TOPEM: a multimodality probe (PET TOF, MRI and MRS) for diagnosis and follow up of prostate cancer.

  1. Introduction

Prostate cancer (PC) is the most common disease and a leading cause of cancer death
Precise disease characterization is needed about cancer location, size, and extent and aggressiveness (1). The current standard for diagnosing PC is transrectal biopsy; however, it is far from perfect. Multimodality imaging can play a significant role by merging anatomical and functional details from simultaneous PET and MRI (and MRS) scans to guide biopsy diagnosis and follow up.Due to sub-optimal prostate imaging geometries, generic scanners prevent separation of the signal from surrounding organs with sensitivity, spatial resolution and contrast inferior to what is achievable with dedicated prostate imagers. Our project is developing an endorectal PET-TOF MRI probe. Exploiting the TOF capability allows an increase in the SNR/NECR and also permits elimination of bladder background (4). The internal probe is used in coincidence with an external dedicated detector and/or a standard PET ring. Performance is dominated by the endorectal detector with improvements in both spatial resolution and efficiency (2,3). The electronics must measure coincidences with a precision of 300 ps or less, and be small enough to be connected to the internal detector. For compactness and MRI compatibility, Silicon Photomultipliers (SiPM) are used. Their time jitter is negligible so the expected time resolution is a direct function of the sqrt of photoelectron number related to the PDE. Extensive ongoing simulation by Geant4 allows study of the scintillator geometry, coupling to the SiPMs and their pixel dimensions.

  1. Detector layout

One of the detector, to be very close to the source has to be a small endorectal probe. The second detector is a standard PET scanner (detectors close to the body of the patient may be added).Fig. 1 a.Detector layout: a small probe in coincidence with a full ring standard PET possibly integrated by few detectors close to the body of the patient; b. the layout in the Geant4 code. c. Spatial resolution as function of the distance and of the resolution of the probe; d. efficiency as function of the dimension of the external PET

a b c d

Fig. 1 a.Detector layout: a small probe in coincidence with a full ring standard PET possibly integrated by few detectors close to the body of the patient; b. the layout in the Geant4 code. c. Spatial resolution as function of the distance and of the resolution of the probe; d. efficiency as function of the dimensione of the external PET

  1. Simulations

A Geant 4 code has been written in order to perform extensive simulations to optimize the detector layout. Preliminary data are available. Fig. 1 a show the layout of the system and b what has been implemented in the Geant4 code(a Zubal phantom for the prostate, a probe of 25 x 50 x 10 mm3 (LSO coupled to two sheets of SipM photodetectors and an half ring of standard PET). Preliminary results of simulations, in Fig. 1 c,d, show the spatial resolution and efficiency obtained with the proposed system. Simulation results show spatial resolution of 1.5 mm - 2 mm for source distances of 10–20 mm, with improved efficiency over external PET. Depending on the reconstructed resolution desired, noise can be reduced by up to ~7x over external ring PET alone giving improvements of effective NEC of ~50x. Simulation of the energy spectra for LSO and LaBr3 continuous crystals shows the advantage of the LaBr3 in terms of energy resolution (15% vs 17%) (see Fig. 3(. The timing also would be better, but the stopping power and spatial resolution wouldn’t favorize LaBr3 for the NECR.

  1. Measurement(s) on prototype(s)

A minidetector prototype has been built (see pictures in Fig.3) and will be tested in a 3 T MRI soon. Continuous as well as pixellated (1 x 1 mm2 and 3 x 3 mm2) LSO scintillator (with different surface treatments for optimizing both the timing and the Depth Of Interaction (DOI), coupled to SiPM arrays (3 x 3 mm2 anode pixel) will be used. A discrete electronics system has been built to perform measurements until the dedicated electronics system, namely the challenging ASIC is available. A continuous LaBr3 scintillator sheet will be also be used for comparison. Timing resolution (only the electronics) of 100 ps has been obtained with the discrete electronic system. The design of front end part of the ASIC (CMOS 130 nm technology) is in advanced stage. Fig. 4 shows basic measurements on SiPM .The figure 4 shows three typical spectra acquired using the same reverse bias voltage (32.0V), but the detector thermostated at different temperatures: a) 13.5C, b) 24.3C, c) 41.5C. The Gain reduction with temperature is due to the variation of breakdown voltage with temperature

Fig. 3. The minidetector and the electronic setup Fig. 4 Temperature dependence of gain for SiPM

for first Fig. 4. “elementary evaluations and

tests on MRI

Fig. 5show sthe energy resolution obtained with two finger LYSO (3 x 3 x 10 mm3) coupled to SiPM Hamamatsu (3 x 3 mm2)

a b

Fig. 5. a. Energy resolution of LYSO (3 x 3 x 10 mm2) coupled to SIPM Hamamatsu (3 x 3 mm2); b. Simulated energy spctra of LYSO and LaBr3(Ce)

  1. Conclusions

A project for designing building and testing an endorectal PET TOF probe compatible with MRI (and MRS) started. Preliminary results are available. More results both form simulation and measurements will be presented at the Conference, namely the ones related to the optimization of the parameters important for the TOF capability

References

1.G. Kellof et al. Challenges in Clincal Prostate Cancer: Role of Imaging. AJR:192, June 2009, pg. 1455

2. N. Clinthorne et al. Multi-resolution image reconstruction for high resolution small animal PET device. IEEE 2003, Nucl Symp. Conf. Rec. 3: 1997-2001

3. W.W. Moses. Time of flight revisited, IEEE TNS Vol 50, N. 5 October 2003, 1325

4.. J.S. Karp et al. Benefit of Time –of-Flight in PET: Experimental and Clinical Resukts, JNM, February 20, 2008/ nume.107.044834

5. W. Moses. Workshop on TOF PET, Baia delle Zagare , September 2009 and personal communication

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