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

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

AFRICA

Echocardiography

Echocardiography is an imaging modality that uses high-

frequency (2–10 MHz) sound waves to image cardiac structures

and to give reproducible information about cardiac structure and

function. Ultrasound is produced when a piezo-electric crystal,

mounted in a transducer, is stimulated by an electric current.

Ultrasound waves are not audible and are harmless to tissue

at the intensities used in diagnostic imaging. The passage of

sound waves depends on the acoustic impedance of tissues.

Most ultrasound waves pass through tissues to deeper structures

further from the surface, but reflected sound returns to strike

the crystal, deforming it and producing electric signals, which

correspond to the degree of deformation.

This electrical

information is transformed so it can be displayed on a cathode-

ray tube as pulses of light. Due to the speed of sound within the

body being relatively constant, the depth of the tissue interface

can be calculated, and reflected echoes are displayed on the

screen on a depth scale.

Blood reflects little sound and appears relatively black/hypo-

echoic compared with the myocardium, which reflects more of

the ultrasound and appears relatively white/hyperechoic. The

heart valves are even more echogenic. Neither bone nor air is

a good transmission medium for ultrasound waves; therefore

as the heart is surrounded by lung and the bony cage of the

thoracic cavity, the ultrasound beam must be aimed through

specific gaps, known as acoustic windows (e.g. parasternal,

apical, subcostal and suprasternal), to produce images of the

heart and vasculature.

Given the lack of ionising radiation,

echocardiography is an attractive first-line investigation for most

forms of CVD encountered in pregnancy.

M-mode and two-dimensional (2D) echocardiography provide

real-time imaging of heart structures throughout the cardiac

cycle; more recently, three-dimensional (3D) echocardiography

has been developed.

Doppler echocardiography provides

information on blood movement inside cardiac structures and

on the haemodynamics

(Fig. 2). Tissue Doppler imaging (TDI)

provides information about movement of cardiac structures.

The relationship between the dynamics of cardiac structures and

the haemodynamics of the blood inside these structures provides

information about cardiac diastolic and systolic function.

Echocardiography is continuously evolving and constantly being

augmented by newer modalities, such as tissue harmonics, speckle

tracking, tissue Doppler strain and tissue characterisation.

To date, there have been no reports of documented adverse

foetal effects from diagnostic ultrasound procedures, including

duplex Doppler imaging.

There are no contra-indications to

echocardiography during pregnancy, and ultrasound is preferred

over X-ray as the primary method of foetal imaging during

pregnancy.

Energy exposure from ultrasonography has been

arbitrarily limited to 94 mW/cm

by the US Food and Drug

Administration (FDA).

Doppler and colour echocardiographic scans work by

concentrating a beam of sound in a small area, and therefore

can cause heating of local tissues if held in the same place for

a long time.

Most scans automatically reduce the power of the

ultrasound beam when Doppler is used, to decrease the intensity.

Nowadays most echocardiographic machines have a low thermal

index and so pose very little risk. Dobutamine is favoured in

pregnancy over adenosine for stress echocardiography.

Cardiovascular computed tomography (CCT)

Computed tomography (CT) is a diagnostic imaging procedure

that uses X-rays to demonstrate cross-sectional images of

the body acquired in different orthogonal planes. The cross-

sections or slices are reconstructed from the measurements of

attenuation coefficients of X-ray beams in the volume of the

object studied.

The fundamental principle of CT is premised

on tissue density traversed by the X-ray beam, which can be

calculated from the attenuation coefficient. In other words,

CT permits reconstruction of tissue density by 2D sections

perpendicular to the axis of the acquisition system. Unlike X-ray

radiography, the detectors of the CT scanner do not produce

an image, but rather measure the transmission of a thin beam

(1–10 mm) of X-rays through a full scan of the body, and the

image of that section is taken from different angles, allowing

retrieval of information on the depth of the tissues imaged.

Complex mathematical algorithms are used to construct an

image from the raw data; a typical CT image is composed of

512 rows, each of 512 pixels, i.e. a square matrix of 512 × 512 =

262 144 pixels (one for each voxel). A typical CCT study gives

0.06 to 0.09 rad.

Similarly, the effective radiation dose for CT

pulmonary angiogram (CTPA) protocols is generally between

2.2 and 7 mSv (0.02–0.07 rad).

Often, for CCT and CTPA, the imaging field of view includes

the lungs and breasts; the radiation dose can be reduced by

Fig. 2.

Echocardiography in a pregnant woman with mitral

stenosis. (A) parasternal long-axis view showing a

deformed, calcified and restricted mitral valve with a

classic ‘hockey-stick’ deformity of the anterior mitral

valve leaflet and a dilated left atrium. (B) continuous-

wave Doppler trace showing a mean gradient of 15.5

mmHg, indicating severe mitral stenosis.

Table 3. Approaches to minimising foetal radiation during

cardiovascular imaging in pregnancy

Restricting the X-ray beam size to as small as is necessary

Choosing the direction of the primary beam so that it is as far away from

the foetus as possible

Ensuring that the overall exposure time is as short as possible

Selecting appropriate exposure factors

Defer abdominal examinations if possible; imaging examinations of the

thorax are associated with negligible risks to the conceptus

Whenever possible, ultrasound is the preferred modality for abdominal

imaging in pregnancy

Magnetic resonance imaging is emerging as an alternative in centres where

it is widely available

Using a lead apron on the table to shield any primary beam from the X-ray

tube reaching the foetus

Calculations of dose by a knowledgeable medical physicist if there is concern

The radiation dose should be kept as low as reasonably achievable

(ALARA principle)