Ionising radiation in medical imaging Essay
Ionising radiation in medical imaging
Contemporary medical practice is heavily reliant on mediconuclear and radiological procedures and investigations. To derive important diagnostic information, medical personnel must carry out investigations which may expose patients and the medical personnel to certain levels of risk. With the continued advancement of medicine and diagnostic examinations, many procedures which utilize relatively high loads of radiation to produce images are becoming more and more popular.
To protect patients and radiologists from the harmful effects of radiation exposure, protection practices and standards are grounded on the understanding that any level of radiation may cause detrimental health effects, including genetic damage and cancer development. However, while protection standards have been crucial in reducing the level of exposure to harmful radiation, these estimates are just approximations. Some authors have argued that these approximations are indeed underestimates and that better estimates of risk should be calculated based on the age and sex of an individual.
These concerns imply that doctors and radiologists must uphold the highest standards for radiation protection, including limiting the number of times a patient subjected to radiologic examination. The use of ionsing radiation in medical imaging began with x-rays discovery in 1895. Basically, ionising radiation consists of the component of electromagnetic spectrum which has sufficient energy to penetrate through matter and dislodge orbital electrons which are then converted into ions and captured in an electromagnetic film. Different types of electromagnetic radiations (gamma rays and x-rays) are utilised in different forms of medical imaging.
Different forms of radiation also have different biologic effects. Imaging modalities such as single photon computed tomography (SPECT), cardiovascular computed tomography (CVCT), positron emission tomography (PET), and x-ray fluoroscopy have become indispensable diagnostic tools in almost every medical establishment. Even though these modalities are necessary for an accurate and timely diagnosis, the emission of both particulate and photon radiation means that the risks and benefits of these diagnostic techniques need to be evaluated.
While it is easy to measure some radiation dosimetry parameters, others can only be estimated by using complex simulations and assumptions models. Generally, radiation dosimetry is presented in terms of physical measurements. Based on literature research, this paper discusses the risks associated with ionising radiation in medical imaging and the measures taken by both the patients and the medical staff to limit the level of radiation dosage. Risks Associated with Ionising Radiation in Medical Imaging
Exposure to ionising radiation in the medical/hospital environment can either exhibit as deterministic or stochastic impacts. Deterministic effects are those effects whose severity is determined by the radiation dose. These effects occur when the radiation exposure goes beyond the dose threshold. The most common example of deterministic effects is skin burns. When the skin is exposed for a long time to radiation, particularly in the course of prolonged fluoroscopic procedures, skin burns may occur.
Patients undergoing electrophysiologic ablation examinations which may take more than one hour are more at risk of suffering skin burns as a result of repeated exposure to relatively high levels of radiation doses. 3 Other deterministic effects include cataract formation, skin erythema, and epilation. Stochastic effects are those effects whose probabilities of occurrence are dependent on the dosage. For instance, effects such as radiation induced carcinogenesis always occur after a person has been exposed to variable doses of radiation for a long time.
Generally, stochastic effects do not have a threshold dose; rather, long term exposure to ionising radiation may cause varying degrees of cell proliferation, division, and differentiation. Thus, it is extremely difficult to establish the exact level of exposure that can be incriminated in the development of cancer or any other associated effect. However, it has been demonstrated that higher doses of ionising radiation may cause chromosomal changes and subsequent malignancies. On the contrary, it should be noted that not all chromosomal alterations cause phenotypic illnesses. Exposure to high doses of radiation may induce malignancy in children, especially the development of leukemia. There are two different theoretical perspectives for discussing the link between medical imaging and cancer development at relatively low exposure levels. According to the linear no-threshold theory, the effects of radiation are not limited to a certain threshold of exposure. In essence, all radiation emitted have the capacity of causing malignancies and the risk increase linearly with dosage.
On the other hand, the linear-quadratic theory states that low doses have an insignificant risk on developing malignancies and that the risk can only increase quantitatively with exposures to high radiation doses. In many international and national legislations concerning radiation and reducing exposure to radioactive emissions, the more conservative linear no-threshold hypothesis have been adopted as opposed to the linear quadratic hypothesis. This consensus ensures that no amount of radiation exposure is taken as being negligible and insignificant when it comes to malignancies and other associated effects.
The Chernobyl disaster and the Hiroshima tragedy are two examples of the effects of high dose radiations that have been widely studied. Studies of the survivors have confirmed the hypothesis that in-utero X ray exposure increases the risk of cancer development. Again, it should be understood that radioactive emissions from nuclear accidents or explosions cannot be compared with the doses of ionising radiation that patients and staff are exposed to during medical imaging. Epidemiologic and experimental evidence suggests that low dose radiations can lead to the development of leukemia and solid tumours. Due to this link, workers in the nuclear industry and health care are often monitored and the level of exposure restricted to not more than 20 mSv per years (100 mSv in five years). The current growth in the use of computed tomography imaging has created renewed interest into the possible risks associated with CT scans. Some studies have established that computed tomography imaging, especially cardiac CT imaging.
According to Moloo (2009), cardiac CT imaging exposes patients to a lifetime risk of developing cancers. A radiation dosage of 2. 3 milliSieverts may result in a lifetime cancer risk of 20 cases per 100,000 women and 8 cases per 100,000 men. The risk to developing lung cancers is higher than that of developing other cancers. Using insurance claims documentation of 1 million clients, other researchers have also been able to establish that more than more than 70% of medical claims were associated with medical imaging procedures.
Notably, CT scans of the abdomen and the pelvis, CT scans of the chest, and myocardial perfusion imaging accounted for the greatest proportion of exposure to cumulative radiation doses. It is also important to note that, even though not widely reported, a single radiation dose of an abdominal or pelvic computed tomography imaging far exceeds the recommended annual background radiation dose. For instance, cardiac CT imaging typically exposes the patient to a radiation dosage of 3-15 mSv, for mammograms the patient is exposed to 0. mSv, while pelvic/abdominal CT imaging exposes the patient to 10mSv. These dosages far exceed the annual background dosages and increase the likelihood of developing cancers. Castranovo (2008) asserts that even though the use of multidetector CT angiography has been extremely important in generating images in a very short time, it should be noted that the population exposure to radiation has risen from a mere 0. 54 mSv to more than 3. 2 mSv. 16-slice computer tomography coronary angiography (CTCA) increases the probability of cancer developing in different patient organs.
These risks call for an evaluation of radiation dosages, particularly in CTCA so as to ensure that the benefits far outweigh the risks associated with the procedure. In a study carried out in the United Kingdom, it was estimated that the course of catheter based coronary angiography exposes patients to fluoroscopic radiation and increases the risk of cancer in 280 per one million patients examined. Exposures to fluoroscopic radiation during electrophysiologic ablation have been estimated to increase the number of fatal malignancies in both men and women. 3
For pregnant mothers, exposure to radiation may cause teratogenesis. High dose variations may cause fetal malformations, central nervous system alterations, particularly mental retardation and microcephaly. Therefore, even though sick mothers may require medical imaging to aid in diagnosis, radiations have a potential adverse effect on the foetus and physicians should critically assess the need merits and demerits of using medical imaging as a diagnostic procedure, especially when there are safer alternatives. Finally, ionising radiations increases the risk of radiation-induced gene mutations.
Germline mutations have the potential of altering future generations. Such radiations increase the frequency of genetic mutations in the population hence potentially altering future germ lines. In other cases, genetic mutations may lead to the development of either physical or physiological deformities, or cause genetic diseases. 12 It widely known that the main consequence of adverse exposure to ionising radiation is hereditary effects. Radioactive damage of gametes may lead to a wide range of mild or serious consequences, including mental defects and death (Edward).
Precautions Required to Protect Against Ionising Radiation Medical professionals have an ethical and professional obligation to protect patients under diagnostic procedures that may expose them to adverse levels of radiation. The primary precautionary measure is training. According to the Ionising Radiations (Medical Exposure) Regulations (IR(ME)R) of 2000 in the United Kingdom, radiologists are required to undergo specialist training on the physics of ionising radiations and the measures which should be taken to protect against radiation exposure.
The legislation also has provisions for patients to report to the IR (ME) R Inspectorate about incidents of radiation overexposure. Since medical radiation accounts for 14 percent of UK’s average annual dose, these legislations are an important regulatory tool protecting both staff and patients from unwarranted exposure to radiation originating from medical imaging procedures. It is also important to reiterate that these laws, as well as the provisions included in the International Commission on Radiological Protection are grounded on the linear threshold theory.
Radiologists have an obligation to expose the patient and the staff to minimal amounts of radiation necessary for the successful completion of the procedure. Where appropriate, the dose of radiation used in cardiovascular computed tomography may be reduced by adjusting the scan parameters to suit every individual patient. Software based modifications can also be installed to reduce the level of exposure to radiation by changing computer tomography scanning protocols. For instance, the x-ray tube radiation output can be reduced by ECG-controlled tube current modulations.
In interventional radiology, the decision to use radiation diagnostic techniques should be justified as per the requirements of the regulatory authority. Patients should never be exposed to radiation unless the practitioner has offered a prescription which complies with relevant national guidelines. In making that prescription, the medical practitioner should be guided by the efficiency of the diagnostic intervention, the benefits and risks associated with the technology, and the availability of other alternative technologies that are less harmful.
With respect to equipment specifications, all authorized diagnostic radiology equipment must meet the compliance requirements set out by national and international regulatory standards. Such specifications must take into account the possibility of human errors, equipment failures, or any other occurrence that may predispose medical practitioners and patients to unhealthy radiation exposure. Radiological procedures such as mammography, dental radiology, and interventional radiology should only be performed by specifically trained personnel using specifically designed imaging systems.
Where appropriate, radiology units should have automatic exposure control systems and automatic brightness controls. These measures optimize patient doses. Operationally, regulatory agencies in the UK are charged with the responsibility of ensuring that the registrants and licensees comply with all the minimum necessary standards. This is achieved by specifying applicable operational parameters such as the types of equipment that can be used, safer procedures for examining the chest, thorax, abdomen, and lumbar spine regions as well as the skull and pelvis.
Other measures may include changing radiation generator parameters such as the tube voltage range and tube loading, changing the focal spot, film processing conditions, and film-screen combination. 15 Conclusion The fact that recent advances in radiological and mediconuclear imaging procedures have become indispensable diagnostic tools is indisputable. Accurate and timely examination of clients through radiologic procedures saves lives and paves the way for scientific based treatment and management of diseases.
The popularity of these techniques continues to soar to their speed and relative ease of use. Recent research has established that the current levels of radiation exposure for diagnostic purposes far supersede what had previously been thought. This realization has created a new wave of debates and studies into the benefits and risks associated with ionising radiation and whether imaging is crucial for accurate diagnosis. 4 Even though an accurate measurement of the incremental risk of ionising radiation in medical imaging is yet to be determined, the uncertainty of the relationship between doses and tissue specific responses should is enough to encourage only the use of low doses. The link between ionising radiation and cancer implies that both physicians and patients need to acknowledge the potential harm that CT imaging causes and strictly implement radiation protection measures.
Every individual should be exposed to radiation dosage based on their phenotypic and physiologic characteristics. Recognizing the gravity of other associated risks such as teratogenesis and radiation induced gene mutations implies that additional care should be taken when using diagnostic imaging. Clinical decision making as regards the utilization of low levels of ionising radiation should be supported by a broad range of modalities which justify the risk-benefit ratio.
University/College: University of Arkansas System
Type of paper: Thesis/Dissertation Chapter
Date: 10 October 2016
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