Rapid Scan Electron Paramagnetic Resonance Opens New Avenues for Imaging Physiologically Important Parameters In Vivo
September 26th, 2016
A new electron paramagnetic resonance (EPR) method, rapid scan EPR (RS-EPR), is demonstrated for 2D spectral spatial imaging which is superior to the traditional continuous wave (CW) technique and opens new venues for in vivo imaging. Results are demonstrated at 250 MHz, but the technique is applicable at any frequency.
The overall goal of Rapid Scan Electron Paramagnetic Imaging, or RSEPRI, is to provide quantitative information on oxygen concentration, pH, redox status and concentration of signaling molecules that are useful for biomedical research. EPRI is a tool that can be used to answer key questions in cancer research regarding the tumor environment. The main advantage of rapid-scan EPR imaging is that you can acquire more information in a much faster time, with a wider variety of probe molecules as compared with continuous wave or CW EPR imaging.
The molecules which are sensitive to redox status or pH are a good example of where rapid-scan EPRI really shines. Begin this procedure with the calculation of rapid-scan experimental conditions as determined in the text protocol. One important piece of rapid-scan is to understand the dependence of the signal on the resonator bandwidth and experimental conditions such as scan frequency sweep width.
To really optimize the experiment you need to understand all three. The rapid-scan coil driver has two amplifiers. When selecting a capacitor, the capacitor box needs to be balanced with an equal capacitance on each side of the box.
The two sides are in series. Unscrew the top cover of the capacitor box and insert capacitors on both sides that are equal to the determined value. Replace the top of the capacitor box and screw it down to ensure it stays on.
Using the front panel of the resonated coil driver, adjust the output frequency until the sinusoidal waveform has the maximum amplitude. To prepare the radicals, remove N-15 PDT from the freezer and allow the container to come to room temperature. Weight out 1.4 milligrams of N-15 PDT using an analytical balance.
Add 1.4 milligrams of N-15 PDT to 15 milliliters of deionized water for a final concentration of 0.5 millimolar. Next, combine 50 milligrams of BMPO with five milliliters of water in a 16 millimeter quartz irradiation tube. Add 100 microliters of 300 millimolar hydrogen peroxide.
Irradiate the mixture in the 16 millimeter quartz irradiation tube with a medium-pressure 450 watt ultraviolet lamp for five minutes. Using a glass transfer pipette, transfer two point five milliliters of the irradiated BMPO-OH solution out of the quartz irradiation tube and into one side of a 16 millimiter quartz sample tube with a three millimeter divider. Transfer the remaining two point five milliliters of irradiated BMPO-OH into the other side of the quartz sample tube with the divider.
A second critical step is to understand the power saturation curve. As scan rate increases, you access higher powers and larger signal amplitudes before your signal saturates. Larger signal amplitudes is another way to decrease your acquisition time.
First, tune the resonator with an aqueous sample of the nitroxide radical by inserting the 15-milliliter sample of 0.5 millimolar N-15 PDT in water into a 16-millimeter quartz electron paramagnetic resonance tube. Insert the quartz tube into the detection side of the cross-loop rapid scan electron paramagnetic resonance or RSEPR resonator. Change the frequency of the instrument source until it matches the frequency of the detection side that contains the sample.
Now, change the frequency of the excitation side to match the frequencies of the experiment source and detection side of the resonator. Change the frequency of the excitation side by turning a variable capacitor within the resonator cavity. To set up the instrument console and main magnet turn on the spectrometer and choose an experiment which records transient data with time on the abscissa.
Perform a power saturation curve on a standard nitroxide radical sample under the same experimental conditions that will be used to look at radicals sensitive to pH or redox status. Once the system is set up, gradients are applied manually or through a computer program for the imaging experiment. EPR signal intensity is directly proportional to the microwave field in the resonator that causes the spin to go from one energy level to the next.
This microwave field is named B1.B1 is proportional to the square root of microwave power. As microwave power is quadrupled, B1 is doubled. In this figure the power is quadrupled from eight to 32 milliwatts, and B1 is doubled from 18 to 36 milliGauss.
A power saturation curve can be constructed by plotting relative amplitude as a function of the square root of microwave power, or if the resonator efficiency is known, B1.The region of this curve that is linear shows the power region where the EPR signal is not saturated or distorted. One of the advantages of a rapid-scan experiment is that the linear power range is extended, represented by the colored dots, in comparison to the regular CW experiment represented by the black dots. Shown here is a two-dimensional spectral spatial image of phantoms consisting of a BMPO-OH adduct separated by three millimeters at a concentration of five micromolar.
A slice through the image shows the spectral shape at 250 megahertz. This image shows the N-14 nitranyl radical which can be used for trapping nitric oxide in vivo in a phantom, where a one point five millimeter thick wall separates two sample chambers. The spectral shape at 250 megahertz is shown here.
The two-dimensional image of pH-sensitive triarylmethyl radicals reflects the difference in spectral features when the phosphate buffer pH equals seven point zero, or the pH equals seven point four. This image shows N-15 dinitroxide in a two-compartment phantom with a 10-millimeter spacer. Initially, both compartments contain 0.5 millimolar probe.
Addition of glutathione to the dinitroxide breaks the reduced linker region and produces two mononitroxides, a change that is reflected in the two-dimensional image. The development of rapid-scan EPR is a total paradigm shift in our ability to study molecules with unpaired electrons. You saw today its application to in vivo EPR, but it also has wonderful possibilities for low-temperature studies of compounds with metals in them, and many other kinds of systems.
For all of the samples that we've studied so far, we've gotten improvements in signal to noise of at least an order of magnitude and sometimes even more.
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Overview
This article presents a novel electron paramagnetic resonance (EPR) method known as rapid scan EPR (RS-EPR) for 2D spectral spatial imaging. RS-EPR offers advantages over traditional continuous wave (CW) techniques, particularly in the context of in vivo imaging applications.
Key Study Components
Area of Science
- Biomedical Imaging
- Electron Paramagnetic Resonance
- Cancer Research
Background
- Rapid Scan Electron Paramagnetic Imaging (RSEPRI) provides quantitative data on various biological parameters.
- It is particularly useful for assessing tumor environments in cancer research.
- The technique allows for faster data acquisition compared to traditional methods.
- It can utilize a wider variety of probe molecules sensitive to redox status and pH.
Purpose of Study
- To enhance imaging techniques for biomedical research.
- To provide detailed information on oxygen concentration, pH, and signaling molecules.
- To improve understanding of the tumor microenvironment.
Methods Used
- Calculation of rapid-scan experimental conditions.
- Assessment of signal dependence on resonator bandwidth.
- Evaluation of experimental conditions such as scan frequency sweep width.
- Application of RS-EPR at 250 MHz, with potential for other frequencies.
Main Results
- Demonstration of RS-EPR's superiority over CW EPR in imaging.
- Ability to acquire more information in a shorter time frame.
- Successful imaging of molecules sensitive to redox status and pH.
- Potential for broader applications in in vivo imaging.
Conclusions
- RS-EPR represents a significant advancement in EPR imaging techniques.
- It opens new avenues for research in cancer and other biomedical fields.
- Future studies may expand its application across various frequencies.
Frequently Asked Questions
Executive Industry Relevance
Rapid-scan EPR imaging enables quantitative, real-time assessment of physiologically critical parameters such as oxygen tension, pH, redox state, and reactive signaling molecules in living systems. This capability supports mechanistic de-risking in oncology and inflammatory disease research by providing direct readouts of microenvironmental conditions that influence therapeutic response and resistance. The method’s enhanced sensitivity, spatial resolution, and acquisition speed improve predictive confidence in preclinical models, facilitating earlier go/no-go decisions in target validation and lead optimization workflows.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of therapeutic hypotheses by mapping oxygenation, pH, and redox dynamics in disease-relevant models.
- Operational Value: Provides quantitative, spatially resolved readouts of microenvironmental biomarkers to clarify pathway activity and target engagement.
- Predictive Value: Supports biological de-risking by correlating probe signal changes with functional phenotypes, improving confidence in target-disease relationships.
Screening & Assay Development
- Assay Readiness: Generates standardized, reproducible spectral-spatial images of radical reporter molecules sensitive to physiological stimuli.
- Quantitative Output: Delivers concentration-dependent signal changes for oxygen, pH, and redox species, enabling dose-response and inhibitor profiling.
- Platform Reuse: Compatible with diverse probe molecules (e.g., BMPO-OH for •OH, nitranyl for NO•, triarylmethyl for pH) across multiple experimental setups.
Translational & Preclinical Research
- Disease Relevance: Directly images tumor hypoxia, acidosis, and oxidative stress—key hallmarks of cancer progression and therapy resistance.
- Translational Continuity: Bridges discovery-phase mechanistic insights with preclinical validation by monitoring the same biomarkers across models.
- Risk-Adjusted Advancement: Enables longitudinal tracking of microenvironmental changes to inform dosing schedules and combination strategies.
Pipeline & Workflow Integration
Rapid-scan EPR fits within the discovery continuum from target hypothesis testing through lead identification to preclinical efficacy assessment, particularly when microenvironmental modulation is a mechanism of action or resistance factor.
- Discovery Biology: Supports hypothesis testing by visualizing how genetic or pharmacological perturbations alter oxygen, pH, or redox parameters in cellular and tissue models.
- Screening: Enables assay development for compounds that modulate hypoxic signaling, antioxidant pathways, or pH-regulating enzymes via measurable EPR shifts.
- Analytics: Provides quantitative, ratiometric measurements of probe signal intensity and spectral shape, allowing comparison across conditions and time points.
- Translational Research: Connects in vitro findings to in vivo relevance by imaging the same radical reporters in phantom and living systems.
- Enterprise Reuse: Functions as a non-destructive, plug-and-play imaging module compatible with existing preclinical imaging suites and adaptable to various probe chemistries.
Operational & Enterprise Impact
- Scientific Value: Increases target validation confidence by reducing ambiguity in microenvironmental contributions to phenotype.
- Operational Value: Improves reproducibility and throughput through faster acquisition and higher signal-to-noise ratio versus CW-EPR.
- Strategic Value: Enhances portfolio triage by identifying compounds that fail due to unfavorable microenvironmental interactions early in development.
- Portfolio Impact: Enables risk-adjusted resource allocation by prioritizing agents with favorable effects on tumor oxygenation, pH normalization, or redox balance.
Implementation Considerations
- Requires expertise in EPR spectroscopy, radical chemistry, and biological sample handling.
- Needs access to a rapid-scan EPR spectrometer with cross-loop resonator and gradient capabilities for 2D imaging.
- Demands standardization of probe preparation, calibration, and environmental controls (temperature, humidity) across sites.
- Involves adaptation considerations for different radical reporters (e.g., pH-sensitive vs. redox-sensitive) and tissue types (phantom, ex vivo, in vivo).
- Practical limitations include probe stability, bioavailability, and potential background signals from endogenous radicals in complex biological matrices.
