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.
Parallel imaging used for cardiovascular imaging (breath hold, real time) and brain (fMRI, single shot EPI diffusion)
