2 Clinical role of echocardiography

Question 1

What are the definitions, and units of measurement, of sound, compression and rarefaction, frequency, wavelength, propagation velocity and amplitude?

Answer 1

Sound is a longitudinal mechanical wave which requires the presence of particles.

In sound waves, there are areas of compression (high pressure in which the particles are more close) and rarefaction (low pressure in which the particles are less close).

Frequency is the number of waves per second, measured in Hz.

Wavelength is the distance between two waves, measured in m.

Propagation velocity is the speed the wave travels through the medium, measured in m/s.

Amplitude is the strength of the wave (baseline to peak), measured in dB.

Question 2

How are wavelength, propagation velocity and frequency related?

Answer 2

Wavelength = propagation velocity / frequency.

Question 3

How is resolution affected by frequency?

Answer 3

The higher the frequency, the shorter the wavelength, the higher the resolution.

Question 4

What are the propagation velocities in the body and the frequencies of ultrasound?

Answer 4

Air = 330m/s
Fat = 1450m/s
Soft tissue = 1540m/s
Blood = 1570m/s
Muscle = 1580m/s
Bone = 3500m/s

Ultrasound frequency is >20,000Hz (2MHz) (audible frequency is 20-20,000Hz).

Question 5

What are the types of reflection?

Answer 5

Specular reflection (mirror reflection) is reflection in one direction which occurs when the reflector is large and smooth (e.g. chambers, valves and vessels). This is angle-dependent.

Backscatter is reflection in multiple direction which occurs when the reflector is small and rough (e.g. RBCs). This is angle-independent. Rayleigh scatter is backscatter which is equal in all directions (e.g. RBCs).

Question 6

What is the difference between reflection and refraction?

Answer 6

Reflection is the change in direction of the ultrasound towards the transducer and refraction is the change in direction of the ultrasound away from the transducer at the boundary of different tissues with different acoustic impedance.

Question 7

What is acoustic impedance and acoustic impedance mismatch?

Answer 7

Acoustic impedance is the resistance to ultrasound transmission.

Acoustic impedance mismatch occurs when ultrasound crosses the boundary between tissues with different acoustic impedances, and the energy is reflected back to the transducer.

Gel decreases acoustic impedance mismatch and hyperinflated lungs increase acoustic impedance mismatch.

Question 8

What is attenuation and half intensity depth?

Answer 8

Attenuation is the loss of energy as ultrasound travels through a medium, measured in dB. The higher the depth, the higher the attenuation. This is due to reflection, scatter and absorption.

The half intensity depth (HID), in soft tissue, is the depth in which the intensity of the ultrasound decreases by 50%.
HID (soft tissue) = 6/f

Question 9

What is the piezo-electric effect?

Answer 9

The piezoelectric effect converts mechanical energy into electrical energy via crystal deformation. Piezoelectric crystals deform when electrical energy is applied. The alternating electrical energy from the transducer causes the piezoelectric crystals to oscillate which generates mechanical ultrasound energy which is transmitted through the body. The reflected mechanical ultrasound energy from the body causes the piezoelectric crystals to oscillate which generates electrical energy which is detected by the transducer.

Question 10

What are the parts of the ultrasound transducer?

Answer 10

Transducers transmit and receive ultrasound.

The piezoelectric elements convert ultrasound (2D transducers have 128 and 3D transducers have 1000s).

The backing layer absorbs ultrasound energy to decrease reverberation/ringing of piezoelectric elements.

The matching layer improves the impedance between the piezoelectric elements and the body to decrease reflection.

The acoustic lens focusses the ultrasound.

The wire transmits information.

The case offers insulation and protection from interference.

Question 11

How do transducers transmit and receive ultrasound to create images?

Answer 11

The transducer transmits short bursts of ultrasound energy, waits, receives the ultrasound energy, and repeats. A small percentage of the ultrasound energy is reflected at interfaces and the transducer calculates the time between the ultrasound being sent and returned. It uses the time, and the propagation velocity, to calculate the distance between the transducer and the reflector. It uses the signal intensity to generate an image.

Question 12

What are the differences between 2D and 3D transducers?

Answer 12

2D transducers use a single plane of ultrasound waves.

3D transducers use multiple planes of ultrasound waves, and generate images with a higher spatial resolution but lower temporal resolution.

Question 13

What are the differences between linear array transducers and curved array transducers?

Answer 13

Linear array transducers organise the elements in a straight line, generate a rectangular image, with a narrower width and a lower depth, are higher frequency with a higher resolution, and may be used for paediatric echo.

Phased array transducers organise the elements in a curved line, generate a sector shaped image, with a wider width and a higher depth, are lower frequency with a lower resolution, and are used for adult echo.

Question 14

What are the Fresnel and Fraunhofer zones and what are their characteristics?

Answer 14

The Fresnel zone is the near zone. It is cylindrical. The near zone is narrow, high intensity, high resolution, and the length is dependent on the frequency.

The Fraunhofer zone is the far zone. It is diverse. The far zone is wide, low intensity and low resolution.

Question 15

How is the near zone affected in image optimisation?

Answer 15

In image optimisation, the higher the frequency and the wider the transducer diameter, the greater the near zone depth, so the higher the resolution.

Question 16

What are side lobes?

Answer 16

Side lobes are low intensity secondary ultrasound signals outside of the primary ultrasound beam.

Side lobes are secondary to energy which travels at different angles to the primary ultrasound pathway and which is reflected by strong reflectors outside of the primary ultrasound beam. This is due to diffraction.

Question 17

What is beam steering and what are the beam steering methods?

Answer 17

Beam steering methods direct and focus the beam.

Mechanical steering involves physically moving the ultrasound transducer. The types of mechanical steering are rotating/oscillating and wobbling array transducers. Rotating transducers involve rotating the transducer to sweep the beam through an area. Wobbling transducers involve a transducer on a motor which rocks back and forward to steer the beam.

Electronic steering involves controlling the timing of electrical energy delivered to the piezoelectric crystals in the transducer. The types of electronic steering include linear array, phased array and curvilinear array transducers. Linear array transducers simultaneously activate elements in the transducer to steer the beam in a linear direction. Phased array transducers use time delays to sequentially activate different elements in the transducer to steer the beam at different angles.

Question 18

What is focusing and what are the transducer focusing methods?

Answer 18

Focussing the ultrasound narrows the ultrasound in the near zone so increases the resolution. However, it widens the ultrasound in the far zone so decreases the resolution. Focussing the ultrasound will not affect the near zone length.

Fixed focusing involves using a fixed point and fixed time delays (limited to known low depths).

Dynamic receive focusing involves introducing time delays to adjust the returning ultrasound at different depths to increase the resolution and improve the image quality. A shorter time delay is required for echos at higher depths and a longer time delay is required for echos at lower depths.

Question 19

What is the focus position?

Answer 19

The focus position is the depth with the highest resolution. The transducer uses electronic focusing methods, in which the timing of sent and returned signals are adjusted, to improve the image quality.

Dual focus uses two focus positions simultaneously or sequentially to increase resolution at two positions, visualising near and far structures.

Question 20

What is ICE?

Answer 20

Intracardiac echocardiography (ICE) visualises the heart from within. It is high resolution and allows assessment of cardiac anatomy and physiology and real time guidance for cardiac procedures (e.g. EP and structural interventions).

Question 21

What is broadband imaging?

Answer 21

Broadband echo imaging uses transducers which transmit and receive ultrasound with a variety of frequencies. Broadband allows a high frequency variety, high resolution, high depth, improved tissue differentiation.

Question 22

What is harmonic imaging?

Answer 22

The reflected ultrasound includes ultrasound at the frequency of the original ultrasound and harmonics (multiples of the original ultrasound frequency).

Second harmonic imaging filters the returning ultrasound to remove the original frequency to generate an image using the second harmonics only. The increased frequencies increase the resolution.

Harmonic imaging decreases noise and artefact and increases the image resolution and quality, particularly for far field structures.

Question 23

What are M-mode and curved M-mode and what are their advantages?

Answer 23

M-mode images the movement on one line. The M-mode zone is narrow so the PRF, and therefore FR, are high. Therefore, M-mode offers a high temporal resolution so improves the assessment of fast moving structures (e.g. valves).

CAMM utilises M-mode and 2D to allow the M-mode line to follow the shape the structure to improve the anatomical and physiological assessment of the structure.

Question 24

What are the pulse repetition frequency, frame rate and scan lines per second?

Answer 24

The PRF is the number of pulses (transmitted by the transducer) per second, measured in Hz.

The FR is the number of frames (images) per second, measured in fps.

Scan lines per frame is the number of beams (lines) required to form an image (frame).

Question 25

What is the relationship between depth and width and PRF and FR, and what are their effects on resolution?

Answer 25

In 2D, the lower the image depth, shorter the distance between the transducer and the sample volume, the shorter the period of time to transmit and receive the ultrasound, so the higher the PRF. In 2D, the narrower the sector width, and the lower the image depth, the fewer the number of scan lines, the shorter the period of time to create a frame, so the higher the FR. The higher the PRF and FR, the higher the temporal resolution but the lower the spatial resolution.

Question 26

What is parallel processing?

Answer 26

3D transducers are able to parallel process by processing the scan lines simultaneously not sequentially. Parallel processing increases the number of scan lines acquired and processed, increasing the FR, increasing the temporal (and spatial) resolution.

Question 27

What are the types of resolution and how are they optimised?

Answer 27

Temporal resolution (FR) is the ability to discriminate between events close in time. The narrower the width and the lower the depth, the higher the temporal resolution. Spatial resolution is the ability to discriminate between structure close in space. The types of spatial resolution are lateral, axial and elevation. Axial resolution is the ability to differentiate between structures on the same axis of the ultrasound. The higher the frequency and the shorter the pulse duration, the higher the axial resolution. Lateral (azimuthal) resolution is the ability to differentiate between structures parallel to one another but perpendicular to the ultrasound. The narrower the ultrasound beam (optimised by focussing the ultrasound) and the lower the gain, the higher the lateral resolution. Elevation resolution is the ability to differentiate between structures with different depths in the elevation direction (z axis), perpendicular to the axial and lateral directions.

Question 28

What is greyscale compression and what is its effect on the image?

Answer 28

Greyscale compression (dynamic range) adjusts the number of shades of grey in the image to adjust the contrast. The higher the dynamic range, the higher the number of shades of grey, the lower the contrast, the higher the resolution. A low dynamic range in used in patients with a high BMI.

Question 29

What are the types of artefacts?

Answer 29

An artefact is the presence of structures on the image but the absence in the heart or vice versa. The types of artefacts are acoustic shadowing, reverberation, beam width and side lobe. Acoustic shadowing is due to the presence of a highly echo reflective structure (e.g. prosthetic mechanical valve) which blocks ultrasound penetration which causes echo dropout (black) in the far field zone. Reverberation artefacts are due to the ultrasound bouncing multiple times between two strong specular reflectors. This delays the return of the ultrasound to the transducer so it misinterprets the location of the structure to be further from the transducer than it truly is. This causes “ghost” images which move with the true structure. Beam width artefacts are due to the machine being unable to differentiate if the returning ultrasound signal is from the centre or the edge of the ultrasound beam. Therefore, highly reflective structures at the edge of the beam in the heart are present at the centre of the beam on the image. Focussing the beam minimises beam width artefacts. Side lobe artefacts are due to side lobe beams (extra beams at the side of the primary beam). Therefore, structures outside of the beam in the heart are present at the centre of the beam on the image. Focussing the beam minimises side lobe artefacts.

Question 30

What are the factors affecting image quality?

Answer 30

The limiting factors are optimisation (width and depth), frequency (wavelength), PRF and FR and lines, characteristics of structures (propagation velocity, acoustic impedance, acoustic impedance mismatch, attenuation), reflection and scatter, focusing and steering, spatial resolution (axial, lateral, elevation) and temporal resolution, artefacts (shadowing, reverberation, side lobe, beam widths) and signal to noise ratio.

Question 31

What are the echo machine controls for image optimisation (power, gain and TCG)?

Answer 31

Transmit power is the level of ultrasound energy transmitted to the patient. The lowest transmit power setting should be used to minimise the risk of mechanical or thermal effects. Gain is the level of signal amplification of the received ultrasound energy to increase the brightness of the image. There is overall gain which increases the gain of the whole image and there are TGCs which increase the gains for part of the image. Gain increases the intensity of low-level signals but increases noise and decreases lateral resolution. Time gain control (TGC) (depth compensation control) compensates for the attenuation of the ultrasound at greater depths. TCG increases the gain of far field signals to create a uniform level of brightness for the image.

Question 32

What are the echo machine controls for image optimisation (logarithmic compression, greyscale compression and processing)?

Answer 32

Logarithmic compression (a method of audio processing) adjusts the dynamic range of the audio so quiet sounds are louder and louder sounds are quieter. Rejecting logarithmic compression maintains the raw dynamic range of the audio. Greyscale compression (dynamic range) (a method of visual processing) adjusts the number of shades of grey in the image to adjust the contrast. The higher the dynamic range, the higher the number of shades of grey, the lower the contrast, the higher the resolution. Pre-processing, processing and post-processing involves processing the image before, during and after image acquisition.

Question 33

What are the echo machine controls for image optimisation (frequency, angle gamma correction)?

Answer 33

The frequency is the number of waves per second. Increasing the frequency decreases the wavelength which increases the resolution but decreases the penetration. The angle is alignment of the ultrasound to the structure. Increasing ultrasound alignment increases signal quality. Gamma correction is a non-linear process which adjusts the brightness and contrast of the images. Decreasing the gamma increases contrast of bright signals and increasing the gamma increases the contrast of dark signals.

Question 34

Why is ultrasound gel used?

Answer 34

Gel fills the air between the transducer and the skin so minimises the acoustic impedance mismatch to maximise ultrasound transmission but gel increases the risk of infection.

Question 35

What are spatial and temporal smoothing?

Answer 35

In spatial and temporal smoothing, a filter is added decrease noise and motion artefacts and increase image quality and uniformity over space and time respectively.

Question 36

How are patients positioned to optimise the images?

Answer 36

Position the patient in the left lateral decubitus position for the parasternal and apical views and the supine position for the subcostal and suprasternal views.

Question 37

How are probes manipulated to optimise the images?

Answer 37

Sweeping (movement on the short axis up and down), sliding (movement on the long axis left and right), rotating (clockwise and counterclockwise), tilting (movement in the short axis forwards and backwards e.g. anterior and posterior) and rocking (movement in the long axis side to side e.g. medial and lateral).

Question 38

How are echo machine controls adjusted to optimise the images?

Answer 38

Increase the depth of the near field zone, decrease the width, decrease the depth, focusing the ultrasound, adjust the gain and TGC, adjust the greyscale compression, decrease the CFD box size and decrease the CWD and PWD scale.

Question 39

What are the standard and non-standard TTE views?

Answer 39

The standard views are the PLAX, RV inflow, RV outflow, PSAX (AV, MV, basal, mid and apical levels), A4C, A5C, A2C, A3C, subcostal and suprasternal. The non-standard views are the high parasternal view to assess the ascending aorta, the subcostal short axis view may used to assess the heart if the PSAX view is suboptimal, the modified subcostal view to assess the hepatic veins and abdominal aorta and the right parasternal view, in the right lateral decubitus position, to assess the ascending aorta and AS.

Question 40

How are images stored and displayed?

Answer 40

The data is acquired, processed and stored. At the transducer, the returning echo signal is processed via amplification, TGC and filtering. At the scan converter, the ultrasound signal is converted to a digital signal and the video signal is converted to a rectangular signal. The data is further processed (post-processing) to be stored in a digital format and/or undergoes digital to analogue processing to create a video signal to be shown on the monitor and/or stored in a videotape or archived on optical discs or hard drives. Archiving offers high data storage, fast data transfer and post-processing. The display devices and display controls are adjusted to allow image analysis.

Question 41

What are the considerations of image storage and display?

Answer 41

Digital Imaging and Communications in Medicine (DICOM) standardises data storage, allowing data to be transferred between systems. Data compression decreases file size for storage and transfer but maintains image quality for analysis. There is the ability to choose the type and number of cardiac cycles, time based or ECG based image acquisition, the ability to review acquired loop and the ability to adjust and quantify images retrospectively.