In order to understand the basic principles of echocardiography, it is extremely important to understand the physical principles behind the process and also the definition of the commonly used terms.
Ultrasound is sound having frequency of > 20,000 cycles/sec. It can be directed in a beam and it obeys the laws of reflection and refraction. It is reflected by objects of small size. Ultrasound beam should be extremely narrow so that it can obtain an ice pick view or slice of the heart.
Resolution is the ability to distinguish or identify two objects that are close together.
Two-dimensional (2-D) echocardiography (Fig. 1.1) is the study of cardiac structures in a two-dimensional view.
Doppler echo is a study of cardiac structures and blood flow profiles using an ultrasound beam. Doppler examination is based on the observation that the frequency of sound increases when a source of sound is moving towards the listener and vice versa.
The same applies when a reflecting object is moving towards a transducer and otherwise. It is used to determine the direction and velocity of RBC with respect to ultrasound beam. The common system used is to encode flow towards the transducer as red and away from transducer as blue [BART (Blue Away; Red Toward)]. The best Doppler information is obtained when the ultrasonic beam is parallel to the moving target (opposite of that for imaging with M mode or 2-D echo). Also better information is obtained with lower frequency transducer compared with a higher frequency transducer.
Continuous wave Doppler is used to assess valvular stenosis and regurgitation and velocity of flow in shunts.
Pulse Doppler information can be used sending a short burst of ultrasound, the frequency of that burst is distorted if the target from which it is reflected is moving. It helps to assess the normal valve functions, LV diastolic function, stroke volume and cardiac output.
The major disadvantage of the pulse Doppler system is that the velocity one can measure is limited. Pulse Doppler system has inherently pulse repetition frequency (PRF)
The PRF determines the ability of the Doppler to detect high frequency Doppler shifts. The inability of the Doppler system to detect high frequency Doppler shifts is known as aliasing. The upper limit of frequency that this limit can detect is known as “Nyquist limit “. This limit is one-half of PRF. If the flow exceeds this limit it is detected as flow in the opposite direction. ‘Alaising’ and ‘wrap around’ may thus add to the confusion. The Nyquist limit of colour flow Doppler imaging is usually lower than ordinary pulsed spectral Doppler.4
Colour flow imaging is pulsed Doppler and therefore is limited in its abilities to measure high velocities. Basically two flow patterns can be detected with spectral Doppler. First is laminar (Figs1.2 and 1.3), i.e. reflecting red blood cells are travelling in same direction with little difference in velocities. The Doppler signal from such a flow indicates a narrow velocity spectrum. Second is turbulent, this happens when blood is flowing through a narrow orifice (Multiple velocities in turbulent flows shown in (Fig. 1.4). As velocity increases aliasing occurs (Fig. 1.5). One of the major functions of colour Doppler system is to identify abnormal flow patterns such as valvular regurgitation and shunts revealed by multiple colour bands.
Note:
- Ultrasonic beam is perpendicular to blood flow-no flow or colour is recorded.
- Ultrasonic beam is parallel to blood flow, e.g. apical views, high velocity, colour is brighter.
- If the velocity is too low flow is not visualized.
Transducers
Transthoracic
There are 4 transthoracic transducers, two phased array and two mechanical. The phased array transducers have a flat surface and no visible moving parts. The phased array transducers vary with the frequency and number of elements in the transducers. Transducers used have varying frequencies. Children frequencies vary 3.5 to 7.0 MHZ. Those with higher frequency has better resolution. In adults, lower frequency probe is used.
The higher frequency transducers are usually smaller. The number of elements varies from 32 to 128. Transducers with more elements are usually larger. The low frequency transducers have better penetration and produce better doppler recordings.5
High frequency transducers give better resolution and finer image.
Transoesophageal echo needs separate probe, adult and paediatric. Transoesophageal transducers are placed in endoscopic instruments. Both mechanical and phased array devices can be used.
Intravascular Transducer
This can be placed in intravascular catheter of almost any size.
Digital Echocardiography
Digital echocardiography is the recording and display of echocardiographic data in digital form. The echocardiographic recording can be viewed and manipulated by computers.
Stress Echocardiography
Stress testing helps to identify latent or known cardiac abnormalities, only become manifest when provoked with some form of stress. Patients with valvular heart disease can show significant hemodynamic changes with stress. Doppler recordings are important under these circumstances
Contrast Echocardiography
When liquid was injected within cardiovascular system, tiny suspended bubbles produce a cloud of echoes.