Magnetic resonance measurement of response to hyperoxia differentiates tumors from normal tissue and may be sensitive to oxygen consumption

Lipid nanocapsules as in vivo oxygen sensors using magnetic resonance imaging

Hypoxia is common occurrence of the tumour microenvironment, wherein heterogeneous gradients of O₂ give rise to tumoural cells which are highly malignant, metastatic, and resistant to therapeutic efforts. Thus, the assessment and imaging of hypoxia is essential for tumour diagnosis and treatment. Magnetic resonance imaging and, more specifically, the quantitative assessment of longitudinal relaxation time enhancement, was shown to enable the mapping of oxygen in tumours with increased sensitivity for lipids as compared to water signal. Unfortunately, this can only be applied to tumours with high lipid content. To overcome this issue, we propose the use of lipid nanocapsules (LNCs). LNCs have been demonstrated as excellent core-shell nanocarriers, wherein the lipidic-core is used for lipophilic drug encapsulation, enabling treatment of highly malignant tumours. Herein, however, we exploited the lipidic-core of the LNCs to develop a simple but effective technique to increase the lipidic content within tissues to enable the assessment and mapping of pO₂. LNCs were prepared using the phase-inversion technique to produce 60 nm sized nanoparticles, and in vitro studies demonstrated the permeability and responsiveness of LNCs to O₂. To evaluate the ability of LNCs to respond to changes in pO₂in vivo, after a hyperoxic challenge, three animal models, namely a normal tissue model (gastrocnemius muscle tissue) and two tumour tissue models (subcutaneous fibrosarcoma and intracerebral glioblastoma) were explored. LNCs were found to be responsive to variation of O₂in vivo. Moreover, the use of MRI enabled the mapping of oxygen gradients and heterogeneity within tumours.

Potential of computer-aided diagnosis of high spectral and spatial resolution (HiSS) MRI in the classification of breast lesions

PURPOSE

To compare the performance of computer-aided diagnosis (CADx) analysis of precontrast high spectral and spatial resolution (HiSS) MRI to that of clinical dynamic contrast-enhanced MRI (DCE-MRI) in the diagnostic classification of breast lesions.

MATERIALS AND METHODS

Thirty-four malignant and seven benign lesions were scanned using two-dimensional (2D) HiSS and clinical 4D DCE-MRI protocols. Lesions were automatically segmented. Morphological features were calculated for HiSS, whereas both morphological and kinetic features were calculated for DCE-MRI. After stepwise feature selection, Bayesian artificial neural networks merged selected features, and receiver operating characteristic (ROC) analysis evaluated the performance with leave-one-lesion-out validation.

RESULTS

AUC (area under the ROC curve) values of 0.92 ± 0.06 and 0.90 ± 0.05 were obtained using CADx on HiSS and DCE-MRI, respectively, in the task of classifying benign and malignant lesions. While we failed to show that the higher HiSS performance was significantly better than DCE-MRI, noninferiority testing confirmed that HiSS was not worse than DCE-MRI.

CONCLUSION

CADx of HiSS (without contrast) performed similarly to CADx on clinical DCE-MRI; thus, computerized analysis of HiSS may provide sufficient information for diagnostic classification. The results are clinically important for patients in whom contrast agent is contra-indicated. Even in the limited acquisition mode of 2D single slice HiSS, by using quantitative image analysis to extract characteristics from the HiSS images, similar performance levels were obtained as compared with those from current clinical 4D DCE-MRI. As HiSS acquisitions become possible in 3D, CADx methods can also be applied. Because HiSS and DCE-MRI are based on different contrast mechanisms, the use of the two protocols in combination may increase diagnostic accuracy.

Dynamic and simultaneous MR measurement of R1 and R2* changes during respiratory challenges for the assessment of blood and tissue oxygenation

This work presents a novel method for the rapid and simultaneous measurement of R1 and R2* relaxation rates. It is based on a dynamic short repetition time steady-state spoiled multigradient-echo sequence and baseline R1 and B1 measurements. The accuracy of the approach was evaluated in simulations and a phantom experiment. The sensitivity and specificity of the method were demonstrated in one volunteer and in four patients with intracranial tumors during carbogen inhalation. We utilized (ΔR2*, ΔR1) scatter plots to analyze the multiparametric response amplitude of each voxel within an area of interest. In normal tissue R2* decreased and R1 increased moderately in response to the elevated blood and tissue oxygenation. A strong negative ΔR2* and ΔR1 response was observed in veins and some tumor areas. Moderate positive ΔR2* and ΔR1 response amplitudes were found in fluid-rich tissue as in cerebrospinal fluid, peritumoral edema, and necrotic areas. The multiparametric approach was shown to increase the specificity and sensitivity of oxygen-enhanced MRI compared to measuring ΔR2* or ΔR1 alone. It is thus expected to provide an optimal tool for the identification of tissue areas with low oxygenation, e.g., in tumors with compromised oxygen supply.

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.

The optimal use of contrast agents at high field MRI

The intravenous administration of a standard dose of conventional gadolinium-based contrast agents produces higher contrast between the tumor and normal brain at 3.0 Tesla (T) than at 1.5 T, which allows reducing the dose to half of the standard one to produce similar contrast at 3.0 T compared to 1.5 T. The assessment of cumulative triple-dose 3.0 T images obtained the best results in the detection of brain metastases compared to other sequences. The contrast agent dose for dynamic susceptibility-weighted contrast-enhanced perfusion MR imaging at 3.0 T can be reduced to 0.1 mmol compared to 0.2 mmol at 1.5 T due to the increased susceptibility effects at higher magnetic field strengths. Contrast agent application makes susceptibility-weighted imaging (SWI) at 3.0 T clinically attractive, with an increase in spatial resolution within the same scan time. Whereas a double dose of conventional gadolinium-based contrast agents was optimal in SWI with respect to sensitivity and image quality, a standard dose of gadobenate dimeglumine, which has a two-fold higher T1-relaxivity in blood, produced the same effect. For MR-arthrography, optimized concentrations of gadolinium-based contrast agents are similar at 3.0 and 1.5 T. In summary, high field MRI requires the optimization of the contrast agent dose in different clinical applications.

CT for intracardiac thrombi and tumors

Although cardiac tumors are rare, they nevertheless represent an important subgroup, the diagnosis of which is challenging for the primary care physician. Symptoms are not characteristic and serious complications including stroke, myocardial infarction and even sudden death from arrhythmia may be the first signs of tumor. The most common primary cardiac neoplasm is the benign myxoma and the most frequent primary malignant lesion is sarcoma. Cardiac metastases from distant primary carcinomas are now frequently encountered. Echocardiography until the past decade was the only consistently reliable and available non-invasive diagnostic tool. New non-invasive CT and MRI exams are changing the diagnostic approach. Echocardiography and angiocardiography with or without coronary arteriography remain routine methods for evaluating cardiac neoplasms. However, CT provides additional diagnostic information and is regarded as essential for adequate staging and treatment planning, particularly when surgical resection is being considered. This chapter reviews the wide spectrum of intracardiac thrombi and tumors including their incidence and imaging characteristics with particular reference to the CT findings and differential diagnosis.

[Use of contrast agent in high-field MRI (3 T)]

The basic diagnostic efficacy of MR contrast medium in the evaluation of primary brain tumors and its clinical usefulness in the detection of brain metastases with single and cumulative triple-dose was compared using a high-field 3 T MR unit and a 1.5 T MR unit. Additionally, the effect of contrast agent on high-resolution MR venography based on the BOLD effect was evaluated at both field strengths. Tumor-brain contrast after gadodiamide administration, as assessed by means of statistical evaluation of MP-RAGE scans and T1-SE images, was significantly higher at 3 T than at 1.5 T. The subjective assessment of cumulative triple-dose 3 T images obtained the best results in the detection of brain metastases, followed by 1.5 T cumulative triple-dose enhanced images. Due to higher spatial resolution, contrast-enhanced MR venography at 3 T showed more details in and around tumors than at 1.5 T, additionally enhanced by stronger susceptibility weighting and higher signal-to-noise ratio at 3 T. In summary, administration of gadolinium-based contrast agent produces higher contrast between tumor and normal brain at 3 T than at 1.5 T, helps to detect more cerebral metastases at 3 T than at 1.5 T in single and cumulative triple dose, and improves MR venography at 3 T with increase in spatial resolution within the same measurement time, thus providing more detailed information.

MR Contrast Agent at High-Field MRI (3 Tesla)

Tumor-to-brain contrast after gadolinium administration using MP-RAGE and T1-SE scans in patients with primary and secondary brain tumors was significantly higher at 3 T than at 1.5 T. The subjective assessment of cumulative triple-dose 3 Tesla images obtained the best results in the detection of brain metastases compared with other sequences followed by 1.5 T cumulative triple-dose enhanced images. In macroadenomas of the hypophysis, contrast-enhanced 3 T MRI was superior to standard MRI in the diagnosis of cavernous sinus infiltration and in visualization of cranial nerves within the cavernous sinus. Due to higher spatial resolution, contrast-enhanced MR venography at 3 T showed more details in and around tumors than at 1.5 T, additionally enhanced by stronger susceptibility weighting and higher signal-to-noise ratio at 3 T. In summary, administration of gadolinium-based contrast agent produces higher contrast between tumor and normal brain at 3 T than at 1.5 T, helps to detect more cerebral metastases at 3 T versus 1.5 T in single and cumulative triple dose, improves the evaluation of macroadenomas of the hypophysis, and makes MR venography at 3 T clinically attractive with increase in spatial resolution within the same measurement time, thus providing more detailed information.

High-resolution three-dimensional contrast-enhanced blood oxygenation level-dependent magnetic resonance venography of brain tumors at 3 Tesla: first clinical experience and comparison with 1.5 Tesla

RATIONALE AND OBJECTIVES

To evaluate the clinical potential of high-resolution 3D contrast-enhanced blood oxygenation level-dependent MR-Venography (CE-MRV) for primary brain tumors and metastases at 3 Tesla (T) in comparison to 1.5 T.

METHODS

Eighteen patients with brain tumors were examined using CE-MRV after application of a standard dose of MRI contrast agent (0.1 mmol/kg gadodiamide). CE-MRV is based on a high-resolution 3D flow-compensated gradient-echo sequence with long echo times that uses the contrast-enhanced blood oxygenation level-dependent effect. This technique was performed using the same volume coverage and acquisition time at both field strengths after performing standard imaging sequences.

RESULTS

The higher spatial resolution of CE-MRV at 3 T showed more details within and around tumors than at 1.5 T. Visibility was enhanced by stronger susceptibility weighting and higher intrinsic signal-to-noise at 3 T. Compared with standard imaging protocols, additional information characterized as tubular and nontubular hypointense structures were found within or around lesions on CE-MRV images.

CONCLUSIONS

Acquisition of CE-MRV data at 3 T enables spatial resolution to be increased within the same measurement time and with the same volume coverage compared with 1.5 T, thus providing more detailed information. The method may also show the potential to estimate oxygen supply of tumors, especially at high field strengths.

Hypoxia imaging in brain tumors

Assessment of the oxygenation status of brain tumors has been studied increasingly with imaging techniques in light of recent advances in oncology. Tumor oxygen tension is a critical factor influencing the effectiveness of radiation and chemotherapy and malignant progression. Hypoxic tumors are resistant to treatment, and prognostic value of tumor oxygen status is shown in head and neck tumors. Strategies increasing the tumor oxygenation are being investigated to overcome the compromising [figure: see text] effect of hypoxia on tumor treatment. Administration of nicotinamide and inhalation of various high oxygen concentrations have been implemented. Existing methods for assessment of tissue oxygen level are either invasive or insufficient. Accurate and noninvasive means to measure tumor oxygenation are needed for treatment planning, identification of patients who might benefit from oxygenation strategies, and assessing the efficacy of interventions aimed to increase the radiosensitivity of tumors. Of the various imaging techniques used to assess tissue oxygenation, MR spectroscopy and MR imaging are widely available, noninvasive, and clinically applicable techniques. Tumor hypoxia is related closely to insufficient blood flow through chaotic and partially nonfunctional tumor vasculature and the distance between the capillaries and the tumor cells. Information on characteristics of tumor vasculature such as blood volume, perfusion, and increased capillary permeability can be provided with MR imaging. MR imaging techniques can provide a measure of capillary permeability based on contrast enhancement and relative cerebral blood volume estimates using dynamic susceptibility MR imaging. Blood oxygen level dependent contrast MR imaging using gradient echo sequence is intrinsically sensitive to changes in blood oxygen level. Animal models using blood oxygen level-dependent contrast imaging reveal the different responses of normal and tumor vasculature under hyperoxia. Normobaric hyperoxia is used in MR studies as a method to produce MR contrast in tissues. Increased T2* signal intensity of brain tissue has been observed using blood oxygen level-dependent contrast MR imaging. Dynamic blood oxygen level-dependent contrast MR imaging during hyperoxia is suggested to image tumor oxygenation. Quantification of cerebral oxygen saturation using blood oxygen level-dependent MR imaging also has been reported. Quantification of cerebral blood oxygen saturation using MR imaging has promising clinical applications; however, technical difficulties have to be resolved. Blood oxygen level dependent MR imaging is an emerging technique to evaluate the cerebral blood oxygen saturation, and it has the potential and versatility to assess oxygenation status of brain tumors. Upon improvement and validation of current MR techniques, better diagnostic, prognostic, and treatment monitoring capabilities can be provided for patients with brain tumors.

Oxygen-enhanced MRI of the brain

Blood oxygenation level-dependent (BOLD) contrast MRI is a potential method for a physiological characterization of tissue beyond mere morphological representation. The purpose of this study was to develop evaluation techniques for such examinations using a hyperoxia challenge. Administration of pure oxygen was applied to test these techniques, as pure oxygen can be expected to induce relatively small signal intensity (SI) changes compared to CO(2)-containing gases and thus requires very sensitive evaluation methods. Fourteen volunteers were investigated by alternating between breathing 100% O(2) and normal air, using two different paradigms of administration. Changes ranged from >30% in large veins to 1.71% +/- 0.14% in basal ganglia and 0.82% +/- 0.08% in white matter. To account for a slow physiological response function, a reference for correlation analysis was derived from the venous reaction. An objective method is presented that allows the adaptation of the significance threshold to the complexity of the paradigm used. Reference signal characteristics in representative brain tissue regions were established. As the presented evaluation scheme proved its applicability to small SI changes induced by pure oxygen, it can readily be used for similar experiments with other gases.

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.

Spectrally inhomogeneous BOLD contrast changes detected in rodent tumors with high spectral and spatial resolution MRI

MRI detects changes in blood-oxygenation-level dependent (BOLD) contrast in tumors caused by tumor oxygenating agents. These changes can be used to guide the design of improved tumor oxygenating treatments (TOXs). The conventional approach to detection of BOLD effects assumes that the water resonance is a single, homogeneously broadened Lorentzian line, and that changes in the T2* of this line owing to changes in deoxyhemoglobin are spectrally homogeneous. This model may not adequately describe BOLD contrast changes in complex water resonances that are often detected in tumors. The present work investigated: (a) whether changes in the water resonance in very small voxels caused by tumor oxygenating agents are spectrally inhomogeneous; and (b) whether high spectral and spatial resolution (HiSS) MRI of the water and fat resonances detects these changes more accurately than conventional gradient-recalled echo (GRE) imaging. Carbogen (95% oxygen, 5% CO2) was used to increase tumor oxygenation. In two tumor models [mammary adenocarcinoma (R3230Ac; n=5) and rhabdomyosarcoma (BA1112; n=5)] proton signals were often complex and inhomogeneously broadened. Spectrally inhomogeneous changes during carbogen breathing occurred in at least 10% of the R3230AC tumor voxels that responded to carbogen and 18% of BA1112 tumor voxels. The largest changes during carbogen breathing in many voxels occurred at frequencies that were significantly different from the frequency of the primary water peak. Carbogen-induced changes in proton T2* detected by simulated GRE and HiSS differed by more than 75% in 67% of voxels in R3230Ac tumors and in 65% of voxels in BA1112 tumors. The spectrally inhomogeneous effects of tumor oxygenating agents may reflect changes in sub-voxelar microenvironements and thus may be important for accurate evaluation of the effects of therapy.

Oxygen-induced MR signal changes in murine tumors

Breathing of 100% oxygen was used to challenge vascular autoregulation in 14 mice with either osteosarcomas (n = 6) or mammary carcinomas (n = 8). Reproducible and statistically significant signal intensity changes of -29 +/- 6% to +35 +/- 3% were observed on heavily T2*-weighted images in the tumors during the oxygen challenge. No significant changes were observed in muscle. For the mammary carcinomas a higher percentage of tumor voxels showed significant signal-intensity decrease (31 +/- 8%) compared to the percentage of voxels showing a signal-intensity increase (22 +/- 3%). In contrast, for the osteosarcomas, a higher percentage of tumor voxels showed signal-intensity increase (52 +/- 9%) compared to the percentage of voxels showing signal-intensity decrease (27 +/- 9%). The regional distribution of these signal intensity changes did not correlate with the signal pattern on T1-, T2-,and T2*-weighted and Gd-DTPA enhanced images acquired without breathing 100% oxygen. Most likely, the signal intensity changes represented the inability of the tumor's neovascularization for autoregulation during the oxygen challenge, particularly in hypoxic regions. Although further investigation is needed, the findings that malignant tumor tissue showed signal intensity changes, whereas normal muscle tissue did not, suggests that this technique may prove useful in distinguishing benign from malignant tissue.

Regional tumor oxygen dynamics: 19F PBSR EPI of hexafluorobenzene

We demonstrate a novel approach to measuring regional tumor oxygen tension using 19F pulse burst saturation recovery echo planar imaging (EPI) relaxometry of hexafluorobenzene. Hexafluorobenzene offers exceptional sensitivity to changes in oxygen tension, and has a single resonance making it ideal for imaging studies. By combining a pulse burst saturation recovery preparation sequence with EPI, the relaxation experiments were performed in approximately 20 min facilitating measurements of dynamic changes in pO2 accompanying interventions. Direct intratumoral administration of hexafluorobenzene permitted labeling of specific regions of interest, and imaging provided maps of pO2, confirming distinct intra tumoral heterogeneity. For a group of three Dunning prostate adenocarcinoma R3327-AT1 tumors interrogation of the central tumor region showed skewed pO2 distributions with considerable radiobiological hypoxia (approximately 90% voxels had pO2 < 15 torr) when rats breathed 33% O2. Altering the inspired gas to pure oxygen caused distributions to shift towards increased pO2 with significant increases in mean oxygen tension (p < 0.05) in two cases. Interrogation of both central and peripheral regions in a fourth tumor showed bimodal distribution for tumor oxygenation including approximately 75% voxels with pO2 > 15 torr. EPI allows the fate of individual voxels to be traced: upon altering the inspired gas to pure oxygen those voxels with baseline pO2 > 30 torr showed significant changes (p < 0.05), whereas those with pO2 < 16 torr showed minimal response. The precision of the measurements, together with the ability to simultaneously examine dynamic changes in multiple regions should provide a useful technique for investigating tumor hypoxia with respect to therapy.

Spectroscopic imaging of the water resonance with short repetition time to study tumor response to hyperoxia

A variety of treatments that modulate tumor oxygen tension are used clinically to improve the outcome of radiotherapy. High resolution, noninvasive measurements of the effects of these treatments would greatly facilitate the development of improved therapies and could guide treatment of cancer patients. Previous work demonstrated that magnetic resonance (MR) gradient echo imaging of the water proton resonance detects changes in T2* and T1 in tumors during hyperoxia that may reflect increased tumor oxygenation. This report describes the use of high resolution MR spectroscopic imaging with short repetition time (TR = 0.2 s) to improve the accuracy with which changes in T2* and T1 are measured. Mammary adenocarcinomas grown in the hind limbs of rats were studied. Carbogen inhalation was used to induce hyperoxia. A single 2-mm slice through the center of tumors and underlying muscle was imaged at 4.7 Tesla with in-plane resolution of approximately 1.2 mm and frequency resolution of 5.8 Hz. The peak integral increased by an average of 6% in tumors during carbogen inhalation suggesting a decrease in T1 (n = 8, P < 0.001). Peak height increased by an average of 15% in tumors during carbogen inhalation (n = 8, P < 0.001). The large difference between increases in peak height and peak integral demonstrates that the width of the water resonance decreased. Assuming a Lorentzian lineshape, an average increase of 12% in T2* was observed in tumors. In muscle, peak integral and peak height increased slightly (about 1.2% and 3%, respectively; P < 0.02) during carbogen inhalation but no significant change in T2* was observed. Spectroscopic imaging detects changes in the water proton resonance in tumors during hyperoxia accurately and reproducibly with high signal-to-noise ratio and allows clear separation of T1 and T2* effects. Increases in T2* may be due to decreased deoxyhemoglobin in tumor blood vessels (i.e., the BOLD effect) and may provide a clinically useful index of increases in tumor oxygenation.

Role of dissolved plasma oxygen in hyperoxia-induced contrast

Currently, hyperoxia is being investigated as a method for producing contrast in magnetic resonance images of the brain, solid tumors, and the eye. However, the underlying physiological mechanisms involved in this type of contrast are still not completely understood. For example, under what conditions would dissolved plasma oxygen contribute to the hyperoxia-induced contrast? Using the eye as a model system, we varied the level of dissolved plasma oxygen and observed different patterns of contrast in the vitreous. The observed contrast changes were consistent with tissue oxygen buffering by hemoglobin at an arterial PO2 of 200 mm Hg and dissolved oxygen offloading at arterial PO2's > 350 mm Hg. These data demonstrate that dissolved plasma oxygen does not become an important contrast mechanism until the arterial oxygen tension exceeds approximately 350 mm Hg. The implication of this result to studies in other organs is discussed.

Hexafluorobenzene: a sensitive 19F NMR indicator of tumor oxygenation

We have surveyed the sensitivity of the spin lattice relaxation rates of the 19F resonances of several perfluorocarbons to changes in oxygen tension and temperature. Hexafluorobenzene was found to exhibit exceptional sensitivity to changes in oxygen tension, and we have exploited this phenomenon to measure tumor oxygen tension following intratumoral injection. When 20 microliters hexaflourobenzene were injected they remained localized and the biodistribution was readily assessed on the basis of combined 1H and 19F three-dimensional MRI. Relaxation measurements indicated a typical baseline oxygen tension of 4.0 +/- 1.5 torr in the central region of a Dunning prostate R3327-AT1 tumor when the rat breathed 66% oxygen. Altering the inspired oxygen concentration to 100% produced a modest increase in pO2 (5.6 +/- 0.7 torr; p < 0.1). Significantly, the precision of these measurements should facilitate NMR investigations of radiobiological hypoxia. Intra-tumoral injection allowed measurements from regions not normally accessible to infused perfluorocarbons and provides an additional approach to measuring tumor oxygenation.

Changes in T2*-weighted images during hyperoxia differentiate tumors from normal tissue

Experiments were performed to determine whether changes in T2*-weighted MR images during and after hyperoxia differentiate tumors from normal tissue. Mammary adenocarcinomas implanted in the right hind limbs of rats were studied. Gradient echo images were obtained at 2 Tesla with an evolution time of 20 ms and a recycle time of 1 s. Breathing gas was either air or 100% O2. Significant increases in image intensity were observed in tumor centers and rims during hyperoxia while much smaller changes were detected in the surrounding muscle. The relaxation rate (1/T2*) in tumors decreased during hyperoxia by an average of 2.5 +/- 1.0 s-1, while in muscle the average change was an increase of 0.6 +/- 2.1 s-1. The largest decreases in relaxation rate were detected in non-necrotic tumor regions with relatively low density of blood vessels. Immediately following hyperoxia significant decreases in intensity were detected in tumors while much smaller decreases were detected in the surrounding muscle.