Fundamental Ultrasound Principles Karen Potts Clinical Scientist Review date Jan 2010 Department of Medical Physics Kent & Canterbury Hospital

Why choose ultrasound as a imaging modality Real time imaging Flexible can react to results Patient tolerate ultrasound well Generally non invasive Non ionising radiation Relatively inexpensive

Why do we need to understand the physics? Acquiring the ultrasound image is very operator dependant compared to other modalities. Understanding the physics –helps to optimise image –recognise artefacts –Be aware of limitations

Sonar Principle

Sound Waves

Waves The movement of individual particles that make up the wave can be separated from the wave movement itself The movement of individual particles that make up the wave can be separated from the wave movement itself Here the particle moves up and down while the wave moves sideways

Sound Waves Sound moves in longitudinal waves The molecules are displaced in the same direction as the wave Where particles are gathered together in a compression the pressure is increased. Where particles are stretched apart at a rarefaction the pressure is lower.

Frequency Frequency is the number of waves that pass any point per second. A frequency of 1 Hertz is one wave travelling past per second.

Wavelength Wavelength is the distance between wave peaks (or troughs). It is easier to measure if we freeze wave motion

Wave Properties The frequency is the number of wavelengths per second measured in Hertz (Hz) Wavelength– length of one complete wave cycle in metres (sometimes called landa ) Pressure

Frequency and Wavelength Frequency and wavelength are locked together If wavelength gets smaller frequency gets higher Number of wave cycles

Higher frequency better detail Small wavelength means you can pick up greater details Higher frequency means better resolution (detail)

Frequency 12MHz (harmonics)

Frequency 8MHz

Ultrasound The human ear responds to frequencies in the range from 20 to Hz (or 20 kHz) Frequencies above 20kHz are known as ultrasound Imaging Ultrasound uses Mega Hertz Bats can hear up to about 60kHz

Ultrasound Attenuation Attenuation  is exponential

Higher frequency lower penetration For identical input intensity a lower frequency will penetrate further before being attenuated to the same output intensity Input intensity Output intensity Input intensity

Images taken with T3000 1) VH or very high frequency 2) VL or very low frequency

Ultrasound Typical ultrasound machine works at 60dB This means an output signal can be sent out Be heavily attenuated until it reaches a target Be weakly reflected depending on the target material Be attenuated again on the way back Only one millionth of the output signal gets back And we still get an image

Where possible change frequency to improve image Typical Ultrasound Frequencies: Deep Body 1.5 to 3.0 MHz Superficial Structures 5.0 to 15.0 MHz Specializedup to 20MHz If you cannot get a good image at depth: lower frequency If you have depth but want more detail: increase frequency

Wave Properties These wave parameters are related Note that for constant speed the wavelength Note that for constant speed c the wavelength –gets shorter if the frequency is increased –gets longer if the frequency is decreased

Interaction of wave and medium The speed through a medium is fixed by the bulk modulus ( and density of a material. The speed c through a medium is fixed by the bulk modulus B (ratio of the change in pressure to the fractional volume compression)and density   of a material.

Speeds of sound The speed of sound c in a given medium remains fixed. Therefore… if decreases f increases (and vice versa) Medium Air Aluminium Water Caster Oil Fat Liver Muscle Soft tissues (average) Speed (ms -1 ) (Temp. = 22˚C)

Waves Which wave will reach the other end quicker? The speed of a wave is fixed in any given medium.

Doppler principles Source of moving sound produces higher frequency waves in front and lower frequency behind. Source of moving sound produces higher frequency waves in front and lower frequency behind.

Doppler

Doppler

Doppler l Note that the most pronounced change l in wavelength or frequency is along l the direction of travel

Ultrasound Doppler v C = f o. o Blood cells moving at velocity v are interrogated by a sound wave moving at velocity c Blood cells moving at velocity v are interrogated by a sound wave moving at velocity c

The Doppler Equation The Doppler shift frequency D f is related to target velocity V by the equation f 0 = doppler carrier frequency f 0 = doppler carrier frequency c = ultrasound propagation velocity c = ultrasound propagation velocity  = doppler angle  = doppler angle

Thank you Please feel free to ask questions