Four-channel surface coil array for 300-MHz pulsed EPR imaging: proof-of-concept experiments

Stretchable receive coil for 7T small animal MRI

Receive coils used in small animal MRI are rigid, inflexible surface loops that do not conform to the anatomy being imaged. The recent trend toward design of stretchable coils that are tailored to fit any anatomical curvature has been focused on human imaging. This work demonstrates the application of stretchable coils for small animal imaging at 7T. A stretchable coil measuring 3.5 × 3.5 cm was developed for acquisition of rat brain and spine images. The SNR maps of the stretchable coil were compared with those of a traditional flexible PCB coil and a commercial surface coil. Stretch and conformance testing of the coil was performed. Ex vivo images of rat brain and spine from the stretchable a coil was acquired using T1 FLASH and T2 Turbo RARE sequences. The axial phantom SNR maps showed that the stretchable coil provided 48.5% and 42.8% higher SNR than the commercial coil for T1-w and T2-w images within the defined ROI. A 33% increase in average penetration depth was observed within the ROI using the stretchable coil when compared to the commercial coil. The ex-vivo rat brain and spine images showed distinguishable anatomical details. Stretching the coil reduced the resonant frequency with reduction in SNR, while the conformance to varying sample volumes increased the resonant frequency with decreased SNR. This study also features an open-source plug-and-play system with preamplifiers that can be used to interface surface coils with the 7T Bruker scanner.

Dual channel EPR excitation coil array for Overhauser-enhanced MRI

This article describes the development of a two-channel surface coil array for Overhauser-enhanced magnetic resonance imaging (OMRI) with the aim of extending the visualization area. The surface coil array consists of two independent surface coil resonators and PIN-diode switches. We utilized the PIN-diode switch to decouple the surface coils. OMRI measurement using a surface coil array was sequentially performed by switching the channels. To evaluate the effectiveness of the surface coil array, we demonstrated OMRI measurements using a phantom filled with nitroxide solution. In addition, in vivo OMRI imaging with a mouse was performed to demonstrate the applicability of our surface coil array to in vivo measurements. As a result, the visualization area obtained with our surface coil array was extended approximately 2-fold compared to the conventional single surface coil. Furthermore, we showed that in vivo imaging with the surface coil array was possible. These results indicate that the surface coil array could enhance the applicability of OMRI imaging.

EPR Everywhere

This review is inspired by the contributions from the University of Denver group to low-field EPR, in honor of Professor Gareth Eaton's 80th birthday. The goal is to capture the spirit of innovation behind the body of work, especially as it pertains to development of new EPR techniques. The spirit of the DU EPR laboratory is one that never sought to limit what an EPR experiment could be, or how it could be applied. The most well-known example of this is the development and recent commercialization of rapid-scan EPR. Both of the Eatons have made it a point to remain knowledgeable on the newest developments in electronics and instrument design. To that end, our review touches on the use of miniaturized electronics and applications of single-board spectrometers based on software-defined radio (SDR) implementations and single-chip voltage-controlled oscillator (VCO) arrays. We also highlight several non-traditional approaches to the EPR experiment such as an EPR spectrometer with a "wand" form factor for analysis of the OxyChip, the EPR-MOUSE which enables non-destructive in situ analysis of many non-conforming samples, and interferometric EPR and frequency swept EPR as alternatives to classical high Q resonant structures.

Effects of Photodynamic Therapy with Redaporfin on Tumor Oxygenation and Blood Flow in a Lung Cancer Mouse Model

Three photodynamic therapy (PDT) protocols with 15 min, 3 h and 72 h drug-to-light time intervals (DLIs) were performed using a bacteriochlorin named redaporfin, as a photosensitizer. Blood flow and pO₂ changes after applying these protocols were investigated in a Lewis lung carcinoma (LLC) mouse model and correlated with long-term tumor responses. In addition, cellular uptake, cytotoxicity and photocytotoxicity of redaporfin in LLC cells were evaluated. Our in vitro tests revealed negligible cytotoxicity, significant cellular uptake, generation of singlet oxygen, superoxide ion and hydroxyl radicals in the cells and changes in the mechanism of cell death as a function of the light dose. Results of in vivo studies showed that treatment focused on vascular destruction (V-PDT) leads to a highly effective long-term antineoplastic response mediated by a strong deprivation of blood supply. Tumors in 67% of the LLC bearing mice treated with V-PDT regressed completely and did not reappear for over 1 year. This significant efficacy can be attributed to photosensitizer (PS) properties as well as distribution and accurate control of oxygen level and density of vessels before and after PDT. V-PDT has a greater potential for success than treatment based on longer DLIs as usually applied in clinical practice.

Wireless implantable coil with parametric amplification for in vivo electron paramagnetic resonance oximetric applications

PURPOSE

To develop an implantable wireless coil with parametric amplification capabilities for time-domain electron paramagnetic resonance (EPR) spectroscopy operating at 300 MHz.

METHODS

The wireless coil and lithium phthalocyanine (LiPc), a solid paramagnetic probe, were each embedded individually in a biocompatible polymer polydimethoxysiloxane (PDMS). EPR signals from the LiPc embedded in PDMS (LiPc/PDMS) were generated by a transmit-receive surface coil tuned to 300 MHz. Parametric amplification was configured with an external pumping coil tuned to 600 MHz and placed between the surface coil resonator and the wireless coil.

RESULTS

Phantom studies showed significant enhancement in signal to noise using the pumping coil. However, no influence of the pumping coil on the oxygen-dependent EPR spectral linewidth of LiPc/PDMS was observed, suggesting the validity of parametric amplification of EPR signals for oximetry by implantation of the encapsulated wireless coil and LiPc/PDMS in deep regions of live objects. In vivo studies demonstrate the feasibility of this approach to longitudinally monitor tissue pO₂ in vivo and also monitor acute changes in response to pharmacologic challenges. The encapsulated wireless coil and LiPc/PDMS engendered no host immune response when implanted for ∼3 weeks and were found to be well tolerated.

CONCLUSIONS

This approach may find applications for monitoring tissue oxygenation to better understand the pathophysiology associated with wound healing, organ transplantation, and ischemic diseases.

Accelerated 4D quantitative single point EPR imaging using model-based reconstruction

PURPOSE

Electron paramagnetic resonance imaging has surfaced as a promising noninvasive imaging modality that is capable of imaging tissue oxygenation. Due to extremely short spin-spin relaxation times, electron paramagnetic resonance imaging benefits from single-point imaging and inherently suffers from limited spatial and temporal resolution, preventing localization of small hypoxic tissues and differentiation of hypoxia dynamics, making accelerated imaging a crucial issue.

METHODS

In this study, methods for accelerated single-point imaging were developed by combining a bilateral k-space extrapolation technique with model-based reconstruction that benefits from dense sampling in the parameter domain (measurement of the T2 (*) decay of a free induction delay). In bilateral kspace extrapolation, more k-space samples are obtained in a sparsely sampled region by bilaterally extrapolating data from temporally neighboring k-spaces. To improve the accuracy of T2 (*) estimation, a principal component analysis-based method was implemented.

RESULTS

In a computer simulation and a phantom experiment, the proposed methods showed its capability for reliable T2 (*) estimation with high acceleration (8-fold, 15-fold, and 30-fold accelerations for 61×61×61, 95×95×95, and 127×127×127 matrix, respectively).

CONCLUSION

By applying bilateral k-space extrapolation and model-based reconstruction, improved scan times with higher spatial resolution can be achieved in the current single-point electron paramagnetic resonance imaging modality.