FPGA based digital signal processing for EPR spectroscopy with an application to MRI

Electron Paramagnetic Resonance (EPR), a magnetic resonance technique similar to nuclear magnetic resonance, detects paramagnetic species such as free radicals. Like Magnetic Resonance Imaging (MRI), EPR can be implemented as an imaging technique for small animals and potentially human applications both in pulsed and continuous wave mode. Typical frequencies used for in vivo applications are about 300 MHz with a corresponding static magnetic field of about 100 G (10mT). As demonstrated with high field MRI imaging systems, a frequency of 300 MHz is applicable for clinical use since the penetration depth of this frequency is high enough to image humans. CW EPR techniques are commonly used since they permit detection of paramagnetic species with large width. Building an EPR spectrometer, as shown in Figure 1, is a challenge. The major goal is to have a high sensitivity receiver, which requires special attention to noise, crosstalk from the transmitter, clock jitter, phase noise, power supply filtering, and high speed measurements and processing. In this research, a new measurement method and its processing in FPGA to improve the sensitivity of the system is investigated

Efficient implementation of 90 degrees phase shifter in FPGA

In this article, we present an efficient way of implementing 90 phase shifter using Hilbert transformer with canonic signed digit (CSD) coefficients in FPGA. It is implemented using 27-tap symmetric finite impulse response (FIR) filter. Representing the filter coefficients by CSD eliminates the need for multipliers and the filter is implemented using shifters and adders/subtractors. The simulated results for the frequency response of the Hilbert transformer with infinite precision coefficients and CSD coefficients agree with each other. The proposed architecture requires less hardware as one adder is saved for the realization of every negative coefficient compared to convectional CSD FIR filter implementation. Also, it offers a high accuracy of phase shift

Embryonic Architecture with Built-in Self-test and GA Evolved Configuration Data

The embryonic architecture, which draws inspirationfrom the biological process of ontogeny, has built-inmechanisms for self-repair. The entire genome is stored in theembryonic cells, allowing the data to be replicated in healthycells in the event of a single cell failure in the embryonic fabric.A specially designed genetic algorithm (GA) is used to evolve theconfiguration information for embryonic cells. Any failed embryoniccell must be indicated via the proposed Built-in Self-test(BIST) the module of the embryonic fabric. This paper recommendsan effective centralized BIST design for a novel embryonic fabric.Every embryonic cell is scanned by the proposed BIST in casethe self-test mode is activated. The centralized BIST design usesless hardware than if it were integrated into each embryoniccell. To reduce the size of the data, the genome or configurationdata of each embryonic cell is decoded using Cartesian GeneticProgramming (CGP). The GA is tested for the 1-bit adder and2-bit comparator circuits that are implemented in the embryoniccell. Fault detection is possible at every function of the cell due tothe BIST module’s design. The CGP format can also offer gate-levelfault detection. Customized GA and BIST are combinedwith the novel embryonic architecture. In the embryonic cell, self-repairis accomplished via data scrubbing for transient errors

Embryonic Architecture with Built-in Self-test and GA Evolved Configuration Data

The embryonic architecture, which draws inspiration from the biological process of ontogeny, has built-in mechanisms for self-repair. The entire genome is stored in the embryonic cells, allowing the data to be replicated in healthy cells in the event of a single cell failure in the embryonic fabric. A specially designed genetic algorithm (GA) is used to evolve the configuration information for embryonic cells. Any failed embryonic cell must be indicated via the proposed Built-in Selftest (BIST) the module of the embryonic fabric. This paper recommends an effective centralized BIST design for a novel embryonic fabric. Every embryonic cell is scanned by the proposed BIST in case the self-test mode is activated. The centralized BIST design uses less hardware than if it were integrated into each embryonic cell. To reduce the size of the data, the genome or configuration data of each embryonic cell is decoded using Cartesian Genetic Programming (CGP). The GA is tested for the 1-bit adder and 2-bit comparator circuits that are implemented in the embryonic cell. Fault detection is possible at every function of the cell due to the BIST module’s design. The CGP format can also offer gate-level fault detection. Customized GA and BIST are combined with the novel embryonic architecture. In the embryonic cell, self-repair is accomplished via data scrubbing for transient errors

Data acquisition and digital signal processing for electron paramagnetic resonance spectrometers

Magnetic resonance imaging (MRI) is one of the most widely used imaging techniques to image the organs of the human body in clinical applications. The MRI used for clinical application is based on Nuclear Magnetic Resonance (NMR), where the interactions of the atomic nuclei with the applied static and radio frequency magnetic fields is exploited. Alternatively, Electron Paramagnetic Resonance Imaging (EPRI) is an attractive technique for imaging tissues based on the electron spins. One of the most important reasons for considering the use of any imaging technique in the clinic application, including EPR, is its ability to provide clinically useful information in a manner that has significant advantages over other approaches. The advantages may be in the uniqueness of the information, or the accuracy of the information, or the ease of obtaining the information. Such capabilities of in vivo EPRI have already been demonstrated in animals, and continues to attract more interest. In vivo EPRI studies in small animals are progressing at a remarkable rate. The EPRI was first detected in 1945 by Yevgeny Zaboisky at Kazan State University in the Soviet Union. In the 1980s, commercial EPR spectrometers became available. The German company Bruker leads the field in the production of commercially available EPR spectrometers, as well as NMR spectrometers and MRI detectors. Today, EPR spectroscopy is done at a wide range of frequencies and magnetic fields. In order to observe the electron resonance, the frequency of the microwaves must correspond to the splitting of the spin states of the electron, which is determined by the strength of the magnetic field. Thus, the strength of the magnetic field required is dependent on the frequency of the radiation used, and vice versa. The dimensions of the antenna, and the magnet are specific to the microwave frequency. For large sample imaging, a Larmor frequency of 300 MHz is sufficient to excite the spin system of the electrons with a corresponding magnetic field of 10 mT whereas an NMR requires at least ten times that of the magnetic field used by EPR spectrometer. A low magneticfield leads to a reduction in size of the magnet and hence cost reduction. There are two ways to exciting the spin system of the electrons: Pulse RF and Continuous Wave (CW) RF. Over the years, most EPR spectrometers were operated at a magnetic field of 0.34T and a RF frequency of 9.5GHz exiting with CW RF. It has been demonstrated in NMR that the pulse technique can offer better sensitivity and increase the information content. Hence, pulse techniques are widely used for EPR systems as well. Today’s pulse EPR systems operate from 3.4T/95 GHz up to 12.8T/360 GHz. Such high frequency spectrometers are useful to improve the sensitivity of small samples. However, for in vivo applications and larger samples, an RF of 300 MHz with a corresponding magnetic field of 10 mT is sufficient. Much development is yet to come for these operational conditions to make the in vivo EPR spectrometers available for clinical applications. For NMR, the pulse techniques are standard by now and it has been demonstrated how it can enormously increase information content and sensitivity compared to CW mode. Pulse EPR on the other hand, leads to technical complications which restrict the feasibility of these sophisticated pulse methods. This is due to the extremely short relaxation times of the electrons. This research focuses on building an EPR spectrometer operating at 10 mT/300 MHz for in vivo applications. Both pulse and CW excitation techniques are explored in this research. The main challenge with the pulse technique is to measure the relaxation times of the sample accurately by increasing the sensitivity of the system. Here, the weak response signal is separated from the RF feed-through by using an orthogonal setup of antennas. This ensures maximum isolation between the RF feed through and the response EPR signal. Measuring the short relaxation times of the electrons at a low Larmor frequency is more challenging. This requires a sophisticated antenna switch on both the transmitter and receiver side. The challenge is that the antenna itself rings down for a few micro-seconds imposing difficulties to measure the relaxation times. The concept of Q switching is explored here to reduce the ring down time. By using an antenna switch, an additional isolation of 10 dB is obtained apart from the 40 dB isolation obtained from the orthogonal arrangement of the transmit and receive antennas. The antenna switches designed has an insertion loss of 0.06 dB and a switching speed of 4 ns. A bandpass filter used for noise reduction is designed with low insertionloss. The bandpass filter is implemented using micro-strip and tuning capacitors. The filter designed has a low insertion loss of 0.9 dB and 0.7 dB for the bandwidths 40 and 55 MHz respectively. The signal to noise ratio is improved by averaging the acquired signal in an Field Programmable Gate Array (FPGA). On the transmitter side of the pulse EPR, a pulse generator is required. For research, an Arbitrary Waveform Generator (AWG) can be used as it is very flexible. But for commercial purpose, a programmable pulse generator is needed. In this work, an FPGA is used to generate the RF pulses. Using FPGAs, only limited frequencies can be generated. This is due to the limitation of the device speed and the device resources. Using synchronous delays, higher frequency pulses (above the maximum clock frequency) is generated. For example, with a Virtex-4 FPGA, this technique allows to generate pulses up to 628 MHz. For this application, RF pulses with a frequency of 300 MHz and a pulse width of 11.55 ns are generated. In addition to the pulse mode, research is also carried out using CW RF. Various detection methods, and receiver architectures are described in the literature. The commercially available CW EPR operates at 0.34 T/ 9.5 GHz. This research work explored the CW EPR at 10 mT/300 MHz with a novel detection technique to improve the sensitivity. Existing EPR spectrometers use a magnetic cavity wherein the detection is done by recording the absorption or derivatives of absorption. In this work, a direct detection of the active signal of the sample is obtained by using an alternating magnetic field. The signal to noise ratio is improved by sampling at the zero crossings of the RF (where the effect is maximum). This is based on the observation that the response of the particles is 90o out of phase with the respect to transmitted excitation pulse. The sampled data are then processed in real time in the FPGA. This gives maximum sensitivity as the sampling is done at the maximum of the effect. To control the various components of the spectrometer and to ensure smooth and fast collection of data, a control software is needed. For this purpose, a custom made firmware for the USB and the PC host control software are developed with various functionalities needed for the measurements. Both the pulse mode and CW mode EPR spectrometers are developed at low magnetic field/low frequency. A complete integration of various components and automation of the software are done to take the system a step closer to the clinical use of EPR operating at 10 mT/300 MHz. However, as with any technology evolution of the system is expected to improve through further optimization

Efficient implementation of 90° phase shifter in FPGA

<p>Abstract</p> <p>In this article, we present an efficient way of implementing 90&#176; phase shifter using Hilbert transformer with canonic signed digit (CSD) coefficients in FPGA. It is implemented using 27-tap symmetric finite impulse response (FIR) filter. Representing the filter coefficients by CSD eliminates the need for multipliers and the filter is implemented using shifters and adders/subtractors. The simulated results for the frequency response of the Hilbert transformer with infinite precision coefficients and CSD coefficients agree with each other. The proposed architecture requires less hardware as one adder is saved for the realization of every negative coefficient compared to convectional CSD FIR filter implementation. Also, it offers a high accuracy of phase shift.</p

Synchronous delay based UWB pulse generator in FPGA

This paper presents an architecture for generating UWB pulses with a high centre frequency accuracy. The architecture allows to generate frequencies twice that of the FPGA clock using synchronous delays and is implementable in all types of FPGA. With a FPGA clock of 150 MHz, we generate RF pulse of 300 MHz with a maximum fractional bandwidth of 30%. The architecture also allows pulse width increment in steps of the clock period