2010B11 Explain the physical principles of ultrasound imaging.



·     Ultrasound waves

·     Ultrasound machine

·     Tissue interactions

·     Imaging properties

·     Doppler effect


Ultrasound waves:


·   Wave of pressurization and rarefaction through a medium

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

·   Speed is ~1540m/s through tissue


·   Speed = distance/time

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


Ultrasound machine:


·   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)


Tissue interactions:

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)


Image properties:


·   Time to detection distance from probe


·   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


·   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)

Doppler effect:


·  Apparent frequency ↑/↓if source is moving towards/away from probe


·  Demonstrate flow and direction of flow

·  Calculate velocity (e.g. aortic jet)

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


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.


·  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)



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