Ultrasound Physics - Chapter 8

Dynamic Receive Focusing

-Another means to reduce the effective sampling volume is dynamic focusing in the receive mode. -by means of TIME DELAY circuitry in the receiver, the returning sound beam-induced signals are refocused when multiple crystals receive the echo -The actual time delay is determined for each crystal by simple geometry and an assumed constant velocity in soft tissue -the depth for receive focus is always known; thus receive DELAY times are constantly changed to yield a continually focused beam at all depths. That is, during the acquisition of scan data, the receive time delays are varied dynamically to sweep the focal zone to each point along the scan line. -a dynamic aperture adjusts the number of crystals along the array to optimize focusing as a function of depth -additional elements are included in the aperture as the depth of the focal zone is increased

Dynamics:

-Because most crystals in the array belong to multiple firing groups, the focusing requirements change and cannot be achieved mechanically -Focusing in the in-plane direction is variable, which allows narrow beam width in this direction at any specified depth along the scan line, whereas the beam width in the elevation direction is not changeable

Principles of focusing

-Electronic focusing involves the superposition of waves and is accomplished by offsetting the firing sequence of various crystals in a group by well defined time DELAYS

Transducer array

-contain multiple active elements -an array comprises a single slab of PZT cut into a collection of separate pieces called elements -each active element is connected by a wire to its own electronic circuitry -a channel is the combination of the active element, wire, and system electronics

Aperture Focusing

-electronic focusing during transmission is also accomplished by VARYING the number of CRYSTALS activated in the segment -the aperture is increased to maintain near field beam width for longer focal lengths. If more crystals are added in the group, then the depth of the near field is extended and the beam width near the transducer is broadened.

aperture and lateral resolution

-importantly, at increased depth, the beam is made narrower by increasing the aperture -large aperture extends the depth of focus -also, increasing the aperture size creates a strongly focused beam, improving LATERAL resolution. Recall, the Focal Zone is inversely proportional to the square of the aperture.

Effective Beam Width

-linear arrays use both transmit focusing and dynamic receive focusing -the effective beam width is the product of the transmit focal zone beam width and the received beam width -since the U/S beam diverges rapidly beyond the focal zone, multiple transmit focal zones are necessary to maintain a narrow beam width along the entire scan line

Transmit Focusing

-operators are allowed to select one of several possible focal lengths -the depth of the focal zone is altered by varying the delay times between crystal excitations -high line density images with multiple focal zones are possible although comes at the expense of frame rate speed

Crystals in Linear Sequential Arrays

1. 200-500 rectangular-shaped strips of PZT material arranged side by side in a line. 2. Each crystal is about 1 wavelength in width. 3. The array is large, up to 10cm long.

Transducer Care

1. A CRACKEDtransducer surface is potentially dangerous to both the operator and patient and must be removed from service. 2. Repeated twisting and bending of the CABLE can lead to transducer malfunction. 3. Immersion in a LIQUID can damage the transducer, since many transducers are not water tight. 4. The preferred CLEANING method is a damp cloth moistened with soap and water. If decontamination of bodily fluids is required, then disinfecting and sterilizing solutions (glutaraldehyde) are used. Still, the sonographer should always check the manufacturer's recommendation for cleaning. 6. Gas sterilization, UV sterilization, dry heat sterilization, autoclaving, and soaking in chlorine beach must all be avoided.

1.5D Array Transducers

1. A major weakness of sequential arrays is the inability to electronically control the slice thickness. The 1.5D transducer provides electronic in the ELEVATION plane. 2. The single row of elements in the conventional linear array is replaced by 3-7 rows of smaller elements in the 1.5D. The additional crystal elements with the time delay channels enable electronic focusing in the elevation plane.

Crystal: Linear Phased Array

1. Although the footprint is small , as many as 100 to 300 elements are arranged along the face of the probe. 2. Each element is rectangular and narrow with a width of ¼ to ½ wavelength.

Hanafy Lens

1. An alternative method for controlling elevation plane is to use a variable thickness crystal called a Hanafy lens. 2. The central portion of the crystal is thin, resonates at high frequency, and forms a superficially focused beam. 3. The crystal becomes thicker, resonates at lower frequency, and contributes to focusing at progressively deeper depths as the outer boundary of the crystal is approached.

Each Transducer type 'category' will be summarized using the following characteristics:

1. Crystals: (Shape and number of active elements) 2. Beam Steering: 3. Beam Focusing: 4. Image Shape: 5. Damaged PZT Crystal: (Consequences)

Damaged PZT Crystals:Linear Phased Array

1. Damage to one of the elements results in INCONSISTENT or erratic beam steering and focusing. 2. The effect of the damaged crystal cannot be precisely determined because of the complex interactive nature of the steering/focusing process.

Transducer Face

1. Damaged face 2. Fluids enter scan head 3. Damage electronics 4. Hazard 5. Remove from service

Electronic Focusing vs Mechanical Focusing

1. Electronic In-plane transmit focal zone Operator selectable Variable 2. Mechanical Fixed Slice thickness more narrow in focal zone

Crystal Element Isolation

1. Elements are designed to transmit and receive beams in isolation but this is not often the case with multiple element arrays. When crystals are not in isolation they are referred to as being 'coupled'. 2. Crystals can be mechanically isolated by using AIR as a spacing material or by designing the crystals so that the WIDTH and LENGTH dimensions are TWICE as long as the THICKNESS. These methods significantly reduce 'cross-talk' between crystals.

Transducer Types and Characteristics

1. Mechanical 2. Linear Sequential Array 3. Curvilinear Sequential Array 4. Linear Phased Array 5. Annular Phased Array 6. Vector Array Other transducers types are discussed but these are the fundamental.

Small Footprint Transducers

1. Phased arrays with small physical dimensions are desirable for neonatal applications and small part scanning, but these transducers therefore have a limited area for assembling piezoelectric elements. 2. Electronic focusing becomes suboptimal as scan range is extended beyond a depth equal to twice the active length of an array. 3. Small footprint transducers, because of their limited range, operate at high frequency (5-20MHz) and have excellent axial resolution. They also have the advantage of reduced weight and relatively wide field of view.

Annular Phased Array summary

1. Pie-shape Field of View 2. Width FOV increases with depth 3. Density lines of sight decrease with depth 4. MECHANICAL steering 5. Electronic focusing in transmit and receive 6. Electronic focusing IN-PLANE and ELEVATION directions

1.5D array summary

1. Rectangular FOV 2. In-plane width = physical length of array 3. Scan lines are parallel and uniform 4. Electronic directional control by sequencing element activation 5. T and dynamic receive electronic focusing 6. Transmit electronic focusing reduces in-plane and elevation beam widths

Summary: Linear Sequential Array

1. Rectangular Field of View 2. Width FOV equals physical array size 3. Parallel lines of sight 4. Steering by sequential element selection 5. Electronic focusing in transmit and receive 6. Slice thickness fixed mechanical focusing

summary of Mechanical Sector

1. Single crystal 2. echanical focus 3. Fixed focal zone 4. Steering Move crystal in arc Transmit pulse by crystal position 5. Sector format

Beam Steering in Linear Sequential Arrays

1. Some but not all of the crystals are fired simultaneously to create each sound beam. 2. Multiple small crystals activated together in the linear array behave in a similar manner to a single crystal of equivalent size.

Beam Focusing:Linear Phased Array

1. Sound beams are electronically focused that allow the sonographer to modify the depth and amount of focusing. 2. Whether the region of clinical interest is superficial or deep, the sonographer can adjust the focal length accordingly. 3. Multiple beams can also be transmitted along the same scan line providing multi-focusing capability.

Beam Steering:Vector Array

1. Steering combines PHASEDand SEQUENTIAL technologies. As in sequential arrays, some but not all of the crystals are fired simultaneously. 2. In manner similar to phased arrays, the electrical signals from the beam former are delayed in a slope spike-line pattern as they excite a group of elements in a vector array. As a result, the beams radiate out in different directions from the face of the transducer.

2D Transducer summary

1. Steering over VOLUME 2. Electronic focusing in two directions 3. 4D acquisition 4. High number of elements 5. Electrical connections 6. Expensive 7. Large size

Digital Beam Former

1. The function of focusing during transmission and reception is executed by a digital beam former and is done so by a combination of TIME DELAY, APODIZATION, INTERPOLATION.. 2. The digital beam former, as opposed to the analog device, allows greater flexibility in beam manipulation with regard to the US field shape, beam width, direction, and intensity.

2D Array Transducers

1. The matrix phased (2D) array is formed by crystal elements configured in MULTIPLE ROWS forming a rectangular plane, totaling a number between 2000 and 9000 elements. 2. The 2D array enables focusing and steering of the beam throughout the sampled VOLUME.

Beam Focusing:Annular Phased Array

1. The primary advantage of annular phased arrays is that MULTIPLE transmit FOCAL ZONES create each scan line of the image. 2. ecall that smaller diameter PZT crystals create shallow focus beams, whereas larger diameter crystals create deeper focus beams. 3. The inner crystal of an annular array creates a shallow focus beam and collects information only from the shallowest depths; all other information from the inner crystal's sound pulse is ignored. The next larger crystal is fired down the same scan line, and information only from its focal zone is stored. This process is repeated for each of the rings.

Curvilinear Array summary

1. Trapezoid Field of View Curved at transducer 2. Width FOV increases with depth 3. Density lines of sight decrease with depth 4. Electronic steering by element selection 5. Electronic focusing in transmit and receive 6. Slice thickness fixed mechanical focusing

Vector Array summary

1. Trapezoid Field of View FLAT at transducer 2. Width FOV increases with depth 3. Density lines of sight decrease with depth 4. Electronic steering by element selection (center) Electronic steering by delay time (edge) 5. Electronic focusing in transmit and receive 6. Slice thickness fixed mechanical focusing

Linear Phased Array summary

1. Trapezoid Field of View 2. Width FOV increases with depth 3. Density lines of sight decrease with depth 4. Electronic steering 5. Electronic focusing in transmit and receive 6. Slice thickness fixed mechanical focusing

Endosonography transducers

1. are specifically designed real-time mechanical, linear array, or phased array transducers mounted on probes that can be inserted into various body cavities. 2. Because the transducers are closer to the area of interest and there is minimal sampling depth, many lines of sight can be collected to improve spatial sampling

Transluminal transducers

1. use small crystals mounted on the end of a catheter to inspect the interior of vessels visually for vascular plaques. 2. These transducers operate at 10-20MHz and produce highly detailed images that aid in the placement of balloons for plaque reduction or laser obliteration.

summary of Mechanical Sector characteristics

6. Pie-shape Field of View 7. Width FOV increases with depth 8 .Density lines of sight decrease with depth 9. Mechanical steering 10. Mechanical focusing in transmit 11. Fixed focusing at specified depth

Beam Steering in Mechanical Sector

A 2D image is built up by rotating the crystal with a motor. The scan plane is created through mechanical steering

Image Shape: in Linear Sequential Arrays

A rectangular image no wider than the transducer is produced. Although a transducer with a 10cm footprint may produce an 8cm image, it cannot create an image wider than 10cm.

Simultaneous Excitation

All crystals have voltage applied at the same time

No focusing

Beam width nearly equal to aperture in near field NFD depends on aperture and frequency Rapid change in beam width after near field

Electronic Focusing Techniques

Because a linear array has a rectangular format, focusing must be applied in two directions to narrow the beam width perpendicular to the direction of propagation -Focusing along the IN-PLANE direction affects the lateral resolution while focusing along the elevation direction determines the thickness of tissue represented by the cross-sectional image Mechanical focusing is used to reduce the width in the ELEVATION direction. The slice thickness varies along the scan line but is typically 3-10mm within the focal zone Electronic focusing narrows the beam width in the in-plane direction

Delay Time Focusing

Crystals have voltage applied at nearly the same time Delay time depends on crystal location and distance to point

Mechanical Focusing

Elevation direction Fixed focal length Slice thickness varies throughout FOV

Curvilinear Array

Expands width of the FOV beyond physical length

Beam Focusing in Curvilinear Array

Focusing is achieved electronically similar to that of the linear array.

Beam Focusing in Linear Sequential Arrays

Focusing is achieved electronically through both transmit and receive focusing that incorporate electronic time delays.

Image Shape in Mechanical Sector

Image is fan or sector shaped similar to a sweeping of a wiper blade. The pulses radiate out like spokes of a wheel from the PZT crystal as it rotates around a single point

Beam Aperture

Increasing aperture with depth of focal zone

Beam Steering:Annular Phased Array

MECHANICAL steering is used by the Annular Array. A 2D image is built up by physically rotating the ringed element array so that it transmits sound beams in different directions.

Phased Array transducer

Multi-element phased array Small footprint All elements activated for transmission Electronic steering Electronic focusing

Beam Scanning

Multiple crystals for transmission to improve beam width at depth Overlap groups to increase number of scan lines across FOV

Electronic Focusing

Multiple elements form beam Alters in-plane beam width Beam width most narrow in focal zone

Damaged PZT : in Linear Sequential Arrays

Only the scan lines that would have been created by the defective crystal are affected

which array have a more severe problem with grating lobes?

PHASED array due to the steering of the US beam.

which transducer create large angle steering?

Phased array

Transmit and Receive Focusing

Receive Transmit Both

which technique that is used to suppress the grating lobes when they exist?

SUBDICING The normal elements are divided into smaller subelements which effectively reduce the center to center distance between transmission points.

Damaged PZT Crystal:Vector Array

Since a Vector array is a combination of a phased and sequential array, any malfunction will behave in a similar manner.

Damaged PZT Crystal in Mechanical Sector

Since there is only one crystal in a mechanical transducer, if it is damaged the entire image is lost.

Beam Steeringin Curvilinear Array

Some but not all of the crystals are fired simultaneously to create a single sound beam. The beam is directed straight out of the crystals but unlike the linear array, the pulses travel in different direction since they are arranged in an arc.

Image Shape in Curvilinear Array

TRAPEZOIDAL and curved at near Transducer

Beam Steering: Linear Phased Array

The 2D image is built up with an electronic steering process called phasing. The sound beams are electronically transmitted in different directions without the use of moving parts and are directed in a pattern similar to the Mechanical Sector.

Crystals: Annular Phased Array

The active element appears DISC-LIKE. Rather than a single element, the disc comprises multiple ring-shaped elements with a common center. Their appearance is similar to the rings that appear on a bull's-eye target.

Beam Focusing in Mechanical Sector

The focal depth is fixed by either a curved active element or an acoustic lens. The focusing occurs in both the horizontal and vertical planes because of the hour-glass shape of the sound beam.

Image Shape:Annular Phased Array

The image is fan or sector shaped like the Mechanical Sector

Image Shape: Linear Phased Array

The image is fan or sector shaped, similar to that produced by the Mechanical Sector.

Crystals in Mechanical Sector

The minimum active elements is one and resembles a coin

SUBDICING

The normal elements are divided into smaller subelements which effectively reduce the center to center distance between transmission points.

Secondary Lobes - Side lobes

There are two types of secondary lobes: Side lobes and Grating lobes. 1. Side lobes are present with all transducer types and result from vibrations and interference phenomena.

Crystal in Curvilinear Array

This array has 120-250 rectangular shaped strips of PZT material side by side in a bowed line. This array is large, with a footprint as long as 10cm, with each crystal about one wavelength in width.

Transmit Focal Zone

Transmit focus applied to specific depth (vary delay times)

Image Shape:Vector Array

Trapezoidal images are created by this transducer. Ahowever unlike curvilinear arrays, the top of the image is FLAT.

Crystal:Vector Array

Vector array is designed with 120 to 250 rectangular-shaped strips of PZT material. The footprint is larger than a phased array yet smaller than a sequential array.

advantages of dynamic receive focusing

WITHOUT a loss in FR or LINE DENSITY unlike transmit focusing

Damaged PZT Crystal in Curvilinear Array

When one active element in a curvilinear array is damaged, only the portion of the image directly below the damaged element is affected.

Damaged PZT Crystal: Annular Phased Array

When one ring of an annular array is damaged, only a portion of the image is lost. This will appear as a HORIZONTALor side to side band of dropout at a particular depth. If the inner most ring is damaged then superficial imaging will be sacrificed; if an outer ring is damaged then a deeper horizontal ban will dropout.

which technique reduces the intensity of side lobes in electronic arrays.

apodization

-dynamic focusing is limited to one fixed depth per transmitted pulse?

is applied for ALL DEPTHS unlike transmit focusing

disadvantage of large angle steering except its larger FOV?

reduces spatial resolution EXCEPT annular array

Grating lobe - Secondary Lobes

unique to crystals arrays, are caused by the regular periodic SPACING of elements within the array. When element-to-element spacing is less than one wavelength then grating lobes are eliminated. However, this is rarely the case.

APODIZATION

uses a variable-strength voltage pulse to the crystals across the aperture during transmit focusing. The excitation voltage to the individual crystals of each segment is maximized at the center and reduced toward the periphery.