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CARDIAC ULTRASOUND - ECHOCARDIOGRAPHY:

Ultrasound


    Sound is made up of pressure waves of different frequencies and measured in Hertz (Hz) or cycles per second. Audible sound is between 20Hz to 20KHz, anything above 20KHz is considered Ultrasound and anything below 20Hz is considered Infrasound. Medical Ultrasound is usually between 2-15MHz depending of it's use. Medical Ultrasound can be used for a multitude of specialties ranging from guiding needle placement, Obstetrics to check on the development of the foetus, imaging of solid organs such as kidneys or testicles, measuring blood flow and to assess the heart. Medical Ultrasound of the heart is commonly referred to as Echocardiography in addition to being called Cardiac Ultrasound.

Echocardiography

    For Echocardiography the ultrasound is artificially created. Piezo-electric crystals are used in the ultrasound probes/transducer. These crystal have a property that if electrical current is applied to them a brief ultrasound pulse is created. Additionally these crystals will create a small electrical signal if they are deformed or compressed.

    So a small electrical current is applied to the piezo-electric crystals  repeatedly and this produces multiple 'pulses' of ultrasound like waves. These ultrasound waves are focussed and directed into the body. Every time the ultrasound wave reaches a tissue density difference, such as blood and bone, some of the ultrasound wave is reflected. The reflected ultrasound deforms/compresses the piezo-electric crystal and generates a small electrical signal. The returning 'echoes' are then processed and displayed in different formats.

    Echocardiography uses ultrasound in the 1.7MHz - 10MHz range. Different frequencies have different properties. The higher the frequency the better the resolution of the images but the ultrasound is limited in how far into the body it can penetrate. The lower the frequency the farther the ultrasound can penetrate but the picture quality is reduced. Most modalities are performed as a Transthoracic echocardiogram (TTE) by placing the ultrasound probe on the chest wall and using a water based gel as a contact medium to allow the ultrasound to penetrate into the body.

Echocardiography modes

    A-Mode:    Amplitude modulation is an early method of displaying the time taken for an 'echo' to return, the depth, versus the amplitude of the signal that represented it's strength.

    B-Mode (2D):    Brightness mode displays spots or dots of varying brightness over time, the brightness is indicative of the strength of the returning echo. A linear array of many piezo-electric crystals are able to scan a plane through the body and can be viewed as a two dimensional image. If the B-mode is repeated at least 20 times a second then the illusion of movement is created and the cardiac structures can be seen in motion. This is the commonest mode of echocardiography as it shows moving two dimensional images of the heart. There are an infinite amount of views that can be obtained but there is a universal standard set of views and under a normal procedure these are observed in a set protocol. These are:

      Parasternal Long Axis
      Parasternal Long Axis RV Inflow
      Parasternal Short Axis (Various levels)
      Apical Four Chamber
      Apical Five Chamber
      Apical Two Chamber
      Apical Long Axis
      Subcostal
      Suprasternal



    Parasternal 2D Apical 4 Chamber 3
    6
    M-Mode:    Motion/Movement mode represents the motion or movement through a single B-Mode scan line. The M-mode will then show the movement of the cardiac structure over time. The M-mode has good temporal resolution so is very good at following rapid movements. This can be correlated with the ECG or respiratory pressure waveforms for more information. M Mode is the mode most commonly used to obtain measurements for cardiac structures such as the septal thickness, size or the aortic root and chamber dimensions. It is also used to assess the opening and closure of the various valves.

    Colour Flow:    In this modality Pulse Wave Doppler signals are used to derive the direction and velocity of blood flow within the heart. Convention has blood flowing towards the probe as red and blood flowing away as blue, the higher the velocity the brighter the respective colour. Blood flowing transversely or perpendicular to the probe will not be shown and turbulent flow may be depicted as green or white.

    Pulse Wave Doppler:    This type of Doppler is able to localise a site of flow and depict the velocity at that point. The point of interest, also known as 'sample volume' is chosen by selecting the area on the 2D image. The sample volume size may be altered slightly as required. Pulse Wave Doppler is also able to determine if the flow is 'laminar', all travelling in the same direction, or turbulent. One disadvantage of Pulsed Wave Doppler is it's inability to measure high velocities. High velocities result in what is known as 'aliasing', if the Doppler signal is aliased then no information about direction of velocity can be obtained.

    Continuous Wave Doppler:    This form of Doppler records velocities along a single line chosen on the 2D image. This method gives a very good resolution of the velocity but no information about the depth or location of the velocity is gained. Care must be taken when recording high velocities that may arise from similar areas and direction Aortic stenosis and Mitral regurgitation for example.

    Tissue Doppler:    Tissue Doppler is very similar to Pulsed Wave Doppler but in this case the velocities of tissue is being measured. These velocities are much lower than blood velocities so the higher velocities are filtered out and the resulting image shows the tissue velocity accurately.

    Transoesophageal Echocardiogram (TOE):    Placing the ultrasound probe on the chest wall places the probe quite far from the heart. The ultrasound has to penetrate a few centimetres of tissue before imaging of the heart can be done. TOE is an ultrasound probe that is attached to a narrow flexible tube that is passed orally and into the oesophagus. It's proximity to the heart is much closer and so provides better resolution images that a transthoracic scan. However the logistics, sterilisation and additional required specialist training make TOE less convenient

    Stress Echocardiogram:    This is a diagnostic procedure that may be used to assess the heart under stress. It is usually done if a subject has new chest pain (angina), chest pains are getting worse or suffered a recent heart attack (myocardial infarction). A routine echocardiogram is performed to attain a subject's normal values. The subject is then encouraged to perform an Exercise ECG, a progressive exercise test, usually under the Bruce Protocol, until the required heart rate is reached. A second echocardiogram is then performed to see how the heart reacted to the exercise and how efficient it was under exercise. If a subject is unable to exercise on a treadmill or bike then a Dobutamine Stress Test may be performed. Dobutamine is given in small doses intravenously and this speeds up the heart rate. The echocardiograms are still done before and after the administration of Dobutamine and cardiac function is assessed.

    3D/4D Echocardiogram:    3D Echo is an emerging modality that uses a matrix array ultrasound probe and powerful processing to provide images in an infinite amount of different planes and reconstruct them to provide a three dimensional image. If the image is moving then it is called a 4D echocardiogram. This form of echo is able to provide detailed anatomical assessment of congenital defects or cardiomyopathies. If Real Time 3D Echocardiography is available then it may be used to monitor intra-cardiac biopsies or catheter delivered devices.

    Contrast Echocardiogram:    This procedure introduces an echo opaque medium into the heart. This 'contrast' medium is made up of micro-bubbles, sometimes manufactured out of gas and a protein shell, that circulates through the heart and provides a highly reflective image. The main application in Contrast echo is for the detection of a Patent Foramen Ovale (PFO), this is where the blood is able to pass from the left atrium to the right directly. Contrast can also be used to detect other flow abnormalities such as cardiac shunts or Tricuspid regurgitation. The commonest indicator for echocardiography is to assess left ventricular function, however, between 5-10% of TTE's are sub-optimal and the use of a Contrast medium enables better delineation of the left ventricle.
     




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