2010B11 Explain the physical principles of ultrasound imaging.

 

List:

·      Basics (7)

·      Tissue interaction (4)

·      Doppler (4)

 

Basics:

Definition

·   Wave of pressurization and rarefaction through a medium >20kHz (probes 2-15MHz)

Equations

·   Speed = distance/time

·   Speed of sound = wavelength / period (where period = 1/frequency)

Probe

·   Curved or linear array of piezoelectric crystals (PZT – lead zirconate titanate)

·   Gel between probe and skin (eliminates reflection at air-skin interface)

Piezoelectric effect

·   Emission: alternating current (AC) -> crystal oscillation -> US wave

·   Detection: reflected US wave -> oscillation -> AC transduction -> computer display

Intermittency

·   Emit, detect, repeat many times per second

Latency

·   distance from probe

Intensity

·   % reflected difference in density or acoustic impedance at tissue interface

·   Water density 1000kg/m3, fat 952kg/m3, muscle 1078kg/m3

Resolution

·   Temporal

·   Spatial (axial and lateral)

·   Contrast

 

Tissue interaction:

Wave fate

·   Reflection -> detection

·   Transmission

·   Diffraction

·   Scattering

·   Absorption as heat

Acoustic attenuation

·   Decay in amplitude with passage through a tissue due to absorption, scattering etc.

·   Units dB/m. Blood 20, fat 60, muscle 180.

·   Equation: I = I0e-ax

o x = distance travelled

o a = attenuation coefficient

§ frequency

§ viscosity

§ 1/elasticity

·   (Note machine may perform time compensation, i.e. correct for acoustic attenuation)

Acoustic impedance

·   Degree of resistance to passage of US wave

·   Unit Rayl (Z)

·   Z = c x density

·   Measure: half power distance

·   <0.1cm air, <1cm bone, 380cm water (Hence gel on skin)

·   Difference in acoustic impedance reflection

·   Reflection coefficient = [(Z2-Z1)/(Z2+Z1)]2

Resolution vs depth

·   ↑ frequency -> ↑tissue interactions per distance ->

o ↑Resolution

o ↑Attenuation -> ↓depth

·   ↓frequency -> opposite

 

Doppler:

Effect

·  Frequency at detector ↑/↓ if relative motion of source towards/away from detector

Use

·  Demonstrate flow and direction of flow

·  Calculate velocity (e.g. aortic jet)

·  Calculate distance by integration (e.g. VTI)

Equation

V = F x C /2F0cosθ

·  F: change in frequency from emission to detection

·  C: speed of sound

·  2x: due to round trip of ultrasound wave

·  F0: emitted frequency

·  cosθ: correct for incident angle

Subtypes

·  Colour mode: recognize movement e.g. arterial flow during IJV cannulation

·  Pulse wave: depth differentiation but not good for high velocity (e.g. LVOT VTI)

·  Continuous wave: for high velocity but no depth differentiation (e.g. LVOT PSV)

 

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