Session 3 - MR Physics Flashcards
(14 cards)
MR Intro
- different techniques for neuroimaging: MR is one of them
- less invasive like MEG, EEG; TMS and PET
- spatial resolution good β> but technically impossible to see neurons even with improved methods
- trying to expand time scale: s to ms β> not yet possible
What is measured in MR?
- protons, elements with nuclear spin of 1/2 (other atmoic nuclei can be detected but are difficult to measure (relaxation too fast))
- usually detect hydrogen because it is so abundant in the universe and body
- MR is blind for electrons
Proton spin
- 1/2 spin, positive charge
- creates magentic field,
- protons behave like compass needle
β> MR interferes with magnetic property - protons are randomly oriented without external magnetic field but align with parallel and anti-parallel to external magentic field B0 (North, South)
Hydrogen atoms in a static magentic field
- precession of the spins with frequency that is proprtional to the magentic field strength Bβ
- Larmor Frequency: Ο = Ξ³*Bβ, gyromagnetic ration Ξ³ = 42MHz/T
β> spins align along external magentic field and they rotate very fast with larmor frequency (42Mio rotations per second) -> increased in MRI - the net magenetisation M is parallel to Bβ (magnetisation vector is parallel to field strength
- number of protons defines magnetisation and temperature (Mβ = ΟβBβ)
The MRI magnet
- very strong: 3T =3.307 tn 60 000 x Earthβs magnetic field (7T = 140 000 x EMF)
- it is continuously switched on even without power
- created by superconductors (electrical resistence extremely small/disappears due to cold helium)
- the closer to the middle, the stronger the magnetic forces (exponentially increasing)
Excitation of the spin system
- protons have specific spin 1/2: short radio frequency pulses rotate magnetisation into transversal plane + rotationβ> induces electrical currents in surrounding wires
- MRI creates field β> relative to this direction: transversal plane is induced when radio frequencies are active β> only then signal can be measured
- shirt RF pulse in resonance frequency rotates magnetisation into transversal plane (longitudgunal magnetisation cannot be measured)
- all spins precess in phase
- 90Β° flip angle
Exclusion criteria
- safety protocol metal check
- claustrophobia
- pacemaker
- implantable cardioverter defibrillator (ICD)
- neurostimulator
- aneurysm clip
- metal implant
- implanted drug infusion device
- forgein metal objects. especially if in or near the eye
- shrapnel or bullet wounds
- permanent cosmetics or tattoos
- dentures/teeth that involve magnetic keepers
- other implants that involve magnets
- medication patch (ie transdermal patch) that contains metal foil
- danger of localised burns due to metallic implants !
Relaxation of NMR signal
Longitudinal relaxation (T1) β> prefers to go back to original orientation
Transversal relaxation (T2<T1): dephasing spins β> sum of signal decreases (we can only detect sum of all signal directions)
- magnetisation reduced, signal weakened
- depends on molecular structure of the material (tissue type) β> different tissues have different relaxation processes (takes longer/shorter, is stringer/weaker) β> can be contrasted
Reception of NMR signal
- electrical current/signal induced in receive coil
β> depends on protons (more protons = stronger signal)
β> disappears within ms - free induction decay (FID)
β> MRI is not very sensitive, voltages induced in receiving antenna are much smaller than needed for excitation (10 000x )
β> MRI machine is surrounded by magnetic shield (metal) to reduce external magnetic fields (frequencies in environment) - the more antenna, the more signal can be detected β> the closer, the better
Slice excitation
- problem: where does the signal come from? it is received from the whole object
β>solution: selective excite certain parts only to localise where the signal comes from - only exicte a certain slice with radio frequency β> same frequency of spins, change strength of magnetic field drom top to bottom (gradient field)
β> above an below normal magnetic field strength (B0) β> this is where Larmar equation applies - can be done for every part of the body
Frequency encoding
two bottles
1. only receive one frequency (signal): constant magnetic field β> decays with time
2. varying magnetic field: mix β> receive two signals (higher and lower)
- bottle on left lower frequency due to weaker magnetic field
- bottle on right higher frequency due to stronger magnetic field
- use gradients to spatially encode the signal
- magentic field strength scales with spin rate β> complex pattern is translated to simple form (IMPORTENT learn this!)
- Fourier transformation: transform from temporal domain (middle) into frequency domain
β> Jean Baptiste Joseph Fourier (1768-1830)
Frequency vs phase coding: getting an image
A. induce magentic field
B: apply RF
C-F: apply three gradients
- changes phases and get frequency which can then be Fourier-transformed
β> three gradients to spatially encode spins by spin frequency manipulation β> repeat several times for multiple brain slices
Why is the MRI loud?
- different magnetic fields want to align so great force acts on coils which will change sound pressure and this can be heard
β> speech experiments are difficult
Image contrasst versus echo time
one way of creating contrast:
- detect signal immediately after excitation (not yet relaxation process) β> homogenous water distribution: no difference can be seen
β> after a few ms relaxation processes will unfold (tissue types differ)
β> iron disturbs magnetic field (tissue types with a lot of iron): some parts will relax faster (less signal) while others stay βconstantβ for longer time
different contrast can be created:
- inversion recovery β> 180Β° degree pulse: free water decay is slower β> white matter vs CSF differ
β> water signal can be suppressed: free water not surrounded by molecules
β> white matter can be suppressed to highlight tumors for example
