October, 2008 J.Brnjas-Kraljević

1. ultrasound diagnostics October, 2008 J.Brnjas-Kraljević

2. Sound waves • sound wave – transport ofmechanical energy through space - byoscillation of particles of elastic medium • audiable sound - frequency range 20 Hz to 20 kHz • infrasound - frequency range  20 Hz resonance of inner organs • ultrasound - frequency range > 20 kHz - resonance of molecules

3. Nature of the sound wave • transport of mechanical energy of oscillation • the source –body oscillating in the elastic medium • energy of oscillation is transferred through space with speed • elastic medium is requisite for the existence of sound-mechanical wave wavelength frequency

4. particle is oscillating in the direction of wave propagation in the medium appear the regions of compression and rarefaction, i.e. with higher and lower pressure 2D presentation of the wave time elongation wave length molecule direction of motion

5. Equation for harmonic wavethe change of elongation in space and time at point M the oscillation is late by t’ with source oscillation what is the angle value for this time interval - wave number

6. Acoustic pressure • the change of acoustic pressure: • intensity of harmonic wave : in diagnosticsI = 10 – 100 mW/cm2

7. Intensity of sound wave • intensity of spherical wave decreases with r2 • plane waves are better for diagnostics acoustic impedance of medium properties of source I1 I2 r1 r2

8. Ultrasonic wave • frequency > 20 kHz, in medicine 1-20 MHz • source: piezoelectric crystal • crystal in electric field oscillates with frequency of alternating field • E = E0 sin t d = d0 sin t source • if stimulated to oscillation by mechanical force - the alternative voltage can be measured at the crystal opposite surfaces • F = F0 sin t U = U0 sin t detector • thickness is determined by wave frequency – resonance

9. Piezoelectric effects • piezoelectric effect – induction of electric charges on the surface of the crystal, which is elastically deformed by external force • change of deformation direction changes the direction of polarization • piezoelectric materials are: quarc (SiO2), tourmaline, different ceramics and some polymers

10. thickness of crystal is l/2 under resonance condition – the intensity of sound wave is the highest the adjustment layer – thickness is l/4 – maximal energy transfer into the tissue impedance adjustmentZ2layer = ZcxZb additional layer of gel removes air bubbles pulse methods – wave energy is transferred in pulses Pulse – defined amount of energy sound isolator – blocks the sound propagation in other directions directed plane wave Piezocrystal is source and detector vod za signal signal tube sound zvučni isolator pozadina background piezo-crystal adjustment layer

11. Ultrasound probe • in diagnostic praxis the frequency is between 2 and 20 MHz. • attenuation and absorption increases with frequency of sound • the choice of best frequency - compromise between better resolution and smaller absorption • resolution increases with increase of frequency • absorption increases with increase of frequency • higher frequencies for surface organs • lower frequencies for deeper structures

12. SPL – space length of pulse –nlPD - time interval – n/nPRF – frequency of repetition time • pulse length is 2 to 3 l • sonic beam is not homogeneous in space– there is a distribution of energy • SPL is changed in tissue – stronger absorption of higher frequencies – the resolution is worse at the spot deeper in the body • PRF - 2-3 kHz – it must be enough time to detect all reflected waves – v/2D • D - depth of imaging determines the resolution highest I = 1W/cm2 Intensity average (SA) <I> =0,3 W/cm2 transductor width

13. Axial and lateral resolution • resolution is determined by configuration of sound field, it is changed with depth in tissue resolution is limited with l – for 3,5 MHz – 0,86 mm • axial resolution – distance between two planes perpendicular to the beam – about 2 l • lateral resolution – distance between two parallel planes – depends on the width of beam - about 10 l • resolution is better for the structures closer to the source • resolution is decreasing – with the distance from the surface • higher frequency – longer Fresnel's zone – better resolution –stronger absorption! Zf=a2/l

14. Transmission depends on ratio of acoustic impedances au ar air Z1r1 Z2r2 water at

15. Reflection and transmission • law of reflection: angle of reflection = angle of incidence • law of refraction: • coefficients of reflection and transmission: r + t =1 • for Z1  Z2 maximum transmission • for Z1 Z2 ili Z1  Z2 maximum reflection

16. Speed and acoustic impedance

17. Intensity level • we do not need absolute value of intensity but its ratio over referent intensity • unit is decibel (dB) • I0 = 10-12 Wm-2 • 20 dB is decrease in intensity 100 times

18. -I=I2-I1=k I1 x -dI=k I dx Attenuation of sound wave in matter I1 I2 I I0 x

19. Absorption of sound wave • due to interaction of sound wave and matter the oscillation amplitude is decreasing: • I  A2 which means: • half value layer x1/2 is determined with I(x1/2) = I0/2 • shorter half value layer means better absorber

20. Basics of ultrasound diagnostics • wave energy in tissue is partially lost due to absorption and scattering • part of energy is lost due to reflection at the boundary of two tissues • the images are formed from the beams reflected of plane surfaces • additional information could be obtained from scattered waves caused by tissue inhomogeneity • it is possible to detect the changes in elastic properties of tissues (consequence of sickness) • elasticity of tissue – connective tissue – high acoustic impedance– blood cells have high impedance – observable in the image • tissue of low transparency - tumors in solid tissue – acoustic shadow - simple diagnosis

21. Doppler's effect • the consequence of source or detector motion is apparent change in detected frequency • frequency shift can be observed if the speed of moving object is lower than the speed of sound wave • approach to the source higher frequency • departure from the source lower frequency • simultaneous approach

22. Ultrasound diagnostics • reflection of wave at the boundary of two different media • the intensities of reflected waves are recorded as a function of time • the image is the distribution of boundaries that areperpendicular to the incident wave • by moving the probe we can record more parallel cross sections - by simultaneous use of more probes we can record the whole body

23. A,B and M mode( the way of presentation) • Amode - intensity of reflected wave is presented with amplitude • B mode-intensity of reflected wave is presented with the brightness of the point; wave of higher intensity – more shining spot on the screen • M mode - used for visualization of moving boundaries, specially heart; it is combination spatial image of echo waves and temporal graphical display

24. Image generation • intensity of reflected wave depends on impendency difference • higher intensity means larger difference • temporal interval of incoming signals is proportional to the distance of reflecting surfaces • in B-mode – higher intensity, due to bright points, but lower resolution • grey scale – bimodal display • each pixel is characterized by number, higher number means stronger reflection • pixel depth has influence on image contrast A mode t B mode t

26. 2D images • cross sections which are recorded are the planes perpendicular on the beam • we use the B-mode • display: grey scale depends on echo intensity – the boundaries of different tissues are white • B-mode, the depth depends on probe parameters • the number of lines in the image is equal to the number of units in the probe - frame • image is reconstituted – frame of speed • if it is high – we get the image in "real time"

27. pulse probe mode mode

28. “Real time” • simultaneous recording with more independent beams, each has a width 1-4 l, dependence on frequency • wave front – in Fresnel zone has the size of probe cross section • the length of lines determines the depth of the image • linear transducer- parallel lines of recording, large aperture, it is not for the heart; sequential starting • aperture – the size of transducer or the number of synchronized elements • phase beam - one sector of space is recorded, line density is decreasing with distance– complicated display, but with smaller window– frequencies 2-10 MHz • image depth is primary determined by attenuation

29. Signal processor Amplifier Amplifier of pulse probe

30. Measurement of flow echo

31. CW Doppler – continuous sonification – the frequency shift of reflected wave is measured • the motion is heard as sound signal • Pulse Doppler – pulse source determines the depth of reflection • frequency analysis is limited on temporal interval between emitted pulse and incoming echo signals – “gating” • limit is – maximum depth and maximum shift, defined PRF

32. by prof.A.Kurjak