MRI Flashcards
Larmor Frequency definition
The precession of a magnetic dipole moment in an external magnetic field
What is T1 relaxation?
The recovery of the longitudinal magnetization vector after the release of the RF pulse.
What is T1 time?
The time it takes for the recovery of 63% of the original NMV after a 90 degree RF pulse.
What kind of material has the fastest T1 time?
Water, the tighter the bond the faster the recovery. Ie: water recovers faster than fat.
What is the difference between T1 recovery and T1 time?
T1 recovery is the return to longitudinal vector after any RF pulse; while T1 time is the recovery of a specific amount (63%) and a given RF strength (90 degrees)
T 0r F? T2 is independent of the strength of the RF pulse or the strength of the magnetic field?
True. T2 depends on how the H protons in water interact with other molecules around them.
What is T2 time?
This is the time required for 37% of the maximum signal to remain.
T1 relaxation is also known as what? (2 things)
This is also known as spin-lattice relaxation, or longitudinal relaxation.
T2 relaxation is also known as what? (2 things)
This is also known as spin-spin relaxation, or transverse relaxation.
How can we tell anterior - posterior relationships in MRI?
The relationship is differentiated by the phase encoding gradient. (This can only be done one row at a time)
How can we tell the if the signal is coming from the left or the right?
The relationship is differentiated by the frequency encoding gradient.
How can we tell in the body where the image is being taken in MRI?
We can tell this by the slice encoding gradient.
How do we change the slice thickness?
This is changed by changing the bandwidth of the RF pulse. (Narrow RF bandwidth gives a narrow slice)
For a T1 image what TR and TE do we need?
For this type of image we need a short TR and a short TE.
For a T2 image what TR and TE do we need?
For this type of image we need a long TR and a long TE.
For a typical T2 image what TR and TE parameters do we need?
For this type of image we need a TR of 2,000 to 4,000 ms, and a TE of 80 to 120 ms.
What is k-space?
This is a matrix, the ‘frequency domain’ repository. This describes 2 dimensional matrix of positive and negative spatial frequency values.
Why do we need short TR?
This sets tissues with different TR times further apart so we can see bigger contrast between the tissues. T1
Why do we need long TR?
We need long TR to eliminate the contrast between tissues based on their T1 relaxation. Allows for full T1 recovery. (TR influences the contrast)
What is the time relationship between T1 and T2 relaxation?
T1 relaxation»_space;> T2 relaxation
Why do we need short TE?
We need short TE to prevent T2 relaxation. (based on de-phasing)
Which scenario gives us a bad signal? (TR & TE)
Short TR & Long TE
For a typical proton density image what TR and TE parameters do we need?
Long TR & short TE.
What is STIR?
Short Time Inversion Recovery. This removes signal from protons with fast T1 relaxation, like fat. so fat is eliminated from the signal.
What is the Gyromagnetic ratio?
The ratio of an atoms magnetic moment to its angular momentum.
What is FLAIR?
Fluid Attenuated Inversion Recovery. This removes signals from protons with slow T2 relaxation, like fluids.
STIR images are good for detecting what?
This image is good for detecting early osteomyelitis. (cuz the signal from fat is eliminated)
FLAIR images are good at detecting what?
This image is good for detecting Multiple Sclerosis (cuz signals from fluids are gone)
Why do we use Fast Advanced SE (FASE or HASTE)
This uses 212 echoes and then fills in the rest of the k-space. We take 212 echoes and reverse and interpolate the early echoes.
If the voxel size is smaller how does this affect the image?
This causes lower signal/ higher noise; and higher spatial resolution.
If the FOV is increased how does this affect the image?
This causes lower spatial resolution, and higher contrast resolution.
When do we use phase wrap?
We use this if the FOV < than the object size.
When do we get a ‘phase wrap artifact’?
This artifact happens when the FOV is smaller than the object
What is a ‘chemical shift’ artifact?
This is when different materials have differing resonance frequencies. Like hydrogen in water vs, hydrogen in lipids.
What is the definition of spin?
Circular movement of an object around a center of rotation.
Definition of angular momentum?
The force that stabilizes the axis’ direction of a body in rotation.
Definition of precession?
Change in orientation of the rotation axis of a rotating body.
In the gradient echo sequence what does a large flip angle give us?
A T1 weighted image.
What is motion artifact in MRI?
This happens when a spin moves during the time between excitation and signal sampling.
