MRI

Question 1

what creates the nuclear magnetic dipoles necessary for MRI?

Answer 1

Spinnings of atoms creates magnetic dipoles

To create these nuclear magnetic dipole moments, an atom needs to have an odd number of nucleons
(ex: hydrogen w/1 proton & no neutrons)

Question 2

Preconditions for imaging atomic nuclei

Answer 2

Nuclei must have an odd number of nucleons

Nuclei must be abundant in body

Question 3

What creates the magnetic field in an MRI

Answer 3

superconductive coils cooled with helium create the homogenous, static B field required

Question 4

What happens when atomic nuclei enter the static magnetic field

Answer 4

Spins of atomic nuclei are normally oriented randomly

When in a magnetic field, the spins align w/ the applied field either in parallel or antiparallel

Excess along B (in the form of an applied field) results in net magnetization (= the sum of the dipole moments)

Question 5

Along what axis does precession occur?

Answer 5

atoms precess or wobble due to magnetic momentum of an atom along the Z access

Question 6

On what does the frequency of precession depend?

Answer 6

Lamor frequency depends on gyromagnetic ratio (for hydrogen = 42.6 MHz/T) and B (magnetic field strength)

The stronger the B field, the faster the precession

Question 7

How are atoms excited in an MRI?

Answer 7

A radio frequency pulse (RF) is sent w/ a frequency that matches the center frequency of the system

Question 8

What is the result of an excitation RF pulse?

Answer 8

RF pulse runs perpendicular to B field (switches net magnetization to xy plane) → causes spins to be in phase

The processing spins induce a current in the receiver coil

Question 9

Which takes longer, T2 relaxation or T1 relaxation?

Answer 9

T1 relaxation is longer

Question 10

What is T1 relaxation?

Answer 10

relaxation in the longitudinal plane (Z axis)

After RF pulse, the spins are flipped toward the xy plane → spins of the protons transfer the energy to the tissue, then flip back toward the Z-axis -→ magnetization in the z axis is therefore recovered

Question 11

What is T2 relaxation?

Answer 11

relaxation in xy plane/transverse decay time constant

after excitation, spins all point in the same direction, but over time, inhomogeneities of B field) cause spins to be in different locations w/ different 𝝎 (spins dephase)

Question 12

What are the gradients produced by the gradient coils?

Answer 12

1) Selective slice excitation (Gss = Gz) → peaks same time as RF pulse
2) Phase encoding gradient (Gpe = Gy) → switched on briefly in anterior-posterior position and when it’s turned off, each proton within the selected slice will spin with the same frequency but with a slightly different phase
3) Frequency encoding gradient (Gro = Gx) → gradient causes variation of precession frequency w/ position, receive sum of signals from spin

Question 13

Which of the following is/are true:

a) flip angle is energy dependent
b) flip angle does not depend on RF pulse strength
c) flip angle is dependent on pulse duration
d) flip angle is dependent on RF pulse strength and duration

Answer 13

a, c, d

Question 14

What do pulse sequences control?

Answer 14

RF excitation
Gradient waveforms
Acquisition
Image reconstruction

Question 15

What are the (general) steps of the spin echo sequence

Answer 15

1) after 90º RF pulse, net magnetization is in xy plane
2) Spins immediately start to de-phase due to T2 relaxation
3) A second RF pulse is given, but this time it’s a 180º pulse
4) 180º pulse causes the spins to re-phase (= a compensation of the static field inhomogeneities)

Question 16

What are the variations of the spin echo sequence?

Answer 16

Multi-echo sequence

Multi-slicing sequence

Inversion recovery sequence

Question 17

How does the multi-echo sequence differ from the standard spin echo sequence?

Answer 17

this time a series of seven 180º pulses are given
→ each 180º pulse generates an echo
→ the k space is divided into 7 segments & each echo fills one line in each segment

Question 18

How does multi-slicing differ from standard spin echo sequence?

Answer 18

As soon as the first repetition is finished, the next repetition is started, but Gss is shifted so that a slice next to the first slice is selected
TR = 540 ms &TE = 30 ms
→ can do 18 slices in one TR

After all slices have been done the first time, go back to first Gss then do second repetition

Question 19

What does the inversion recovery sequence do?

How is it done?

Answer 19

Increases T1 contrast

Usually long TR and short TE
During the first 180º excitation pulse, the net magnetization is flipped to the Mz axis (no magnetization in xy plane yet)
=> only T1 recovery happening
T1 relaxation allowed to occur for a certain amount of time (=TI)

Basically = a spin echo sequence preceded by an additional 180º pulse

Question 20

How do gradient echo sequences occur?

Answer 20

1) Slice selected w/ Gss
2) Excitation pulses sent
3) Gro turned on w/ negative polarity
→ polarity switch has the same effect as a 180º RF pulse
4) Spins de-phase
5) Gro switched to positive polarity
6) Spins rephase → are in phase again
7) Singal sampling during Gro

Question 21

What do gradient echo sequences allow us to measure?

Answer 21

T2* relaxation (sensitive to local fluctuating and static field inhomogeneities)

Question 22

Compare spin- to gradient-echo sequences

Answer 22

In spin, echo is caused by 180º RF pulse + gradient fields
In gradient, echo is caused by gradient fields

The flip angle in spin echo is 90º while it is variable, but less than 90º in gradient (the smaller the FA, the shorter repetition time TR)

Spin echo is time consuming whereas gradient echo is fast

Spin echo gives T2 contrast while gradient echo gives T2* contrast

Spin echo is insensitive to static field inhomogeneities and sensitive to motion artifacts
Gradient echo is sensitive to static field inhomogeneities but insensitive to motion artifacts

Question 23

On what does T1 contrast depend?

Answer 23

T1 relaxation