MR Physics 1 Flashcards
What do MRI scanners consist of?
Main magnet- use superconducting magnets which generate much larger fields
Ferromagnetic blocks & shim coils- both inside the bore- even out the magnetic field, keep it homogenous
Gradient coils (amplifiers)- spatially modulate the main magnetic field in a predictable way causing the Larmor frequency of spins to vary as a function of position
Radiofrequency transmitter and receiver- excite and detect the MRI scanner
Computer- generates the image using information from the scanner
What is the typical magnetic field strength in MRI?
Usually between 0.5 and 1.5 tesla
Is a higher magnetic field strength better than low magnetic field strength?
Yes
Means the signal being read by the coils and transmitted to the computer is increased
= better image due to less obstruction from noise
= higher spatial and temporal resolution
What are gradient coils in MRI?
Loops of wire or thin conductive sheets on a cylindrical shell inside the bore
When an electrical current passes through these coils, the result is a secondary magnetic field
This gradient field distorts the main magnetic field in a slight but predictable pattern
How many gradients are there?
3 - one for each axis (X, Y & Z)
What is the fundamnetal basis of MRI scanning?
MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field
When a radiofrequency current is then pulsed through the patient, the protons are stimulated, and spin out of equilibrium, straining against the pull of the magnetic field
When the radiofrequency field is turned off, MRI sensors detect the energy released as the protons realign with the magnetic field
What is nuclear spin?
Nuclei with unpaired neutrons and/or protons (odd numbers) have nuclear spin
Spin combines with nuclear charge to produce a magnetic moment
Spin + charge = tiny magnet
Protons align in what two orientations?
‘Spin up’ and ‘spin down’ states
If the electron spins clockwise on its axis, it is described as spin-up, if it spins anti-clockwise it is spin-down
These two states have a difference in energy that is proportional to the magnetic field strength
What is a magnetic moment?
The microscopic magnetic field originating from nuclear spin
What is the larmour equation?
The precession frequency of the nuclear spins, or the resonance frequency for nuclear magnetic transitions
The precession frequency obeys the equation:
f0 = y . B0 (Larmor equation)
How many spins give rise to a signal?
Only a small amount of the spins give rise to a signal: few per million (ppm)
What is the magnetisation vector?
When the body is placed in a strong magnetic field (the MRI scanner), the protons’ axes all line up
This uniform alignment creates a magnetic vector oriented along the axis of the MRI scanner
The net magnetisation vector in MRI is the summation of all the magnetic moments of the individual hydrogen nuclei
We can treat excess spins in the lower energy level as a single magnetisation vector aligned to the main magnetic field
The predominant signal from the body is due to what?
Water and fat
What is chemical shift in MRI?
The chemical shift refers to signal intensity alterations that result from inherent differences in the resonant frequencies of precessing protons
Chemical shift is due to the differences between resonance frequencies of fat and water.
It occurs in the frequency-encode direction where a shift in the detected anatomy occurs because fat resonates at a slightly lower frequency than water
How do we detect the MRI signal?
To produce ‘signal’, MRI scanner interacts with protons in the body
Randomly orientated protons become aligned with the powerful magnetic field in the bore of the scanner
A rapidly repeating sequence of radiofrequency pulses produced by the scanner causes ‘excitation’ and ‘resonance’ of protons
As each radiofrequency pulse is removed, the protons ‘relax’ to realign with the magnetic field, and as they do so they give off radiofrequency ‘signal’ which is detected by the scanner and transformed into an image.
How can RF pulses control magnetisation?
By changing their strength and/or duration
How do we detect the MRI signal?
As each radiofrequency pulse is removed, the protons ‘relax’ to realign with the magnetic field, and as they do so they give off radiofrequency ‘signal’ which is detected by the scanner and transformed into an image
How do we record the signal (induction)?
Nuclear spins precess like a spinning magnets
Changing magnetisation induces voltage in detector coil
Measure rate of oscillation as a voltage change
How do we record the signal (free induction decay)?
Changing magnetisation induces voltage in detector coil
Detect analogue oscillating voltage
Analogue signal digitised at discreet points – analogue to digital converter (ADC)
Sampling rate > 2 highest frequency- Nyquist condition
What is the MRI pulse sequence?
An MRI pulse sequence is a programmed set of changing magnetic gradients
Each sequence will have a number of parameters e.g. TR, TE, 2D vs 3D, diffusion weighted etc
Summarise MR Physics
The ‘bulk’ magnetic moment of the water protons aligns and ‘precesses’ about the direction of the field at a frequency dependent on the Larmor equation
A radiofrequency field B1 applied perpendicular to the main magnetic field can supply the correct energy pulse to tip the water magnetic moment away from alignment with the field direction
Different pulses can be used to control the magnetisation
The precessing magnetic moment generates a field which can be detected by a tuned receiver coil
The analogue signal is discreetly sampled as a Free Induction Decay (FID)
What changes image contrast?
Relaxation
When the pulse is switched off spin are not at equilibrium - more spins in the upper energy level
Once the radiofrequency field is switched off the nuclei undergo two processes:
T2 (sometimes called spin-spin or transverse relaxation)
T1 (sometimes called spin-lattice or longitudinal relaxation)
-due to microscopic motion of water and are collectively known as relaxation.
What is T2 relaxtion?
Signal decay
Spins experience tiny difference in magnetic field strength due to neighbouring spins
Each spin has a slightly different frequency from other spins
As spins precess at different frequencies to magnetisation, bulk vector spreads out, dephases in the XY plane losing signal
T2: loss of signal due to each spin signal cancelling each out
Decay rate is exponential with a time constant T2
What is T2* relaxation?
T2 effects transverse magnetisation in the xy plane
Signal decay rate exponential with a time constant T2 (ms).
T2* = T2 loss + extra loss due to variation in applied magnetic field (e.g imperfect magnet)
T2* always shorter than T2, i.e. signal disappears faster
Influences the appearance of the image
What is T1 relaxation?
Requires the correct energy (frequency) for exchange to occur
Over a longer period the magnetic moments realign with the magnetic field with an exponential recovery time constant T1
Bulk magnetisation vector returns to the z-axis
Recovery rate exponential with a time constant T1
99.5% of the signal is recovered in 5*T1
What happens after the RF pulse?
T1 and T2 relaxation occur simultaneously and return to equilibrium
Sumarise T1, T2 and T2* relaxation
After an rf pulse the magnetisation is in a non-equilibrium state
Signal decays back to equilibrium via relaxation mechanisms
T1: relaxation along the z-axis. Spins return to lower energy
T2: relaxation in the xy plane. Variation in magnetic field produced by the spins and is an intrinsic property of the sample
T2*: as T2 but affected by locally distortions in the applied magnetic field
T1/T2/T2* offer the ability to change image contrast to highlight anatomical features
