Imaging investigations
In physics, electromagnetic energy is broadly categorized into ionizing and non-ionizing energy. Ionizing radiation ionizes matter by detaching electrons from atoms or molecules.
These energy sources may directly cause mutations due to DNA or RNA damage. In general, fetal exposure of more than 5-10 centigray (cGy) should be avoided. In contrast, non-ionizing energy as it is used in medical diagnostics does not usually pose a direct (deterministic) or indirect (stochastic) risk and it is often safely used in pregnancy. Non-ionizing energy may produce some heat in tissues. Examples of non-ionizing radio waves or sound waves include magnetic resonance imaging (MRI) and ultrasonography.The ionizing radiation effects on the mother and fetus can be categorized into deterministic and stochastic effects. Deterministic effects are dose dependent and cause cellular damage with loss of organ development or function. Stochastic effects are usually associated with low-dose ionizing radiation and have a longer-term random element to it, causing genomic damage, which over time may lead to secondary cancers. In a workup for a patient with a malignancy, the aim is to determine whether a solid tumour has progressed to systemic spread by means of careful imaging investigations. Fetal radiation exposure remains a concern due to possible teratogenic effects of radiation exposure. The so-called ALARA (‘as low as reasonably achievable') principle should be used when radiation exposure is considered in a pregnant patient (5). A threshold dose for the deterministic effects is generally regarded as 100 milligray (mGy) to a fetus. Any dose above this may lead to congenital malformations, intellectual disability, and even fetal death.
The American Association of Physicists in Medicine recommends that adequate shielding is necessary for the use of X-rays and computed tomography (CT) investigations (Table 72.1).
MRI is usually considered to be safe during pregnancy.
However, the use of gadolinium is not recommended. Gadolinium is regardedTable 72.1 Fetal irradiation dose for different diagnostic tests
| Diagnostic test | I Fetal irradiation dose (mGy) |
| Chest X-ray | 0.0006 |
| AbdominalX-ray | 1.5-2.6 |
| Computed tomography (CT) chest | 0.1-13 |
| CT abdomen | 8-30 |
| Positron emission tomography (PET) | 1.1-2.43 |
by the United States Food and Drug Administration as a category C drug. Category C means that animal studies have shown adverse fetal effects and there are no adequate and well-controlled studies in humans. Potential benefits may warrant use of the drug in pregnant women despite potential risks because it crosses the placenta. In some reported studies there were no adverse effects on fetal development after inadvertent administration of gadolinium.
Ultrasound is generally regarded as safe in pregnancy. The ‘Essential Steps in the Management of Obstetric Emergencies' (ESMOE) training programme states in clinical practice guidelines in 2010 that there is also a theoretical risk of fetal heating and possible cavitation with the use of MRI and it should best be avoided during the first trimester.