Chapter 8 REALTIME ULTRASOUND TRNSDUCERS 02102020 1 Types

Chapter 8 REAL-TIME ULTRASOUND TRNSDUCERS 02/10/2020 1

Types of real time scanners Ø Mechanical sector scanner Ø Electronic linear scanner Ø Electronic sector scanner 02/10/2020 2

Mechanical scanner q Contact scanner: v Rotating well v Rucker type q Liquid path scanner: q Can be direct path (with moving transducer) or mirror path (with fixed transducer) v Single or multiple crystal are incremented in a linear or sector. v Be used an acoustic mirror to alter the ultrasonic path. 02/10/2020 3

Contact scanner 02/10/2020 4

Characteristics of contact scanners q Scanning between tight areas is possible. q To have excellent images. q Their fixed focal length restricts the best lateral resolution to a limited range of depth. 02/10/2020 5

Linear arrays q Sequential linear arrays q segmental linear arrays 02/10/2020 6

Electronic focusing technique Length: form one dimension of the cross sectional image. q Be applied in two dimension: Ø Width: determines the slice thickness. Ø Mechanical method focus the beam in the width direction by curving the crystal or acoustic lens. q Focusing in the elevation direction is done by curving or more usual way of acoustic lens q 02/10/2020 7

Principles of focusing Electronic focusing involves superposition of ultrasound waves. q The relative position of crystal in the group dictates the focusing requirements for that crystal. q Done by offsetting the firing of crystal in a group by a small time delay ( ). q 02/10/2020 8

Principles of focusing 02/10/2020 9

TRANSMIT FOCUSING q Mechanical system focal depth is fixed. q Can be selected one of the several possible focal zone in the linear array units (phased linear array). q In phased linear array focal zone is altered by varying the delay times between crystal firing. q Multiple transmit focusing results in high resolution image but reduce frame rate FR=c/(2 Rnnx) 02/10/2020 10

Aperture focusing q Based on beam aperture, if more crystal are added in the group, depth of the near field is extended. q Aperture focusing is characterized by f-number: 02/10/2020 11

Aperture focusing q Optimal focusing occurs with an f-number of 2. q Focusing narrows the beam width in the region around the focal point. q To maintain the depth of field as focal length is increased, the aperture size (d) must also increased. 02/10/2020 12

Aperture focusing 02/10/2020 13

Dynamic receive focusing Received echo from large distance has less delay time. q By means of additional delay circuitry, the returning echo beam is refocused. q q 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) 02/10/2020 14

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Ø Effective beam width: it is a combination of transmitted and received pulse width Ø Multiplexers: To reduce the number of wiring multiplexer is used. 02/10/2020 16

Curvilinear arrays Ø Used to extend FOV Ø The radius is usually 25 to 100 mm Ø As other sweep the beam by firing multiple crystals in a groups 02/10/2020 17

Phase array q According to the geometric configuration of crystal elements, contain: Ø Linear Ø Annular Ø Rectangular q All of the crystals in the array are excited nearly simultaneously. 02/10/2020 18

Linear phase array q To overcome certain limitations in segmental linear array (such as small field of view and small line of sight) q Number of crystals can be small (16) or large (256) and all the crystals are exited at the same time q The design produces a narrow field with a sector format, as large as 90 degree FOV q Steering the beam throughout the field of view allow for data collection along. q Electronic focusing is done in transmit and receive mode q Focusing in elevation mode is done mechanically q Mainly used in echocardiography 02/10/2020 19

Transmit steering q In linear phase array, all the transducers act as a single line of sight q By altering the timing sequence of the excitation pulses, the direction of propagation of transmitted beam can be varied to any desired scan angle. q The scan angle is the angle between direction of propagation and normal to the center of the probe 02/10/2020 20

Ø The principle of transmit steering is shown Ø The time delay between the transducer for each wave front angle is; 02/10/2020 21

Transmit focusing Focusing can be performed for each depth and point by varying the delay time between the excitation of the elements. Ø Time delay=(r 1 -r 2)/c Ø Ø Transmit focusing is limited to one focal point for each transmitted beam Ø To change the focal point the delay timing should be altered 02/10/2020 22

Dynamic focusing Ø Dynamic focusing during reception is possible as in other scanners Ø Dynamic focusing improve lateral resolution 02/10/2020 23

Compound linear array q To be incorporate characteristics from both the linear array and linear phased array. q Multiple crystals are fired at one time to steer the beam in various directions. q Scan lines for the center of the field are obtained by directing the beam perpendicular to the array. q At the extremes the beam is steered at wide angle by phase method q The field of view is trapezoidal in shape. 02/10/2020 24

Annular Phase array q Have a central crystal surround by concentric rings of additional crystals. q Electronic focusing of very small region at a specified depth (about 1 -3 mm) along the beam axis is done. q Steering is accomplished by reflecting the beam from a moving mirror or by rotating the transducer mechanically without mirror. q The beam with is the same at different directions 02/10/2020 25

Problems with electronic arrays q Secondary lobes: It is assumed that the reflected echoes are from the main lob direction. Secondary lobs can cause artifacts. q Two types of lobs exist: Ø Side lobs: results form the radial vibration and interference. Ø Can be reduced by lowering the voltage toward the periphery (apodization) 02/10/2020 26

Ø Ø Grating lobs: Results form regular periodic spacing of the elements in the array Ø The specific angular location of the grating lobs is: Sinθ =(mλ/x) Ø 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 Ø Subdicing also reduce grating lobe Ø Crystal element isolation Ideally adjacent crystal should not excite or receive echo related to the neighbor q In reality cross talk in both excitation and reception occurs q q 02/10/2020 27

Side Lobes & Grating Lobes Artifact 02/10/2020 28

Ø Ø Ø 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. . 02/10/2020 29

Ø Beam steering and focusing in endovaginal sonography: Ø Steering can be done in up and down manner. Ø The images are sagital Ø The rotation to get image from other direction can be done manually Ø Probe can be single or an array of small elements Ø 02/10/2020 30

Ø Scanning techniques in endosonography Ø Transesophageal probe: to scan the heart Ø Transluminal transducers: used for transvascular image (f-1020 MHz) Ø Henafy lens Ø TD array transducers Ø Transducer care 02/10/2020 31