103. FMRI: DIFFUSION Flashcards
1
Q
- Define: Diffusion.
A
- this is the term used to describe moving molecules
- these molecules are moving due to random thermal
motion
THIS IS KIND OF MOTION:
- is called Brownian Motion
2
Q
- What is Brownian Motion restricted by?
A
IT IS RESTRICTED BY BOUNDARIES SUCH AS:
- ligaments
- membranes
- macromolecules
- pathology
3
Q
- Define: Diffusion Coefficient.
A
- this is a parameter
- it describes the rate of diffusion in tissues
4
Q
- What other kinds of motion of molecules can we find in FMRIs?
A
- microcirculation
5
Q
- How is diffusion affected by Gradients?
A
WHEN STRONG GRADIENTS ARE APPLIED:
- diffusion is minimal
6
Q
- Define: Apparent Diffusion Coefficient (ADC).
A
- this is the rate at which molecules diffuse
- it is measured in mm² per second
TISSUES WILL HAVE A HIGH APPARENT DIFFUSION COEFFICIENT:
- if they experience diffusion that is free
TISSUES WILL HAVE A LOW APPARENT DIFFUSION COEFFICIENT:
- if they experience diffusion that is restricted
7
Q
- What are some characteristics of a High Apparent Diffusion Coefficient?
A
- There is free Diffusion
- There is a Low Signal Intensity regarding the Diffusion
Weighted Images that are acquired
EXAMPLE:
- Cerebral Spinal Fluid
8
Q
- What are some characteristics of a Low Apparent Diffusion Coefficient?
A
- There is restricted Diffusion
- There is a High Signal Intensity regarding the Diffusion
Weighted Images that are acquired
EXAMPLE:
- Grey Matter
- White Matter
9
Q
- What is the mathematical relationship between the Apparent Diffusion Coefficient and the Diffusion Weighted Signal?
A
- they are inversely a proportional
- an increase in one leads to a decrease in the other
10
Q
- How are Diffusion Weighted Images acquired?
A
- they are acquired by sensitising the motion of the
molecules - this is done by using strong gradients
11
Q
- How do we make use of strong gradients?
A
- two equal gradients are applied to either side of the
180° Radio Frequency Pulse - this happens in a Spin Echo Sequence
12
Q
- How are the gradients desgined?
A
- they are designed to cancel each other out
- this happens if the spins do not move
- this is in order to obtain Diffusion Weighted Images
13
Q
- Determine the factors that can be seen in this image?
A
- G = gradient
- t = time that the gradient is applied
- T = period of the pulse
- TE = echo time
14
Q
- What does the amount of signal attenuation depend on?
A
- Amplitude
- The directions of the applied gradients
- The Gradient Difference (Gdiff)
- The Apparent Diffusion Coefficient (ADC)
15
Q
- What equation do we use to measure the degree (attenuation) of the diffusion?
A
b = b-value
= this is the b factor
= it is the degree of diffusion
= it is measured in s / mm²
y = Gyromagnetic Ratio
= MHz / T
G= Gradient Amplitude
= mT / m
δ = Gradient Duration
= m / s
Δ = the time between the two pulses
= this is the time between the 90° and 180° Radio
Frequency Pulses
16
Q
- In which axes (planes) are Diffusion Gradients applied in?
A
- they are applied in the X, Y and Z axes
- they are combined to produce a Diffusion Weighted
Image
17
Q
- Do the Diffusion gradients need to be strong or weak?
A
- they need to be strong
- this is so that they achieve enough Diffusion Weighting
18
Q
- How is Diffusion Sensitivity controlled?
A
- it is controlled by the b factor
- this is what determines the Diffusion Attenuation
- this is the degree at which the diffusion happens
19
Q
- What is the average b-value?
A
- it ranges from 550 s/mm²
- to 1000 s/mm²
20
Q
- What happens as the b-value increases?
A
- the Diffusion Weighting Increases
- they are inversely proportional
21
Q
- Other than the Diffusion Weighting, what else does the b-value control?
A
- it controls the contrast in the images
22
Q
- What effect does a high b-value have on contrast?
A
- high b-values will exaggerate the differences in a
tissue’s Apparent Diffusion Coefficient
23
Q
- What do we typically use Diffusion Weighted Images to diagnose?
A
- strokes
- this is because they are very sensitive
24
Q
- How can we tell that a stroke is present using a Diffusion Weighted Image?
A
- the area where the stroke is happening will be bright
- this is because there is decreased diffusion
- this is due to infraction
25
Q
- What happens in early strokes?
A
- the cells will swell
- they absorb water from the extracellular space
- the diffusion is restricted
26
Q
- What do the changes in the Diffusion Weighted Image help us to identify after someone has had a stroke?
A
- the locality of the stroke
- the extent of the impact of the stroke
NB:
- this is seen before we can use other modes of imaging
for more information
27
Q
- Can we see the development of a stroke using CT and T2 Weighted Images?
A
- no
28
Q
- Which other body parts do we image using Diffusion Weighted Images?
A
- the liver
- the prostate
- the breast
29
Q
- What do we use Diffusion Weighted Images to differentiate between?
A
- we differentiate between malignant and benign lesions
- we differentiate between solids and cystic areas
