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Sensing Systems and Signal Processing
Dr Richard
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Ultrasonic bio-imaging
Introduction
Biomedical ultrasound uses sound image the body.
The frequencies are well above the audible range and are typically in the few MHz range.
Ultrasound is non-ionising – the energy the sound carries is sufficiently low that it does not cause damage to the tissue it is propagating in (there is an exception to this for HIFU where it is used to destroy cancerous tissue).
Ultrasound scanners provide real time, low cost, with reasonable resolution and contrast and are widely used in hospitals around the world for a wide range of applications.
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Introduction
Transducer emits ultrasound pulse.
Differences in acoustic impedances between tissues causes a partial reflection of the sound. These travel back to the transducer and are seen as echoes in the time trace.
The time of the echo allows the depth of the object to be calculated.
The amplitude (at that depth) of the echo tells you something about its acoustic properties.
transducer
Sound emitted
Introduction
You could scan the transducer around (in a line) to produce a cross sectional image or B scan.
However, transducer arrays are usually used so that the scanning can be done electronically.
The transducer array emits ultrasound pulse on each element in turn. It records the trace and then moves onto the next element. Each signal is processed and forms an image of x vs z (space vs depth)
This gives an image of a slice through the object.
Transducer
Sound emitted
Echo locations
Introduction
Using ‘Phased arrays’ scanning with no moving parts is possible.
Electronic steering of a beam is possible by changing the delay of the excitation pulse for each element in the transducer array
This allows complex control of the beam, enabling full 3D scanning of a volume.
http://www.ob-ultrasound.net/somers.html
Scanning for the BPD
Biparietal Diameter
Morphology: the “Biparietal Diameter”
Among the most accurate 2nd trimester measures of gestational age.
Measured from beginning to inside aspect of fetal skull
Using this same image the fetal head circumference obtained The BPD can be used to determine gestational age with a 95% confidence of 10 to 14 days.
If the gestational age is already known with precision (1st trimester ultrasound scan), then the BPD can be used to evaluate fetal growth.
Liquid to bone interface
Ultrasound bio-imaging
Femur length
Used to determine gestational age
more useful in evaluating foetal weight
It is also useful as a marker for foetal malformation and genetic abnormality.
Many, though not all, trisomy 21 foetuses have shortened femurs.
Twins and other multiple gestations can usually be identified fairly early in pregnancy.
They may be seen with two separate gestational sacs (diamniotic, dichorionic twins) or occupying the same gestational sac (monochorionic twins).
Ultrasound baby-imaging!
Other Applications
Many other applications for ultrasound in the biomedical field
Anesthesiology – for guiding needles
Cardiology – imaging heart valve function
Opthalmology – seeing inside the eye
Urology – bladder imaging and function
Nephrology (kidneys) – kidney diseases
Musculoskeletal – tendon, muscle, nerves, ligament, soft tissues masses and bone surfaces.
Impedance matching:
From liquid or solid to air => poor impedance matching, low sound propagation
No good for lung imaging
(though they can be filled with liquid **saturated with oxygen!)
Spatial resolution limited by wavelength of sound and matrix inversion of multiple unknown sources
Temporal resolution limited.
As frequency rises leads to high attenuation and low propagation distances
Absolutely safe, so far as 30+ years of imaging have demonstrated!
Mature technology now used for “art” as well as medical science – moving (4D) ultrasound and 3D-printed babies!
Ultrasound limitations:
0 5 10 15 20 25 30 35 40 45
Gestational Week
Biparietal Diameter
Femur Length
Head Circumference
Circumference
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Gestational Week
Biparietal Diameter
Femur Length
Head Circumference
Circumference
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