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

Hypoxia signaling in cancer: Implications for therapeutic interventions

Hypoxia is a persistent physiological feature of many different solid tumors and a key driver of malignancy, and in recent years, it has been recognized as an important target for cancer therapy. Hypoxia occurs in the majority of solid tumors due to a poor vascular oxygen supply that is not sufficient to meet the needs of rapidly proliferating cancer cells. A hypoxic tumor microenvironment (TME) can reduce the effectiveness of other tumor therapies, such as radiotherapy, chemotherapy, and immunotherapy. In this review, we discuss the critical role of hypoxia in tumor development, including tumor metabolism, tumor immunity, and tumor angiogenesis. The treatment methods for hypoxic TME are summarized, including hypoxia-targeted therapy and improving oxygenation by alleviating tumor hypoxia itself. Hyperoxia therapy can be used to improve tissue oxygen partial pressure and relieve tumor hypoxia. We focus on the underlying mechanisms of hyperoxia and their impact on current cancer therapies and discuss the prospects of hyperoxia therapy in cancer treatment.

T(2)∗ relaxation times of intraductal murine mammary cancer, invasive mammary cancer, and normal mammary gland

PURPOSE

This study investigates the feasibility of T(2)∗ to be a diagnostic indicator of early breast cancer in a mouse model. T(2)∗ is sensitive to susceptibility effects due to local inhomogeneity of the magnetic field, e.g., caused by hemosiderin or deoxyhemoglobin. In these mouse models, unlike in patients, the characteristics of single mammary ducts containing pure intraductal cancer can be evaluated.

METHODS

The C3(1)SV40Tag mouse model of breast cancer (n = 11) and normal FVB∕N mice (n = 6) were used to measure T(2)∗ of normal mammary gland tissue, intraepithelial neoplasia, invasive cancers, mammary lymph nodes, and muscle. MRI experiments were performed on a 9.4T animal scanner. High resolution (117 microns) axial 2D multislice gradient echo images with fat suppression were acquired first to identify inguinal mammary gland. Then a multislice multigradient echo pulse sequence with and without fat suppression were performed over the inguinal mammary gland. The modulus of a complex double exponential decay detected by the multigradient echo sequence was used to fit the absolute proton free induction decay averaged over a region of interest to determine the T(2)∗ of water and fat signals.

RESULTS

The measured T(2)∗ values of tumor and muscle are similar (∼15 ms), and almost twice that of lymph nodes (∼8 ms). There was a statistically significant difference (p < 0.03) between T(2)∗ in normal mammary tissue (13.7 ± 2.9 ms) and intraductal cancers (11 ± 2.0 ms) when a fat suppression pulse was applied.

CONCLUSIONS

These are the first reported T(2)∗ measurements from single mammary ducts. The results demonstrated that T(2)∗ measurements may have utility for identifying early pre-invasive cancers in mouse models. This may inspire similar research for patients using T(2)∗ for diagnostic imaging of early breast cancer.

Hyperthermically induced changes in high spectral and spatial resolution MR images of tumor tissue--a pilot study

This pilot study investigated the feasibility of using MRI based on BOLD (blood-oxygen-level-dependent) contrast to detect physiological effects of locally induced hyperthermia in a rodent tumor model. Nude mice bearing AT6.1 rodent prostate tumors inoculated in the hind leg were imaged using a 9.4 T scanner using a multi-gradient echo pulse sequence to acquire high spectral and spatial resolution (HiSS) data. Temperature increases of approximately 6 °C were produced in tumor tissue using fiber-optic-guided light from a 250 W halogen lamp. HiSS data were acquired over three slices through the tumor and leg both prior to and during heating. Water spectra were produced from these datasets for each voxel at each time point. Time-dependent changes in water resonance peak width were measured during 15 min of localized tumor heating. The results demonstrated that hyperthermia produced both significant increases and decreases in water resonance peak width. Average decreases in peak width were significantly larger in the tumor rim than in normal muscle (p = 0.04). The effect of hyperthermia in tumor was spatially heterogeneous, i.e. the standard deviation of the change in peak width was significantly larger in the tumor rim than in normal muscle (p = 0.005). Therefore, mild hyperthermia produces spatially heterogeneous changes in water peak width in both tumor and muscle. This may reflect heterogeneous effects of hyperthermia on local oxygenation. The peak width changes in tumor and muscle were significantly different, perhaps due to abnormal tumor vasculature and metabolism. Response to hyperthermia measured by MRI may be useful for identifying and/or characterizing suspicious lesions as well as guiding the development of new hyperthermia protocols.

HiSStology: high spectral and spatial resolution magnetic resonance imaging detection of vasculature validated by histology and micro-computed tomography

High spectral and spatial resolution (HiSS) data, acquired with echo-planar spectroscopic imaging (EPSI), can be used to acquire water spectra from each small image voxel. These images are sensitive to changes in local susceptibility caused by superparamagnetic iron oxide particles (SPIO); therefore, we hypothesized that images derived from HiSS data are very sensitive to tumor neovasculature following injection of SPIO. Accurate image registration was used to validate HiSS detection of neovasculature with histology and micro-computed tomographic (microCT) angiography. Athymic nude mice and Copenhagen rats were inoculated with Dunning AT6.1 prostate tumor cells in the right hind limb. The tumor region was imaged pre- and post-intravenous injection of SPIO. Three-dimensional assemblies of the CD31-stained histologic slices of the mouse legs and the microCT images of the rat vascular casts were registered with EPSI. The average distance between HiSS-predicted regions of high vascular density on magnetic resonance imaging and CD31-stained regions on histology was 200 μm. Similarly, vessels identified by HiSS in the rat images coincided with vasculature in the registered microCT image. The data demonstrate a strong correlation between tumor vasculature identified using HiSS and two gold standards: histology and microCT angiography.

Fourier component imaging of water resonance in the human breast provides markers for malignancy

The purpose of this paper is to demonstrate that voxels with inhomogeneously broadened water resonances, as revealed by high spectral and spatial resolution (HiSS) MRI, correlate with underlying tumor pathology findings, and thus carry diagnostically useful information. Thirty-four women with mammographically suspicious breast lesions were imaged at 1.5 T, using high-resolution echo-planar spectroscopic imaging. Fourier component images (FCIs) of the off-peak spectral signal were generated, and clusters of voxels with significant inhomogeneous broadening (broadened clusters) were identified and correlated to biopsy results. Inhomogeneously broadened clusters were found significantly more frequently in malignant than in benign lesions. A larger percentage of broadened cluster voxels were found inside the malignant versus benign lesions. The high statistical significance for separation of benign and malignant lesions was robust over a large range of post-processing parameters, with a maximum ROC area under curve of 0.83. In the human breast, an inhomogeneously broadened water resonance can serve as a correlate marker for malignancy and is likely to reflect the underlying anatomy or physiology.

Sensitivity to tumor microvasculature without contrast agents in high spectral and spatial resolution MR images

Contrast-enhanced (CE)-MRI is sensitive to cancers but can produce adverse reactions and suffers from insufficient specificity and morphological detail. This research investigated whether high spectral and spatial resolution (HiSS) MRI detects tumor vasculature without contrast agents, based on the sensitivity of the water resonance line shape to tumor blood vessels. HiSS data from AT6.1 tumors inoculated in the hind legs of rats (N = 8) were collected pre- and post-blood pool contrast agent (iron-oxide particles) injection. The waterline in small voxels was significantly more asymmetric at the tumor rim compared to the tumor center and normal muscle (P < 0.003). Composite images were synthesized, with the intensity in each voxel determined by the Fourier component (FC) of the water resonance having the greatest relative image contrast at that position. We tested whether regions with high contrast in FC images (FCIs) contain vasculature by comparing FCIs with CE-MRI as the "gold standard" of vascular density. The FCIs had 75% +/- 13% sensitivity, 74% +/- 10% specificity, and 91% +/- 4% positive predictive value (PPV) for vasculature detection at the tumor rim. These results suggest that tumor microvasculature can be detected using HiSS imaging without the use of contrast agents.

Quantitative analysis of water proton spectral lineshape: a novel source of contrast in MRI

Previous work in this laboratory has demonstrated improved anatomic and functional images produced from high spectral and spatial resolution (HiSS) MRI of the water proton signal. The present work tests the hypothesis that different Fourier components of the water resonance represent anatomically and/or physiologically distinct populations of water molecules within each small image voxel. HiSS datasets were acquired from tomatoes and rodent tumors at 4.7 T using echo-planar spectroscopic imaging (spatial and spectral resolutions were 117-150 microm and 1.5-3.1 Hz, respectively). Images of each Fourier component of the water resonance (referred to as Fourier component images, or FCIs) were produced. FCIs at frequencies offset from the peak of the water resonance ('off-peak' FCIs) were compared to images of the Fourier component with largest amplitude, i.e. the water peak-height image. Results demonstrate that off-peak FCIs differ significantly from the water peak-height image and that water resonances are often asymmetric. These results show that water signal at various frequency offsets from the peak of the water resonance come from water molecules in different anatomic/physiologic environments. Off-peak FCIs are a new source of structural and functional information and may have clinical utility.

Fat suppression with spectrally selective inversion vs. high spectral and spatial resolution MRI of breast lesions: Qualitative and quantitative comparisons

PURPOSE

To compare conventional fat-suppressed MR images of the breast to images derived from high spectral and spatial resolution MR data. Image quality and the level of fat suppression are compared qualitatively and quantitatively.

MATERIALS AND METHODS

Women with suspicious breast lesions found on X-ray mammography were imaged on 1.5 Tesla GE SIGNA scanners. High spectral and spatial resolution (HiSS) data were acquired using echo-planar spectroscopic imaging. Images with intensity proportional to the water signal peak height in each voxel were synthesized. Conventional fat-suppressed images were acquired using a frequency selective inversion method. The experimental (HiSS) and conventional images were compared by experienced radiologists to evaluate the quality of fat suppression. In addition, fat suppression and image quality were evaluated quantitatively.

RESULTS

Fat suppression, tumor edge delineation, lesion conspicuity, and image texture were improved in the peak height images derived from HiSS data.

CONCLUSION

The results demonstrate that the water peak height images obtained from HiSS data potentially could improve the quality of fat suppression, detection and diagnosis of breast cancer. HiSS allowed detection of lesions and evaluation of lesion morphology prior to contrast media injection. J. Magn. Reson. Imaging 2006. (c) 2006 Wiley-Liss, Inc.

Comparison of high-resolution echo-planar spectroscopic imaging with conventional MR imaging of prostate tumors in mice

High spectral and spatial resolution (HiSS) MRI of rodent tumors has previously been performed using conventional spectroscopic imaging to obtain images with improved contrast and anatomic detail. The work described here evaluates the use of much faster echo-planar spectroscopic imaging (EPSI) to acquire HiSS data from rodent tumor models of prostate cancer. A high-resolution EPSI pulse sequence was implemented on a 4.7 T Bruker scanner. Three-dimensional EPSI data were Fourier-transformed along the k-space and temporal (free-induction decay) axes to produce detailed water and fat spectra associated with each small image voxel. The data were used to generate images of spectral parameters, e.g. peak-height images for each small voxel. Two variants of EPSI were performed; gradient-echo or spin-echo excitation with EPSI readout. These imaging methods were tested in commonly used rodent prostate cancers, including seven mice implanted with non-metastatic AT2.1 (n=3) and metastatic AT3.1 (n=4) prostate tumors on the hind leg, and 10 mice implanted with LNCaP prostate cancers in situ. The peak-height images derived from EPSI datasets provide more detailed tumor anatomy, improved signal-to-noise and contrast-to-noise ratios compared with the gradient-echo or spin-echo images at all echo times. The results suggest that HiSS MRI data from small animal models of prostate cancer can be acquired using EPSI, and that this approach improves imaging of heterogeneous tissue and vascular environments inside the tumors compared with conventional MR techniques.

Anatomical and functional brain imaging using high-resolution echo-planar spectroscopic imaging at 1.5 Tesla

High-resolution echo-planar spectroscopic imaging (EPSI) of water resonance (i.e. without water suppression) is proposed for anatomic and functional imaging of the human brain at 1.5 T. Water spectra with a resolution of 2.6 Hz and a bandwidth of 333 Hz were obtained in small voxels (1.7 x 1.7 x 3 mm3) across a single slice. Although water spectra appeared Lorentzian in most of the voxels in the brain, non-Lorentzian broadening of the water resonance was observed in voxels containing blood vessels. In functional experiments with a motor task, robust activation in motor cortices was observed in high-resolution T2* maps generated from the EPSI data. Shift of the water resonance frequency occurred during neuronal activation in motor cortices. The activation areas appeared to be more localized after excluding the voxels in which the lineshape of the water resonance had elevated T2* and became more non-Lorentzian during the motor task. These preliminary results suggest that high-resolution EPSI is a promising tool to study susceptibility-related effects, such as BOLD contrast, for improved anatomical and functional imaging of the brain.

Increased oxygenation of intracranial tumors by efaproxyn (efaproxiral), an allosteric hemoglobin modifier: In vivo EPR oximetry study

PURPOSE

To determine quantitatively the changes in oxygenation of intracranial tumors induced by efaproxiral, an allosteric hemoglobin modifier. Efaproxiral reduces hemoglobin-oxygen binding affinity, which facilitates oxygen release from hemoglobin into surrounding tissues and potentially increases the pO(2) of the tumors.

METHODS AND MATERIALS

The study was performed on 10 male Fisher 344 rats with 9L intracranial tumors. Electron paramagnetic resonance (EPR) oximetry was used to measure quantitatively the changes in the pO(2) in the tumors. Lithium phthalocyanine (LiPc) crystals were implanted in the tumors and in the normal brain tissue in the opposite hemispheres. We monitored the cerebral pO(2) starting 7 to 10 days after the tumor cells were implanted. NMR imaging determined the position and size of tumor in the brain. After an initial baseline EPR measurement, efaproxiral (150 mg/kg) was injected intravenously over 15 minutes, and measurements of tumor and normal brain oxygen tension were made alternately at 10-minute intervals for the next 60 minutes; the procedure was repeated for 6 consecutive days.

RESULTS

Efaproxiral significantly increased the pO(2) of both the intracranial tumors and the normal brain tissue on all days. The maximum increase was reached at 52.9 to 59.7 minutes and 54.1 to 63.2 minutes after injection, respectively. The pO(2) returned to baseline values at 106 to 126.5 minutes after treatment. The maximum tumor and normal tissue pO(2) values achieved after efaproxiral treatment from Day 1 through Day 6 ranged from 139.7 to 197.7 mm Hg and 103.0 to 135.9 mm Hg, respectively. The maximum increase in tumor tissue pO(2) values from Day 2 to Day 5 was greater than the maximum increase in normal tissue pO(2).

CONCLUSION

We obtained quantitative data on the timing and extent of efaproxiral-induced changes in the pO(2) of intracerebral 9L tumors. These results illustrate a unique and useful capability of in vivo EPR oximetry to obtain repeated noninvasive measurements of tumor oxygenation over a number of days. The information on the dynamics of tumor pO(2) after efaproxiral administration illustrates the ability of efaproxiral to increase intracranial tumor oxygenation.

Fourier components of inhomogeneously broadened water resonances in breast: a new source of MRI contrast

High spectral and spatial resolution (HiSS) MR data were acquired at 1.5 T using echo-planar spectroscopic imaging from patients with suspicious breast lesions. The water resonances in small voxels are inhomogenously broadened and often have distinct components. Images were calculated with intensity proportional to the Fourier components of the water resonance in each voxel at different offsets from the peak frequency. The results demonstrate that in breast the off-peak Fourier component images of water are qualitatively different from those derived from the peak height of the water resonance. These differences most likely reflect underlying anatomy or physiology. In conventional images, the superposition of the various Fourier components of the water signal may cause loss of detail. The synthesis of water Fourier component images from high spectral and spatial resolution data may provide a new form of contrast, and increase sensitivity to subvoxel physiology and anatomy.

Effect of RSR13, an allosteric hemoglobin modifier, on oxygenation in murine tumors: an in vivo electron paramagnetic resonance oximetry and bold MRI study

PURPOSE

RSR13, an allosteric modifier of hemoglobin, reduces hemoglobin-oxygen binding affinity facilitating oxygen release from hemoglobin, resulting in increases in tissue pO(2). The purpose of this study was noninvasively to monitor the time course and effect of RSR13 on tumor oxygenation, directly using in vivo electron paramagnetic resonance (EPR oximetry), and indirectly using blood oxygen level dependent magnetic resonance imaging (BOLD MRI).

METHODS AND MATERIALS

The study was performed in transplanted radiation-induced fibrosarcoma tumors (RIF-1) in 18 female C3H/HEJ mice, which had two lithium phthalocyanine (LiPc) deposits implanted in the tumor when the tumors reached about 200-600 mm(3). Baseline EPR measurements were made daily for 3 days. Then, for 6 consecutive days and after an initial baseline EPR measurement, RSR13 (150 mg/kg) or vehicle (same volume) was injected intraperitoneally, and measurements of intratumoral oxygen were made at 10-min intervals for the next 60 min. In each mouse, every third day, instead of EPR oximetry, BOLD MRI measurements were made for 60 min after administration of the RSR13.

RESULTS

Based on EPR measurements, RSR13 produced statistically significant temporal increases in tumor pO(2) over the 60-min time course, which reached a maximum at 35-43 min postdose. The average time required to return to the baseline pO(2) was 70-85 min. The maximum increase in tumor tissue pO(2) values after RSR13 treatment from Day 1 to Day 5 (8.3-12.4 mm Hg) was greater than the maximum tumor tissue pO(2) value for Day 6 (4.7 mm Hg, p < 0.01). The maximum increase in pO(2) occurred on Day 2 (12.4 mm Hg) after RSR13 treatment. There was little change in R(2)*, indicating that the RSR13 had minimal detectable effects on total deoxyhemoglobin and hemoglobin-oxygen saturation.

CONCLUSION

The extent of the increase in tumor pO(2) achieved by RSR13 would be expected to lead to a significant increase in the effectiveness of tumor radiotherapy. The lack of a change in the BOLD MRI signal suggests that the tumor physiology was largely unchanged by RSR13. These results illustrate a unique and useful capability of in vivo EPR oximetry and BOLD MRI to obtain repeated measurements of tumor oxygenation and physiology. The dynamics of tumor pO(2) after RSR13 administration may be useful for the design of clinical protocols using allosteric hemoglobin effectors.

The effect of varying spectral resolution on the quality of high spectral and spatial resolution magnetic resonance images of the breast

PURPOSE

To evaluate the effect of varying spectral resolution on image quality of high spectral and spatial resolution (HiSS) images.

MATERIALS AND METHODS

Eight women with suspicious breast lesions and six healthy volunteers were scanned using echo-planar spectroscopic imaging (EPSI) at 1.5 Tesla with 0.75- to 1-mm in-plane resolution and 2.3- to 2.6-Hz spectral resolution. Time domain data were truncated to obtain proton spectra in each voxel with varying (2.6-83.3 Hz) resolution. Images with intensity proportional to water signal peak heights were synthesized. Changes in water signal line shape following contrast injection were analyzed.

RESULTS

Fat suppression is optimized at approximately 10-Hz spectral resolution and is significantly improved by removal of wings of the fat resonance. This was accomplished by subtracting a Lorentzian fit to the fat resonance from the proton spectrum. The water resonance is often inhomogeneously broadened, and very high spectral resolution is necessary to resolve individual components. High spectral resolution is required for optimal contrast in anatomic features with very high T(2)* (e.g., within a lesion) and for detection of often subtle effects of contrast agents on water signal line shape.

CONCLUSION

Despite a trade-off between the spectral resolution and signal-to-noise ratio, it is beneficial to acquire data at the highest spectral resolution currently attainable at 1.5 Tesla.

Using High Spectral and Spatial Resolution Bold MRI to Choose the Optimal Oxygenating Treatment for Individual Cancer Patients

We evaluate whether high spectral and spatial resolution (HiSS) BOLD MRI can correctly rank the effects of three tumor-oxygenating treatments on radiosensitivity in BA1112 rhabdomyosarcomas (n = 5). Significant decreases in spectral linewidth predict that treatment with carbogen gas combined with a perfluorocarbon emulsion will increase radiosensitivity more than either treatment alone, which agrees with the known effects of these treatments on hypoxic fraction. High-resolution maps show that tumor response to each treatment is spatially heterogeneous, and that there is a paradoxical response to the treatments in 7-12% of tumor pixels. Because HiSS MRI emphasizes changes in necrotic and/or hemorrhagic regions, it is more sensitive to oxygenation changes compared to conventional MRI. These results demonstrate that HiSS MRI is a practical, noninvasive method that could be used to choose the treatment that maximizes the size and extent of increases in tumor oxygenation for individual patients.

Effect of carbogen on tumor oxygenation: combined fluorine-19 and proton MRI measurements

PURPOSE

Blood oxygen level dependent (BOLD) contrast in magnetic resonance imaging (MRI) has been widely used for noninvasive evaluation of the effects of tumor-oxygenating agents. However, there have been few tests of the validity of this method. The goal of the present work was to use the T(1) of fluorine-19 in perfluorocarbon (PFC) emulsions as a "gold standard" for comparison with BOLD MRI. MATHODS AND MATERIALS: Rats bearing R3230AC tumors implanted in the hind limb were injected with an emulsion of perfluoro-15-crown-5-ether for 2-3 days before experiments, which ensured that the PFC emulsion concentrated in the tumors. We correlated changes in tumor oxygenation caused by carbogen inhalation measured by (1)H BOLD MRI with quantitative (19)F measurements. The (19)F spin-lattice relaxation rate R(1) (= 1/T(1)) was measured to determine initial oxygen tension (pO(2)) in each image pixel containing the PFC, and changes in pO(2) during carbogen (95% O(2), 5% CO(2)) breathing. In a second carbogen breathing period, changes in water signal linewidth were measured using high spectral and spatial resolution imaging. (19)F and (1)H measurements were used to classify pixels as responders to carbogen (pixels where oxygen increased significantly) or nonresponders (no significant change in tumor oxygenation).

RESULTS

The (19)F and (1)H measurements agreed in 65% +/- 11% of pixels (n = 14). Agreement was even stronger among pixels where (1)H showed increased oxygenation; (19)F measurements agreed with (1)H measurements in over 79% +/- 11% of these pixels. Similarly, there was strong agreement between the two modalities in pixels where (19)F reported no change in pO(2); (1)H also showed no changes in 76% +/- 18% of these pixels. Quantitative correlation of changes T(2)* (DeltaT(2)*) in (1)H and changes R(1) (DeltaR(1)) in (19)F was weak during carbogen breathing, and averaged over the whole tumor was approximately 0.40 for 14 experiments. However, the spatial patterns of (1)H and (19)F changes were qualitatively very similar. In hypoxic regions that were identified based on long (19)F T(1) (>2.53 s), (19)F and (1)H MRI agreed that carbogen had relatively weak effects.

CONCLUSIONS

These results suggest that (1)H BOLD MRI reliably identifies increases in tumor pO(2). In hypoxic regions where increases in pO(2) are most desirable, carbogen was ineffective. The data suggest that (19)F and (1)H MRI can be used individually or in combination to guide the design of improved tumor-oxygenating agents.

Oxygen-limited thermal tolerance in Antarctic fish investigated by MRI and (31)P-MRS

The hypothesis of an oxygen-limited thermal tolerance was tested in the Antarctic teleost Pachycara brachycephalum. With the use of flow-through respirometry, in vivo (31)P-NMR spectroscopy, and MRI, we studied energy metabolism, intracellular pH (pH(i)), blood flow, and oxygenation between 0 and 13 degrees C under normoxia (PO(2): 20.3 to 21.3 kPa) and hyperoxia (PO(2): 45 kPa). Hyperoxia reduced the metabolic increment and the rise in arterial blood flow observed under normoxia. The normoxic increase of blood flow leveled off beyond 7 degrees C, indicating a cardiovascular capacity limitation. Ventilatory effort displayed an exponential rise in both groups. In the liver, blood oxygenation increased, whereas in white muscle it remained unaltered (normoxia) or declined (hyperoxia). In both groups, the slope of pH(i) changes followed the alpha-stat pattern below 6 degrees C, whereas it decreased above. In conclusion, aerobic scope declines around 6 degrees C under normoxia, marking the pejus temperature. By reducing circulatory costs, hyperoxia improves aerobic scope but is unable to shift the breakpoint in pH regulation or lethal limits. Hyperoxia appears beneficial at sublethal temperatures, but no longer beyond when cellular or molecular functions become disturbed.

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.

Changes in oxygenation of intracranial tumors with carbogen: a BOLD MRI and EPR oximetry study

PURPOSE

To examine, using blood oxygen level dependent (BOLD) MRI and EPR oximetry, the changes in oxygenation of intracranial tumors induced by carbogen breathing.

MATERIALS AND METHODS

The 9L and CNS-1 intracranial rat tumor models were imaged at 7T, before and during carbogen breathing, using a multi-echo gradient-echo (GE) sequence to map R(2)*. On a different group of 9L tumors, tissue pO(2) was measured using EPR oximetry with lithium phthalocyanine as the oxygen-sensitive material.

RESULTS

The average decline in R(2)* with carbogen breathing was 13 +/- 1 s(-1) in the CNS-1 tumors and 29 +/- 4 s(-1) in the 9L tumor. The SI vs. TE decay curves indicate the presence of multiple components in the tumor. Tissue pO(2) in the two 9L tumors measured was 8.6 +/- 0.5 and 3.6 +/- 0.6 mmHg during air breathing, and rose to 20 +/- 7 and 16 +/- 4 mmHg (mean +/- SE) with carbogen breathing. Significant changes were observed by 10 minutes, but changes in pO(2) and R(2)* continued in some subjects over the entire 40 minutes.

CONCLUSION

EPR results indicate that glial sarcomas may be radiobiologically hypoxic. Both EPR and BOLD data indicate that carbogen breathing increases brain tumor oxygenation. These data support the use of BOLD imaging to monitor changes in oxygenation in brain tumors.

Breast MR imaging with high spectral and spatial resolutions: preliminary experience

The authors evaluated magnetic resonance (MR) imaging with high spectral and spatial resolutions (HSSR) of water and fat in breasts of healthy volunteers (n = 6) and women with suspicious lesions (n = 6). Fat suppression, edge delineation, and image texture were improved on MR images derived from HSSR data compared with those on conventional MR images. HSSR MR imaging data acquired before and after contrast medium injection showed spectrally inhomogeneous changes in the water resonances in small voxels that were not detectable with conventional MR imaging.

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.

Enhancement of tumor oxygenation and radiation response by the allosteric effector of hemoglobin, RSR13

Prior studies using pO(2) microelectrodes have shown that RSR13, an allosteric modifier of hemoglobin, increases tissue oxygenation in vivo. Recently, measurements of tissue oxygenation have been performed by many investigators using blood oxygen level-dependent magnetic resonance imaging (BOLD MRI). In this study, we tested the hypothesis that the BOLD MRI signal ratio in tumors will change after administration of RSR13. NCI-H460 human lung carcinoma cells were used as a xenograft in athymic nude mice. Mice with 1-cm(3) tumors in the flank were anesthetized and mounted on the MRI apparatus, and various doses of RSR13 were administered intraperitoneally (i.p.). MR images were then acquired at 10-min intervals for up to 60 min after injection. The effect of RSR13 on tumor response was studied using the same mouse xenograft model with tumor growth delay measurements. RSR13 increased the MRI signal ratio [Intensity(t)/Intensity(t = 0)] in a dose-dependent manner, with maximum increases occurring 30 min after RSR13 was administered. An RSR13 dose of 200 mg/kg proved to be optimum. Since the MRI signal ratio has been shown previously to be linearly related to tissue oxygenation, the changes in the MRI signal ratio can be attributed to changes in tumor oxygen levels. Using a 200-mg/kg dose of RSR13, with a 10-Gy dose of radiation administered to tumors 30 min later, enhancement of radiation-induced tumor growth delay by RSR13 was observed (enhancement factor = 2.8). Thus our MRI results support and verify the previously reported RSR13-induced increase in tumor oxygenation obtained using pO(2) microelectrodes. Based upon these results and other previous studies, the mechanism of enhancement of the effect of radiation by RSR13 probably involves an increase in tumor oxygenation.

Differentiation of nonmetastatic and metastatic rodent prostate tumors with high spectral and spatial resolution MRI

MR images can be acquired with high spectral and spatial resolution to precisely measure lineshapes of the water and fat resonances in each image voxel. Previous work suggests that the high-resolution spectral information can be used to improve image contrast, SNR, sensitivity to contrast agents and to physiologic and biochemical processes that affect local magnetic susceptibility gradients. The potential advantages of high-resolution spectroscopic imaging (SI) suggest that it might be useful for early detection and characterization of tumors. The present experiments evaluate the use of high-resolution SI to discriminate between metastatic and nonmetastatic rodent Dunning prostate tumors. SI datasets were obtained at 4.7 Tesla with an in-plane resolution of 350-500 micron in a single 1.0-mm slice, and 6-8 Hz spectral resolution, before and after i.v. injection of an iron oxide contrast agent. Images of water signal peak height in nonmetastatic tumors were smoother in the tumor interior than images of metastatic tumors (P <.004 by t-test) before contrast media injection. This difference was stronger in contrast-enhanced images (P <.0004). In addition, the boundary between the tumor and muscle was more clearly demarcated in nonmetastatic than metastatic tumors. Combinations of image texture, tumor edge morphology, and changes in T2* following contrast media injection improved discrimination between metastatic and nonmetastatic tumors. The data presented here do not demonstrate that effective discrimination between metastatic and nonmetastatic tumors depends on the use of high-resolution SI. However, the results suggest that SI and/or other MR methods that provide similar contrast might be used clinically for early and accurate detection of metastatic disease.

Effects of different levels of hypercapnic hyperoxia on tumour R 2 ∗ and arterial blood gases

The hypercapnia induced by carbogen (95% O(2)/5% CO(2)) breathing, which is being re-evaluated as a clinical radiosensitiser, causes patient discomfort and hence poor compliance. Recent preclinical and clinical studies have indicated that the CO(2) content might be lowered without compromising increased tumour oxygenation and radiosensitisation. This preclinical study was designed to see if lower levels of hypercapnia could evoke similar decreases in the transverse relaxation rate R(2)* of rodent tumours to those seen with carbogen breathing. The response of rat GH3 prolactinomas to 1%, 212% and 5% CO(2) in oxygen, and 100% O(2) breathing, was monitored by non-invasive multi-gradient echo MRI to quantify R(2)*. As the oxygenation of haemoglobin is proportional to the blood p(a)O(2) and therefore in equilibrium with tissue pO(2), R(2)* is a sensitive indicator of tissue oxygenation. Hyperoxia alone decreased R(2)* by 13%, whilst all three hypercapnic hyperoxic gases decreased R(2)* by 29%. Breathing 1% CO(2) in oxygen evoked the same decrease in R(2)* as carbogen. The DeltaR(2)* response is primarily consistent with an increase in blood oxygenation, though localised increases in tumour blood flow were also identified in response to hypercapnia. The data support the concept that levels of hypercapnia can be reduced without loss of enhanced oxygenation and hence potential radiotherapeutic benefit.

Effects of nicotinamide and carbogen on tumour oxygenation, blood flow, energetics and blood glucose levels

Both host carbogen (95% oxygen/5% carbon dioxide) breathing and nicotinamide administration enhance tumour radiotherapeutic response and are being re-evaluated in the clinic. Non-invasive magnetic resonance imaging (MRI) and 31P magnetic resonance spectroscopy (MRS) methods have been used to give information on the effects of nicotinamide alone and in combination with host carbogen breathing on transplanted rat GH3 prolactinomas. Gradient recalled echo (GRE) MRI, sensitive to blood oxygenation changes, and spin echo (SE) MRI, sensitive to perfusion/flow, showed large signal intensity increases with carbogen breathing. Nicotinamide, thought to act by suppressing the transient closure of small blood vessels that cause intermittent tumour hypoxia, induced a small increase in blood oxygenation but no detectable change in perfusion/flow. Carbogen combined with nicotinamide was no more effective than carbogen alone. Both carbogen and nicotinamide caused significant increases in the nucleoside triphosphate/inorganic phosphate (betaNTP/Pi) ratio, implying that the tumour cells normally receive sub-optimal substrate supply, and is consistent with either increased glycolysis and/or a switch to more oxidative metabolism. The most striking observation was the marked increase in blood glucose (twofold) induced by both nicotinamide and carbogen. Whether this may play a role in tumour radiosensitivity has yet to be determined.

MRI measurements correctly predict the relative effects of tumor oxygenating agents on hypoxic fraction in rodent BA1112 tumors

PURPOSE

We evaluate whether magnetic resonance imaging (MRI) with blood oxygenation level-dependent (BOLD) contrast correctly predicts the relative effects of tumor-oxygenating agents on hypoxic fraction in BA1112 rhabdomyosarcomas in WAG/Rij rats.

METHODS AND MATERIALS

The response of ten tumors to carbogen (95% O(2)/5% CO(2)), a perfluorocarbon emulsion (PFC), and the combination of PFC + carbogen was studied with high spectral and spatial resolution MR imaging of the water resonance at 4.7 Tesla. Decreases in MR signal linewidth indicate increases in tumor blood oxygen levels.

RESULTS

Average MR signal linewidth was decreased 2.0% by carbogen, 2.5% by PFC + air, and 4.9% by PFC + carbogen. PFC + carbogen caused a larger linewidth decrease than either treatment alone (p < 0.04 by ANOVA). Maps of pixels responding to treatment indicate that combining PFC with carbogen significantly enlarges the area of the tumor in which oxygen levels are increased (p < 0.01 by ANOVA).

CONCLUSION

MRI predicts that PFC + carbogen will increase radiosensitivity more than either treatment alone; this agrees with the known effects of these treatments on hypoxic fraction. Utilizing MRI to choose the treatment that maximizes the size and extent of increases in tumor oxygenation could reduce hypoxic fraction.

Uptake of a superparamagnetic contrast agent imaged by MR with high spectral and spatial resolution

Conventional MRI implicitly treats the proton signal as a single, narrow Lorentzian. However, water signals in vivo are often in homogeneously broadened and have multiple resolvable components. These components represent discrete populations of water molecules within each pixel which are affected differently by physiology and contrast agents. Accurate measurement of each component of the water resonance can improve anatomic and functional MR images and provide insight into the structure and dynamics of subpixelar microenvironments. This report describes high spectral and spatial resolution (HiSS) MR imaging of rodent prostate tumors before and after injection of a superparamagnetic contrast agent. HiSS datasets were used to synthesize images in which intensity is proportional to peak height, peak frequency, and linewidth. These images showed anatomic features which were not clearly delineated in conventional T(2) and gradient echo images. HiSS images obtained after injection of the contrast agent showed T *(2) and T(1) changes which were not seen in conventional images. These changes are associated with microvessel density and permeability. The results suggest HiSS with superparamagnetic contrast agents has the potential to improve characterization of tumors.

Preservation of tumour oxygen after hyperbaric oxygenation monitored by magnetic resonance imaging

Hyperbaric oxygen (HBO) has been proposed to reduce tumour hypoxia by increasing the dissolved molecular oxygen in tissue. Using a non-invasive magnetic resonance imaging (MRI) technique, we monitored the changes in MRI signal intensity after HBO exposure because dissolved paramagnetic molecular oxygen itself shortens the T1 relation time. SCCVII tumour cells transplanted in mice were used. The molecular oxygen-enhanced MR images were acquired using an inversion recovery-preparation fast low angle shot (IR-FLASH) sequence sensitizing the paramagnetic effects of molecular oxygen using a 4.7 tesla MR system. MR signal of muscles decreased rapidly and returned to the control level within 40 min after decompression, whereas that of tumours decreased gradually and remained at a high level 60 min after HBO exposure. In contrast, the signal from the tumours in the normobaric oxygen group showed no significant change. Our data suggested that MR signal changes of tumours and muscles represent an alternation of extravascular oxygenation. The preserving tumour oxygen concentration after HBO exposure may be important regarding adjuvant therapy for cancer patients.

Response of hepatoma 9618a and normal liver to host carbogen and carbon monoxide breathing

The effects of hyperoxia (induced by host carbogen [95% oxygen/5% carbon dioxide breathing] and hypoxia (induced by host carbon monoxide [CO at 660 ppm] breathing) were compared by using noninvasive magnetic resonance (MR) methods to gain simultaneous information on blood flow/oxygenation and the bioenergetic status of rat Morris H9618a hepatomas. Both carbogen and CO breathing induced a 1.5- to 2-fold increase in signal intensity in blood oxygenation level dependent (BOLD) MR images. This was due to a decrease in deoxyhemoglobin (deoxyHb), which acts as an endogenous contrast agent, caused either by formation of oxyhemoglobin in the case of carbogen breathing, or carboxyhemoglobin with CO breathing. The results were confirmed by observation of similar changes in deoxyHb in arterial blood samples examined ex vivo after carbogen or CO breathing. There was no change in nucleoside triphosphates (NTP)/P(i) in either tumor or liver after CO breathing, whereas NTP/P(i) increased twofold in the hepatoma (but not in the liver) after carbogen breathing. No changes in tumor intracellular pH were seen after either treatment, whereas extracellular pH became more alkaline after CO breathing and more acid after carbogen breathing, respectively. This tumor type and the liver are unaffected by CO breathing at 660 ppm, which implies an adequate oxygen supply.

Flow and oxygenation dependent (FLOOD) contrast MR imaging to monitor the response of rat tumors to carbogen breathing

Gradient recalled echo (GRE) images are sensitive to both paramagnetic deoxyhaemoglobin concentration (via T2*) and flow (via T1*). Large GRE signal intensity increases have been observed in subcutaneous tumors during carbogen (5% carbon dioxide, 95% oxygen) breathing. We term this combined effect flow and oxygenation-dependent (FLOOD) contrast. We have now used both spin echo (SE) and GRE images to evaluate how changes in relaxation times and flow contribute to image intensity contrast changes. T1-weighted images, with and without outer slice suppression, and calculated T2, T2* and "flow" maps, were obtained for subcutaneous GH3 prolactinomas in rats during air and carbogen breathing. T1-weighted images showed bright features that increased in size, intensity and number with carbogen breathing. H&E stained histological sections confirmed them to be large blood vessels. Apparent T1 and T2 images were fairly homogeneous with average relaxation times of 850 ms and 37 ms, respectively, during air breathing, with increases of 2% for T1 and 11% for T2 during carbogen breathing. The apparent T2* over all tumors was very heterogeneous, with values between 9 and 23 ms and localized increases of up to 75% during carbogen breathing. Synthesised "flow" maps also showed heterogeneity, and regions of maximum increase in flow did not always coincide with maximum increases in T2*. Carbogen breathing caused a threefold increase in arterial rat blood PaO2, and typically a 50% increase in tumor blood volume as measured by 51Cr-labelled RBC uptake. The T2* increase is therefore due to a decrease in blood deoxyhaemoglobin concentration with the magnitude of the FLOOD response being determined by the vascular density and responsiveness to blood flow modifiers. FLOOD contrast may therefore be of value in assessing the magnitude and heterogeneity of response of individual tumors to blood flow modifiers for both chemotherapy, antiangiogenesis therapy in particular, and radiotherapy.

Causes and effects of heterogeneous perfusion in tumors

A characteristic of solid tumors is their heterogeneous distribution of blood flow, with significant hypoxia and acidity in low-flow regions. We review effects of heterogeneous tumor perfusion are reviewed and propose a conceptual model for its cause. Hypoxic-acidic regions are resistant to chemo- and radiotherapy and may stimulate progression to a more metastatic phenotype. In normal tissues, hypoxia and acidity induce angiogenesis, which is expected to improve perfusion. However, aggressive tumors can have high local microvessel density simultaneously with significant regions of hypoxia and acidosis. A possible explanation for this apparent contradiction is that the mechanisms regulating growth and adaptation of vascular networks are impaired. According to a recent theory for structural adaptation of vascular networks, four interrelated adaptive responses can work as a self-regulating system to produce a mature and efficient blood distribution system in normal tissues. It is proposed that heterogeneous perfusion in tumors may result from perturbation of this system. Angiogenesis may increase perfusion heterogeneity in tumors by increasing the disparity between parallel low- and high-resistance flow pathways. This conceptual model provides a basis for future rational therapies. For example, it indicates that selective destruction of tumor vasculature may increase perfusion efficiency and improve therapeutic efficacy.

Tumour response to hypercapnia and hyperoxia monitored by FLOOD magnetic resonance imaging

Flow and oxygenation dependent (FLOOD) MR images of GH3 prolactinomas display large intensity increases in response to carbogen (5% CO2/95% O2) breathing. To assess the relative contributions of carbon dioxide and oxygen to this response and the tumour oxygenation state, the response of GH3 prolactinomas to 5% CO2/95% air, carbogen and 100% O2 was monitored by FLOOD MRI and PO2 histography. A 10-30% image intensity increase was observed during 5% CO2/95% air breathing, consistent with an increase in tumour blood flow, as a result of CO2-induced vasodilation, reducing the concentration of deoxyhaemoglobin in the blood. Carbogen caused a further 40-50% signal enhancement, suggesting an additional improvement due to increase blood oxygenation. A small 5-10% increase was observed in response to 100% O2, highlighting the dominance of CO2-induced vasodilation in the carbogen response. Despite the large FLOOD response, non-significant increases in tumour pO2 were observed in response to the three gases. Tissue pO2 is determined by the balance of oxygen supply and demand, hence increased blood flow/oxygenation may not necessarily produce a large increase in tissue PO2. The FLOOD response is determined by the level of deoxygenation of blood, the size of this response relating to vascular density and the potential of high-oxygen content gases to improve the oxygen supply to tumour tissue.

Magnetic resonance imaging techniques for monitoring changes in tumor oxygenation and blood flow

The application of functional magnetic resonance (MR) imaging techniques to the measurement of oxygenation and blood flow in tumors is described. Gradient recalled echo MR imaging (GRE-MRI) offers a real-time noninvasive method for monitoring tumor response to vasomodulators such as carbogen (95% O2/5% CO2) breathing in attempts to overcome tumor hypoxia and improve treatment efficacy. Although the response is tumor-type dependent, increases in signal intensity of up to 100% have been observed in several animal tumor types. Responses are also seen in human tumors. The observed increases in GRE-MRI signal intensity are due to a combination of a reduction of deoxyhemoglobin in the blood causing changes in the MR imaging relaxation time T2* and changes in blood flow and may also reflect the capillary density. Thus, the magnitude of the GRE image intensity change gives an indication of the potential response of an individual tumor to treatments that aim to improve tissue oxygenation and therefore how the tumor may respond to therapy. In addition, carbogen breathing by the host has been shown to increase the uptake and efficacy of chemotherapeutic agents in animal tumors.

Fast spectroscopic imaging of water and fat resonances to improve the quality of MR images

RATIONALE AND OBJECTIVES

The authors evaluated whether fast spectroscopic imaging of water and fat resonances can produce high-quality anatomic magnetic resonance (MR) images of rodent tumors and human breast.

MATERIALS AND METHODS

Fast MR spectroscopic images of eight rats with mammary tumors were acquired by using a 4.7-T MR unit equipped with self-shielded gradient coils. MR spectroscopic images of four human breasts were acquired with a 1.5-T MR unit.

RESULTS

Artifacts due to eddy currents were minimal. Images synthesized from MR spectroscopic data, in which intensity was proportional to water signal peak height, were similar to T2-weighted MR images. Boundaries of rodent mammary tumors are similar but not identical on peak height-weighted and T2-weighted images. MR spectroscopic images of human breast showed improved detail compared to gradient-echo MR images.

CONCLUSION

Preliminary results suggest that incorporation of fast MR spectroscopic imaging methods into many standard clinical MR imaging procedures may substantially improve image quality.