2010B11 Explain the physical principles of ultrasound imaging.

 

List:

·     Ultrasound waves

·     Ultrasound machine

·     Tissue interactions

·     Imaging properties

·     Doppler effect

 

Ultrasound waves:

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)

 

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. Water 5, Blood 20, fat 60, muscle 180.
(difference gives rise to posterior acoustic enhancement)

·  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: Rayl (Z) = Pa.s/m

·  Impedance (Z) = velocity x density

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

(i.e. reflection ∝ Δ acoustic impedance – e.g. air & skin, hence gel)

 

Image properties:

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)

Doppler effect:

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)

 

 

Feedback welcome at ketaminenightmares@gmail.com