Probe evolution
Single disc, mechanically steered, annular array, electronic array
Mechanical transducers
-started out as thin disk shaped crystal 6-19mm in diameter
- fixed focal zone with varying configuration
+ linear translation
+ wobbler
+ oscillating element
+ oscillating mirror
Linear translation
- oldest technology
- moved crystal assembly along track to SWEEP the tissue
Wobbler
- fixed and element onto a motor that rotated back and forth to obtain field of view
- very popular type of probe that produced a SECTOR image
- fluid filled and moving parts risked production of air bubbles
Linear field of view
- to achieve linear FOV probes use a reflector or acoustic mirror to direct the sound into the patient
- OSCILLATING MIRROR that moves with a stationary element
Rotating wheel
- several crystals that spin and only pulse when the element passes the FOV
Annular array
- biggest advances in transducer technology
- probe still mechanically steered you could electronically change the focus
Annular array - focusing
- uses concentric disco to allow for the focusing of the beam
- changing the disc that is fired you changed the diameter which varies the focal depth
- wider th crystal the larger the distance to the focal point
Annular array - limitation
- still has the same limitation for mechanically steered probes
- inherit to annular arrays was grating lobes
- grating lobes are low energy beams that radiate away from main axis and make artifact
+ grating lobes are called side lobes in single disk element
Electronic arrays
- have multiple small elements aligned in linear rows
- 2nmain types
+ phased sector array
+ phased linear array ( and curved) - have wide bandwidths and need flexibility to perform many exams
Electronic array - phasing
- uses nanosecond time delays to change when the voltage excites the crystal
- purpose to control the Beam using Huygens principle and allow for beam focusing snd steering
Electronic arrays - focusing and steering
- accomplished with phasing of voltages
- steering of beam can help direct th sound at an Angle for sector array
- vector scanning is same as steering the beam
Electronic arrays - phased sector array s
- all crystals (128) are fired at nearly the same time to steer beam across the face of the probe
- sector FOV can be as large as 90’ or as small as 30’ with the thought of scanning the ribs as reason for the shape
Electronic arrays - linear arrays
- have relatively large footprints with 64-100 elements in a row
- FOV is rectangular shaped but can be trapezoidal
- no scan line divergence
- highenfrequencies
Segmental sequencing
- group of crystals fired to crate a beam followed by a second group one crystal down
Electronic array - curved linear
- same principle as linear array
- small rectangular elements that are curved not straight
- beam angles are like spoke of a wheel
- large footprint and FOV
- see nearfield and Farfllied
- disadvantage = resolution in far field bad from space between beams
Apodization
- crystal can ring in undesirable ways along length and width
- potential of creating off axis beams called grating lobes that make artifact
- apodization weakens the voltages from the inside out to prevent this
Multidimensional probes
- obtaining 3D and 4D in 2 ways
+ free hand using traditional array probe
+ volumetric probe
- beam swept over area of interest to obtain information for 3D r construction
1.5 and 2D probes
- 2D probes are actually 1D with only one row of crystals
- 1.5D probes can adjust focus in x,y, and z planes and have 1 1/2 rows
Specialty probes
- many probes used for biopsies, intervention, endocavitary exams
- intravascular probe scan from within the lumen
- transesophageal echoes probes for heart
Continues wave probes
- dedicated Czw probes have 2 crystals one transmits the other receives
- probes are air backed and have continuous AC driving voltage
- vascular called pencil probe and echo is ped-off probe
