Label-free imaging of human brain tissue at subcellular resolution for potential rapid intra-operative assessment of glioma surgery.

Background: Frozen section and smear preparation are the current standard for intraoperative histopathology during cancer surgery. However, these methods are time-consuming and subject to limited sampling. Multiphoton microscopy (MPM) is a high-resolution non-destructive imaging technique capable of optical sectioning in real time with subcellular resolution. In this report, we systematically investigated the feasibility and translation potential of MPM for rapid histopathological assessment of label- and processing-free surgical specimens. Methods: We employed a customized MPM platform to capture architectural and cytological features of biological tissues based on two-photon excited NADH and FAD autofluorescence and second harmonic generation from collagen. Infiltrating glioma, an aggressive disease that requires subcellular resolution for definitive characterization during surgery, was chosen as an example for this validation study. MPM images were collected from resected brain specimens of 19 patients and correlated with histopathology. Deep learning was introduced to assist with image feature recognition. Results: MPM robustly captures diagnostic features of glioma including increased cellularity, cellular and nuclear pleomorphism, microvascular proliferation, necrosis, and collagen deposition. Preliminary application of deep learning to MPM images achieves high accuracy in distinguishing gray from white matter and cancer from non-cancer. We also demonstrate the ability to obtain such images from intact brain tissue with a multiphoton endomicroscope for intraoperative application. Conclusion: Multiphoton imaging correlates well with histopathology and is a promising tool for characterization of cancer and delineation of infiltration within seconds during brain surgery.

Assessment of the effect of therapy in a rat model of glioblastoma using [18F]FDG and [18F]FCho PET compared to contrast-enhanced MRI.

OBJECTIVE: We investigated the potential of [18F]fluorodeoxyglucose ([18F]FDG) and [18F]Fluoromethylcholine ([18F]FCho) PET, compared to contrast-enhanced MRI, for the early detection of treatment response in F98 glioblastoma (GB) rats. METHODS: When GB was confirmed on T2- and contrast-enhanced T1-weighted MRI, animals were randomized into a treatment group (n = 5) receiving MRI-guided 3D conformal arc micro-irradiation (20 Gy) with concomitant temozolomide, and a sham group (n = 5). Effect of treatment was evaluated by MRI and [18F]FDG PET on day 2, 5, 9 and 12 post-treatment and [18F]FCho PET on day 1, 6, 8 and 13 post-treatment. The metabolic tumor volume (MTV) was calculated using a semi-automatic thresholding method and the average tracer uptake within the MTV was converted to a standard uptake value (SUV). RESULTS: To detect treatment response, we found that for [18F]FDG PET (SUVmean x MTV) is superior to MTV only. Using (SUVmean x MTV), [18F]FDG PET detects treatment effect starting as soon as day 5 post-therapy, comparable to contrast-enhanced MRI. Importantly, [18F]FDG PET at delayed time intervals (240 min p.i.) was able to detect the treatment effect earlier, starting at day 2 post-irradiation. No significant differences were found at any time point for both the MTV and (SUVmean x MTV) of [18F]FCho PET. CONCLUSIONS: Both MRI and particularly delayed [18F]FDG PET were able to detect early treatment responses in GB rats, whereas, in this study this was not possible using [18F]FCho PET. Further comparative studies should corroborate these results and should also include (different) amino acid PET tracers.

Assessment of tumor treatment response using active contrast encoding (ACE)-MRI: Comparison with conventional DCE-MRI.

PURPOSE: To investigate the validity of contrast kinetic parameter estimates from Active Contrast Encoding (ACE)-MRI against those from conventional Dynamic Contrast-Enhanced (DCE)-MRI for evaluation of tumor treatment response in mouse tumor models. METHODS: The ACE-MRI method that incorporates measurement of T1 and B1 into the enhancement curve washout region, was implemented on a 7T MRI scanner to measure tracer kinetic model parameters of 4T1 and GL261 tumors with treatment using bevacizumab and 5FU. A portion of the same ACE-MRI data was used for conventional DCE-MRI data analysis with a separately measured pre-contrast T1 map. Tracer kinetic model parameters, such as Ktrans (permeability area surface product) and ve (extracellular space volume fraction), estimated from ACE-MRI were compared with those from DCE-MRI, in terms of correlation and Bland-Altman analyses. RESULTS: A three-fold increase of the median Ktrans by treatment was observed in the flank 4T1 tumors by both ACE-MRI and DCE-MRI. In contrast, the brain tumors did not show a significant change by the treatment in either ACE-MRI or DCE-MRI. Ktrans and ve values of the tumors from ACE-MRI were strongly correlated with those from DCE-MRI methods with correlation coefficients of 0.92 and 0.78, respectively, for the median values of 17 tumors. The Bland-Altman plot analysis showed a mean difference of -0.01 min-1 for Ktrans with the 95% limits of agreement of -0.12 min-1 to 0.09 min-1, and -0.05 with -0.37 to 0.26 for ve. CONCLUSION: The tracer kinetic model parameters estimated from ACE-MRI and their changes by treatment closely matched those of DCE-MRI, which suggests that ACE-MRI can be used in place of conventional DCE-MRI for tumor progression monitoring and treatment response evaluation with a reduced scan time.

Minibeam radiation therapy at a conventional irradiator: Dose-calculation engine and first tumor-bearing animals irradiation.

PURPOSE: Minibeam radiation therapy (MBRT) is a novel therapeutic strategy, whose exploration was hindered due to its restriction to large synchrotrons. Our recent implementation of MBRT in a wide-spread small animal irradiator offers the possibility of performing systematic radiobiological studies. The aim of this research was to develop a set of dosimetric tools to reliably guide biological experiments in the irradiator. METHODS: A Monte Carlo (Geant4)-based dose calculation engine was developed. It was then benchmarked against a series of dosimetric measurements performed with gafchromic films. Two voxelized rat phantoms (ROBY, computer tomography) were used to evaluate the treatment plan of F98 tumor-bearing rats. The response of a group of 7 animals receiving a unilateral irradiation of 58 Gy was compared to a group of non-irradiated controls. RESULTS: The good agreement between calculations and the experimental data allowed the validation of the dose-calculation engine. The latter was first used to compare the dose distributions in computer tomography images of a rat's head and in a digital model of a rat's head (ROBY), obtaining a good general agreement. Finally, with respect to the in vivo experiment, the increase of mean survival time of the treated group with respect to the controls was modest but statistically significant. CONCLUSIONS: The developed dosimetric tools were used to reliably guide the first MBRT treatments of intracranial glioma-bearing rats outside synchrotrons. The significant tumor response obtained with respect to the non-irradiated controls, despite the heterogenous dose coverage of the target, might indicate the participation of non-targeted effects.

A novel Cerenkov luminescence tomography approach using multilayer fully connected neural network.

Cerenkov luminescence tomography (CLT) has been proved as an effective tool for various biomedical applications. Because of the severe scattering of Cerenkov luminescence, the performance of CLT remains unsatisfied. This paper proposed a novel CLT reconstruction approach based on a multilayer fully connected neural network (MFCNN). Monte Carlo simulation data was employed to train the MFCNN, and the complex relationship between the surface signals and the true sources was effectively learned by the network. Both simulation and in vivo experiments were performed to validate the performance of MFCNN CLT, and it was further compared with the typical radiative transfer equation (RTE) based method. The experimental data showed the superiority of MFCNN CLT in terms of accuracy and stability. This promising approach for CLT is expected to improve the performance of optical tomography, and to promote the exploration of machine learning in biomedical applications.

Photoacoustic ratiometric assessment of mitoxantrone release from theranostic ICG-conjugated mesoporous silica nanoparticles.

A theranostic nanosystem based on indocyanine green (ICG) covalently conjugated to mesoporous silica nanoparticles (MSNs) loaded with the anticancer drug mitoxantrone (MTX) is proposed as an innovative photoacoustic probe. Taking advantage of the characteristic PA signal displayed by both ICG and MTX, a PA-ratiometric approach was applied to assess the drug release profile from the MSNs. After complete in vitro characterization of the nanoprobe, a proof-of-concept study has been carried out in tumour-bearing mice to evaluate in vivo its effectiveness for cancer imaging and chemotherapeutic agent delivery.

Performance Characteristics of Photoacoustic Imaging Probes with Varying Frequencies and Light-delivery Schemes.

Photoacoustic imaging (PAI) is an emerging biomedical imaging technique that utilizes a combination of light and ultrasound to detect photoabsorbers embedded within tissues. While the clinical utility of PAI has been widely explored for several applications, limitations in light penetration and detector sensitivity have restricted these studies to mostly superficial sites. Given the importance of PA signal generation and detection on light delivery and ultrasound detector frequency, there is an ongoing effort to optimize these parameters to enhance photoabsorber detection at increased depths. With this in mind, in this study we examined performance benchmarks of a commercially available PAI/ultrasound linear array system when using different imaging frequencies and light delivery schemes. A modified light fiber jacket providing focused light delivery (FLD) at the center of the probe was compared with the built-in fiber optics lining the length of the probe. Studies were performed in vitro to compare performance characteristics such as imaging resolution, maximum imaging depth, and sensitivity to varying hematocrit concentration for each frequency and light delivery method. Monte Carlo simulations of each light delivery method revealed increased light penetration with FLD. In tissue-mimicking phantoms, vascular channels used to simulate blood vessels could be visualized at a depth of 2.4 cm when lowering imaging frequency and utilizing FLD. Imaging at lower frequencies with FLD also enabled enhanced detection of varying hematocrit concentration levels at increased depths, although lateral imaging resolution was reduced. Finally, a proof of concept in vivo probe comparison study in a mouse tumor model provided supportive evidence of our in vitro results. Collectively, our findings show that adjusting imaging frequency and applying FLD can be a straightforward approach for improving PAI performance.

InVivo Molecular Ultrasound Assessment of Glioblastoma Neovasculature with Endoglin-Targeted Microbubbles.

Objectives: Glioblastoma, as one of the most malignant cancer in the world, usually shows substantially increased angiogenesis. Endoglin (CD105), which is an alternative proangiogenic growth factor, has been remarkably upregulated on the proliferating glioblastoma neovasculature. However, little is known on the noninvasive assessment of the expression levels of CD105 during glioblastoma progression. Herein, we investigated the potential of the molecular ultrasound imaging for the noninvasive assessment of the expression levels of the biomarker CD105 during the glioblastoma progression. Materials and Methods: The CD105-targeted perfluorocarbon-containing lipid-shelled microbubbles (MBs) were prepared. A parallel flow chamber was employed, in which the CD105-targeted and non-targeted MBs were tested across the CD105 ± expression cell lines. In vivo molecular US imaging was conducted based on a subcutaneous xenograft tumor model (n=9). Finally, the statistical analysis was conducted to quantitatively correlate the attachment numbers of MBs in the parallel flow chamber test with the CD105 expression levels of the cells in the flow cytometry test and the in vivo molecular ultrasound signals with the ex vivo expression levels of CD105 in the immunohistochemical test. Results and Discussion: The attachment numbers of the CD105-targeted MBs significantly correlated with the CD105 expression levels of the cells in the parallel flow chamber test. There was a good correlation between the in vivo molecular ultrasound signals with the CD105-targeted MBs and the ex vivo expression levels of CD105 in the immunohistochemical test. The results indicate that the molecular US imaging is much potential to assess the progression of the glioblastoma neovasculature noninvasively.

Unsupervised Segmentation of 5D Hyperpolarized Carbon-13 MRI Data Using a Fuzzy Markov Random Field Model.

Hyperpolarized MRI with 13C-labelled compounds is an emerging clinical technique allowing in vivo metabolic processes to be characterized non-invasively. Accurate quantification of 13C data, both for clinical and research purposes, typically relies on the use of region-of-interest analysis to detect and compare regions of altered metabolism. However, it is not clear how this should be determined from the five-dimensional data produced and most standard methodologies are unable to exploit the multidimensional nature of the data. Here we propose a solution to the novel problem of 13C image segmentation using a hybrid Markov random field model with continuous fuzzy logic. The algorithm fully utilizes the multi-dimensional data format in order to classify each voxel into one of six distinct classes based on its metabolic characteristics. Bayesian priors fully incorporate spatial, temporal and ratiometric contextual information whilst image contrast from multiple spectral dimensions are considered concurrently by using an analogy from color image segmentation. Performance of the algorithm is demonstrated on in silico data, where the superiority of the approach over a reference thresholding method is consistently observed. Application to in vivo animal data from a pre-clinical subcutaneous tumor model illustrates the ability of the MRF algorithm to successfully detect tumor location whilst avoiding image artifacts. This work has the potential to assist the analysis of human hyperpolarized 13C data in the future.

Multislice CEST MRI improves the spatial assessment of tumor pH.

PURPOSE: Multislice maps of extracellular pH (pHe) are needed to interrogate the heterogeneities of tumors and normal organs. To address this need, we have developed a multislice chemical exchange saturation transfer (CEST) MRI acquisition method with a CEST spectrum-fitting method that measures in vivo pHe over a range of 6.3 to 7.4. METHODS: The phase offset multiplanar (POMP) method was adapted for CEST fast imaging with steady-state free precession (FISP) MRI to acquire multiple image slices with a single CEST saturation pulse. The Bloch-McConnell equations were modified to include pH based on a calibration of pH and chemical exchange rate for the contrast agent iopamidol. These equations were used to estimate the pixel-wise pHe values throughout the multislice acidoCEST MR images of the tumor, kidney, bladder, and other tissues of a MDA-MB-231 tumor model. RESULTS: Multislice acidoCEST MRI successfully mapped a gradient of pHe from 6.73 to 6.81 units from the tumor core to rim, and also mapped a gradient of pHe 6.56 to 6.97 across the mouse kidney. The bladder was found to be pHe 6.3. CONCLUSION: AcidoCEST MRI with POMP acquisition and Bloch-McConnel analysis can map pHe in multiple imaging slices through the tumor, kidney, and bladder. This multislice evaluation facilitates assessments of spatial heterogeneity of tissue pHe. Magn Reson Med 78:97-106, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

TM1-IR680 peptide for assessment of surgical margin and lymph node metastasis in murine orthotopic model of oral cancer.

Treatment outcome after surgical removal in oral carcinoma is poor due to inadequate methodologies available for marking surgical margins. Even though some methodologies for intraoperative margin assessment are under clinical and preclinical trials for other solid tumours, a promising modality for oral cancer surgery is not developed. Fluorescent-based optical imaging using Near Infrared (NIR) dyes tagged to tumour specific target will be an optimal tool for this purpose. One such target, Gastrin Releasing Peptide Receptor (GRPR) was selected for the study, and its binding peptide, TM1-IR680, was tested for its efficacy for surgical margin prediction in murine orthotopic model of oral cancer, derived from primary samples. Here, for the first time in a preclinical analysis, we show that the size and margin of oral cancer can be predicted, as revealed by 3D-imaging. Interestingly, the peptide was sensitive enough to detect lymph nodes that harboured dispersed tumour cells before colonization, which was impossible to identify by conventional histopathology. We recommend the use of TM1-NIR dyes alone or in combination with other technologies to improve the clinical outcome of oral cancer surgery.

DW-MRI and (18) F-FLT PET for early assessment of response to radiation therapy associated with hypoxia-driven interventions. Preclinical studies using manipulation of oxygenation and/or dose escalation.

Early markers of treatment response may help in the management of patients by predicting the outcome of a specific therapeutic intervention. Here, we studied the potential value of diffusion-weighted MRI (DW-MRI) and (18)F-fluorothymidine ((18)F-FLT), markers of cell death and cell proliferation respectively, to predict the response to irradiation. In addition, dose escalation and/or carbogen breathing were used to modulate the response to irradiation. The studies were performed on two hypoxic rat tumor models: rhabdomyosarcoma and 9L-glioma. The rats were imaged using MRI and PET before and two days after the treatment. In both tumor models, changes in ADC (apparent diffusion coefficient) and (18)F-FLT SUV (standardized uptake value) were significantly correlated with the tumor growth delay. For both tumor models, the ADC values increased in all irradiated groups two days after the treatment while they decreased in the untreated groups. At the same time, the uptake of (18)F-FLT increased in the untreated groups and decreased in all treated groups. Yet, ADC values were not sensitive enough to predict the added value of dose escalation or carbogen breathing in either model. Change in (18)F-FLT uptake was able to predict the higher tumor response when using increased dose of irradiation, but not when using a carbogen breathing challenge. Our results also emphasize that the magnitude of change in (18)F-FLT uptake was strongly dependent on the tumor model.

Contrast-enhanced ultrasound with perflubutane in the assessment of anti-angiogenic effects: early prediction of the anticancer activity of bevacizumab in a mouse xenografted model.

To investigate the feasibility of contrast-enhanced ultrasound (CEUS) with perflubutane for evaluating anti-angiogenic effects, we assessed the contrast enhancement of mice xenograft treated with bevacizumab. SJSA-1 implanted mice were imaged before and 2, 6, 9 and 13 d after initiation of bevacizumab or saline treatment. Intra-tumoral perfusion areas were quantified by binarizing the ultrasound images and the micro-vessel density was observed by CD31 immunohistochemistry. As a result, the perfusion area and its ratio in the tumor were smaller in the bevacizumab group than the control group at 9 and 13 d, although tumor size was not significantly different. CD31-positive areas were smaller in the bevacizumab group than the control group and correlated well with the ratio of intra-tumoral perfusion areas. CEUS with perflubutane was found to have potential for early prediction of the anti-cancer activity of bevacizumab, and the perfusion area measured by binarized ultrasound images could be used as an indicator.

Characterization of [(99m)Tc]Duramycin as a SPECT Imaging Agent for Early Assessment of Tumor Apoptosis.

PURPOSE: We investigated the usefulness of [(99m)Tc]duramycin for monitoring early response to cancer therapy in mice, with an eye towards clinical translation. PROCEDURES: [(99m)Tc]Duramycin was injected in healthy CD1-/- mice to estimate human [(99m)Tc]duramycin radiation dose. [(99m)Tc]Duramycin single-photon emission computed tomography (SPECT) imaging of apoptosis was evaluated in a mouse model of colorectal cancer treated with irinotecan and validated ex vivo using autoradiography, cleaved caspase-3, and TdT-mediated dUTP nick-end labeling (TUNEL) histology of the tumors. RESULTS: The mean effective dose was estimated to be 3.74 × 10(-3) ± 3.43 × 10(-4) mSv/MBq for non-purified and 3.19 × 10(-3) ± 2.16 × 10(-4) mSv/MBq for purified [(99m)Tc]duramycin. [(99m)Tc]Duramycin uptake in vivo following therapy increased significantly in apoptotic irinotecan-treated tumors (p = 0.008). Radioactivity in the tumors positively correlated with cleaved caspase-3 (r = 0.85, p < 0.001) and TUNEL (r = 0.92, p < 0.001) staining. CONCLUSION: [(99m)Tc]Duramycin can be used to detect early chemotherapy-induced tumor cell death, and thus, may be a prospective candidate for clinical SPECT imaging of tumor response to therapy.

Preclinical assessment of early tumor response after irradiation by positron emission tomography with 2-amino-[3-¹¹C]isobutyric acid.

The positron emission tomography (PET) probe, 2-amino-[3-¹¹C]isobutyric acid ([3-¹¹C]AIB), is reported to accumulate less in inflammatory lesions than 2-deoxy-2-[¹8F]fluoro-D-glucose ([¹8F]FDG) and has the potential for evaluation of the efficacy of radiotherapy. To determine whether [3-¹¹C]AIB is useful to monitor early metabolic change in tumors after radiotherapy, we evaluated the temporal change in [3-¹¹C]AIB tumor uptake, tumor volume, histological features and expression of amino acid transporters early after radiotherapy in a mouse tumor model. PET with [3-¹¹C]AIB was conducted in mice bearing a subcutaneous tumor (SY, derived from small cell lung cancer) in two schedules: schedule 1, before (day -1) and after (days 1 and 3) 15 Gy of radiation and schedule 2, days -1, 1 and 5. [3-¹¹C]AIB tumor uptake tended to increase on day 1 after irradiation and decreased thereafter. Tumor uptake was not correlated with tumor volume in schedule 1. Although tumor uptake was correlated with tumor volume in schedule 2, this correlation was lost when the day 5 data of greatly reduced tumor volumes were excluded. In a separate group of tumor-bearing mice, excised tumor sections were stained with terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) or anti-Ki-67 antibody. There was no correlation between tumor uptake and percentages of TUNEL- or Ki-67-positive cells. Expression of amino acid transporters, SLC38A1, SLC38A2 and SLC38A4, was determined by real-time RT-PCR. SLC38A1 and SLC38A2 were expressed in SY tumors, and a significant correlation was observed between [3-¹¹C]AIB tumor uptake and SLC38A1 expression. In conclusion, early change in [3-¹¹C]AIB tumor uptake after irradiation reflected the temporal change in amino acid transporter expression, while it was independent of change in tumor volume, apoptosis and cell proliferation. PET with [3-¹¹C]AIB has the potential for use in non-invasive evaluation of early metabolic change after irradiation before morphological change of tumors.

Dual-energy computed tomography for the assessment of early treatment effects of regorafenib in a preclinical tumor model: comparison with dynamic contrast-enhanced CT and conventional contrast-enhanced single-energy CT.

OBJECTIVES: The potential diagnostic value of dual-energy computed tomography (DE-CT) compared to dynamic contrast-enhanced CT (DCE-CT) and conventional contrast-enhanced CT (CE-CT) in the assessment of early regorafenib treatment effects was evaluated in a preclinical setting. METHODS: A rat GS9L glioma model was examined with contrast-enhanced dynamic DE-CT measurements (80 kV/140 kV) for 4 min before and on days 1 and 4 after the start of daily regorafenib or placebo treatment. Tumour time-density curves (0-240 s, 80 kV), DE-CT (60 s) derived iodine maps and the DCE-CT (0-30 s, 80 kV) based parameters blood flow (BF), blood volume (BV) and permeability (PMB) were calculated and compared to conventional CE-CT (60 s, 80 kV). RESULTS: The regorafenib group showed a marked decrease in the tumour time-density curve, a significantly lower iodine concentration and a significantly lower PMB on day 1 and 4 compared to baseline, which was not observed for the placebo group. CE-CT showed a significant decrease in tumour density on day 4 but not on day 1. The DE-CT-derived iodine concentrations correlated with PMB and BV but not with BF. CONCLUSIONS: DE-CT allows early treatment monitoring, which correlates with DCE-CT. Superior performance was observed compared to single-energy CE-CT. KEY POINTS: • Regorafenib treatment response was evaluated by CT in a rat tumour model. • Dual-energy contrast-enhanced CT allows early treatment monitoring of targeted anti-tumour therapies. • Dual-energy CT showed higher diagnostic potential than conventional contrast enhanced single-energy CT. • Dual-energy CT showed diagnostic potential comparable to dynamic contrast-enhanced CT. • Dual-energy CT is a promising method for efficient clinical treatment response evaluation.

Stratification of 18F-labeled PET imaging agents for the assessment of antiangiogenic therapy responses in tumors.

UNLABELLED: Successful antiangiogenic therapies have been developed for the treatment of various cancers, but not all patients respond. Therefore, the early determination of therapy efficacy is essential for patient management. This study was done to evaluate the utility of various PET imaging biomarkers for early determination of the response to therapy with the antiangiogenic agent axitinib, a multiple receptor tyrosine kinase inhibitor, in tumors with diverse biologic characteristics. METHODS: Mice bearing U87-MG and MDA-MB-231 subcutaneous tumors were treated with axitinib (25 mg/kg intraperitoneally daily for 10 d), and tumor volumes were assessed with caliper measurements. The animals were concurrently imaged longitudinally with (18)F-FDG, 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT), and 2-(18)F-fluoroethyl-triazolyl conjugated c(RGDyK) peptide ((18)F-FtRGD) to determine the optimal radiopharmaceutical for measuring the early treatment response in the 2 tumor types. RESULTS: Daily administration of axitinib successfully retarded the growth of both U87-MG and MDA-MB-231 subcutaneous tumors, with significant differences in tumor volumes being observed from day 7 after therapy on. (18)F-FDG revealed a treatment efficacy response only at day 10 after treatment in both U87-MG tumor-bearing and MDA-MB-231 tumor-bearing animals. (18)F-FLT afforded earlier detection of the therapy response, revealing a significant difference between drug- and vehicle-treated animals at day 3 for animals bearing U87-MG tumors and at day 7 for animals bearing the more slowly growing MDA-MB-231 tumors. (18)F-FtRGD showed a rapid change in tumor retention that reached significance by day 7 in U87-MG tumor-bearing animals; in contrast, no significant difference in tumor retention was observed in MDA-MB-231 tumor-bearing animals. CONCLUSION: Longitudinal imaging with different radiopharmaceuticals displays various characteristics in different tumor types, depending on their biologic characteristics. Such studies may provide clinically important information to guide patient management and monitor the response to antiangiogenic therapy with the optimum noninvasive imaging agent in the relevant cancer type.

Early assessment of the efficacy of temozolomide chemotherapy in experimental glioblastoma using [18F]FLT-PET imaging.

UNLABELLED: Addition of temozolomide (TMZ) to radiation therapy is the standard treatment for patients with glioblastoma (GBM). However, there is uncertainty regarding the effectiveness of TMZ. Considering the rapid evolution of the disease, methods to assess TMZ efficacy early during treatment would be of great benefit. Our aim was to monitor early effects of TMZ in a mouse model of GBM using positron emission tomography (PET) with 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT). METHODS: Human glioma cells sensitive to TMZ (Gli36dEGFR-1) were treated with sub-lethal doses of TMZ to obtain cells with lower sensitivity to TMZ (Gli36dEGFR-2), as measured by growth and clonogenic assays. Gli36dEGFR-1 and Gli36dEGFR-2 cells were subcutaneously (s.c.) or intracranially (i.c.) xenografted into nude mice. Mice were treated for 7 days with daily injection of 25 or 50 mg/kg TMZ. Treatment efficacy was measured using [(18)F]FLT-PET before treatment and after 2 days. Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) were used to determine tumor volumes before treatment and after 7 days. RESULTS: A significant difference was observed between TMZ and DMSO treated tumors in terms of variations of [(18)F]FLT T/B ratio as soon as day 2 in the i.c. as well as in the s.c. mouse model. Variations of [(18)F]FLT T/B uptake ratio between days 0 and 2 correlated with variations of tumor size between days 0 and 7 (s.c. model: n(tumor) = 17 in n(mice) = 11, P<0.01; i.c. model: n(tumor/mice) = 9, P<0.01). CONCLUSIONS: Our results indicate that [(18)F]FLT-PET may be useful for an early evaluation of the response of GBM to TMZ chemotherapy in patients with glioma.

Simultaneous attenuation and scatter corrections from the projections in small animal PET imaging.

Attenuation and scatter corrections are important in quantitative positron emission tomography (PET) imaging even in small animals such as mice and rats. In this work we describe a simple and efficient model to correct for both scatter and attenuation in a single operation. The model aims to solve the equation M=(A+F) P for the primaries P, corrected for attenuation and scatter, based on the measured coincidences M, the matrix of compensation for attenuation A and on the scatter fractions F issued from all emitting sources and contributing to M. The scatter functions are analytically calculated using Klein-Nishina formula, the scanner geometry and the detection efficiencies. This method was applied in measured data of line sources and hot spots phantoms as well as in rat heart and tumors and compared to Monte Carlo based simulations and to the single scatter simulation model developed by Watson et al. The corrected data showed a quantitative contrast and signal to noise ratio enhancement with respect to the uncorrected data. In terms of results, our method is comparable to that of Watson et al. The Monte Carlo simulations, where the primaries and the scattered events were separately registered, confirmed the accuracy of the new approach.

Assessment of molecular imaging of angiogenesis with three-dimensional ultrasonography.

Molecular imaging (MI) with ultrasonography relies on microbubble contrast agents (MCAs) adhering to a ligand-specific target for applications such as characterizing tumor angiogenesis. It is projected that ultrasonic (US) MI can provide information about tumor therapeutic response before the detection of phenotypic changes. One of the limitations of preclinical US MI is that it lacks a comprehensive field of view. We attempted to improve targeted MCA visualization and quantification by performing three-dimensional (3D) MI of tumors expressing αvß3 integrin. Volumetric acquisitions were obtained with a Siemens Sequoia system in cadence pulse sequencing mode by mechanically stepping the transducer elevationally across the tumor in 800-micron increments. MI was performed on rat fibrosarcoma tumors (n  =  8) of similar sizes using MCAs conjugated with a cyclic RGD peptide targeted to αvß3 integrin. US MI and immunohistochemical analyses show high microbubble targeting variability, suggesting that individual two-dimensional (2D) acquisitions risk misrepresenting more complex heterogeneous tissues. In 2D serial studies, where it may be challenging to image the same plane repeatedly, misalignments as small as 800 microns can introduce substantial error. 3D MI, including volumetric analysis of inter- and intra-animal targeting, provides a thorough way of characterizing angiogenesis and will be a more robust assessment technique for the future of MI.