MRI Flashcards
Energy of wave with frequency w
E_w = hw/2 pi
Energy difference between levels
delta E = gamma h B0 / 2 pi
Larmor frequency
w = gamma B0
T1 relaxation
Spin lattice relaxation - longitudinal.
Magnetization vector returns to equilibrium, caused by loss of energy of nuclei to surroundings - given as heat not signal
T1 relaxation equation
Mz(t) = M0[1-exp(-t/T1)]
(after 180 degree pulse Mz(t) = M0[1-2exp(-t/T1)])
Is T1 or T2 shorter
T2 is shorter
In solution T2 can approach T1, in vivo T2 can be 5-10 times shorter
T2 relaxation
Spin-spin relaxation - transverse.
Decay of observable transverse magnetization, nuclei randomly moving together or further apart means magnetic moment affects other nuclei and experienced B field - causes dephasing.
T2 relaxation equation
Mxy(t)=Mxy(0)exp(-t/T2)
T2* relaxation
Caused by spatial inhomogeneities in the field - causes addition dephasing. Constant, not temporal or random.
Is T2* or T2 shorter?
T2* is shorter
Spin echo
Rephases with 2 pulses, 90 degree and then 180 degree. Rephases T2*, can’t help T2.
Gradient echo
Rephases using gradients - not rephased T2 star, gradient echo is weighted by T2*.
3 gradients
Phase encoding
Readout (frequency encoding)
Slice selection
Readout gradient
Frequency encoding
Magnetic field gradient across scanner so w changes dependent on location. Spatially encoded spins. Frequency tells you where you are - amplitude tells you how many.
Phase encoding gradient
Orthogonal to readout gradient. First time most negative gradient up to most positive. Changes phase dependent on y position then switch off gradient, phase shift remains.
Effect of PE gradient strength on signal
No gradient, no dephasing, large signal
Small gradient, smaller signal
Large gradient, larger dephasing, zero signal.
When structures match gradient - high signal.
Testing different frequencies.
Image formation process steps
Polarise
Excite
Encode
Detect
Transform
What causes the noise hazard?
Vibration of the gradient coils due to rapidly switching electrical currents
What causes peripheral nerve stimulation
High current, voltage and switching rate
Bodily tissues are electrically conductive
Resultant electric and induced fields stimulate nerves and muscles.
Gradient direction and body position alter sites of PNS.
How can we prevent PNS and noise hazard
Reduce magnitude and frequency of coil vibration and lower gradient switching
Reduce dB/dt.
RF field hazard, how is it measured and what raises the risk
A fraction of energy deposited in body as heat - main risk is thermal heating leading to burns.
Measured in terms of specific absorption rate.
Raised by induced currents in: coiled cables, implanted cables, implanted medical devices, tissue loops.
Why do RF burns occur
As a result of excessive heat deposition
Polarise step
Separate energy levels of nucleus leading to thermal polarisation of nuclear spins between energy levels
Excite and detect
RF transmit coil transmits excitation pulse and range of local RF receiver coils detect MR signal
Encode
gradient coils, which are switchbale, impose linear gradients on field such that resonant frequency becomes linearly dependent on location in scanner bore
Transform
computer reconstruction and image storage system used to perform FT of data giving final image
Centre/outside of k-space
Centre: low frequencies, contrast
Edges: high frequency, edges
