Alexander M. Grant

NEMA NU 2‐2012 performance studies for the SiPM‐based ToF‐PET component of the GE SIGNA PET/MR system

PURPOSE The GE SIGNA PET/MR is a new whole body integrated time-of-flight (ToF)-PET/MR scanner from GE Healthcare. The system is capable of simultaneous PET and MR image acquisition with sub-400 ps coincidence time resolution. Simultaneous PET/MR holds great potential as a method of interrogating molecular, functional, and anatomical parameters in clinical disease in one study. Despite the complementary imaging capabilities of PET and MRI, their respective hardware tends to be incompatible due to mutual interference. In this work, the GE SIGNA PET/MR is evaluated in terms of PET performance and the potential effects of interference from MRI operation. METHODS The NEMA NU 2-2012 protocol was followed to measure PET performance parameters including spatial resolution, noise equivalent count rate, sensitivity, accuracy, and image quality. Each of these tests was performed both with the MR subsystem idle and with continuous MR pulsing for the duration of the PET data acquisition. Most measurements were repeated at three separate test sites where the system is installed. RESULTS The scanner has achieved an average of 4.4, 4.1, and 5.3 mm full width at half maximum radial, tangential, and axial spatial resolutions, respectively, at 1 cm from the transaxial FOV center. The peak noise equivalent count rate (NECR) of 218 kcps and a scatter fraction of 43.6% are reached at an activity concentration of 17.8 kBq/ml. Sensitivity at the center position is 23.3 cps/kBq. The maximum relative slice count rate error below peak NECR was 3.3%, and the residual error from attenuation and scatter corrections was 3.6%. Continuous MR pulsing had either no effect or a minor effect on each measurement. CONCLUSIONS Performance measurements of the ToF-PET whole body GE SIGNA PET/MR system indicate that it is a promising new simultaneous imaging platform.

Prototype positron emission tomography insert with electro-optical signal transmission for simultaneous operation with MRI

The simultaneous acquisition of PET and MRI data shows promise to provide powerful capabilities to study disease processes in human subjects, guide the development of novel treatments, and monitor therapy response and disease progression. A brain-size PET detector ring insert for an MRI system is being developed that, if successful, can be inserted into any existing MRI system to enable simultaneous PET and MRI images of the brain to be acquired without mutual interference. The PET insert uses electro-optical coupling to relay all the signals from the PET detectors out of the MRI system using analog modulated lasers coupled to fiber optics. Because the fibers use light instead of electrical signals, the PET detector can be electrically decoupled from the MRI making it partially transmissive to the RF field of the MRI. The SiPM devices and low power lasers were powered using non-magnetic MRI compatible batteries. Also, the number of laser-fiber channels in the system was reduced using techniques adapted from the field of compressed sensing. Using the fact that incoming PET data is sparse in time and space, electronic circuits implementing constant weight codes uniquely encode the detector signals in order to reduce the number of electro-optical readout channels by 8-fold. Two out of a total of sixteen electro-optical detector modules have been built and tested with the entire RF-shielded detector gantry for the PET ring insert. The two detectors have been tested outside and inside of a 3T MRI system to study mutual interference effects and simultaneous performance with MRI. Preliminary results show that the PET insert is feasible for high resolution simultaneous PET/MRI imaging for applications in the brain.

Simultaneous PET/MR imaging with a radio frequency‐penetrable PET insert

Purpose: A brain sized radio frequency (RF)‐penetrable PET insert has been designed for simultaneous operation with MRI systems. This system takes advantage of electro‐optical coupling and battery power to electrically float the PET insert relative to the MRI ground, permitting RF signals to be transmitted through small gaps between the modules that form the PET ring. This design facilitates the use of the built‐in body coil for RF transmission and thus could be inserted into any existing MR site wishing to achieve simultaneous PET/MR imaging. The PET detectors employ nonmagnetic silicon photomultipliers in conjunction with a compressed sensing signal multiplexing scheme, and optical fibers to transmit analog PET detector signals out of the MRI room for decoding, processing, and image reconstruction. Methods: The PET insert was first constructed and tested in a laboratory benchtop setting, where tomographic images of a custom resolution phantom were successfully acquired. The PET insert was then placed within a 3T body MRI system, and tomographic resolution/contrast phantom images were acquired both with only the B0 field present, and under continuous pulsing from different MR imaging sequences. Results: The resulting PET images have comparable contrast‐to‐noise ratios (CNR) under all MR pulsing conditions: The maximum percent CNR relative difference for each rod type among all four PET images acquired in the MRI system has a mean of 14.0 ± 7.7%. MR images were successfully acquired through the RF‐penetrable PET shielding using only the built‐in MR body coil, suggesting that simultaneous imaging is possible without significant mutual interference. Conclusions: These results show promise for this technology as an alternative to costly integrated PET/MR scanners; a PET insert that is compatible with any existing clinical MRI system could greatly increase the availability, accessibility, and dissemination of PET/MR.

Performance characterization of compressed sensing positron emission tomography detectors and data acquisition system.

In the field of information theory, compressed sensing (CS) had been developed to recover signals at a lower sampling rate than suggested by the Nyquist-Shannon theorem, provided the signals have a sparse representation with respect to some base. CS has recently emerged as a method to multiplex PET detector readouts thanks to the sparse nature of 511 keV photon interactions in a typical PET study. We have shown in our previous numerical studies that, at the same multiplexing ratio, CS achieves higher signal-to-noise ratio (SNR) compared to Anger and cross-strip multiplexing. In addition, unlike Anger logic, multiplexing by CS preserves the capability to resolve multi-hit events, in which multiple pixels are triggered within the resolving time of the detector. In this work, we characterized the time, energy and intrinsic spatial resolution of two CS detectors and a data acquisition system we have developed for a PET insert system for simultaneous PET/MRI. The CS detector comprises a 2 x 4 mosaic of 4 x 4 arrays of 3.2 x 3.2 x 20 mm(3) lutetium-yttrium orthosilicate crystals coupled one-to-one to eight 4 x 4 silicon photomultiplier arrays. The total number of 128 pixels is multiplexed down to 16 readout channels by CS. The energy, coincidence time and intrinsic spatial resolution achieved by two CS detectors were 15.4±0.1% FWHM at 511 keV, 4.5 ns FWHM and 2.3 mm FWHM, respectively. A series of experiments were conducted to measure the sources of time jitter that limit the time resolution of the current system, which provides guidance for potential system design improvements. These findings demonstrate the feasibility of compressed sensing as a promising multiplexing method for PET detectors.

All-optical encoding of PET detector signals

Typical PET scanners use many electronic readout channels and dedicated electronic coincidence processing boards that contribute significantly to system complexity and costs. We have developed a method of optically encoding position, energy, and arrival time of annihilation photon interactions in PET detectors with fast coherent optical pulse (100 ps FWHM) trains from telecommunications-grade lasers, and propose a method that, if successful, will enable multiplexing the entire system output to a single optical fiber output channel. This will allow the elimination of much of the currently used processing electronics, achieving coincidence time resolutions of <300 ps FWHM while decreasing system complexity. We constructed a single channel proof of concept system using fast 10 Gbps off-the-shelf optical components and a silicon photomultiplier (SiPM) coupled to an LSO:Ce crystal. Using this system we demonstrated the encoding of position, energy, and arrival time into four coherent optical pulses. We measured extremely low timing jitter for the optical pulses (3 ps) and collected preliminary 137Cs energy spectra (energy resolution ∼17% FWHM at 662 keV - not optimized yet) with all-optical pulse encoding. Optical encoding and multiplexing could greatly facilitate the construction of high resolution PET scanners with thousands of detector channels.

Analog filtering methods improve leading edge timing performance of multiplexed SiPMs

Multiplexing many SiPMs to a single readout channel is an attractive option to reduce the readout complexity of high perfromance time of flight (TOF) PET systems. However, the additional dark counts and shaping from each SiPM cause significant baseline fluctuations in the output waveform, degrading timing measurements using a leading edge threshold. This work proposes a simple analog filtering network to reduce the baseline fluctuations in highly multiplexed SiPM readouts. With 16 SiPMs multiplexed, the FWHM coincident timing resolution for single 3mm × 3mm × 20mm LYSO crystals was improved from 401 +/−4ps to 248 +/− 5ps. With 4 SiPMs multiplexed, using a 20mm length, 2 layer DOI array of LYSO crystals the mean time resolution was improved from 277 +/− 6ps to 217 +/−4ps using a ADCMP572 comparator for timing pickoff. All experiments were performed at room temperature with no active temperature regulation. This results show a promising technique for the construction of high performance multiplexed TOF PET readout systems with simple analog leading edge timing pickoff.

RF-transmissive PET detector insert for simultaneous PET/MRI

Positron emission tomography (PET) and magnetic resonance imaging (MRI) have revolutionized the disease characterization as they enable the simultaneous measurement of functional and anatomical information of the body. However, the whole body simultaneous PET/MRI has been limited by its high cost. To address this issue, we are developing an RF-transmissive PET insert system that can be inserted into an MRI system without requiring modifications to the MR hardware system. Our PET system prototype consists of 16 PET detector modules in a 32 cm ring pattern with small gaps between them. By using electro-optical signal transmission technology, the PET insert is electrically floating relative to the MRI RF ground which allows the PET detector insert to be RF-penetrable. The inter-modular gaps and the electrical floating enable the RF fields of MRI body coil to pass through with some attenuation. We performed 2D electromagnetic simulations and experiments in a 3-T MR system to understand the degree to which a ring of electrically floating Faraday cages facilitates the RF transmissivity. The electromagnetic simulation results showed that the grounded PET insert blocks the RF field while the electrically floating PET insert allows the RF field to uniformly transmit through the gaps with some attenuation. The MRI attenuation experiments showed that the transmit attenuation was -3.47 dB and similarly the receive attenuations were -3.40 and -3.94 dB for GE and SE sequences, respectively. We have shown from both simulations and experiments that the RF field of the MRI body coil can penetrate a PET ring through small inter-modular gaps, when the PET ring is electrically floating with respect to the MR system.

Studies of electromagnetic interference of PET detector insert for simultaneous PET/MRI

We are developing a brain positron emission tomography (PET) system prototype with long optical cables to minimize mutual interference between our PET components and magnetic resonance imaging (MRI) system. Our PET system consists of 16 PET detector modules which are placed in Faraday cages spaced equally in a 32 cm diameter ring. By using 20 m length optical cables rather than electrical connections, the Faraday cage ground can float relative to the MRI RF ground which permits the RF field to transmit through PET ring. This could eliminate the need for custom RF coils in whole body inserts, or the need for a custom transmit coil in brain insert PET/MRI designs. The aim of this study is to investigate the feasibility of PET detectors with a floating ground from measurements of electromagnetic interference (EMI) shielding and numerical analyses of RF field attenuation under different conditions. The shielding effectiveness equation shows that a copper plate of 30 μm (~4×Skin Depth) thickness shields approximately 120 dB (99.9999 %) of both the 66.7 MHz analog-to-digital converter sampling frequency of the interior PET electronics and the 127.7 MHz Larmor frequency of the exterior 3-T MRI RF coil. Simulation results using ANSOFT Maxwell showed that a larger gap between PET detectors or a shorter height of PET Faraday cage results in less RF field attenuation. The two side plates of adjacent PET Faraday cage act as a capacitor. When the gap increases or height shrinks, capacitive impedance increases which then results in less RF power dissipation and thus more RF field transmission inside field of view (FOV). Simulation results showed 25 dB increase of the transmission level when the gap was increased by 2 mm and height was decreased by 20 mm. Further MR-compatibility analysis will be performed by acquiring MR images with the shielded PET detector ring inserted.

RF-Penetrable PET insert for simultaneous PET/MR imaging

Integrated PET/MRI enables simultaneous measurement of molecular, functional and anatomical information of the body in one combined scan, providing physicians and researchers with multi-parameter information. However, the long-term impact of integrated PET/MRI is limited by the high cost of the current commercial systems, which require the users to purchase both PET and MR subsystems, which are permanently integrated. We are developing a RF-penetrable PET insert technology to address this challenge, and a prototype brain-dedicated insert system has been built to evaluate the technology. The insert system consists of 16 detector modules assembled into a ring of an inner diameter of 32 cm. A total number of 2,048 3.2 mm × 3.2 mm × 20 mm LYSO crystals coupled one-to-one to 2,048 SiPM pixels are implemented in the system. An intrinsic spatial resolution of below 2.3 mm has been achieved by measuring the coincidence point spread functions of a 500 μm positron-emitting point source with two electronically collimated detectors. A custom resolution phantom with hot rods ranging from 2.8 to 5.2 mm diameter has been acquired and reconstructed to evaluate the image spatial resolution of the system. The sizes of the smallest resolvable hot rods in the reconstructed images were 2.8 mm and 4.2 mm, when the phantom was placed at the center of the fleld-of-view and 9-cm off-center trans-axially, respectively. An energy resolution of 15.6 % FWHM at 511 keV and a coincidence time resolution of 5.2 ns FWHM have been achieved, limited by the outdated 2008 SiPM technology employed. The variation of the energy resolution and coincidence time resolution stays within a range of 0.5 % and 80 ps over 3 hours, demonstrating the stability of the system.

Optical encoding and multiplexing of detector signals with dual threshold time-over-threshold

We have previously demonstrated a method of optical pulse processing for PET detectors in which position, timing, and energy data is encoded into a series of fast (~100 ps) optical pulses at 1550 nm. This optical encoding method can potentially support a very high multiplexing ratio, allowing the majority of the readout electronics to be concentrated into a single high speed digitizer channel that records and decodes the event data from every detector, making it suitable for high resolution PET systems with many detectors. In our previous prototype, single threshold time-over-threshold (ToT) was used to encode energy and timing as it was relatively simple to implement with few components, minimizing added timing jitter. However, single threshold ToT has major drawbacks that degraded both timing and energy resolution, resulting in poor performance overall. In this work we replace single threshold ToT with a dual threshold ToT method using a custom circuit with only a few high speed, low jitter components, greatly improving timing and energy performance. With dual threshold ToT we can fully encode and multiplex position, timing, and energy information from two detector channels on just a single optical fiber with excellent timing resolution (160 ps FWHM with 3×3×5 mm3 LSO and Hamamatsu SiPMs), suitable for time-of-flight.