pO2 Maps Research Articles

TH-E-BRA-04: Feasibility of Mapping Transient Tumor Hypoxia Using in Situ Activation PET Imaging: A Simulation Study

Purpose: Detecting transient tumor hypoxia in real‐time is desirable and has been challenging in radiation therapy. Here, we explore the feasibility of mapping tumor transient hypoxia in real‐time using in situ activation by high‐energy photons.Methods: The simulation used spatial uptake of a hypoxia tracer (e.g. 64Cu‐ATSM) in PET or autoradiograph as a surrogate for micro‐vessel density, which was then used to generate a steady state oxygen tension (pO2) map using a reactiondiffusionmodel. The 15O in situ activation map by high‐energy photons was generated using Ten Haken model based on the steady state pO2 map with the addition of element composition. The activation map includes both mobile and immobile 15O. The mobile 15O component was isolated by subtracting the fast decay portion from the overall decay curve. The simulation process was coded in Matlab using photon energies of 20 – 50MeV. Results: The diffusionmodel shows heterogeneous distribution of pO2 ranges from 28 – 1.9 mmHg for a pancreas tumormodel, depending on simulation baseline values, tumor vasculature structure and architecture. The threshold of 63% of the mean image intensity on the in situ activation PET image was found to reasonably agree with original hypoxia map using 5 mmHg as a cutoff value. The simulated transient hypoxic fraction was comparable with measurements reported using 64Cu‐ATSM autoradiography, e.g., measurement of 0.166 ± 0.067 versus simulation of 0.157. Conclusions: It is feasible to detect transient tumor hypoxia with oxygen in situ activation using high‐energy photons. This opens door for tracking tumor transient hypoxia in real‐time during the delivery of radiation therapy. Immediate future work includes quantifying the in situ activation PET (e.g., 15O activity in terms of pO2 in mmHg), and in‐vivo measurement of the in situ activation.

Electron Paramagnetic Resonance Imaging of Tumor pO2

Electron paramagnetic resonance imaging (EPRI) can be used to noninvasively and quantitatively obtain three-dimensional maps of tumor pO₂. The paramagnetic tracer triarylmethyl (TAM), a substituted trityl radical moiety, is not toxic to animals and provides narrow isotropic spectra, which is ideal for in vivo EPR imaging experiments. From the oxygen-induced spectral broadening of TAM, pO₂ maps can be derived using EPRI. The instrumentation consists of an EPRI spectrometer and 7T magnetic resonance imaging (MRI) system both operating at a common radiofrequency of 300 MHz. Anatomic images obtained by MRI can be overlaid with pO₂ maps obtained from EPRI. With imaging times of less than 3 min, it was possible to monitor the dynamics of oxygen changes in tumor and distinguish chronically hypoxic regions from acutely hypoxic regions. In this article, the principles of pO₂ imaging with EPR and some relevant examples of tumor imaging are reviewed.

Hypoxia Imaging of Tumor and Spatiotemporal Analysis during Oxygen Inhalation

SUMMARY Tumor hypoxia is considered a potential therapeutic problem because it reduces the effects of radiation therapy. Clinical experience has shown that long-term tumor oxygenation cannot be achieved with oxygen inhalation, but the mechanisms behind this phenomenon remain unknown. In this study, we designed an optical system for evaluating spatiotemporal changes in tissue oxygen tension (pO2) by phosphorescence quenching. The system can measure continuous changes in pO2 at a fixed point and can also perform two-dimensional mapping of pO2 in any part of the tumor tissue. We implanted tumor tissue in a dorsal skinfold chamber (DSC) of C57BL/6 mice and observed tumor growth. The mice received oxygen inhalation and pO2 was measured. Tumor pO2 increased after inhalation but the oxygen level was not maintained despite continuous inhalation of pure oxygen; the tumor returned to a hypoxic state. These results mimic the clinical experience of oxygen inhalation treatment in radiation therapy. Our system reproduces the repeat hypoxic phenomenon in a murine tumor model and can be used to determine the mechanisms of oxygen metabolism in tumors.

SU‐E‐T‐08: A PO2‐Based Vascular Tumour Model for Iterative Radiobiological Modeling

Purpose: A 2D model of a tumour incorporating the microvasculature environment is developed and used in an iterative manner to model the changing levels of oxygen through its volume over the course of a radiation therapy treatment. Methods: Tumour vasculature is created by iteration, using published pO2 histograms. A 4 mm2 vessel map is randomly generated then convolved with an oxygen‐spreading kernel to generate pO2 map. A cost function is calculated based on the relative square differences between the input and newly created pO2 histograms. Vessel number and spatial position are optimized. Radiation therapy treatment fractions are modelled by iteration. A linear‐quadratic surviving fraction response map is generated incorporating local hypoxia reduction factors (HRFs) that are calculated from the pO2 map. A threshold, dose‐dependent response that modifies the microvasculature environment is also introduced. Vessel, pO2, and HRF maps are updated before calculating the i+1 map of tumour response. Parameters for a squamous cell carcinoma were incorporated for the initial testing. Results: Weighting the cost function allowed us to keep differences in the extremely hypoxic bin (0 – 5 mmHg) and fully oxic bin to less than 1 % of the overall cell count. Differences of as much as 12 % in intermediate bins may indicate limitations of the model to conform to the measured data. The total surviving fraction (TSF) increased by 123% in the vascular response model without any specific vascular dose enhancement compared to the stationary vascular case. Vascular dose enhancement by factors of 1.5 and 2.0 further increased the TSF by 26 % and 50 %, respectively. Conclusions: This work provides a framework for answering questions relating to tumour vasculature response during radiation therapy. These preliminary results suggest potential for caution in enhancing dose to tumour vasculature.

Imaging Cycling Tumor Hypoxia

Cycling hypoxia is now a well-recognized phenomenon in animal and human solid tumors. Cycling hypoxia can exist more than 100-μm distances from a microvessel, and some of these regions have been shown to exist adjacent to normal tissue. Fluctuations in pO(2) of approximately 20 mm Hg can occur with periodicities of minutes to hours and even days. These fluctuations have been attributed to changes in erythrocyte flux, perfusion, and also development of newer vascular networks. Cycling hypoxia has been shown to induce the expression of hypoxia-inducible transcription factor-1α (HIF-1α) and also confer tumor cells and tumor vascular endothelial cells with enhanced prosurvival pathways, making tumors less responsive to radiation and chemotherapy. Imaging of cycling hypoxia in tumors can provide capabilities to help plan appropriate treatment, by taking into account the magnitude and frequency of fluctuations and also their locations adjacent to normal tissue. Electron paramagnetic resonance imaging (EPRI) provides the ability to distinguish chronic and cycling hypoxic regions and has the required spatial and temporal resolutions to provide quantitative maps of tumor pO(2). EPRI can serve as a valuable tool in examining tumor pO(2) longitudinally in response to treatment and in an experimentally chosen time window to spatially map fluctuations in pO(2) noninvasively in animal models of implanted or orthotopic tumors, with a potential for human applications.

Noninvasive mapping of spontaneous fluctuations in tumor oxygenation using MRI

Acute hypoxia (transient cycles of hypoxia-reoxygenation) is known to occur in solid tumors and may be a poorly appreciated therapeutic problem as it can be associated with resistance to radiation therapy, impaired delivery of chemotherapeutic agents, or metastasis development. The objective of the present study was to use MR 19F relaxometry maps to analyze the spontaneous fluctuations of partial pressure of oxygen (pO2) over time in experimental tumors. The pO2 maps were generated after direct intratumoral administration of a fluorine compound (hexafluorobenzene) whose relaxation rate (1/T1) is proportional to the % O2. The authors used a SNAP inversion-recovery sequence at 4.7 T to acquire parametric images of the T1 relaxation time with a high spatial and temporal resolution. Homemade routines were developed to perform regions of interest analysis, as well as pixel by pixel analysis of pO2 over time. The authors were able to quantify and probe the heterogeneity of spontaneous fluctuations in tumor pO2: (i) Spontaneous fluctuations in pO2 occurred regardless of the basal oxygenation state (i.e., both in oxygenated and in hypoxic regions) and (ii) spontaneous fluctuations occurred at a rate of 1 cycle/12-47 min. For validation, the analysis was performed in dead mice for which acute changes did not occur. The authors thereby demonstrated that 19F MRI technique is sensitive to acute change in pO2 in tumors. This is the first approach that allows quantitative minimally invasive measurement of the spontaneous fluctuations of tumor oxygenation using a look-locker approach (e.g., SNAP IR). This approach could be an important tool to characterize the phenomenon of tumor acute hypoxia, to understand its physiopathology, and to improve therapies.

Retinal Tissue Oxygen Tension Imaging in the Rat

To report an imaging technique for measurement of oxygen tension (PO2) in retinal tissue and establish its feasibility for measuring retinal PO2 variations in rat eyes by adjusting the fraction of inspired oxygen (FiO2). A narrow laser line was projected at an angle on the retina, and phosphorescence emission was imaged after intravitreal injection of an oxygen-sensitive molecular probe. A frequency-domain approach was used for phosphorescence lifetime measurements. Retinal PO2 maps were computed from phosphorescence lifetime images, and oxygen profiles through the retinal depth were derived in rats in conditions of 10%, 21%, and 50% FiO2. Retinal PO2 measurements were repeatable, and variations in outer and inner retina PO2 at different locations along the image were not significant (P>or=0.3). Maximum outer retinal PO2 obtained in 10%, 21%, and 50% FiO2 were significantly different (P<0.0001). Maximum outer retinal PO2 correlated with systemic arterial PO2 (R=0.70; P<0.0001). The slope of the outer retina PO2 profile correlated with maximum outer retinal PO2 (R=0.84; P<0.0001). Mean inner retina PO2 correlated with maximum outer retinal PO2 (R=0.88; P<0.0001). A technique has been developed for quantitative mapping of retinal tissue oxygen tension with the potential to enable sequential monitoring of retinal oxygenation in health and disease.

A short-breath-hold technique for lung pO2 mapping with 3He MRI.

A pulse-sequence strategy was developed for generating regional maps of alveolar oxygen partial pressure (pO2) in a single 6-sec breath hold, for use in human subjects with impaired lung function. Like previously described methods, pO2 values are obtained by measuring the oxygen-induced T1 relaxation of inhaled hyperpolarized 3He. Unlike other methods, only two 3He images are acquired: one with reverse-centric and the other with centric phase-encoding order. This phase-encoding arrangement minimizes the effects of regional flip-angle variations, so that an accurate map of instantaneous pO2 can be calculated from two images acquired a few seconds apart. By combining this phase-encoding strategy with variable flip angles, the vast majority of the hyperpolarized magnetization goes directly into the T1 measurement, minimizing noise in the resulting pO2 map. The short-breath-hold pulse sequence was tested in phantoms containing known O2 concentrations. The mean difference between measured and prepared pO2 values was 1 mm Hg. The method was also tested in four healthy volunteers and three lung-transplant patients. Maps of healthy subjects were largely uniform, whereas focal regions of abnormal pO2 were observed in diseased subjects. Mean pO2 values varied with inhaled O2 concentration. Mean pO2 was consistent with normal steady-state values in subjects who inhaled 3He diluted only with room air.

Oxygen Transport in Brain Tissue

Oxygen is essential to maintaining normal brain function. A large body of evidence suggests that the partial pressure of oxygen (pO(2)) in brain tissue is physiologically maintained within a narrow range in accordance with region-specific brain activity. Since the transportation of oxygen in the brain tissue is mainly driven by a diffusion process caused by a concentration gradient of oxygen from blood to cells, the spatial organization of the vascular system, in which the oxygen content is higher than in tissue, is a key factor for maintaining effective transportation. In addition, a local mechanism that controls energy demand and blood flow supply plays a critical role in moment-to-moment adjustment of tissue pO(2) in response to dynamically varying brain activity. In this review, we discuss the spatiotemporal structures of brain tissue oxygen transport in relation to local brain activity based on recent reports of tissue pO(2) measurements with polarographic oxygen microsensors in combination with simultaneous recordings of neural activity and local cerebral blood flow in anesthetized animal models. Although a physiological mechanism of oxygen level sensing and control of oxygen transport remains largely unknown, theoretical models of oxygen transport are a powerful tool for better understanding the short-term and long-term effects of local changes in oxygen demand and supply. Finally, emerging new techniques for three-dimensional imaging of the spatiotemporal dynamics of pO(2) map may enable us to provide a whole picture of how the physiological system controls the balance between demand and supply of oxygen during both normal and pathological brain activity.

METHODS FOR STUDYING THE PHYSIOLOGY OF KIDNEY OXYGENATION

1. An improved understanding of the regulation of kidney oxygenation has the potential to advance preventative, diagnostic and therapeutic strategies for kidney disease. Here, we review the strengths and limitations of available and emerging methods for studying kidney oxygen status. 2. To fully characterize kidney oxygen handling, we must quantify multiple parameters, including renal oxygen delivery (DO2) and consumption (VO2), as well as oxygen tension (Po2). Ideally, these parameters should be quantified both at the whole-organ level and within specific vascular, tubular and interstitial compartments. 3. Much of our current knowledge of kidney oxygen physiology comes from established techniques that allow measurement of global kidney DO2 and VO2, or local tissue Po2. When used in tandem, these techniques can help us understand oxygen mass balance in the kidney. Po2 can be resolved to specific tissue compartments in the superficial cortex, but not deep below the kidney surface. We have limited ability to measure local kidney tissue DO2 and VO2. 4. Mathematical modelling has the potential to provide new insights into the physiology of kidney oxygenation, but is limited by the quality of the information such models are based on. 5. Various imaging techniques and other emerging technologies have the potential to allow Po2 mapping throughout the kidney and/or spatial resolution of Po2 in specific renal tissues, even in humans. All currently available methods have serious limitations, but with further refinement should provide a pathway through which data obtained from experimental animal models can be related to humans in the clinical setting.

The Pervasive Presence of Fluctuating Oxygenation in Tumors

Tumor hypoxia is a persistent obstacle for traditional therapies in solid tumors. Strategies for mitigating the effects of hypoxic tumor cells have been developed under the assumption that chronically hypoxic tumor cells were the central cause of treatment resistance. In this study, we show that instabilities in tumor oxygenation are a prevalent characteristic of three tumor lines and previous characterization of tumor hypoxia as being primarily diffusion-limited does not accurately portray the tumor microenvironment. Phosphorescence lifetime imaging was used to measure fluctuations in vascular pO(2) in rat fibrosarcomas, 9L gliomas, and R3230 mammary adenocarcinomas grown in dorsal skin-fold window chambers (n = 6 for each tumor type) and imaged every 2.5 minutes for a duration of 60 to 90 minutes. O(2) delivery to tumors is constantly changing in all tumors, resulting in continuous reoxygenation events throughout the tumor. Vascular pO(2) maps show significant spatial heterogeneity at each time point, as well as between time points. The fluctuations in oxygenation occur with a common periodicity within and between tumors, suggesting a common mechanism, but have tumor type-dependent spatial patterns. The widespread presence of fluctuations in tumor oxygenation has broad ranging implications for tumor progression, stress response, and signal transduction, which are altered by oxygenation/reoxygenation events.

Low-field paramagnetic resonance imaging of tumor oxygenation and glycolytic activity in mice

A priori knowledge of spatial and temporal changes in partial pressure of oxygen (oxygenation; pO(2)) in solid tumors, a key prognostic factor in cancer treatment outcome, could greatly improve treatment planning in radiotherapy and chemotherapy. Pulsed electron paramagnetic resonance imaging (EPRI) provides quantitative 3D maps of tissue pO(2) in living objects. In this study, we implemented an EPRI set-up that could acquire pO(2) maps in almost real time for 2D and in minutes for 3D. We also designed a combined EPRI and MRI system that enabled generation of pO(2) maps with anatomic guidance. Using EPRI and an air/carbogen (95% O(2) plus 5% CO(2)) breathing cycle, we visualized perfusion-limited hypoxia in murine tumors. The relationship between tumor blood perfusion and pO(2) status was examined, and it was found that significant hypoxia existed even in regions that exhibited blood flow. In addition, high levels of lactate were identified even in normoxic tumor regions, suggesting the predominance of aerobic glycolysis in murine tumors. This report presents a rapid, noninvasive method to obtain quantitative maps of pO(2) in tumors, reported with anatomy, with precision. In addition, this method may also be useful for studying the relationship between pO(2) status and tumor-specific phenotypes such as aerobic glycolysis.

Simultaneous Imaging of Cortical Partial Oxygen Pressure and Anatomic Structures Using a Transparent Optical Sensor Foil

Reliable information of cerebral oxygenation is-besides the monitoring of the intracranial pressure-of eminent interest when treating patients with brain injuries. In this study, we introduce a new, fast, and sensitive method capable of determining the cortical partial oxygen pressure on the surface of the cortex using a special sensor foil. The introduced method exploits the O2-dependent phosphorescence of a thin sensor foil, which is excited by a short light-emitting diode flash. The optical signal is registered by a charge-coupled device camera and analyzed with PC-based software. The adequacy of this method was tested in 10 animals. The sensor device was placed directly over the cortex after craniotomy and removal of the dura. Arterial oxygen pressure was systematically varied by modifying the ventilation gas mixture. A total of 225 measurements were performed within 4 regions of interest. Obtained results were sufficient in each case. The pO2 over the cortex correlated well with arterial pO2. Measurements over arteries showed a correlation coefficient of 0.72 (P<0.001), over veins 0.58 (P<0.001), over cortical parenchyma 0.46 (P<0.001), and in a larger region of interest containing vessels and cortical tissue 0.59 (P<0.001). The frequency of the measurements was 7 Hz with a single measurement covering an area of 30 x 30 microm. For the first time, nearly online pO2 maps of a brain cortex can be generated, allowing simultaneously also separate measurements over distinct anatomic structures yielding a good spatial resolution.

Transcutaneous pO2 measurement during tourniquet-induced venous occlusion using dynamic phosphorescence imaging

A sufficient oxygen supply in skin grafts requires a functioning microcirculation. Venous occlusion impairs the microcirculation and is therefore a major threat of healing. Luminescence life time imaging (LLI) enables the non-invasive and two-dimensional assessment of the transcutaneous oxygen partial pressure (p(tc)O2). In the current trial this new device was applied for monitoring of venous congestion. A tourniquet on the upper arm was inflated up to 40-50 mmHg and released after 10 min in eight healthy volunteers. The p(tc)O2 was measured at the lower arm every minute prior to, during and up to 10 min after cuff occlusion (40 degrees C applied skin temperature) using LLI of platinum(II)-octaethyl-porphyrin immobilized in a polystyrene matrix. For validation the polarographic Clark electrode technique was applied in close proximity and measurement was performed simultaneously. p(tc)O2 measurements prior to (Clark: 50.68+/-5.69 mmHg vs. LLI: 50.89+/-4.96 mmHg) and at the end of the venous congestion (Clark: 16.41+/-4.54 mmHg vs. LLI: 23.82+/-3.23 mmHg) did not differ significantly using the Clark electrode vs. LLI. At the initial congestion respectively reperfusion phase the Clark electrode measured faster decreases respectively increase of p(tc)O2 due to oxygen consumption of this method. This experimental trial demonstrates the applicability of LLI to quantify the p(tc)O2 under changing venous blood flow. The use of planar transparent sensors allows the non-invasive generation of two-dimensional maps of surface pO2 what makes this method particular suitable for monitoring of skin grafts.

Luminescent dual sensor for time-resolved imaging of pCO2 and pO2 in aquatic systems

An optical dual sensor for the two-dimensional detection of pCO2 and pO2 is described. Tris(tetraoctylammonium)-8-hydroxypyrene-1,3,6-trisulfonate ((TOA)3HPTS) acting together with the lipophilic buffer tetraoctylammonium hydrogen carbonate ((TOA)HCO3) as pCO2-sensing system along with the oxygen indicator tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) bis(3-(trimethylsilyl)-1-propanesulfonate) (Ru(dpp)3TMS2) are incorporated into a single layer ethyl cellulose matrix. A second layer of black silicone rubber served as an optical isolation. The two indicators were simultaneously excited with a 460-nm LED, and a fast-gateable CCD camera was used as the detector. The time-gated imaging scheme enables the mapping of pCO2 and pO2 within one measurement, where images in three different time windows during and after a series of square-shaped excitation pulses are recorded. A numerical evaluation method for the resolution of the single parameter maps from these three overall images is described. The response of the sensor has been optimized for use in aquatic systems.

Alveolar oxygen partial pressure and oxygen depletion rate mapping in rats using 3He ventilation imaging

A hyperpolarized 3He ventilation imaging protocol was implemented to assess alveolar pO2 values and the oxygen depletion rate in rats. The imaging protocol, which is based on spiral k-space sampling, was designed to acquire a high signal-to-noise ratio (SNR) T1-weighted ventilation series of images in a single breath-hold. Simulations were performed to estimate the accuracy and dependence of the pO2 imaging protocol on the image SNR and the RF flip-angle determination. The imaging protocol was validated in vitro in phantoms and in vivo in rats. Imaging sessions were carried out for different inhaled O2 concentrations ranging from 20% to 40%. Parametric maps of alveolar pO2 and oxygen depletion rate were generated from the series of images. For each investigated animal, the differences in measured alveolar pO2 values are in agreement with the changes in inhaled O2 concentration. The oxygen depletion rates, ranging between 0.7 and 8.0 mbar s-1, are in close agreement with the published values for healthy rats.

Time-Resolved pH/pO2 Mapping with Luminescent Hybrid Sensors

A method for simultaneous and referenced 2D mapping of pH and pO2 is described. The experimental setup combines a fast gateable CCD camera as detector, a LED as excitation light source and a single-layer sensor membrane as optical transducer. The planar optode comprises a lipophilic fluorescein derivative (lifetime approximately 5 ns) and platinum(II) mesotetrakis(pentafluorophenyl)porphyrin (approximately 70 micros in the absence of a quencher) immobilized in a hydrogel matrix. Depending on the fluorescent pH indicator, a pH transition in the physiological range (pH 6-pH 8) or in the near-basic region (pH 7-pH 9) can be achieved. The measuring scheme involves the time-resolved acquisition of images in three windows during a series of square-shaped excitation pulses. A method allowing the calculation of both parameters from these three images is presented. The pH/pO2 hybrid sensor incorporating the pH indicator 2',7'-dihexyl-5(6)-N-octadecyl-carboxamidofluorescein was characterized in detail. The pH and pO2 were determined with a maximum deviation of 0.03 pH unit and 6.5 hPa pO2, respectively, within the range of pH 7.6-pH 8.7 and 0-200 hPa pO2 in test measurements. The ionic strength (IS) cross-sensitivity was found to be relatively small (pH/IS < 3.5 x 10(-4) mM(-1) and pO2/IS <-0.053 hPa mM(-1) at a transition from 0.5 to 0.1 M IS). Whereas a strong temperature effect on the sensor signal was observed (DeltapH/DeltaT = 0.011-0.034 K-1 and DeltapO2/DeltaT = 1.85-7.17 hPa K-1 in the range from 277 to 308 K). Examples of pH/pO2 images obtained in natural marine sediment are presented.

Electron paramagnetic resonance imaging of tumor hypoxia: Enhanced spatial and temporal resolution for in vivo pO2 determination

The time-domain (TD) mode of electron paramagnetic resonance (EPR) data collection offers a means of estimating the concentration of a paramagnetic probe and the oxygen-dependent linewidth (LW) to generate pO2 maps with minimal errors. A methodology for noninvasive pO2 imaging based on the application of TD-EPR using oxygen-induced LW broadening of a triarylmethyl (TAM)-based radical is presented. The decay of pixel intensities in an image is used to estimate T2*, which is inversely proportional to pO2. Factors affecting T2* in each pixel are critically analyzed to extract the contribution of dissolved oxygen to EPR line-broadening. Suitable experimental and image-processing parameters were obtained to produce pO2 maps with minimal artifacts. Image artifacts were also minimized with the use of a novel data collection strategy using multiple gradients. Results from a phantom and in vivo imaging of tumor-bearing mice validated this novel method of noninvasive oximetry. The current imaging protocols achieve a spatial resolution of approximately 1.0 mm and a temporal resolution of approximately 9 s for 2D pO2 mapping, with a reliable oxygen resolution of approximately 1 mmHg (0.12% oxygen in gas phase). This work demonstrates that in vivo oximetry can be performed with good sensitivity, accuracy, and high spatial and temporal resolution.

Tumour oxygen dynamics measured simultaneously by near-infrared spectroscopy and 19F magnetic resonance imaging in rats**Presented in part at the 12th annual meeting of the International Society of Magnetic Resonance in Medicine, Kyoto, 2004.

Simultaneous near-infrared spectroscopy (NIRS) and magnetic resonance imaging (MRI) were used to investigate the correlation between tumour vascular oxygenation and tissue oxygen tension dynamics in rat breast 13762NF tumours with respect to hyperoxic gas breathing. NIRS directly detected global variations in the oxygenated haemoglobin concentration (Δ[HbO2]) within tumours and oxygen tension (pO2) maps were achieved using 19F MRI of the reporter molecule hexafluorobenzene. Multiple correlations were examined between rates and magnitudes of vascular (Δ[HbO2]) and tissue (pO2) responses. Significant correlations were found between response to oxygen and carbogen breathing using either modality. Comparison of results for the two methods showed a correlation between the vascular perfusion rate ratio and the mean pO2 values (R2 > 0.7). The initial rates of increase of Δ[HbO2] and the slope of dynamic pO2 response, d(pO2)/dt, of well-oxygenated voxels in response to hyperoxic challenge were also correlated. These results demonstrate the feasibility of simultaneous measurements using NIRS and MRI. As expected, the rate of pO2 response to oxygen is primarily dependent upon the well perfused rather than poorly perfused vasculature.

In vivo mapping of the effects of radiotherapy on neovascular permeability using dynamic contrast enhanced magnetic resonance imaging (DCE-MRI)

3550 Background: Neoadjuvant therapy is an integral part of rectal cancer management. Prediction of response to radiotherapy (RT) is highly relevant for both further surgery and prognosis in these patients. In a rat colorectal cancer model, we studied the use of DCE-MRI with a new macromolecular rapid clearance contrast agent (P792) to visualize tumor vascular permeability as a surrogate marker for angiogenesis and evaluate the effect of RT. Methods: Male WAG/Rij rats were injected with 2x106 CC531 cells in the hind leg. Once the tumor reached a diameter of 1 cm, DCE-MRI with high temporal resolution was performed before and 5 days after 5x500 cGy of external RT. The DCE-MRI data were used to generate parametric tissue permeability maps of the endothelial transfer constant (Ktrans) according to the Tofts-Kermode two compartment mathematical model. Separate region of interest (ROI) analyses were performed of the entire tumor (T), periferal vascular rim (P) and tumor core (C). In order to differentiate permeability changes from flow and oxygenation effects of RT, fiber optic tissue pO2 and microcirculatory laser blood flow (LBF) histogram mapping was performed in each tumor before and after RT using a stepwise micromanipulator technique. Results: The biophysical properties of P792 allowed excellent discrimination between tumor and normal tissue. Mean Ktrans (ml/min/1000g tissue) value changes measured over all pixels in a ROI were as follows: T: 24.03 vs 5.47 (p<0.05); P: 29.7 vs 10.4 (p<0.05); and C: 8.65 vs 0.47 (p<0.05) before and after radiotherapy. Microcirculatory LBF (arbitrary units) decreased from 117.3 to 84.7 (p<0.001) after RT. All tumors were severely hypoxic with a mean tissue pO2 over a 4 mm trajectory of 3.5 mm Hg before and 5.6 mm Hg after RT (p<0.001). Conclusions: High temporal resolution DCE-MRI with P792 allows non-invasive imaging and quantification of neovascular permeability in this colorectal cancer model. Administration of RT significantly reduces permeability and microcirculatory flow. RT enhanced oxygenation, but all tumors remained severely hypoxic after radiotherapy. DCE-MRI could have an important role in therapy monitoring. No significant financial relationships to disclose.