W4L1 - fMRI Physics

Positron Emission Tomography (PET). Exposes patient to ionizing radiation whereas fMRI does not

1.5 - 9 Tesla (3T for most)

3T = 60000x Earth’s 65 microtesla

• ) Participant is placed on the bed and moved into the magnet (No metals) • ) Participants can see projection via. mirror mounted on head coil

Responses can be given via scanner-compatible keys, joystick, touchpad, etc.

The head coil is used to send radio frequency pulses and also functions as a receiver Head position is fixed to avoid any movement

Structures image of the brain

> 70% of brain consists of h20. H+ can be thought as small bar magnets, “precessing” like a spinning top about an axis

• 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.

Z-axis

Magnetisation along the Z-axis can’t be measured > Tilt the magnetisation vector by RF pulse perpendicular to magnetic field via head coil

Matches the precession frequency of protons

• ) 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)

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)

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

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

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.

“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

“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

“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

Fourier transformation to reconstruct the entire space. Takes about 1-3 seconds