eLFH - Nuclear Magnetism and MRI Flashcards
Spin definition
Imbalance of number of protons and neutrons within a nucleus gives atoms spin
Nuclear magnetism definition
Result from combination of a positive electric charge + spin
Why is hydrogen the isotope that is imaged in MRI
Has nuclear magnetism
Highly abundant throughout soft tissues (water in body)
Also produces strongest MR signal with single proton in nucleus
Magnetic resonance mechanism
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
Net magnetism definition
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
Precession definition
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
Larmor equation
Rate of precession - measured in Hz
Resonance definition
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
Requirement for resonance to occur
Delivery of energy at same rate of oscillation / precession
In MRI this refers to the Larmor frequency
Relaxation definition
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
Two main types of relaxation in MRI
T1
T2
T1 relaxation aka
Spin lattice relaxation
T1 relaxation definition
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
T1 relaxation quantification
One T1 time is the point at which 63% of the tissue has returned to equilibrium
Factors which affect T1 relaxation time
Type of tissue due to chemical makeup of each tissue varying
Magnetic field strength
T2 relaxation aka
Spin-spin relaxation
aka
Free induction decay
T2 definition
Time taken for the loss of phase coherence of the net magnetisation, following the rotation of the net magnetisation
Loss of phase coherence of net magnetisation definition
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
Effect of loss of phase on MR signal
Loss of phase results in decreased MR signal
T2 relaxation quantification
One T2 time is time taken for 63% of the original signal to be lost
Factors which affect T2 relaxation time
Unique for each tissue type
NOT field strength dependent (unlike T1 relaxation)
How are T1 and T2 relaxation used for MR images
Both T1 and T2 relaxation occurs at the same time
Relative difference in relaxation times are used to produce contrast on MR images
Pulse sequence definition
Slices of tissue excited with specifically measured and times radio-frequency pulses
Factors which determine the T1 and T2 influence on the contrast of the images from MR
Repetition time (TR)
Echo time (TE)
Echo time (TE) definition
Time between tissues being excited until a signal is detected
Repetition time (TR) definition
Time between each radio-frequency pulse sequence
Repetition time differentiates tissues according to which type of relaxation more with shorter TR
Differentiates tissue according to T1 relaxation
T1 weighted images
Echo time differentiates tissues according to which type of relaxation more with shorter TE
Differentiates tissue according to T2 relaxation
T2 weighted images
Process by which MR signals from tissues are converted into recognisable images
Fourier transformation
Use of IV paramagnetic contrast media
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
