Reducing Radiation Exposure in Computed Tomography Angiography - Dissertation Example

Reducing Radiation Exposure in Computed Tomography Angiography April 8, Table of Contents Introduction 3 Background 3 Risks 4 Reducing Exposure 6 Conclusions 10 References 11 Reducing Radiation Dose in Computed Tomography Angiography Introduction Computed tomography scans are a modern imaging technology that has greatly assisted the diagnosis process by allowing visualization of internal structures (Gopal & Budoff 2009). Computed tomography scans are used in many emergency department differential diagnoses, including those for chest and abdominal pain, to triage patients and determine surgery needs (Broder et al. 2010). In the last twenty years, the diagnostic use of computed tomography scans has increased by a thousand percent (Smith et al. 2007). These scans do, obviously, make use of ionizing radiation to produce their images, which can be harmful to human health in even small dosages. Protecting the patient from harm is the cornerstone of the profession of radiology, and reducing radiation exposure while maintaining a high level of image quality and therefore diagnostic ability is vital (Australian Institute of Radiography 2002). Key to this goal is understanding the use of computed tomography scans, the risks of their radiological effects, and the methods available to reduce exposure. Reducing exposure to radiation is the only way to make this important technology safer. Background There are several types of computed tomography scans used today. Computed tomographic angiography is one of these specialized types of scans, and it is also one of the most interesting with regard to dosage. The use of a computed tomography angiography scan allows the practitioner to view a patients coronary arteries, their level of function, and even the quality of the arterial lumen (Gopal & Budoff 2009). Computed tomography angiography scans are important to study when looking at radiation dosage because the scan area by definition includes such radiosensitive areas as the thyroid, the vascular tissue of the breasts, and of course the coronary arteries (Costantino et al. 2008). Moving even further into imaging specialization for viewing the heart and related tissues is the technique of retrospective image acquisition, which produces images using 64-multi-row detector computed tomography angiography. This method scans the heart in overlapping slices, which allows it to show cardiac motion as well as all parts of the cardiac cycle (Gopal & Budoff 2009) Computed tomography angiography can be used as a tool in the diagnosis of acute chest pain. This is a concern within the emergency department, because acute chest pain can be as benign as gas or heartburn or as possibly deadly as pulmonary embolism (Nauffal Manzur 2006). Patients presenting with these symptoms must be triaged and diagnosed quickly, as the more dangerous possibilities have very low survival rates if the condition is not treated properly and immediately. It is also used in the diagnosis of such conditions as aortic dissection, and could even become the standard for diagnosis of such chronic heart problems as coronary artery disease. This is due in part to the development of newer and faster computed tomography scanners. These machines have such rapid rotation times and such powerful detectors that the entire chest can be imaged in the course of a single thirty-second breath hold (Johnson et al. 2007). Risks Despite the many benefits of the increased use of computed tomography and computed tomography angiography scans, it is exactly this increase that increases the associated risk. More patients being scanned means more patients being exposed to the radiation used by this machinery. According to the fifth Biological Effects of Ionizing Radiation report, there are eight cases of cancer related to ionizing radiation reported per 10,000 computed tomography examinations performed (Costantino et al. 2008). While this may not seem like a large number, when put into perspective with other diagnostic procedures the need to increase safety becomes clear. Computed tomography (computed tomography) scans make up only three to five percent of imaging or radiological diagnostic tests, but they are responsible for thirty five to forty five percent of the radiation dose in the patient population from such tests (Imhof 2003). A woman of average height, weight, and chest size will receive a radiation dosage from a computed tomography angiography scan that is equivalent to a lifetimes worth of two-view mammograms; in other words, a single computed tomography angiography scan exposes the patient to radiation levels that are ten to twenty five times higher than a single mammogram (Costantino et al. 2008). Several medical organizations, both throughout Europe and in the United States, agree that computed tomography scans are considered high-dose therapies, and that over-exposure to radiation even for therapeutic usage can be harmful. These organizations also state that exposure to therapies in this category should be limited whenever possible (Imhof 2003). The dosages corresponding to different scanning therapies are published by the corresponding agencies in each area, and have been confirmed through various research studies. Some examples of dosages determined through research, all approximate ranges, include: 3.2 mSv for pulmonary angiography; 1.2 mSv for ventilation–perfusion scintigraphy;1.6–7 mSv for nongated computed tomography angiography, and 10.5 ± 5.2 mSv for pediatric computed tomography angiography (Kritsaneepaiboon et al. 2009); (Johnson et al. 2007). These dosages are affected by many factors, including the type of scan, the machinery used, and the size of the patient. For example, the radiation dosage for an ECG-gated computed tomography angiography scan is higher than that for a similar though non-gated scan (Johnson et al. 2007). multi-detector computed tomography angiography results in higher radiation dosages than “prospective acquisition” methods such as Electron Beam Tomography, though computed tomography provides a better image quality (Gopal & Budoff 2009). An example of machinery effect can be seen in the fact that a four-detector row protocol, with 1 mm collimation, will increase the patients radiation exposure from thirty to one hundred percent over a single-detector row protocol with 5 mm collimation (Schoepf & Costello 2004). Reducing Exposure Many of the methods to reduce radiation exposure in patients are already practiced, or can be very easily put into place without changing scan protocols or reducing diagnostic effectiveness. For example, the use of bismuth-metal chest shields, whenever possible and practical, can drastically reduce the exposure of the patients thyroid and breast tissue during a computed tomography angiography (Hurwitz et al. 2009; Imhof 2003). Computed tomography angiography scans should only be used when they are the best diagnostic choice and not, therefore, simply as a general screening method. Similarly, higher dosage scans should be used only when very specific diagnostic information is needed (Costantino et al. 2008). The previously mentioned higher-dosage ECG-gated scans, for instance, should only be used if imaging of the pulmonary or coronary arteries is absolutely needed for diagnosis (Johnson et al. 2007). Also, scan protocols should focus on producing those images that show the affected area in enough detail to meet diagnostic requirements, not the most information or the highest quality. Radiologists should make use of the scanning protocol and the image quality with the lowest “maximal diagnostic acceptable signal to noise ratio” (Imhof 2003). The biggest step towards reducing patient exposure to radiation related to the use of computed tomography scans is simple awareness on the part of radiologists and physicians to the dangers of overuse or misuse of this technology (Schoepf & Costello 2004). Radiologist should also be aware of dosages indicated by their machinery or dosage charts. Research has shown that dosages shown by computed tomography machines that provide such data (and chart-calculated values) are reasonably equivalent to measured quantities and so can be used as a fair estimate of patient exposure (Imhof 2003). Finally, radiologists should understand how the engineering and design of the scanners being used can affect the dosages received by the patient, and change their protocols to allow for this (Hamberg et al. 2003). Different scanners have different focal point distances, for example, and protocols cannot simply be transferred wholesale from one machine to another. Lowering power power levels in the machinery will also reduce radiation exposure during a scan. There exists a current threshold at 24 kW. At this threshold, there is a discontinuity in the linear relationship between increasing current and increasing radiation dose, because of the increase in focal point size. Using current above this level results in a ten percent increase over expected radiation dosage for the corresponding wattage; awareness of this can help in designing safer protocols that avoid this threshold whenever possible. Just as with current, radiation exposure for the patient also increases linearly with voltage. If current is high, reducing voltage can help reduce dosage. The converse is true as well, so that reducing current when a high voltage is being used will also reduce the radiation exposure. This is another factor to consider when designing safer and lower dosage protocols (Hamberg et al. 2003). Another possibility for lowering the required current without sacrificing image quality is the use of automatic tube current modulation (Hurwitz et al. 2009). For example, the machinery can be equipped with devices that modulate tube output in relation to the shape and x-ray attenuation of structure being scanned, so that highly absorbent structures can be imaged without increasing the risk to less absorbent tissues (Schoepf & Costello 2004). A final possibility is the use of a a lower mAs setting, which could be used to offset a dosage increase from higher tube voltage, if such a higher voltage is required (Imhof 2003). Updating equipment and using higher-quality machinery will help to reduce dosage. Reducing radiation exposure can be done by decreasing scan time, as a longer scan will, obviously, correlate to a higher radiation dose (Hamberg et al. 2003). The use of faster machinery that requires only a single pass, instead of multiple, can greatly reduce exposure. Such scanners may also be able to make use of a higher pitch, 0.3 instead of the 0.2 that is traditional, which will reduce radiation exposure and contrast medium requirements without sacrificing image quality (Johnson et al. 2007). Other studies agree that radiation dosage decreases with increasing pitch, and so a higher pitch should be used when possible (Imhof 2003). With these considerations, one can see that better equipment can increase image quality while reducing the radiation exposure. However, higher detector-row technology may not always correlate to lower dosage, as is true when comparing four detector-row machines to sixteen detector-row machines (Schoepf & Costello 2004). Increased computing power also allows the use of thin section slides, so that a greater noise per individual slide section is possible still producing images clear enough for diagnostic purposes (Schoepf & Costello 2004). Computing technology has also made possible the design and use of noise-reducing filters. However, some radiation-reducing technology and methods require high levels of patient cooperation, as well as longer breath-holds for chest imaging (Imhof 2003). Protocols must be designed to allow for non-compliant patients if such a situation arises, for patients that are incompetent due to age or brain dysfunction. One final method to reduce radiation exposure is to take steps to reduce the size of the scanned area. The first method is to reduce the scan output to allow for the girth of the individual patient. For example, the tube output or current can be adjusted to allow for the body size differences between individual patients, without a corresponding loss in image quality. Less radiation will still penetrate a smaller patients girth and provide an image of diagnostic quality. This allows for smaller adults to be given a smaller dose of radiation, more appropriate to their size (Schoepf & Costello 2004). Pediatric protocols should also be designed to take the patients age and weight into consideration, as pediatric patients often receive dosages that are high for their relative size. This is especially important when it is considered that such pediatric patients are at higher risk for the neoplasms that result from this exposure (Kritsaneepaiboon et al. 2009). Other methods to reduce radiation exposure by reducing scan angle involve better measurements of the actual area that must be imaged in order to successfully diagnose the patient. Coronary calcium scanning can help to better locate the positions of the coronary arteries (especially the highly variable left coronary artery) than the current method of estimation based on physiological landmarks from a scout scan. This can be used to more precisely note the areas requiring imaging and reduce the scan volume (unnecessary slices were found to be 28± 9% of total), which reduces the dosage of radiation that the patient receives by 22±10% (Gopal & Budoff 2009). The coronary calcium scan itself adds radiation exposure but this is offset by the radiation savings of the reduced scan volume. However, limiting the scan angle based on the z-axis, or vertical axis, of the scan area can sometimes result in a reduced collection of information necessary for diagnosis; care must be used in the design of these procedures to ensure that the scan does in fact image the entire required area (Broder et al. 2010). Conclusions Computed tomography scans are arguably one of the more important diagnostic tools in modern medicine. These scans allow a physician, with the help of the radiology team, to see inside a patient and observe exactly what is occurring in high detail. computed tomography angiography scans, and more specifically multi-detector computed tomography angiography scans, even allow visual observation of cardiac motion. These advances have greatly increased the speed of emergency department diagnoses for such symptoms as chest pain, as well as increasing the proportion of correct diagnoses arrived at under these high-pressure conditions. However, no gain comes without risk, and this is true of radiological medicine. The increased use of computed tomography and computed tomography angiography scanning has increased the related incidence of cancer from the radiation exposure. Therefore, all efforts must be undertaken, by both radiologists and other physicians, to reduce these exposures by limiting scan quality and area to only that which is absolutely necessary, not performing unnecessary or unnecessarily high-dosage scans, being aware of the risks of performing such scans, making use of the best available equipment, and finally writing protocols and operating equipment in ways that take into account factors such as power levels and patient size. Further research into these areas is clearly indicated, as while much is known about the factors that increase exposure, not enough data is available on methods to reduce it. References Broder, J.S. et al., 2010. Prospective Double-Blinded Study of Abdominal-Pelvic Computed Tomography Guided by the Region of Tenderness: Estimation of Detection of Acute Pathology and Radiation Exposure Reduction. Annals of Emergency Medicine, 56(2), p.126-134. 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Radiation Dose Savings for Adult Pulmonary Embolus 64-MDCT Using Bismuth Breast Shields, Lower Peak Kilovoltage, and Automatic Tube Current Modulation. American Journal of Roentgenology, 192(1), p.244-253. Available at: http://www.ajronline.org/cgi/doi/10.2214/AJR.08.1066 [Accessed April 7, 2011]. Imhof, H., 2003. Spiral CT and radiation dose. European Journal of Radiology, 47(1), p.29-37. Available at: http://www.ejradiology.com/article/S0720-048X(02)00232-2/abstract [Accessed April 7, 2011]. Johnson, T.R.C. et al., 2007. ECG-Gated 64-MDCT Angiography in the Differential Diagnosis of Acute Chest Pain. American Journal of Roentgenology, 188(1), p.76-82. Available at: http://www.ajronline.org/cgi/doi/10.2214/AJR.05.1153 [Accessed April 7, 2011]. Kritsaneepaiboon, S. et al., 2009. MDCT Pulmonary Angiography Evaluation of Pulmonary Embolism in Children. American Journal of Roentgenology, 192(5), p.1246-1252. 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