Risk due to radiation

Radiation effects on humans may be:

There are legal regulations which guide the use of diagnostic radiation (see Appendix I). These are the basic principles:

Justification is particularly important when considering the irradiation of women of reproductive age, because of the risks to the developing fetus. The mammalian embryo and fetus are highly radiosensitive. The potential effects of in-utero radiation exposure on a developing fetus include prenatal death, intrauterine growth restriction, small head size, mental retardation, organ malformation and childhood cancer. The risk of each effect depends on the gestational age at the time of the exposure and the absorbed radiation dose.² Most diagnostic radiation procedures will lead to a fetal absorbed dose of less than 1 mGy for imaging beyond the maternal abdomen/pelvis and less than 10 mGy for direct abdominal/pelvic or nuclear medicine imaging.⁷ There are important exceptions which result in higher doses. Computed tomography (CT) scanning of the maternal pelvis may result in fetal doses below the level thought to induce neurologic detriment to the fetus, but, theoretically, may double the fetal risk for developing a childhood cancer.⁸

Almost always, if a diagnostic radiology examination is medically indicated, the risk to the mother of not doing the procedure is greater than the risk of potential harm to the fetus.⁹ However, whenever possible, alternative investigation techniques not involving ionizing radiation should be considered before a decision is taken to use ionizing radiation in a female of reproductive age. It is extremely important to have a robust process in place that prevents inappropriate or unnecessary ionizing radiation exposure to the fetus. Previous guidelines recommended that all non-emergency examinations that would involve irradiation of the lower abdomen or pelvis in women of child-bearing age be restricted to the first 10 days (10-day rule) or 28 days (28-day rule) following the onset of a menstrual period.¹⁰ This led to some potential practical difficulties, e.g. for women who denied recent sexual intercourse, for those with an irregular menstrual cycle or for those who had been taking the oral contraceptive pill. There are also concerns:

Joint guidance from the National Radiological Protection Board, the College of Radiographers and the Royal College of Radiologists recommends the following:¹¹

When a female of reproductive age presents for an examination in which the primary beam irradiates the pelvic area, or for a procedure involving radioactive isotopes, she should be asked whether she is or might be pregnant. If the patient cannot exclude the possibility of pregnancy, she should be asked if her menstrual period is overdue. Her answer should be recorded and, depending on the answer, the patient assigned to one of the following four groups:

If the examination is necessary, a technique that minimizes the number of views and the absorbed dose per examination should be utilized. However, the quality of the examination should not be reduced to the level where its diagnostic value is impaired. The risk to the patient of an incorrect diagnosis may be greater than the risk of irradiating the fetus. Radiography of areas that are remote from the pelvis and abdomen may be safely performed at any time during pregnancy with good collimation and lead protection.

Risk due to the contrast medium

The risks associated with administration of iodinated contrast media and magnetic resonance imaging (MRI) contrast are discussed in detail in Chapter 2 and guidelines are given for prophylaxis of adverse reactions to intravascular contrast.

Contraindications to other contrast media, e.g. barium, water-soluble contrast media for the gastrointestinal tract and biliary contrast media, are given in the relevant sections.

Risks due to the technique

Skin sepsis can occur at the needle puncture site.

Specific contraindications to individual techniques are discussed with each procedure.

Aftercare

May be considered as:

Complications

Complications may be considered under the following three headings:

Patient positioning

The resolution of gamma camera images is critically dependent upon the distance of the collimator surface from the patient, falling off approximately linearly with distance. Every effort should, therefore, be made to position the camera as close to the patient as possible. For example, in posterior imaging with the patient supine, the thickness of the bed separates the patient from the camera, as well as interposing an attenuating medium. In this case, imaging with the patient sitting or standing directly against the camera is preferable.

Patient immobilization for the duration of image acquisition is very important. If a patient is uncomfortable or awkwardly positioned, they will have a tendency to move gradually and perhaps imperceptibly, which will have a blurring effect on the image. Point marker sources attached to the patient away from areas being examined can help to monitor and possibly permit correction for movement artefact.

‘Oldendorf’ bolus injection³

For some investigations the Oldendorf technique is recommended to provide an abrupt bolus:

Radiation safety

Special instructions should be given to patients who are breast feeding regarding expression of milk and interruption of feeding.² Precautions may have to be taken with patients leaving hospital or returning to wards, depending upon the radionuclide and activity administered. These precautions were reviewed following the introduction of the Ionising Radiation Regulations 1999 and the adoption of lower dose limits to members of the public, and appropriate guidance has been published.⁴

Static field

The strength of the static magnetic field used in MRI is measured in units of gauss or tesla (10000 gauss = 1 tesla (T)). The earth’s magnetic field is approximately 0.6 gauss. Current guidelines on MRI field strength are given in Table 1.2.

Table 1.2 Guidelines on whole-body exposure to static magnetic fields⁴

Gradient field

Radiofrequency field

Radiofrequency energy at frequencies above 10 MHz, deposited in the body during an MRI examination, will be converted to heat, which will be distributed by connective heat transfer through blood flow. Energy deposited in this way is calculated as the average energy dissipated in the body per unit of mass and time, the specific absorption rate (SAR). Guidelines on radiofrequency exposure are designed to limit the rise in temperature of the skin, body core and local tissue to levels below those where systemic heat overload or thermal-induced local tissue damage occur. The extent to which local temperature rises depends on the SAR and on the blood flow and vascularity of the tissue. Areas of particular concern include those most sensitive to heat such as the hypothalamus and poorly perfused regions such as the lens of the eye.

For whole-body exposures, no adverse health effects are expected if the increase in body core temperature does not exceed 1 °C. In the case of infants and those with circulatory impairment, the temperature increase should not exceed 0.5 °C. With regard to localized heating temperatures, measured temperature in focal regions of the head should be less than 38 °C, of the trunk less than 39 °C, and in the limbs less than 40 °C.⁵

Controlled and restricted access

Access to the scanning suite should be limited. Areas should be designated as restricted (5 gauss line) and, closer to the scanner, controlled (10 gauss line). No patient with a pacemaker should be allowed to enter the restricted area. Any person who enters this area should be made aware of the hazards; in particular the ‘missile effect’. Any person entering the controlled area should remove all loose ferromagnetic materials, such as paperclips and pens, and it is advisable that wristwatches and any magnetic tape or credit cards do not come near the magnet. Other considerations include the use of specially adapted cleaning equipment, wheelchairs and trolleys. Fire extinguishers and all anaesthetic or monitoring equipment must be constructed from non-ferromagnetic materials.

Implants

As mentioned above, persons with pacemakers must not enter the restricted area. All other persons must be screened to ensure there is no danger from implanted ferromagnetic objects, such as aneurysm clips, prosthetic heart valves, intravascular devices or orthopaedic implants. Where an object is not known to be ‘magnet safe’, then the person should not be scanned. Lists of safe and unsafe implants are available⁸ and should be consulted for each individual object.

Specific questioning is also advisable to assess the risk of shrapnel and intraocular foreign body (Table 1.3).

Pregnancy

The developing fetus and embryo are susceptible to a variety of teratogens, including heat. Sensitivity varies during gestation and is usually greatest during organogenesis. Although no conclusive evidence of teratogenesis exists in humans, scanning should be avoided, particularly during the first trimester, unless alternative diagnostic procedures would involve the exposure of the fetus to ionizing radiation. Before a pregnant patient is accepted for an MRI examination, a risk–benefit analysis of the request should be made. Pregnant staff members are advised to remain outside the controlled area (10 gauss line) and to avoid exposure to gradient or RF fields.

MRI contrast agents should not be routinely administered to pregnant patients (see Chapter 2).

Occupational exposure

The safety of MRI workers is regulated by the EU Medical Devices Directive (amend. Direct 93/42/EEC) and established safety standards have been set by the International Electrotechnical Commission (IEC/EN 60601-2-33, 2nd amendment 2007).