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)
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
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