Positron emission tomography (PET) is a sensitive research tool that utilizes radiolabeled tracers for viewing molecular targets and biochemical pathways in vivo. The ability to image tissue perfusion, glucose metabolism, drug/receptor interactions, and gene expression have made it a particularly effective tool in oncology, cardiology and neuroscience research. A brief description of the physics of PET imaging is viewable elsewhere on this site. The functional imaging achieved with PET can be combined with anatomical information obtained from the same animal using microCT or MRI to refine the identification of sites of PET radiotracer uptake.

There are two imaging rooms dedicated to PET scanning; one contains our microPET II and microCT scanners, and the other, a Focus 120 and microCAT II. Both rooms have injection stations and veterinary anesthesia systems for induction and maintenance of isoflurane anesthesia. Other support equipment include a dose calibrator (CRC 15R; Capintec) for measurement of radiotracer dose, a mouse tail illuminator (Braintree Scientific) for tail vein injection, a warming light and a circulating water heating pad.

MicroPET II

This custom-built PET scanner is a second-generation tomograph developed and fabricated in the laboratory of Simon Cherry (1,2). It succeeds the original microPET scanner that was designed and constructed at U.C.L.A in the late 1990’s by a group of collaborators headed by Dr. Cherry (3). That prototype was the design concept for the first commercial small animal PET scanners produced in 2000 by Concorde Microsystems, Inc. (acquired by Siemens, 2005). MicroPET II has approximately 4-fold greater resolution and sensitivity than the prototype scanner.

A custom-built microCT has recently been constructed in Dr. Cherry’s laboratory (4). It was designed to integrate with the microPET II scanner to provide a dual modality system for anatomic and molecular imaging. It can be used as a stand-alone CT scanner, or in tandem with the microPET II scanner. In the latter configuration, the gantry can simply be advanced into the CT scanner after a PET scan has been acquired. Fusion of the PET and CT images is achieved by the use of predetermined transformation matrices, eliminating the need for fiducial markers.

A second design requirement implemented in the microCT was reduction of radiation dose, achieved by using a high-sensitivity scintillator screen. The accumulated radiation dose from combined PET and CT, particularly when animals undergo repeated scans, may have biological effects, therefore the microCT was designed to reduce the radiation dose delivered by a CT scan.

Focus 120 (F120)

The F120 is a third generation small animal PET scanner produced by Siemens Medical Solutions, Inc (formerly CTI Concorde Microsystems). Compared to the previous generation R4 model, which has the same ring diameter and axial length, the sensitivity and volumetric resolution are improved by more than 50% and 250%, respectively (5). The longer axial length of the F120 compared to microPET II makes it more suitable for whole body mouse imaging, which requires 2 bed positions with microPET II. The animal bed from the F120 can be easily removed and attached to our microCAT II scanner to acquire CT anatomical images for coregistration with a PET image. Fiducial markers visible on both PET and CT are placed on or near the animal for precise co-registration of the images.

MicroPET P4

The P4 PET scanner has slightly lower spatial resolution that the other two microPET scanners, but has a larger bore to accommodate larger animals (6). It has been in routine use at UC Davis since 2002, and has been used for studies in mice, rats and non-human primates. It is located in a satellite facility at the California National Primate Research Center (CNPRC) and is maintained and operated by our staff. The instrument has been used exclusively for non-human primate imaging for the last 2 years. The animals used at this facility can be routinely handled and cared for by the CNPRC veterinary staff during PET imaging.

microPET Specifications Comparison

microPET	-P4	-F120	-II
Animal Port Diameter	22 cm	12 cm	15 cm
Axial Field of View	8 cm	7.6 cm	4.9 cm
Timing Window	2,6,10,14 or 18 nsec	2, 6,10, or 14 nsec	2, 6,10, or 14 ns
Energy Window	Variable: 0 – 810 keV	Variable: 0 – 814 keV	Variable: 0 – 814 keV
Absolute System Sensitivity	2.25 %	7%	2.26%
Reconstructed Spatial Resolution	≈1.8 mm	≈1.2 mm	≈1.0 mm
Reconstructed Volumetric Resolution	≈6.4	≈2	≈1.4

MODALITY	MODEL	MANUFACTURER	RESOLUTION
PET	Focus 20	Siemens	~1.3mm
PET	P4	Siemens	~1.8mm
PET	microPETI	Custom-built	~1.8mm
SPECT	microSPECT	Siemens	~1mm
CT	microCAT II	Siemens	25-150 um
CT	microCT	Custom-built	~ 300um
MRI	Biospec 7T	Bruker	50-250 um
Ultrasound	Sequoia	Siemens	~ 120 um
Optical	IVIS 100	Xenogen	~ 2mm
Autoradiography	Storm 860	Amersham Biosciences	50-100 um

References

Tai YC, Chatziioannou A, Yang YF, Silverman RW, Meadors K, Siegel S, Newport D, Stickel JR, Cherry SR. MicroPET II: design, development, and initial performance of an improved microPET scanner for small-animal imaging. Phys Med Biol; 48:1515-37, 2003.

Yang Y, Tai Y-C, Siegel S, Newport DF, Bai B, Li Q, Leahy RM, Cherry SR. Optimization and performance evaluation of the microPET II scanner for in vivo small-animal imaging. Phys Med Biol; 49:2527-45, 2004.

Cherry SR, Shao Y, Siegel S et al. MicroPET: a high resolution PET scanner for imaging small animals. IEEE Trans Nucl Sci ; 44:1161-6, 1997.

Liang H, Yang Y, Yang K, Wu Y, Boone JM, Cherry SR. A microPET/CT system for in vivo small animal imaging. Phys Med Biol ; 52:3881-94, 2007.

Kim JS, Lee JS, Im KC, Kim SJ, Kim S-Y, Lee DS, Moon DH. Performance measurement of the microPET Focus 120 scanner. J Nucl Med; 48:1527-35, 2007.

Tai YC, Chatziioannou A, Siegel S, Young J, Newport, D, Goble RN, Nutt RE, Cherry SR. Performance evaluation of the microPET P4: a PET system dedicated to animal imaging. Phys Med Biol; 46:1845-62, 2001.

For additional Information:

• Phelps ME. Positron emission tomography provides molecular imaging of biological processes. Proc Natl Acad Sci; 97:9226-33, 2000.
• Cherry SR. In vivo molecular and genomic imaging: new challenges for imaging physics. Phys Med Biol; 49:R13-48, 2004.
• Cherry SR. The 2006 Henry N. Wagner Lecture: Of mice and men (and positrons) – advances in PET imaging technology. J Nucl Med; 47:1735-45, 2006.
• Cherry SR. Multimodality in vivo imaging systems: twice the power or double the trouble? Annu Rev Biomed Eng; 8:35-62, 2006.