MRI

Question 1

2 types of relaxation mechanisms

Answer 1

Spin lattice
Spin spin

Question 2

Spin spin relaxation

Answer 2

Hydrogen ions no longer precess in phase , due to field inhomogeneity and spin coupling where field of oscillating nuclei affects nearby nuclei

Question 3

How does moment in the xy and z plane change after the 90 degree pulse?

Answer 3

Exponential decay/growth

Question 4

Magnetic field gradient

Answer 4

Allows imaging using the nuclear magnetic resonance signal
Magnetic field strength changes in proportion to the direction of

Question 5

Magnetic field gradient description of the B0 field provided by the MRI scanner along the main field of the magnet

Answer 5

The magnetic field strength becomes a function of distance

Question 6

Formula for the magnetic field strength along z axis

Answer 6

Constant base magnetic field strength + distance along x * gradient of x + same for y and z

Question 7

Why is the gradient created?

Answer 7

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

Question 8

Process to create an MRI image using NRI technique

Answer 8

Slice selection
Phase encoding
Frequency encoding

Question 9

Slice selection

Answer 9

A 90 degree pulse is applied at a frequency for all protons which rotate into the transverse plane

Question 10

What is the 90 degree pulse really equal to?

Answer 10

It will oscillate over a range of frequencies which is the bandwidth, which depends on the pulse length

Question 11

How does the range of frequencies affect the slice thickness?

Answer 11

Higher bandwidth, thicker slice

Question 12

How to select the plane for the slice in the MRI ?

Answer 12

Change the gradient
Change the frequency

Question 13

What happens to the selected slice?

Answer 13

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

Question 14

What happens after the pulse is applied?

Answer 14

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

Question 15

What happens after slice selection?

Answer 15

Within the slice the larmor frequencies and gradients are manipulated further

Question 16

Phase encoding

Answer 16

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

Question 17

How does x value vary the precession of hydrogen nucleus?

Answer 17

Higher x, higher larmor frequency

Question 18

How does the y value affect precession of hydrogen nucleus?

Answer 18

Higher y, higher increase in the phase value (angle between the z axis and magnetisatin vector

Question 19

What affects the contrast of each image?

Answer 19

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)

Question 20

Spin pulse echo sequence

Answer 20

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.

Question 21

How does the magnetization vector in the transverse direction reach a maximum value?

Answer 21

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

Question 22

Echo in spin echo pulse

Answer 22

Produced by current passing through coil

Question 23

T2*

Answer 23

Inherent properties of the material + field properties