Introduction

In human and animal MRI, physiological motions can cause detectable B₀ field fluctuations in the region of interest, which, if not corrected, can lead to severe image artifacts. Real-time B₀ field monitoring and correction is highly desirable because the expected data integrity should be preserved prior to image reconstruction. Several methods^1,2 have been employed to accomplish field stability by setting up a feedback loop in which the B₀ field is monitored and corrected during MRI scans with a minimum feedback latency in 20 -100 ms. However, a fundamental shortcoming of these methods is that the components of the feedback loops are distributed over different electronic devices, boards and even subsystems with feedback data processing performed on CPUs. The unnecessary inter-device data routing and slower processing increase the feedback latency. To address the feedback latency, in this abstract, we describe the design for a multifunctional digital transceiver and demonstrate its potential applications to real-time zero-order B₀ field stabilization.

Methods

Our strategy for decreasing the feedback latency is to integrate a digital transmitter and receiver, a pulse sequencer, signal and image processors on a single device to constitute a multifunctional transceiver. An MRI electronic system built on such a multifunctional transceiver will not only have a greatly-simplified architecture, but also make it possible to establish real-time interactions among the signal and image processors, the transmitter and the receiver so to enable real-time B₀ field monitoring and correction. In this work we developed a new transmitter module and integrated it with a previous digital receiver^3,4 with a newly-designed multiple-slice image reconstruction module. We used a NI USRP-2940R board (National Instruments, Inc., Austin, TX, USA) as a hardware platform. Analog signal processing is performed with the analog front end; while the multifunctional transceiver architecture is implemented on an on-board FPGA (Fig. 1). On the receiver portion, an input baseband signal, consisting of in-phase (I) and quadrature (Q) components, is digitized and delivered to the digital demodulator controlled by a numerically-controlled oscillator (NCO). New modules (green) are developed and implemented to perform frequency detection and correction, data correction and image reconstruction. The data flow within the receiver is controlled by Rx Control module. On the transmitter portion, the I/Q components of a digital pulse are generated by a pulse sequencer module and mixed with a second NCO, resulting in a baseband analog pulse which is routed to the analog front end for up-conversion modulation to generate an RF pulse. The pulse attributes are specified by the pulse sequencer which is essentially a FIFO buffer storing a pulse envelope. The data flow within the transmitter is controlled by Tx Control module. We used LabView (National Instruments, Inc., Austin, TX USA) software package as a development platform. All the modules were designed around an FPGA chip XC7K410T (Xilinx Inc., San Jose, CA USA). IF signals were sampled at 120 MS/s with 14-bit resolution; digital signals were processed at 80 MHz.

Experiments and Discussion

The multifunctional transceiver was tested with a series of gradient-echo (FLASH) imaging experiments when incorporated with our Bruker BioSpec 94/30USR spectrometer. The input port of the FPGA board was directly connected to the output at the pre amplifier of the spectrometer; while the output port was directly connected to the input of the RF power amplifier of the spectrometer so that the RF route completely bypassed the RF transmitter, RF receiver and digitizer boards installed on the spectrometer. A typical FLASH sequence was slightly modified to allow for acquiring a FID signal for frequency detection and a gradient-echo signal for imaging. To evaluate the effectiveness of the real-time B₀-field correction, we performed the following imaging experiments. At the beginning of every MRI scan, the receiver frequency was offset randomly from one phase-encoding step to the next to simulate zero-order B₀ field fluctuations. The Frequency Detection module detected the frequency shift from the FID signal, which was immediately used to adjust the receiver NCO. Data Correction module performed signal phase correction. With this setup, the frequency values can be determined in 75 ns; while the receiver needs ~0.6 ms to update the local oscillator frequency. Some of the resulting images of rabbit brain shown in Fig.2 demonstrate that image ghosting artifacts due to the random frequency offsets are eliminated with frequency compensation and phase correction. These results verify that the multifunctional transceiver functions properly and can perform image reconstruction with real-time frequency and phase compensation. It is important to note that the frequency detection and compensation, data correction and image reconstruction were all performed on the same FPGA. The correct images were generated before they were transferred to a host computer. With the integration strategy, the multifunctional transceiver can eliminate unnecessary inter-device data routing, and thus dramatically reduce the feedback latency, making it possible to perform real-time system adjustments or data correction in response to unpredictable and irreproducible system or field perturbations in intra scans. Although this single-channel multifunctional transceiver is suitable only for the zero-order B₀ field stabilization, it could be expanded to accommodate multiple channels suitable for the higher-order B₀ field stabilization.

Acknowledgements

This work was supported by NIH grant R21EB024852.

References

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