MRI Flashcards
advantages of clinical MRI
- excellent soft tissue contrast with high resolution
- display of several images and oblique cuts
- no ionising radiation
what is the most challenging problem in abdominal MRI
breathing motion and movement
what happens to spin when external magnetic field is introduced
spins will align either parallel or anti-parallel state
more spins in the parallel or anti-parallel state and why
parallel - because lower energy
what is larmor frequency
precessing frequency of magnetic moment of proton
which state spins are more unstable
anti-parallel - higher energy state
what happens when a 90 deg RF pulse is injected
- “in excess” spins will absorb the external energy and jump to the unstable high energy state (from parallel to anti-parallel)
- all nuclei become “in phase” in the x-y plane
- amount of energy causing the NMV flip down onto x-y plane
signal is detected only when spins are precessing in which plane
xy plane
what happens when RF pulse is removed
- generation of free induction decay (FID) signal
- amplitude of NMV is exponentially decaying
without 90 deg RF pulse, spin dephase occurs due to
inhomogeneity of local magnetic field
what happens during T1 relaxation
MNV flips back towards its equilibrium position after removal of RF pulse, excessive energy is released and dissipated as heat in the tissue lattices
factors affecting T1 relaxation
- T1 relaxation increases with the complexity of the lattice: very easily give away energy to surrounding lattice structure, T1 relaxation time decreases
- T1 relaxation decreases as the lattice becomes increasingly spare
rank tissue from the fastest T1 relaxation time to the longest T1 relaxation time
fat
liver
kidney
spleen and white matter
muscle
gray matter
csf
short or long TE maximises T1 relaxation
short TE
what happens during T2 relaxation
magnetic moments at different locations in space are subjected to a slightly different magnetic field strength, thus precess with slightly different frequencies, causing loss of phase coherence and spin dephasing
how to counter spin dephasing (T2) due to magnetic field
applying a 180deg rephasing pulse
how to recover spin dephasing due to activation of magnetic gradients
gradient reversal
how to recover spin dephasing due to internal local field of tissue
- cannot be recovered because it is fluctuating
- however, internal local field of tissue is responsible for generation of tissue contrast on MR images
what can be determined from T2 relaxation time
- tells us about the internal local field strength of a particular type of hydrogen-containing tissue
- stronger internal field = higher spin dephasing speed = shorter T2 relaxation time
what type of tissues have shorter T2 relaxation time
solid tissue
- rigid molecular structure have strong IF
- rapid loss of phase coherence of magnetic moments
rank the tissues of shortest to longest T2 relaxation time
liver parenchyma
muscle
kidneys
spleen
fat
white matter
grey matter
CSF
short or long TE maximises T2 effect
long TE
what is the purpose of image pulse sequence
- rephase the dephased spins
- remove magnetic inhomogeneity effects by 180deg RF pulse
- produce signal or echo that contains decay characteristics of different tissue
- enable manipulation of different TE and TR setting to produce different types of contrast weighting images
- spatial encoding
how are signals being rephased
- 180deg RF pulse (Spin Echo)
- gradient reversal (GRE)
what is TR
- repetition time
- time from application of one RF pulse to the next
- measured is ms
- affects length of relaxation period allowed after application of one RF pulse to the beginning of the next
what is TE
- echo time
- time between RF excitation pulse and the collection of signal
- affects length of the T2 relaxation period allowed after the removal of an RF excitation pulse and the peak of the signal receiver coil
- measured in ms
what is the maximum TR for a T1 weighted image
< 600 ms
tissues with short T1 values appear
hyperintense
what is the approximate TR to suppress T1 effect in a T2weighted image
2000msec
what is the minimum TE to enhance T2 effect
> 30ms
on T2 weighted image, tissues with long T2 values appear
hyperintense
what is the minimum TR to minimise T1 effect in PD weighted image
> 2000ms
what is the maximum TE to minimise any T2 effect in PD weighted image
> 20 ms
slice thickness can be selected by
- adjusting RF bandwidth
- keeping RF pulse bandwidth unchanged but using a
*steeper gradient, exciting a thinner slice
*shallower gradient, exciting a thicker slice
what are the steps to obtaining spatial information
- slice selection
- phase encoding
- frequency encoding
what happens during the phase encoding step
y-axis gradient turns on very briefly, causing a transient difference in precessing speed in different rows
results:
- spins along the same column will have same frequency but varies in phase between rows
- phase value of spins in the same column contain “pseudo frequency” - which can be decided later by Fourier transformation to generate back Y coordinate information
what happens during the frequency encoding step
assuming no phase encoding was done,
spins in the different column varies -> which can be decoded later by Fourier transform to generate x-coordinate information
peripheral lines of k-space contributes mainly to
spatial resolution
what does low pass filter do
- cuts our all high spatial frequency data from k-space
- results in a lack of details on the image, although coarse contrast of image remains
what does high pass filter do
- removes low spatial frequencies data from k-space
- resultant image shows little contrast, yet edge definition remains
- fine details of the image are contained in the high spatial frequency data
