A 64-Channel Transmitter for Investigating Parallel Transmit MRI

Abstract

Multiple channel radiofrequency (RF) transmitters are being used in magnetic resonance imaging to investigate a number of active research topics, including transmit SENSE and B(1) shimming. Presently, the cost and availability of multiple channel transmitters restricts their use to relatively few sites. This paper describes the development and testing of a relatively inexpensive transmit system that can be easily duplicated by users with a reasonable level of RF hardware design experience. The system described here consists of 64 channels, each with 100 W peak output level. The hardware is modular at the level of four channels, easily accommodating larger or smaller channel counts. Unique aspects of the system include the use of vector modulators to replace more complex IQ direct digital modulators, 100 W MOSFET RF amplifiers with partial microstrip matching networks, and the use of digital potentiometers to replace more complex and costly digital-to-analog converters to control the amplitude and phase of each channel. Although mainly designed for B(1) shimming, the system is capable of dynamic modulation necessary for transmit SENSE by replacing the digital potentiometers controlling the vector modulators with commercially available analog output boards. The system design is discussed in detail and bench and imaging data are shown, demonstrating the ability to perform phase and amplitude control for B(1) shimming as well as dynamic modulation for transmitting complex RF pulses.

Another alternate integrated circuit approach to modulation of radiofrequency transmission signals in magnetic resonance imaging

“…The SMM was designed primarily using offthe-shelf ICs. For the same benefits as presented by Feng et al 9 an RFIC vector modulator (HPMX-2005, Hewlett Packard, Santa Rosa, CA, USA) was chosen for phase control. For amplitude fine tuning, a Monolithic Microwave Integrated Circuit (MMIC) attenuator (MAAVSS0006, M/ A-Com Technology Solutions Inc., Lowell, MA, USA) was selected.…”

“…[8][9][10] Although similar in function, the engineering designs that have been investigated are substantially different. Several research groups have presented their RF signal modulation hardware in the literature.…”

“…Several research groups have presented their RF signal modulation hardware in the literature. [8][9][10] Although similar in function, the engineering designs that have been investigated are substantially different. For example, Scott et al 8 used a polyphase quadrature filter and signal multipliers as a basis to achieve the desired RF phase adjustments, reporting their results on a 1.5 T MRI system.…”

“…For example, Scott et al 8 used a polyphase quadrature filter and signal multipliers as a basis to achieve the desired RF phase adjustments, reporting their results on a 1.5 T MRI system. Feng et al 9 and Shooshtary et al 10 utilized commercially available broadband integrated circuits (IC) with digital step attenuators and gain amplifiers for their 64 and 32 channel systems at 3 T and 7 T respectively. Given that this work is not exhaustive, other design options are worth considering.…”

Eight‐channel parallel transmit‐receive system for 7 T MRI with optically controlled and monitored on‐coil current‐mode RF amplifiers

“…11,12 To facilitate pTx, new hardware has been developed that allows independent control of the amplitude and phase of the multiple fields generated in a transmission (Tx) coil array. [13][14][15][16][17][18] Recently, pTx hardware has been fully integrated into commercial high-field scanner infrastructure. [19][20][21] Like most pTx hardware, this is designed with a conventional, and not necessarily optimal, Tx architecture similar to that used for quadrature Tx coils.…”