MRI Flashcards
where does the signal come from in MRI?
Protons within our body, mainly from the hydrogen nuclei as this is so abundant.
what is the spatial resolution of MRI?
1mm
what does the MRI machine consist of?
A large superconductive magnet usually at 1.5 or 3Tesla
Coils which a radiofrequency current is put through
Magnetic field gradients to help localise the MR signals
how do protons spin when there is no magnetic field?
they have an intrinsic spin which produces a positive charge called the magnetic moment.
The magnetic moment is unaligned and in different directions.
Net magnetisation = 0
what happens to the protons when the patient is put into a magnetic scanner?
The protons magnetisation experiences an aligning force and they will either align with the magnetisation (low energy state) or in the opposite direction to the magnetisation (high energy state)
More align in the low energy state with the magnetisation than in the opposite direction (high energy state) this creates a non zero net magnetisation with the longitudional (Z) axis
what happens when a radiofrequency pulse is applied?
The pt is in the scanner so the protons net magnetisation is with the longitudinal z axis and we don’t measure the longitudinal magnetization so we need to flip it to transverse magnetization (Mxy)
The radiofrequency pulse causes this flip and makes the net magnetization in the transverse plane.
The magnetization in the transverse plane induces a signal voltage in the recieving coils
when the rf pulse is stopped the transverse magnetization begins to decay and nuclei return back to longitudinal magnetisation.
What is T1 recovery/ spin lattice relaxation?
When the nuclei are in the transverse plane they are jostled by the surrounding lattice and as this happens they give energy to the molecules.
As they give energy to these molecules they return back to longitudinal magnetisation
T1 is the time taken for Mz (longitudinal magnetisation) to recover to 63% of it’s original value
fat and protein: short T1 (quick return to Mz)
Water/CSF: long t1
what is T2 decay/spin-spin relaxation?
After the RF pulse stops the magnetic property of each nuclei alters the magnetic field reducing the amount of transverse magnetization.
The rate the nuclei loose coherence and their net transverse magnetization reduces is exponential and called free induction decay. This is determined by the amount of magnetic interaction between spins of nuclei
T2 is the time taken for transverse magnetisation to decay to 37% of it’s original value and depends on the local magnetic field.
Bone: short T2
water: long T2
what is the spin echo sequence?
- the first 90 pulse is applied and Mxy is at it’s highest
- Mxy decays and some protons loose this faster than others creating leaders and laggers
- 180 RF pulse is applied and Mxy is high
- echo is the signal that you measure after the 180 pulse
what is TE?
time to echo
during this time there is loss of Mxy (depends on T2)
what are the times in T1 weighted imaging?
Short TR and short TE
why is a T1 weighted image T1 weighted?
t1- recovery of Mz from Mxy after the RF pulse
The higher the Mz when the RF pulse is applied the higher the Mxy will be.
This means the longer the repetition time the stronger Mz is when the pulse is applied and the stronger Mxy will be. This means the TR determines the T1 signal
to maximise the contrast of T1 properties in different tissues TR is set at the point where there is the biggest different in T1 of different molecules which is at a short TR.
why does water appear dark on T1 weighted images?
Water has a long T1 (Takes a long time for Mz to return) but the TR is short when the 180 pulse is applied the Mz is low as it takes a long time to recover. This means the new Mxy will be low and the signal weak.
what are the timings on a T2 weighted image?
Long TE, long TR
why is a T2 weighted image T2 weighted?
It has a long TE time meaning more time for Mxy decay because the echo is took and signal received.
To maximise T2 contrast at the point where tissue has the greatest difference in T2 decay which is at a long TE but this has to be balanced with the signal still being negligable.
why is water bright on T2 weighted imaging?
Water has a long T2 meaning Mxy takes a long time to decay.
As the TE is long in T2 weighted imaging this is the tissue with the most Mxy so gives a stronger signal.
what are good clinical applications for T2 weighted imaging?
Processes that cause oedema (degeneration, cell breakdown, membrane damage)
demyelination is bright on T2 weighted so MS appears as bright lesions
what are the properties in proton density sequences for MRI?
Long TR, short TE
what is proton density imaging?
instead of imaging the magnetic characteristics of the hydrogen nuclei it focuses on the number of nuclei in the area being imaged
why does proton density imaging have a long TR and short TE?
the long TR reduces contrast from T1
Short TE reduces contrast from T2 as there hasn’t been much time for Mxy decay.
why does proton density imaging provide good info from MSK issues?
there is variation in the proton density between structures. ie in the brain the proton density is similiar in all the structures.
what are disadvantages of MRI?
the radiofrequency pulse can cause warming of tissue but scanners are designed to limit this
metal objects can be pulled into the bore
people with certain implants can’t be scanned ie pacemakers
expensive
can be hard to access
what are advantages of MRI?
high detail
no exposure to ionising radiation
no clearly demonstrated biological effects.
what type of weighting is MRI angiography?
t1 weighted
why does flowing blood provide a good signal on MRI angiography?
its set to T1 weighted parameters (short TE, Short TR)
In stationary tissue when repeated radiofrequency pulses are put in Mz doesn’t have enough time to recover in between so Mxy in the corresponding pulse will be low. When there is blood flow the new fluid hasn’t been exposed to previous pulses so has a larger Mz and therefore a larger Mxy after the pulse has been applied. Therefore it will have a higher signal.
what is weighting of MRCP and why?
T2 weighted as fluid is bright meaning the hepatobiliary system will send a brighter signal
what is the weighting of MR lymphangiography?
T2 weighted so fluid shines brighter allowing you to see lymph
what generates the signal contrast in diffusion weighted imaging?
differences in the brownian motion of water diffusion
when there is diffusion of water the signal is repressed. When there is restriction of diffusion there is signal
what are advantages of diffusion weighted imaging?
provides a quantitative diffusion coefficient this can allow for more accuracy than qualititative visual changes ie increases with neurodegeneration. Can also pick up on changes earlier than with conventional imaging
You can measure the diffusion coefficient in different directions of the white matter pathway. In white matter there is anisotropy meaning diffusion isn’t equal in all directions as it’s limited perpendicular to fibres. You can work out the direction of the white matter fibres from anisotropy using tractography
what are clinical applications of diffusion weighted imaging?
acute brain ischaemia: DWI combined with MRI angiography can identify salvageable areas of tissue
Acute stroke: swelling means restricted diffusion so ADC decreases but T2 is the same
chronic stroke: membrane damage means increased diffusion and ADC increases. T2 increases
cellular tumour: diffusion coefficient decreases
what does MR neurography image?
highlights peripheral nerves from their surroundings
heavy T2 with fat suppression
what are clinical uses of MR neurography?
can see nerve compression and signal changes
carpal tunnel
brachial plexus injury
what is MR spectroscopy?
allows you to measure the concentration of certain molecules within the imaged region by targeting different nuclei
what are different metabolites in MR spectroscopy?
myoinositol: a glial marker raised in glioma
GABA/glutamine: raised in hepatic encephalopathy
lactate: shows anerobic respiration and therefore areas of ischaemia and seizures
Lipids: raised in necrotic tumours
