10/10/2011. Objectives. Ultrasound. Pulse Echo. Frequency. Understand Basic Physics of Ultrasound. Principles of Ultrasound.

Transcription

1 Objectives Principles of Ultrasound Dr Zunaid Karim Consultant Rheumatologist Mid Yorkshire NHS Trust Visiting Senior Lecturer Leeds Institute of Molecular Medicine Understand Basic Physics of Ultrasound Pulse Echo principle Frequency and wavelength Resolution Artefacts Doppler Doppler artefacts Apply knowledge to help improve image quality, avoid pitfalls Ultrasound Sound above the acoustic spectrum that can be heard by a human being. Humans 20 Hertz (Hz) Hz. Medical diagnostic ultrasound:1 25 MHz Pulse Echo Ultrasound waves are generated by a transducer Orginal quartz, now crystals of lead zirconate titanate (Piezo-electric effect) Electrical pulse through crystal causes it to contract, changing currents cause it to expand, leads to sound (pulse) Returning sound wave hits transducer causing movement of crystal, generating electric current (echo) Computer technology converts the information into images Frequency Transducer Number of complete cycles per unit of time Man-made transducer frequency is predetermined by design One cycle per second = 1 Hertz (Hz) One million Hertz = 1 megahertz (MHz) E.g.: a 12.5 MHz transducer= 12,500,000 cycles per second 1

2 Wavelength Distance between each cycle of sound Frequency and Wavelength Wavelength and frequency inversely related Higher frequency shorter wavelength, greater resolution but less penetration & vice versa Attenuation Reduction in power and intensity of sound as it travels through tissue Transducer Frequencies 2.5 MHz Deep abdomen, OB/Gyn 3.5 MHz General abdomen, OB/Gyn Higher frequencies attenuate, or are absorbed, faster than lower frequencies (less tissue penetration) Time Gain Compensation (TGC) used to adjust for this 5.0 MHz 7.5 MHz 10.0 MHz + Bandwidth- range Vascular, Breast, Gyn Hip, Breast, Thyroid Rheum US, Breast, Thyroid, 2

3 Tranmission or reflection? Transmission: when a pulse passes through one tissue into another Reflection: when an ultrasound pulse is reflected at a boundary How much? Acoustic impedance: related to the density of the material and the speed of sound in the material (= Density x Speed) Greater the difference in impedance between tissues, the more sound will be reflected rather than transmitted Medium Acoustic impedance Impedance (in standard unit) air water 1.50 blood 1.59 fat 1.38 muscle 1.70 bone 6.50 Reflection Soft tissue- S.tissue 1-2% Soft tissue - bone 50% Soft tissue - gas 99% Acoustic impedance Acoustic impedance skin air gel skin Acoustic impedance Acoustic impedance gel skin fluid gel skin fluid bone 3

4 A-mode (Amplitude) Transducer sends a sound wave, spike represents tissue interface and amplitude of reflection B-mode (Brightness) (grey-scale ultrasound) Amplitude of signal changed into dot, larger signal brighter Axial Spatial resolution Dependent on wavelength, higher frequency better Lateral (not as good as axial) Dependent on bandwidth, narrower better Higher gain, higher bandwidth Focus Focus Focus Multiple Zone Focussing Improves spatial resolution Temporal resolution Ability to detect movement over time Synonymous with frame rate Increase frame rate, better Reduce depth Reduce focus zones Reduce density (worse spatial resolution) Narrow image 4

5 Grey scale resolution Ability to detect subtle difference in echo Different shades of grey Dynamic range Low speckly High smooth 5

6 Artefacts Echoes which either by depth, direction or amplitude do not correspond to a real tissue target Anisotropy Pulse needs to be sent perpendicular to tissue being assessed, if not, reflected echo may not be detected Anistropy Anisotropy Sound wave reflected but not detected by transducer so read as absorbed (black dot) Adjust probe to minimise this, by keeping ROI parallel to probe Beam Steer Transducer sends signal out at angle (as opposed to perpendicular) 6

7 Enhancement When pulse crosses a structure with low attenuation (eg. fluid), echoes are enhanced distally i.e. brighter Edge Artefact Edge- (Refraction) occurs as the pulse crosses edge of tissues of different densities, refraction (or bending) results in a shadow distal to the interface Acoustic Shadow Low signal intensity after interface with very high acoustic impedance difference e.g. soft tissue and calcific deposit Doppler Effect Sound waves increase in frequency when they reflect from objects (e.g. red blood cells) moving towards the transducer, and decrease in frequency when they reflect from objects moving away from the transducer. Doppler Detection of movement by measuring a frequency shift in the returned echo Possible to calculate flow rate and direction of blood flow - Anaemia (& PC), temperature, exercise, probe pressure, Raynaud s 7

8 Doppler Power uses the strength of a returned sound wave from anything that is moving to give the position and brightness - Amplitude, good for low flow such as synovium Colour Doppler uses the amount of frequency shift from anything that is moving to determine the speed of movement (average) Spectral gives the velocity of flow within a vessel by only analysing a specific section within the vessel Frequency Independent of grey scale Higher- more detailed image of vessels, less penetration Lower- more penetration but grainier Doppler image (larger, less clear) Frequency Frequency Frequency Doppler- what helps form signal Power of the sound wave sent (pulse) Number of moving particles Depth of tissue being assessed Pulse repetition frequency and wall filter Amplification of the returned signal (echo)-power Doppler gain Box size-frame rate 8

9 Pulse repetition frequency PRF High PRF- higher velocities, filters remove low flow, less noise Lower PRF- increases sensitivity to lower flow, gives machine more time, also more time for deeper signal to return Lower PRF, lower filter Lower filter greater sensitivity but more movement artefact PRF PRF Gain Independent of grey scale, determines sensitivity to flow Increase gain, increases signal returning from machine Increases sensitivity Lower gain, reduces noise and motion artefacts, BUT also sensitivity Box size Larger box, reduces frame rate and impact on sensitivity Reduces risk of false positive from mirror, or reverberation artefact above Increase till get background noise, then reduce till just gone 9

10 Like grey scale Focal Zone Keep to region of interest Echoes generated in focal zone have higher amplitudes Artefacts Random noise- all electrical circuits will produce this, avoid by lowering gain just below setting that has little or no noise Motion- any movement of patient, ultrasonographer or probe Mirror- any highly reflective surface like bone (same as for Grey scale) Blooming- colour beyond the vessel Reverberation- repeated like grey scale (full box) Mirror and reverberation Summary A lot of Physics! Necessary evil if you want to get the best out of machine and avoid artefacts Torp-Pedersen S T, Terslev L Ann Rheum Dis 2008;67: Courtesy of Lene Terslev Questions? 10