Unit 8 Flashcards
What is signal?
RF received from relaxation of excited protons
What is noise?
RF received from background electrical interference and thermal motion in a patient
What is SNR?
signal to noise ratio; measure of image quality
What is contrast to noise ratio?
ratio between SNR of adjacent tissues
What is spatial resolution?
ability to distinguish points as separate and distinct
What is crosstalk?
mixing of signal from adjacent slices
What is cross excitation?
transmitted RF that leaks over from adjacent slices
What is respiratory compensation?
method whereby shallow phase encoding are acquired between breaths
What is phase/frequency oversampling?
method whereby extra encoding steps are acquired, but discarded prior to image reconstruction
Why is respiratory compensation used?
reduce respiratory motion artifact
Why is phase/frequency oversampling used?
reduce wrap-around
How is SNR affected by TR?
proportionally
How is SNR affected by TE
inversely proportional
How is SNR affected by slice thickness
proportionally
How is SNR affected by FOV?
proportionally
How is SNR affected by image matrix?
inversely proportional
How is SNR affected by Number of Signal Avgs (NSA)?
proportionally
How is SNR affected by slice gap?
proportionally
How is SNR affected by receiver bandwidth?
inversely proportionally
Describe phase mismapping/ghosting artifact
ghosting/blurring of image along phase econding axis
What causes phase mismapping/ghosting?
motion - resp, blood, swallowing
Describe wrap around artifact
anatomy is placed in wrong posistion in image
What causes wrap around?
FOV too narrow in phase direction
Describe partial volume avg artifact
blurred edges of structures
What causes partial volume avg?
large voxel size
Describe Chemical shift artifact
dark edge on one side of structures, and bright on the opposite in frequency direction
What causes chemical shift?
the different precessional frequencies of fat and water
Describe chemical misregistration artifact
dark lining around tissue interfaces on GE
What causes chemical misregistration?
fat and water precessions being out of phase
Describe truncation/Gibbs artifact
low intensity bands that run parallel to tissue interface in phase direction
What causes truncation/gibbs?
undersampled tissue interfaces
Describe data clipping artifact
signal loss of image; exaggerated/inverted contrast
What causes data clipping?
image data is beyond the range of the receiver
What parameters affect voxel volume?
FOV, slice thickness, matrix size
What parameters affect contrast and SNR?
TR, TE, flip angle
What parameters affect scan timeand SNR?
TR, NSA, matrix size
What parameters affect number of slices available during each TR?
TE, receive bandwidth
What parameters affect resolution and SNR?
slice thickness, matrix size, FOV
How much does SNR increase by doubling NSA?
44%
How does slice gap affect SNR?
greater gap = less noise
How is scan time calculated?
=TR x #phase encoding steps x NSA
How is slice encoding done for 3D imaging?
phase encoding steps are performed in the slice direction
What are advantages to 3D imaging?
thinner slices, no slice gap, higher SNR, manipulate into different orientations
What is a disadvantage to 3D imaging?
longer scan time
How do you compensate for motion/ghosting artifact?
swap phase and frequency direction, presatureation, resp comp, flow comp, gating
How do you compensate for wrap-around?
oversample, presaturate
How do you compensate for chemical shift?
increase bandwidth, decrease FOV, decrease voxel size
How do you compensate for chemical misregistration?
SE squences, GE w/ TE when fat and water in phase
Why is GE more susceptible to chemical misregistration?
there isn’t a refocusing pulse
How do you compensate for truncation?
inc phase encoding steps
How do you compensate for cross-excitation?
increase slice gap, interleaved slice excitation
