Physics Review - Part 5

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Questions from the class discussion

What portion of the ultrasound system (from the flow chart) controls the transducer?

The pulser and the beam former

Doppler is best achieved at 0° or 90°?

What happens at 90°?

you lose the doppler signal

What's required for flow to occur in a blood vessel?

a pressure gradient and a path way.

The larger the gradient, the _____________.

more flow it contains

In a stenosis, where does turbulence occur, in the stenosis, before or after the stensosis?

after the stenosis

Where is your pressure the lowest in a stenosis?

in the stenosis, because this is where the velocities are the highest.

What kind of flow is bullet shaped?

laminar

With laminar flow you have concentric rings of flow. Where would the fastest flow be, in the middle or on the edges?

in the middle

What kind of flow is the same across the vessel?

plug

The Reynold's number is a predictor of?

turbulence

What is the Reynold's number that indicates turbulence?

2000

What has the greatest effect on pressure in a vessel?

vessel radius

Name two useful artifacts.

posterior acoustic enhancement and shadowing

Grating lobes occur from ________ transducers.

array

Side lobes occur from ________ transducers.

mechanical

How do we get rid of side lobes and grating lobes?

subdicing and apodization

What is subdicing?

making the element into smaller pieces. When we make it into smaller pieces, it makes for smaller wavelets that create constructive interference.

What are wavelets and constructive interference connected to?

Huygen's principle

What is apodization?

We vary the amount of strength going to the elements where usually the outside elements get weaker signals than the inside, so we don't have all of the extraneous power going to the outside of the beam. Varying the excitation to the inner most crystals, to reduce grating lobes.

What is the purpose of harmonics?

To provide better resolution.

What are the tradeoffs of harmonics?

Pulse inversion harmonics reduces frame rate, which causes temporal degradation

What do you do to change your contrast resolution (what would you use), and if you want to change the range of your dynamic signal, what would you use? Both changes use the same change.

compression -- changes both dynamic range and contrast resolution

What's the advantage of PW over CW Doppler?

Range resolution - PW can identify where the return signal is coming from.

What's the advantage of CW over PW Doppler?

Measurement of high velocities without aliasing

What is aliasing equal to?

1/2 PRF

What's 1/2 PRF referred to as?

Nyquist limit

How do you get rid of aliasing?

Use a CW probe, change the scale (changing the scale is like changing the PRF), change to a lower frequency transducer, use a shallower imaging depth.

How often should QA be performed?

regularly

What is an AIUM phantom?

An older phantom that is no longer used much.

How is a tissue phantom better than the older AIUM phantom?

It can be scanned from multiple plans and can evaluate more of the gray scale.

What kind of calibrations and testing can be done with a tissue phantom?

horizontal calibrations, lateral calibrations, axial resolution, and attenuation.

What cannot be measured with a tissue phantom?

density or power -- it can only image the visual qualities and the accuracy of our measurements.

Name something that can measure the intensity of the sound beam.

Hydraphone or a radiation force balance.

What is a hydraphone?

It is a little transducer.

The tissue phantom can measure ______ resolution, _________ resolution, and ________ resolution. It cannot measure _________ resolution.

axial, lateral, contrast
temporal

What kind of phantom can measure elevational resolution?

a slice thickness phantom

What is b-mode?

gray scale

what is a-mode?

amplitude mode

what is m-mode?

motion mode - movement of a reflector with respect to time.

There are no known bioeffects as long as temperatures are kept to less than a _______ change.

2 deg C

With diagnostic ultrasound, temperature changes are kept at ___________.

1 deg C or less

With a sound wave, which is more dangerous --focused or unfocused?

unfocused, temperature elevations are more likely to occur.

When temperature elevations occur, where will most of the absoprtion occur?

in bone

What's your order of imaging adjustments for patient safety?

TGC, then gain, then power

There are different ways of focusing, what are they?

electronic focusing, internal mechanical focusing, and external mechanical focusing.

How does electrical focusing work?

by curving the electrical firing pattern

How does external mechanical focusing work?

by using a lens or mirror

How does internal mechanical focusing work?

by curving the crystal

Which of the receiver functions can be changed by the sonographer?

amplification, compensation, compression, and rejection

Which of the receiver functions cannot be changed by the sonographer?

demodulation (rectification and smoothing)

What is the range of signals from lowest to highest?

Dynamic range

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