Ch.20 - MRI: Understanding the Principles and Recognizing the Basics

Question 1

When the RF pulse is turned off:

Answer 1

The displaced protons relax and realign with the main magnetic field, producing a radiofrequency signal (the echo) as they do so.

• -> Receiver coils receive this signal (or echo) given off by the excited protons.
• -> Computer reconstructs the information from the echo to generate an image.

Question 2

The main magnet in an MRI scanner:

Answer 2

Is usually a superconducting magnet that is cooled to extremely low temperatures in order to carry the electrical current.

Question 3

There are 3 major types of coils within the MRI scanner:

Answer 3

1. Volume.
2. Surface.
3. Gradient coils.

Question 4

Pulse sequences:

Answer 4

Consist of a set of imaging parameters that determine the way a particular tissue will appear.

Question 5

The two main pulse sequences that ALL MRI pulse sequences are based on:

Answer 5

Are called spin echo (SE) and gradient recalled echo (GRE).

Question 6

T1 and T2:

Answer 6

Time constants:
T1 –> The longitudinal relaxation time.
T2 –> The transversal relaxation time.

Question 7

TR is:

Answer 7

The repetition time between 2 RF pulses.

A short TR creates a T1-weighted image.

Question 8

TE is:

Answer 8

The echo time between a pulse and its resultant echo.

A long TE creates a T2-weighted image.

Question 9

On T1-weighted image, what is typically bright:

Answer 9

1. Fat.
2. Hemorrhage.
3. Proteinaceous fluid.
4. Melanin.
5. Gadolinium.

Question 10

On T2-weighted images, what is bright:

Answer 10

1. Fat.
2. Water.
3. Edema.
4. Inflammation.
5. Infection.
6. Cysts.
7. Hemorrhage.

Question 11

In summary, fat is bright in:

Answer 11

Fat is T1-bright + T2-bright.

Question 12

In summary, water is:

Answer 12

T1-dark and T2-bright.

Question 13

Suppression is:

Answer 13

A feature of MRI that will cancel out or eliminate signal from certain tissues and is most often used for fat.

• -> Although, normally T1-bright, fat will be DARK in T1-weighted fat-SUPPRESSED images.
• -> Fat suppression is particularly useful for tissue characterization after administration of gadolinium.

Question 14

MC IV contrast agent in clinical MRI:

Answer 14

Gadolinium.

Question 15

Gadolinium effect on T1-relax time:

Answer 15

Gadolinium shortens the T1-relaxation time of hydrogen nuclei yielding a brighter signal.

Question 16

What is enhanced after gadolinium administration:

Answer 16

Vascular structures such as tumors and areas of inflammation ENHANCE after gadolinium administration and become more conspicuous.

Question 17

MRI in pregnancy:

Answer 17

Preferred in the 2nd/3rd trimester.

Gadolinium is CONTRAINDICATED.

Question 18

Nephrogenic systemic fibrosis:

Answer 18

A debilitating fibrotic disease that can occur in patients with renal insufficiency who receive IV gadolinium-based contrast agents.
–> Gadolinium is typically avoided in patients with severe renal disease.

Question 19

RF pulse:

Answer 19

A RF pulse, transmitted by a transmitter coil, displaces the protons from their original alignment with the external magnetic field of the scanner.