722 Module 1

Echocardiographic principles

Attenuation

Reduction in amplitude and intensity. Increases with depth and frequency. Occurs due to absorption, reflection, scattering, diffraction and refraction.

Absorption

Converts energy from the sound beam into heat.

Processes that redirect the beam causing structures to be displaced or not in view:

diffraction and refraction

Processes that help in ultrasound image production:

reflection and scattering

What factors influence attenuation:

Properties of the medium (attenuation coefficient)

Distance travelled (cm)

frequency of the sound beam (MHz)

Attenuation equation

a (dB/cm) x L (cm) x f (MHz)

Decibels (dB)

Relative difference between two acoustic signals of different intensities based on a log scale.

Decibel equation

= 10 log (intensity of the beam at any point) / (initial intensity of the beam)

What factors determine absorption?

medium viscosity

medium relaxation time

frequency of sound

What factors influence reflection?

acoustic properties of the two tissues

angle of incidence

reflecting surface

What determines the amplitude of a reflected sound beam?

The difference in acoustic impedance between two materials. Greater impedance difference = increased reflection of the incident beam

Equation for reflection amplitude

R = (Z2 - Z1) / (Z2 + Z1)

Specular reflector

Large smooth surface (eg. epicardium) that produces strong echoes

Non-specular reflector

Rough surface (eg. myocardium) that causes scattering of the sound beam, meaning only some echoes return creating granular images.

Rayleigh scatters

Occurs when the sound waves encounter structures smaller than their wavelength (eg. RBCs) causing the energy to scatter equally in all directions leading to weak reflections resulting in grainy ‘speckle’ texture in the images.

Refraction

Bending of the sound beam when the sound beam passes into a tissue with different speed of sound. Causes structures to be misplaced in ultrasound.

Snell’s law equation

Sinθi / Sinθt = c1/c2

C1 = C2

Beam travels in a straight path, no refraction.

C1 > C2

Fast to slow. Angle of incidence is greater than angle of transmission.

C1 > C2

Slow to fast. Angle of incidence smaller than angle of transmission.

Diffraction

When the sound wave passes through an opening or around a barrier causing the sound beam to spread out and intensity decreases.

Piezoelectric transducer

Converts electrical signals into acoustic energy and vice versa. Transmitter and a receiver. Crystal expands and contracts.

Range equation

D = C x t / 2

(D = depth to reflector

C = propagation speed

t = time taken for signal to return to probe)

Spatial resolution

Ability to detect echoes from two closely spaced reflectors and to display them as being separate. Divided into axial and lateral.

Axial resolution

Ability to distinguish structures parallel to the beam. Improved with shorter pulse duration. Increasing the frequency decreases spatial pulse length.

Lateral resolution

Ability to distinguish side by side structures, perpendicular to the beam. The more narrow the sound beam the better the lateral resolution. (ie. by focusing the beam or using a high frequency transducer)

Contrast resolution

Ability to differentiate differences in echogenicity between structures and display those as different shades of grey. Improved by: image optimisation, reduction of artefacts, native tissue harmonics.

Temporal resolution

Ability to accurately show the position of moving structures over time. Increased frame rate = increased temporal resolution. Enhanced by: decreasing imaging depth (increases PRF), narrowing sector width (reduces number of scan lines) and using only one focal zone.

Frame rate

Number of times a sweep of the sound beam is done per second by the transducer.

Frame rate equation

FR = PRF/n

(pulse repetition frequency/number of lines per frame)

Range equation rearranged for t

t = 2 x D / c

time for one line

Frame rate using range equation

FR = c / (2 x D x n)

Axial resolution equation

spatial pulse length (SPL) / 2

Spatial pulse length equation

SPL = wavelength x number of cycles in a pulse

Relationship between frequency and wavelength

Inversely proportional.

Decreasing the wavelength increases the frequency.

Beam width equation

BW = (2.4 x wavelength x focal length) / diameter of the element

Relationship between beam width and lateral resolution

Inversely proportional. Decreasing beam width increases lateral resolution.

Harmonics

Multiples of the fundamental frequency.

Dynamic range/compression

Determines the range of echo intensities displayed and how they are mapped to the grey scale. Wider dynamic range = more shades of grey and vice versa.

Focal zone

Where the ultrasound beam is narrowest and therefore lateral resolution is best.