Lecture 2 - MRI scanners, safety and physics Flashcards
1
Q
What does an MRI system consist of?
A
- Magnetic = A large magnet to generate the magnetic field B(0)
- Resonance = Radiofrequency coils- these electromagnetic coils are used to generate and receive energy at the samples resonant frequency. Only used during image acquisition
- Imaging = Gradient coils- ultimate goal of MRI is image formation. These electromagnetic coils create a controlled spatial variation in the strength of the magnetic field - so different parts of space contribute to differently to measured signal overtime (only used during image acquisition)
- Shim coils = to make the magnetic field as homogenous as possible
- A computer = to reconstruct the radio signals into the final image. Data transfer and storage computer. Physiological monitoring, stimulus display, and behavioural recording hardware
2
Q
MAGNETIC: static field coils?
A
- Scanner contains large parallel coilings of wires through which a current is passed.
- These generate the main constant magnetic field (B0), which gives the scanner its field strength (e.g. 3T).
- For fMRI, field strength can be 1.5T, 3T, 7T. Travels along z-axis
- Uses a super conducting magnet.
- Maintaining a large magnetic field needs a lot of energy: accomplished using superconductivity, which involves trying to reduce resistance in wires to almost 0. This is done by bathing wires in a continuous supply of liquid helium at -269.1 degrees C.
3
Q
RESONANCE: Radiofrequency coils?
A
- RF coils are used to transmit RF pulse and receive electro-magnetic signal from the protons’ transverse field as they process in phase
- Electrical coils tuned to radio signals send energy into the brain and record an emitted “echo”
- Coils are application specific (e.g. 12-channel head coil, 16-channel breast coil)
- Head coil: homogenous signal, moderate SNR (signal-to-noise ratio)
- Surface coil: highest signal at hotspot, high SNR at hotspot
4
Q
IMAGING: Gradient coils?
A
- Gradient coils arranged on all 3 axis to vary the magnetic field B0 in each dimension used, one in each of the cardinal directions
- Allow spatial encoding of the MR signal
- Because the field strength will be different at different parts of the body, the frequency of precession will also be different
- Applying a RF pulse at the frequency will only resonate the nuclei at that location
- A gradient is applied in all 3 dimensions to enable a location to be narrowed down to a single pixel
5
Q
MRI safety - what can go wrong?
A
- Pacemaker malfunctions leading to death
- Blinding to movement of metal in the eye
- Dislodgement of aneurysm clip
- Projectile injuries, e.g. from oxygen canister (most common injuries)
- Gun pulled out of policeman’s hand…
6
Q
Issues in MRI safety
A
- Known acute risks: projectiles, rapid field changes, RF heating, claustrophobia, acoustic noise - can reach 120 dB (should be given ear plugs)
- Potential acute/chronic risks: current induction in tissue at high fields? Changes in the developing brain?
- Epidemiological studies of chronic risks: extended exposure to magnetic fields does not cause harm
- Difficulty in assessing subjective experience
7
Q
Recipe for MRI
A
1) Put subject in big magnetic field (leave them there)
2) Transmit radio waves into subject (about 3ms)
3) Turn off radio wave transmitter
4) Receive radio waves re-transmitted by subject
5) Store measured radio-wave data vs. time (now go back to (2) to get some more data)
6) Process raw data to reconstruct images
7) Allow subject to leave scanner
8
Q
Basis of MRI Physics
A
- Human body composed of 70% H20
- MRI relies on properties of hydrogen atoms to produce images
- H nucleus is composed of a single proton = spinning charged particle
- So all hydrogen atoms will act like little magnets
- The abundance of water in the human body makes this very powerful
9
Q
Protons/Spin/Precession?
A
- Protons possess a positive charge
- Protons possess a spin.
- Positive electrical charge spins around with it, create its own electrical current and magnetic field
- Normally protons are aligned in a random fashion. BUT when we put them in a strong external magnetic field, they align with it - some parallel (pointing up), some anti-parallel (pointing down)
- Protons don’t just lie there in their parallel of anti-parallel positions -> instead, they move in a certain way - this type of movement is called precession
- The stronger the magnetic field, the higher the precession frequency, a relationship that is mathematically described in the Larmor equation: f = gammaB0 (gamma = gyromagnetic ratio)
10
Q
What happens during Excitation/Resonance/Relaxation?
A
- Excitation = magnetisation can be moved or rotated by applying “excitation” magnetic fields (radio frequency pulses)
- Resonance = Magnetisation will “resonate” at a frequency proportional to magnetic field strength
- Relaxation = The oscillations die out, i.e. magnetisation “relaxes” back to equilibrium - speed of relaxation is tissue dependant. This is what gives out different signals and our MRI images.
11
Q
What happens when a radio frequency pulse that has the same frequency as the precession frequency of the protons is sent in during EXCITATION?
A
- Can cause resonance, transfer energy to the protons, which results in:
- Lifting some protons to a higher level of energy (they point down) resulting in decreasing the magnetisation along the z-axis, the so called LONGITUDINAL MAGNETISATION
- Causes protons to process in synch, in phase. This establishes a new magnetisation in the x-y-plane, a new TRANSVERSAL MAGNETISATION, which moves around with the precessing protons
- Summary: the RF pulse causes longitudinal magnetisation to decrease and establishes a new transversal magnetisation (+ depending on the RF pulse, longitudinal magnetisation may even totally disappear)
12
Q
What happens when the RF pulse is switched off?
A
- Longitudinal magnetisation increases again; this longitudinal relaxation is described by a time constant T1 = the longitudinal relaxation time
- Transversal magnetisation decreases and disappears; this transversal relaxation is described as a time constant T2 = the transversal relaxation time. This is cause of spin-spin interactions (T2 decay). OR because of both spin-spin interactions and local field inhomogeneities (T2*)
- Different tissues have different T1 and T2 relaxation times: fat has short T1, water has long T1. This is what gives an endogenous contrast in MRI
13
Q
What we are expected to know regarding MRI physics in a nutshell…
A
- MRI is based on the magnetisation properties of atomic nuclei. A powerful, uniform, external magnetic field is employed to align the protons that aren normally randomly orientated within the water of the tissue being examined
- This alignment (or magnetisation) is next disrupted by introduction of RF pulse
- The nuclei return to through various relaxation processes and in so doing emit RF energy.
- After a certain period following the initial RF, the emitted signals are measured
- Fourier transformation is used to convert the frequency info contained in the signal from each location in the imaged plane to corresponding intensity levels, which are then displayed as shades of grey in a matrix arrangement of pixels
- By varying the sequence of RF pulses applied and collected, different types of images are created
- REPETITION TIME (TR) = the amount of time between successive pulse sequences applied to the same slice
- TIME OF ECHO (TE) = the time between the delivery of the RF pulse and the receipt of the echo signal
- Tissue can be characterised by 2 different relaxation times - T1 and T2.
- T1 (longitudinal relaxation time) is the time constant which determines the rate at which excited protons return to equilibrium
- T2 (transverse relaxation time) is a measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field
14
Q
How do we know where in space a signal is coming from?
A
- SO we know that the precession frequency of a proton depends on the strength of the magnetic field
- If this strength is different from point to point in a patient, then protons with different places precess with different frequencies.
- As they precess with different frequencies, the resulting MR signal from different locations also has a different frequency.
- By the frequency received, we can assign a signal to a certain location
15
Q
How to measure the RF energy at an appropriate point in time (receive RF) to optimise tissue contrast?
How to make data look like a brain?
A
- Choose TR (repetition time) or TE (time of echo) to get T1-weighted or T2* weighted images
- Convert RF frequencies represented in k-space to representation of physical space
