2010B11 Explain the physical principles of ultrasound imaging.

· Ultrasound waves

· Ultrasound machine

· Tissue interactions

· Imaging properties

· Doppler effect

Definition

· Wave of pressurization and rarefaction through a medium

· Frequency is >20kHz (but probes are 2-15MHz)

· Speed is ~1540m/s through tissue

Equations

· Speed = distance/time

· Propagation velocity = wavelength / period
(where period = 1/frequency)

Ultrasound machine:

Probe	· Array of piezoelectric crystals (PZT = lead zirconate titanate) · Matching layer (reduces impedance gradient between probe and body) · Acoustic lens (focuses waves toward field) · Damping block (attenuates oscillation of crystals) · Metal and plastic housing (protection and acoustic insulation)
Piezoelectric effect	· Emission: alternating current -> crystal oscillation -> emitted US wave · Detection: reflected US wave -> crystal oscillation -> alternating current · Emission and detection are repeated many times per second
Regular modes	· A (amplitude): one crystal, display graph of amplitude vs depth (rare use) · M (motion): one crystal, display ‘ice pick’ vs time (for timing, e.g. TAPSE) · B (brightness): array of crystals, 2D display; still or real-time · 3D: 2D images-> volumetric dataset; still or real-time
Doppler modes	· Colour: indicate direction and velocity (not quantity) of flow (e.g. regurg) · Pulse wave: measure velocity at a specified location (e.g. LVOT VTI) · Continuous wave: measure all velocities along a line (e.g. through the AV)

Wave fate

· Reflection -> detection by probe

· Transmission

· Diffraction

· Scattering

· Absorption as heat

Acoustic attenuation

· Decay in amplitude with passage through a tissue

· Occurs due to absorption, scattering etc.

· Can be overcome by ‘time gain compensation’

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

· Attenuation (I) = I₀e^-ax

o x = distance travelled

o a = attenuation coefficient

§ ∝ frequency

§ ∝ viscosity

§ ∝ 1/elasticity

Acoustic impedance

· Degree of resistance to passage of US wave

· Units: half power distance (<0.1cm air, <1cm bone, 380cm water)
High impedance of air -> use ultrasound gel

· Impedance (Z) = velocity x density

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

(i.e. reflection ∝ Δ acoustic impedance)

Latency	· Time to detection ∝ distance from probe
Intensity	· Intensity of signal ∝ amount reflected · Reflection occurs if Δ acoustic impedance = Δ tissue density · Water density 1000kg/m³, fat 952kg/m³, muscle 1078kg/m³ · Reflection back to probe is greatest if object is perpendicular to USS waves
Resolution	· Temporal: shallow field (↓latency), narrow field (↑pulse repetition freq) · Axial: ↓depth (↑frequency -> ↑tissue interactions) · Lateral: ↓sector width (↑line density*), focal point (i.e. USS convergence)
Trade off	· Frequency ∝ tissue interactions ∝ attenuation · ↑Freq: ↑resolution but ↓depth · ↓Freq: ↓resolution but ↑depth

(*phased array transducer only)

Definition	· Apparent frequency ↑/↓if source is moving towards/away from probe
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 · V: velocity · ∆F: change in frequency between emission and detection · C: speed of sound · 2x: ultrasound wave does a return trip · F₀: emitted frequency · cosθ: correction for incident angle N.B. ultrasound does not correct for angle. Need <20° angle for <6% inaccuracy.
Modes	· Colour mode: recognize abnormal motion (e.g. valvular regurgitation) · Pulse wave: specified depth, low velocity (e.g. LVOT VTI) · Continuous wave: unspecified depth, high velocity (e.g. LVOT PSV)