Lecture 12: Brain Imaging Flashcards
MRI uses the signal from water protons. What are these?
The nuclei of the hydrogen atoms in a water molecule.
What are the two key properties of a hydrogen nucleus?
- spin
- nuclear magnetic moment (aligns with magnetic field)
In an MRI, protons will adopt one of two orientations. What are they? And what are their energy states?
- Parallel orientation - low energy state
* Anti-parallel orientation - high energy state
What does the application of the Radio Frequency pulse do?
Causes the magnetisation of the protons to change direction.
When the protons then relax and realign this leads to what? What is this detected by?
Leads to a voltage change detected by a receiver coil.
Rate of relaxation differs across tissues. What does this give?
Image contrast (fat, water, etc.)
What are the two processes proton relaxation occurs by?
T1 and T2 relaxation.
Which type of relaxation is longitudinal relaxation?
T1 relaxation.
What is transverse relaxation?
Transverse relaxation (type T2) involves the loss of magnetisation in the xy plane.
What is longitudinal relaxation?
Longitudinal relaxation (type T1) is the return of magnetisation in the z plane.
In T2 weighted imaging, the image contrast is determined by what?
The T2 relaxation time.
In T2 weighted imaging does white or grey matter appear dark?
White matter appears dark and grey matter appears bright.
What type of substance is T2 usually used to show?
Fluids.
In T1 weighted imaging, the image contrast is determined by what?
The T1 relaxation time.
In T1 weighted imaging, does white or grey matter appear dark?
Grey matter appears dark and white matter appears bright.
What is T1 weighted imaging mainly used for?
Anatomical images.
Perfusion weighted imaging usually needs an injection of what?
A contrast agent.
In perfusion weighted imaging, when is there no need for a contrast agent?
When arterial spin labelling is used.
