fast MRI and its applications Flashcards
give some dis of fast mri
low SNR
low spatial resolution
short time for T1 recovery/t2 decay
increased costs e.g software
improvement in MRI speed due to:
phased array coils
new gradient tech (fast switching and stronger gradients)
faster sequenced
what are 3 main issues with short TR
- tissues with long t1 dont fully recover
- images become increasingly t1 weighted
- compromises image quality
FLASH sequence is first sequence imaging technique to reduce flipping angle
- optimal flip angle (for certain tissue) to receive largest signal (with short TR) given by ERNEST equation
there are 5 sequences used for fast MRI, what are they and state how each of them work
- multi echo spin echo
- multi slice spin echo
- fast spin echo
- echo planar imaging (gradient echo) / gradient echos in general
- multishot spin echo / EPI (spin echo)
3 pros and negatives of using fast spin echo
pro:
short imaging time
allows breath-holding imaging
less susceptibility
cons:
lower SNR
HIGH SAR ( HEATING)
Longer ETL alters image contrast
2 main issues with EPI spin echo
- can hardware de/rephase using FEG that fast
- can software readout the signals that fast
whats the rough scan time for EPI
30-100MS/SLICE
EPI imaging are single shot sequences (1 RF), WHAT ARE 3 PROS OF EPI imaging
- low SAR
- signal losses associated with reduced TR eliminated
- high sensitivity to susceptibility (important in fMRI)
NOTE: images from fast MRI look much better under homogenous magnetic field so shimming is very important
image matrix in k-space is hermitian, what does this mean
holds conjugate symmetry
- top half of k-space contains info that errors bottom half same goes for left and right
- can measure half and estimate the other (half-fourier)
what 3 negatives are associated with estimating k-space
- reduced spatial resolution
- low contrast
- more artifacts
note: noise, in-homogeneities, patient effects etc introduce phase variations cross matrix making assumption of conjugate symmetry wrong
- corrections are made by camping into 2nd half of matrix and assuming phase variation is linear
fractional NEX (number of excitations). Aqquire fraction of the k-space and assume the rest (as they are mirrored)
- fractional echo shortens the minimum TE
3 pros for spiral imaging
- more efficient k-space trajectory for fast MRI (tho not as fast as EPI)
- better immunity to flow artefacts (no gradient at centre of k-space)
- allows more room for magnetisation prep e.g diffuse weighting
what is parallel imaging
multiple receiver coils (phased array coils) to simultaneously get data, decreasing amount of k-space data needed, using coil sensitivity maps for reconstruction
what are 2 forms of parallel imaging
SENSE - sensitivity encoding
GRAPPA - generalised auto calibrating partial parallel acquisition
what is SENSE/how does it work
combines reduced Fourier encoding with coil sensitivity patterns to make artefact free image
- applies knowledge of sensitivities of coil elements to calculate aliased signal component at each point
what is mSENSE
modified SENSE
- additional lines acquired at centre of k-space for each coil during scan
what 2 aspects of scanner hardware are required for parallel imaging
phased array coils
gradients
coil array in parallel imaging are used for both what?
- signal reception
- signal encoding
coil array requirements:
- multiple receiver channels, amplifiers, AD converters, computer memory, significant post processing to combine images
phased array coils have many surface coils, give 2 pros and cons for this
adv- goos SNR, large FOV
con- multiple receiver channels costly, coil coupling
- the stronger/steeper the gradient, the faster the acquisition time
what is a slew rate
The slew rate (AKA “rise time”) is the speed at which the gradient reaches its maximum amplitude.
