Chapter 8 REAL-TIME ULTRASOUND TRNSDUCERS

1. Chapter 8REAL-TIME ULTRASOUND TRNSDUCERS

2. Types of real time scanners • Mechanical sector scanner • Electronic linear scanner • Electronic sector scanner

3. Mechanical scanner • Contact scanner: • Rotating well • Rucker type • Liquid path scanner: • Can be direct path (with moving transducer) or mirror path (with fixed transducer) • Single or multiple crystal are incremented in a linear or sector. • Be used an acoustic mirror to alter the ultrasonic path.

4. Contact scanner

5. Characteristics of contact scanners • Scanning between tight areas is possible. • To have excellent images. • Their fixed focal length restricts the best lateral resolution to a limited range of depth.

6. Linear arrays • Sequential linear arrays • segmental linear arrays

7. Length: form one dimension of the cross sectional image. Be applied in two dimension: Width: determines the slice thickness. Mechanical method focus the beam in the width direction by curving the crystal or acoustic lens. Focusing in the elevation direction is done by curving or more usual way of acoustic lens Electronic focusing technique

8. Electronic focusing involves superposition of ultrasound waves. The relative position of crystal in the group dictates the focusing requirements for that crystal. Done by offsetting the firing of crystal in a group by a small time delay ( ). Principles of focusing

9. Principles of focusing

10. Mechanical system focal depth is fixed. Can be selected one of the several possible focal zone in the linear array units (phased linear array). In phased linear array focal zone is altered by varying the delay times between crystal firing. Multiple transmit focusing results in high resolution image but reduce frame rate FR=c/(2Rnnx) TRANSMIT FOCUSING

11. Based on beam aperture, if more crystal are added in the group, depth of the near field is extended. Aperture focusing is characterized by f-number: Aperture focusing

12. Aperture focusing • Optimal focusing occurs with an f-number of 2. • Focusing narrows the beam width in the region around the focal point. • To maintain the depth of field as focal length is increased, the aperture size (d) must also increased.

13. Aperture focusing

14. Received echo from large distance has less delay time. By means of additional delay circuitry, the returning echo beam is refocused. It is not confined to a fixed depth. Can be operate at different depth by varying the delay time in the received mode (beam formation) Dynamic receive focusing

16. Effective beam width: it is a combination of transmitted and received pulse width • Multiplexers: To reduce the number of wiring multiplexer is used.

17. Used to extend FOV The radius is usually 25 to 100mm As other sweep the beam by firing multiple crystals in a groups Curvilinear arrays

18. Phase array • According to the geometric configuration of crystal elements, contain: • Linear • Annular • Rectangular • All of the crystals in the array are excited nearly simultaneously.

19. To overcome certain limitations in segmental linear array (such as small field of view and small line of sight) Number of crystals can be small (16) or large (256) and all the crystals are exited at the same time The design produces a narrow field with a sector format, as large as 90 degree FOV Steering the beam throughout the field of view allow for data collection along. Electronic focusing is done in transmit and receive mode Focusing in elevation mode is done mechanically Mainly used in echocardiography Linear phase array

20. In linear phase array, all the transducers act as a single line of sight By altering the timing sequence of the excitation pulses, the direction of propagation of transmitted beam can be varied to any desired scan angle. The scan angle is the angle between direction of propagation and normal to the center of the probe Transmit steering

21. The principle of transmit steering is shown • The time delay between the transducer for each wave front angle is;

22. Focusing can be performed for each depth and point by varying the delay time between the excitation of the elements. Time delay=(r1-r2)/c Transmit focusing is limited to one focal point for each transmitted beam To change the focal point the delay timing should be altered Transmit focusing

23. Dynamic focusing during reception is possible as in other scanners Dynamic focusing improve lateral resolution Dynamic focusing

24. Compound linear array • To be incorporate characteristics from both the linear array and linear phased array. • Multiple crystals are fired at one time to steer the beam in various directions. • Scan lines for the center of the field are obtained by directing the beam perpendicular to the array. • At the extremes the beam is steered at wide angle by phase method • The field of view is trapezoidal in shape.

25. AnnularPhase array • Have a central crystal surround by concentric rings of additional crystals. • Electronic focusing of very small region at a specified depth (about 1-3mm) along the beam axis is done. • Steering is accomplished by reflecting the beam from a moving mirror or by rotating the transducer mechanically without mirror. • The beam with is the same at different directions

26. Problems with electronic arrays • Secondary lobes: It is assumed that the reflected echoes are from the main lob direction. Secondary lobs can cause artifacts. • Two types of lobs exist: • Side lobs: results form the radial vibration and interference. • Can be reduced by lowering the voltage toward the periphery (apodization)

27. Grating lobs: • Results form regular periodic spacing of the elements in the array • The specific angular location of the grating lobs is: Sinθ =(mλ/x) • The only effective parameters is x (centre to centre distance between elements) • When x is less than λ then θ become grater than 90 degree and grating lobe diminish • Subdicing also reduce grating lobe • Crystal element isolation • Ideally adjacent crystal should not excite or receive echo related to the neighbor • In reality cross talk in both excitation and reception occurs

28. Side Lobes & Grating Lobes Artifact

29. Digital beam former: Is more better than analog in manipulating signals and data • Small-Footprint transducers: Usually used for neonate and hence is a small probe. • The useful depth is twice the size of the transducer • Composite material transducers: is formed by dicing the piezoelectric material into an array of rectangular pillars and filling the interspaces with epoxy resin • Broadband transducers: Composite piezoelectric materials formulated for lowering the acoustic impedance of the crystal. The matching layer is also improved and results for a wider bandwidth • Endosonography: design of small crystals results to make a small probe to use for linear array for endorectal; transesophageal, transvaginal ..

30. Beam steering and focusing in endovaginal sonography: • Steering can be done in up and down manner. • The images are sagital • The rotation to get image from other direction can be done manually • Probe can be single or an array of small elements

31. Scanning techniques in endosonography • Transesophageal probe: to scan the heart • Transluminal transducers: used for transvascular image (f-10-20MHz) • Henafy lens • TD array transducers • Transducer care