MRI Flashcards
2 types of relaxation mechanisms
Spin lattice
Spin spin
Spin spin relaxation
Hydrogen ions no longer precess in phase , due to field inhomogeneity and spin coupling where field of oscillating nuclei affects nearby nuclei
How does moment in the xy and z plane change after the 90 degree pulse?
Exponential decay/growth
Magnetic field gradient
Allows imaging using the nuclear magnetic resonance signal
Magnetic field strength changes in proportion to the direction of
Magnetic field gradient description of the B0 field provided by the MRI scanner along the main field of the magnet
The magnetic field strength becomes a function of distance
Formula for the magnetic field strength along z axis
Constant base magnetic field strength + distance along x * gradient of x + same for y and z
Why is the gradient created?
So that only a few hydrogen nuclei precess at a frequency equally equal to the frequency at which the applied field at 90• is rotating
Process to create an MRI image using NRI technique
Slice selection
Phase encoding
Frequency encoding
Slice selection
A 90 degree pulse is applied at a frequency for all protons which rotate into the transverse plane
What is the 90 degree pulse really equal to?
It will oscillate over a range of frequencies which is the bandwidth, which depends on the pulse length
How does the range of frequencies affect the slice thickness?
Higher bandwidth, thicker slice
How to select the plane for the slice in the MRI ?
Change the gradient
Change the frequency
What happens to the selected slice?
It is now in the transverse axis, and when a coil is put through it in the radial direction it will have a current through it, so there will be a signal emitted
What happens after the pulse is applied?
Gradient is reversed, so the nuclei that were precessing a bit faster will precess slower and vice versa, the B1 pulse is removed.
Gradient reversal
What happens after slice selection?
Within the slice the larmor frequencies and gradients are manipulated further
Phase encoding
A perpendicular field is applied so that the new larmor frequency, and there is an increase in the phase of rotation of the magnetization vector
The y-gradient is shut off, and the x-gradient is switched on
The starting point of rotation depends on the y value
How does x value vary the precession of hydrogen nucleus?
Higher x, higher larmor frequency
How does the y value affect precession of hydrogen nucleus?
Higher y, higher increase in the phase value (angle between the z axis and magnetisatin vector
What affects the contrast of each image?
The magnitude will depend on the number of hydrogen nuclei within that small volume: the local value of the density
Local value of T1/T2: rate at which vertical radial and horizontal transverse rate decays
The relative co tribulation depends on radia frequency and gradient fields (pulse sequence)
Spin pulse echo sequence
Initially, the M magnetization vector is allligned with the B0 field
Another B1 field is applied to force the magnetization vector to precess about the xy plane
But it precedes around z axis as well
Due to relaxation mechanisms, magnetization vector builds back up to vertical and phase becomes incoherent again. The rate of relaxation will depend on the type of material.
The magnetization vector of the nucleus in the xy plane will decay depending on T2*.
After a period of time a 180 degree pulse is applied, causing the slower nuclei to precess at a higher rate than the faster ones. After another period of time, the faster will catch up with the slower ones, and all the spins are rotating at the same phase again.
How does the magnetization vector in the transverse direction reach a maximum value?
When the Mxy vector is flipped so it reflects by the x axis
The faster spinning nuclei will be slower
And the slower will be faster
Eventually they will reach phase coherence
Resonance occurs
Echo in spin echo pulse
Produced by current passing through coil
T2*
Inherent properties of the material + field properties
