inversion recover (STIR/FLAIR) Flashcards
What is the purpose of inversion recovery pulse sequences
to null / negate signal coming from a specific tissue
which recovery sequence is utilised to negate signal coming from a specific tissue
t1 recovery / longitudinal recovery
explain the process of inversion recovery sequence
- keep in mind the back bone of a regular spin echo sequence ( 90, 180, TE, TR)
- prior to the spin echo, a 180 degree RF pulse is applied which allows the protons to precession beyond the 90 degree angle (as energy is transferred to it)
- as it goes beyond the 90 degree angle, you now have increasing vectors pointing ANTI PARALLEL to the longitudinal magnetisation
(remember that there are already vectors pointing parallel and anti parallel but because most point parallel the net magnetisation is in that direction)
- so the 180 degree flip simply makes the vector predominantly in the anti parallel direction of the main magnetic field and so it has 100 % NEGATIVE magnetisation
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why does inversion recovery only affect/ deal with T1 recovery and not T2 decay
- due to the 180 degree RF pulse, we have negative longitudinal magnetisation
- as the 180 RF pulse is stopped, the protons do not recovery/ flip back through the 90 degree angle going back to the main longitudinal magnetisation (increasing and decreasing signal along the way) BUT
- the protons sitting in the anti parallel magnetic field begin to go out of phase with each other and interact with the lattice (spin-lattice interaction)
- so as this happens, they lose their net magnetisation in the anti parallel direction and begin to realign with the main magnetic field
- this ‘recovery’ does not flip back into the 90 degree angle and there is not gain or loss in signal
- there is only gain in longitudinal magnetisation which is why only t1 recovery is affected during inversion recovery
why is there a point of ZERO magnetisation as a result of the 180 degree flip in inversion recovery
- after the 180 RF pulse is stopped
- dephasing of protons in anti parallel direction increases = decreasing in net magnetisation in anti parallel direction and simultaneous gain in longitudinal magnetisation
- eventually the size of the loss in anti parallel magnetisation and gain in logitudinal magnetisation is EQUAL and so they cancel out giving ZERO magnetisation in any direction
using inversion recovery, we can figure out the time point in which certain tissues cross the certain threshold from negative to positive magnetisation and then regain longitudinal magnetisation
why does CSF have a longer t1 recovery
- less spin lattice interaction = slower dephasing of protons = longer for t1 recovery to take place
why is the differences between t1 in different tissues amplified due to inversion recovery pulse sequence
- following a normal spin echo sequence, 90 RF pulse is applied to the longitudinal magnetisation vector
- but with inversion recovery, the 90 RF pulse is given at the point where there is ZERO longitudinal magnetisation (as anti parallel and parallel magnetisation are equal and cancel out)
- this hence doubles the t1 time from regular as time is needed to gain longitudinal magnetisation as well as flip into the transverse plane
how is inversion recovery used to suppress fat for example
- keeping in mind that once the inversion sequence is given, there will be a point in time that there is 0 magnetisation in the longitudinal plane as it cancels out with the anti parallel decay
- so fat experiences faster longitudinal recovery so it will reach the point of 0 magnetisation faster than CSF
- if a 90 RF pulse is applied at this point in time, it causes the flip of ONLY CSF into the transverse plane as fat has no longitudinal magnetisation in that point of time
- hence you will only receive a signal from CSF whilst fat gets supressed
what is STIR
short TI (tau) inversion recovery
- this is when fat gets surpressed and CSF has high signal
- signal from fat can be negated and there is a bright signal coming from muscle or CSF
Why is inversion recovery / STIR good for imaging things with artefacts
- because following the initial 180 degree flip, it follows the same spin echo sequence where there is another 180 degree flip to make up for the local inhomonogeniety
- this accommodates for the presence of metal artefacts or chemical shift artefacts / similar
that is why STIR would be good for imaging something close to a hip replacement that has metal etc
what is the main negative of STIR
- has a longer acquisition time due to the additionally 180 degree pulse
why can you not use gadolinium with STIR
because STIR utilises a short t1 in order to gain certain signals for the image
- the purpose of gadolinium as a contrast is to shorten T1 time for increased contrast
- so if you use STIR with gadolinium, it will null the signal from the tissues that have gadolinium within it
STIR also causes a reduction in signal to noise ratio so u need to weight the pros and cons when utilising it
why do lesions/ CSF/ fluid filled structured tend to look much brighter on STIR despite having very long T1
- The inversion allows a high signal to be received from fluid despite the very long T1 as it has the additive effects of T1 and T2
what would happen if you were to increase the time of inversion recovery so that it matched to the threshold of CSF (point in which theres ZERO magnetisation in CSF) ? Also known as FLAIR
- Fluid attenuation inversion recovery
- if time of inversion is moved so that the 90 RF pulse is now given at the null point of CSF
- this means that there is no signal in the transverse plain of CSF after application of 90 degree flip as there is 0 magnetisation in the longitudinal plane of CSF
- hence fluid on the image will show up as extremely dark whilst fat and muscle will show up brighter
- the sequence also takes longer (as it takes a longer time for CSF to reach null point)
how can FLAIR help visualisation of lesions in the brain
- without flair, CSF shows up bright on the image hence you cannot tell if there is a lesion in the brain where it is close to fluid (ventricles) as it may or may not be fluid filled
- adding flair surpasses the brightness of CSF and so if the lesion near fluid areas remain bright, you know that it is not fluid filled and is in fact potentially malignant
if you were to suppress an image using STIR and the lesion suddenly goes dark, you can assume the lesion is made of fat
if you use FLAIR and the lesion suddenly goes dark, you can assume the lesion is fluid filled etc
this is how the concept of FLAIR and STIR is used to identify structures
most common type of reconstructed images produces a modulus image, what is the difference between a modulus and non-modulus display
modulus =
zero signal intensity - black
non-modulus =
zero signal intensity - mid-grey
what is the time of inversion for FAT
0.69T1 fat
if the time of repetition is less than 1500ms, how does this affect the signal to noise ratio
TR < 1500ms =
- decreased SNR
- decreased number of slices within TR
increasing TE is ok as T1 and T2 contrast are ‘additive’, but number of slices within TR internal is reduced
the TI time constant for any tissue occurs at 69% recovery of the tissue so it has a value of 0.69TI
what determines the size of signal regarding tissue
proton density
what determines the properties of tisseus
relaxation times
what determines motion
blood flow
