Cardiovascular Journal of Africa: Vol 27 No 2 (March/April 2016)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 27, No 2, March/April 2016

100

AFRICA

from MR in pregnancy, the current guidelines of the FDA

require labelling of MR devices to indicate that the safety of

MRI with regard to the foetus ‘has not been established’.

Nuclear cardiovascular imaging

Diagnostic nuclear medicine investigations also involve ionising

radiation. Unlike X-rays, nuclear techniques involve the

inhalation, ingestion or injection of a small quantity of a

radioactive isotope bound in a substance that targets a particular

organ, for example the heart. The gamma radiation emitted

by the radioactive isotope is detected outside the body by

electronic receptors of a gamma camera, which displays images

or functional data about the heart.

The most commonly used radioisotope, technetium-99m

(

99m

Tc), is a metastable daughter product following negative

beta decay of molybdenum-99.

99m

Tc decays to

Tc with a half-

life of six hours, releasing a mono-energetic gamma photon of

140 keV.

Nuclear studies that may be performed during pregnancy

include ventilation-perfusion scintigraphy for diagnosis of

pulmonary embolism, myocardial perfusion imaging where

99m

may be combined with several compounds that localise to active

myocardial cells, allowing ischaemic areas of the heart to be

determined, and, less commonly, cardiac ventriculography where

99m

Tc can be used to evaluate cardiac function (ejection fraction)

by imaging the ventricles. The dose of radiation passed on to the

foetus during a ventilation-perfusion scan is about 0.05 rad.

Along with conventional gamma scintigraphic imaging, the

two major nuclear imaging techniques are positron-emission

tomography (PET) and single photon-emission computed

tomography (SPECT). Both imaging modalities are now

standard in the major nuclear medicine services.

PET is based on the principle of positron annihilation by

using radionuclides that decay through positive beta decay.

Positrons generated by the decay combine with an electron and

annihilate, releasing two photons, with energies of 0.51 MeV, in

the process. The photons are released in opposite directions.

The most commonly used compound for PET imaging is

fluoro-2-deoxyglucose (

FDG), which is initially metabolised

within the cell, is unable to progress to the citric acid cycle, and

is not easily excreted by the cell.

Hence, cells that have a high

glucose metabolism concentrate,

FDG, can then be imaged.

The sections are reconstructed by algorithms, similar to but more

complex than those used for conventional CT, to accommodate

the 3D acquisition geometries.

Correction by considering the

physical phenomena provides an image representative of the

distribution of the tracer within the heart. In PET scanning, an

effective dose of the order of 8 mSv is delivered to the patient.

SPECT imaging is based on detectors that rotate around the

patient to obtain a digital representation of a 3D radioactive

distribution of the chest. The injected radioactive tracers emit

during their disintegration, gamma photons, which are detected

by an external detector after passing through the surrounding

tissue.

In SPECT, the main radioactive isotopes are

99m

Tc,

iodine and thallium-201 (which is used primarily for studies on

the heart). To increase the sensitivity and resolution of SPECT

systems, converging channel collimators were developed.

Both PET and SPECT benefit from electrocardiographic

gating used to enhance tomographic myocardial scintigraphy.

Therefore, the radioactivity from the myocardium and the

electrical activity of the heart are coupled. Depending on the

procedure, the mother and baby will generally receive a small

radiation dose with SPECT. It is unlikely that any diagnostic

nuclear medicine investigation would result in the radiation dose

of the foetus approaching 20 mGy. It is ideal that radioactive

isotopes are avoided during pregnancy. However, if there is a real

clinical need for such imaging to be performed, the risk to the

mother and foetus is minimal.

Contrast agents

A variety of oral and intravascular contrast agents are used with

X-ray and MR procedures. Radiopaque agents used with CT

and conventional radiography contain derivatives of iodine and

have not been studied comprehensively in human pregnancy.

However, iohexol, iopamidol, iothalamate, ioversol, ioxaglate

and metrizamide have been studied in animals and do not appear

to be teratogenic.

Neonatal hypothyroidism has been associated with some

iodinated agents taken during pregnancy.

Therefore iodine-

based contrast agents are relatively contra-indicated in

pregnancy, unless absolutely essential for a correct diagnosis.

Studies requiring views before and after the administration of

contrast agents will necessarily have greater radiation exposure.

While most contrast agents pass into the breast milk, they have

not been associated with problems in nursing babies.

Despite

in vitro

concerns, iodinated contrast agents seem safe to use in

pregnancy.

Radioactive isotopes of iodine are mutagenic and

are absolutely contra-indicated during the pregnancy.

Paramagnetic contrast agents used during CMR have not

been studied systematically in pregnant women. Animal studies

have demonstrated increased rates of spontaneous abortion,

skeletal abnormalities, and visceral abnormalities when given

at two to seven times the recommended human dose.

It is not

clear whether gadolinium-based contrast agents are excreted

into human breast milk. It is important to emphasise that

gadolinium-based contrast agents have not been associated with

any harm in human pregnancy.

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The 2007 American College of Radiology (ACR) guidance for

safe MR practices (expanded and updated in 2013) recommends

that intravenous gadolinium should be avoided in pregnancy and

should only be used if absolutely essential, until there is further

information about these agents.

74,75

Consequently, the FDA has

classified gadolinium as a category C drug, meaning it can be

considered in pregnancy ‘if the potential benefits justify the

potential risks to the fetus’.

Safety counselling

When a pregnant mother considers any radiation exposure,

the most prominent question in her mind is likely to be, ‘Is this

safe for my baby?’ To answer this question, the physician must

carefully choose words that will help a patient understand the real,

although very small, risks of exposure. The general population’s

total risk of spontaneous abortion, major malformations, mental

retardation and childhood malignancy is approximately 286 per

1 000 deliveries. Exposing a foetus to 0.50 rad adds only about

0.17 cases per 1 000 deliveries to this baseline rate, or about one

additional case in 6 000.

Such numbers often do not make much