MRI

Question 1

Larmor Frequency definition

Answer 1

The precession of a magnetic dipole moment in an external magnetic field

Question 2

What is T1 relaxation?

Answer 2

The recovery of the longitudinal magnetization vector after the release of the RF pulse.

Question 3

What is T1 time?

Answer 3

The time it takes for the recovery of 63% of the original NMV after a 90 degree RF pulse.

Question 4

What kind of material has the fastest T1 time?

Answer 4

Water, the tighter the bond the faster the recovery. Ie: water recovers faster than fat.

Question 5

What is the difference between T1 recovery and T1 time?

Answer 5

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)

Question 6

T 0r F? T2 is independent of the strength of the RF pulse or the strength of the magnetic field?

Answer 6

True. T2 depends on how the H protons in water interact with other molecules around them.

Question 7

What is T2 time?

Answer 7

This is the time required for 37% of the maximum signal to remain.

Question 8

T1 relaxation is also known as what? (2 things)

Answer 8

This is also known as spin-lattice relaxation, or longitudinal relaxation.

Question 9

T2 relaxation is also known as what? (2 things)

Answer 9

This is also known as spin-spin relaxation, or transverse relaxation.

Question 10

How can we tell anterior - posterior relationships in MRI?

Answer 10

The relationship is differentiated by the phase encoding gradient. (This can only be done one row at a time)

Question 11

How can we tell the if the signal is coming from the left or the right?

Answer 11

The relationship is differentiated by the frequency encoding gradient.

Question 12

How can we tell in the body where the image is being taken in MRI?

Answer 12

We can tell this by the slice encoding gradient.

Question 13

How do we change the slice thickness?

Answer 13

This is changed by changing the bandwidth of the RF pulse. (Narrow RF bandwidth gives a narrow slice)

Question 14

For a T1 image what TR and TE do we need?

Answer 14

For this type of image we need a short TR and a short TE.

Question 15

For a T2 image what TR and TE do we need?

Answer 15

For this type of image we need a long TR and a long TE.

Question 16

For a typical T2 image what TR and TE parameters do we need?

Answer 16

For this type of image we need a TR of 2,000 to 4,000 ms, and a TE of 80 to 120 ms.

Question 17

What is k-space?

Answer 17

This is a matrix, the ‘frequency domain’ repository. This describes 2 dimensional matrix of positive and negative spatial frequency values.

Question 18

Why do we need short TR?

Answer 18

This sets tissues with different TR times further apart so we can see bigger contrast between the tissues. T1

Question 19

Why do we need long TR?

Answer 19

We need long TR to eliminate the contrast between tissues based on their T1 relaxation. Allows for full T1 recovery. (TR influences the contrast)

Question 20

What is the time relationship between T1 and T2 relaxation?

Answer 20

T1 relaxation&raquo_space;> T2 relaxation

Question 21

Why do we need short TE?

Answer 21

We need short TE to prevent T2 relaxation. (based on de-phasing)

Question 22

Which scenario gives us a bad signal? (TR & TE)

Answer 22

Short TR & Long TE

Question 23

For a typical proton density image what TR and TE parameters do we need?

Answer 23

Long TR & short TE.

Question 24

What is STIR?

Answer 24

Short Time Inversion Recovery. This removes signal from protons with fast T1 relaxation, like fat. so fat is eliminated from the signal.

Question 25

What is the Gyromagnetic ratio?

Answer 25

The ratio of an atoms magnetic moment to its angular momentum.

Question 26

What is FLAIR?

Answer 26

Fluid Attenuated Inversion Recovery. This removes signals from protons with slow T2 relaxation, like fluids.

Question 27

STIR images are good for detecting what?

Answer 27

This image is good for detecting early osteomyelitis. (cuz the signal from fat is eliminated)

Question 28

FLAIR images are good at detecting what?

Answer 28

This image is good for detecting Multiple Sclerosis (cuz signals from fluids are gone)

Question 29

Why do we use Fast Advanced SE (FASE or HASTE)

Answer 29

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.

Question 30

If the voxel size is smaller how does this affect the image?

Answer 30

This causes lower signal/ higher noise; and higher spatial resolution.

Question 31

If the FOV is increased how does this affect the image?

Answer 31

This causes lower spatial resolution, and higher contrast resolution.

Question 32

When do we use phase wrap?

Answer 32

We use this if the FOV < than the object size.

Question 33

When do we get a ‘phase wrap artifact’?

Answer 33

This artifact happens when the FOV is smaller than the object

Question 34

What is a ‘chemical shift’ artifact?

Answer 34

This is when different materials have differing resonance frequencies. Like hydrogen in water vs, hydrogen in lipids.

Question 35

What is the definition of spin?

Answer 35

Circular movement of an object around a center of rotation.

Question 36

Definition of angular momentum?

Answer 36

The force that stabilizes the axis’ direction of a body in rotation.

Question 37

Definition of precession?

Answer 37

Change in orientation of the rotation axis of a rotating body.

Question 38

In the gradient echo sequence what does a large flip angle give us?

Answer 38

A T1 weighted image.

Question 39

What is motion artifact in MRI?

Answer 39

This happens when a spin moves during the time between excitation and signal sampling.