Chapter 4 - MRI

Question 1

What is the Larmour frequency? What is it dependent on?

Answer 1

A fixed frequency of precession (angular momentum) that protons have in the static external magnetic field (Bo). A higher strength Bo will result in a higher frequency.

Question 2

What is the Larmour frequency of protons in a 1.5T MR machine?

Answer 2

63.8 MHz, or 63,800,000 cycles per second.

Question 3

Why is a Faraday cage used in MRI?

Answer 3

Because the MR signal measured is low amplitude and needs to be isolated from outside environmental radiofrequency “noise” for accurate signal reading. A defect in the faraday cage will cause a zipper artifact.

Question 4

What are the 3 methods that allow for MR signal localization in 3D space?

Answer 4

Slice selection gradient (z axis) using a specific Larmour frequency.

Phase Encoding (x or y axis) applied between RF and TE.

Frequency Encoding (x or y axis) applied during TE/signal reading

Question 5

What 2 main settings are adjusted to create different MR sequences?

Answer 5

Echo Time (TE) and Repetition Time (TR)

Question 6

What is T1 relaxation?

Answer 6

Spin-lattice relaxation, or T1 relaxation, is energy lost to the local environment from the high energy anti-parallel protons after the RF pulse was applied. It is the exponential regrowth of 63% of the longitudinal magnetization Mz vector.

Question 7

What is T2 relaxation?

Answer 7

Spin-spin relaxation, or T2 relaxation, is dephasing of the protons in precession due to local microenvironmental magnetic inhomogeneities after the RF pulse is applied. It is exponential decay of Mxy vector.

Question 8

Is T1 relaxation rapid or slow in fat? In water? What does this mean in terms of their signal intensities?

Answer 8

T1 relaxation is rapid in fat, making the signal hyperintense/bright.

T1 relaxation is slow in free water, making the signal hypointense/dark.

Question 9

What are 2 fat saturation MR sequences? How do they differ?

Answer 9

Short Tau Inversion Recovery (STIR) is an older method that can be used in low field and high field MR systems. It cannot be used for post-contrast scans.
FatSat sequences (ex: DIXON) are only available with high field MR systems, and can be used for post-contrast scans. FatSat are useful in MSK studies to highlight bone edema from medullary fat (similar for muscle pathology).

Question 10

Brain MR is superior to CT in what ways?

Answer 10

Superior soft tissue contrast.
Higher sensitivity for vascular and inflammatory diseases, congenital brain malformations, and subtle cranial nerve pathology.

Question 11

What are 4 contraindications to performing an MR study?

Answer 11

1. Pacemaker
2. Metallic implants in the region of interest.
3. Metallic foreign bodies.
4. Pregnancy.

Question 12

Susceptibility artifact is caused by ___________. What sequences is it most severe in?

Answer 12

Metallic implants
Blood/hemorrhage (iron)
Gas, or interfaces between gas and bone.

Most notable in gradient echo sequences (T2* or SWI), and more pronounced in high field machines.

Question 13

The MR signal strength/amplitude produced is proportional to _________

Answer 13

Magnetic field strength (Bo)

Question 14

How does gadolinium produce contrast?

Answer 14

It is a paramagnetic molecule with 7 unpaired electrons. This enhances the local magnetic field, resulting in shorter T1 recovery times in adjacent tissues, and thus an increased signal.

Question 15

Most diseases result in asymmetric pathology except which 3 causes (focus on brain MR)?

Answer 15

Nutritional, toxic, and metabolic diseases are often bilaterally symmetric.

Question 16

What 6 areas can be evaluated in the brain to assess appropriate T1 contrast enhancement on MR?

Answer 16

Choroid plexuses 
Large veins
Pituitary gland 
Trigeminal nerve ganglion
Nasal sinuses
Salivary glands

Question 17

What are the MR features of cytotoxic edema in T1 and T2 sequences?

Answer 17

Intracellular water accumulation, often due to ischemia, infarct, or post-ictal changes. Well-marginated areas of homogeneously T2 hyperintense/T1 iso-to-hypointense lesions in areas of high metabolic activity (gray matter where the neurons live). Rarely causes mass effect. Poor contrast enhancement unless a lesion (ex: neoplasm) is present.

Question 18

What are the MR features of vasogenic edema in T1 and T2 sequences?

Answer 18

Interstitial/extracellular fluid accumulation due to increased vascular permeability. Poorly-marginated areas of homogeneously T2 hyperintense/T1 iso-to-hypointense lesions predominantly in white matter. Can be large and cause significant mass effect. Poor contrast enhancement unless a lesion (ex: neoplasm) is present.

Question 19

Hyperacute hemorrhage (within a few hours) will appear ________ on T1 and ________ on T2.

Answer 19

Oxyhemoglobin will be T1 isointense and T2 hyperintense.

Question 20

Acute hemorrhage (hours to days) will appear ________ on T1 and ________ on T2.

Answer 20

Deoxyhemoglobin will be T1 iso-to-hypointense and T2 hypointense.

Question 21

Early subacute hemorrhage (first few days) will appear ________ on T1 and ________ on T2.

Answer 21

Methemoglobin intracellulalry will be T1 hyperintense and T2 hypointense.

Question 22

Subacute-to-chronic hemorrhage (weeks-months) will appear ________ on T1 and ________ on T2.

Answer 22

Extracellular methemoglobin will be T1 hyperintense and T2 hyperintense.

Question 23

Remote hemorrhage (weeks-years) will appear ________ on T1 and ________ on T2.

Answer 23

Ferritin and hemosiderin will be T1 iso-to-hypointense and T2 hypointense.