with medical physicists to reduce the radiation dose whenever possible, while in the meantime, radiologists and physicians should increase their awareness of the risk of radiation dose to patients when choosing CT as the first line technique. I have learnt that 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, so it may not be always possible. To summarize, ever since the advent of CT scanning, it has emerged as one of the most important diagnostic imaging modalities in our times. However, concerns have been raised about the risk of radiation exposure from CT scans. During the course of my training, I have learnt that 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). Other simple strategies, which I have realized are possible, are, to replace CT with other modalities like ultrasound and magnetic resonance imaging (MRI), whenever possible, and by decreasing the number of CT studies that are prescribed. Apparently, I have learnt valuable information about CT technique, especially the awareness of radiation dose and strategies to reduce dose during CT scans. I will surely apply this knowledge to my clinical practice in the future and keep the radiation dose “as low as reasonably achievable.” Read More