A recent study has reported the first use of Cerenkov radiation to optically image a mouse (Robertson R, Germanos MS, Li C, Mitchell GS, Cherry SR, Silva MD.  Phys Med Biol 54:N355-65, 2009).  Cerenkov radiation occurs when high energy particles such as positrons momentarily travel faster than light in tissues.  The radiation is produced in a continuous spectrum from near ultraviolet through the visible spectrum and hence can be imaged using sensitive CCD methods optimized for optical imaging of small animals.  In a further investigation, Cerenkov Luminescent Tomography (CLT) was used to noninvasively image the ¹⁸F-FDG concentration in a mouse.  Briefly, the animal was injected with ¹⁸F-FDG (352 μCi) 2.5 hours before performing a 15-min PET scan (μPET II).  MicroCT (Inveon SPECT/CT) and optical (Xenogen, IVIS 100) images were also acquired and the images overlaid.  In the left figure, the colored regions are the optically imaged concentration of ¹⁸F-FDG reconstructed with CLT from the surface measurements of optical photons produced by Cerenkov radiation.   The regions correspond to the heart and urinary bladder which characteristically display high ¹⁸F-FDG concentrations in the PET scan on the right (CT and PET).  CLT is a new molecular imaging tool for preclinical studies that may provide a high-throughput, low cost PET alternative.  But more than that, it may be anticipated that the detection and utilization of Cerenkov radiation may lead to further advances in molecular imaging techniques. 

Multiple microPET (Focus 120) mouse images from a high-throughput screening study of [¹⁸F] labeled-peptides targeting the a_vb₆ integrin. In vitro assays had identified 42 peptides with significant affinity and/or selectivity to a_vb₆, and all compounds (+ 1 control) were evaluated in vivo with microPET on 11 consecutive days.  Two mice were scanned simultaneously with each peptide (60 min dynamic scan post-injection and static scan at 180 min).  A key finding was that the 4 most promising compounds identified in vivo did not correspond to the most promising compounds identified preliminarily in vitro.  The image was awarded Siemens’ Most Innovative Preclinical Image of the Year, 2007.   (Gagnon, MKJ, Marik J, Hausner SH, Abbey CK, Sutcliffe JL; High-throughput screening of molecular imaging agents using microPET.  54th Society of Nuclear Medicine Annual Meeting, Washington, D.C; June, 2007)

MicroCT (MicroCAT II) image of a moray eel partial skeleton of a preserved specimen.  Bones of the skeleton were rendered by a surface rendering algorithm and false coloration was used to distinguish selected anatomical features.  The investigator can use the surface mesh of the skull in a finite element analysis algorithm to study where forces are applied to the skull during biting in moray eels.  This project uses 3D models generated from microCT scans to determine how variations in biting behavior of eels affects stresses on the skull, and how that may have affected the evolution of skull shape in both closely related as well as disparate lineages. (RS Mehta, Section of Evolution and Ecology, U.C.Davis)

The inhibitory effect of the chemopreventative agent, rapamycin, on the development of a pre-malignant mammary lesion was assessed in a mouse model.  Four weeks after receiving mammary tissue xenografts, animals were scanned (PET/FDG) for 8 successive weeks.  Rapamycin was administered for 2 weeks, then stopped for 2 weeks, and finally readministered for 3 weeks.  Coronal maximum intensity projections (MIP) of one animal, and a corresponding transverse slice through the mammary lesion (arrowhead), show lesion changes during the 8 weeks of imaging. The summary graph indicates that rapamycin was effective in reducing lesion size, but that cessation of treatment resulted in recurrence of lesions, which were more resistant to a second round of treatment with rapamycin.  The study illustrates one of the advantages of non-invasive imaging by PET:  use of a small number of animals in drug development studies to investigate both the primary question of therapeutic efficacy as well as secondary questions regarding 1) possible recurrence at completion of treatment and 2) efficacy during repeated treatments. (Abbey CK, Borowsky AD, Gregg JP, Cardiff RD, Cherry SR. J Mammary Gland Biol Neoplasia 11:137-49, 2006)

A pilot CT study (MicroCAT II) was performed to develop optimal image acquisition parameters and image processing techniques for measurement of adipose tissue in mice.  Anesthetized mice were imaged with microCT (40 kVp, 400 uAmp, 800 ms projections, 201 projections) without a contrast agent.  The segmented images depict adipose tissue (red) in relation to the skeleton (yellow) in lean and obese mouse models.  Ex-vivo measurement of fat mass confirmed close agreement with the volume of adipose tissue imaged by microCT.

This project examined the neuroendocrine basis of social bonding in a primate model.  Titi monkeys are monogamous and form strong emotional bonds between pair-mates. The PI used functional neuroimaging to investigate baseline differences in regional brain metabolism of pair-bonded and non-bonded animals. Conscious animals were administered ¹⁸F-FDG and were active for 35 min before being anesthetized for a 1-hr PET (microPET P4) brain scan.  For MRI (1.5 Tesla), a 25-30 min brain scan was acquired.  Image registration software (RView) was used to fuse each animal’s MRI and PET image to accurately identify anatomical regions and measure regional brain glucose metabolism.   The figure shows selected image slices from MRI (top) and PET (center) scans in a pair-bonded male, and the coregistered images (bottom).  Pair-bonded males showed lower relative glucose metabolism than lone males in all three of the areas identified on the MRI scan.  (Bales KL, Mason WA, Catana C, Cherry SR, Mendoza SP. Brain Res 1184:245-53, 2007)

This study was designed to investigate the kinetics of enhanced green fluorescence protein-polymorphonuclear leukocyte (EGFP-PMN) influx into a cutaneous wound site using fluorescence imaging.  Transgenic mice, in which EGFP-PMN was knocked into the lysozyme gene, were imaged (Xenogen IVIS 100) at multiple time points after skin wounding.  PMN influx was rapid during the first 12 hr and reached a maximum between 1 and 2 days, followed by a gradual decline over succeeding days.  (Kim M-H, Liu W, Borjesson DL et al. J Invest Dermatol 128:1812-20, 2008)

MicroPET (Focus 120) imaging was used to track the biodistribution of liposomes radiolabeled with 18F-fluorodipalmitin (18F-FDP) in a rat model.  Liposome-encapsulated 18F-FDP, prepared on-site, was administered intravenously and imaged during a 90-min dynamic scan using continuous bed motion.  Liposomes (average size = 45 nm) remained in the circulation at near-constant levels for the duration of the scan, providing a detailed map of the vasculature as seen in this maximum intensity projection image.  Visualization of the lungs, liver, kidney and spleen reflect the vascular blood flow in these organs.  Liposomes can be prepared with surface peptides or antibodies to target specific cell surface receptors.  (Marik J, Tartis MS, Zhang H, Fung JY, Kheirolomoom A, Sutcliffe JL, Ferrara KW.  Nucl Med Biol 34:165-71, 2007)

The formalin-preserved head of a macaw was imaged by microCT (MicroCAT II) as part of a pilot project to assess the feasibility of studying how the relationships between the individual bones of the skull and jaw are altered during opening and closing the beak.  The image required 2 bed positions, and the acquired image was processed using an isosurface algorithm to provide a 3-dimensional impression of the bones of the skull and jaw.

The biodistribution of the chemotherapeutic agent, paclitaxel, was studied by microPET (Focus 120) in a rat model.  ¹⁸F-paclitaxel, prepared on-site, was administered intravenously and after 10 min the animal was dynamically imaged during a 60-min continuous bed motion scan.  The time-segmented image set characterizes the biodistribution of free paclitaxel:  once taken up by the liver it is solubilized in bile and secreted into the proximal small intestine, eventually entering the large intestine. (Ferrara lab)