AJR August 2016

Richard Ting                Assistant Professor of Pharmacology                Molecular Imaging Innovations Institute                Department of Radiology                Weill Cornell Medicine

Dual PET and Near-Infrared Fluorescence Imaging Probes as Tools for Imaging in Oncology

This article, published in the August 2016 issue of the American Journal of Roentgenology, describes the current state of positron emission tomography (PET) and fluorescence (FL) multimodality probe development in oncology.

Does your research reveal any preferred imaging combinations that stand out because they have highly synergistic properties?

Positron emission tomography (PET) and fluorescence (FL) on a single probe are a highly synergistic imaging combination. Both contrast agents have approved counterparts that are being used in patients. The two techniques have similar sensitivities, and can be detected at sub-nanomolar quantities. This allows us to image without concern for toxicity or contraindication. At sub-microgram/kilogram doses, some of our most toxic agents are considered non-lethal.

The two modalities complement each other well. PET allows us to image quantitatively through deep tissue, enabling us to view delivery and clearance of an agent. FL is less quantitative through deep tissue, but at the superficial level, FL allows us to image millimeter detail. FL enables clearer visualization of a margin in a surgical field, or biomarker-specific heterogeneity within a histological sample. This detail is difficult to image with a stand-alone PET agent. Both technologies can help guide surgeries, and allow surgeons to corroborate successful resection. This can be important if one modality is less obvious. For example, a surgery may take longer than expected, scintigraphy may not be clear, or a surgeon could resect a generous margin that can obscure an optical signal.

Your article suggests that the full potential of combined PET and fluorescence imaging has yet to be realized. Are there any limits of PET fluorescence imaging that are not likely to be overcome through advanced technology?

Regarding potential: There are many standalone FL and PET agents that are already FDA approved, so we believe that it is inevitable that someone will combine PET and FL in a PET/FL agent for patient use. The two modalities are clearly non-toxic. To the best of my knowledge, I cannot think of a single instance where a PET or FL agent, used at its appropriate imaging concentration, has caused adverse mortality or even morbidity.

PET and FL data are already acquired (and processed) in digital formats (as opposed to film), so these data are immediately available for further processing in advanced applications like computer-guided surgery. This is probably true of many other imaging modalities, but I like how the near-infrared optical wavelengths that are needed to excite fluorophores are already so similar to those being used in computer-guided collision avoidance. Avoiding unnecessary, accidental contact is just as important in surgery as it is in self-driving cars.

Regarding limits: Predicting the limits of a technology is not easy. The ability to image two PET isotopes that are injected into a patient at the same time is a current limitation of PET. The ability to image a fluorophore through very deep or opaque tissue, like the skull, is a limitation with FL. PET is expensive, while FL imaging equipment is not. But these limitations are compensated for in a PET/FL probe, where we combine technologies with complementary limits. Keep in mind that we have not identified the hard limits of PET and FL yet. Super resolution FL and enzyme cofactor-linked FL activation/deactivation are hot fields that will continue to push the limits of FL imaging. There are techniques for deconvoluting deep tissue fluorescent spectra. Time-of-flight PET and PET/MR imaging, and MR imaging reconstruction will advance PET resolution.

What are the advantages of PET/FL combinations?

Both PET and FL can be used in low doses, so complicating contraindications like nephrogenic systemic fibrosis would not need to be considered in a PET/FL probe. Patients would not have to be pre-screened to receive PET/FL contrast. PET is useful in deep tissue, and its quantitative properties are not affected by opaque tissue like bone.

PET allows for dynamic, non-invasive imaging. PET isotopes are also a significant improvement over older single photon emission isotopes; detection is more sensitive and PET-isotope decay is generally quick, therefore long-term ionizing radiation exposure is not a concern.

FL on the other hand, persists in tissue and does not decay. FL can extend the temporal window over which an agent can be visualized. One can theoretically visualize a FL- labeled section over and over again in a histopathology slide years after sample preparation. In open surgery, FL can be used to see fine structure that is not visible by PET. FL can be multiplexed, i.e., different biomarker specific agents that are tagged with different fluorophores can be visualized in a single sample simultaneously.

Did your research result in any information that was unexpected?

Prior to conducting PET/FL experiments, we feared that the resolution of PET would be so great that it would completely obviate the need for a PET/FL probe. We were wrong about this. The degree to which FL and PET moieties complement one another genuinely surprised us.

As we expected, PET is better than FL for deep tissue resolution. For example, a subcutaneous tumor could be superficially visualized in the FL mode, but would disappear if we flipped over our specimen and tried to visualize it through the animal. PET is much better for whole-body deep tissue detection. The resolving power of FL really becomes useful in an open surgical field. We could see fine, sub-millimeter structure like lymph tracks or nerve bundles within an open surgical site that could not be resolved in a PET/CT scan. During pre-clinical surgery, this FL connectivity data is crucial in helping us decide how to expand a surgical site so that the tissue of interest could be properly resected. Both FL and PET could be used to confirm a successful surgery. We particularly appreciate the fact that we could store our resections in paraffin blocks for months (after which PET-isotope decay is complete), but still see FL signal persist in FL histology at 40 and 200 X magnification.

Fortunately, our fears were not realized. PET and FL agents comprehensively expand our ability to image an agent non-invasively on the whole-body, deep-tissue level all the way down to the subcellular, organelle level. In this age of highly personalized medicine involving tumor boards and the identification of intratumoral homogeneity, this data will undoubtedly be more useful.