The physics of ultrasound Dr Graeme Taylor Guy s & St Thomas NHS Trust
Physics & Instrumentation Modern ultrasound equipment is continually evolving This talk will cover the basics
What will be covered? The basic properties of ultrasound Generating and detecting it The common imaging modalities A, B & M mode Spectral Doppler Colour flow & Power Doppler Safety issues
What is ultrasound? Just a normal sound wave but at a higher frequency than we can hear (>1 MHz) It is a pressure wave which travels through a medium In medical imaging, longtitudinal waves are commonly used
What are its major properties? It travels through a medium with a characteristic wave velocity It reflects off interfaces between media of different acoustic impedance It refracts (bends) when passing obliquely between media of different sound velocity It scatters in media containing small objects It is absorbed by most media
Sound velocities Air about 300 m/s Water about 1450 m/s Soft tissue about 1560 m/s Fat, muscle, blood all have slightly different velocities Bone about 3000 m/s It takes about 6 μsec for sound to travel through 1 cm of soft tissue
Sound reflection When sound travels across a junction between two media, some bounces back and some travels onward This depends on the product of the density ρ and sound velocity c of each medium ( its impedance = ρc) As the impedances differ more, so more is reflected and less transmitted. No sound travels across air-tissue boundary
Sound refraction Unlike X rays sound does not always travel in a straight line though imagers assume that is what s happening When passing at an oblique angle between media with different velocity, the sound can change direction (like light) This can distort the ultrasound image Important when using it to guide interventions!
Scattering Scatter (sound bouncing off in all directions) occurs when a medium contains lots of small (<1 wavelength) items of a different impedance. This is what gives tissues their greyness on an ultrasound image (Wavelength at 1 MHz = 1.5mm)
Absorption This is where energy is lost to the tissues It means deeper structures return weaker echoes Soft tissue absorption is approximately proportional to frequency Bone has high absorption, nearly proportional to the square of frequency To image deep structures use lower frequency
Absorption the consequences All imagers need to compensate for absorption with depth Imaging close to the surface can use higher frequencies (up to 10 MHz) Large folk need to be imaged at lower frequency Imaging through bone (skull) is only practicable at 2 MHz or less Resolution Depth compromise
Imagers some available modes B Scan Colour flow mapping Spectral Doppler
Imagers not forgetting M mode B Scan M mode Used to visualise the movement of moving reflecting objects
The basis of imaging - echo ranging The fundamental principle is that of Directional pulse-echo location The distance to a sound reflector is proportional to the delay in the echo Depth α delay time = distance / sound velocity
Instrumentation A Scan To make a successful A (amplitude) Scan tracing one needs: a short ultrasound pulse a narrow ultrasound beam A scan Echo Amplitude Time
Instrumentation B Scan To make a successful B (brightness) Scan image one also needs: a way to steer/move beam to cover a two dimensional area All scanners assume some fixed sound velocity is it correct for your patient
What frequency to use? Shorter sound pulses have better resolution but higher frequency content So the shortest pulse is limited by the absorption of sound in tissue To obtain echoes from deep structures one needs to use low frequency pulses 1-4 cm deep use 7-12 MHz 10-20 cm deep use 2-5 MHz (Guide depth resolution ~ 2 wavelengths)
How big a transducer? The beam width is defined by the shape of the active transducer A small transducer has a narrow beam close up, but widens out & weakens quickly A trade off between resolution and penetration
Instrumentation Swept gain Swept gain or depth gain compensation must be used to compensate for quieter distant echoes Echo amplitude Depth This is inbuilt in all scanners but assumes some average attenuation which may be wrong Gain increased with time after transmission
Practical Transducers The all electronic transducer is the basis of the modern ultrasound B scanner No moving parts Array of small elements Wide frequency range Electronically controlled
Instrumentation - transducers The all electronic transducer is a large array of small elements Each is individually wired to the scanner
Transducers Each element is a simple piezo-electric transducer electrical sound electrical They are not individually directional They must work in unison
Instrumentation - transducers The transducer is made directional by a beam former an electronic process which synchronously controls the action of many of the independent elements simultaneously To make a simple forward looking directional beam - just use plenty of elements together
Instrumentation linear array To model a rectilinear scanner just repeat pulse-echo cycle with different sets of active elements Linear array
Instrumentation phased array To model a sector transducer, one only needs to add short time delays between firing elements and also on receiving ehoes. sector This effectively bends the beam Repeating the pulse-echo sequence with differing delays gives a sector scan
Focal zones It is possible to narrow the beam width at a certain depth for better resolution This is done by adding symmetrical time delays to inner elements in the group selected when transmitting and receiving Greater delay difference = closer focus
Multiple focal zones Can optimise beam width of pulse for a given depth range (the focal zone) To improve resolution at various depths, make separate images, each with different focal zone placement, then cut-&-paste This reduces the frame rate
Real-time compounding If one can CHANGE DIRECTION of the transducers line-of-sight, one can interrogate tissue from different directions & combine to give better image Removes speckle improves echoes from non-aligned specular reflectors
Image showing advantage of compounding Conventional image of breast (Speckle & shadow) With compounding SonoCT
transducer bandwidth A transducer s bandwidth refers to the range of frequencies which it can transmit & receive Older transducers typically had limited bandwidths This limited their use to a fixed depth range and resolution F
wide bandwidth Wideband transducers have been developed, along with scanners which control the range of frequencies transmitted and received at any one time Vary receive bandwidth with depth of echo One transducer - various centre frequencies Harmonic imaging - Send at low frequency, receive at 1 st harmonic
Modern image possibilities
Spectral Doppler Basic B Scan techniques do not work well with blood, it appears dark and it travels too fast Movement sensitive techniques are needed Spectral Doppler Colour Flow Mapping Power Doppler Christian Andreas Doppler
Spectral Doppler Red cells scatter back a small proportion of incident ultrasound MOVING cells will reflect ultrasound with a DOPPLER SHIFT in FREQUENCY This small shift is proportional to the red cell velocity θ
Spectral Doppler If the transmit frequency is in the low MegaHertz range, then the Doppler-shift frequency for blood flow is in the audio range Basic B scan equipment can be adapted, and it is helped if the transducer has a wide bandwidth and the beam can be steered to get a good Doppler angle Usually Pulsed Doppler is provided
Duplex - Spectral Doppler Spectral Doppler techniques use similar pulses, beam shaping & steering methods to Bscan The basic difference is that the pulse-echo process is repeated over and over, but for just one line-of-site.
Duplex Spectral Doppler These two techniques are combined with a method for displaying the sound spectrum - a sonogram Note it is common to show the vertical axis in cm/sec though it really is only known in khz!
Duplex - Colour flow The data is colour coded according to frequency (velocity) and superimposed on the B scan image Be aware of direction of flow detection and aliasing
Duplex - Power Doppler Velocity and direction can sometimes confuse, and one can choose Power Doppler mode This displays the areas where flow is detected but with the brightness proportional to the POWER of the movement detected. Good for slow flow and when vessels hard to see on B scan
Some safety issues Thermal & mechanical effects Significant deleterious bio-effects on either patients or operators of diagnostic ultrasound procedures have not been reported in literature ALARP (as low as reasonably practicable) MI mechanical index (likelihood of cavitation) TI thermal index (1.0 = 1ºC temperature rise)
Some safety issues Use BMUS GUIDELINES FOR THE SAFE USE OF DIAGNOSTIC ULTRASOUND EQUIPMENT Eg: avoid - an embryo less than eight weeks after conception; - the head, brain or spine of any foetus or neonate; - an eye (in a subject of any age).
Imaging summary B SCAN pulse echo, short pulses, displays anatomy M MODE displays mechanical motion SPECTRAL DOPPLER displays flow signal at one point COLOUR FLOW displays flow over image area