Strategies for Reducing Exposure to Radiation - Report Example

Introduction I thought that i had quite a lot of knowledge about CT scanning before my placement but I learnt much more during my experience in radiology department. Being an employee at the radiology department requires much precautionary methods for the safety of self and others, specially the patients. I realized during my placement in the radiology department, that working with high radiation machines such as the ones used for CT scans puts even more responsibility on the technicians about the safety and protection of the subjects exposed to these radiations. The amount of radiation exposure also depends on the facility, machine, and operator. The As Low as Reasonably Achievable (ALARA) principle has always guided radiology department’s approach to the dosages of radiation used to acquire CT images. I found while working that age and gender differences are an added complication. Each group needs a different kind of supervision and results are affected by different factors. Strategies for Reducing Exposure to Radiation Working in the Radiology department, CT scan section, I strongly felt that there is a general lack of awareness regarding the actual risks involved in the technique. I found out a lot about a variety of dose reducing strategies and their application. During my experience, I learnt that certain strategies which are very helpful against the harmful effects of radiation, and are provided with the CT scan softwares were not being applied. Some of these strategies involve changes in the acquisition parameters like: kVp, gantry rotation time, milli ampere pitch and collimation. The setting of CT parameters plays an important role in determining the radiation dose received during a CT study. The first step in this regard is to establish what information is desired from an examination along with the associated minimum level of contrast to noise that is acceptable for diagnostic purposes. If lesser contrast is required and greater noise can be tolerated, then a lower tube current (lower milli amperes) should be used. Tube current is directly proportional to dose, therefore tube current reduction will result in a lower dose. Gantry rotation time also has a similar effect on radiation dosage. This can be advantageous when a quicker acquisition is needed, for instance in unstable patients. I also learnt that decreasing the peak voltage can also result in a lower dose of radiation as long as the tube current is not increased to compensate. A higher pitch leads to a decreased amount of time in exposure to an anatomic part. By increasing the pitch, the dose of radiation to the patient is decreased. Collimation using strategy “acquire thin and view thick” is very dose-efficient because whereas the display images will be thick, low acquisition parameters can be used for the acquisition. The source images will be noisy; but the thick images used for review will not. The source images with higher spatial resolution will be available if required to interpret an unresolved finding on the thick review images and as source images for 2D and 3D reformats. But as I learnt, this “acquire thin and review thick” approach is not only efficient in terms of radiation dose but also improves workflow because fewer images need to be reviewed. CT Dose Modulation During my experience i found out about many different strategies that are being used. Dose modulation is a technique in which the CT scanner automatically adjusts the tube current according to the patient’s attenuation. I was told that this is done in order to achieve the same image quality for the least possible tube current. Dose modulation is capable of providing a reduction in radiation dose without significant image compromise. Image Post Processing as an Additional Consideration for Dose Reduction I was briefed during my experience about image post processing. What I have learnt about it is that Isotropic resolution offered by modern multi detector row CT scanners, allows high-quality image reconstruction and can help diminish the number of scans needed; automatically reducing patient dose. I was told that previously the radiologists used to acquire both axial and direct coronal images for our sinus CT studies. But, they now acquire the images in helical mode in the axial plane with a section thickness of 0.625 mm and reconstruct them in the coronal plane. In this way, the patients undergo 1 scanning instead of 2. Dose reduction strategies for specific patient groups I also noticed some special precautions being taken while CT scanning certain specific patient groups: Pregnant patients During my placement period I noticed, that the common indications for CT scanning in a pregnant patients are suspected appendicitis, pulmonary embolism, and urinary tract calculi. I learnt that in order to minimize radiation exposure to the foetus, it is important to find out if the necessary diagnostic information can be obtained from an alternative non-radiation imaging modality. For non-acute symptoms, radiologists and physicians must also decide on whether or not immediate CT scanning is required. I also found out that in some cases the CT scan is absolutely necessary. Therefore, in case of an unavoidable abdominal-pelvic CT scan which directly irradiates the foetus, scan parameters (such as wider beam collimation, higher pitch, and lower mAs, kV and scan range) must be selected to reduce the foetal dose. Paediatric patients Paediatric CT scans are quite common. I noticed the radiologists always took special care with children. When I asked them as to why they do this, I was told the risk of cancer in children due to radiation exposure is about two to three times higher than it is in adults because paediatric patients have a longer life expectancy and their organs are more sensitive to radiation damage. I further researched and found out that for newborns, the risk of cancer induction is essentially the same as in the second and third trimester of pregnancy. The best way to reduce the radiation dose to paediatric patients is obviously to avoid unnecessary CT exams and to search for alternative diagnostic imaging modalities with less or no exposure to ionizing radiation. I learnt that paediatric protocols with scanning parameters specifically designed for children must be used. These protocols usually include tube current modulation, a child-size bowtie filter and scanning field of view (FOV) and a weight/size-based technique chart that can determine the appropriate kV and/or mAs for each patient. Automatic tube current modulation and manual technique charts are quite popular. In addition, i noticed that lower kV values were applied. I found out further that lower kV values must be used, depending on patient size and clinical indication. For paediatric patients, due to less attenuation in the body, the noise level does not increase significantly with the decrease of kV for the same radiation dose. Therefore, for iodine contrast-enhanced exams, a lower kV is often used to improve the contrast enhancement without increasing the noise. Due to this the image quality improvement or dose reduction is much more significant in paediatric patients than in adult patients. I was told that lower-kV techniques have also been widely researched on and their popularity is increasing among paediatric CT scans. Cardiac patients I already knew that radiation dose in cardiac CT is more complex issue as compared to non-cardiac CT applications. This was confirmed during my experience. I found out that there are two major reasons for this complexity. The first reason is that dose, noise, and pitch have different relationships in cardiac CT compared to non-cardiac spiral CT. The second reason is that dose in cardiac CT can be dependent on the patient’s heart rate (HR), depending on the equipment used. As i noticed, in non-cardiac multi–detector CT (MDCT), noise depends on pitch. However, in cardiac spiral CT, this is not the case. In these, noise is independent of pitch and depends only on the tube current-time product (mAs). I also learnt, that since only a partial amount of the projection data from one gantry rotation is used for image reconstruction (to optimize the temporal resolution), relatively high mAs values are needed to provide an acceptable noise level for cardiac CT imaging, especially for cardiac CTA exams, which require the use of thin slices for better visualization of the coronary arteries. I noticed radiologists using a combination of relatively high mAs values with the low pitch values required in cardiac CT (dose is proportional to mAs/pitch), which explains why cardiac CT exams are associated with a higher radiation dose. During my experience with cardiac cases of CT Scan, I learnt that ECG-based tube current modulation is an important dose reduction tool in cardiac CT. The percent dose reduction using ECG-based tube current modulation is higher for patients with slow heart rates as compared to those with high heart rates. On inquiry i found out that this is so because the maximum mA time period is a smaller percentage of the R-R time interval at low heart rate. Additional dose reductions can be achieved using prospective ECG-triggering. Selective In-plane Shielding I also learnt that selective shielding of radiation sensitive tissues and organs during CT scanning has been proposed in many researches. Commercial products to implement this are also available in market. However, i was told that the use of such products is not generally recommended because the dose reduction they provide can be readily achieved by decreasing x-ray tube current, which does not introduce noise or increase beam-hardening artefacts. I noticed that radiologists use shields made of thin sheets of flexible latex impregnated with bismuth, shaped to cover the eye lens, thyroid, or breasts during brain, cervical spine, or chest CT exams respectively. Justification for CT scan Finally, I have learnt that each CT exam must be appropriate for the individual patient. Justification is a shared responsibility between requesting clinicians and radiologists. Hence for medical exposures, the primary tasks of the imaging community are to coordinate with the ordering clinicians in order to direct patients to the most appropriate imaging modality for the required diagnostic task. I now know that the imaging community must make sure that all technical aspects of the examination are optimized to obtain the required level of image quality while keeping the doses as low as possible. Thus, a CT exam is performed only when the radiation dose is deemed to be justified by the potential clinical benefit to the patient. CT scan protocols Updates in CT scan protocols are very common and i noticed this during my experience. This is done by CT scanner manufactures and radiologists to keep radiation dose as low as possible to the patients. Role of Hospitals I felt that hospitals also play an important role in dose reduction during CT Scanning. Hospitals should obtain CT scanners that have options of systematic implementation of dose modulation. Conclusion Due to my placement in the radiology department I discovered a lot of new information. Firstly, that CT scanning equipment is a major contributor to the radiation dose received in radiology departments. There are many strategies available to reduce the radiation dose associated with CT protocols. These strategies involve changes in the acquisition parameters like: kVp, gantry rotation, time, milliampere, pitch. I further learnt that such strategies however, always involve a compromise between image quality and radiation dose. More recently, CT manufacturers have introduced techniques of dose reduction such as dose modulation, which result in a decrease in dose to patients without significant image compromise. In light of my experience so far, I recommend the systematic implementation of dose modulation for all CT studies to ensure reduced exposure to radiation without compromising the image quality. Read More