MR Physics Flashcards
Why is the area under the selection re-phase gradient, half the area under the selection gradient?
Transverse magnetisation builds up gradually while the 90o pulse is being applied, so the AVERAGE de-phasing effect is 50%
Describe what the purpose of frequency encoding is
- Used to obtain SPATIAL INFORMATION along one direction in the object under study
- MR signal is measured while a magnetic field gradient is applied across the sample
- Spins now precess at slightly different frequencies during free evolution (echo formation) depending on their location along the gradient axis
- FT of the MR signal gives a 1D projection of the whole slice along the gradient direction
Describe what the purpose of Phase encoding is
- Phase encoding is used to obtain spatial information along the THIRD DIRECTION of the object under study (orthogonal to frequency encoding direction and slice-selection direction)
- Gradient is applied in the phase encoding direction. Gradient applied during free precession. This results in spins precessing at slightly different speeds.
- Once the field is switched off, the spins precess at the larmor frequency, but now with different phase along the phase encoding axis
- Gradients cause dephasing of the spins, where gradients of increasing strength are used in the y-axis from the midpoint, resulting in more and more dephasing
- Y axis location is built from multiple phase encoding space to populate the rows in k-space, where the rows are populated by the net magnitisation strength over time in the x-axis
How is resolution defined in your image?
- The matrix size: i.e., the number of phase encoding steps and the number of sample bins in the readout gradient
What is receiver bandwidth? How does this relate to SNR?
The range of frequencies within the designated FOV in the frequency encoding plane
SNR = 1/BW
What is SAR?
- Specific absorption rate (SAR) is the rate that electromagnetic energy in the radiofrequency pulses is absorbed by tissues during MR image acquisition measured in watts per kilogram (W/kg)
What are the risk factors for increased SAR?
Specific absorption rate proportionately increases with certain parameters:
- square of the Larmor frequency or B0, i.e. worse as main field increases
- square of the B1 pulse, worse with larger flip angles
- size and shape of the patient: larger SAR with obesity
- RF pulses per unit time: greater SAR with FSE/TSE
- contact with wall of bore
What changes can you make to imaging the patient to reduce the SAR?
Precautions to reduce the SAR to patients can include:
- Taking breaks between high SAR sequences
- alternating between low SAR and high SAR sequences
- reducing the flip angle
- reducing slice numbers
- reducing pulse number and duration
- reducing pulse frequency
- ensuring the patient is lightly dressed
- ensuring scanner ventilation system is turned on
What is the Antenna effect?
- one-dimensional implants tend to exhibit the so-called antenna effect; that is, a pronounced maximum of the scattered E-field at their distal end, whose intensity peaks when the electrical length of the implant is in the range of one-quarter to one-half of the RF wavelength in tissue.
*therefore if you double the field, you half the critical length
What is the typical critical length for RF heating at 1.5T
Approx 20 cm
