Parallel (Multi-) Transmit RF
Several manufacturers recently started offering "multi-transmit" technology. What is it and how does it work?
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Principle of RF multi-transmit technology. Two or more coil elements are independently powered and controlled, resulting in increased speed and RF homogeneity. (Diagram courtesy of Canon).
Use of dual-transmit RF to reduce standing wave (dielectric shading) artifacts. (Courtesy Philips Medical Systems)
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Multi-transmit or parallel transmission technology is one of the most exciting recent developments in MRI. In many ways it is the natural extension of widely used parallel receiver imaging applied to optimize properties of the transmitted (B1) field.
The basic idea is relatively simple: instead of the entire RF coil transmitting a single B1 field, divide the coil into separate independently powered and controlled elements that each produce their own B1 subfields. The sum of these subfields constitutes the net B1 field experienced by the tissue. In this manner it is possible to more carefully control the homogeneity of RF excitation as well as the distribution of magnetic and electric fields produced in tissue. |
This has the practical application of reducing standing wave (dialectric effect) shading artifacts as well as minimizing RF-energy deposition in tissues.
B1 field generated by a dual-transmit RF coil
In multi-transmit systems power is distributed to the ports of the RF transmit coil (typically a TEM resonator) using two or more independent channels. The design of such systems requires rigorous control over timing, phase, power and amplitude of each channel, as well as various safety adaptations.
All RF-transmitters are synchronized with sub-nanosecond precision and each channel is driven by its own solid state RF-amplifier. Each channel has its own transmit/receive (T/R) switch to allow bimodal operation if desired. Alternatively, the resonator can be operated purely in transmit mode with separate surface coils used as receiver elements.
An amplifier shutdown unit is required to avoid potentially hazardous increase in tissue heating should a single channel fail. Circulators are needed to protect amplifiers against reflected or coupled power from coil elements.
Prior to imaging a calibration sequence (typically a 3D gradient echo field phase mapping technique) is required to to optimize the power, amplitude, phase and waveforms of the individual RF sources to each patient’s anatomy. This process is called adaptive RF-shimming. Notwithstanding the time involved in this calibration, total imaging times with 2-channel RF systems may be reduced on certain high specific absorption rate (SAR) sequences by 30% or more by allowing more slices per TR.
All RF-transmitters are synchronized with sub-nanosecond precision and each channel is driven by its own solid state RF-amplifier. Each channel has its own transmit/receive (T/R) switch to allow bimodal operation if desired. Alternatively, the resonator can be operated purely in transmit mode with separate surface coils used as receiver elements.
An amplifier shutdown unit is required to avoid potentially hazardous increase in tissue heating should a single channel fail. Circulators are needed to protect amplifiers against reflected or coupled power from coil elements.
Prior to imaging a calibration sequence (typically a 3D gradient echo field phase mapping technique) is required to to optimize the power, amplitude, phase and waveforms of the individual RF sources to each patient’s anatomy. This process is called adaptive RF-shimming. Notwithstanding the time involved in this calibration, total imaging times with 2-channel RF systems may be reduced on certain high specific absorption rate (SAR) sequences by 30% or more by allowing more slices per TR.
References
Brink WM, Gulani V, Webb AG. Clinical applications of dual-channel transmit MRI: a review. J Magn Reson Imaging 2015; 42:855-869.
Grissom WA, Sacolick L, Vogel MW. Improving high-field MRI using parallel excitation. Imaging Med 2010; 2:675-693.
Harvey PR, Hoogeveen RM. Multi transmit parallel RF transmission technology. Philips Healthcare. 2009.
Katscher U. Basic and tailored RF shimming in a multi-transmit whole body MR system. PIERS Online, 2008; 4(7):781-784.
Wald LL, Adalsteinsson E. Parallel transmit technology for high field MRI. Siemens Medical Systems, Magnetom FLASH, 2009; January:124-129.
Webb AG, Collins CM. Parallel transmit and receive technology in high-field magnetic resonance neuroimaging. Int J Imaging Syst Technol 2010; 20:2–13.
Brink WM, Gulani V, Webb AG. Clinical applications of dual-channel transmit MRI: a review. J Magn Reson Imaging 2015; 42:855-869.
Grissom WA, Sacolick L, Vogel MW. Improving high-field MRI using parallel excitation. Imaging Med 2010; 2:675-693.
Harvey PR, Hoogeveen RM. Multi transmit parallel RF transmission technology. Philips Healthcare. 2009.
Katscher U. Basic and tailored RF shimming in a multi-transmit whole body MR system. PIERS Online, 2008; 4(7):781-784.
Wald LL, Adalsteinsson E. Parallel transmit technology for high field MRI. Siemens Medical Systems, Magnetom FLASH, 2009; January:124-129.
Webb AG, Collins CM. Parallel transmit and receive technology in high-field magnetic resonance neuroimaging. Int J Imaging Syst Technol 2010; 20:2–13.
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