MEDS12001 Lecture Notes - Lecture 4: Fresnel Zone, Acoustic Impedance, Lead Zirconate Titanate
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WHAT IS A TRANSDUCER?
• a device capable of converting energy
from one form to another (typically
converting input energy to output energy
• this conversion of energy is bidirectional
TRANSDUCER DESIGN
• physical housing assembly
• electrical connections
• piezoelectric element
• backing material
• acoustic lens
• impedance matching layer
THE PIEZOELECTRIC CRYSTAL/ELEMENT
• used to produce and detect ultrasound
waves
• special property to expand and contract
• traditionally quartz, now PZT (lead
zirconate-titanate)
o poling
• Pure wave and XD clear crystals
o Wider bandwidth and improved
sensitivity
• Principle of piezoelectricity
POLING
CRYSTAL PROPERTIES
• Able to convert electrical energy to sound
energy and vice versa
• Be efficient to maximise energy transfer
• Low acoustic impedance
• Wide bandwidth
• High sensitivity
OPERATING FREQUENCY OF THE CRYSTAL
Determined by:
• Propagation speed of the element
material
• Thickness of the transducer element
(thinner elements operate at higher
frequencies)
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SENSITIVITY AND BANDWIDTH
NEW WAVE CRYSTALS/TRANSDUCERS
• Near perfection alignment
• More responsive to compression and rare
faction than traditional PZT
• Good electromagnetic coupling efficiency
• Better penetration and better image
quality
DAMPING MATERIAL (BACKING LAYER)
Pros:
• Improves resolution
• Broadens bandwidth
Cons:
• Reduces the ultrasound amplitude and
thus decreases the sensitivity of the
system
MATCHING LAYER:
• A material placed on the transducer face
to improve sound transmission across the
element-tissue boundary
o Impedance of the element is 20
times that of the tissue
• Impedance of matching material is
intermediate between those of the
transducer element and tissue
COUPLING MEDIUM
• Eliminates air layer between skin and
transducer
• Facilitates passage of sound in and out of
the tissue
• Z value between matching layer of probe
and skin
INVASIVE TRANSDUCERS
• Designed to enter the body via the vagina,
rectum, oesophagus, or blood vessel
• Allow higher frequencies to be used
• Improve resolution
SOUND BEAM
• The width of a pulse as it travels away
from the transducer
• The transducer produces a sound beam
with a width that varies according to the
distance from the transducer face
• Intensity is NOT uniform throughout the
beam
BEAM PARAMETERS
• Near (Fresnel)
zone
• Near zone length
(NZL)
• Far (Fraunhofer)
zone
• Focal length
• Focal zone
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NEAR ZONE
• Also call Fresnel zone or near
field
• Region extending from the
transducer to the minimum beam
width
• Beam width decreases with
increasing distance from the
transducer
NEAR ZONE LENGTH
• Determined by the size
and operating
frequency of the
element or group of
elements
• Increases with
increasing frequency
• Increases with increase
in size of the element
(aperture)
FAR ZONE
• Also far field or Fraunhofer
zone
• The region that lies beyond a
distance of one near zone
length
• Beam width increases with
increasing distance from the
transducer
FOCUSING
• Improves resolution
• Only
accomplished in
the near zone
• Beam width is
decreased in the
focal region and
widens in the
region beyond
• Focal length is
the distance from
the transducer to
the focal region
FOCAL POINT
• The intensity of the beam is increased at
the focal point
• Intensity = power/area
• Better resolution
ARRAYS
• A complete scan of the ultrasound beam is
called a frame
• Real-time sonography presents images
(frames) in a rapid sequential format
• Electronic scanning is performed with
arrays
• Arrays are operated in two ways, called
sequencing and phasing
• The elements are arranged in a straight or
curved line
find more resources at oneclass.com
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Document Summary
What is a transducer: a device capable of converting energy from one form to another (typically converting input energy to output energy this conversion of energy is bidirectional. Transducer design: physical housing assembly, electrical connections, piezoelectric element, backing material, acoustic lens impedance matching layer. Crystal properties: able to convert electrical energy to sound energy and vice versa, be efficient to maximise energy transfer, low acoustic impedance, wide bandwidth, high sensitivity. Determined by: propagation speed of the element material, thickness of the transducer element (thinner elements operate at higher frequencies) The piezoelectric crystal/element: used to produce and detect ultrasound waves special property to expand and contract traditionally quartz, now pzt (lead zirconate-titanate, poling, pure wave and xd clear crystals, wider bandwidth and improved sensitivity, principle of piezoelectricity. Sensitivity and bandwidth: a material placed on the transducer face to improve sound transmission across the element-tissue boundary. Impedance of the element is 20 times that of the tissue.
