A Sub-Microsecond Switch Enabling SWIFT ²³Na Imaging at 10.5 T

To enable sodium SWIFT imaging, which is a zero-echo time imaging technique, at ultra-high magnetic fields through the development of custom electronics hardware, and to showcase this capability with in vivo imaging results.

The custom hardware developed consists of a high-speed optical trigger with 10 ns resolution, an in-bore PIN diode driver capable of sourcing high current, and RF switches (both an in-line switchable attenuator and transmit/receive switch) optimized to achieve sub-microsecond switching speeds while also producing isolation between the transmitter and receiver.

Practical switching speeds of 0.6 μs (receive to transmit) and 1.7 μs (transmit to receive) are achieved on the RF switches, limited by transient-induced spurious emissions from the low noise amplifier. Transmit to receive isolation of 115 dB is achieved over a 1 MHz bandwidth, with 120 dB of isolation at the Larmor frequency. This was crucial to suppress unblanked RFPA noise. SWIFT images of the human wrist with 1.5 mm isotropic resolution were acquired in less than 5 min to demonstrate the utility of sodium SWIFT imaging at ultra-high field. SWIFT imaging signal-to-noise ratio in a reference phantom with heterogeneous sodium concentrations was comparable to ultra-short echo time imaging, which provided assurance that hardware had achieved the necessary specifications.

The feasibility of sodium SWIFT imaging at ultra-high field was established, and a musculoskeletal imaging application was demonstrated. The success of this work enables further development of SWIFT at high and ultra-high fields for imaging of sodium, proton, and other nuclei.