# 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

### 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.

laminar

in the middle

plug

turbulence

2000

array

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

1/2 PRF

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.

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

gray scale

amplitude mode

### what is m-mode?

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

2 deg C

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

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)

Dynamic range

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