MR 4 Flashcards
Describe a magnetic field gradient varying along x
It varies from left to right
Give the equation for a magnetic field gradient in the x-direction
Describe a magnetic field gradient varying along y
It varies from back to nose
Give the equation for a magnetic field gradient in the y-direction
Describe a magnetic field gradient varying along z
It varies from toes to head
Give the equation for a magnetic field gradient in the z-direction
What is the impact of applying a magnetic field gradient?
Spins within the field will precess at slightly different frequencies.
Give the equation for the local magnetic field experienced by a spin in the presence of a magnetic field gradient
B(r) = local magnetic field
B₀ = applied static field
G = gradient
r = position
Give the equation for the Larmor frequency of a nucleus in the presence of a magnetic field gradient
ω = Larmor frequency
γ = Gyromagnetic ratio
B₀ = applied static field
G = gradient
r = position
What is the isocentre?
The position at which spins precess at the Larmor frequency.
_______ _____ are designed to produce linearly varying magnetic fields.
Gradient coils
Which gradient coil produces a gradient along z?
Maxwell pairs
Which gradient coil produces a gradient along x or y?
Straight wires
How can a magnetic gradient be produced in an arbitrary direction (i.e. at an angle)?
By using a linear combination of x and y gradients.
Describe an MRI scanner that contains gradient coils for gradients along x, y, and z
Why is MRI acoustically noisy?
Magnetic field gradients are created by currents that flow within the magnetic field and alternate rapidly, causing the gradient coils to vibrate.
How does slice select work?
A magnetic field gradient, G, is applied in the direction of the chosen plane. An RF pulse is also applied over a narrow bandwidth of frequencies (∆ω) at the same time as the gradient to excite spins with that range of Larmor frequencies.
Give the equation for the bandwidth of frequencies chosen in slice selection
∆ω = bandwidth of frequencies
γ = Gyromagnetic ratio
G = gradient
∆z = position range
How are RF pulses created?
By modulating the carrier frequency at audio frequencies to generate the pulse shape (i.e. by multiplying the carrier frequency by an audio window).
What is the frequency equivalent of multiplying two functions in time?
Convolution in the frequency space.
How can a specific, narrow range of frequencies be excited?
- Choose a carrier frequency (ω₀) for the RF frequency.
- Use an RF pulse with a bandwidth, ∆ω.
- Fourier transform the signals to give a square wave centred at the Larmor frequency in frequency space that can be used to excite the frequencies.
How can the thickness of an imaging slice be altered?
By varying the shape of the RF pulse. The longer the pulse (∆ω), the narrower the bandwidth.
How can the slice position of an imaging slice be altered?
By varying the carrier frequency. The carrier frequency represents the centre of the slice.
Give the equation that relates time to frequency
ω = frequency
t = time
Give the equation that relates space to spatial frequency
k = spatial frequency
λ = space
What is k-space?
An array of numbers representing spatial frequencies in an image.
Why is k-space important in MRI imaging?
k-space must be filled to generate an MRI image.
How does back-projection reconstruction (BPR) work for MRI imaging?
- A slice is selected by applying a gradient in the z-direction.
- Another gradient then is applied along in a different direction so that precessional frequency depends on this gradient.
- The signal depends on the density of spins, ρ(x), at a given position. This spin density is encoded using a third ‘readout’ gradient.
- This is repeated for many angles to map out lines of k-space.
- The projections are Fourier Transformed to process an image.
What is the benefit of applying many gradients to generate an MRI image?
The signal can be localised
What applications are best suited to back projection reconstruction?
Imaging things with a short T₂* like the lungs.
What are the 3 main drawbacks of back projection reconstruction?
- The low frequencies (central) are well sampled but the high frequencies are poorly sampled.
- Images can be badly affected by inhomogeneities in the main magnetic field (B₀).
- The total scan time is long.
Define signal decay
The bright signal at the centre of k-space which drops away.
What is spin-warp imaging?
A method of imaging that uniformly samples k-space. It uses an encode gradient initially (AB line) which causes a phase shift, then a frequency encode gradient (BC line) is applied.
What are the advantages of spin-warp imaging?
- Most widely used imaging sequence
- Robust
- Field errors cause distortion rather than blurring
What are the disadvantages of spin-warp imaging?
- Long imaging times
- Motion causes artefacts (exemplified by long imaging times)
What is echo-planar imaging?
An imaging technique that uniformly samples k-space similar to spin-warp imaging, however, it samples all of k-space in one FID by taking a spin echo and then changing the phase encoding until k-space is filled.
What are the advantages of echo planar imaging?
- Total scan time is very short (<100 ms)
- No motion artefacts
- Dynamic processes can be scanned
- Quantitative imaging
What are the disadvantages of echo planar imaging?
- Low intrinsic signal-to-noise ratio
- Low spatial resolution
- Sensitive to ‘ghost’ artefacts
- Very noisy unless low inductance gradient coils are used
- Sensitive to distortions
- Sensitive to signal loss
