C1A: Transducer Evolution Flashcards by M B

Did old static scanners used one or multiple crystals? And what shape was the crystal?

One crystal which was disc shaped

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Do modern days probes use a single or an array of crystals?

An array of multiple crystals

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How was the sound steered in early probes?

It was mechanically steered (the crystal had to rotate)

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Is the focal zone fixed for mechanical transducers?

Yes

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What is the names of the earliest probe technology where the crystal was mechanically moved across a track?

Linear translation

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Describe a wobbler probe?

A wobbler probe has a fixed element on a motor that rotates back and fourth

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What type of image does a wobbler probe produce?

Sector type image

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What is the main downside of the construction of a wobbler probes?

They were filled with fluid and the moving parts produced air bubbles

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With mechanical transducers what are the two items you can’t change?

Frequency

Focus

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How do mechanical probes obtain a linear FOV?

By using a reflector or acoustic mirror to direct sound into the patient

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How does an oscillating mirror probe work? What about oscillating element

It has a mirror that moved and a stationary element… opposite for oscillating element

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How do rotating wheel probes work?

They have several crystals that spin and only pulse when the element passes the FOV

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Why was the annular array probe one of the biggest advances in probe technology?

You had the ability to change the focus

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How was an annular array probes steered?

Mechanically

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How are annular array probes constructed?

It uses concentric discs to allow focusing of the beam

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How does firing the different consecutive layers of an annular array probe effect the focus?

The wider the diameter of the concentric layer, the deeper the focal point and vice versa

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How are annular array probes focused?

Electronically

But they’re still mechanically steered

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What is one of the most significant problems with annular array probes?

The production of grating lobes

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What are grating lobes called in mechanical probes?

They’re called side lobes and occur due to radial vibration

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What are grating lobes caused by in annular away probes?

Caused by length and width vibration

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What are grating lobes?

low energy beams that radiate away from the main axis and contribute to artifacts

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How is the element arranged in electronic array probs?

They have many small elements aligned in a linear row

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What are the two main types of arrays?

Phased array and Phased linear array

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What is phasing?

Phasing is using nanosecond time deals to change when the voltage stimulate the crystal

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Whats the purpose of phasing?

allows for beam focusing and steering

Whats another term that describes steering the sound beam?

Vector Scanning

How will an unsteered beam travel in comparison to a steered beam?

The unsteered beam will travel in a straight line, the steered beam with be angled one way or another.

What determines which way the beam angles?

Whichever side of the probe is stimulated first, the sound will travel away from the side (e.g. if L side is stimulated first, the beam will travel to the R side)

How are the elements fired in a sector phased array probe?

All crystals are fired at nearly the same time to steer the beam across the face of the probe

What is the range of the FOV for the sector phased array probes?

90-30 degrees

Approximately how many crystals does a sector phased array probe have?

~128

Approximately how many crystals does a linear array probe have?

~64-200 elements arranged in a row

What shape is the FOV for a linear array probe?

Rectangular but can also be trapezoidal if scanning in vector mode

Do linear array probes have scan line divergence?

Are linear array probes associated with higher or lower frequencies?

Higher

Why is it bad to fire only a single element when scanning?

Firing a small source results in more divergence which causes a broad beam. A broad beam gives poor resolution and sensitivity (specifically, we have poor lateral resolution)

Does firing a single element effect our ability to focus and steer the beam?

Yes, it takes the ability away completely

What is our overall field of view composed of?

Each individual scan line firing

What effect does firing multiple elements in a group or segment result in?

A larger group means less divergence so better resolution (lateral) and sensitivity. It also allows us to steer and focus the beam electronically.

What is the downside of fining multiple elements in a group of segment?

We have fewer scan lines which reduces spatial resolution and can cause gaps between beams

How do we fix gaps caused by firing elements in groups?

We use segmental sequencing

How does segmental sequencing work?

We fire a group of crystals to create a beam and then we fired a second group which is only one crystal down the row. This causes the beams to overlap so their are no empty spaces between the beams

How are the crystals in a curved linear array probe different than a linear array probe?

It‘s rectangular shaped elements are curved on one end instead of flat

What is the advantage of a curved linear probe?

It has a large footprint and FOV and has good near field and far field resolution

What are the disadvantages of a curved linear probe?

Resolution in the far field can be degraded as the distance between beams in increased

How do we correct for grating lobes?

Apodization

What is apodization?

A process in which the voltages of the crystals are gradually weakened from inner to outer... this reduces cross talk (e.g. outside crystals dont ring as much as the inner ones)

What are the two ways to obtain 3D and 4D images?

Freehand, using a traditional array probe AND Volumetric probe

How do volumetric probes work?

The beam is swept over the area of interest to get in info needed for 3D/4D (can be mechanically or electronically swept)

How are 1.5D probes different than 1D probes?

1.5D probes allow you to focus the beam in the x, y AND z planes which reduce artifacts (especially beam width artifacts) and improves the image

How are the elements arranged in 1.5 D probes?

There are 3 rows of crystals

1D probes are used for what type of scanning?

What kind of backing to CW probes have? And what kind of voltage do they use?

They're air backed and have a continuous AC driving voltage

What is the 'focal zone' or sample volume for CW?

Zone of sensitivity

What type of bandwidth is associated with electronic array probes?

Wide bandwidth

What limits the number of frequencies that be can drive the probe to ring at?

The size of the bandwidth and attenuation

What type of frequencies are too weak to be detected by the probe?

Any frequencies that have an amplitude of less than half of the resonance frequency

What is another name for the usable bandwidth?

The 6 dB bandwidth

When referring to bandwidth, what value is equal to half of the amplitude or 1/4 the intensity of the resonant frequency?

6 dB

A drop in what number is equal to a reducing of half the amplitude?

6 dB

Whats the equation for fraction bandwidth?

FB= Bandwidth/Frequency

A probe with a faction bandwidth of what value is considered to be a broadband design?

80%

What is another term to describe bandwidth?

Quality Factor or Q factor

What is the equation for Q factor?

Q= Frequency/Bandwidth

What kind of Q factor do we want for 2D scanning? And for Doppler?

Low Q factor for 2D (because we want a short pulse and a wide bandwidth) High Q factor for Doppler (because we want a long pulse and a narrow bandwidth)

Whats the relationship between pulse length and bandwidth?

Inverse Short pulse = wide bandwidth Long pulse = narrow badwidth

Whats the relationship between bandwidth and Q factor?

Inverse

Modes requiring more sensitivity (CW Doppler) will have what type of bandwidth?

Narrow bandwidth (which means higher Q factor)

2D probes are used for what type of scanning?

3D and 4D

What happens to the focal point if the outer crystals of a probe are fired before the inner crystals?

The focal point will be closer to the probe

What are two distinct qualities of a vector probe?

1) they are capable of expanding the probes field of view | 2) they allow the operator to steer the Doppler colour box

Does segmental sequencing increase the number of scan lines we’re sending out

Yes