Jerome J. Griesmer

Design and performance evaluation of a whole-body Ingenuity TF PET–MRI system

The Ingenuity TF PET-MRI is a newly released whole-body hybrid PET-MR imaging system with a Philips time-of-flight GEMINI TF PET and Achieva 3T X-series MRI system. Compared to PET-CT, modifications to the positron emission tomography (PET) gantry were made to avoid mutual system interference and deliver uncompromising performance which is equivalent to the standalone systems. The PET gantry was redesigned to introduce magnetic shielding for the photomultiplier tubes (PMTs). Stringent electromagnetic noise requirements of the MR system necessitated the removal of PET gantry electronics to be housed in the PET-MR equipment room. We report the standard NEMA measurements for the PET scanner. PET imaging and performance measurements were done at Geneva University Hospital as described in the NEMA Standards NU 2-2007 manual. The scatter fraction (SF) and noise equivalent count rate (NECR) measurements with the NEMA cylinder (20 cm diameter) were repeated for two larger cylinders (27 cm and 35 cm diameter), which better represent average and heavy patients. A NEMA/IEC torso phantom was used for overall assessment of image quality. The transverse and axial resolution near the center was 4.7 mm. Timing and energy resolution of the PET-MR system were measured to be 525 ps and 12%, respectively. The results were comparable to PET-CT systems demonstrating that the effect of design modifications required on the PET system to remove the harmful effect of the magnetic field on the PMTs was negligible. The absolute sensitivity of this scanner was 7.0 cps kBq(-1), whereas SF was 26%. NECR measurements performed with cylinders having three different diameters, and image quality measurements performed with IEC phantom yielded excellent results. The Ingenuity TF PET-MRI represents the first commercial whole-body hybrid PET-MRI system. The performance of the PET subsystem was comparable to the GEMINI TF PET-CT system using phantom and patient studies. It is conceived that advantages of hybrid PET-MRI will become more evident in the near future.

Design considerations for a new solid-state gamma-camera: Soltice

This paper presents an alternative to a conventional gamma camera. The proposed system is based on the combination of a new detection material - CZT - and a new acquisition geometry - 1D detector with slat collimator. The slat collimator offers a better compromise to the usual sensitivity-resolution tradeoff. Solid-state detection offers significantly improved energy resolution. The combination of the two elements, and the development of better reconstruction tools, has the potential of bringing a new level of performance to nuclear medicine imaging. The targeted system performance includes a spatial resolution of 5 mm FWHM at 10 cm from the detector surface and an energy resolution better than 5% FWHM, both at 140 keV. While the results are still preliminary and incomplete, they more than justify further investigation.

Dynamic Load Balancing on Distributed Listmode Time-of-Flight Image Reconstruction

A major obstacle in performing listmode reconstruction in PET imaging is the increased computation time compared to a conventional frame or histogrammed reconstruction. To overcome this challenge in a clinical setting, it is desirable to distribute the reconstruction task to multiple nodes. A previous work investigated the impact of high performance communication networks and focused mainly on static distribution. In practice, optimal static load balancing is difficult. Therefore we have developed a dynamic load balancing approach, which is flexible and can easily be adapted to a varying number of nodes, and the performance of which is not constrained by variation of the load levels of nodes needed for other tasks or by asymmetric network. In this approach, one of the nodes is designated as the distributor, whose task is to partition the events into small chunks and then distribute those chunks to other nodes for processing. Other nodes, which do the actual data processing, are called workers. Each worker requests a new chunk of data to process upon completion of an old one. In case of the OSEM algorithm, when all chunks have been processed in a subset, the workers are synchronized and the image is updated. This image forms the basis for the next subset. This system has been deployed in the Philips GEMINI-TF PET/CT system. For a whole-body patient scan of 150M events, the event processing time with 8 Xeon 3.6GHz dual-processor computers amounts to approximately 9 minutes for 3 iterations.

PET performance of the GEMINI TF PET — MR: The world's first whole body PET — MRI scanner

The GEMINI TF PET-MRI (Philips Healthcare, Cleveland, OH) is a newly released whole body hybrid imaging system with a Philips Achieva 3T system and a Philips TF (TruFlight) PET. We report the standard NEMA NU2 measurements for the scanner. Compared to PET-CT, modifications to the PET were made to avoid mutual system interference and deliver uncompromising performance which is equivalent to the standalone systems. The PET gantry was redesigned to introduce magnetic shielding for the PMTs. Stringent electromagnetic noise requirements of the MR system necessitated the removal of PET gantry electronics to be housed in the PET-MR equipment room. All PET calibrations and measurements were done with the MRI ramped to 3T, shimmed and calibrated. Measurements were performed as described in NEMA standard manual. GEMINI TF PET-CT typical NEMA values were compared to PET-MR (n≥4 for PET-MR data). Spatial resolution and sensitivity were comparable to PET-CT results. Timing resolution and energy resolution of the PET-MR system (not NEMA standard measurements) were measured to be 520 ps and 12%, respectively. Image quality measurements performed with IEC phantom yielded results comparable to PET-CT. The NEMA results obtained with PET-MR are equivalent to typical GEMINI TF PET-CT results. System energy and timing resolution were comparable to PET-CT demonstrating the effect of magnetic shielding to maintain the PMTs in normal flux levels. In conclusion, we report the design of a whole-body hybrid PET-MRI system where PET performance is comparable to standalone GEMINI TF PET-CT system.

Whole-body PET-MR imaging system initial calibration results

PET-MR imaging systems have been discussed and written about for many years. Philips is the first to develop a whole body PET-MR imaging system. The approach taken followed the realization that a sequential pair of scans, with both imaging scanners in the SAME ROOM could be achieved in far less time and cost than a simultaneous PET-MR and would achieve most of the potential benefits of the two modalities. We have designed, built and calibrated a pair of fully functional imagers, obtaining optimal imaging from both modalities, but having solved all of the difficult interaction issues that have previously prevented operation in the same room.

Measurements of entrance-surface vs. conventional single-ended readout of a monolithic scintillator

Availability of compact high-gain, low-noise Silicon Photomultipliers (SiPM) prompts us to examine readout sensors on the entrance surface (SES) as compared to the conventional single-ended readout with sensors on the opposing surface. We measured detector response statistics versus 3D position for these configurations using an 8×8 SiPM array on a 15-mm-thick by 32-mm-wide LYSO block. We calibrate an independently distributed multivariate-normal likelihood model and use it to generate maximum-likelihood estimates of 3D interaction position. Spatial resolution improved 14% and timing resolution improved 10% for the SES device. Bias was unaffected. Photodetection efficiency of our prototype SiPM may have limited further improvement in positioning and timing performance. In future work, we will look to utilize SiPM arrays with enhanced photodetection efficiency.

Validation of GATE Monte Carlo simulations of the Philips GEMINI TF and TruFlight Select PET/CT scanners based on NEMA NU2 standards

The objective of this study is to validate the in-house GATE simulations of the Philips GEMINI TF and TruFlight Select PET scanners and evaluate their accuracy for further research and optimization of current and future PET products. GATE results are compared to experimental data obtained according to the National Electrical Manufacturers Association (NEMA) NU2-2007 standards. A detailed implementation of the geometrical and functional models of the scanners and the NEMA phantoms was conducted, allowing the evaluation of the simulated absolute sensitivity, spatial resolution, count rates and the image quality of both systems. All Monte Carlo data production was performed according to the NEMA protocols. Simulated data were converted into the Philips list-mode format and analyzed using the same software tools as in the quality control step of the production line. Good agreement was found between the simulated results and the measured data from both scanners. This validation study represents an important step towards the use of these tools as an aid for the optimization of the current acquisition protocols and the validation of reconstruction and data correction techniques.

Maximum area sampling scheme for SOLSTICE rotating slat collimator system

A concept of a slat collimator mounted directly on a rotating Solid-State detector has been previously described (2001). The detector consists of a long and narrow CZT assembly spinning to get 2D imaging and revolving around the object to build 3D reconstruction. One fully expects the spinning of a linear detector to produce a circular field of view, which in turn may render the 3D reconstruction more elaborate and user visualization more complex. Choices are either to define the inscribed square (or rectangle) inside the circular field-of-view, or, more economically, define a different spinning scheme that would allow non-circular area of interest. This paper describes such a scheme based on a special case of the curve of constant width: the Reuleaux triangle. Implementation and performance are discussed.