MR 2 Flashcards
Give the equation for the energy required to excite transitions between Zeeman energy levels
∆E = required energy
ω = Larmor frequency
γ = Gyromagnetic ratio
B = magnetic field
The frequency required to excite transitions between Zeeman energy levels is in the _____________ (__) range.
Radiofrequency
RF
How are MRI scanners shielded from external radiofrequency signals?
The scanner is placed within a Faraday cage to provide RF shielding.
What are the two effects of the RF B₁ field (a field oscillating at RF frequency) on magnetic moments?
- It alters spin states
- It brings spins into phase
What is used to generate a B₁ field?
An RF coil tuned to the Larmor frequency
Which direction is the B₁ field applied in?
The x-direction to flip the equilibrium magnetisation
How does the RF B₁ field alter spin states?
An RF pulse can be applied at a given angle, energy and frequency, changing the populations of the energy levels.
What happens when the B₁ field RF pulse applied is at 90º to the B₀ field?
When the B₁ field RF pulse is applied in the x-direction (at 90º to the B₀ field), the populations of energy levels are equalised and M is reduced to 0 in the z-direction.
How does the RF B₁ field bring spins into phase?
If the B₁ field is applied perpendicular to the B₀ field (in the x-direction) it causes the spins to precess coherently because they have a net x-y component of magnetisation.
Describe a linearly polarised field
A sinusoidal alternating current is driven through a single loop at the Larmor frequency, generating an oscillating sinusoidal B₁ field perpendicular to the B₀ field.
Describe a circularly polarised field
A sinusoidal alternating current is driven through a pair of loops at the Larmor frequency, generating a rotating B₁ field.
Is linear or circular polarisation more efficient?
Circular
Why is circular polarisation more efficient?
An oscillating B₁ field is equivalent to two B₁ fields rotating at ±ω with a clockwise component on-resonance and an anticlockwise component off-resonance.
Linear polarisation does not pick up the off-resonance component but circular polarisation picks up both components so no power is wasted.
What frame is the B₁ field usually considered in?
A rotating frame
What are the coordinates of the rotating frame compared to a lab frame?
Lab frame: (x, y, z)
Rotating frame: (x’, y’, z)
Give the equation that relates a stationary frame to a rotating frame
LHS = position in the lab frame
ω x M = rotation of frame
Final term = position in rotating frame
What causes the net magnetisation to precess?
Torque
Give the equation for the precession of the magnetisation about the B₁ field in the lab frame
M = net magnetisation
γ = Gyromagnetic ratio
B = magnetic field
Give the equation for the precession of the magnetisation about the B₁ field in the rotating frame
M = net magnetisation
γ = Gyromagnetic ratio
B = magnetic field
ω = Larmor frequency
[term] = effecting magnetic field
Give the equation for the precession of the magnetisation about the B₁ AND B₀ fields in the rotating frame
M = net magnetisation
γ = Gyromagnetic ratio
B = magnetic field (= B₁i + B₀k)
ω = Larmor frequency
Give the equation for the precession of the magnetisation about the B₁ AND B₀ fields in the rotating frame if the system is ON-RESONANCE
M = net magnetisation
γ = Gyromagnetic ratio
B₁ = magnetic field
Give the equation for the flip angle through which magnetisation is rotated
α = flip angle
γ = Gyromagnetic ratio
B₁ = strength of RF pulse
τ = duration of RF pulse
What plane does a 90º RF pulse flip the magnetisation into?
From the z plane into the x-y plane.
What plane does a 180º RF pulse flip the magnetisation into?
From the z plane into the -z plane (inversion).
Generally, an αº pulse acting on equilibrium magnetisation will reduce M_z from M₀ to ________, and increase M_xy to ________.
M₀cos(α)
M₀sin(α)
What risks are associated with RF radiation?
It can induce heating in tissue. In cases where tissue is touching itself skin burns can occur.
Give 3 ways that tissue heating from RF radiation can be reduced
- Low humidity
- Cool environment
- Good airflow over the skin
How is the signal detected in NMR?
Once the RF pulse is turned off, the rotating transverse magnetisation (M_xy) precesses about B₀ at the Larmor frequency. This induces an oscillating EMF in a receiver (pick-up) coil along y.
What is Free Induction Decay (FID)
The short-lived sinusoidal signal that decays exponentially following excitation of a sample by an RF pulse. It represents the rotating transverse magnetisation precessing about B₀ when the B₁ is turned off and is detected by a pick-up coil.
Describe the shape of Free Induction Decay
Exponential decay with a time constant of the ‘transverse relaxation time, T₂*’.
What is transverse relaxation time, T₂*?
A property of tissue (time)
Which processes require us to know the exact frequency of the received signal in NMR?
- Encoding images
- Performing spectroscopy in analytical NMR
What is phase-sensitive detection (PSD)?
The comparison of the detected NMR signal to a reference frequency (ω₀). After being low-pass filtered, this gives a precise signal in the audio range that can be digitised and stored. This signal is the difference between the frequencies.
Phase-sensitive detection _________ the signal.
Demodulates
What is quadrature phase-sensitive detection?
A form of PSD that allows the difference in positive and negative frequency to be detected (e.g. 49 and 51 Hz for a 50 Hz reference).
How is quadrature PSD different to PSD in terms of the data collection?
Quadrature uses 2 PSDs with a 90º phase shift rather than one. This means that it can collect real and imaginary components.
What are the 3 steps to produce an MRI image?
- Produce RF pulse (transmit B₁)
- Detect NMR signal (detect/receive)
- Gradients
Describe how an RF pulse is produced (1)
- The computer sends the desired pulse to the waveform controller which produces the appropriate analogue audio frequency modulation, defining the shape of the RF pulse.
- The frequency synthesizer produces the carrier RF frequency which is generally set to be the mean Larmor frequency of the whole sample.
- The carrier frequency is multiplied by an RF pulse envelope in the mixer, giving an RF-modulated pulse which is passed to the RF amplifier and then sent to the RF transmit coil.
Are different coils used to transmit and detect NMR signals?
Yes (usually)
Why is the receiver gated off during the application of an RF pulse?
To avoid damage
Describe how an NMR signal is detected (2)
- The reference signal at the RF frequency is passed through a quadrature splitter, producing two signals 90º out of phase.
- The signal is detected by an RF receive coil and then sent to pre-amp.
- The signal is then mixed in the two quadrature PSDs, demodulating the signal to the audio-frequency range.
- The signal is sent to two analogue-to-digital converters (ADCs) and back to the computer to be processed (Fourier Transform).
Describe how gradients are used in MRI scanners (3)
Magnetic field gradients are used to encode images. They are controlled by a waveform controller.
Describe the layout of an MRI scanner
Describe the position of the three types of coils in an MRI scanner
