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18 Cards in this Set

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What is resolution?
Ability of an imaging system to differentiate between structures and display them as separate.
What is spatial resolution?

Ability to display 2 structures close together as separate. (Axial, Lateral)


Depends on wavelength of the sound producing the image.


5MHz beam has a wavelength of 0.3mm=Cant resolve objects less than 0.33mm apart.

What affects axial resolution?

Depends of spatial pulse length.


Short pulse= Good axial resolution.


Best resolution is achieved at half spatial pulse length.


Pulse length=1mm structures that are less than 0.5mm apart are not resolved.

What factors affect spatial pulse length and axial resolution?

Frequency: Pulse is 2 wavelengths (short wavelength=shorter SPL.


Increase frequency= Smaller wavelength


High Frequency= shorter SPL=Increased axial resolution.


For structures to be resolved they must be separated by 2 pixels.


Field of view= As FOV increases, distance representing 2 pixels increases


Increased FOV=Decreased axial resolution.

What does lateral resolution depend on?

Narrow beam=good lateral resolution




Beam width determines the size of echoes displayed and structures must be separated by distance greater than the beam width for them to be resolved.




Beam width must fit between 2 structures for them to be resolved and return no echoes between them.

Which is the better resolution lateral or axial?
Axial
How does focussing affect resolution?

Best lateral resolution is at the focal point of the beam.


Beam width increases the further away it is from the focal point.


Length of focal zone All affect beam width.


No. of zones

What is contrast resolution?
Ability to distinguish between body tissue and display them as shades of grey.
What affects contrast resolution?

Transducer design= High freq= smaller slice thickness=better contrast res.


Analog to digital conversion= Voltage signal representing an echo is converted into a binary No.


During the conversion there is compression and filtering of signal which affects the No. of grey scales.


General settings: Gain, TGC, Processing, Harmonics (improves contrast), Compound imaging (Reduces contrast).

What is temporal resolution?

Ability to display events which occur at different times as separate events.


Important when looking at the heart (fast moving)

What is temporal resolution determined by?
Frame rate.
What does frame rate depend on?

Line density


Depth


Sector width


PRF.

How do you calculate frame rate?

PRF


--------


Lines per frame.


EG 1000 pulses/s


--------------------- =10 frames s


100 lines/frame

What happens to the PRF when depth is increased?
Decreases the PRF.
What happens when the line density is increased?
Increased lines per frame.
How do you improve spatial resolution?

Increase frequency


Focus the ultrasound beam at the point of interest.

How do you improve contrast resolution?

Highest frequency


Correct TGC and overall gain.


Use an appropriate preset.

How do you improve temporal resolution?
Image at the highest frame rateReduce sector width and depthSingle focal zone.