What is echo?

CHAPTER 1 What is echo?

1.1 BASIC NOTIONS

Echocardiography (echo) – the use of ultrasound to examine the heart – is a safe, powerful, non-invasive and painless technique.

Echo is easy to understand as many features are based upon simple physical and physiological facts. It is a practical procedure requiring skill and is very operator dependent – the quality of the echo study and the information derived from it are influenced by who carries out the examination!

This chapter deals with:

• Ultrasound production and detection

• The echo techniques in common clinical use

• The normal echo

• Who should have an echo.

Ultrasound production and detection

Sound is a disturbance propagating in a material – air, water, body tissue or a solid substance. Each sound is characterized by its frequency and its intensity. Frequency is measured in hertz (Hz), i.e. in oscillations per second, and its multiples (kilohertz, kHz, 10³ Hz and megahertz, MHz, 10⁶ Hz). Sound of frequency higher than 20 kHz cannot be perceived by the human ear and is called ultrasound. Echo uses ultrasound of frequencies ranging from about 1.5 MHz to about 7.5 MHz. The nature of the material in which the sound propagates determines its velocity. In the heart, the velocity is 1540 m/s. The speed of sound in air is 330 m/s.

The wavelength of sound equals the ratio of velocity to frequency. In heart tissue, ultrasound with a frequency of 5 MHz has a wavelength of about 0.3 mm. The shorter the wavelength, the higher the resolution. As a rough estimate, the smallest size that can be resolved by a sound is equal to its wavelength. On the other hand, the smaller the wavelength of the sound, the less its penetration power. So a compromise has to be struck between resolution and penetration. A higher frequency of ultrasound can be used in children since less depth of penetration is needed.

Ultrasound results from the property of certain crystals to transform electrical oscillations (varying voltages) into mechanical oscillations (sound). This is called the piezoelectric effect (Fig. 1.1). The same crystals can also act as ultrasound receivers since they can effect the transformation in the opposite direction (mechanical to electrical).

Fig. 1.1 Piezoelectric effect.

The repetition rate is 1000/second. Each transmitting and receiving period lasts for 1 ms. Transmission accounts for 1 μs of this time. The remaining time is spent in ‘receiving’ mode.

At the core of any echo machine is this piezoelectric crystal transducer. When varying voltages are applied to the crystal, it vibrates and transmits ultrasound. When the crystal is in receiving mode, if it is struck by ultrasound waves, it is distorted. This generates an electrical signal which is analysed by the echo machine. The crystal can receive as long as it is not transmitting at that time. This fixes the function of the crystal – it emits a pulse and then listens for a reflection.

When ultrasound propagates in a uniform medium, it maintains its initial direction and is progressively absorbed or scattered. If it meets a discontinuity such as the interface of 2 parts of the medium having different densities, some of the ultrasound is reflected back. Ultrasound meets many tissue interfaces and echo reflections occur from different depths. Some interfaces or tissues are more echo-reflective than others (e.g. bone or calcium are more reflective than blood) and these appear as echo-bright reflections.

Two quantities are measured in an echo:

1. The time delay between transmission of the pulse and reception of the reflected echo

2. The intensity of the reflected signal, indicating the echo-reflectivity of that tissue or tissue–tissue interface.

The signals that return to the transducer therefore give evidence of depth and intensity of reflection. These are transformed electronically into greyscale images on a TV screen or printed on paper – high echo reflection is white, less reflection is grey and no reflection is black.

1.2 VIEWING THE HEART

Echo studies are carried out using specialized ultrasound machines. Ultrasound of different frequencies (in adults usually 2–4 MHz) is transmitted from a transducer (probe) which is placed on the subject’s anterior chest wall. This is transthoracic echo (TTE). The transducer usually has a line or dot to help rotate it into the correct position to give different echo views. The subject usually lies in the left lateral position and ultrasound jelly is placed on the transducer to ensure good images. Continuous electrocardiograph (ECG) recording is performed and phonocardiography may be used to time cardiac events. An echo examination usually takes 15–20 min.

Echo ‘windows’ and views (Fig. 1.2)

There are a number of standard positions on the chest wall for the transducer where there are ‘echo windows’ that allow good penetration by ultrasound without too much masking and absorption by lung or ribs.

Fig. 1.2 The main echo ‘windows’.

A number of sections of the heart are examined by echo from these transducer positions, which are used for 2 main reasons:

1. There is a limitation determined by the anatomy of the heart and its surrounding structures

2. To produce standardized images that can be compared between different studies.

Useful echo information can be obtained in most subjects, but the study can be technically difficult in:

• Very obese subjects

• Those with chest wall deformities

• Those with chronic lung disease (e.g. chronic airflow limitation with hyperinflated lungs or pulmonary fibrosis).

Rarely, an echo study is impossible.

A number of ‘echo views’ are obtained in most studies. ‘Axis’ refers to the plane in which the ultrasound beam travels through the heart.

Left parasternal window.

(2nd–4th intercostal space, left sternal edge):

1. Long-axis view (Figs 1.3, 1.4). Most examinations begin with this view. The transducer is used to obtain images of the heart in long axis, with slices from the base of the heart to the apex. The marker dot on the transducer points to the right shoulder.

2. Short-axis views (Figs 1.5, 1.6). Without moving the transducer from its location on the chest wall and by rotating the transducer through 90º so the marker dot is pointing towards the left shoulder, the heart is cut in transverse (short-axis) sections. By changing the angulation on the chest wall, it is possible to obtain any number of short-axis views, but the standard 4 are at the level of the aortic valve (AV), mitral valve (MV), left ventricular papillary muscles and left ventricular apex (Figs 1.5, 1.6).

Fig. 1.3 Parasternal long-axis view. Arrows show chordae.

Fig. 1.4 Parasternal long-axis view.

Fig. 1.5 Parasternal short-axis views: (a) Aortic valve level. The pulmonary valve is shown (arrow). (b) Mitral valve level. The anterior (A) and posterior (P) leaflets are shown. Mitral orifice (O). (c) Papillary muscles (arrows) level.

Fig. 1.6 Parasternal short-axis views.

Apical window.

(Cardiac apex):

1. 4-chamber view (Figs 1.7a, 1.8a). The transducer is placed at the cardiac apex with the marker dot pointing down towards the left shoulder. This gives the typical ‘heart-shaped’ 4-chamber view (Fig. 1.7a).

2. 5-chamber (including aortic outflow) (Figs 1.7b, 1.8b). By altering the angulation of the transducer so the ultrasound beam is angled more anteriorly towards the chest wall, a ‘5-chamber’ view is obtained. The 5th ‘chamber’ is not a chamber at all but is the AV and ascending aorta. This is useful in assessing aortic stenosis (AS) and aortic regurgitation (AR).

3. Long-axis and 2-chamber views (Figs 1.7c, 1.8c). By rotating the transducer on the cardiac apex it is possible to obtain apical long-axis and 2-chamber views which show different segments of the left ventricle (LV).

Fig. 1.7 Apical views: (a)

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Tags: Echo Made Easy

Jun 11, 2016 | Posted by admin in CARDIOLOGY | Comments Off

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