Echocardiography

Echocardiography has advanced to such a stage that it is able to provide excellent anatomical and physiological data. It is thus used in the definitive establishment of normality (e.g. innocent murmurs) and in the accurate diagnosis of structural cardiac disease. It also remains the chief method for following progressive lesions and determining the timing of surgery. Its principle limitations are that the accuracy is operator dependent, poor imaging of the peripheral pulmonary arteries and descending aorta and that in older subjects image quality may be poor.

Principles

Sound is emitted from the echo transducer and the time taken for the reflected sound (echo) to return is proportional to the distance traveled. The computer maps the echos on a line & then takes another sample on a different line thus building up a 2D picture approx 50 times a second.
The frequencies commonly used (2-10MHz) are much higher than the audible range of 2-18KHz. Higher frequencies allow better resolution (eg. lateral resolution of a 5 MHz is 2 mm compared to 3mm of 3MHz whilst axial resolution varies from 0.5 mm to 1 mm) but tissue penetration is poorer.

In children all parts of the heart can be imaged either through rib spaces, subcostal area or suprasternal notch (echo windows). The anatomy, wall and cavity sizes easily determined. The principle limitation of echocardiography is that the accuracy is critically dependant on the operator’s skill.

2D Echo

The 2D pictures are taken from the various echo windows and give "standard views" to build up a complete picture of the cardiac anatomy. Not all chambers are visible in every view.

Apical 4 Chamber

The transducer is held at the apex of the heart and angled towards the right shoulder. The 4 chambers are readily seen and both the mitral and tricuspid valves. If the transducer is angled anteriorly then the aorta is also visualised. This view is shows the ventricular septum well to look for septal defects.

Parasternal Long Axis

The transducer is ;placed in just to the left of the mid to upper sternal border. The right ventricular outflow region, the ventricular septum and the left atrium and ventricular are well visualized. This is one of the best views to obtain an M Mode and hence information on cardiac function.

Parasternal Short Axis

From the parasternal long axis view the transduce is rotated 90° to point towards the left shoulder. The aorta and coronary artery origins are seen well in cross section. The PA is also seen along with the branches and a PDA if it is present

Subcostal View

This is a good view to see lovely images - especially in babies as no ribs or lung tissue obscures the view. The atrial septum is particularly well seen. Unfortunately the transducer is the furthest from the heart and in older children and adults the distance may be too great to allow detailed imaging.

Arch View

This is obtained by sliding the transducer towards the upper sternal edge and suprasternal notch. It allows the ascending aorta, arch and neck vessels to be imaged.

M Mode

Used to measure accurately chamber size at various times in cardiac cycle. Consider it as if several pencils were attached on the line seen on the 2D image & a piece of paper drawn rapidly across - the line moves according to the cardiac motion of that specific point in the heart.

The M Mode measures the interventricular wall thickness in systole (IVSs & IVSd ) and diastole, the LV posterior wall thickness in systole and diastole (LVPWd & LVPWs) and the LV cavity in systole and diastole (LVDs & LVDd) which allows calculation of the shortening fraction (FS) by dividing the difference between LVDd & LVDs into the LVDd and is a measure of LV contractility.

Doppler

Sound waves at a constant frequency are emitted from the transducer when an electric current is passed through a crystal. They are reflected back by the red blood corpuscles - if the blood is traveling towards the transducer the wavelength is compressed (upper diagram), the converse is true if blood is traveling away from the transducer (lower diagram).

The velocity of blood flow is calculated from the wavelength difference. The velocity can be converted into a pressure difference by the Bernoulli equation - this allows gradients across valves or narrowed arteries to be measured accurately. The pressure drop = 4V².
eg the velocity (V) is 3 m/s which predicts a 36 mm Hg pressure gradient.

Colour Doppler

Over the area selected by the sonographer the machine analyses the Doppler flows using the pulsed wave principle, codes the results using colour (by convention red towards the probe, blue away) and overlays the colour map on the 2D image. This allows rapid assessment of blood flow direction and an assessment of its velocity.

The example opposite shows blood flowing through a mid septal VSD.

Transesophageal Echocardiography

This is performed under sedation in adults but the majority of children prefer a general anaesthetic. The TEE probe is place down the oesophagus and lies immediately behind the left atrium. As the probe is thus very near to the cardiac structures and there are no other structures in the way the image quality is excellent. It is particularly beneficial in those in whom precordial images are poor – especially the adult congenital population. It is much more sensitive than transthoracic echocardiography in the investigation of endocarditis.

Transesophgeal echo

PFO demonstrated on TEE

Doppler Tissue Imaging

Doppler signals can be adjusted to display wall motion rather than blood motion. This allows detailed scrutiny of wall motion (contractility) and a more accurate assessment of myocardial function. The Doppler is overlaid on the 2D image or presented in M mode. This digital data then allows accurate measurement of wall motion and is especially useful in ischaemic heart disease.

3D Echocardiography

The data set is acquired by a “sweep” of the transducer from one or preferably more "echo windows". Computing power has now advanced such that it is possible to obtain images in real time. The digital data set allows images to be constructed offline that are not possible to obtain in “real life” allowing better demonstration of anatomy. The volume information allows flows to be calculated at sites in the heart of great vessels to allow cardiac output to be assessed. Unfortunately the procedure is still time consuming, continues to be limited by adequate echo windows and hard and software shortcomings and has yet to live fully up to expectations

Other Areas

Do not forget that ultrasound is an excellent way to look for effusions - both pericardial and pleural. It is safe and easily repeatable and the same machine for echocardiography can be used.

This page was last edited 16/2/2004

Acknowledgements