Ultrasonography Jan Šprindrich 3 LF UK Praha RDG klinika 2006

Ultrasound in medicine History : - since 1940 therapeutic applications - physiotherapy frequency kHz - since 1970 diagnostic applications ultrasonography ( echography ) – frequency MHz Physics : high-frequency longitudinal sound waves propagated by compression and relaxation of molecules principle: reflexion – echo on impedace interfaces ( boundaries ) of tissues Frequency is inversely proportional to the wavelength

Ultrasound waves are produced in an US transducer Generation: piezo-electric effect ( crystals or synthetic materials vibrate by electric current ). Reception: converse piezo-electric effect Transducers are both transmitters and receivers of ultrasound US waves in ultrasonography : frequency range 2 – 15 MHz Audible sound – frequency range 16 Hz – 20 kHz

Scanning probes Linear array of small transducers is emitting parallel waves and the scan is rectangular The resolution is good near to the probe, in deep tissues is worse. Linear array

Scanning probes These scans are fan-shaped and originate through a very small acoustic window They are useful in upper abdomen and in cardiology. Thanks to the fan shape they have a good resolution in deeper layers Sector probe

Scanning probes Modified linear array – curvilinear transducer fan-beam with a broader apex Relative good images of surface and deeper layers. Convex transducer

Scanning probes Circular disposition of transducers Good resolution in near zone Good access through narrow acoustic window – intercostal, transfontanellar etc. Anular array probe

Propagation of US waves in soft tissues In the US beam molecules are vibrating and exchanging energy. This corresponds to compression followed by relaxation of molecules in the US field

Speed of propagation of US waves In frequencies over 1 MHz are the corresponding wavelengths shorter than 1 mm. On the bone surfaces are the US waves reflected.

Part is reflected on the boundaries ( difference in acoustic impedance ) Part is scattered when hitting the structures smaller than its wavelength Part is deviated when the angle of incidence is not 90 o Part is absorbed and transformed in the heat US waves penetrating in the tissues

Penetration of US waves in soft tissues is inversely proportional to frequency. Structural resolution is directly proportional to frequency. the scattering of waves in the tissues can be limited by focusation of the US beam

Acoustic impedance Impedance – resistance of the environment against the propagation of acoustic waves – is proportional to the density of the environment and to the velocity of propagation of the waves. Z = At tissue boundaries part of the waves is reflected and other part is attenuated. The amount of the reflected waves depends on the difference between acoustic impedance of both tissues.

Ultrasound image Piezoelectric transducer is emitting the pulses of waves of a very short duration. In the meantime the reflected waves are registered. Transducers are both transmitters and receivers of ultrasound. Reflected waves are then transformed to electric signals. After amplification they are stored in the computer and further displayed on a monitor.

US image registration of reflected echoes

Ultrasound image Highly reflecting structures are bright – we call them hyperechogenic Poorly reflecting structures are dark – we call them hypechogenic Not reflecting structures are black – we call them anechogenic Two structures with the same intensity of echoes are called isoechogenic Two-dimensional map of all the tissues that have been in the beam.

US scanner ( unit ) - stationary - mobile - portable - transducers (probes) - registration – hard copy unit - coupling agent - gel Equipment :

Registration modes of US examination A mode (amplitude) 1D curve - peaks B mode (brightness) 2D sectional image M mode (motion) displays motion D mode (doppler) flow assessment Combined modes ( duplex, triplex ) Accessory modes ( colour coding, 3D )

A mode registration peaks on the time-base line of the cathode ray tube peak = amplitude = echo

A scan in ophtalmology ocular metrics probe coupling gel amotion of retine

A scan in obstetrics metrics of pelvis and fetus

Origin of B mode - 2 D image B scan : basic US imaging mode static dynamic – real time

Real-time B mode Images are constructed and presented on the monitor very quickly – this mode displays motion ( as fluoroscopy in X-ray examinations ). Images change with each movement of the transducer It is possible to freeze the displayed image, holding it stationary so that it can be studied, measured if necessary and registered as a hard copy. Black and white images are presented in 256 degrees of gray depending on the resolution of the monitor. 2D dynamic examination

Rectangular image obtained by linear array probe abdomen - appendix

Fan-shaped image from a convex probe kidney - cyst

Acoustic shadow behind the intestinal air bubble kidney examination Inspirium pushes the air bubble aside

Acoustic shadowing behind the reflecting surface Gall-bladder stones – tail of comet images

Examination of tendons – perpendicular planes transverse longitudinal Achilles´ tendon

Examinations by endocavitary probes endorectal endovaginal TEE prostate examination by endorectal probe

Complementary value of US and X-ray examination in mammology US : hyperechogenic Rx : shadow US : anechogenic Rx : shadow

M mode in cardiology registration of mitral valve movements

Johann Christian Doppler professor of mathematics Polytechnic school in Prague In 1842 formulated his principle : Frequency of acoustic waves emitted from a mobile source is changing if registered by stationary observer DOPPLER´S PRINCIPLE

Reflection of waves from the moving interface frequency of reflected waves is changing

D – mode registration measurement of blood flow in vessels ( direction, type, velocity, flow volume ) US waves are reflected on circulating blood cells frequency shift

Doppler´s principle the difference of frequencies is proportional to the speed of blood circulation f v = frequency of emitted waves f p = frequency of reflected waves

Doppler examination By continuous US emission (CW) By pulsed US emission (PW) plus B mode Duplex mode

Registration of D mode examination Amplitude is proportional to the speed Colours are marking the direction of flow and their shades the speed of circulation (CFM – colour flow mapping) Duplex mode : B mode + PW Doppler Triplex mode : B mode + CFM + spectral curve. spectral curvecolour mapping

Registration of measured values Cursor of sampling volume is placed in the lumen of vessel Blood flow is represented by curve Amplitude expresses the velocity of flow ( V ) Longitudinal axis measures time Flow to the probe is registered above the x-axis, flow from the probe below the x-axis.

Spectral curve of arterial flow measurement of flow velocity

Doppler examination and haemodynamics flow volume q = flow volume A = cross-sectional area v = mean velocity of flow laminar flow type of flow turbulent flow behind the stenosis

Doppler examination of vessels colour flow coding triplex mode art. carotis communis art. et venae tibiales post.

Colour flow mapping – transplants transplanted kidney - evidence of vascularisation

Colour flow mapping – obstetrics head of the fetus in utero – circle of Willis umbilical cord

U ltrasound contrast agents Small air-bubbles covered by polysaccharides They increase the echogenity of circulating blood in vessels and capillaries The signal-to-noise ratio is ameliorated ( higher amplitude of signals ) Their effect has a short duration

Ultrasound contrast agents Doppler – CFM – liver – before and after application of Levovist

Technical means improving the quality of US image Imaging be means of harmonic frequencies THI - tissue harmonic imaging ( SHI )

Tissue Harmonic Imaging - THI Images are constructed from multiples of a basic frequency (so called harmonic tones). They increase in deeper layers This technique brings more details of remote structures. conventional US image THI image

Ultrasound 3D image fetus in utero, 31 weeks of gestation

Biological effects of ultrasound I. Thermic index (TI) II. Acoustic intensity (SPTA) III. Mechanical index (MI) Technical parameters of US units determinating the biologic effects

Heating of tissues as a factor of risk Biological effects of ultrasound Embryonal tissue is more sensitive, critical temperature is 39,5 o C. In adults the critical temperature is 41 o C. As confirmed by control tests and phantom measurements diagnostic ultrasonographic generators never reach these critical levels. Thermic index is measuring the intesity of heating

Biological effects of ultrasound Cavitation effect – in strong high frequency vibrations the partial pressure and volume of microscopic gas bubbles in tissues is changing This may lead to the mechanical damage of cellular membranes or cytoplasmatic structures of cells. Mechanical index is an indicator of this danger. Non-thermic effects as factor of risk

Advantages of US minimal invasivity minimal discomfort of the patient minimal side-effects and complications acceptable price of US scanners compared to other digital imaging modalities minimal operating costs

Prerequisites of a valuable ultrasound examination good scanner and suitable probes sufficient knowledge of anatomy and hemodynamics experience of an examinator optimalisation of the US image correct setting of all parameters and measurements

Disadvantages of US subjective assessment of examination time-consuming examination anatomical orientation sometimes difficult, limited by the shape of a probe artefacts of the US image

Routine indications of ultrasonography Abdominal organs - abdominal ultrasonography internal medicine, surgery, pediatrics Pelvis – gynecology, obstetrics, urology Heart – cardiology Vessels – angiology Brain and extracranial vessels – neurology Locomotion system – orthopedics, traumatology, rheumatology Others : ophtalmology, endocrinology, mammology et al.

US can be used for soft-tissue examinations in every situation, where X-rays pose a threat of an inadequate radiation risk. evidence of pregnancy – fetus, 8 weeks of gestation

Thank you for attention ! First historical US image: pig´s heart, 1954, Lund