Advanced MRI

Question 1

Why is anatomical imaging using MRI used?

Answer 1

Excellent soft tissue contrast, can show tumours in areas of soft tissue
No ionising radiation

Question 2

What are the limitations of using MRI?

Answer 2

Geometric distortions due to scanner limitations and patient body geometry and composition
Long scan times
Grey levels not equal to electron density

Question 3

Why do individual molecules move randomly in solution?

Answer 3

Thermal energy

Brownian motion

Question 4

How is the average distance which a group of molecules will move calculated?

Answer 4

r = (6Dt)^(1/2)

Question 5

What are the requirements of the pair of diffusion-weighting gradient pulses?

Answer 5

Positioned either side of the 180 pulse
Equal amplitude
Equal duration
Applied in same direction

Question 6

What is the equation for the signal intensity in a pulsed gradient spin echo scan?

Answer 6

S/S0 = e^(-bD)

Question 7

What is the equation for the diffusion weighting term in the pulsed gradient spin echo scan?

Answer 7

b = gamma^2 G^2 delta^2 (DELTA - delta/3)
Where G = gradient strength
delta = gradient duration
DELTA = gradient separation

Question 8

What is the apparent diffusion coefficient (ADC)?

Answer 8

This is the diffusion which occurs when the water diffusion is limited by cellular or sub-cellular structures

Question 9

What are ADC maps and diffusion weighted whole body imaging scans used for?

Answer 9

ADC - guage effect of therapy - changing cellularity, necrosis
DWIBS - highlight tissue with reduced diffusion - tumours with high cell density
- detect metastises
- gives PET like appearance
- free-breathing acquisition

Question 10

What is anisotropic diffusion caused by?

Answer 10

Some biological tissues with anisotropic structures limit the diffusion of water in certain directions

Question 11

What is anisotropy diffusion used for?

Answer 11

Brain imaging as diffusion is easy along axon but hard across it

Question 12

What is diffusion tensor imaging used for?

Answer 12

Defining tumour boundaries as data suggests tumours spread along tracts

Question 13

Why is perfusion measured?

Answer 13

Critical parameter of early disease related changes as tumours:
Increase metabolic rate and therefore need a lot of vasculator to maintain energy requirements
Added value over structural imaging as improved pathalogical characterisation
Biomarker for therapeutic intervention

Question 14

Why is gadolinium used as a contrast agent?

Answer 14

Reduces water T1
Contrast agent delivered to tissue is proportional to perfusion rate
Rapid scanning so can do dynamic contrast enhanced (DCE) MRI

Question 15

What is DCE MRI used to report on?

Answer 15

Vessel density
Vessel permeability/surface area
Extracellular volume

Question 16

What type of plots are used to determine if a tumour is malignant in DCE?

Answer 16

Enhancement vs time curves

Question 17

What is the Toft’s model?

Answer 17

A model that uses the DCE MRi to estimate the proportion of a drug reaching the tissue from the plasma from the surrogate of the contrast

Question 18

How does dynamic susceptibility contrast work and what is it used for?

Answer 18

T2* weighted contrast
Exploits magnetic susceptibility of i/v Gd contrast agent
Used for neurological applications as it remains intravascular

Question 19

How does arterial spin labelling work?

Answer 19

Magnetic label generated by scanner
Inverts or saturates magnetisation in/near the region of interest
Observe the inflow of unlabelled/labelled blood in the region

Question 20

What phenomena does fMRI rely on and how does it work?

Answer 20

The BOLD effect
Neuronal activity alters blood flow and oxygenation
Change in oxygenation alters T2* of tissue
Can correlate change in signal intensity with functional task