W4L1 - fMRI Physics Flashcards
What is the first method for brain imaging. Why is it used less now
Positron Emission Tomography (PET). Exposes patient to ionizing radiation whereas fMRI does not
What is the Telsa field for fMRI
1.5 - 9 Tesla (3T for most)
fMRI magnetic field vs Earth
3T = 60000x Earth’s 65 microtesla
What are the tools required for fMRI. Include Steps
- ) Participant is placed on the bed and moved into the magnet (No metals)
- ) Participants can see projection via. mirror mounted on head coil
How is the experiment controlled from outside the scanner room
Responses can be given via scanner-compatible keys, joystick, touchpad, etc.
What is head coil used for
The head coil is used to send radio frequency pulses and also functions as a receiver
Head position is fixed to avoid any movement
What does MRI do
Structures image of the brain
What is the role of proton in MRI. What is it thought of
> 70% of brain consists of h20. H+ can be thought as small bar magnets, “precessing” like a spinning top about an axis
Physics of MRI: what happens when protons are put into very strong magnetic field in MRI scanner
- Aligned parallel/anti-parallel (not perfectly aligned)
- Not static (Still precessing randomly)
- Precession frequency of proton depends on the strength of the magnetic field.
Individual photon recessing at same frequency but different points in space.
The axis along which the magnetization is build up in the scanner is called …
Z-axis
Why can’t we use the Z-axis. What should we do then?
Magnetisation along the Z-axis can’t be measured
> Tilt the magnetisation vector by RF pulse perpendicular to magnetic field via head coil
What is the amplitude of the RF pulse
Matches the precession frequency of protons
What does the RF pulse do to protons. 3 Effects
- ) Protons to absorb energy.
- ) Tilts magnetization vector (Z) to the transversal plane (X & Y)
- ) Align precession of spins > protons’ rotations phase coherent (each proton doing same thing)
What can the transversely rotating magnetization vector do?
The transversely rotating magnetization vector can then be recorded as a signal: The head coil is used to send the RF pulses, but it is also the receiver (Just a signal now)
What happens after we got the transversely rotating magnetization vector and the signal recording. What happens then to the protons?
Off the RF pulse.
Transversal magnetization decays: RELAXATION
- ) Protons emit excess energy
- ) Transversal magneitzation (X & Y) disappear and longitudinal mangeitzation reestablished
- ) Protons loses phase coherence– the protons emit the excess energy
During the relaxation phase, what happens
The summed effect of many protons relaxing can be measured.
- Transversal magnetization decays with different speeds depending on the tissue
> One Reason: Lose coherence because other protons in environment influence. Signal from diff protons out of phase with each other and cancel each other out.
Structural brain image depends on WHEN signal is recorded during this process
How do we reconstruct brain image from signals. What is the premise
Need a way to decide where exactly our signal comes from,
Premise: Protons will absorb energy from RF pulses only when RF pulse frequency = precession frequency
Gradients: Causing the magnetic field to vary linearly, we can cause the resonance frequency to vary throughout the brain.
How do we reconstruct z-axis
“Slice selecting gradient”
- Vary the gradient field along the z-axis and know that different slices were exposed to different field strengths
- Different protons are in different magnetic fields, their precession frequencies will be different > only one slice will be excited at a time using a specific RF pulse, because for the others, the precession frequency will not be matched
- By exciting one slice at a time, we get the z-coordinate of all resulting signals
How do we reconstruct y-axis
“Frequency Encoding Gradient”
- Change the magnetic field within this slice found in z-axis (vary the gradient along the y-axis)
- Protons in each slice also have different precession frequencies
How do we reconstruct x-axis
“phase encoding gradient”
- Very briefly using a gradient along the x-axis causes protons to “speed up” their precession according to the strength of the magnetic field for a very short time
- When switching off, all protons are all back to the same precessing frequency, but they are “out of phase” with each other
- By measuring the phase signal, we now also get the x-coordinate of the resulting signal
Knowing x-y-z axis, how do we do it.
Fourier transformation to reconstruct the entire space. Takes about 1-3 seconds
