Very High Resolution Small Very High Resolution Small Animal PETAnimal PET

Don J. BurdetteDon J. Burdette

Department of PhysicsDepartment of Physics

What is PET?What is PET?

Answer: Cancerous cells have a Answer: Cancerous cells have a higher metabolism than normal higher metabolism than normal

cells, thus cells, thus PET scans detect PET scans detect cancer. cancer.

Other Uses for PET include detection Other Uses for PET include detection of Cardiovascular Disease, of Cardiovascular Disease, Alzheimer’s disease, Parkinson’s Alzheimer’s disease, Parkinson’s disease, epilepsy, and other disease, epilepsy, and other neurological disorders.neurological disorders.

PET stand for Positron Emission TomographyPET stand for Positron Emission Tomography It is a leading medical imaging technique (more than 200,000 PET scans in It is a leading medical imaging technique (more than 200,000 PET scans in

over 700 institutions performed last year in the US) over 700 institutions performed last year in the US) Unlike MRI and X-Ray (which image bodily structure) PET creates images of Unlike MRI and X-Ray (which image bodily structure) PET creates images of

metabolic processesmetabolic processesQuestion: Why is imaging metabolic processes important?Question: Why is imaging metabolic processes important?

An image of damaged heart (left) compared to a normal heart (right). The damaged heart has undergone a heart attack. Single slice

Example of a PET image

How does PET work?How does PET work?

How does PET work?How does PET work?

• Typical Resolution of human full body scans about 10mm

• There exist other applications for PET technology outside human imaging

Small Animal PET SystemsSmall Animal PET Systems

• biomedical, pharmaceutical, and biomedical, pharmaceutical, and genetic manipulation researchgenetic manipulation research

• Mice are used as human disease Mice are used as human disease modelsmodels

• A single mouse can be used to A single mouse can be used to tract disease development and tract disease development and treatment (Important for treatment (Important for genetically altered specimens)genetically altered specimens)

Transgenic mouse showing cells with albumin gene switched on

Uses of Small Animal PET systems:

Challenges for Small Animal PET: • Need resolution of less than 1mm

• Lower doses, high efficiency

Possible Solutions:• smaller detection elements

• higher efficiency detectors

• Increase solid angle of the detector

Example of a Small Animal PET image

MicroPET R4 from University of Michigan’s MicroPET R4 from University of Michigan’s PET centerPET center

• Consists of 8 X 8 array of individual LSO scintillation crystals coupled to 64-channel PMT

• 15 cm detection ring diameter

• Typical System resolution = 1.8 mm

• Image of two 1.1-1.2mm capillary tubes filled with Flourine-18 source

Silicon DetectorsSilicon DetectorsCharacteristics of ideal Silicon Characteristics of ideal Silicon

detector for use in PET:detector for use in PET:• Thick detector to increase efficiency Thick detector to increase efficiency

(1mm compared to typical 0.3mm (1mm compared to typical 0.3mm thickness)thickness)

• Small detection pads for excellent Small detection pads for excellent spatial resolutionspatial resolution

• Custom made readout chips including Custom made readout chips including an amplifier, and sample-hold switchan amplifier, and sample-hold switch

• Chips have trigger logic to allow Chips have trigger logic to allow independent silicon operationindependent silicon operation

Silicon Detector composed of an array of 32 X 16 pads 1mm thick by 1.4mm X 1.4mm in area

• Small Detection Pixels yield excellent spatial resolution of the absorbed photons

• Timing resolution of 200ns (Coincidence Window) Americium-241 spectrum

collected by silicon detector

Experimental Experimental Set-upSet-up

Mechanical Set-up

In collaboration with the University of Michigan

Reconstructing the Data for a Reconstructing the Data for a Simulated Disk SourceSimulated Disk Source

Simple Back-Projection of the data (just drawing the lines)

Fourier transforming this image into frequency space…

Reconstructing the Data for a Reconstructing the Data for a Simulated Disk SourceSimulated Disk Source

Projection of previous image with and without filtering

• Blue line = unfiltered data

• Green line = filtered data

• The narrower the peak, the higher the resolution = sharper image

Reconstructing the Data for a Reconstructing the Data for a Simulated Point SourceSimulated Point Source

Image of Disk Source after filtering and transforming back to position space

Reconstructing the Data for a Reconstructing the Data for a Simulated Disk SourceSimulated Disk Source

Original Image Filtered Image

Experimental ResultsExperimental Results

Our small animal PET set-upOur small animal PET set-up Resolution of 0.7mm Resolution of 0.7mm

From microPET R4 set-upFrom microPET R4 set-up Resolution of 1.8mm Resolution of 1.8mm

Images of Flourine-18 source contained in two 1.1-1.2mm capillary tubes with a wall thickness of 0.2mm.

Conclusion and Future WorkConclusion and Future Work Our prototype Small Animal PET achieves a system Our prototype Small Animal PET achieves a system

resolution below the 1mm goal demonstrating the resolution below the 1mm goal demonstrating the usefulness of silicon in Small Animal PET applications.usefulness of silicon in Small Animal PET applications.

Future Work includes:Future Work includes:• Add a stack of silicon detectors to increase efficiencyAdd a stack of silicon detectors to increase efficiency• Improve rate capabilities by decreasing coincidence Improve rate capabilities by decreasing coincidence

windowwindow This can be accomplished by taking advantage of the This can be accomplished by taking advantage of the

Compton scattered photon and using a secondary Compton scattered photon and using a secondary detector with faster coincidence timing (Silicon timing detector with faster coincidence timing (Silicon timing 200ns coincidence window, timing from some scintillation 200ns coincidence window, timing from some scintillation crystals approach 60ns)crystals approach 60ns)

ReferencesReferences Miles N. Wernick and John N. Aarsvold, editors. Miles N. Wernick and John N. Aarsvold, editors.

Emission Tomography: The Fundamentals of Emission Tomography: The Fundamentals of PET and SPECT. PET and SPECT. Elsevier, Acad Press, 2004.Elsevier, Acad Press, 2004.

Glenn F. Knoll. Glenn F. Knoll. Radiation Detection and Radiation Detection and Measurement. Measurement. John Wiley and Sons, Inc. 1989.John Wiley and Sons, Inc. 1989.

Christof Knoess. Performance evaluation of the Christof Knoess. Performance evaluation of the Micropet R4 pet scanner of rodents. Micropet R4 pet scanner of rodents. Eur J Nucl Eur J Nucl Med Mol Imaging, Med Mol Imaging, 2003.2003.

Special Thanks to Neal Clinthorne, Klaus Special Thanks to Neal Clinthorne, Klaus Honscheid, Harris Kagan, Sang-June Park, and Honscheid, Harris Kagan, Sang-June Park, and Joseph Regensburger Joseph Regensburger