MRI & MRS

Question 1

what is contrast

Answer 1

the ability to distinguish between differences in signal between anatomical or pathological regions

Question 2

CT contrast

Answer 2

tissue dependent attenuation of x-rays

Question 3

CT of bone

Answer 3

BRIGHT

Question 4

CT of fluid

Answer 4

DARK

Question 5

uses of CT

Answer 5

haemorrhagic stroke, traumatic brain injury

Question 6

disadvs of CT

Answer 6

not good at distinguishing between tissue structures, so use MRI

Question 7

CSF on T2 weighed MRI

Answer 7

BRIGHT

Question 8

Fatty tissues on T2 weighted MRI

Answer 8

DARK

Question 9

CSF on T1 weighted MRI

Answer 9

DARK

Question 10

what is a magnetic moment produced by

Answer 10

The positive charge of a spinning proton produces a magnetic moment

Question 11

how is an MR image generated

Answer 11

• signal comes from water
• the hydrogen atom in water has a proton in its core
• the nucleus of the proton is positively charged
• this spins on its axis and produces a magnetic moment
• in an MR scanner, the magnetic moment lines up with strong magnetic field in the scanner
• can manipulate this signal using radiofrequency pulses to generate an MR image

Question 12

safety issues for MRI

Answer 12

no ferromagnetic objects in exam room, e.g. scissors, stethoscope, wheel chair

Question 13

Contraindications of MRI

Answer 13

• pacemakers
• infusion pumps
• 1st trimester pregnancy
• Aneurysm clips
• Metallic foreign bodies

Question 14

MRI signal source

Answer 14

from the protons in the water and fat in the tissue, intensity depends on: water content, tissue structure, blood flow, perfusion, diffusion, paramagnetics

Question 15

T1 and T2 relaxation times vary between

Answer 15

different tissues and pathology

Question 16

principle of T2 relaxation times

Answer 16

The longer the echo time, the weaker the signal

Question 17

T2 relaxation in tissues

Answer 17

bound water decays quickly & free water decays slowly

• as tissue structure changes, the position of the curve does and so we can see changes in MR image
• lesions show up brighter increasing in water content

Question 18

T1 relaxation times

Answer 18

generate a pulse, have to wait till magnetisation comes back
T1 of brain tissue is short: magnetisation comes back quickly

Question 19

Fatty tissue in T1

Answer 19

short T1, bright

Question 20

differences between grey and white matter in T1 MRI

Answer 20

White: much shorter T1 than grey (much more rigid structure)
Myelinated structures have a reduced T1
Can see delineation if grey matter

Question 21

contrast agents

Answer 21

water in the vicinity of contrast agents, reduction in T1 or T2 relaxation time and so the signal is reduced

Question 22

why can contrast agents pass through blood vessels in the brain?

Answer 22

blood vessels in the brain are of poor quality, large molecules such as contrast agents can pass through the poorly developed vascular walls

Question 23

MRS

Answer 23

A 1H spectrum of biochemicals can be obtained from a localised region of brain using three slice selective processes

Question 24

chemical shift

Answer 24

• around the proton is an electron cloud
• electron cloud shields the proton from external magnetic field
• shielding changes depending on chemical structure

Question 25

MR for water vs fat

Answer 25

High frequency signal for water, lower for fat

Question 26

NAA

Answer 26

found predominantly in neurons, marker for viable neurons, reduced in pathology

Question 27

tCho

Answer 27

cell membrane metabolism (growth and degradation), elevated in tumors and gliosis

Question 28

Lipids

Answer 28

membrane breakdown products, macrophages: can indicate necrotic high-grade tumour