In Vivo Application of Proton-Electron Double-Resonance Imaging

This work was partially supported by NIH grants 1ZIABC010477-14 (MKC), CA194013 (VVK), CA192064 (VVK), U54GM104942 (VVK); by KAKENHI grant 16H05113 (H.U.) from the Japan Society for the Promotion of Science (HU) and start-up grant from the WVCTSI (VVK).Peer reviewedPostprin

Imaging of Glucose Metabolism by 13C-MRI Distinguishes Pancreatic Cancer Subtypes in Mice

Metabolic differences among and within tumors can be an important determinant in cancer treatment outcome. However, methods for determining these differences non-invasively in vivo is lacking. Using pancreatic ductal adenocarcinoma as a model, we demonstrate that tumor xenografts with a similar genetic background can be distinguished by their differing rates of the metabolism of 13C labeled glucose tracers, which can be imaged without hyperpolarization by using newly developed techniques for noise suppression. Using this method, cancer subtypes that appeared to have similar metabolic profiles based on steady state metabolic measurement can be distinguished from each other. The metabolic maps from 13C-glucose imaging localized lactate production and overall glucose metabolism to different regions of some tumors. Such tumor heterogeneity would not be not detectable in FDG-PET

Dynamic Imaging of Glucose and Lactate Metabolism by C-13-MRS without Hyperpolarization

Abstract Metabolic reprogramming is one of the defining features of cancer and abnormal metabolism is associated with many other pathologies. Molecular imaging techniques capable of detecting such changes have become essential for cancer diagnosis, treatment planning, and surveillance. In particular, 18F-FDG (fluorodeoxyglucose) PET has emerged as an essential imaging modality for cancer because of its unique ability to detect a disturbed molecular pathway through measurements of glucose uptake. However, FDG-PET has limitations that restrict its usefulness in certain situations and the information gained is limited to glucose uptake only.13C magnetic resonance spectroscopy theoretically has certain advantages over FDG-PET, but its inherent low sensitivity has restricted its use mostly to single voxel measurements unless dissolution dynamic nuclear polarization (dDNP) is used to increase the signal, which brings additional complications for clinical use. We show here a new method of imaging glucose metabolism in vivo by MRI chemical shift imaging (CSI) experiments that relies on a simple, but robust and efficient, post-processing procedure by the higher dimensional analog of singular value decomposition, tensor decomposition. Using this procedure, we achieve an order of magnitude increase in signal to noise in both dDNP and non-hyperpolarized non-localized experiments without sacrificing accuracy. In CSI experiments an approximately 30-fold increase was observed, enough that the glucose to lactate conversion indicative of the Warburg effect can be imaged without hyper-polarization with a time resolution of 12s and an overall spatial resolution that compares favorably to 18F-FDG PET

EPR-based oximetric imaging : a combination of single point-based spatial encoding and T1 weighting

Purpose: Spin-lattice relaxation time (T1)-weighted time-domain EPR oximetry is reported for in vivo applications using a paramagnetic probe, a trityl-based Oxo71. Methods: The R1 dependence of the trityl probe Oxo71 on pO2 was assessed using single point imaging (SPI) mode of spatial encoding combined with rapid repetition, similar to T1-weighted MRI, where R1 was determined from 22 repetition times ranging from 2.1–40.0 μs at 300 MHz. The pO2 maps of a phantom with three tubes containing 2 mM Oxo71 solutions equilibrated at 0%, 2%, and 5% oxygen were determined by R1 and apparent spin-spin relaxation rate (R2*) simultaneously. Results: The pO2 maps derived from R1 and R2* agreed with the known pO2 levels in the tubes of Oxo71. However, the histograms of pO2 revealed that R1 offers better pO2 resolution than R2* in low pO2 regions. The standard deviations of pixels at 2% pO2 (15.2 mmHg) were about 5 times lower in R1-based estimation than R2*-based estimation (mean ± SD: 13.9 ± 1.77 mmHg and 18.3 ± 8.70 mmHg, respectively). The in vivo pO2 map obtained from R1-based assessment displayed a homogeneous profile in low pO2 regions in tumor xenografts, consistent with previous reports on R2*-based oximetric imaging. The scan time to obtain the R1 map can be significantly reduced using three repetition times ranging from 4.0‒12.0 μs. Conclusion: Using the SPI modality, R1-based oximetry imaging with useful spatial and oxygen resolutions for small animals was demonstrated

Metabolic and physiologic imaging biomarkers of the tumor microenvironment predict treatment outcome with radiation or a hypoxia-activated prodrug in mice.

Pancreatic ductal adenocarcinoma (PDAC) is characterized by hypoxic niches that lead to treatment resistance. Therefore, studies of tumor oxygenation and metabolic profiling should contribute to improved treatment strategies. Here we define two imaging biomarkers that predict differences in tumor response to therapy: 1) partial oxygen pressure (pO2), measured by EPR imaging; and 2) [1- 13C] pyruvate metabolism rate, measured by hyperpolarized 13C MRI. Three human PDAC xenografts with varying treatment sensitivity (Hs766t, MiaPaCa-2, and Su.86.86) were grown in mice. The median pO2 of the mature Hs766t, MiaPaCa-2, and Su.86.86 tumors was 9.1±1.7, 11.1±2.2, and 17.6±2.6 mmHg, and the rate of pyruvate-to-lactate conversion was 2.72±0.48, 2.28±0.26, and 1.98±0.51 min-1 , respectively (n=6, each). These results are in agreement with steady state data of matabolites quantified by mass spectroscopy and histological analysis indicating glycolytic and hypoxia profile in Hs766t, MiaPaca-2, and Su.86.86 tumors. Fractionated radiation therapy (5 Gy x 5) resulted in a tumor growth delay of 16.7±1.6 and 18.0±2.7 days in MiaPaca-2 and Su.86.86 tumors, respectively, compared to 6.3±2.7 days in hypoxic Hs766t tumors. Treatment with gemcitabine, a first-line chemotherapeutic agent, or the hypoxia-activated prodrug TH-302 was more effective against Hs766t tumors (20.0±3.5 and 25.0±7.7 days increase in survival time, respectively) than MiaPaCa-2 (2.7±0.4 and 6.7±0.7 days) and Su.86.86 (4.7±0.6 and 0.7±0.6 days) tumors. Collectively, these results demonstrate the ability of molecular imaging biomarkers to predict the response of PDAC to treatment with radiation therapy and TH-30

Time-domain EPR imaging with slice selection

Synopsis: The slice selection imaging has advantages of reducing imaging time and obtaining optimum dynamic range in image for EPR imaging as well as for MRI. However, the slice selection using a selective pulse, which is used in MRI, is difficult to implement in EPR imaging because of ultra-fast relaxation time compared to gradient settling time. Therefore, we used a modulated gradient field to achieve slice selection in pulsed EPR imaging in this study. We demonstrated the slice selection imaging with tubes and a living mouse to show the effect of slice selection in pulsed EPR imaging. Introduction: In MRI, slice-selection is accomplished using a selective pulse in presence of a slice selective gradient. The spatial encoding and other functional properties of the spins in the selected slice are carried out by the subsequent refocusing pulses and phase or frequency encoding gradients. Such slice selection is difficult in pulsed EPR imaging, due the ~microsecond relaxation times of unpaired electrons which are shorter than gradient settling times. An alternative mode of slice-selection however is feasible by applying a modulated gradient along one of the directions1,2. The selected slice is located at the ‘zero-crossing’ of the modulated gradient. Its thickness depends on the modulation amplitude and frequency. Such slice selection can reduce the imaging time by an order of magnitude since only 2D images are measured, and the slice location can be changed by physically translating the resonator or electrically off-setting the center of the oscillatory gradient. Phantom and in vivo results are shown. Modalities which image a small number of m slices We have incorporated this approach of slice-selection using sinusoidally modulated gradients to generate a set of 2D images of slices that can greatly reduce the measurement time and can thus allow improvement in the SNR and resolution in the selected slices without additional measurement time. Methods: We employ the single point imaging (SPI) scheme, by which two and three dimensional in vivo EPR imaging and relaxation based oximetry have been carried out routinely in our laboratory. In this development, we use the same imaging equipment operating at 300 MHz, with an additional provision of applying a low frequency (100 Hz) sinusoidal field along one of the gradient axes at nominal AC amplitude of about 10 mT/m. The modulation of the gradient along a particular axis introduces inhomogeneity along that axis everywhere except around the midpoint at which the amplitude is zero. A two-dimensional phase encoding in a plane perpendicular to the axis of the modulated gradient retains coherent phase information only from the narrow slice at the center with spin distribution on either side of the slice undergoing total loss of coherence and does not contribute to the detected signal (Fig. 1). As proof of principle we made a phantom consisting of three tubes(4 mm i.d) filled to different levels with 3 mM Oxo633 (a stable trityl radical with a narrow single ESR absorption) and placed at a spacing interval of 10 mm as shown in Fig. 2A. Two dimensional images were obtained by single point imaging with a maximum gradient of 15 mT/m along the three planes.In addition to above phantom experiment, we performed in vivo experiments to investigate how dynamic range was improved using a mouse hind leg. Along with the mouse leg, we placed a TCNQ tube which produces strong signal as shown in Fig. 3A. The mouse was injected 75 mM oxo63 intravenously. Two dimensional images were obtained by single point imaging with a maximum gradient of 8 mT/m. In order to investigate the minimum slice thickness that can be achieved, we filled a 14 mm glass cuvette (the ones used in optical spectroscopy) with 2 mM Oxo63 and placed the cuvette at the center of the resonator along Y-direction. The EPR spectra were obtained when the Z-gradient was modulated at 100 Hz with a gradual increase in amplitude from 0 to 2 volt. Results and Discussion: When we carried out the 2D phase encoding in the XY plane with the Z-gradient being modulated at 100 Hz at amplitude of 1.4 volt, we saw only the tube centered at z-coordinate of zero (Fig. 2D). The other two tubes did not produce any signal due to the inhomogeneity imposed by the modulated Z-gradient. By shifting the resonator such that the other tubes were brought to the center sequentially, we could get slices showing exclusive images of each tube. Figure 3B and 3C shows the comparison between images acquired with conventional 2D projection and slice-selection methods. The distribution of Oxo63 acquired by 2D projection imaging was interfered by strong signal of TCNQ, while the slice selection image showed only the distribution of Oxo63. This suggested the optimum dynamic range in signal intensity was achieved by slice selection technique. The minimum slice thickness that could be achieved was around 1.7 mm at and above 1.8 volt. Conclusion: With modulation gradient, we have demonstrated the slice selection in pulsed EPR imaging and succeeded to obtain slice selected images with optimum dynamic range in signal intensity. The method will also enable the study functional dynamics in the images with improved temporal resolution.References:1. Hinshaw WS. Spin mapping:application of moving gradients to NMR. Phys Lett A. 1974; 48: 87–88. 2.Sato-Akaba H, Abe H, Fujii H, Hirata H. Slice-selective images of free radicals in mice with modulated field gradient electron paramagnetic resonance (EPR) imaging. Magn Reson Med. 2008; 59: 885-890. 3.Ardenkjaer-Larsen JH, Laursen I, Leunbach I, Ehnholm G, Wistrand LG, Petersson JS, Golman K. EPR and DNP properties of certain novel single electron contrast agents intended for oximetric imaging. J Magn Reson 1998; 133: 1–12.Joint Annual Meeting ISMRM-ESMRMB 201