1. Hartmut F.-W. Sadrozinski RESMDD06 Radiography Studies for Proton CT M. Petterson, N. Blumenkrantz, J. Feldt, J. Heimann, D. Lucia, H. F.-W. Sadrozinski, A. Seiden, D. C. Williams SCIPP, UC Santa Cruz, CA 95064 USA V. Bashkirov, R. Schulte Loma Linda University Medical Center, CA 92354 USA M. Bruzzi, D. Menichelli, M. Scaringella, C. Talamonti INFN and Univ. of Florence, Italy G.A.P. Cirrone, G. Cuttone, D. Lo Presti, N. Randazzo, V. Sipala INFN Sezione di Catania, Italy Motivation for pCT Tracking Study: Most likely Path inside Phantom Calorimeter Studies: Imaging using Energy Loss

2. Hartmut F.-W. Sadrozinski RESMDD06 Motivation for Proton Computed Tomography (pCT) Proton radiation therapy is one of the most precise forms of non-invasive image-guided cancer therapy. Well defined range of protons in material, low entrance dose, dose maximum (“Bragg peak”) rapid distal dose fall-off, Limitation to the precision: The use of x-ray computed tomography (CT) for imaging. The resulting uncertainties can lead to range errors from several millimeters up to more than 1 cm depending on the anatomical region treated. In addition, during the treatment the patient might move and thus one needs to verify the position of the target tissue in the beam. In both cases, a CT system using the protons themselves is the solution.

3. Hartmut F.-W. Sadrozinski RESMDD06 Proposed pCT System R. W. Schulte, et al.,, “Conceptual design of a proton computed tomography system for applications in proton radiation therapy”, IEEE Trans. Nucl. Sci., vol 51, no.3, pp 866 – 875, June 2004. Challenges for pCT For pCT we need to determine for every single proton : Where did it go? How much energy did it loose?

4. Hartmut F.-W. Sadrozinski RESMDD06 Apparatus Measure Positions, Angles and Energy Loss of Single Protons Use Loma Linda U MC proton beam (here 200 MeV) Single Proton Tracking in 10 Si planes: Single-sided, 192 strips, 236  m pitch, [GLAST 97 B.T.) Module = x-y pair with 90 o rotated strips Entrance and exit telescopes + 1 “roving” inside absorber Energy Measurement in Calorimeter One CsI crystal 5 cm x 5 cm x 15 cm Common Readout of Silicon Strip + Calorimeter into FPGA Si has binary readout with Time-over-Threshold (ToT) CsI read out with Photodiode into Charge-to-Time-Converter (CTC) Position Resolution: 70  m Angular Resolution: 5 mrad

5. Hartmut F.-W. Sadrozinski RESMDD06 Tracking Studies External Tracking of Proton predicts Path inside Absorber (MLP) “The most likely path of an energetic charged particle through a uniform medium” D C Williams Phys. Med. Biol. 49 (2004) 2899–2911 To verify the “Banana”: Measure displacement with “Roving Plane”at 3 depth N. Blumenkrantz et al.,, “Prototype Tracking Studies for Proton CT ”, to appear in IEEE Trans. Nucl. Sci.,

6. Hartmut F.-W. Sadrozinski RESMDD06 Correlation between “Roving plane #2” and exit parameters Tracking Studies Multiple Coulomb Scattering (MCS) at work

7. Hartmut F.-W. Sadrozinski RESMDD06 Multiple Coulomb Scattering (MCS) overcome Analytical calculation of the most likely path MLP (open symbols: the size of the symbol is close to the MLP spread ). Good agreement data – MLP (~300  m) but systematically growing difference with larger displacements: need M.C. Simulation Resolution inside Absorber better than 500  m vs. MLP width of 380  m Using exit angle improves resolution: by ~20% RMS = 490um MLP width = 380 um Displacement from incoming direction in the “Roving planes” as a function of exit displacement bins of 500  m (all angles). MLP < 500  m Localization within Absorber

8. Hartmut F.-W. Sadrozinski RESMDD06 Energy Loss Studies Image Low-contrast Phantom = voids in PMMA Phantom = 12 PMMA plates (each 1.25 cm thick ), 6th has holes Phantom 2 SSD each (x-y)Calorimeter p

9. Hartmut F.-W. Sadrozinski RESMDD06 E beam = 201.1 MeV E beam = 100 MeV E beam = 0 MeV (Pedestal) Calorimeter Calibration and Resolution Energy Response with Proton Beams without PMMA Gaussian fit to the falling slope of the CTC spectrum ( ¼ of protons are useful in the determination of the mean energy)

10. Hartmut F.-W. Sadrozinski RESMDD06 Calorimeter: Work in Progress Energy Loss in PMMA, Energy Resolution and Straggling PMMA Data not described by NIST -> Leakage ? MC underway. Energy resolution  E in CsI (measured and corrected for pedestal) Energy resolution in PMMA factor 2 to large to be caused by energy straggling. Leakage? MC underway.

11. Hartmut F.-W. Sadrozinski RESMDD06 Proton Images Energy Loss ~ Integrated Stopping Power ~  d Phantom Holes Diam. 1.0 cm, depth 1.25 cm (D) Diam. 0.6 cm, depth 0.6 cm (B, F) Reconstructed energy (pixel size 2.4 mm x 2.4 mm) Reconstructed energy (pixel size 1.2 mm x 1.2 mm). Black boxes = target and control pixel Resolve phantom into 2D pixels of size d and fit for the energy mean. When adding the depth of the target voids d, construct 3D voxels used in CT Energy contrast ~ density difference* voxel size  d

12. Hartmut F.-W. Sadrozinski RESMDD06 Required Dose for Image Reconstruction Determination of Fluence Limit by Data Reduction by factor n = 2,…, 64 Pixel size: 1.2 mm x 1.2 mm Fluence limit : n = 8-16 (blurred image and many white pixels = no valid fit) 1 rst Fluence Limit: number of protons in pixel > 10! n=2 n=32n=16 n=8n=4 n=64

13. Hartmut F.-W. Sadrozinski RESMDD06 Determination of Dose for the ~10 mm Voxel Data Reduction by factor n = 1,…, 200 Pixel size: 8 mm x 8 mm Fluence limit is reached at about n=64 Required significance S >2 n = 2 4 8 16 n = 32 64 128 200 Pattern Recognition relies on Significance (energy contrast/resolution)

14. Hartmut F.-W. Sadrozinski RESMDD06 Dose –Contrast – Voxel Size Dose ~ Fluence = Number of Protons / Voxel size Dose to the patient during imaging depends on the square of the effective energy resolution (including beam straggling). Proton energy resolution needs to be better than energy straggling (~1%)

15. Hartmut F.-W. Sadrozinski RESMDD06 Energy mean error can be improved with larger dose Error on Energy Mean vs. Energy Resolution Error on the mean is approximately equal to the RMS/sqrt(N) Revisit after improving calorimeter leakage. (Note that suspected leakage will not be a factor in a larger calorimeter system)

16. Hartmut F.-W. Sadrozinski RESMDD06 Clinical Interpretation Dose D – Voxel Size d – Density Variation  Dose D for two voxel sizes d: d [cm]D [mGy] 1.22.8*10 -5 0.62.5*10 -4 Ratio d 3 0.13 D 0.1 Expect after reduction of calorimeter leakage: resolution = straggling the dose will be reduced by factor 4.

17. Hartmut F.-W. Sadrozinski RESMDD06 Conclusions We performed beam tests with the elements of a prototype pCT system High resolution tracker using silicon strips Crystal calorimeter Fast DAQ Tracking studies show that the location within the phantom can be determined using external telescopes to a precision of better than 500  m. Calorimeter studies show that imaging is possible using the energy loss of the protons. By reducing the number of protons, relative fluence limits are derived, which can be expressed as minimum doses to image a voxel with a density difference. They scale well as a function of voxel size d (~1/d3), as predicted. MC is needed to take into account the finite extension of the prototype detectors on the energy scale and energy resolution (P. Cirrone’s talk).