Sequencing_and_Phasing_PP_Notes

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Electrical Engineering

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Oct 30, 2023

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Sequencing and Phasing Transducer Arrays Phasing ● Applying voltage pulses to all of the elements separately with small time delays ● Allows the path of the beam to be changed ○ Scanning ○ Steering ○ Focusing Steering by Phasing ● Voltage pulses applied to all elements with small time differences ● Delay pattern alters the orientation of the wavefront, causing beam to “steer” to a particular direction Scanning by Phasing ● Uses beam steering technique to scan electronically ● Pulse applied to each element with small time delays ● Pattern is repeated, changing time delays to individual elements causing the angle of the wavefront to change ● Beam travels perpendicular to wavefront, sweeping the beam through the tissue ● Produces a “sector” shaped image ● Transducer that scans by phasing is called a phased array Phasing to Focus ● Phasing can be applied to sequenced arrays to provide electronic focal control ● Phased linear array ○ Phased focus control added to a linear sequenced array 1 i'sweeping rising 7 u so small minescule Huggenson principal still applies v8 EYES Nga th Jonty scanned by phasing

● Phased convex array ○ Phased focus control added to a convex sequenced array Focusing ● Circular array ○ The focusing can be done in all directions transverse to the beam direction (i.e. in the imaging plane and transverse to the plane) ● Annular focusing ○ Focusing can be done in all directions both in plane and transverse to the plane ● Linear focusing ○ Focusing can only be done in one direction - the imaging plane Electronic Focusing ● Beam can be focused electronically by phasing ● Curvature added to delay pattern ● More curvature = focus closer ● Less curvature = focus deeper ● Focusing cannot occur in the far field ● Phasing provides electronic control of the location of the focus ● Multiple focuses can be used to achieve a long focal zone ● Each pulse can only focus at one depth requiring multiple pulses ● Decreases frame rate (temporal resolution) ● To have multiple focal zones: ○ You need separate pulses for each focal zone represented ○ These separate pulses must then be combined to create one image 2 Htt a line no crystals bleep so need hmm cense t Temporal resolution

○ Therefore: it takes several pulses to produce one frame - this is why our temporal resolution is affected Vector Array ● A linear sequenced array that emits pulses from different starting points (sequencing) and in different directions (phasing) ● Produces a sector shaped image with a blunted (flat) top ● Typically has a smaller footprint than that of a linear array ● Focusing by phasing, sector shape produced by phasing ( also called linear phased array) Dynamic Focusing ● Electronic outputs of the elements are timed so that the array “listens” at a particular depth ● Also called the receiving focus depth ● “Receiving focus depth” - can be continually changed to optimize resolution ● Electronic focusing in the plane along the line of the transducer elements improves lateral resolution as well as sensitivity by increasing the amount of energy in the focal zone Effect of Phasing on Beam Width ● Beam width determined by: ○ Aperture size ○ Focal length ○ Wavelength ● Focus is tighter with a closer focus (increased curvature in delay pattern) ● Beam width at focus increases as focal length increases 3 between yes 1 I A Linear flattop sequence B Linear flattop phasing beam being steered Miami when superfocal use stand off pad or move away and angle back Breast work stand off Abdomen more over

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● For longer focal lengths, aperture must be wider to maintain same beam width Dynamic Aperture ● More elements used to increase size of aperture ● Fewer elements used to decrease size of aperture ● Performed automatically by machine (dynamic) ● Wider beam width produces a longer focal zone - better lateral resolution Slice-thickness ● Approximately equal to the size of the transducer (side view) ● Also called section thickness ● Sometimes referred to as the elevational plane ● Focusing is the need to control the section thickness artifacts and improve the spatial resolution of the image Slice-thickness Focusing ● Control is obtained in one of two ways: ○ By a fixed focal length lens attached to the array (Edelman calls this external focusing) ■ The fixed lens can be set at different depths depending on transducer frequency and what depth you plan to be scanning ■ This is VERY important to note for small parts work ■ For breast use 10mHz+ (elevational plane at 1.5cm) ■ For vascular 5mHz+ (elevational plane at 3.0 - 4.0cm) ○ Also controlled by curving the ultrasound element (Edelman calls this internal focusing) 4 when scanning 3 butdrop depth machine will add more crystals to get down there I crystal deep Zhen movement is there but not really 93

Two-Dimensional Arrays ● Many manufacturers are developing two-dimensional arrays to improve poor section-thickness performance ● Array consists of a two-dimensional matrix of elements ● Allows an electronic focusing of the thickness of the beam (3rd dimension) Terms Denoting Construction ● Mechanical ● Array ○ Linear ○ Convex (curved) Terms Denoting Operation ● Sequencing - used to scan beam ● Phasing - used to scan , steer , or focus the beam ○ Steering is used with Doppler and color flow to change the direction of the beam to optimize the Doppler angle ● Vector - indicates both sequencing and phasing being used to scan beam Transducer Arrays ● How to keep your linear transducers straight? ● If the transducer SCANS by: ○ Sequencing = rectangle ○ Phasing = sector 5 round annular how its built when asking Scanning choosier

TYPE SCANNED BY FOCUSED BY IMAGE DISPLAY Linear Array Sequencing phasing Rectangle Convex Array Sequenced phasing Curved to match face of probe Phased Array Phasing Phasing Sector Vector Array Sequenced & phasing Phasing Vector ● All modern transducers that steer, do so by phasing ● All modern transducers are focused by phasing ● Mechanical transducers were NEVER able to focus (registry question) 6

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