Physics Test #2

A transducer converts one form of ______ to another

energy

ex. motor, lightbulb, energy in your body

Ultrasound transducers convert ______ energy into ______ energy, and vice versa

electric, ultrasound

Ultrasound transducers operate on the ___________ principle

piezoelectric

Single-element transducers are in the form of ______

disks

The _________ of a transducer element changes when voltage is applied to its faces

thickness

The term transducer is used to refer to a transducer _________ or to a transducer ___________

element, assembly

A transducer element is part of what?

A transducer assembly

An electric voltage pulse, when applied to a transducer, produces an ultrasound pulse of a ________ that is equal to that of the voltage pulse

frequency

The resonance frequency of an element is determined by its what?

Thickness

Operating frequency decreases when transducer element thickness is what?

Increased

The addition of damping material to a transducer reduces the number of _________ in the pulse, thus improving ______________. It increases ___________.

cycles, axial resolution, bandwidth

Damping material reduces the _______ of the transducer and __________ of the diagnostic system

efficiency, sensitivity

Ultrasound transducers typically generate pulses of how many cycles?

2 or 3, but 2 is the most common

For a particular transducer element material, if a thickness of 0.4mm yields an operating frequency of 5 MHz, the thickness required for an operating frequency of 10 MHz is what?

0.2mm

Which of the following transducer frequencies would have the thinnest elements? 2, 3, 5, 7, 10

10 MHz

The matching layer on the transducer surface reduces _______ caused by impedance differences/mismatch

reflection

A coupling medium on the skin surface eliminates reflection caused by

air

True or false: damping lengthens the pulse

False

True of false: damping increases efficiency

False

The damping layer is in the front or back of the element?

Back

The matching layer is in the front or the back of the element

Front

The matching layer has what kind of impedance?

Intermediate

Elements in linear arrays are in the form of what?

Rectangles

Transducer assemblies are also called:

transducers, probes, scanheads

Operating frequency is also known as?

Resonance frequency

Mixtures of piezoelectric ceramic and a nonpiezoelectric polymer are called what?

composites

To operate a transducer at more than one frequency requires what?

Broad bandwidth

Is it practical to attempt to operate a 5-MHz transducer with a bandwidth of 1MHz at 6MHz?

No, frequency is outside the bandwidth

Is it practical to attempt to operate a 5MHz transducer with a bandwidth of 2.5 at 3 and 7 MHz?

No, frequencies are outside bandwidth

A focused beam is divided into what three regions?

Near zone, focal zone, and far zone

What is the region between the transducer and the focal region?

Near zone length

Transducer size is also called what?

Aperture

Near zone length increases with increasing what?

Aperture and frequency

Which transducer element has the longest near zone?

6mm, 5MHz

6mm, 7MHz

8mm, 7MHz

8mm, 7MHz

A higher frequency transducer produces a ________ near zone length

longer

A smaller aperture produces a _________ near zone length

shorter

True or false: a transducer with a near zone length of 10 cm can be focused at 12 cm?

False

Which of the following transducers can focus at 6cm?

5MHz, NZL of 5cm

4MHZ, NZL of 6cm

4MHz, NZL of 10cm

4MHz, NZL of 10cm

(Can’t focus into/at 6cm)

Sound may be focused by using what?

curved element, lens, phased array

True or false: focusing reduces the beam diameter at all distances from the transducer

False

The distance from a transducer to the location of the narrowest beam width produced by a focused transducer is called what?

Focal length

Transducer arrays are transducer assemblies with several what?

Transducer elements

Linear arrays can scan beams by doing what?

Sequencing element groups

A phased linear array with a single line of elements can focus in what dimension?

1D

Focusing in section thickness can be accomplished with _____ elements or a ____

curved, lens

Electronic focusing in section thickness requires multiple rows of what?

Elements

Linear, phased array, or convex array? Voltage pulses are applied in succession to groups of elements across the face of a transducer

Linear and convex array

Linear, phased, or convex array? Voltage pulses are applied to most or all elements as a group, but with small time differences

Phased array

If the elements of a phased array are pulsed in rapid succession from right to left, the resulting beam is:

steered left

If the elements of a phased array are pulsed in rapid succession from outside in, the resulting beam is _______

focused

________ and ______ describe how arrays are constructed

Linear and convex

_________, _________, and _________ describe how arrays are operated

Sequenced, phased, vector

Shorter time delays between elements fired from outside in results in ____ curvature in the emitted pulse and a ______ focus

less, deeper

A rectangular image is a result of linear scanning of the beam. This means that pulses travel in ______ direction from _____ starting points across the transducer face

the same, different

A sector image is a result of sector steering of the beam. This means that pulses travel in ______ directions from a common _____ at the transducer face

different, origin

In ____ and ____ arrays, pulses travel out in different directions from different starting points on the transducer face.

convex, vector

What does this transducer assembly part do? Cable

conducts voltage pulses

What does this transducer assembly part do? Damping material

reduces pulse duration

What does this transducer assembly part do? Piezoelectric element

converts voltage pulses to sound pulses

What does this transducer assembly part do? Matching layer

reduces reflection at transducer surface

What improves sound transmission from the transducer element into the tissue?

Matching layer and coupling medium

A 5MHz unfocused transducer with an element thickness of 0.4mm, an element width of 13mm, and a near-zone length of 14cm produces two-cycle pulses. Determine the following:

1. Operating frequency if thickness is reduced to 0.2mm:

2. Axial resolution at 5MHz (spl/2):

3. Depth at which lateral resolution is best (at the focus):

4. Lateral resolution at 14 cm:

5. Lateral resolution at 28 cm:

6. This transducer can be focused at depths less than:

1. 10 MHz

2. 0.15mm

3. 14 cm

4. 6.5mm

5. 13mm

6. 14 cm

Lateral resolution is improved by what?

Focusing

For an unfocused transducer, the best lateral resolution (minimum beam width) is ________ the transducer width. This value of lateral resolution is found at a distance from the transducer face that is equal to what?

half, near zone length

For a focused transducer, the best lateral resolution is found where?

In the focal region (minimum bandwidth)

An unfocused 3.5MHz, 13mm transducer will yield a minimum beam width (best LR) of what?

6.5mm

An unfocused 3.5MHz, 13mm transducer produces 3 cycle pulses. The axial resolution in soft tissue is what?

0.7mm

True or false: if my axial resolution is 0.7mm and my lateral resolution is 6.5mm, axial resolution is better.

True

True or false axial resolution is often not as good as lateral resolution in diagnostic ultrasound

False

The two resolutions may be comparable in the _____ region of a strongly focused beam

focal

True or false beam diameter may be reduced in the near zone by focusing

True

True or false beam diameter may be reduced in the far zone by focusing

False

What does element thickness determine?

Axial and lateral resolution, operating frequency

What does element width and shape (flat or curved) determine?

Lateral resolution

What does damping determine?

Axial resolution

The principle on which ultrasound transducers operate is the ____

piezoelectric effect

Which of the following is not decreased by damping?

Refraction, pulse duration, spatial pulse length, efficiency, sensitivity

Refraction

Which 3 things determine beam diameter for a disk transducer?

frequency, aperture, distance from disk face

The lower and upper limits of the frequency range useful in diagnostic ultrasound are determined by what

resolution and penetration requirements

The range of frequencies useful for most applications of diagnostic ultrasound is what?

2-35 MHz

What transducer type is for each display format?

1. linear array

2. phased linear array

3. phased array

4. convex array

5. vector array

Sound is confined into ______, the source (aka the originating emitter of the sound) is a ________

beams, transducer

Transducers form the connecting link between what?

The ultra sound tissue interactions

Label the matching layer, piezoelectric element, and damping material

What does piezoelectric mean?

To press, electron/amber (not naturally electric)

When was the piezoelectric effect discovered?

1880s by the Curies

When is the piezoelectric effect common?

In crystalline materials with positive and negative charges on each molecule called dipoles

Describe the piezoelectric effect

Electrically charged plates, one negative and one positive, are placed on either side of a slab of PZT crystal, and heated above the “curie” temperature and dipoles align

Then cooled below Curie temperature while charge is still being applied, and crystals are locked into position

This is why we never head probes because dipoles will unalign/misalign

What does the piezoelectric effect have to do with ultrasound?

Crystal changes thickness when voltage is applied, alternating polarity causes expansion and contraction of the crystal, which creates mechanical vibrations

What is the most common piezoelectric element in the US?

Lead Zirconate Titanate (PZT): a ceramic material, made to have piezoelectric properties in the lab through heating, Curie Point

Non-piezoelectric compounds are combined to make composites to improve what?

bandwidth, sensitivity, and resolution

What is piezoelectricity?

When some materials are deformed from applied pressure, they produce a voltage… this works in the opposite direction when an applied voltage produces pressure… reduces impedance mismatch

What are the advantages of CMUT over PZT?

broader bandwidth

improved resolution

work well with electronic circuits

miniaturization

3D

cost effective

What is CMUT?

Capacitive Micromachines Ultrasound Transducer

• 2 electricity conducting layers facing each other, 1 fixed and 1 flexible

• Separated by a vacuum sealed gap and insulating layer

• Alternating voltage applied = flexible layer moves quickly toward and away from fixed layer = US wave created

What is the transducer element?

The physical piece of piezoelectric element that does the energy conversion

What shape is a single transducer element?

Disc

What transducer element is numerous rectangular shaped elements?

Linear array

What are some names of transducer elements?

Piezoelectric element, active element, crystal

Describe PZT operation

Cycles of alternating voltage

2-3 cycles = single cycle driving voltage

5-20 cycles = longer driving voltage (Doppler)

Frequency of the sound is equal to

the frequency of the driving voltage