Ultrasound diagnostics Lecture outline Ø Physical properties of ultrasound and acoustic parameters of medium Ø Ultrasonography • Impulse reflection method • A-mode – one-dimensional • B-mode – two-dimensional • M-mode • Basic characteristics of US images • Interventional sonography • Echocontrast agents • Harmonic imaging • Principle of 3D imaging Ø Doppler flow measurement • Principle of Doppler effect • Principle of blood flow measurement • CW Doppler system • Systems with pulsed wave – PW Doppler • Duplex and Triplex methods • Power Doppler method • Tissue Doppler Imaging (TDI) Ø Ultrasonic densitometry Ø Patient Safety: reducing Ultrasound ‘Doses’ Ultrasound diagnostics Ø Ultrasound diagnostics started to develop in early 50‘ of 20th century. It allows to obtain cross-sectional images of the human body which can also include substantial information about its physiology and pathology. Ø Ultrasound diagnostics is based mainly on reflection of ultrasound waves at acoustical interfaces Ø We can distinguish: – Ultrasonography (A, B and M mode, 3D and 4D imaging) – Doppler flow measurement, including Duplex and Triplex methods (Duplex, Colour, Triplex, Power) – Tissue Doppler imaging – Ultrasound densitometry Interactions of US with Tissue Ø Reflection (smooth homogeneous interfaces of size greater than beam width, e.g. organ outlines) Ø Rayleigh Scatter (small reflector sizes, e.g. blood cells, dominates in non-homogeneous media) Ø Refraction (away from normal from less dense to denser medium, note opposite to light, sometimes produces distortion) Ø Absorption (sound to heat) – absorption increases with f, note opposite to X-rays – absorption high in lungs, less in bone, least in soft tissue, again note opposite to x-rays Ø Interference: ‘speckles’ in US image result of interference between Rayleigh scattered waves Ø Diffraction Acoustic parameters of medium Speed of US c depends on elasticity and density r of the medium: K - modulus of compression in water and soft tissues c = 1500 - 1600 m.s^-1, in bone about 3600 m.s^-1 Acoustic parameters of medium Ø Near field (Fresnel area) – this part of US beam is cylindrical – there are big pressure differences in beam axis Ø Far field (Fraunhofer area) – US beam is divergent – pressure distribution is more homogeneous Ø Increase of frequency of US or smaller probe diameter cause shortening of near field - divergence of far field increases Degree of reflectivity – echogenity. The images of cystic (liquid-filled) and solid structures are different. According to the intensity of reflection we can distinguish structures: hyperechogenic, izoechogenic, hypoechogenic, anechogenic. Ø Solid structures – acoustic shadow (caused by absorption and reflection of US) Ø Air bubbles and other strongly reflecting interfaces cause repeating reflections (reverberation, „comet tail“). Ultrasonography Interventional sonography Ø Interventional sonography is used mainly for guiding punctures Ø diagnostic – thin needle punctures to take tissue samples for histology Ø therapeutic – for aspiration of a cyst or an abscess content or an exudate etc. Ø Puncture can be done by „free hand“ – the probe is next to the puncture site – or the puncture needle is guided by a special probe attachment. Four-dimensional (4D) image The fourth dimension is time Doppler flow measurement The Doppler effect (frequency shift of waves formed or reflected at a moving object) can be used for detection and measurement of blood flow, as well as, for detection and measurement of movements of some acoustical interfaces inside the body (foetal heart, blood vessel walls) Ultrasonic densitometry It is based on both the measurement of speed of ultrasound in bone and the estimation of ultrasound attenuation in bone. In contrast to X-ray methods, ultrasound densitometry also provides information on the structure of bone and its elastic properties. ØThe speed of ultrasound depends on the density and elasticity of the measured medium. The anterior area of the tibia and the posterior area of the calcaneus are frequently used as places of measurement. The speed of ultrasound is given by the quotient of measured distance and the transmission time. ØUltrasound attenuation depends on the physical properties of the given medium and the frequency of the ultrasound applied. For the frequency range 0.1 - 1 MHz the frequency dependence is nearly linear. Attenuation is currently expressed in dB/MHz/cm. ØClinical importance: diagnostics of osteoporosis Ultrasonic densitometry Patient Safety: reducing Ultrasound ‘Doses’ Prudent use of Ultrasound Ø US is non-ionising BUT since many bioeffects of ultrasound have not yet been studied fully, ‘prudent’ use is recommended Ø ALARA – as low as reasonably achievable (exposure) Ø In practice ‘prudent’ = justification + optimisation Biological Effects Ø Possible bioeffects: inactivation of enzymes, altered cell morphology, internal haemorrhage, free radical formation … Ø Mechanisms of bioeffects: – Mechanical effects • Displacement and acceleration of biomolecules • Gas bubble cavitation (stable and transient) – see the lecture on biological effects of ultrasound – Elevated tissue temperatures (absorption of ultrasound and therefore increase in temperature high in lungs, less in bone, least in soft tissue) Ø All bioeffects are deterministic with a threshold (cavitation) or without it (heating). Output Power from Transducer Øvaries from one machine to another ØIncreases as one moves from real-time imaging to colour flow Doppler ØM-mode output intensity is low but dose to tissue is high because beam is stationary Risk Indicators Ø To avoid potentially dangerous exposures, two indices were introduced. Their values (different for different organs) are often displayed on device screens and should not be exceeded. Ø Thermal Index (TI): TI = possible tissue temperature rise if transducer is kept stationary – TIS: soft tissue path – TIB: bone near focus of beam – TIC: Cranium (near surface bone) Ø Mechanical Index (MI): measure of possible mechanical bioeffects Justification ØNo commercial demos on human subjects ØNo training on students ØNo ‘see baby just for fun’ or excessive screening in obstetrics Optimisation of ‘Dose’ 1 Ø Minimise TI and MI and use appropriate index (TIS, TIB, TIC), care in cases when these underestimate Ø Check acoustic power outputs on manual Ø Use high receiver gain when possible as opposed to high transmit power Ø Start scan with low transmit power and increase gradually Optimisation of ‘Dose’ 2 Ø Avoid repeat scans and reduce exposure time Ø Do not hold transducer stationary Ø Greater care when using contrast agents as these increase the possibility of cavitation Ø Exceptional care must be taken in applying pulsed Doppler in obstetrics Ø Regular quality control of the ultrasound device Authors: Vojtěch Mornstein, Ivo Hrazdira, Pavel Grec Content collaboration and language revision: Carmel J. Caruana Graphical design: Lucie Mornsteinová Last revision: June 2009