In vivo oxygen tension and temperature: simultaneous determination using 19F NMR spectroscopy of perfluorocarbon

Development of a compact NMR system to measure pO2 in a tissue-engineered graft

Cellular macroencapsulation devices, known as tissue engineered grafts (TEGs), enable the transplantation of allogeneic cells without the need for life-long systemic immunosuppression. Islet containing TEGs offer promise as a potential functional cure for type 1 diabetes. Previous research has indicated sustained functionality of implanted islets at high density in a TEG requires external supplementary oxygen delivery and an effective tool to monitor TEG oxygen levels. A proven oxygen-measurement approach employs a 19F oxygen probe molecule (a perfluorocarbon) implanted alongside therapeutic cells to enable oxygen- and temperature- dependent NMR relaxometry. Although the approach has proved effective, the clinical translation of 19F oxygen relaxometry for TEG monitoring will be limited by the current inaccessibility and high cost of MRI. Here, we report the development of an affordable, compact, and tabletop 19F NMR relaxometry system for monitoring TEG oxygenation. The system uses a 0.5 T Halbach magnet with a bore diameter (19 cm) capable of accommodating the human arm, a potential site of future TEG implantation. 19F NMR relaxometry was performed while controlling the temperature and oxygenation levels of a TEG using a custom-built perfusion setup. Despite the magnet's nonuniform field, a pulse sequence of broadband adiabatic full-passage pulses enabled accurate 19F longitudinal relaxation rate (R1) measurements in times as short as ∼2 min (R1 vs oxygen partial pressure and temperature (R2 > 0.98)). The estimated sensitivity of R1 to oxygen changes at 0.5 T was 1.62-fold larger than the sensitivity previously reported for 16.4 T. We conclude that TEG oxygenation monitoring with a compact, tabletop 19F NMR relaxometry system appears feasible.

Oxygen-Carrying Capacity of Perfluorohexyloctane, a Novel Eye Drop for Dry Eye Disease

Objective

One-hundred percent perfluorohexyloctane (PFHO) is a water-free, preservative-free eye drop approved by the Food and Drug Administration in the United States for the treatment of dry eye disease. PFHO has shown relief of dry eye signs and symptoms in clinical trials and has potent antievaporative action in vitro. The objective of this study was to measure the level of oxygen in PFHO.

Methods

T1 relaxation times (time taken for proton spins to translate from a random alignment to an alignment with the main magnetic field) for fluorine-19 in perfluorohexyloctane were measured using fluorine-19 nuclear magnetic resonance spectroscopy. The level of oxygen was interpolated from published data.

Results

The hydrogen-1 and fluorine-19 nuclear magnetic resonance spectra of PFHO were well resolved and the resonance assignments and intensities were as expected. The T1 values calculated for the CF₃ group resonance in the current study was 0.901 seconds and 1.12 seconds at 25 °C and 37 °C, respectively. The T1 values for the CF₂ group resonances increased by 17% to 24% with an increase in temperature from 25 °C to 37 °C. The mean (SD) partial pressure of oxygen in PFHO was calculated to be 257 (36) mm Hg and 270 (38) mm Hg at 25 °C and 37 °C, respectively.

Conclusions

The current study confirms that PFHO contains a significant amount of oxygen, more so than that calculated for tears in equilibrium with air. Once instilled on the eye, PFHO is not expected to be a barrier to the oxygen necessary for a healthy cornea and may in fact deliver nonreactive oxygen to the cornea to facilitate healing in patients with dry eye disease.

MR-oximetry with fat DESPOT

PURPOSE

The R₁ relaxation rate of fat is a promising marker of tissue oxygenation. Existing techniques to map fat R₁ in MR-oximetry offer limited spatial coverage, require long scan times, or pulse sequences that are not readily available on clinical scanners. This work addresses these limitations with a 3D voxel-wise fat R₁ mapping technique for MR-oximetry based on a variable flip angle (VFA) approach at 3 T.

METHODS

Varying levels of dissolved oxygen (O₂) were generated in a phantom consisting of vials of safflower oil emulsion, used to approximate human fat. Joint voxel-wise mapping of fat and water R₁ was performed with a two-compartment VFA model fitted to multi-echo gradient-echo magnitude data acquired at four flip angles, referred to as Fat DESPOT. Global R₁ was also calculated. Variations of fat, water, and global R₁ were investigated as a function of the partial pressure of O₂ (pO₂). Inversion-prepared stimulated echo magnetic resonance spectroscopy was used as the reference technique for R₁ measurements.

RESULTS

Fat R₁ from Fat DESPOT was more sensitive than water R₁ and global R₁ to variations in pO₂, consistent with previous studies performed with different R₁ mapping techniques. Fat R₁ sensitivity to pO₂ variations with Fat DESPOT (median O₂ relaxivity r_{1, O2} = 1.57× 10^-3 s^-1 mmHg^-1) was comparable to spectroscopy-based measurements for methylene, the main fat resonance (median r_{1, O2}= 1.80 × 10^-3 s^-1 mmHg^-1).

CONCLUSION

Fat and water R₁ can be measured on a voxel-wise basis using a two-component fit to multi-echo 3D VFA magnitude data in a clinically acceptable scan time. Fat and water R₁ measured with Fat DESPOT were sensitive to variations in pO₂. These observations suggest an approach to 3D in vivo MR oximetry.

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

Development of a Highly Responsive Organofluorine Temperature Sensor for 19F Magnetic Resonance Applications

Temperature can affect many biological and chemical processes within a body. During in vivo measurements, varied temperature can impact the accurate quantification of additional abiotic factors such as oxygen. During magnetic resonance imaging (MRI) measurements, the temperature of the sample can increase with the absorption of radiofrequency energy, which needs to be well-regulated for thermal therapies and long exposure. To address this potentially confounding effect, temperature can be probed intentionally using reporter molecules to determine the temperature in vivo. This work describes a combined experimental and computational approach for the design of fluorinated molecular temperature sensors with the potential to improve the accuracy and sensitivity of ¹⁹F MRI-based temperature monitoring. These fluorinated sensors are being developed to overcome the temperature sensitivity and tissue limitations of the proton resonance frequency (10 × 10^-3 ppm °C^-1), a standard parameter used for temperature mapping in MRI. Here, we develop (perfluoro-[1,1'-biphenyl]-4,4'-diyl)bis((heptadecafluorodecyl)sulfane), which has a nearly 2-fold increase in temperature responsiveness, compared to the proton resonance frequency and the ¹⁹F MRI temperature sensor perfluorotributylamine, when tested under identical NMR conditions. While ¹⁹F MRI is in the early stages of translation into clinical practice, development of alternative sensors with improved diagnostic abilities will help advance the development and incorporation of fluorine magnetic resonance techniques for clinical use.

Prognostic Value of Fluorine-19 MRI Oximetry Monitoring in cancer

Hypoxia is a key prognostic indicator in most solid tumors, as it is correlated to tumor angiogenesis, metastasis, recurrence, and response to therapy. Accurate measurement and mapping of tumor oxygenation profile and changes upon intervention could facilitate disease progression assessment and assist in treatment planning. Currently, no gold standard exists for non-invasive spatiotemporal measurement of hypoxia. Magnetic resonance imaging (MRI) represents an attractive option as it is a clinically available and non-ionizing imaging modality. Specifically, perfluorocarbon (PFC) beacons can be externally introduced into the tumor tissue and the linear dependence of their spin-lattice relaxation rate (R1) on the local partial pressure of oxygen (pO₂) exploited for real-time tissue oxygenation monitoring in vivo. In this review, we will focus on early studies and recent developments of fluorine-19 MRI and spectroscopy (MRS) for evaluation of tumor oximetry and response to therapy.

Investigation of the Post-Synthetic Confinement of Fluorous Liquids Inside Mesoporous Silica Nanoparticles

Perfluorocarbon (PFC) filled nanoparticles are increasingly being investigated for various biomedical applications. Common approaches for PFC liquid entrapment involve surfactant-based emulsification and Pickering emulsions. Alternatively, PFC liquids are capable of being entrapped inside hollow nanoparticles via a postsynthetic loading method (PSLM). While the methodology for the PSLM is straightforward, the effect each loading parameter has on the PFC entrapment has yet to be investigated. Previous work revealed incomplete filling of the hollow nanoparticles. Changing the loading parameters was expected to influence the ability of the PFC to fill the core of the nanoparticles. Hence, it would be possible to model the loading mechanism and determine the influence each factor has on PFC entrapment by tracking the change in loading yield and efficiency of PFC-filled nanoparticles. Herein, neat PFC liquid was loaded into silica nanoparticles and extracted into aqueous phases while varying the sonication time, concentration of nanoparticles, volume ratio between aqueous and fluorous phases, and pH of the extraction water. Loading yields and efficiency were determined via ¹⁹F nuclear magnetic resonance and N₂ physisorption isotherms. Sonication time was indicated to have the strongest correlation to loading yield and efficiency; however, method validation revealed that the current model does not fully explain the loading capabilities of the PSLM. Confounding variables and more finely controlled parameters need to be considered to better predict the behavior and loading capacity by the PSLM and warrants further study.

Assessing Oximetry Response to Chimeric Antigen Receptor T-cell Therapy against Glioma with 19F MRI in a Murine Model

Purpose

To assess the cell-specific, intracellular partial pressure of oxygen (Po₂) dynamics of both tumor and chimeric antigen receptor (CAR) T cells in a murine immunotherapy model.

Materials and Methods

Human glioblastoma cells or human T cells were intracellularly labeled with perfluorocarbon nanoemulsion droplet sensors prior to in vivo injection in severe combined immunodeficient mice to measure Po₂ in the two cell types in response to treatment. Two main sets of experiments were performed: (a) mice were injected in the flank with perfluorocarbon-labeled human glioblastoma cells and were then inoculated with either CAR T cells or untransduced T cells or were untreated 5 days after tumor inoculation; and (b) mice with unlabeled glioblastoma tumors were inoculated with perfluorocarbon-labeled CAR T cells or untransduced T cells 5 days after tumor inoculation. Longitudinal fluorine 19 (¹⁹F) spin-lattice relaxation time measurements of the tumor mass were used to ascertain absolute Po₂ in vivo. Results were analyzed for significance using an analysis of variance, a linear mixed-effect model, and a Pearson correlation coefficient test, as appropriate.

Results

The intracellular tumor cell Po₂ temporal dynamics exhibited delayed, transient hyperoxia at 3 days after infusion of CAR T cells, commensurate with significant tumor cell killing and CAR T-cell infiltration, as observed by bioluminescence imaging and histologic findings. Conversely, no significant changes were detected in CAR or untransduced T-cell intracellular Po₂ over time in tumor using these same methods. Moreover, it was observed that the total ¹⁹F tumor cell signal quenches with treatment, consistent with rapid tissue clearance of probe from apoptotic tumor cells.

Conclusion

Cell-specific Po₂ measurements using perfluorocarbon probes can provide insights into effector cell function and tumor response in cellular immunotherapeutic cancer models.Keywords: Animal Studies, MR-Imaging, MR-Spectroscopy, Molecular Imaging-Cancer, Molecular Imaging-Immunotherapy Supplemental material is available for this article. © RSNA, 2021See also commentary by Bulte in this issue.

Perfluorocarbons-Based 19F Magnetic Resonance Imaging in Biomedicine

Fluorine-19 (¹⁹F) magnetic resonance (MR) molecular imaging is a promising noninvasive and quantitative molecular imaging approach with intensive research due to the high sensitivity and low endogenous background signal of the ¹⁹F atom in vivo. Perfluorocarbons (PFCs) have been used as blood substitutes since 1970s. More recently, a variety of PFC nanoparticles have been designed for the detection and imaging of physiological and pathological changes. These molecular imaging probes have been developed to label cells, target specific epitopes in tumors, monitor the prognosis and therapy efficacy and quantitate characterization of tumors and changes in tumor microenvironment noninvasively, therefore, significantly improving the prognosis and therapy efficacy. Herein, we discuss the recent development and applications of ¹⁹F MR techniques with PFC nanoparticles in biomedicine, with particular emphasis on ligand-targeted and quantitative ¹⁹F MR imaging approaches for tumor detection, oxygenation measurement, smart stimulus response and therapy efficacy monitoring, et al.

Molecular oxygen loading in candidate theranostic droplets stabilized with biocompatible fluorinated surfactants: Particle size effect and application to in situ19F MRI mapping of oxygen partial pressure

OBJECTIVE

Perfluorocarbon nano- and micron-sized emulsions are a new field of investigation in cancer treatment due to their ability to be used as imaging contrast agents, or as delivery vectors for pharmaceuticals. They also demonstrated capability to enhance the efficiency of high intensity focused ultrasound thermo-therapy. In the context of new biomedical applications we investigated perfluorooctyl bromide (PFOB) theranostic droplets using ¹⁹F NMR. Each droplet contains biocompatible fluorinated surfactants composed of a polar Tris(hydroxymethyl)aminomethane head unit and hydrophobic perfluorinated tail (abbreviated as F-TAC). The influence of the droplet size on the oxygen loading capacity was determined from longitudinal relaxation (T₁) data of ¹⁹F NMR signal.

MATERIAL AND METHODS

Liquid PFOB and five samples of PFOB droplets of average diameter 0.177, 0.259, 1.43, 3.12 and 4.53 µm were tested with different oxygen levels. A dedicated gas exchange system was validated to maintain steady state oxygen concentrations, including a spatial gradient of oxygen concentration. A prototyped transmit-receive switchable ¹⁹F/¹H quadrature coil was integrated on a 3 T clinical scanner. The coil is compatible with focused ultrasound sonication for future application. A spectroscopy FID inversion-recovery (IR) sequence was used to measure the T₁ value per sample and per value of equilibrium oxygen pressure. Pixel wise, spatial T₁ mapping was performed with magnetization prepared 2D gradient echo sequences in tissue mimicking gels doped with theranostic droplets.

RESULTS

Experimental data indicated that the longitudinal relaxation rate of ¹⁹F signal of the investigated theranostic droplets depended approximately linearly on the oxygen level and its slope decreased with the particle size according to a second order polynomial over the investigated range. This semi-empirical model was derived from general thermodynamics and weak electrostatic forces theory and fitted the experimental data within 0.75% precision. The capacity of oxygen transportation for the described theranostic droplets tended to that of pure PFOB, while micron-sized droplets lost up to 50% of this capacity. In a specific setup producing a steady state gradient of oxygen concentration, we demonstrated spatial mapping of oxygen pressure gradient of 6 kPa/mm with 1 mm in-plane resolution.

CONCLUSION

The size-tunable PFOB theranostic droplets stabilized with F-TAC surfactants could be characterized by ¹⁹F MRI in a clinical setup readily compatible with interventional in vivo studies under MR guidance. Current precision and spatial resolution of T₁ mapping are promising. A potential challenge for further in vivo studies is the reduction of the imaging time.

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

PURPOSE

Spin-lattice relaxation rate (R₁ )-based 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 partial oxygen pressure (pO₂ ) was assessed using single-point imaging mode of spatial encoding combined with rapid repetition, similar to T₁ -weighted MRI, for which R₁ was determined from 22 repetition times ranging from 2.1 to 40.0 μs at 300 MHz. The pO₂ maps of a phantom with 3 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 SDs 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 3 repetition times ranging from 4.0 to 12.0 μs.

CONCLUSION

Using the single-point imaging modality, R₁ -based oximetry imaging with useful spatial and oxygen resolutions for small animals was demonstrated.

Oxygenation Imaging by Nuclear Magnetic Resonance Methods

Oxygen monitoring is a topic of exhaustive research due to its central role in many biological processes, from energy metabolism to gene regulation. The ability to monitor in vivo the physiological distribution and the dynamics of oxygen from subcellular to macroscopic levels is a prerequisite to better understand the mechanisms associated with both normal and disease states (cancer, neurodegeneration, stroke, etc.). This chapter focuses on magnetic resonance imaging (MRI) based techniques to assess oxygenation in vivo. The first methodology uses injected fluorinated agents to provide quantitative pO₂ measurements with high precision and suitable spatial and temporal resolution for many applications. The second method exploits changes in endogenous contrasts, i.e., deoxyhemoglobin and oxygen molecules through measurements of T ₂* and T ₁, in response to an intervention to qualitatively evaluate hypoxia and its potential modulation.

Extending the Scope of 19F Hyperpolarization through Signal Amplification by Reversible Exchange in MRI and NMR Spectroscopy

Fluorinated ligands have a variety of uses in chemistry and industry, but it is their medical applications as ¹⁸F-labelled positron emission tomography (PET) tracers where they are most visible. In this work, we illustrate the potential of using ¹⁹F-containing ligands as future magnetic resonance imaging (MRI) contrast agents and as probes in magnetic resonance spectroscopy studies by significantly increasing their magnetic resonance detectability through the signal amplification by reversible exchange (SABRE) hyperpolarization method. We achieve ¹⁹F SABRE polarization in a wide range of molecules, including those essential to medication, and analyze how their steric bulk, the substrate loading, polarization transfer field, pH, and rate of ligand exchange impact the efficiency of SABRE. We conclude by presenting ¹⁹F MRI results in phantoms, which demonstrate that many of these agents show great promise as future ¹⁹F MRI contrast agents for diagnostic investigations.

Oxygen Sensing with Perfluorocarbon-Loaded Ultraporous Mesostructured Silica Nanoparticles

Oxygen homeostasis is important in the regulation of biological function. Disease progression can be monitored by measuring oxygen levels, thus producing information for the design of therapeutic treatments. Noninvasive measurements of tissue oxygenation require the development of tools with minimal adverse effects and facile detection of features of interest. Fluorine magnetic resonance imaging (¹⁹F MRI) exploits the intrinsic properties of perfluorocarbon (PFC) liquids for anatomical imaging, cell tracking, and oxygen sensing. However, the highly hydrophobic and lipophobic properties of perfluorocarbons require the formation of emulsions for biological studies, though stabilizing these emulsions has been challenging. To enhance the stability and biological loading of perfluorocarbons, one option is to incorporate perfluorocarbon liquids into the internal space of biocompatible mesoporous silica nanoparticles. Here, we developed perfluorocarbon-loaded ultraporous mesostructured silica nanoparticles (PERFUMNs) as ¹⁹F MRI detectable oxygen-sensing probes. Ultraporous mesostructured silica nanoparticles (UMNs) have large internal cavities (average = 1.8 cm³ g^-1), facilitating an average 17% loading efficiency of PFCs, meeting the threshold fluorine concentrations needed for imaging studies. Perfluoro-15-crown-5-ether PERFUMNs have the highest equivalent nuclei per PFC molecule and a spin-lattice (T₁) relaxation-based oxygen sensitivity of 0.0032 mmHg^-1 s^-1 at 16.4 T. The option of loading PFCs after synthesizing UMNs, rather than traditional in situ core-shell syntheses, allows for use of a broad range of PFC liquids from a single material. The biocompatible and tunable chemistry of UMNs combined with the intrinsic properties of PFCs makes PERFUMNs a MRI sensor with potential for anatomical imaging, cell tracking, and metabolic spectroscopy with improved stability.

Theranostic Probes for Targeting Tumor Microenvironment: An Overview

Long gone is the time when tumors were thought to be insular masses of cells, residing independently at specific sites in an organ. Now, researchers gradually realize that tumors interact with the extracellular matrix (ECM), blood vessels, connective tissues, and immune cells in their environment, which is now known as the tumor microenvironment (TME). It has been found that the interactions between tumors and their surrounds promote tumor growth, invasion, and metastasis. The dynamics and diversity of TME cause the tumors to be heterogeneous and thus pose a challenge for cancer diagnosis, drug design, and therapy. As TME is significant in enhancing tumor progression, it is vital to identify the different components in the TME such as tumor vasculature, ECM, stromal cells, and the lymphatic system. This review explores how these significant factors in the TME, supply tumors with the required growth factors and signaling molecules to proliferate, invade, and metastasize. We also examine the development of TME-targeted nanotheranostics over the recent years for cancer therapy, diagnosis, and anticancer drug delivery systems. This review further discusses the limitations and future perspective of nanoparticle based theranostics when used in combination with current imaging modalities like Optical Imaging, Magnetic Resonance Imaging (MRI) and Nuclear Imaging (Positron Emission Tomography (PET) and Single Photon Emission Computer Tomography (SPECT)).

Molecular mechanisms of hypoxia in cancer

PURPOSE

Hypoxia is a condition of insufficient oxygen to support metabolism which occurs when the vascular supply is interrupted, or when a tumour outgrows its vascular supply. It is a negative prognostic factor due to its association with an aggressive tumour phenotype and therapeutic resistance. This review provides an overview of hypoxia imaging with Positron emission tomography (PET), with an emphasis on the biological relevance, mechanism of action, highlighting advantages, and limitations of the currently available hypoxia radiotracers.

METHODS

A comprehensive PubMed literature search was performed, identifying articles relating to biological significance and measurement of hypoxia, MRI methods, and PET imaging of hypoxia in preclinical and clinical settings, up to December 2016.

RESULTS

A variety of approaches have been explored over the years for detecting and monitoring changes in tumour hypoxia, including regional measurements with oxygen electrodes placed under CT guidance, MRI methods that measure either oxygenation or lactate production consequent to hypoxia, different nuclear medicine approaches that utilise imaging agents the accumulation of which is inversely related to oxygen tension, and optical methods. The advantages and disadvantages of these approaches are reviewed, along with individual strategies for validating different imaging methods. PET is the preferred method for imaging tumour hypoxia due to its high specificity and sensitivity to probe physiological processes in vivo, as well as the ability to provide information about intracellular oxygenation levels.

CONCLUSION

Even though hypoxia could have significant prognostic and predictive value in the clinic, the best method for hypoxia assessment has in our opinion not been realised.

Development and Validation of Noninvasive Magnetic Resonance Relaxometry for the In Vivo Assessment of Tissue-Engineered Graft Oxygenation

Techniques to monitor the oxygen partial pressure (pO2) within implanted tissue-engineered grafts (TEGs) are critically necessary for TEG development, but current methods are invasive and inaccurate. In this study, we developed an accurate and noninvasive technique to monitor TEG pO2 utilizing proton (1H) or fluorine (19F) magnetic resonance spectroscopy (MRS) relaxometry. The value of the spin-lattice relaxation rate constant (R1) of some biocompatible compounds is sensitive to dissolved oxygen (and temperature), while insensitive to other external factors. Through this physical mechanism, MRS can measure the pO2 of implanted TEGs. We evaluated six potential MRS pO2 probes and measured their oxygen and temperature sensitivities and their intrinsic R1 values at 16.4 T. Acellular TEGs were constructed by emulsifying porcine plasma with perfluoro-15-crown-5-ether, injecting the emulsion into a macroencapsulation device, and cross-linking the plasma with a thrombin solution. A multiparametric calibration equation containing R1, pO2, and temperature was empirically generated from MRS data and validated with fiber optic (FO) probes in vitro. TEGs were then implanted in a dorsal subcutaneous pocket in a murine model and evaluated with MRS up to 29 days postimplantation. R1 measurements from the TEGs were converted to pO2 values using the established calibration equation and these in vivo pO2 measurements were simultaneously validated with FO probes. Additionally, MRS was used to detect increased pO2 within implanted TEGs that received supplemental oxygen delivery. Finally, based on a comparison of our MRS data with previously reported data, ultra-high-field (16.4 T) is shown to have an advantage for measuring hypoxia with 19F MRS. Results from this study show MRS relaxometry to be a precise, accurate, and noninvasive technique to monitor TEG pO2 in vitro and in vivo.

In Vivo Chemiluminescent Imaging Agents for Nitroreductase and Tissue Oxygenation

Tissue oxygenation is a driving parameter of the tumor microenvironment, and hypoxia can be a prognostic indicator of aggressiveness, metastasis, and poor response to therapy. Here, we report a chemiluminescence imaging (CLI) agent based on the oxygen-dependent reduction of a nitroaromatic spiroadamantane 1,2-dioxetane scaffold. Hypoxia ChemiLuminescent Probe 2 (HyCL-2) responds to nitroreductase with ∼170-fold increase in luminescence intensity and high selectivity for enzymatic reductase versus other small molecule reductants. HyCL-2 can image exogenous nitroreductase in vitro and in vivo in living mice, and total luminescent intensity is increased by ∼5-fold under low oxygen conditions. HyCL-2 is demonstrated to report on tumor oxygenation during an oxygen challenge in H1299 lung tumor xenografts grown in a murine model as independently confirmed using multispectral optoacoustic tomography (MSOT) imaging of hemoglobin oxygenation.

19F Oximetry with semifluorinated alkanes

This work examines the variation of longitudinal relaxation rate R1(= 1/T1) of the ¹⁹F-CF₃-resonance of semifluorinated alkanes (SFAs) with oxygen tension (pO₂), temperature (T) and pH in vitro. Contrary to their related perfluorocarbons (PFCs), SFA are amphiphilic and facilitate stable emulsions, a prerequisite for clinical use. A linear relationship between R1 and pO₂ was confirmed for the observed SFAs at different temperatures. Using a standard saturation recovery sequence, T1 has been successfully measured using fluorine ¹⁹F-MRI with a self-constructed birdcage resonator at 9.4 T. A calibration curve to calculate pO₂ depending on T and R1 was found for each SFA used. In contrast to the commonly used PFC, SFAs are less sensitive to changes in pO₂, but more sensitive to changes in temperature. The influence of pH to R1 was found to be negligible.

Molecular imaging of the tumor microenvironment for precision medicine and theranostics

Morbidity and mortality from cancer and their associated conditions and treatments continue to extract a heavy social and economic global burden despite the transformative advances in science and technology in the twenty-first century. In fact, cancer incidence and mortality are expected to reach pandemic proportions by 2025, and costs of managing cancer will escalate to trillions of dollars. The inability to establish effective cancer treatments arises from the complexity of conditions that exist within tumors, the plasticity and adaptability of cancer cells coupled with their ability to escape immune surveillance, and the co-opted stromal cells and microenvironment that assist cancer cells in survival. Stromal cells, although destroyed together with cancer cells, have an ever-replenishing source that can assist in resurrecting tumors from any residual cancer cells that may survive treatment. The tumor microenvironment landscape is a continually changing landscape, with spatial and temporal heterogeneities that impact and influence cancer treatment outcome. Importantly, the changing landscape of the tumor microenvironment can be exploited for precision medicine and theranostics. Molecular and functional imaging can play important roles in shaping and selecting treatments to match this landscape. Our purpose in this review is to examine the roles of molecular and functional imaging, within the context of the tumor microenvironment, and the feasibility of their applications for precision medicine and theranostics in humans.

Perfluorocarbon nanoparticles for physiological and molecular imaging and therapy

Herein, we review the use of non-nephrotoxic perfluorocarbon nanoparticles (PFC NPs) for noninvasive detection and therapy of kidney diseases, and we provide a synopsis of other related literature pertinent to their anticipated clinical application. Recent reports indicate that PFC NPs allow for quantitative mapping of kidney perfusion and oxygenation after ischemia-reperfusion injury with the use of a novel multinuclear (1)H/(19)F magnetic resonance imaging approach. Furthermore, when conjugated with targeting ligands, the functionalized PFC NPs offer unique and quantitative capabilities for imaging inflammation in the kidney of atherosclerotic ApoE-null mice. In addition, PFC NPs can facilitate drug delivery for treatment of inflammation, thrombosis, and angiogenesis in selected conditions that are comorbidities for kidney failure. The excellent safety profile of PFC NPs with respect to kidney injury positions these nanomedicine approaches as promising diagnostic and therapeutic candidates for treating and following acute and chronic kidney diseases.

Visualization of immune cell infiltration in experimental viral myocarditis by (19)F MRI in vivo

OBJECTIVE

This paper introduces a new approach permitting for the first time a specific, non-invasive diagnosis of myocarditis by visualizing the infiltration of immune cells into the myocardium.

MATERIALS AND METHODS

The feasibility of this approach is shown in a murine model of viral myocarditis. Our study uses biochemically inert perfluorocarbons (PFCs) known to be taken up by circulating monocytes/macrophages after intravenous injection.

RESULTS

In vivo (19)F MRI at 9.4 T demonstrated that PFC-loaded immune cells infiltrate into inflamed myocardial areas. Because of the lack of any fluorine background in the body, detected (19)F signals of PFCs are highly specific as confirmed ex vivo by flow cytometry and histology.

CONCLUSION

Since PFCs are a family of compounds previously used clinically as blood substitutes, the technique described in our paper holds the potential as a new imaging modality for the diagnosis of myocarditis in man.

MR - eyes for cancer: looking within an impenetrable disease

Probe development is a critical component in cancer imaging, and novel probes are making major inroads in several aspects of cancer detection and image-guided treatments. Intrinsic MR probes such as signals from metabolites and their chemical shifts have been used for more than a decade to understand cancer physiology and metabolism. Through the integration of technology, molecular biology, and chemistry, the last few years have witnessed an explosion of extrinsic probes for molecular and functional imaging of cancer that, together with techniques such as CEST and hyperpolarization, have significantly expanded the repertoire of MR techniques in basic and translational investigations of many different aspects of cancer. Furthermore, incorporation of MR probes into multifunctional nanoparticles and multimodality imaging platforms have opened new opportunities for MR in image-guided diagnosis and therapy of cancer. Here we have provided an overview of recent innovations that have occurred in the development of MRI probes for molecular and functional imaging of cancer. Although most of these novel probes are not clinically available, they offer significant promise for future translational applications. In this review, we have highlighted the areas of future development that are likely to have a profound impact on cancer detection and treatment.

In vivo MRI cell tracking using perfluorocarbon probes and fluorine-19 detection

This article presents a brief review of preclinical in vivo cell-tracking methods and applications using perfluorocarbon (PFC) probes and fluorine-19 ((19) F) MRI detection. Detection of the (19) F signal offers high cell specificity and quantification ability in spin density-weighted MR images. We discuss the compositions of matter, methods and applications of PFC-based cell tracking using ex vivo and in situ PFC labeling in preclinical studies of inflammation and cellular therapeutics. We also address the potential applicability of (19) F cell tracking to clinical trials.

In vivo intracellular oxygen dynamics in murine brain glioma and immunotherapeutic response of cytotoxic T cells observed by fluorine-19 magnetic resonance imaging

Noninvasive biomarkers of anti-tumoral efficacy are of great importance to the development of therapeutic agents. Tumor oxygenation has been shown to be an important indicator of therapeutic response. We report the use of intracellular labeling of tumor cells with perfluorocarbon (PFC) molecules, combined with quantitative ¹⁹F spin-lattice relaxation rate (R₁) measurements, to assay tumor cell oxygen dynamics in situ. In a murine central nervous system (CNS) GL261 glioma model, we visualized the impact of Pmel-1 cytotoxic T cell immunotherapy, delivered intravenously, on intracellular tumor oxygen levels. GL261 glioma cells were labeled ex vivo with PFC and inoculated into the mouse striatum. The R₁ of ¹⁹F labeled cells was measured using localized single-voxel magnetic resonance spectroscopy, and the absolute intracellular partial pressure of oxygen (pO₂) was ascertained. Three days after tumor implantation, mice were treated with 2×10⁷ cytotoxic T cells intravenously. At day five, a transient spike in pO₂ was observed indicating an influx of T cells into the CNS and putative tumor cell apoptosis. Immunohistochemistry and quantitative flow cytometry analysis confirmed that the pO₂ was causally related to the T cells infiltration. Surprisingly, the pO₂ spike was detected even though few (∼4×10⁴) T cells actually ingress into the CNS and with minimal tumor shrinkage. These results indicate the high sensitivity of this approach and its utility as a non-invasive surrogate biomarker of anti-cancer immunotherapeutic response in preclinical models.

Rapid quantification of oxygen tension in blood flow with a fluorine nanoparticle reporter and a novel blood flow-enhanced-saturation-recovery sequence

We present a novel blood flow-enhanced-saturation-recovery (BESR) sequence, which allows rapid in vivo T1 measurement of blood for both (1)H and (19)F nuclei. BESR sequence is achieved by combining homogeneous spin preparation and time-of-flight image acquisition and therefore preserves high time efficiency and signal-to-noise ratio for (19)F imaging of circulating perfluorocarbon nanoparticles comprising a perfluoro-15-crown-5-ether core and a lipid monolayer (nominal size = 250 nm). The consistency and accuracy of the BESR sequence for measuring T1 of blood was validated experimentally. With a confirmed linear response feature of (19)F R1 with oxygen tension in both salt solution and blood sample, we demonstrated the feasibility of the BESR sequence to quantitatively determine the oxygen tension within mouse left and right ventricles under both normoxia and hyperoxia conditions. Thus, (19)F BESR MRI of circulating perfluorocarbon nanoparticles represents a new approach to noninvasively evaluate intravascular oxygen tension.

Quantitative tissue oxygen measurement in multiple organs using 19F MRI in a rat model

Measurement of individual organ tissue oxygen levels can provide information to help evaluate and optimize medical interventions in many areas including wound healing, resuscitation strategies, and cancer therapeutics. Echo planar (19) F MRI has previously focused on tumor oxygen measurement at low oxygen levels (pO(2)) <30 mmHg. It uses the linear relationship between spin-lattice relaxation rate (R(1)) of hexafluorobenzene (HFB) and pO(2). The feasibility of this technique for a wider range of pO(2) values and individual organ tissue pO(2) measurement was investigated in a rat model. Spin-lattice relaxation times (T(1) = 1/R(1)) of hexafluorobenzene were measured using (19) F saturation recovery echo planar imaging. Initial in vitro studies validated the linear relationship between R(1) and pO(2) from 0 to 760 mmHg oxygen partial pressure at 25, 37, and 41°C at 7 Tesla for hexafluorobenzene. In vivo experiments measured rat tissue oxygen (ptO2) levels of brain, kidney, liver, gut, muscle, and skin during inhalation of both 30 and 100% oxygen. All organ ptO(2) values significantly increased with hyperoxia (P < 0.001). This study demonstrates that (19) F MRI of hexafluorobenzene offers a feasible tool to measure regional ptO2 in vivo, and that hyperoxia significantly increases ptO2 of multiple organs in a rat model.

Simultaneous dual-nuclei imaging for motion corrected detection and quantification of 19F imaging agents

Fluorine MRI offers broad potential for specific detection and quantification of molecularly targeted agents in diagnosis and therapy planning or monitoring. Because non-proton MRI applications lack morphological information, accompanying proton images are needed to elucidate the spatial tissue context. Furthermore, low concentrations typical of targeted molecular imaging agents require long examinations for signal averaging during which physiological motion may lead to blurring, underestimation in signal quantification, and erroneous localization of the agent distribution. Novel methods for truly simultaneous acquisition of dual-nuclei MR data are presented that offer efficient and precise anatomical localization of fluorine signals using accurate motion correction based on contemporaneous proton signals. The feasibility of simultaneous dual-nuclei MRI motion correction and corresponding dual-resolution reconstruction, providing nuclei-specific spatial resolution to retrospectively optimize the balance between signal-to-noise ratio and resolution, is shown on a clinical 3 T MR system.

Imaging tumor hypoxia by magnetic resonance methods

Tumor hypoxia results from the negative balance between the oxygen demands of the tissue and the capacity of the neovasculature to deliver sufficient oxygen. The resulting oxygen deficit has important consequences with regard to the aggressiveness and malignancy of tumors, as well as their resistance to therapy, endowing the imaging of hypoxia with vital repercussions in tumor prognosis and therapy design. The molecular and cellular events underlying hypoxia are mediated mainly through hypoxia-inducible factor, a transcription factor with pleiotropic effects over a variety of cellular processes, including oncologic transformation, invasion and metastasis. However, few methodologies have been able to monitor noninvasively the oxygen tensions in vivo. MRI and MRS are often used for this purpose. Most MRI approaches are based on the effects of the local oxygen tension on: (i) the relaxation times of (19)F or (1)H indicators, such as perfluorocarbons or their (1)H analogs; (ii) the hemodynamics and magnetic susceptibility effects of oxy- and deoxyhemoglobin; and (iii) the effects of paramagnetic oxygen on the relaxation times of tissue water. (19)F MRS approaches monitor tumor hypoxia through the selective accumulation of reduced nitroimidazole derivatives in hypoxic zones, whereas electron spin resonance methods determine the oxygen level through its influence on the linewidths of appropriate paramagnetic probes in vivo. Finally, Overhauser-enhanced MRI combines the sensitivity of EPR methodology with the resolution of MRI, providing a window into the future use of hyperpolarized oxygen probes.

Quantitative cardiovascular magnetic resonance for molecular imaging

Cardiovascular magnetic resonance (CMR) molecular imaging aims to identify and map the expression of important biomarkers on a cellular scale utilizing contrast agents that are specifically targeted to the biochemical signatures of disease and are capable of generating sufficient image contrast. In some cases, the contrast agents may be designed to carry a drug payload or to be sensitive to important physiological factors, such as pH, temperature or oxygenation. In this review, examples will be presented that utilize a number of different molecular imaging quantification techniques, including measuring signal changes, calculating the area of contrast enhancement, mapping relaxation time changes or direct detection of contrast agents through multi-nuclear imaging or spectroscopy. The clinical application of CMR molecular imaging could offer far reaching benefits to patient populations, including early detection of therapeutic response, localizing ruptured atherosclerotic plaques, stratifying patients based on biochemical disease markers, tissue-specific drug delivery, confirmation and quantification of end-organ drug uptake, and noninvasive monitoring of disease recurrence. Eventually, such agents may play a leading role in reducing the human burden of cardiovascular disease, by providing early diagnosis, noninvasive monitoring and effective therapy with reduced side effects.

In vivo observation of intracellular oximetry in perfluorocarbon-labeled glioma cells and chemotherapeutic response in the CNS using fluorine-19 MRI

Preclinical development of therapeutic agents against cancer could greatly benefit from noninvasive markers of tumor killing. Potentially, the intracellular partial pressure of oxygen (pO(2) ) can be used as an early marker of antitumor efficacy. Here, the feasibility of measuring intracellular pO(2) of central nervous system glioma cells in vivo using (19) F magnetic resonance techniques is examined. Rat 9L glioma cells were labeled with perfluoro-15-crown-5-ether ex vivo and then implanted into the rat striatum. (19) F MRI was used to visualize tumor location in vivo. The mean (19) F T(1) of the implanted cells was measured using localized, single-voxel spectroscopy. The intracellular pO(2) in tumor cells was determined from an in vitro calibration curve. The basal pO(2) of 9L cells (day 3) was determined to be 45.3 ± 5 mmHg (n = 6). Rats were then treated with a 1 × LD10 dose of bischloroethylnitrosourea intravenously and changes in intracellular pO(2) were monitored. The pO(2) increased significantly (P = 0.042, paired T-test) to 141.8 ± 3 mmHg within 18 h after bischloroethylnitrosourea treatment (day 4) and remained elevated (165 ± 24 mmHg) for at least 72 h (day 6). Intracellular localization of the perfluoro-15-crown-5-ether emulsion in 9L cells before and after bischloroethylnitrosourea treatment was confirmed by histological examination and fluorescence microscopy. Overall, noninvasive (19) F magnetic resonance techniques may provide a valuable preclinical tool for monitoring therapeutic response against central nervous system or other deep-seated tumors.

An oxygen-consuming phantom simulating perfused tissue to explore oxygen dynamics and (19)F MRI oximetry

OBJECTIVE

This study presents a reproducible phantom which mimics oxygen-consuming tissue and can be used for the validation of (19)F MRI oximetry.

MATERIALS AND METHODS

The phantom consists of a haemodialysis filter of which the outer compartment is filled with a gelatin matrix containing viable yeast cells. Perfluorocarbon emulsions can be added to the gelatin matrix to simulate sequestered perfluorocarbons. A blood-substituting perfluorocarbon fluid is pumped through the lumen of the fibres in the filter. (19)F relaxometry MRI is performed with a fast 2D Look-Locker imaging sequence on a clinical 3T scanner.

RESULTS

Acute and perfusion-related hypoxia were simulated and imaged spatially and temporally using the phantom.

CONCLUSIONS

The presented experimental setup can be used to simulate oxygen consumption by somatic cells in vivo and for validating computational biophysical models of hypoxia, as measured with (19)F MRI oximetry.

Liquid perfluorocarbons as contrast agents for ultrasonography and (19)F-MRI

Perfluorocarbons (PFCs) are fluorinated compounds that have been used for many years in clinics mainly as gas/oxygen carriers and for liquid ventilation. Besides this main application, PFCs have also been tested as contrast agents for ultrasonography and magnetic resonance imaging since the end of the 1970s. However, most of the PFCs applied as contrast agents for imaging were gaseous. This class of PFCs has been recently substituted by liquid PFCs as ultrasound contrast agents. Additionally, liquid PFCs are being tested as contrast agents for (19)F magnetic resonance imaging (MRI), to yield dual contrast agents for both ultrasonography and (19)F MRI. This review focuses on the development and applications of the different contrast agents containing liquid perfluorocarbons for ultrasonography and/or MRI: large and small size emulsions (i.e. nanoemulsions) and nanocapsules.

Pre-clinical PET/MR: technological advances and new perspectives in biomedical research

INTRODUCTION

Combined PET/MRI allows for multi-parametric imaging and reveals one or more functional processes simultaneously along with high-resolution morphology. Especially in small-animal research, where high soft tissue contrast is required, and the scan time as well as radiation dose are critical factors, the combination of PET and MRI would be beneficial compared with PET/CT.

DEVELOPMENT

In the mid-1990's, several research groups used different approaches to integrate PET detectors into high-field MRI. First, systems were based on optical fibres guiding the scintillation light to the PMT's, which reside outside the fringe magnetic field. Recent advances in gamma ray detector technology, which were initiated mainly by the advent of avalanche photodiodes (APD's) as well as the routine availability of fast scintillation materials like lutetium oxyorthosilicate (LSO), paved the way towards the development of fully magnetic-field-insensitive high-performance PET detectors.

TECHNOLOGY

Current animal PET/MR technologies are reviewed and pitfalls when engineering a full integration of a PET and a high-field MRI are discussed. Compact PET detectors can be integrated in small-bore, high-field MRI tomographs. Detailed performance evaluations have shown that the mutual interference between the two imaging systems could be minimized. The performance of all major MR applications, ranging from T1- or T2-weighted imaging up to echo-planar imaging (EPI) for functional MRI (fMRI) or magnetic resonance spectroscopy (MRS), could be maintained, even when the PET insert was built into the MRI and acquiring PET data simultaneously. Similarly, the PET system performance was not influenced by the static magnetic field or applied MRI sequences.

APPLICATIONS

Initial biomedical research applications range from the combination of functional information from PET with the anatomical information from the MRI to multi-functional imaging combining metabollic PET and MRI data.

DISCUSSION

Compared to other multi-modality approaches PET/MR offers a multitude of complementary function and anatomical information. The ability to obtain simultaneous PET and MRI data with this new imaging modality could have tremendous impact on small animal imaging research.

Fluorocarbon nanoparticles as multifunctional drug delivery vehicles

The history and current status of fluorocarbon nanoparticles in biomedicine is briefly reviewed. The deficiencies of current fluorocarbon nanoparticle formulations are highlighted. Strategies to remedy such deficiencies and to functionalize fluorocarbon nanoparticles are presented. Potential applications of fluorocarbon nanoparticles as multifunctional drug delivery vehicles are discussed. The strength of fluorocarbon nanoparticles as drug delivery vehicles is that they integrate drug delivery with non-invasive MR imaging so that the biodistribution of the pharmaceutical entity (drug + delivery vehicle) can be monitored in real time. This, in turn, permits the physician to adjust treatment plan for each patient based on his/her actual response to the ongoing treatment.

Molecular imaging of hypoxia

Hypoxia, a condition of insufficient O2 to support metabolism, occurs when the vascular supply is interrupted, as in stroke or myocardial infarction, or when a tumor outgrows its vascular supply. When otherwise healthy tissues lose their O2 supply acutely, the cells usually die, whereas when cells gradually become hypoxic, they adapt by up-regulating the production of numerous proteins that promote their survival. These proteins slow the rate of growth, switch the mitochondria to glycolysis, stimulate growth of new vasculature, inhibit apoptosis, and promote metastatic spread. The consequence of these changes is that patients with hypoxic tumors invariably experience poor outcome to treatment. This has led the molecular imaging community to develop assays for hypoxia in patients, including regional measurements from O2 electrodes placed under CT guidance, several nuclear medicine approaches with imaging agents that accumulate with an inverse relationship to O2, MRI methods that measure either oxygenation directly or lactate production as a consequence of hypoxia, and optical methods with NIR and bioluminescence. The advantages and disadvantages of these approaches are reviewed, along with the individual strategies for validating different imaging methods. Ultimately the proof of value is in the clinical performance to predict outcome, select an appropriate cohort of patients to benefit from a hypoxia-directed treatment, or plan radiation fields that result in better local control. Hypoxia imaging in support of molecular medicine has become an important success story over the last decade and provides a model and some important lessons for development of new molecular imaging probes or techniques.

A Versatile High Speed 250 MHz Pulse Imager for Biomedical Applications

A versatile 250 MHz pulse electron paramagnetic resonance (EPR) instrument for imaging of small animals is presented. Flexible design of the imager hardware and software makes it possible to use virtually any pulse EPR imaging modality. A fast pulse generation and data acquisition system based on general purpose PCI boards performs measurements with minimal additional delays. Careful design of receiver protection circuitry allowed us to achieve very high sensitivity of the instrument. In this article we demonstrate the ability of the instrument to obtain three dimensional images using the electron spin echo (ESE) and single point imaging (SPI) methods. In a phantom that contains a 1 mM solution of narrow line (16 μT, peak-to-peak) paramagnetic spin probe we achieved an acquisition time of 32 seconds per image with a fast 3D ESE imaging protocol. Using an 18 minute 3D phase relaxation (T(2e)) ESE imaging protocol in a homogeneous sample a spatial resolution of 1.4 mm and a standard deviation of T(2e) of 8.5% were achieved. When applied to in vivo imaging this precision of T(2e) determination would be equivalent to 2 torr resolution of oxygen partial pressure in animal tissues.

Basics concepts and clinical applications of oxygen-enhanced MR imaging

Oxygen-enhanced MR imaging is a new technique, and its physiological significance has not yet been fully elucidated. This review article covers (1) the theory of oxygen enhancement and its relationship with respiratory physiology; (2) design for oxygen-enhanced MR imaging sequencing; (3) a basic study of oxygen-enhanced MR imaging in animal models and humans; (4) a clinical study of oxygen-enhanced MR imaging; and (5) a comparison of advantages and disadvantages of this technique with those of hyperpolarized noble gas MR ventilation imaging. Oxygen-enhanced MR imaging provides not only the ventilation-related, but also respiration-related information. Oxygen-enhanced MR imaging has the potential to replace nuclear medicine studies for the identification of regional pulmonary function, and many investigators are now attempting to adapt this technique for routine clinical studies. We believe that further basic studies as well as clinical applications of this new technique will define the real significance of oxygen-enhanced MR imaging for the future of pulmonary functional imaging and its usefulness for diagnostic radiology and pulmonary medicine.

Electron Paramagnetic Resonance Oxygen Images Correlate Spatially and Quantitatively with Oxylite Oxygen Measurements

Tumor oxygenation predicts cancer therapy response and malignant phenotype. This has spawned a number of oxymetries. Comparison of different oxymetries is crucial for the validation and understanding of these techniques. Electron paramagnetic resonance (EPR) imaging is a novel technique for providing quantitative high-resolution images of tumor and tissue oxygenation. This work compares sequences of tumor pO2 values from EPR oxygen images with sequences of oxygen measurements made along a track with an Oxylite oxygen probe. Four-dimensional (three spatial and one spectral) EPR oxygen images used spectroscopic imaging techniques to measure the width of a spectral line in each image voxel from a trityl spin probe (OX063, Amersham Health R&D) in the tissues and tumor of mice after spin probe injection. A simple calibration allows direct, quantitative translation of each line width to an oxygen concentration. These four-dimensional EPR images, obtained in 45 minutes from FSa fibrosarcomas grown in the legs of C3H mice, have a spatial resolution of approximately 1 mm and oxygen resolution of approximately 3 Torr. The position of the Oxylite track was measured within a 2-mm accuracy using a custom stereotactic positioning device. A total of nine images that involve 17 tracks were obtained. Of these, most showed good correlation between the Oxylite measured pO2 and a track located in the tumor within the uncertainties of the Oxylite localizability. The correlation was good both in terms of spatial distribution pattern and pO2 magnitude. The strong correlation of the two modalities corroborates EPR imaging as a useful tool for the study of tumor oxygenation.

Clinical oxygen-enhanced magnetic resonance imaging of the lung

SUMMARY

Oxygen-enhanced magnetic resonance (MR) ventilation imaging is a new technique, and the full extent of its physiologic significance has not been elucidated. This review article includes (1) theory of oxygen enhancement; (2) respiratory physiology; (3) oxygen-enhanced MR imaging (MRI) sequence design; (4) basic study of oxygen-enhanced MRI in animal models and humans; (5) clinical study of oxygen-enhanced MRI; and (6) merits and demerits of the technique in comparison with hyperpolarized noble gas MR ventilation imaging. Oxygen-enhanced MRI provides not only ventilation-related information but also respiration-related information. Although application of oxygen-enhanced MR ventilation imaging to patients with pulmonary diseases has been limited, oxygen-enhanced MRI offers the possibility of demonstrating regional pulmonary function and substituting for nuclear medicine ventilation-perfusion study, when combined with MR perfusion imaging. We believe that further basic studies and clinical applications of this new technique will define the real significance of oxygen-enhanced MR ventilation imaging in the future of pulmonary functional imaging and its usefulness for diagnostic radiology.

Dynamic breast tumor oximetry: the development of prognostic radiology

A novel pre clinical approach to evaluating tumor oxygen dynamics was recently introduced (Am. J. Clin. Oncol. 24, 462-466 (2001)). FREDOM (Fluorocarbon Relaxometry using Echo planar imaging for Dynamic Oxygen Mapping) allows maps of tumor pO(2) including 50 - 150 individual locations simultaneously to be produced with typical in plane resolution of 1.25 mm in 6.5 mins. The technique has been applied extensively in rat prostate tumors and is now demonstrated in the rat breast 13762NF adenocarcinoma. When anesthetized rats breathed 33% oxygen, mean baseline pO(2) was in the range 17 +/- 2 (se) torr to 74 +/- 4 torr with mean value for nine tumors 46 +/- 8 torr. However, small tumors (< 2.2 cm(3)) were significantly better oxygenated with mean pO(2) = 63 +/- 7 torr than large tumors (> 2.4 cm(3)) with mean pO(2) 24 +/- 5 torr (p < 0.002). Switching the inhaled gas to oxygen or carbogen produced a significant and rapid increase in mean pO(2) for both small and larger tumors (p < 0.05). Given the increasing evidence that tumor oxygenation is related to therapeutic outcome, we believe this approach to measuring tumor oxygen dynamics can be of value in predicting response to therapy, evaluating adjuvant interventions designed to modulate response to therapy, and in providing "Prognostic Radiology".

Applications of dynamic contrast enhanced MRI in oncology: measurement of tumor oxygen tension

A new model based on an extension of the Krog's cylindrical model was developed to calculate tumor oxygen tension (pO(2)) from the H-1 dynamic contrast enhanced MRI (DCE-MRI) measurements. The model enables one to calculate the tumor pO(2) using the vascular volume fraction (f(b)) obtained by the DCE-MRI. The proposed model has three parameters. For small values of f(b) one assumes that there exists a linear relationship between and f(b). The constant of proportionality in this case is given by C(1) - the oxygen tension per vascular volume fraction. For larger values of f(b) a modified version of Krogh model using two parameters is developed and here C(2) - is the integrated blood oxygen tension, and C(3) - given by the combination of the oxygen diffusion coefficient, solubility of oxygen in the tissue, capillary radius, and tissue metabolic consumption rate. The parameters of the model can be determined by performing simultaneous in-vivo F-19 MRI oxygen tension measurement and dynamic Gd-DTPA enhanced MRI on the same tumor. Dynamic MRI data can be used with a compartmental model to calculate tumor vascular volume fraction on a pixel by pixel basis. Then tumor oxygen tension map can be calculated from the vascular volume fraction by the extended Krogh model as described above. In the present work, the model parameters were determined using three rats bearing Walker-256 tumors and performing simultaneous F-19 and DCE MRI on the same tumor. The parameters obtained by fitting the model equation to the experimental data were: C(1) = 983.2 +/- 133.2torr, C(2) = 58.20 +/- 2.4 torr, and C(3) = 1.7 +/- 0.1 torr. The performance of the extended Krogh model was then tested on two additional rats by performing both F-19 and DCE-MRI studies and calculating the pO(2) (H-1) using the model and comparing it with the pO(2) (F-19) obtained from the F-19 MRI. It was found that the measurements obtained by both techniques had a high degree of correlation [pO(2) (H-1) = (1.01 +/- 0.07) pO(2) (F-19) + (0.91 +/- 0.05) and r=0.96], indicating the applicability of the proposed model in determining pO(2) from the DCE-MRI.

19F-MRI of perflubron for measurement of oxygen partial pressure in porcine lungs during partial liquid ventilation

A method for in vivo measurement of oxygen partial pressure (pO2) in porcine lungs during partial liquid ventilation (PLV) with perflubron (PFOB) was developed. A pulse sequence for high-resolution MRI of the distribution of PFOB in the lung after intratracheal administration was developed as well. Moreover, quantitative measurements of longitudinal relaxation time T(1) of (19)F resonances for assessment of regional pO2 are described. Due to the need to acquire data during a single expiratory breathhold, only low SNRs were achieved in vivo. Therefore, simulations were performed to investigate the influence of background noise on T(1) values calculated from data with low SNR. Based on these simulations, a postprocessing strategy was developed to correct for systematic errors by background noise prior to quantitative analysis. Results of a pilot study in pigs under conditions of PLV are presented.