Integration of digital signal processing technologies with pulsed electron paramagnetic resonance imaging

Abstract

The integration of modern data acquisition and digital signal processing (DSP) technologies with Fourier transform electron paramagnetic resonance (FT-EPR) imaging at radiofrequencies (RF) is described. The FT-EPR system operates at a Larmor frequency (L(f)) of 300MHz to facilitate in vivo studies. This relatively low frequency L(f), in conjunction with our approximately 10MHz signal bandwidth, enables the use of direct free induction decay time-locked subsampling (TLSS). This particular technique provides advantages by eliminating the traditional analog intermediate frequency downconversion stage along with the corresponding noise sources. TLSS also results in manageable sample rates that facilitate the design of DSP-based data acquisition and image processing platforms. More specifically, we utilize a high-speed field programmable gate array (FPGA) and a DSP processor to perform advanced real-time signal and image processing. The migration to a DSP-based configuration offers the benefits of improved EPR system performance, as well as increased adaptability to various EPR system configurations (i.e., software configurable systems instead of hardware reconfigurations). The required modifications to the FT-EPR system design are described, with focus on the addition of DSP technologies including the application-specific hardware, software, and firmware developed for the FPGA and DSP processor. The first results of using real-time DSP technologies in conjunction with direct detection bandpass sampling to implement EPR imaging at RF frequencies are presented.

Development and testing of a CW-EPR apparatus for imaging of short-lifetime nitroxyl radicals in mouse head

“…CW-EPR imaging is a practical solution to the visualization of free radical molecules that have a short relaxation time in animal subjects, and several methods are available to reduce the acquisition time of EPR imaging for small rodents: (i) pulsed EPR method [21][22][23][24], (ii) rapid-scan EPR method [25][26][27][28], (iii) EPR imaging using spinning magnetic field gradients [29,30], and (iv) uniform distribution of projections [12][13][14][15]. While pulsed EPR imaging has been intensively developed for animal imaging over the past decade, it is limited to free radical species that have a longer relaxation time, on the order of microseconds.…”

Digitally generated excitation and near-baseband quadrature detection of rapid scan EPR signals

“…X-band pulsed EPR has recently been performed using microwaves generated with an AWG [13, 25]. One other lab, in parallel with our efforts, has digitally generated the incident RF and directly digitized the CW EPR signal at 300 MHz [26, 27]. The current work makes the major step to CW at X-band, which was not feasible until the latest developments in digital technology.…”

“…The current work makes the major step to CW at X-band, which was not feasible until the latest developments in digital technology. Down-conversion and subsampling techniques, which are also extensively used in NMR [1] and MRI, have been used by the Hyde lab for recording multiharmonic X-band CW EPR [15, 28, 29] [30], by the Ohio State lab for L-band CW EPR [14], and by the NCI lab to digitize an FID [26]. An AWG was used to control pulse timing in a 26.5-40 GHz pulsed EPR spectrometer [31].…”

Digital EPR with an arbitrary waveform generator and direct detection at the carrier frequency

“…Exponential growth in processing power and memory capacity of modern digital electronics facilitates transition from the homodyne detection that is commonly used in EPR spectrometers to digital detection either at an intermediate frequency, IF [1–6], or directly at the carrier frequency [7–10]. Digital detection has several advantages.…”

“…An IF and sub-sampling have been used for CW, saturation recovery, ELDOR, and FID detection at X-band [4–6]. Direct detection with sub-sampling was used for FID imaging at 300 MHz [7,8] and for CW with multiple harmonics at L-band [19]. …”