Lecture 3 - Transducers, Fields,

How can you visually tell what the strength of an echo is?

the grey level it is assigned

Stronger echoes are assigned brighter greys (whites)

Weaker echoes are assigned darker colours

How is grey level effected by the strength of the echo?

stronger echoes (reflections) are assigned a brighter grey whilst weaker echoes are assigjned a darker grey

Why does fluid appear as darker than tissue on an echo?

the reflections/ echoes are weaker

What is a phased array transducer?

• multiple small individually controlled piezoelectric elements

• The beam can be electronically steered or focused by altering the timing of activation of individual piezoelectric elements. Delayed activation or phasing can steer the beam/ adjust the focal point.

• small face to enable imaging in small places such as in between ribs

What specific material enables the production of ultrasound?

ceramic piezoelectric crystals which have dual property: when electric current is applied can both produce and receive ultrasound waves.

How many transducer elements (piezoelectric crystals) are in a phased array probe?

68-80

How do piezoelectric crystals generate ultrasound ?

• electric current is applied

• piezoelectric crystals change shape

• alternating current causes them to rapidly oscillate producing vibrations which are ultrasound

What is the structure of a transducer?

1. acoustic lense

2. matching layer

3. piezoelectric tranducer

4. backing layer (surrounded by front and back electrode)

What is the purpose of the matching layer?

• matching layer impedance (Z) is the geometric mean of the Z of ceramic and soft tissue.

• reduce the difference in impedance between the PZT crystal layer and the soft tissue to minimise internal reflections from the probe and maximise amount of ultrasound transmitted into the patient.

What is the purpose of the backing layer?

• high impedance

• absorbs the ultrasound waves being transmitted behind the probe. Dampens ringing and reverberation

• reduce unwanted reflections not from the tissues/ area of interest

What is the near field called?

Fresnel zone

What shape are the beams in rhe near filed

cylindrical

pWhat formula governs the length of the near field

aperture diameter ² / wavelength . Which explains why as frequency increases, wavelenght reduces and the field gets longer

What is the far field called?

Fraunhofer zone

What is the focal point?

• the point at which the beamwidth is the narrowest = point of greatest lateral resolution

What is the beam doing in the fresnel zone?

converging

the spatial res is better

What does the beam do in the far field Fraunhofer zone?

diverge

spatial res worsens

Which part of the transducer focuses the beam?

the acoustic lens

What happens to the beam when it is focused?

• make each beam as thin as possible

• narrower beam width, higher resolution

but diverge more in the far zone

By what other mechanism is the beam focused?

electronically by the sonographer

Can we adjust the fresnel zone?

Yes, if we increase the depth of the Fresnel zone then we expand the area where the spatial resolution is sharper,

How does the transducer steer the beam?

• electronic delays

• altering the timing for activating the individual elements

How does the length of the fresnel zone change with a higher frequency transducer?

• longer fresnel zone, shorter fraunhofer zone (worse resolution in the far field)

How many PZT elements in a 3D probe?

thousands

In short how do 3D transducers work?

scan organs in 3 planes and display simultaneously

Briefly what type of probe is used in TOE?

multiplane probe which can be rotated 180º to cut through the heart in every plane

WHy can TOE provide better image quality?

TOE views the heart from the oesophagus

• probe closer to heart so US doesn’t have to penetrate as deeply

• so higher frequencies can be used

• which provide greater spacial resolution

• depth is reduced so higher frame rate??

• better visualisation more posterior structures such as of LAA and pulmonary veins

What are some modern advances in ultrasound machine ?

-portable mobile plug in machines

What is one major drawback of portable mobile US machine?

• data stored on the machine more likely to be lost or stolen as its portable

• resolution poorer

• patient confidentiality

What is a different type of probe being developed?

• butterfly probe which uses microchips not piezoelectric crystals to produce ultrasound

Why is beam divergence a problem?

• reduced lateral resolution which can effect the image quality

How does the beam diverge?

• beyond the focal point

What happens to resolution if you have multiple focal points?

spatial res improves, temporal res reduces because takes double/ tipple as long to receieve signals from multipe focal points.