Tissue Supression - 6.2.24 Flashcards
What are the advantages of Fat signal?
Useful in displaying anatomical boundaries of organs
Eg exiting nerve roots
What are the disadvantages of the Fat signal?
- May cause small structures to be hidden
Eg optic nerves surrounded by retro-orbital fat - high fat signal masks uptake of contrast agent
Eg breast, bone marrow
What are thee advantages of STIR?
Independent of field in homogeneities, so good for large FOV applications, or in presence of local field distortion (eg metal)
Often, the only choice for low field systems
What are the disadvantages of STIR ?
Limited tissue contrast and long acquisition time (Long TR)
Should not be used with Gad since this shortens the T1 of tissues that contain the contrast therefore will be nulled along with fat
Can only produce fat suppressed T2 w images
What is the TE and TR of STIR?
TR - 1800
TE - 20
What is the TR, TE and TI of FLAIR?
TR - 6000
TE - 120
TI - 2000
What is Double IR used for?
Used to suppress signal from blood in SE
creates black blood imaging sequences in CMR
What is Triple IR used for?
Used to suppress signal from blood and fat in SE
creates black blood imaging which has fat suppression sequences in cMRI
What is the Chemical shift for a 1.5 T scanner
At 1.5 T = 64 mhz
Chemical shift = 3.5 ppm
Frequency diff at 1.5 T =. (3.5 x 10^-6) x (64 x 10^6) = 220 Hz
What is Chemical shift?
The difference in resonance frequency of protons and water molecules
What is FAT-SAT in full?
Fat saturation
What is ChemSAT in full?
Chemical saturation
What does SPIR stand for?
Spectral Inversion recovery
What does SPECIAL stand for?
SPECtral Inversion At Lipid
What does CHESS pulse stand for?
Chemical shift Elective pulse
What is CHESS pulse?
Long 90’ pulse
Narrow range of RF frequencies
Centred on W0fat
Fat protons excited
Water protons inexcited
What are the advantages of Fat sat ?
Simple - can be added to almost any sequence
Doesn’t alter image contrast ( except fat)
Works well at high field strengths (> 1T) and small FOVs
What are the disadvantages of Fat sat?
Increases acquisition time or reduces number of slices
Increases SAR due to extra RF pulses per TR
Does not work well at low field strengths (<0.5T)
Patchy fat suppression due to
- poor magnetic field homogeneity
- large FOV
- metallic implants
What is the Water selective approach?
- Frequency selective pulse for water
- does not excite the lipid protons so no need to suppress
- Pulse more complex - Known as Binomial RF pulse
= eg water excitation (WE) or proSet - not slice selective so usually combined with 3D image acquisition
- can also be made slice selective by using spectral-spatial selective RF pulse
What factors are part of Silicon only sequences?
Various methods that only show silicon
Used to diagnose implant ruptures
Small frequency shift between fat and silicon
Frequency selective fat saturation of limited use
What sequences are used for Silicon sequences?
Uses STIR to suppress fat signal
Use frequency selective saturation pulse to suppress water signal
What is Chemical shift - Dixon (SE and GE)?
- Uses 2 or 3 TEs
- using the in-phase (IP) and out-of-phase (OP) techniques
- combine images to get water only (W) and fat only (F) images
What a re the advantages of chemical shift - Dixon (SE and GE)?
Good fat sat in general
What are the disadvantages of Chemical shift - Dixon (SE and GE)?
- fat and water images can be mislabelled due to fit failing ( called “ fat-water swap)
- can fail near inhomogeneous areas such as neck and around metal
- sequences are longer as need larger TR to fit in extra echoes, makes higher res or larger coverage challenging in reasonable time
What are the 2 Chemical Clift Dixon equations and how are they rearranged?
IP = W + F OP = W - F
W = 1/2 [IP + OP]. F = 1/2 [ IP - OP]
What is Subtraction?
- Used for contrast -enhanced imaging
- A pre-contrast image set (mask) is subtracted from the post-contrast image set
- removes tissue that is not affected my the contrast
What are the Disadvantages of subtraction?
Decreased SNR
Affected by patient motion
What are Spatial saturation slabs?
AKA - Sat bands
- the band is exited using a spatially-selective 90’ RF pulse
- large dephasing (spoiler) gradients are then applied to ensure it contributes no signal.
- the spatial saturation band is applied to each TR period prior to slice selection.
What are sat bands used for?
Removes areas of signal which may cause an artefact
Removes signals from flowing blood
