The aim of this study was to develop a prototype PET detector module for a combined small animal positron emission tomography and magnetic resonance imaging (PET/MRI) system. The most important factor for small animal imaging applications is the detection sensitivity of the PET camera, which can be optimized by utilizing longer scintillation crystals. At the same time, small animal PET systems must yield a high spatial resolution. The measured object is very close to the PET detector because the bore diameter of a high field animal MR scanner is limited. When used in combination with long scintillation crystals, these small-bore PET systems generate parallax errors that ultimately lead to a decreased spatial resolution. Thus, we developed a depth of interaction (DoI) encoding PET detector module that has a uniform spatial resolution across the whole field of view (FOV), high detection sensitivity, compactness, and insensitivity to magnetic fields.
The approach was based on Geiger mode avalanche photodiode (G-APD) detectors with cross-strip encoding. The number of readout channels was reduced by a factor of 36 for the chosen block elements. Two 12 × 2 G-APD strip arrays (25μm cells) were placed perpendicular on each face of a 12 × 12 lutetium oxyorthosilicate crystal block with a crystal size of 1.55 × 1.55 × 20 mm. The strip arrays were multiplexed into two channels and used to calculate the x, y coordinates for each array and the deposited energy. The DoI was measured in step sizes of 1.8 mm by a collimated 18F source. The coincident resolved time (CRT) was analyzed at all DoI positions by acquiring the waveform for each event and applying a digital leading edge discriminator.
All 144 crystals were well resolved in the crystal flood map. The average full width half maximum (FWHM) energy resolution of the detector was 12.8% ± 1.5% with a FWHM CRT of 1.14 ± 0.02 ns. The average FWHM DoI resolution over 12 crystals was 2.90 ± 0.15 mm.
The novel DoI PET detector, which is based on strip G-APD arrays, yielded a DoI resolution of 2.9 mm and excellent timing and energy resolution. Its high multiplexing factor reduces the number of electronic channels. Thus, this cross-strip approach enables low-cost, high-performance PET detectors for dedicated small animal PET and PET/MRI and potentially clinical PET/MRI systems.
This work was funded by the German Research Association (DFG PI771/1-3) and the Swiss Werner Siemens-Foundation. The authors thank Mathew Divine and Magdalena Rafecas (Werner Siemens Imaging Center) for critical review and discussion of the paper, Matthias Schmand, Nan Zhang, and Johannes Breuer (Siemens Medical Solutions) for their many helpful discussions. The authors also thank Carlo Tintori (CAEN) for customer support with the digitizer electronics.
Eckart Lorenz passed away during the review process. We appreciate his contributions, discussions and will cherish his memory.
A patent application with the international application number (PCT/EP2012/057580) has been filed. The authors declare no conflict of interest.
II. MATERIAL AND METHODS
II.A. G-APD strip detector
II.B. Detector setup
II.C. Measurement setup
II.D. Acquisition electronics
II.E. Crystal identification, energy, and DoI encoding
II.F. Coincident resolved time
III.A. Crystal identification, energy resolution, and DoI encoding
III.B. Coincident resolved timing
- Positron emission tomography
- Scintillation detectors
- Spatial resolution
- Magnetic field sensors
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