MRI basics

Question 1

What are the protons in tissues that spin around their axes?

Dewey

Answer 1

Hydrogen protons.

They generate their own micromagnetic environments.

Question 2

What happens when hydrogen protons are brought into a strong external magnetic field?

Dewey

Answer 2

They rearrange under its influence.

A slight majority of protons will align with the magnetic field.

Question 3

When the tissue magnetization vector is parallel with the main magnetic field, can it be used for imaging?

Dewey

Answer 3

Nope, it can’t, they need to be separated.

Question 4

Does the magnetic field influence the spinning motion of the hydrogen protons?

Dewey

Answer 4

Yep, it does, the hydrogen protons wobble under its influence.

Similar to influence of gravity in a spinning top.

Question 5

R……….. p……. are applied to manipulate tissue magnetization so it can be separated from the main magnetic field.

Dewey

Answer 5

Radiofrequency pulses.

Question 6

According to Faraday’s law, any change in the m………. environment of a coil of w…… will cause an electric signal.

Dewey

Answer 6

Magnetic
Wire

That is the basis of image formation in MRI.

Question 7

What is the most common flip angle of the tissue magnetization vector, used in spin echo sequences?

Dewey

Answer 7

90.

Transverse plane, perpendicular to main magnetic field.

Question 8

Basic spin echo sequences

T/F. Each SE sequence starts with a 90° RF pulse, followed by a 180° pulse applied exactly halfway between the initial 90° pulse and the generation of the signal (echo).

Dewey

Answer 8

True!

SE sequences are T1-W, T2-W and proton density.

Question 9

What’s the term for ‘generation of the signal’?

Dewey

Answer 9

Echo.

Question 10

What does TE stand for?

Dewey

Answer 10

Time of echo

It’s the time between the 90° pulse and the echo.

Question 11

What does TR stand for?

Dewey

Answer 11

Time of repetition.

The time between successive 90° pulses.

Question 14

In T1-weighting sequence, a short/long TR and TE are chosen.

Dewey

Answer 14

Short.

Short TR: maximize differences in T1 relaxation between tissues.

Short TE: to minimize T2 effects.

Question 15

T/F

In a T2-weighting sequence, long TE and TR are chosen.

Dewey

Answer 15

True.

Long TE: maximize differences in T2 relaxation between tissues.

Long TR: to minimize T1 relaxation effects.

Question 16

T/F

In a T2-W sequence, tissues with longer T2 relaxation times (such as fluid) appear brighter.

Dewey

Answer 16

True.

Bc they still maintain their transverse magnetization.

Question 17

If fat has a short T2 relaxation time, why appears hyperintense on today’s T2-W MRI studies?

Dewey

Remember: T2-W, long T2 relaxation time = hyperintense.

Answer 17

Bc conventional T2-W SE sequences have been replaced with shorter fast spin echo (FSE) or turbo spin echo (TSE) sequences.

Which means: additional pulses are applied, and fat appears hyperintense.

Question 18

T/F

Proton density (PD) weighting is a basic spin echo sequence and reflects the actual amount of hydrogen protons in a certain tissue volume.

Dewey

Answer 18

Correct.

It’s achieved by choosing a long TR and a short TE.

Question 19

T/F

Fat has a very short relaxation time, fluid has the longest relaxation time, and soft tissues are intermediate.

Dewey

Answer 19

True.