Radiation Exposure Of Medical Imaging

Function

Quantification of Radiation

Before understanding the biological effects of radiation, 1 should familiarize themselves with 2 important medical terms in radiology: absorbed radiation dose and effective dose.

Absorbed Dose

It’s the amount of energy that radioactive waves deposit in any material 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 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. It measures how much-absorbed radiation dose has a biological effect on tissues. The dose equivalent is used when measuring the effective radiation dosage in a specific organ or tissue. In contrast, the effective dose measures the effective radiation dosage of the whole body.[4] Both of these quantities are expressed in Sieverts (Sv) and are measured by estimating data collected from personal dosimeters.

Equivalent dose= Absorbed dosage x Tissue weighting factor.

The tissue weighting factor varies from 1 organ or tissue to another and reflects the organ's sensitivity to radiation.[4] 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.[5]

Health Risks of Radiation Exposure

Two types of responses, Tissue reaction and stochastic effect, have been associated with ionizing radiation exposure.[6]

• Tissue reaction (Deterministic effect) is when 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 attempt division or differentiation unsuccessfully. Skin burn and cataract development are examples of tissue reactions.[7] The lens of the eye is the most radiosensitive tissue in the body. Posterior subcapsular changes are typical of radiation exposure.[8] 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.[9]
• 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 radiation exposure and the development of carcinoma, but the latent period extends up to a decade or 2 in most cases.[10] This type of damage is more prevalent in low-dose radiation exposure, which usually occurs due to routine medical imaging.

Radiation Exposure Monitoring

Lif TLD badges or rings quantify the absorbed radiation dose that a healthcare worker acquires while performing diagnostic or therapeutic procedures requiring radiation use. Lif crystals store radiation energy. At the end of the monitoring period, these badges or rings are melted, and the energy stored is released as visible light, which allows the determination of radiation exposure. These badges can detect radiation exposure of as low as 1 mrem. The International Council on Radiation Protection (ICRP) recommends wearing 2 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.[11] The dosimeter worn at the collar or the left shoulder level can also be used to determine the radiation exposure to the eyes' lens or the unshielded skin.[12] The effective dose can be calculated by summing the calculated equivalent dose.

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

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

Occupational Dose Limitation

The ICRP and the National Council on Radiation Protection and Measurements (NCRP) provide guidelines regarding the health and safety aspects of ionizing radiation exposure relevant to patients and healthcare providers.[13] According to ICRP, 20 mSv/year averaged over 5 years (ie, 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, the occupational dose limit is 50 mSv in any 1 year, and the lifetime limit is 10 mSv, multiplied by the individual’s age in years. ICRP also defines effective dose limits related to certain body organs, ie, 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. No dose limitation is recommended in any rescue operation (Procedures reducing mortality and morbidity) where the benefits of procedures outweigh the risks of occupational radiation exposure. Otherwise, every effort should be made to minimize 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, healthcare 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.[14][15]

Clinical Significance

“As low as radiation exposure” (ALARA) is the guiding principle of diagnostic and interventional procedures using radiation. The principle's application is limited to reducing radiation exposure and includes using personal protective equipment (PPE). In an interventional lab, the greatest radiation exposure source to healthcare workers is scattering from the patient. Anything that reduces patient radiation exposure indirectly reduces the healthcare worker’s radiation exposure. On the other hand, reducing 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: The duration of the procedure and the timing of contact with patients are important factors determining the radiation exposure of healthcare workers. Minimizing the time during which the patient is exposed to radiation minimizes radiation exposure to the operator and other staff members. Similarly, taking a history before and after the radiologic procedure also reduces exposure.

Increase Distance: Radiation exposure is inversely proportional to the distance between the operator and the radiation source. It decreases the inverse square root of the distance between both. Positioning oneself on the patient's 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 hindering radiation transmission from a radiation source to the person. These Shielding methods are not only limited to the personal level, ie, the use of PPE, but are also employed during the construction of hospitals. PPE includes protective eyeglasses, lead aprons, gloves, scrub caps, and thyroid collars.[16][17] Keeping body physique variations in mind, PPE should be adjusted to ensure proper fitting and subsequent radiation protection. Healthcare workers should be frequently asked about the integrity and fitting of PPE. Lead aprons are available in 1-piece and 2-piece (vest and skirt) options. During pregnancy, pregnancy aprons are available to encase the enlarging abdomen. Lead aprons to reduce the penetrating radiation dose to 2% to 10%, depending upon the thickness of the apron. 0.25 mm and 0.5 mm lead aprons to reduce the penetrating radiation dose to 10% and 2%, respectively.[18]    

Role of the Hospital Facility

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

• First and foremost, hospital management must minimize radiation exposure. One possible method of achieving this is by implementing shielding methods both at the architectural level, eg, placing heavy aggregate concrete around the walls of X-ray rooms to absorb radiation, and at the personal level, by providing a sufficient supply and ensuring PPE usage inside the facilities.[19] Another method is ensuring that no medical personnel or patient becomes unnecessarily exposed to radiation at any time.
• Every hospital facility should assess workers' radiation exposure and provide them with periodic feedback. Also, each worker who is expected to receive more than 10% of the applicable dose limit should be required to wear 1 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.[20]
• Hiring staff members with 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 available anytime and operated according to the manual’s instructions.
• Optimization of radiation exposure relevant to the imaging system's 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.[21]

Other Issues

Tips for Interventionalists

An interventionalist could be related to interventional cardiology, interventional radiology, neurosurgery, or orthopedic surgery. Regardless of the specialty, the primary operator is responsible for controlling radiation exposure while performing procedures.[22] 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, every effort should be made to minimize radiation exposure by using non-radiologic modalities (eg, ultrasound imaging) and the lowest possible radiation dose during the procedure. Recent advancements in imaging modalities ensure the provision of low-dose modes. The half-dose mode reduces the entrance radiation dosage to half without affecting imaging quality.[23] Even when there is an absolute necessity to use radiologic imaging studies, preference should be given to the imaging modality that uses the least amount of radiation, eg, fluoroscopy over digital subtraction angiography.[21]
• Preferable use of pulse mood instead of continuous fluoroscopy should be sought whenever possible.[24] In pulse mood, multiple short X-ray pulse emissions produce images compared to continuous fluoroscopy mode. Using a constant frame rate (7.5 images/second) compensates for the loss of temporal resolution and ensures the smooth transition of images.[25] 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 healthcare 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.[26] The operator and other staff members should try to position themselves in an area where the radiation scattering is minimal, ie, near the image receptor side.[26] The radiation receptor side should be placed closest to the patient to avoid radiation dispersion and enable the lowest radiation dosage to be captured. Reducing the field of view using appropriate collimation enhances image accuracy and lowers radiation scattering.[27]

Enhancing Healthcare Team Outcomes

Tips for Fluoroscopy Suite Staff

Fluoroscopy suite staff members are exposed to higher doses of radiation than 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.
• It should be compulsory to wear an appropriate lead apron and thyroid shield eye protection glasses during procedures and use a personal dosimeter.
• The lab management staff should periodically calibrate and inspect the fluoroscopy lab to ensure enough protective devices, lead aprons, and thyroid collars.[28]
• Radiation physicists should be involved during initial setup, periodic quality control, and lab staff education. Radiation physicists' opinions should be sought to monitor radiation exposure and evaluate the root cause of excessive radiation exposure.[29]

Tips for Nuclear Medicine Staff

In nuclear medicine studies, radiation is detected from radioisotopes injected inside the body compared to radiologic studies in which radiation is emitted from external sources. Considering this fact makes it easier to understand that healthcare 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 pharmaceuticals. Technetium 99m carries the highest risk of radiation exposure out of all positron emission tomography pharmaceuticals.[30] The mean daily effective dose for positron emission tomography 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.[31] In addition to general precautions, using semiautomated injectors, patient video tracking, and shielded syringes can reduce radiation exposure.[32][33][34]

Nursing, Allied Health, and Interprofessional Team Monitoring

Tips for Other Lab Staff

Echocardiographers, paramedical staff, and anesthesia services should be integral to 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 radiologic procedures is a significant source of radiation exposure to echocardiographers.[35] Efforts should be made to delay or schedule echocardiography 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 eg eyes (cataracts), and skin (burns). This increased risk is not only for patients but also for the health care providers. The pros and cons of radiation usage should be considered and weighed individually on a case-to-case basis. Using personal protective equipment, employing shielding methods, strict implementation of ICRP and NCRP guidelines regarding radiation control, and educating healthcare professionals of possible risks and precautions can substantially reduce the health hazards of radiation exposure.