eLFH - Nuclear Magnetism and MRI

Question 1

Spin definition

Answer 1

Imbalance of number of protons and neutrons within a nucleus gives atoms spin

Question 2

Nuclear magnetism definition

Answer 2

Result from combination of a positive electric charge + spin

Question 3

Why is hydrogen the isotope that is imaged in MRI

Answer 3

Has nuclear magnetism

Highly abundant throughout soft tissues (water in body)

Also produces strongest MR signal with single proton in nucleus

Question 4

Magnetic resonance mechanism

Answer 4

In tissues, each hydrogen atom has own tiny magnetic field in different orientations

Strong magnetic field applied to tissue and majority of H+ ions orientate with the strong magnetic field

Radio-frequency energy applied for resonance

Relaxation as radio-frequency energy stopped

MR signals detected and interpreted into images

Question 5

Net magnetism definition

Answer 5

Sum of magnetisation of all the hydrogen protons in the tissue in the same direction as the main magnetic field

Small proportion of H+ protons align against magnetic field due to inherent energy within some protons allowing them to oppose magnetic field

Question 6

Precession definition

Answer 6

Hydrogen protons that don’t fully align with main magnetic field

Align at slight angle with property of spin

Spins with motion in which the axis of rotation describes a cone

Question 7

Larmor equation

Answer 7

Rate of precession - measured in Hz

Question 8

Resonance definition

Answer 8

Absorption of energy by oscillating / precessing systems

Results in net magnetisation rotating away from direction of main magnetic field

Rotation / radio-frequency of 90 degrees often used in MRI as gives maximal tissue signal as is perpendicular to main magnetic field

Question 9

Requirement for resonance to occur

Answer 9

Delivery of energy at same rate of oscillation / precession

In MRI this refers to the Larmor frequency

Question 10

Relaxation definition

Answer 10

Radio-frequency energy switched off after resonance

Net magnetisation returns to equilibrium with the main magnetic field and loses energy to surrounding tissue

2 main types to relaxation process that occur simultaneously

Question 11

Two main types of relaxation in MRI

Answer 11

T1
T2

Question 12

T1 relaxation aka

Answer 12

Spin lattice relaxation

Question 13

T1 relaxation definition

Answer 13

Time taken for the rotated net magnetisation to realign with the main magnetic field

Energy transfer takes place when the nuclei of a tissue interact with one another

Question 14

T1 relaxation quantification

Answer 14

One T1 time is the point at which 63% of the tissue has returned to equilibrium

Question 15

Factors which affect T1 relaxation time

Answer 15

Type of tissue due to chemical makeup of each tissue varying

Magnetic field strength

Question 16

T2 relaxation aka

Answer 16

Spin-spin relaxation

aka

Free induction decay

Question 17

T2 definition

Answer 17

Time taken for the loss of phase coherence of the net magnetisation, following the rotation of the net magnetisation

Question 18

Loss of phase coherence of net magnetisation definition

Answer 18

Immediately following radio-frequency pulse, protons are spinning in phase with one another at 90 degrees to main magnetic field

As time progresses, interactions of the protons cause some to speed up and others to slow down relative to each other

This results in loss of phase of the individual magnetic moments

Question 19

Effect of loss of phase on MR signal

Answer 19

Loss of phase results in decreased MR signal

Question 20

T2 relaxation quantification

Answer 20

One T2 time is time taken for 63% of the original signal to be lost

Question 21

Factors which affect T2 relaxation time

Answer 21

Unique for each tissue type

NOT field strength dependent (unlike T1 relaxation)

Question 22

How are T1 and T2 relaxation used for MR images

Answer 22

Both T1 and T2 relaxation occurs at the same time

Relative difference in relaxation times are used to produce contrast on MR images

Question 23

Pulse sequence definition

Answer 23

Slices of tissue excited with specifically measured and times radio-frequency pulses

Question 24

Factors which determine the T1 and T2 influence on the contrast of the images from MR

Answer 24

Repetition time (TR)

Echo time (TE)

Question 25

Echo time (TE) definition

Answer 25

Time between tissues being excited until a signal is detected

Question 26

Repetition time (TR) definition

Answer 26

Time between each radio-frequency pulse sequence

Question 27

Repetition time differentiates tissues according to which type of relaxation more with shorter TR

Answer 27

Differentiates tissue according to T1 relaxation

T1 weighted images

Question 28

Echo time differentiates tissues according to which type of relaxation more with shorter TE

Answer 28

Differentiates tissue according to T2 relaxation

T2 weighted images

Question 29

Process by which MR signals from tissues are converted into recognisable images

Answer 29

Fourier transformation

Question 30

Use of IV paramagnetic contrast media

Answer 30

Used in conjunction with T1 weighted MR imaging for lesion characterisation

Contrast media have effect of shortening T1 relaxation times
More vascular structures more clearly demonstrated - e.g. vascular tumours