9.4T Magnetic Resonance Imaging and Spectroscopy and duel modality PET/MRI

The MR core facility currently utilizes a 9.4T 20-cm bore magnet imaging/spectroscopy system operating at 400 MHz for proton imaging experiments alongside a 7T/30 scanner. Ultra high-resolution ex vivo MRI can also be performed with a 11.7T micro MRI scanner. The core has a new PET/MRI insert that allows simultaneous co-registered acquisition of both in vivo PET/MR imaging in mouse and sequential PET/MR acquisition for rats. We provide a wide array of non-invasive, high-resolution, quantitative imaging-based capabilities for metabolic and genetic characterization of tumors and their microenvironment in small-animals. We perform MRI phenotypic screening of tumor incidence, progression and therapy-induced regression in xenograft models and transgenic/knock-out models; detection and localization of tumor hypoxia and re-oxygenation; evaluation of tumor cellularity, vascularity and the its response under treatment; measurement of the time-dependent biodistribution of new radionuclide diagnostic and therapeutic agents.

In addition, in vivo metabolite quantitation using x-nuclei in vivo MR spectroscopy imaging is available after custom setup. This allows monitoring of drugs with labels that are MR visible. Other labels, including iron and gadolinium which are MR detectable through relaxation effect, are used as MR contrast agents. These have potential for tagging of compounds and cells for in vivo tracing.

Techniques Available and Services Provided

• In vivo acquisition and reconstruction of high-resolution images in small animals (up to cat size).
• In vivo and Ex vivo diffusion imaging
• In vivo cine cardiac imaging
• Off-line Image processing.
• Ex vivo imaging and analysis of tissues.
• Metabolite quantitation of both perfused cells and animals.
• In vivo drug and organ metabolism.
• Estimates of tumor perfusion.
• A complete preclinical histopathology/immunochemistry for radioactive tissue specimens.
• PMOD kinetic modeling software and other processing tools for robust advanced quantitative image analysis.

microPET, microCT, X-SPECT, and Optical Imaging

The R4 microPET (Concorde Microsystems, Knoxville, TN) is a dedicated high-resolution small-animal (rodent) 3D PET scanner employing lutetium orthooxysilicate (LSO) scintillation crystal detectors. The linear and volume spatial resolution of the microPET is approximately 2 mm and 8 mm³, respectively. microPET imaging generally requires intravenous administration of activities of the order of 0.1 mCi and image acquisition times of five to ten minutes per animal. The microCAT II microCT (Imtek Corp, Oak Ridge, TN) is a dedicated small-animal (i.e., mouse) CT scanner. microCT studies require ~10 min per animal and yield spatial resolution of the order of 100 microns, delivering a radiation dose of the order of 20 cGy. As with radiographic imaging generally, bone-soft tissue, and lung-soft tissue contrasts are excellent but soft contrast-soft tissue contrast is poor without intravenous contrast agents.

The Xenogen IVIS Optical Imaging System (Xenogen Corp, Alameda, CA) is based on a cooled high-sensitivity charge coupled device (CCD) camera for quantitatively measuring photons (light). The system specifically and quantitatively images the expression of the luciferase (Luc) gene, the so-called reporter gene, in mice. With the recent installation of a newly available, fluorescent light source and four-filter set, imaging of fluorescent probes such as green fluorescent protein (GFP) and red fluorescent probes (RFP) in vivo with this system can now be performed.

The X-SPECT (Gamma Medica, Northridge, CA) is a dedicated small-animal (mouse and rat) SPECT-CT scanner for non-invasive, ultra-high-resolution imaging in vivo of conventional (non positron-emitting) radiotracers and ultra-high-resolution CT scans for anatomic registration. Imaging may be performed in a “whole-body” mode with parallel-hole collimation at a resolution of ~4 mm or in “local” mode with pinhole collimation at a resolution of ~1 mm. X-SPECT imaging requires administered activities of the order of 1 mCi and image acquisition times of several minutes (for planar imaging) to 40 minutes (for tomographic imaging); for the CT studies, the imaging times (~10 min), resolution (~100 microns), and radiation doses (~20 cGy) are similar to those for the microCAT II studies. All imaging studies require animals to be fully anesthetized, and isofluorane anesthesia systems (provided by the facility) are generally used.

Techniques Available and Services Provided

• Acquisition and reconstruction of planar, SPECT, and PET images of single-photon- and positron-emitting radiotracer distributions, microCT imaging, and optical (bioluminescent and fluorescent) imaging of mice and rats.
• Image processing (digital filtering, ROI analysis, etc.) and computer storage.
• Mathematical analysis (e.g., curve-fitting, compartmental modeling) of time-activity data.
• Harvesting of tissues at necropsy and scintillation well-counting ex vivo of radioactivity in blood and tissue samples.
• Phosphor plate-based quantitative autoradiography of tissue sections (with harvesting, freezing, and sectioning of tissue samples using the facility’s cryostatic microtome).
• Consultation on experimental design, including choice of the radiotracer, handling of radioactivity, and compliance with associated institutional and regulatory requirements.

4.7-T Magnetic Resonance Imaging and Spectroscopy

The MRI/MRS facility currently utilizes a 4.7-T 33-cm bore magnet imaging/spectroscopy system operating at 200 MHz for 1-H (standard) imaging experiments, which will shortly be upgraded to a 7.0 T magnet. To optimize imaging on this instrument, specially designed radiofrequency coils for each experiment to enhance signal to noise and spatial resolution are routinely built. The spatial resolution limit of the current system is 1 mm x 100m x 100m. It is primarily utilized for rats and mice but it can accommodate animals as large as cats. Coronal, sagittal, and axial slices can be obtained routinely. The images can be obtained in either a high-spatial resolution mode or at somewhat lower resolution for deriving tracer perfusion and diffusion information. In contrast to other modalities, the images are quantitative and the estimated volume by MRI can directly and accurately provide tumor or organ volume as has been shown for multiple orthotopic tumor models. In addition, radiofrequency coils have been designed and fabricated in-house for simultaneous imaging of multiple mice (up to 13) with medium resolution (~1.5mm x 250m x 250m).

The Core has a new PET/MRI insert that allows simultaneous acquistion of  both PET and MR in vivo imaging in mouse. In summer 2018 a new 9.4T MRI scanner will be available for rodent studies.

A new instrument will be installed in 2002 that will provide higher-resolution (~200m x 75m x 75m) images at much greater speeds, with additional pulse sequences including oblique slice capability. In addition, in vivo metabolite quantitation using 31P, 13C, 19F NMR spectroscopy is available. This allows monitoring of drugs with labels that are NMR visible, particularly if labeled with/or having a naturally occurring fluorine atom. Other labels, including iron and gadolinium, are readily detected and require the cooperation of a chemist. These have potential for tagging of compounds and cells for in vivo tracing. Both imaging and spectroscopy can be done dynamically or with single studies for detecting various metabolites. Dynamic studies can provide pharmacokinetic, metabolic, and potentially perfusion data.

Techniques Available and Services Provided

• In vivo acquisition and reconstruction of high resolution images in different orientations (axial, sagittal, and coronal) in small animals (up to cat size).
• Image processing.
• Ex vivo imaging and analysis of tissues.
• Metabolite quantitation of both perfused cells and animals.
• In vivo drug and organ metabolism.
• Estimates of tumor perfusion.
• A complete preclinical histopathology/immunochemistry for radioactive tissue specimens.
• PMOD kinetic modeling software for robust advanced quantitative imaging processing tools.