comprises a single slap of PZT cut into a collection of separate pices called elements. Each active element is connected by a wire to its own electronic circuitry
Mechanical Transducer
single,circular: disc shaped
fan or sector-shape
fixed or mechanical focusing
damage - entire image lost
Types of array transducers
Linear
annular
convex
Linear phased arrays
small footprint
100-300 elements, side by side, rectangular and narrow
electronic steering (phasing)
electronic beam focus
damage - inconsistent focusing
Phased array
always means adjustable or multi-focus
Beam former
the electonics within the ultrasound system that create a pattern
Annular phased arrays
disc shaped element
mechanical steering
multiple focal zones (not adjustable)
inner circle - shollow scan
sector shape image
damage - horizontal or side to side band
Linear sequential arrays
large footprint
120 - 250 piezoelectric elements:
some bur not all crystals are fired simultaneously
transmit and receive focusing (electronic)
rectangular image
damage - vertical line
Convex (curved) arrays
120-250 rectangular elements
some but not all crystals are fired simultaneously
electronic beam focus
blunt sector shape
damage - vertical line
dynamic receiving focus
Vector arrays
120-250 rectangular elements
small footprint
electronic beam steering
electronic focusing
trapezoidal image
Resolution
accuracy in imaging
Slice thickness
elevational resolution
Image resolution
three dimensional space:
side to side
shallow to deep
above and below
Side lobes
off axis sound beams from a single element transducer
Grating lobes
off axis sound beams created by array transducers
Apodization
stronger electrical signals are used to excite the innner crystals, annd progressiely weaker electrical spikes excite the outer crystals. this diminishes side lobes
Variable aperture (dynamic aperture)
changing the number of crystals along the face of the probe used to transmit pulses and receive reflctions
(number of elements used to receive reflected echoes.
Temporal resolution
accuracy in time
the ability to precisely position moving structures from instant to instant
determined by frame rate
higher frame rate improves temporal resolution
Frame rate
the ability of the system to create numerous frames each second
measured in Hz
determined by speed of sound and imaging depth
Tframe
the time it takes to make a single image
inversely related to frame rate
sonographer setting that control frame rate
imaging depth
number of pulses in each picture
Imaging depth
shallow imaging increases frame rate and improves temporal resolution
Number of pulses per image
pulses per frame and frame rate are inversely related
multi vs. single focus
sector size
lines per angle of sector
Multi focus
negative effect on temporal resolution
improves accuracy of the individual image
superior lateral resolution
Sector size
increase in sector size decreases temporal resolution
Line density
space between sound beams
high line density decreases temporal resolution
improved spatial resolution
dynamic range
a method of reporting the extent to which a signal can vary and still be accurately measured
measured in dB
relative measurement or ratio
dynamic range of components
transducer 120
receiver 100 - 120
scan converter 40 - 50
display 20 - 30
archive 10 - 30
Harmonic imaging
the creation of an image from sound reflctions at twice the frequency of the transmitted sound
arise from no -linear behavior
Fundamental frequency
sound created by the transducer and transmitted into the body
Fundamental image
the image created by rocessing reflections that have the same frequency as the transmitted sound
Harmonic image
image created by processing reflections that are twice the fundamental frequency
Linear behavior
proportional or symmetrical
systems respond in an even manner
Non-linear behavior
irregular or disproportionate
system behaves unevenly
Contrast agents
microbubbles that are entrapped in a shell that are ingested or injected into the circulation
have a different acoustic fingerprint than blood or tissue and therefore create strong reflections that actually "light up" blood chambers and vessels
Contrast agent requirements
1. safe
2. metabolically inert
3. long lasting
4. strong reflector of ultrasound
5. small enough to pass through capillaries
Contrast harmonics
when an ultrasound pulse interacts with microbubbles, a small amount of energy in converted from the fundamental frequency to the harmonic frequency
contrast harmonics are created during reflection
Resonance
uneven behavior caused when the microbubble within the sound beam grows or shrinks in relation to the pressure
Mechanical index (MI)
the amount of contrast harmonics produced is estimated by MI
MI=peak negative pressure/sqaure roote of frequency
high frequency -less pressure- lower MI
high mechanical index - bubbles expand and break apart
Contrast agent characteristic
nature of the outer shell
gas that fills the microbubbles
Tissue harmonics
as a sound wave travels in the body, a miniscule amount of energy is converted from the fundamental frequency to the harmonic frequency
faster through compressions - slower through rarefactions