1. +
Basic Ultrasound Physics
Dr. A. Ferguson

2. +
Objectives
 Level 1 Knowledge Components
 Ultrasound physics, terminology, and safety
 Equipment care, ultrasound techniques, and
controls

3. +
The transducer
Acoustic lens
Impedance
CABLE matching
to skin
Damper material
Piezoelectric crystal
(sends and receives)

4. +
The ultrasound wave
Wavelength
Amplitude (dB)
V=f
Velocity m/s
1540 m/s approx
Delivered in pulses (bursts)
• Length of pulse varies
• Frequency of pulse varies
1 second: cycles/second = frequency(Hz)
Clinical use varies from 2.5-20MHz

5. +
Velocity in tissue
Medium Velocity of US (m/sec)
Air 330
Fat 1450
Water 1480
Soft tissue 1540
Kidney 1560
Blood 1570
Muscle 1580
Bone 4080

6. +
The ultrasound beam
Beam width
Near zone
Focal zone
Divergence
angle
Side lobes
Unfocused transducer Focused transducer

7. +
Ultrasound beam lobes
Feldman M K et al. Radiographics 2009;29:1179-1189

8. +
Resolution
 Axial (along length of beam) – most precise
 Smallest resolvable distance = 2 x
 Higher frequency = better resolution
 Independent of depth
 Lateral (across beam)
 Varies with depth
 Within focal zone may be as good as axial
 Elevational (within the slice)
 Slice might be 3-8mm wide with some probes
 Strong reflectors at edges may appear in centre
 Contrast (shades of gray)

9. +
Ultrasound/tissue interaction
Transducer
Skin
1 2
Scattering Reflection
Tissue interface
3 4
Refraction Attenuation

10. +
Scattering
 Structures with radius < wavelength scatter US
 e.g. RBCs and micro-structures within tissues
 Scattering is multidirectional
 Only small portion of incident US gets back to probe
 Scattering from RBCs contributes to DOPPLER effect
 Tissue scattering results in speckled appearance

11. +
Reflection
 Critical to image generation
 Depends on:
 Angle of beam relative to tissue
 Change in acoustic impedance* across boundary
 Smooth tissue boundaries act almost as mirrors
 Called “specular reflectors” e.g. pleura
* Acoustic impedance = tissue density x US velocity in the tissue

12. +
Acoustic impedance
Medium Acoustic impedance*
Air 0.0004
Lung 0.18
Fat 1.34
Liver 1.65
Blood 1.65
Kidney 1.63
Muscle 1.71
Bone 7.8
* x106 Rayls

13. +
Refraction
 Waves deflected passing through interface
 Can be useful in focusing US waves
 Results in artefacts

14. +
Attenuation
 Loss of US energy as it passes through tissue
 Depends on
 Attenuation coefficient of tissue
 Frequency of transducer
 Distance from transducer
 Intensity of transmitted US
 AIR has a very large attenuation coefficient
 Lower frequencies penetrate better than high

15. +
Attenuation values
Medium Half-power distance (cm)
Water 380
Blood 15
Soft-tissue (non-muscle) 1-5
Muscle 0.6-1
Bone 0.2-0.7
Air 0.08
Lung 0.05

16. +
Image artifacts
 Poor image quality
 Images of structures that are either
 Not there at all
 Present in a different location than image suggests
 Lack of visualisation of structures
 Images that differ in size or shape from reality
 Some artifacts are clinically useful

17. +
Image artifacts
 Acoustic shadowing
 Acoustic enhancement
 Refraction artifact
 Reverberation artifact
 Comet-tail artifact
 Mirror-image artifact
 Ghosting artifact
 Beam-width artifact
 Ring-down artifact
 Speed-displacement artifact

18. +
Beam-width artifact
Adjust focal zone
Grey dot assumed to be in main beam area Grey dot outside beam
Area of interest outside focal zone Area of interest inside focal zone
Feldman M K et al. Radiographics 2009;29:1179-1189

19. +
Side-lobe artifact
Black dot signal may return from multiple side-lobes
resulting in duplication on screen
Feldman M K et al. Radiographics 2009;29:1179-1189

20. +
Reverberation artifact
US bounces back and forth between two strong reflectors
Feldman M K et al. Radiographics 2009;29:1179-1189

21. +
Ring-down artifact
Ring of bubbles with fluid trapped centrally. Fluid vibrations detected as strong
signal and displayed as line behind true source.
Feldman M K et al. Radiographics 2009;29:1179-1189

22. +
Mirror-image artifacts
US beam bounces between structure and deeper strong reflector e.g. diaphragm.
This means probe receives signals as if from same object on other side of reflector.
Feldman M K et al. Radiographics 2009;29:1179-1189

23. +
Speed-displacement artifact
Discontinuous diaphragm sign
Part of beam encounters tissue where velocity is much lower than 1540 m/s,
e.g. fat. Returning signal appears to come from deeper in body.
Feldman M K et al. Radiographics 2009;29:1179-1189

24. +
Refraction artifact
Refraction at an interface between two objects makes the deeper object
appear in false location.
Feldman M K et al. Radiographics 2009;29:1179-1189

25. +
Acoustic shadowing
Strong attenuator means weak beam beyond = shadow
Feldman M K et al. Radiographics 2009;29:1179-1189

26. +
Acoustic enhancement
Signal behind weak attenuator is stronger than at same level in adjacent tissues.
Gives impression of brighter structures deep to low attenuator =enhancement
Feldman M K et al. Radiographics 2009;29:1179-1189

27. + Probe types
Sector Linear array Curved array

28. + Use of Gain
Near field Far field
Attenuation
Original
Max
Gain
Min
Processed
Time-gain compensation (TGC)