REAL-TIME IMAGING PRINCIPLES AND INSTRUMENTATION
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| Mechanical real time transducers/scanners | First generation scanners
.Water Path scanners
.liquid scanners
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| PRINCIPLES | . A-mode .static B-mode
.the data (echoes) are acquired one frame at a time.
. A-mode you are only imaging one line of sight.
. static B-mode you acquire the image by moving the transducer as it acquires one line of sight at a time.
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| LIQUID PATH SCANNERS | . liquid path transducer the crystals are put in a liquid .water
. alcohol.
. 2 kinds of liquid path scanners.
.nonreflecting
. reflecting path.
.The crystals can be
. single
. multiple
in either sector or linear format.
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| LIQUID PATH SCANNERS | . nonreflecting transducers are called
. “inline” transducers.
. Sound path is direct from the crystal to the reflector and back.
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| LIQUID PATH SCANNERS | . The reflected transducer the sound is reflected off a mirror before traveling into the patient.
.Reflecting transducer can either move the crystal or move the mirror to direct the sound beam.
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| LIQUID PATH SCANNERS ADVANTAGES | . loss of image from the first mm of scan data is not lost like in the contact scanners
.The design allows strongly focused large diameter crystals to improve lateral resolution.
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| Real time SCANNERS Real time scanners include all of the following: | .Mechanical scanners
.Linear array
.Curvilinear
.Phased
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| PRINCIPLES /REAL TIME IMAGING | For real-time gray-scale imaging you must acquire the data in a very rapid fashion to give the perception of motion.
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| PRINCIPLES /REAL TIME IMAGING | The ultrasound beam is swept or steered through the area of interest by
. mechanical
. electronic means in a repetitive automated fashion.
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| PRINCIPLES /REAL TIME IMAGING | Instead of a single sonogram with multiple lines of sight, as in static scans, multiple sonograms are formed, each consisting of multiple lines of sight.
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| MECHANICAL TRANSDUCERS | .early days a single disc shaped crystal transducer was used .
. could only look straight ahead and could only view a different region of the body being manually angled.
. one-dimensional scanning technique
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| MECHANICAL TRANSDUCERS | .Over time able to see more regions “simultaneously”
. became two-dimensional data
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| MECHANICAL TRANSDUCERS | The first major class of scanners
. mechanical scanners
. contact scanners
. the simplest
. least expensive.
. NO LONGER USED
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| MECHANICAL SCANNERS | . Real-time mechanical transducers employed disc shaped crystal attached to a motor
1.The transducer wobbles back and forth is one
2.Has multiple crystals that rotate on a wheel.
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| MECHANICAL SCANNERS | .The single crystal wobbling model, when the crystal is at the extreme left position data collection is initiated.
. The crystal is excited and the sound beam is directed along the line of site.
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| MECHANICAL SCANNERS | STEERING is MECHANICAL
The active element is moved by a motor, oscillating crystal or mirror through a pathway automatically creating a scan plane.
.The image is built up by this motor
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| MECHANICAL SCANNERS | FOCUSING
The focus
.is fixed to a specific depth .cannot be changed.
.Curvature of the acoustic lens .focuses the beam
.This limits the range of resolution .
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| MECHANICAL SCANNERS | . more lines of sight in the top of the image.
. than the deeper areas .Resolution deep in the patient is not as good.
.Resolution shallower is better
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| MECHANICAL SCANNERS Conventional or fixed focusing is achieved 2 different ways: | .Internal focusing with the use of a curved active element
.External focusing with the use of an acoustic lens
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| MECHANICAL SCANNERS The image shape | . fan
. sector shaped
.If there is a defective crystal it will destroy the entire image! (Because there is only one crystal if it breaks you loose the entire image!)
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| ARRAYS( Todays transducers are arrays.) | . multiple element
.instead of single-element.
. consists of a group of closely spaced piezoelectric elements .Each with its own electrical connection to the US instrument.
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| ARRAYS | .This enables the elements to be excited
. individually
. groups to produce a beam.
.There are 4 types of arrays:
Linear (Also Linear Sequential, Segmental)
.Curvilinear
.Phased
.Annular
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| ARRAYS Arrays provide two advantages: | 1. They enable electronic beam steering.
. beams are swept across the imaging field with no mechanical parts
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| Arrays provide two advantages: | 2. They enable electronic focusing and beam formation .Providing very effective control of the focal distance and the beam width throughout the imaged field.
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| ANNULAR PHASED ARRAY TRANSDUCERS | These transducers have 5 to 10 concentric rings piezoelectric elements
. Do-nut shaped
. That are excited sequentially from inside to out, resulting in circular symmetry of the beam.
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| ANNULAR PHASED ARRAY TRANSDUCERS | The data acquired is 3 dimensional
.it can only be displayed in 2 dimensions.
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| ANNULAR PHASED ARRAYS | Element:
Small diameter rings have a shallow focus but diverge rapidly
.Large diameter rings have a deep focal length
.Selected focal zones use inner crystals for shallow regions .outer crystals for deep regions
.Image is fan or sector shaped
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| ANNULAR ARRAY TRANSDUCERS | Focusing:
Phasing provides electronic focusing in all planes at all depths
.provides optimal lateral resolution at all depths
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| ANNULAR ARRAY TRANSDUCERS | Steering:
Steering is performed mechanically.
If there is a defective crystal it will cause horizontal (side to side) band of drop out.
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| ANNULAR ARRAY TRANSDUCERS | An annular array transducer is made with
. concentric crystal elements, . .the shape of an annulus.
The ultrasound beam can be focused and is 3 dimensional.
.This allows the focus to be varied electronically .
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| ANNULAR ARRAY TRANSDUCERS | .The transducer assembly is oscillated from side to side, or bounced off an oscillating mirror to produce a sector scan.
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| ANNULAR ARRAY TRANSDUCERS | .The beam width is the same. .slice thickness direction
. in plane direction
. the beam converges symmetrically to the focal point (like an ice cream cone). (Sector shaped)
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| LINEAR ARRAYS | In the late 1970s electronic real-time instruments were developed.
.Images with higher quality .faster frame rates
.were possible
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| LINEAR ARRAYS | The lines of site were increased .improved image quality
.electronic arrays are not subjected to the wear that mechanical systems have .
.transducer lasted longer
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| LINEAR ARRAYS | .more expensive
.better quality of the images
.the ability to change the focal zone and have multiple focal zones
. makes these instruments superior to the mechanical ones.
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| LINEAR SEQUENTIAL ARRAYS | .(Switched array) has a large acoustic footprint
.creates rectangular images
.Image is no wider than the transducer
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| LINEAR SEQUENTIAL ARRAYS | .A linear sequential array has 120-250 rectangular shape strips of piezoelectric material. .arranged side by side in a line.
. active elements in linear sequential arrays are much larger than those found in linear phased arrays.
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| LINEAR SEQUENTIAL ARRAYS | Each crystal is about 1 wavelength in width
. large = 10 cm long.
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| SEQUENTIAL LINEAR ARRAYS The first type of sequential linear transducer | 1.sequential
non-segmental
2.linear array.
.Multiple crystals are arranged in a straight row
.each produces a US beam and then receives the returning echoes
. for data collection along one line of sight.
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| SEQUENTIAL LINEAR ARRAYS | .The crystals are activated in a sequential fashion .
.forms the individual lines of sight
.number of crystals in the array determines the number of lines of sight for each image.
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| SEQUENTIAL LINEAR ARRAYS | If the array contains 130 individual crystals that are fired (excited) one at a time in sequence then 130 lines of sight are acquired.
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| 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.
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| SEQUENTIAL LINEAR ARRAY | the geometry of these transducers is large (in the lateral direction) the lateral dimension of the scan can be accomplished without beam steering.
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| SEQUENTIAL LINEAR ARRAY | 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
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| SEQUENTIAL LINEAR ARRAY | .duration of the time delay between each crystal activation is set by the maximum scanning depth (13 μs for each cm in depth).
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| SEQUENTIAL LINEAR ARRAY | For example, if you are scanning 15 cm into the patient: The first crystal is activated and 195 μs later (13 μs/cm x 15 cm) the second crystal is excited.
This continues until all 130 crystals have been activated.
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| SEQUENTIAL LINEAR ARRAYS | The rate at which the sequence of firing all 130 crystals is repeated determines the frame rate.
The 15 cm depth limits the pulse repetition frequency (PRF) to 5000 pulses per second (pps).
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| SEQUENTIAL LINEAR ARRAYS | With 130 lines per frame(lpf) a maximum frame rate of 38 images per second is obtained.
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| SEQUENTIAL LINEAR ARRAYS ELEMENT SHAPE | .Each element is rectangular
.A few elements (5-10) but not all are fired exactly the same time
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| SEQUENTIAL LINEAR ARRAYS ELEMENT SHAPE | The sound wavelets from the multiple crystals interfere with each other to create a single sound beam that travels straight ahead into the body, then the next group is fired.
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| SEQUENTIAL LINEAR ARRAYS ELEMENT SHAPE | .the physical dimension of the crystal dictate the width of the US beam.
Linear arrays consist of many small crystals along a row
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| SEQUENTIAL LINEAR ARRAYS ELEMENT SHAPE | The major problem with sequential linear arrays is that the small crystal size produces a short narrow near field and a rapidly diverging far field
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| SEQUENTIAL LINEAR ARRAYS FOCUSING | .Sequential arrays have a fixed transmit and receive focus without any steering
.Image shape is rectangular no wider than the transducer
.If you have a defective crystal there will be drop out from superficial to deep
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| SEQUENTIAL LINEAR ARRAYS SEGMENT | (segment) of crystals in the linear array, the crystals act in concert to produce a deeper near field and less diverging far field compared to a single crystal acting alone.
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| SEQUENTIAL LINEAR ARRAYS SEGMENT | The segmental linear array creates fewer lines of site so the spatial mapping is not as good as with the sequential linear array with more lines of site.( Another name is Step down Linear array)
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| SEQUENTIAL LINEAR ARRAYS SEGMENT | The resultant ultrasound field, however, provides a more favorable beam pattern.
For example, assume that 64 individual crystals are fired in groups of four.
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| SEQUENTIAL LINEAR ARRAYS SEGMENT | The first four crystals are fired simultaneously and then 195 μs (the time required for the round trip at 15 cm depth) and then the next 4 crystals are fired and so on until all 64 have been fired.
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| SEQUENTIAL LINEAR ARRAYS SEGMENT | .You will get 16 lines of site that are spaced widely apart.
.The overall beam pattern will be improved but the resolution will be worse because of the fewer lines of site.
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| SEQUENTIAL LINEAR ARRAYS SEGMENT | To achieve good lateral resolution at great depths a combination of large crystal size and high line of sight density is required.
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| SEQUENTIAL LINEAR ARRAYS SEGMENT | This is accomplished by using a step-down segmental array
If you have an array of 64 crystal elements, and fire 4 at a time and each group was offset by one crystal, a total of 61 lines of site are acquired for each image.
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| SEQUENTIAL LINEAR ARRAYS | The segmental array with 64 - 200 crystal elements produces good temporal resolution (high frame rate) and good spatial resolution ( beam size and number of lines of sight).
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| SEQUENTIAL LINEAR ARRAYS | The field of view is rectangular with the in field length equal to the length of the transducer.
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| SEQUENTIAL LINEAR ARRAYS | Additional adjustments in focusing are necessary to reduce the beam width and improve lateral resolution.
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| SEGMENTAL LINEAR ARRAY | These transducers were physically large and consisted of many elements
(200 or more) They had no steering capability therefore the image shape was rectangular
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