MRI - Image Weighting and Contrast Flashcards
What are the two categories that diagnostic imaging contrast depends on?
Intrinsic and extrinsic.
List the intrinsic and extrinsic parameters MRI contrast depends on.
Intrinsic parameters:
Cannot be changed -they are inherent to the body’s tissues.
1) T1 recovery
2) T2 decay
3) Proton density
4) FLOW
5) ADC (apparent diffusion coefficient) - measures the magnitude of diffusion (of water molecules) within cerebral tissue.
Extrinsic parameters:
Can be manipulated by MRT.
1) TR (repetition time)
2) TE (echo time)
3) FA (flip angle)
4) ECHO TRAIN LENGTH/turbo factor
5) b-value
What factors determine the T1 and T2 relaxation times of a tissue?
The inherent energy of the tissue
How closely packed the molecules are
How well the molecules tumbling rate matches the Lamor .
Factors that determine the T1 and T2 relaxation times of a tissue.
Describe the inherent energy of the tissue:
If the inherent energy is low then the molecular lattice act like a sponge and are more able to absorb energy from hydrogen nuclei during relaxation.
In high inherent energy tissues they are like kitchen paper and cannot easily absorb energy from the hydrogen nuclei.
This is NB in the T1 relaxation process.
Factors that determine the T1 and T2 relaxation times of a tissue.
Describe how closely packed the molecules are:
In closely packed molecule tissues the energy exchange is more efficient than when the molecules are wide apart.
This is NB in T2 decay where energy is exchanged from one hydrogen nucleus to another.
Factors that determine the T1 and T2 relaxation times of a tissue.
Describe how well the molecules tumbling rate matches the Lamor frequency.
If there is a good match between the two then of course the energy exchange is more efficient than with a bad match where it is not so efficient.
NB with T1 and T2 relaxation processes.
Why does fat have short T1 and T2 relaxation times?
Fat and water appears differently on MRI images, because the hydrogen in fat recovers faster along the longitudinal axis than water and also loses transverse magnetization faster than water.
List four points regarding T1 recovery in fat:
Low inherent energy so easily absorbs energy into its lattice from hydrogen nuclei.
Slow molecular tumbling allows it to recover relatively fast because the molecular tumbling rate matches the Lamor frequency and has efficient energy exchange form surrounding nuclei to the surrounding molecular lattice.
Magnetic moments of fat nuclei are able to relax and regain their longitudinal relaxation quickly.
The NMV of fat realigns rapidly with Bo and therefore the T1 time of fat are short.
List four points regarding T1 recovery in water:
High inherent energy so cannot absorb energy easily into its lattice from the hydrogen nuclei.
High molecular tumbling rate resulting in less efficient T1 recovery because the tumbling rate does not match the Larmor frequency so does not allow efficient energy exchange from hydrogen nuclei to the surrounding molecular lattice.
Magnetic moments take longer to relax and regain longitudinal magnetization.
The NMV of water takes longer to realign with B0 so T1 if water is long.
Define the term “weighting”.
T1, T2 mechanisms and inherent PD contrast is present in any MR image simultaneously and contribute to the image contrast, so we have to “weight” an image in order to make one process dominant and diminish the other two.
In order to determine whether an image is T1, T2 or PD weighted , specific TR and TE values need to be selected for each given pulse sequence.
Describe T1 Weighting:
On a T1 weighted image the contrast predominately depends on the T1 times between fat and water (all other tissues have intermediate signal).
TR controls how far each vector recovers before it is excited by the next RF pulse.
The TR must be short enough in a T1 image so that neither fat nor water has sufficient time to return fully to B0.
What will happen if the TR is too long?
T1 relaxation will be complete in both tissues and the differences are not demonstrated on the image.
Describe T2 Weighting:
The contrast is predominately dependent on the differences in T2 times between fat end water (all other tissues has and intermediate signal).
TE controls the amount of T2 decay that is allowed to occur before the signal is received.
Therefore the TE must be long enough in a T2 weighted image to give both fat and water time to decay so that the differences between fat and water can be demonstrated on the image.
What will happen if the TE is too short?
Neither fat or water will have time to decay and therefore the differences between fat and water will not be demonstrated on the image.
For T2 weighted images the TE must be long because TE controls the amount of T2 weighting.
Describe Proton Density Weighting:
In these images the main determining factor for image contrast is the difference in the numbers of protons per unit volume in the patient tissues.
There is always some form of proton density present to some extent.
In order to achieve PD images we have to diminish the T1 and T2 contrast.
When we use a long TR both fat and water will recover fully in their longitudinal magnetization so in that way diminishing T1 weighing.
A short TE does not give fat and water enough time to decay so diminishing T2 weighting.
Draw graphs from MRI - Image Weighting and Contrast Slides 11-19.
(Draw graphs from slides 11-19)
