Very High Resolution Small Animal PET

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Very High Resolution Small Animal PET

• Don J. Burdette
• Department of Physics

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What is PET?

• PET stand for Positron Emission Tomography
• It is a leading medical imaging technique (more
  than 200,000 PET scans in over 700 institutions
  performed last year in the US)
• Unlike MRI and X-Ray (which image bodily
  structure) PET creates images of metabolic
  processes
• Question Why is imaging metabolic processes
  important?

• Answer Cancerous cells have a higher metabolism
  than normal cells, thus PET scans detect cancer.
• Other Uses for PET include detection of
  Cardiovascular Disease, Alzheimers disease,
  Parkinsons disease, epilepsy, and other
  neurological disorders.

Example of a PET image
An image of damaged heart (left) compared to a
normal heart (right). The damaged heart has
undergone a heart attack. Single slice
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How does PET work?
Photons
Electron
-

Radioactive Tracer
Positron
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How does PET work?

• Typical Resolution of human full body scans
  about 10mm
• There exist other applications for PET
  technology outside human imaging

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Small Animal PET Systems
Uses of Small Animal PET systems
Example of a Small Animal PET image

• biomedical, pharmaceutical, and genetic
  manipulation research
• Mice are used as human disease models
• A single mouse can be used to tract disease
  development and treatment (Important for
  genetically altered specimens)

Challenges for Small Animal PET

• Need resolution of less than 1mm
• Lower doses, high efficiency

Possible Solutions
Transgenic mouse showing cells with albumin gene
switched on

• smaller detection elements
• higher efficiency detectors
• Increase solid angle of the detector

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MicroPET R4 from University of Michigans PET
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

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Silicon Detectors

• Characteristics of ideal Silicon detector for use
  in PET
• Thick detector to increase efficiency (1mm
  compared to typical 0.3mm thickness)
• Small detection pads for excellent spatial
  resolution
• Custom made readout chips including an amplifier,
  and sample-hold switch
• Chips have trigger logic to allow independent
  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
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Experimental Set-up
In collaboration with the University of Michigan
PC
Silicon Detector
Data Aquisition Module
Coincidence Logic
Intermediate Board
Intermediate Board
Collimated Source
Si Trigger 1
Si Trigger 2
Si Signal
Source
Si Signal
Mechanical Set-up
Si Detector
Si Detector
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Reconstructing the Data for a Simulated Disk
Source
Simple Back-Projection of the data (just drawing
the lines) Fourier transforming this image into
frequency space
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Reconstructing the Data for a Simulated Disk
Source

• Blue line unfiltered data
• Green line filtered data
• The narrower the peak, the higher the resolution
  sharper image

Projection of previous image with and without
filtering
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Reconstructing the Data for a Simulated Point
Source
Image of Disk Source after filtering and
transforming back to position space
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Reconstructing the Data for a Simulated Disk
Source
Original Image
Filtered Image
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Experimental Results

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

• From microPET R4 set-up
• 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.
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Conclusion and Future Work

• Our prototype Small Animal PET achieves a system
  resolution below the 1mm goal demonstrating the
  usefulness of silicon in Small Animal PET
  applications.
• Future Work includes
• Add a stack of silicon detectors to increase
  efficiency
• Improve rate capabilities by decreasing
  coincidence window
• This can be accomplished by taking
  advantage of the Compton scattered photon and
  using a secondary detector with faster
  coincidence timing (Silicon timing 200ns
  coincidence window, timing from some
  scintillation crystals approach 60ns)

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References

• Miles N. Wernick and John N. Aarsvold, editors.
  Emission Tomography The Fundamentals of PET and
  SPECT. Elsevier, Acad Press, 2004.
• Glenn F. Knoll. Radiation Detection and
  Measurement. John Wiley and Sons, Inc. 1989.
• Christof Knoess. Performance evaluation of the
  Micropet R4 pet scanner of rodents. Eur J Nucl
  Med Mol Imaging, 2003.
• Special Thanks to Neal Clinthorne, Klaus
  Honscheid, Harris Kagan, Sang-June Park, and
  Joseph Regensburger