Synergies Between Calorimetry and PET

1. Outline: Fundamentals of PET Comparison of Calorimetry & PET Areas of Common Interest Conclusions Synergies Between Calorimetry and PET William W. Moses Lawrence Berkeley National Laboratory March 26, 2002

2. Step 1: Inject Patient with Radioactive Drug • Drug is labeled with positron(+) emitting radionuclide. • Drug localizes in patient according to metabolic properties of that drug. • Trace (pico-molar) quantities of drug are sufficient. • Radiation dose fairly small(<1 rem). Drug Distributes in Body

3. • Interesting Biochemistry Easily incorporated into biologically active drugs. • 1 Hour Half-Life Maximum study duration is 2 hours. Gives enough time to do the chemistry. • Easily Produced Short half life  local production. Ideal Tracer Isotope 18F 2 hour half-life 15O, 11C, 13N 2, 20, & 10 minute half-lives

4. Step 2: Detect Radioactive Decays Ring of Photon Detectors • Radionuclide decays, emitting +. • + annihilates with e– from tissue, forming back-to-back 511 keV photon pair. • 511 keV photon pairs detected via time coincidence. • Positron lies on line defined by detector pair (known as a chord or a line of response or a LOR). Detect Pairs of Back-to-Back 511 keV Photons

5. Multi-Layer PET Cameras Scintillator Tungsten Septum Lead Shield • Can image several slices simultaneously • Can image cross-plane slices • Can remove septa to increase efficiency (“3-D PET”) Planar Images “Stacked” to Form 3-D Image

6. 1-Dimensional Horizontal Projection 1-Dimensional Vertical Projection Step 3: Reconstruct with Computed Tomography 2-Dimensional Object By measuring all 1-dimensional projections of a 2-dimensional object, you can reconstruct the object

7. Why Do Computed Tomography? Planar X-Ray Computed Tomography Separates Objects on Different Planes Images courtesy of Robert McGee, Ford Motor Company

8. Attenuation Correction + Source • Use external + source to measure attenuation. • Attenuation (for that chord) same as for internal source. • Source orbits around patient to measure all chords. • Measure Attenuation Coefficient for Each Chord • Obtain Quantitative Images

9. 500 ps timing resolution  8 cm fwhm localization Time-of-Flight Tomograph • Can localize source along line of flight. • Time of flight information reduces noise in images. • Time of flight tomographs have been built with BaF2 and CsF. • These scintillators force other tradeoffs that reduce performance. c = 1 foot/ns Not Compelling with Present Technology...

10. • NMR “Sees” Structure with 0.5 mm Resolution • PET “Sees” Metabolism with 5.0 mm Resolution NMR & PET Images of Epilepsy NMR PET

11. PET Images of Cancer Brain Heart Bladder Treated Tumor Growing Again on Periphery Metastases Shown with Red Arrows Normal Uptake in Other Organs Shown in Blue

12. PET Camera Design • Typical Parameters • Detector Module Design

13. PET Cameras • Patient port ~60 cm diameter. • 24 to 48 layers, covering 15 cm axially. • 4–5 mm fwhm spatial resolution. • ~2% solid angle coverage. • $1 – $2 million dollars. Images courtesy of GE Medical Systems and Siemens / CTI PET Systems

14. Early PET Detector Element BGO Scintillator Crystal (Converts  into Light) Photomultiplier Tube (Converts Light to Electricity) 10 — 30 mm high (determines axial spatial resolution) 30 mm deep (3 attenuation lengths) 3 — 10 mm wide (determines in-plane spatial resolution)

15. Modern PET Detector Module 4 PMTs (25 mm square) • Saw cuts direct light toward PMTs. • Depth of cut determines light spread at PMTs. • Crystal of interaction found with Anger logic (i.e. PMT light ratio). 50 mm BGO Scintillator Crystal Block (sawed into 8x8 array, each crystal 6 mm square) 50 mm 30 mm Good Performance, Inexpensive, Easy to Pack

16. Crystal Identification with Anger Logic Profile through Row 2 Uniformly illuminate block. For each event, computeX-Ratio and Y-Ratio,then plot 2-D position. Individual crystals show up as dark regions. Profile shows overlap (i.e. identification not perfect). Y-Ratio X-Ratio Can Decode Up To 64 Crystals with BGO

17. Fundamental Limits of Spatial Resolution • Dominant Factor is Crystal Width • Limit for 80 cm Ring w/ Block Detectors is 3.6 mm

18. Tangential Projection Radial Projection Radial Elongation • Penetration of 511 keV photons into crystal ring blurs measured position. • Effect variously known as Radial Elongation, Parallax Error, or Radial Astigmatism. • Can be removed by measuring depth of interaction.

19. • Digitize Arrival Time (latch 500 MHz clock — 2 ns accuracy) • Identify Crystal of Interaction & Measure Energy • Correct Energy and Arrival Time (based on crystal) • Maximum “Singles” Event Rate is 1 MHz / Detector Module PET Front End Electronics Custom ASIC Off the Shelf RAM “Singles”Event Word Energy ADC PMT A X ADC PMT B Analog ASIC FPGA Y PMT C ADC • Position • Time PMT D Time TDC If Energy Consistent with 511 keV,Send Out “Singles” Event Word (Position & Time)

20. • Search for “Singles” in Time Coincidence (~10 ns window) • Strip Off Timing Information • Format “Coincidence” Event Word (chord location) • Maximum “Coincidence” Event Rate is 10 MHz / Camera Singles 0 Singles n PET Readout Electronics Off the Shelf From Each Camera Sector “Coincidence”Event Word . . . Fiber Optic Interface FPGAs • Locationof Chord Search for Coincidences, Send Out “Coincidence” Event Word (Position of Chord)

21. Similarities and DifferencesBetween Calorimetry & PET • Similarities • The PET World Picture...

22. Similarities Between Calorimeters and PET Calorimeter PET Camera • Cylindrical Gamma Ray Detectors • High Efficiency, Hermetic • Segmented, High Density Scintillator Crystals • High Performance Photodetectors • High Rate, Parallel Readout Electronics

23. The PET World Picture: Need to Image 0.000000511 TeV* Photons Signal Levels Are Very Low *511 keV

24. No Pair Production / EM Showers • Compton scatter in patient produces erroneous coincidence events. • ~15% of detected events are scattered in 2-D PET(i.e. if tungsten septa used). • ~50% of events are scatteredin 3-D Whole Body PET. Scatter Length ≈ 10 cm • Compton Scatter is Important Background • Use Energy to Reject Scatter in Patient

25. Patient Radiation Dose is Limited! • Cannot Increase Signal Source Strength • Image Noise Is Limited by Counting Statistics

26. Competitive Commercial Market CMS Calorimeter PET Camera • $60 Million (parts cost) • 72,000 Channels • $833 / Channel • $1 Million (parts cost) • 18,400 Channels • $54 / Channel In a PET Camera: • Scintillator crystals are ~25% of total parts cost • Photomultiplier tubes are ~25% of total parts cost • No other component is >10% of total parts cost Cost is Very Important

27. Detect 511 keV Photons With(in order of importance): • >85% efficiency • <5 mm spatial resolution • “low” cost (<$100 / cm2) • “low” dead time (<1 µs cm2) • <5 ns fwhm timing resolution • <100 keV fwhm energy resolution PET Detector Requirements Based on Current PET Detector Modules

28. Synergies... • Scintillators • Photodetectors • Electronics • Computation

29. New Scintillators Developed Recently PbWO4 LSO Image courtesy of E. Auffray, CERN Image courtesy of C. Melcher, CTI PET Systems • Discovered in ~1992. • Approximately 10 years of R&D before large scale production. • Development efforts driven by end users, but included efforts of luminescence scientists, spectroscopists, defects scientists, materials scientists, and crystal growers. Very Strong Parallels...

30. PbWO4 Lu2SiO5 Density (g/cc): 8.3 7.4 Attenuation Length (cm): 0.9 1.2 Light Output (phot/MeV): 200 25,000 Decay Time (ns): 10 40 Emission Wavelength (nm): 420 420 Radiation Hardness (Mrad): >10 10 Dopants: Y, Nd Ce Cost per cc: $1 >$25 Scintillator Properties Different Tradeoffs Required

31. Avalanche Photodiode Arrays Hamamatsu Photonics RMD, Inc. Advantages: • High Quantum Efficiency  Energy Resolution • Smaller Pixels  Spatial Resolution • Individual Coupling  Spatial Resolution Challenges: • Dead Area Around Perimeter • Signal to Noise Ratio • Reliability and Cost

32. Calorimetry PET High Gain?: Yes Yes High QE / Blue Sensitivity?: Yes Yes Radiation Hardness?: Yes No Nuclear Counter Effect?: Yes No Timing Signal (low C)?: No Yes High Packing Density?: No Yes Sensitive to Leakage Current?: ~ Yes APD Requirements Different Tradeoffs Required

33. Calorimetry PET Low Noise Analog Amplifier?: Yes Yes Low Power Consumption?: Yes Yes Mixed-Mode Custom ICs?: Yes Yes Real-Time Data Correction?: Yes Yes Highly Parallel Readout?: Yes Yes High Data Rate?: Yes Yes Electronics Requirements Many Similarities

34. Calorimetry PET Radiation Damage?: Yes No Analog Dynamic Range: High Low Self-Generated Timing Signal?: No Yes Asynchronous Inputs?: No Yes Event Size / Complexity?: High Low Multiple Trigger Levels?: Yes No “Good” Event Rate?: kHz MHz Electronics Requirements Different Tradeoffs Required

35. Calorimetry PET Significant Computation?: Yes Yes Monte Carlo Simulation?: Yes Yes Large Programming Project?: Yes Yes Complexity of Analysis?: High Low Data Set Size?: TB–PB GB Time to Finish Analysis?: Years Minutes FDA Certification Required?: No Yes Computation Requirements Different Tradeoffs Required

36. Many Synergies Exist Between HEP & PET Scintillators, detectors, electronics, computing, … Tools & experience are particularly valuable PET is a Mature, Commercial Technology Innovations will only be used if they areclearly superior (not just novel) All requirements must be met Cost is very important Difficult to Transfer Identical Technology Need to optimize for PET tradeoffs Final Thoughts