2010B11 Explain the physical principles of ultrasound imaging.

 

List:

·     Ultrasound waves

·     Ultrasound probe

·     Tissue interactions

·     Basic measurement

·     Doppler imaging

 

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

Modes

·   B (brightness) mode: an ice-pick view (not used)

·   M (motion) mode: an ice-pick view across time (best for timing e.g. FS)

·   2D mode: single imaging plane, either video or still (best all-purpose)

·   3D mode (for complex valve lesions and babies)

 

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:

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

Resolution

Types of resolution:

·   Temporal (frame rate): best if simple mode and shallow field of view

·   Axial: best if shallow field of view (since higher frequency)

·   Lateral: best if narrow field of view (since less dispersal)

·   Contrast: user-adjustable

Optimal resolution:

·   Simple mode (less processing)

·   Shallow field (higher frequency -> more interactions per unit depth)

·   Narrow field (less dispersal -> more interactions per unit width)

·   Focal point (junction between near and far field; only if phased array)

·   Waves hit object at 90 degrees (greatest reflection back toward probe)

·   Waves hit probe at 90 degrees (least reflection away from probe)
(Cf. doppler imaging: accuracy best if waves in line with motion)

tl;dr put the object of interest in the middle of screen

Trade off

·   Frequency tissue interactions attenuation

·   ↑Freq: ↑resolution but ↓depth

·   ↓Freq: ↓resolution but ↑depth

 

Doppler effect:

Definition

·  Apparent frequency ↑/↓if object 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)

 

 

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