Ultrasound

+

Basic Ultrasound PhysicsDr. A. Ferguson

+Objectives

Level 1 Knowledge Components

Ultrasound physics, terminology, and safety

Equipment care, ultrasound techniques, and controls

+The transducer

Damper material

Piezoelectric crystal(sends and receives)

Acoustic lens

Impedancematching to skin

CABLE

+The ultrasound wave

Am

plit

ude (

dB

)

Wavelengthl

1 second: cycles/second = frequency(Hz)Clinical use varies from 2.5-20MHz

Velocity m/s1540 m/s approx

Delivered in pulses (bursts)• Length of pulse varies• Frequency of pulse varies

V = fl

+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

+The ultrasound beam

Near zone

Unfocused transducer

Divergenceangle

Side lobes

Beam width

Focal zone

Focused transducer

+Ultrasound beam lobes

Feldman M K et al. Radiographics 2009;29:1179-1189

+ResolutionAxial (along length of beam) – most precise

Smallest resolvable distance = 2 x l 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)

+Ultrasound/tissue interaction

Transducer

Scattering

Reflection

Refraction

Attenuation

Tissue interface

Skin

1 2

3 4

+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

+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

+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

+Refraction

Waves deflected passing through interface

Can be useful in focusing US waves

Results in artefacts

+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

+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

+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

+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

+Beam-width artifact


Grey dot assumed to be in main beam areaArea of interest outside focal zone

Adjust focal zone

Grey dot outside beamArea of interest inside focal zone

+Side-lobe artifact


Black dot signal may return from multiple side-lobes resulting in duplication on screen

+Reverberation artifact


US bounces back and forth between two strong reflectors

+Ring-down artifact


Ring of bubbles with fluid trapped centrally. Fluid vibrations detected as strong signal and displayed as line behind true source.

+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.

+Speed-displacement artifact


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.

Discontinuous diaphragm sign

+Refraction artifact


Refraction at an interface between two objects makes the deeper object appear in false location.

+Acoustic shadowing


Strong attenuator means weak beam beyond = shadow

+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

+ Probe typesSector Linear array Curved array

+ Use of Gain

GainMin

Max

Near field Far field

Attenuation

Time-gain compensation (TGC)

Pro

cess

ed

Ori

gin

al

Documents

Ultrasound