Lecture 6: Transducers
Annular Array
An array consists of a *group of closely spaced* piezoelectric elements each with its own electrical connection to the US instrument. This enables the elements to be excited individually or in groups to produce a beam. Today's transducers are arrays. These are *multiple element** instead of single- element.
Ped-Off/Non Imaging Probe
Blind Doppler probe (Pedof, pencil probe). It is a single element probe, sometimes the crystal is broken into two halves Beam is symmetric in elevation and lateral Has a broad depth of view Very narrow . There is no image . It is manually steered has a fixed focus for transmit and receive.
Dynamic Receive Focusing
Receive focusing is applied, but this is not controlled by the operator -It is done dynamically, the focus changing rapidly as echoes arrive from deeper and deeper reflectors. - Dynamic receive focusing is *applied at all depths,* but does not slow down the frame rate. It changes continuously
Sequencing
Sequencing refers to exciting groups of elements in a specific pattern to linearly scan a region. Sequencing is commonly used with large linear and curved linear arrays.
What is the difference in side lobes and grating lobes?
Side lobes are possible *with any type of transducer system*, both single element and array. -If present they are located directly alongside the main beam.
Disadvantages of Mechanical
Parts wear out (mechanical), Parts break, Motion artifacts, Limited temporal resolution, *Excessive grating lobes,* More expensive electronics necessary.
Mechanical Steering
*STEERING is MECHANICAL* The active element is moved by a motor, oscillating crystal or mirror Sound beams are built up, and as the crystal moves sound beams are emitted. The motor sweeps the beam back and forth to collect lines of sight. Hence the scan plane is created through mechanical steering.
Steering of Mechanical Array
*Steering:* Steering is performed *mechanically.* If there is a defective crystal, it will cause a horizontal (side to side) band of drop out.
Linear Phased Arrays
- Same as sequential only with a phased transducer, If there are 64 elements, then 64 elements are excited at the same time. - Elements are side by side. - Same type of focusing and steering as a phased array. - These can also be sequential depending on manufacturer.
Side Lobes
All transducers produce side lobes. *They are usually low intensity compared to the main beam*
MECHANICAL TRANSDUCERS
In the early days of US a *single disc shaped crystal* transducer was used. - These simple transducers could only look straight ahead and could only view a different region of the body by being manually angled. - In essence it was one-dimensional scanning technique
Phased Array
*Arrays provide two advantages:* They enable electronic beam steering, beams are swept across the imaging field with no mechanical parts. They enable *electronic focusing* and beam formation, providing very effective control of the focal distance and the beam width throughout the imaged field.
Multiple Focus Zones
*Disadvantage* is it diminishes temporal resolution. *Advantage* is better lateral resolution
Fan or Sector Shaped
*Element:* Small diameter rings have a shallow focus, but diverge rapidly. Large diameter rings have a deep focal length. *Image is fan or sector shaped*
Endosonography Probes
*Endosonography transducers:* Endovaginal, Endorectal, and Endoesophageal
Fixed Focusing
*FOCUSING:* The focus in these transducers is fixed(Conventional) to a specific depth and cannot be changed. Focused either internally (curved element) or externally (acoustic lens)
Focusing a Sequential Array
*FOCUSING:* Sequential arrays have a *fixed transmit and receive focus without any steering.* Pulses are sent down parallel lines.
Focusing of Mechanical Array
*Focusing:* Primary advantage of annular arrays is the *multiple transmit focal zones.* Selected focal zones use inner crystals for shallow planes and outer crystal for deep regions.
What is the difference in side lobes and grating lobes?
*Grating lobes, on the other hand, are found only with array transducers*. Rather than being situated directly alongside the main beam, if present they are off at a fairly large angle.
Side Lobes
*Higher frequency transducers reduce the number and intensity of side lobes.*
Convex Array
A convex array has *120-250 rectangular shaped* strips of PZT material arranged side by side in a bowed line. (Arc) Beam lines are not parallel, but emerge at different angles, like the spokes of a wheel
Problems with Arrays
A major problem with electronic array transducers is the formation of secondary lobes of sound waves. These secondary lobes are created at the transducer and radiate outward at various angles to the main beam
Dynamic Receive Focusing
A much more accurate image can be created when the US receiver introduces *time delays* to some of the electrical signals during reception
Vector Arrays
A vector array Is a combination of linear sequential and linear phased array Sloped electrical time delay patterns can be introduced that steer the sound beam in many directions *Trapezoidal image Is created* * It steered and focused electronically*
Vector Arrays
A vector array has *120-250 rectangular shaped* strips of PZT arranged side by side in a line *The footprint can be small .* These are useful in abdominal scans such as kidney (small parts).
Early Sequential Array
Early scanner probe was bulky to fit on the abdomen e Images from early real time scanners had obstrusive scan lines, low dynamic range and resolution
Grating Lobes
Grating lobes are secondary lobes of beam energy that occur because the transducer is not a continuous surface, but diced into small elements. Grating lobes, if present, shoot the ultrasound off at a *large angle from the main energy beam*. This can add ghost images of the object being scanned.
Grating Lobes
Grating lobes can be reduced in strength and importance by designing the array so that there is *no greater than 1/2 wavelength* between the centers of individual elements. Sometimes it is not possible to meet this design requirement and grating lobes appear at noticeable levels
Early Sequential Array
If you have a defective crystal, there will be drop out from superficial to deep
Linear Sequential Array
The Linear sequential array has *a large acoustic footprint and creates rectangular images* (Each element is rectangular also). Image is no wider than the transducer. A linear sequential array has *120-250* rectangular shape strips of piezoelectric material arranged *side by side in a line.*
Convex Arrav
The array is large, acoustic footprint can be as long as 10cm. These may be sequential or phased array *Images are blunted sector shaped.* *Focusing is electronic and steering is in many directions*
Sequencing
The first crystal is excited then waits for the echoes to return and then the next crystal and the next until all the crystals have been activated. - This forms one image (one frame). Then the whole process is repeated for the next frame.
Fan or Sector Shaped
The image shape is *fan or sector shaped*, this is similar to the sweeping of a wiper blade across windshield. *Mechanical transducers have only a single crystal, the element resembles a coin** If there is a defective crystal it will destroy the entire image!
Linear Switched Array
The linear switched array is a group of elements which can *be turned either on or off* through electronic switches, most of these 200 elements. These could only be steered manually (by angling the probe) (No longer use)
Problems with Arrays
The machine assumes it comes from the main line of sight and places the echo at the appropriate spot These secondary lobes are also responsible for clutter which degrades the image. There are two main types of secondary lobes; *side lobes and grating lobes.* Lobes degrade lateral resolution
Transluminal
The miniaturization of electronics makes transducers small enough to be inserted on the tip of a catheter and used to image the internal structure of vessels. These are very high frequency transducers that get excellent spatial resolution *(10 to 20 MHZ).*
Dynamic Receive Focusing
The optimal time delays used during receive focusing change, depending upon the depth at which the reflection was created -Delay patterns during receive focusing *change continuously*
Sequencing
The rate at which the sequence of firing all 130 crystals is repeated determines the frame rate.
Multiple Focus Zones
The simultaneous *multiple transmit focal zones*, greatly expand the focal region of the instrument. .But this *lowers the frame rate* because of the multiple sound pulses at each different depth along the beam.
Phased Array/Pie Shaped
These have a Small footprint (Ple shaped display) (Cardlac). Unlike the linear and convex array transducers, *all the elements in the scanhead* are involved in producing the sound beam and recelving the echoes for each acoustic line of sight.
Transmit Steering
These phase delays between the excitation pulses drive the elements of a phased array transducer. Electronic steering allows for varying incident angles
Mechanical Annular Array
These transducers *have five to ten concentric rings piezoelectric elements (bul's eye).** Elements are excited sequentially from inside to out, resulting in circular symmetry of the beam.
Transesophogeal/Vaginal Probes
These transducers are capable of using very high line density and very high frame rates because the depth of penetration is very shallow. -This results in much improved imaging of the heart
Phased Array
These transducers have a number of elements generally between *64-128 elements* or more that are packed tightly in a small area rather than being side by side . This allows the face of the transducer to be much smaller than the linear array we *can image between the ribs with these. Cardiac*
Problems with Arrays
This produces artifacts in the image when they strike an interface and return to the transducer since the transducer doesn't know where the echo originates from
Transmit Steering
To steer the beam *electronically,* tiny time shifted pulses are applied to each element of the aperture (window) . In essence each transducer element acts as an individual transducer producing a spherical wave
Transducers
Types: Mechanical Electronic Transducers Linear arrays Convex linear array Phased arrays Vector array Annular array
MECHANICAL TRANSDUCERS
Unlike newer transducers, these mechanical scanners have *moving** parts to generate the scan. These transducers were some of the first contact scanners
Electronic Transmit Focusing
Unlike the focal distance of a single element transducer, the focal distance of an array may be *varied electronically* by changing the electronic delay sequence (nano-second time delays) Arrays allow the selection of the transmit focus distance by the sonographer
Multiple Focus Zones
When the sonographer activates the multiple focus zones, the number of pulses transmitted down each scan line increases, and the number of pulses required to make an image also increases,
Transmit Steering
sound beams produced by a phased array are *electronically steered* at various angles with respect to the array surface - Steering for each beam is achieved by introducing *nano-second time delays* in the excitation pulses applied to individual elements in the array
