JNM: Simultaneous PET/3D FOT is feasible

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Phantom and in vivo experiments have demonstrated the feasibility of simultaneous PET and 3D fluorescence optical tomography (FOT) imaging, according to research published in the August issue of the Journal of the Nuclear Medicine.

“A perfect biomedical imaging modality would provide anatomic, functional, physiologic and molecular information,” the study authors wrote. “Integrated PET and 3D FOT imaging has unique and attractive features for in vivo molecular imaging applications.”

Thus, Changqing Li, PhD, and colleagues from the department of biomedical engineering at University of California, Davis, designed, built and evaluated a simultaneous PET and 3D FOT system.

They noted that the design of their FOT system is compatible with many existing small-animal PET scanners. The system comprises an aluminum conical mirror that is used to view the whole-body surface of a mouse with an electron-multiplying charge-coupled device camera when a collimated laser beam is projected on the mouse to stimulate fluorescence.

The researchers used a diffusion equation to model the propagation of optical photons inside the mouse body, and 3D fluorescence images were reconstructed iteratively from the fluorescence intensity measurements measured from the surface of the mouse. Insertion of the conical mirror into the gantry of a small-animal PET scanner allowed simultaneous PET and 3D FOT imaging.

In the phantom experiment, the distribution of the radionuclide and fluorophore were “accurately reconstructed” for location, wrote the authors, who added that several enhancements are planned. These enhancements include more excitation locations across multiple surfaces of the phantoms, excitation and detection at multiple wavelengths, increased cooling of the EMCCD camera to reduce the noise and improved rejection of ambient light.

Li and colleagues evaluated the mutual interactions between PET and 3D FOT experimentally. PET has “negligible effects” on 3D FOT performance, according to the authors. The inserted conical mirror introduces a reduction in the sensitivity and noise-equivalent count rate of the PET system and increases the scatter fraction. They also performed PET-FOT phantom experiments and an in vivo experiment using both PET and FOT.

For the in vivo study, Li and colleagues reported that the simultaneously acquired datasets showed the capability to localize signal in both FOT and PET. “This example is challenging for FOT, as it involves a systemically administered, non-activatable probe, with a broad distribution throughout the body,” the researchers reported. The example represents one of the least favorable cases for reconstruction.

In the future, they plan to develop a new mirror to reduce photon attenuation and scatter in PET, extension to multispectral FOT and the use of a multimodal phantom to compute the rigid registration of the PET and FOT image volumes and account for any small variability in mirror positioning within the PET scanner. Also, they plan to use this system to study multimodality imaging probes and complementary pairs of radiotracers and fluorescent probes in vivo.