Radiation Exposure Of Medical Imaging


Introduction

It is a general consensus that ionizing radiation is oncogenic in nature. Much of this agreement is based upon observation of increased incidence of carcinoma in a population surviving a nuclear attack or in uranium miners exposed to radiation at the workplace. The amount of radiation used by imaging modalities is negligible as compared to the abovementioned exposures. For instance, in the United States, people are exposed to average annual background radiation levels of about 3 mSv; exposure from a chest X-ray is about 0.1 mSv, and exposure from a whole-body computerized tomography (CT) scan is about 10 mSv, and that’s one of the reasons why physicians usually miscalculate the potential risks associated with the radiation exposure while performing procedures using radiologic imaging.[1][2] This article will attempt to explain how to quantify radiation, the biological effect of radiation, risks to health care workers as a result of radiation exposure, and certain recommendations and tips for various medical professionals.

Radiation is defined as a moving form of energy. It can be classified into two categories, i.e., ionizing and non-ionizing type. Ionizing radiations can be further classified into electromagnetic radiation (matter less) and particulate radiation.

Electromagnetic radiations are energy packets (photons) traveling in the form of a wave. Basic examples of electromagnetic radiation are x-rays and gamma rays. Particulate radiation consists of a beam of particles that can be either charged or neutral. Electromagnetic radiations have high energy and can easily penetrate body tissues. Ionizing radiation is mainly used for diagnostic purposes.

Function

Quantification of Radiation

Before understanding the biological effects of radiation, one should get familiarized with two important medical terminologies in radiology, i.e., absorbed radiation dose and effective dose.

Absorbed Dose

It’s the amount of energy that radioactive waves deposit in any material through which they pass. The unit to measure the dosage of deposited energy is rad (radiation absorbed dose) or Gray (Gy). An absorbed dose of 1 rad means 1000 ergs get absorbed in 1 gram of material after radiation exposure.[3] Gray is a newer International (SI) unit to measure the absorbed dose. The relationship between both units is described as below:

1 Gy = 100 rad

Absorbed dose does not measure the biological effects of radiation on human tissues. For this purpose, an effective dose or dose equivalent is used.

Effective Dose (Dose Equivalent)

Dose equivalent or Effective dose combines the amount of radiation absorbed and the biological effects of radiation. They measure how much of absorbed radiation dose actually have a biologic effect on tissues. Dose equivalent is used when measuring the effective radiation dosage in a specific organ or tissue, while the effective dose is used to measure the effective radiation dosage of the whole-body.[4] Both of these quantities are expressed in Sieverts (Sv) and are measured by the estimation of data collected from personal dosimeters.

Equivalent dose= Absorbed dosage x Tissue weighting factor.

The tissue weighting factor varies from one organ or tissue to another and reflects the sensitivity of the organ to the radiation.[5] The effective dosage is calculated by summing up the equivalent dosage of all exposed organs or tissues.

Issues of Concern

Mechanism of Radiation Associated Damage

Ionizing radiation affects the human body by causing damage at the atomic or molecular level leading to cellular damage. They can cause damage to the vital organelles of cells resulting in cell death, or they can damage human Deoxyribonucleic acid (DNA) either directly or indirectly. Direct effects occur when ionizing radiation comes directly in contact with molecules of DNA, leading to DNA strand breaks. Indirect effects are related to the ionization of molecules. Ionizing radiation causes the formation of hydroxyl ions at the cellular level by ionizing water molecules. These hydroxyl ions interact with DNA leading to strand breaking or base damage. Adequate DNA repair mechanisms usually repair these DNA damages immediately, and if the damage cannot be repaired, these cells undergo apoptosis. The absence of DNA repair mechanisms or faulty repair of DNA leads to Genetic mutations that result in carcinoma formation.[6]

Health Risks of Radiation Exposure

Two types of responses, Tissue reaction, and stochastic effect, have been associated with ionizing radiation exposure.[7]

  • Tissue reaction (Deterministic effect) is the one in which the severity of damage increases proportionately with an increase in radiation dose, and it usually occurs when radiation dose exceeds a particular threshold. Tissue reactions are largely caused by the death or radiation-induced reproductive sterilization of large numbers of cells. Normally there is a latent period between radiation exposure and the appearance of visible effects. Clinical expression of radiation effects is not evident until these cells unsuccessfully attempt division or differentiation. Skin burn and cataract development are examples of tissue reactions.[8] The lens of the eye is the most radiosensitive tissue in the body. Posterior subcapsular changes are typical of radiation exposure.[9] The latency period between radiation exposure and cataract development varies inversely with radiation dosage. During prolonged fluoroscopic procedures, radiation exposure can result in skin burns, usually with fluoroscopy times longer than 60 minutes. Necrosis, fibrosis, Bone marrow suppression, organ atrophy, Sterility, and subfertility are other examples of tissue reactions.[10]
  • Stochastic effects, including cancer and hereditary effects, are caused by a mutation or other permanent changes in which the cell remains viable. Although the severity of the response observed in the stochastic effect is not dependent upon the radiation dosage threshold, the probability of stochastic effect increases with an increase in radiation dosage. There is a variable latency period between the radiation exposure and development of carcinoma, but the latent period extends up to a decade or two in most cases.[11] This type of damage is more prevalent in low dose radiation exposure that usually occurs due to routine medical imaging.

Radiation Exposure Monitoring

Lif TLD badges or rings are used to quantify the absorbed radiation dose that a health care worker acquires while performing diagnostic or therapeutic procedures that require radiation use. Lif crystals store radiation energy. At the end of the monitoring period, these badges or rings are melted, and energy stored is released as visible light, which allows the determination of radiation exposure. These badges are able enough to detect radiation exposure of as low as 1 mrem. International Council on Radiation Protection (ICRP) recommends wearing two personal dosimeters in the interventional lab. One is worn at the neck or left shoulder level outside the apron, while the other is at the waist level inside the apron.[12] The dosimeter worn at the collar or the left shoulder level can also be used to determine the radiation exposure to the lens of the eyes or the unshielded skin.[13] The effective dose can be calculated by summing the calculated equivalent dose.

  • Effective dose = 0.025 H (Neck) + 0.5 H (Waist)

Although effective dose value, derived from dosimeter readings, overestimates absorbed radiation dose by 100 times yet, due to health risks associated with radiation exposure, it’s still recommended to calculate it.

Occupational Dose Limitation

International Commission on Radiological Protection (ICRP) and National Council on Radiation Protection and Measurements (NCRP) provides guidelines regarding the health and safety aspects of ionizing radiation exposure in relevance to patients and healthcare providers.[14] According to ICRP, 20 mSv/year averaged over a period of 5 years (i.e., a limit of 100 mSv in 5 years) is the maximum occupational effective dose, with no annual effective dosage exceeding 50 mSv/year. According to NCRP, 50 mSv in any one year and a lifetime limit of 10 mSv multiplied by the individual’s age in years is the occupational dose limit. ICRP also defines effective dose limits related to certain body organs, i.e., 150 mSv for the lens of the eye, 500 mSv for the skin (average dose over 1 cm of the most highly irradiated area of the skin), and 500 mSv for the hands and feet. In any rescue operation (Procedures reducing mortality and morbidity), where the benefits of procedures outweigh the risks of occupational radiation exposure, no dose limitation is recommended. Otherwise, every effort should be made to minimize the radiation exposure below 50% of the maximum annual occupational dosage limit.

Pregnancy

During pregnancy, radiation exposure poses an extra risk to the fetus due to its teratogenic potential, especially if exposure occurs during the first trimester of pregnancy. For this purpose, health care workers at risk of radiation exposure should notify hospital authorities, and a dosimeter badge should be worn under the lead apron at the waist level at all times to monitor radiation exposure. Readings from the dosimeter should be checked periodically. ICRP provides a strict guideline regarding radiation exposure control and recommends that radiation exposure to a fetus should not exceed greater than 1 mSv during the whole pregnancy. The NCRP recommends limiting occupational radiation exposure of the fetus as low as reasonably achievable but no more than 5 mSv during the entire pregnancy and 0.5 mSv per month of the pregnancy.[15][16]

Clinical Significance

“As low as radiation exposure” (ALARA) is the guiding principle of diagnostic and interventional procedures using radiation. The application of the principle is limited to the reduction of radiation exposure and includes the use of personal protective equipment (PPE). In an interventional lab, the greatest radiation exposure source to health care workers is scattering from the patient. Anything that reduces patient radiation exposure will indirectly reduce the health care worker’s radiation exposure. On the other hand, the reduction of radiation exposure should not affect the quality of the procedure. In general, a reduction in radiation exposure can be made by implementing the following principle while performing any procedure:

Reduce Time: Duration of procedure and timing of contact with patients is an important factor determining the radiation exposure to the health care workers. Minimization of time during which the patient is exposed to radiation minimizes radiation exposure to the operator and other staff members. Similarly, taking a history before the radiologic procedure rather than after the procedure also reduces exposure.

Increase Distance: Radiation exposure is inversely proportional to the distance between the operator and radiation source. It decreases the inverse square root of distance between both. Positioning oneself on the patient side opposite the radiation source decreases radiation exposure substantially.

Use Shielding: This exposure control method reduces the effect of radiation exposure by placing a physical object providing hindrance to radiation transmission from a radiation source to the person. These Shielding methods are not only limited to the personal level, i.e., use of PPE, but are also employed during the construction of hospitals. PPE includes protective eyeglasses, lead aprons, gloves, scrub caps, thyroid collars.[17][18] Keeping body physique variations in mind, PPE should be adjusted to ensure proper fitting and subsequent radiation protection. Health care workers should be frequently asked about the integrity and fitting of PPE. Lead aprons are available in one piece and two pieces (vest and skirt) options. During pregnancy, pregnancy aprons are available to encase the enlarging abdomen. Lead aprons reduce the penetrating radiation dose to 2% to 10%, depending upon the thickness of the apron. 0.25 mm and 0.5 mm lead apron to reduce the penetrating radiation dose to 10% and 2%, respectively.[19]    

Role of the Hospital Facility

For facilities participating in the Medicare program, the Centers for Medicare & Medicaid Services (CMS) has established minimum standards for hospital radiologic services and accreditation requirements for freestanding advanced diagnostic imaging facilities. States and/or accreditation organizations may have additional requirements that go beyond the CMS requirements. In complying with these requirements, facilities can ensure the adoption of quality assurance and quality control modalities for each of their programs. Some practical suggestions for minimization of radiation exposure are given below:

  • First and foremost, the duty of hospital management is to minimize radiation exposure. One possible method of achieving it is by implementing shielding methods both at the architectural level, e.g., placement of heavy aggregate concrete around the walls of x-ray rooms to absorb radiations, and at the personal level by providing a sufficient supply and also ensuring usage of PPE inside the facilities.[20] Another method is ensuring that no medical personal or patient becomes unnecessarily exposed to radiation at any given time.
  • Every hospital facility should assess the radiation exposure of workers and provide periodic feedback to them. Also, each worker who is expected to receive more than 10% of the applicable dose limit should be required to wear one or more dosimeters. Any interventionalist whose monthly dosimeter reading exceeds occupational dose limits should be asked to avoid performing further procedures for the next few weeks. The International Council on Radiation Protection recommends that the advice of a medical physicist be sought to interpret monitoring results.[21]
  • Hiring staff members having adequate knowledge and training to ensure the production of quality images at appropriate patient doses resulting in a decreased probability of repeating procedures. The equipment’s operating manual should be made available at any time and should be operated according to the manual’s instruction.
  • Optimization of radiation exposure in relevance to the imaging system performance should be done. The goal here should be that the optimal dose should neither too high nor too low and should not affect the quality of the imaging study.
  • Facilities should use diagnostic reference levels and achievable doses as quality improvement tools by collecting and assessing radiation dose data and comparing them to diagnostic reference levels and achievable doses. Each facility should also submit its radiation dose data to a national registry.
  • The radiation safety officer must strictly enforce badge compliance, monitor and record fluoroscopy time, review individual radiation exposure, and investigate higher radiation exposure causes in case of higher readings.[22]

Other Issues

Tips for Interventionalists

An interventionalist could be related to interventional cardiology, interventional radiology, neurosurgery, and orthopedic surgery. Irrespective of the field of specialty primary operator is responsible for controlling radiation exposure while performing procedures.[23] An interventionalist should consider the following recommendations to minimize radiation exposure.

  • The risk-benefit ratio of X-ray use has to be considered for every patient and every procedure. Although ionizing radiation is most commonly used in interventional procedures yet, every effort should be made to minimize radiation exposure by using non-radiologic modalities (e.g., ultrasound imaging) and using the lowest possible radiation dose during the procedure. Recent advancement in imaging modalities ensures the provision of low dose modes. The use of half-dose mode ensures a reduction of the entrance radiation dosage to half without affecting imaging quality.[24] Even when there is an absolute necessity of using radiologic imaging studies, preference should be given to the imaging modality that uses the least amount of radiation, e.g., fluoroscopy over digital subtraction angiography.[22]
  • Preferable use of pulse mood instead of continuous fluoroscopy should be sought whenever possible.[25] In pulse mood, multiple short X-ray pulse emissions produce images, as compared to continuous fluoroscopy mode. The use of a constant frame rate (7.5 images/second) compensates for the loss of temporal resolution and ensures the smooth transition of images.[26] The major benefit of using pulse mood is the minimization of radiation exposure. The foot pedal should be engaged only when the imaging information is required.
  • Any intervention that reduces the radiation scattering subsequentially reduces the health care worker’s radiation exposure. Gantry position is a major determinant of radiation scattering. Scattering of radiation is maximum when the gantry position is > 30 degrees in left or right anterior oblique angulation or greater than 15 degrees in cranial angulation.[27] The operator and other staff members should try to position themselves in an area where the scattering of radiation is minimum, i.e., near the image receptor side.[27] The radiation receptor side should be placed closest to the patient to avoid dispersion of radiations and also to enable capturing the lowest radiation dosage. The reduction of field of view by using appropriate collimation enhances image accuracy and lowers radiation scattering.[28]

Enhancing Healthcare Team Outcomes

Tips for Fluoroscopy Suite Staff

Fluoroscopy suite staff members are exposed to higher doses of radiation as compared to nuclear medicine staff. In addition to the general recommendations mentioned above, the following precautions can further reduce radiation exposure to the lab staff.

  • Every staff member must avoid direct exposure to the primary beam as much as possible.
  • Wearing an appropriate lead apron and thyroid shield eye protective glasses during procedures and using a personal dosimeter should be made compulsory.
  • The lab management staff should periodically calibrate and inspect the fluoroscopy lab to ensure there are enough protective devices, lead aprons, and thyroid collars.[29]
  • Radiation physicists should be involved during initial setup, periodic quality control, and lab staff’s education. Radiation physicist opinion should be sought to monitor radiation exposure and to evaluate the root cause of excessive radiation exposure.[30]

Tips for Nuclear Medicine Staff

In nuclear medicine studies, radiations are detected from radioisotopes injected inside the body compared to radiologic studies in which radiations emit from external sources. Considering this fact makes it easier to understand that health care workers are at low risk of radiation exposure in nuclear medicine as compared to interventional lab staff, and that is the reason that their effective dose is highly unlikely to exceed occupational dose limits. The highest radiation exposure in nuclear medicine is associated with exposure to positron emission tomography (PET) pharmaceuticals. Out of all PET pharmaceuticals, Technetium 99m carries the highest risk of radiation exposure.[31] The mean daily effective dose for PET technologists is approximately 14 mSv, and the effective dose per minute of close contact (<2 m) with a radioactive source is approximately 0.5 mSv/min.[32] In addition to employing general precautions, usage of semiautomated injectors, patient video tracking, and shielded syringes can further reduce radiation exposure.[33][34][35]

Nursing, Allied Health, and Interprofessional Team Monitoring

Tips for Other Lab Staff

It is desirable that echocardiographers, paramedical staff, and anesthesia services be an integral part of the interventional lab or procedure room. Echocardiographers are at more risk as they have to position themselves towards the head end of the bed near the radiologic source. Therefore, they should use personal protective devices to minimize radiation exposure. Scattered radiation after the radiologic procedures is a significant source of radiation exposure to the echocardiographers.[36] Efforts should be made to delay echocardiography or schedule it before the nuclear medicine procedure. Pregnant echocardiographers should be asked to avoid performing the procedure during pregnancy. Another suggestion is that the echocardiographers should take turns to perform procedures to avoid repeated radiation exposure.

Ionizing radiation has revolutionized Diagnostic and interventional aspects of medicine at the cost of increased risk of carcinoma and other side effects on the body tissues e.g eyes (Cataracts) and skin (Burns). This increased risk is not only for patients but for the health care providers as well. The pros and cons of radiation usage should be considered and weighed individually on a case-to-case basis. The use of personal protective equipment, employment of shielding methods, strict implementation of ICRP and NCRP guidelines regarding radiation control, and educating health care professionals of possible risks and precautions can substantially reduce the health hazards associated with radiation exposure. 



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Classification of Radiation
Classification of Radiation
Contributed by Salman Akram
Article Details

Article Author

Salman Akram

Article Editor:

Yuvraj S. Chowdhury

Updated:

11/21/2021 11:01:48 PM

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