Feasibility study of portable multi-energy computed tomography with photon-counting detector for preclinical and clinical applications.

In this study, preclinical experiments were performed with an in-house developed prototypal photon-counting detector computed tomography (PCD CT) system. The performance of the system was compared with the conventional energy-integrating detector (EID)-based CT, concerning the basic image quality biomarkers and the respective capacities for material separation. The pre- and the post-contrast axial images of a canine brain captured by the PCD CT and EID CT systems were found to be visually similar. Multi-energy images were acquired using the PCD CT system, and machine learning-based material decomposition was performed to segment the white and gray matters for the first time in soft tissue segmentation. Furthermore, to accommodate clinical applications that require high resolution acquisitions, a small, native, high-resolution (HR) detector was implemented on the PCD CT system, and its performance was evaluated based on animal experiments. The HR acquisition mode improved the spatial resolution and delineation of the fine structures in the canine's nasal turbinates compared to the standard mode. Clinical applications that rely on high-spatial resolution expectedly will also benefit from this resolution-enhancing function. The results demonstrate the potential impact on the brain tissue segmentation, improved detection of the liver tumors, and capacity to reconstruct high-resolution images both preclinically and clinically.

Therapy region monitoring based on PET using 478 keV single prompt gamma ray during BNCT: A Monte Carlo simulation study.

We confirmed the feasibility of using our proposed system to extract two different kinds of functional images from a positron emission tomography (PET) module by using an insertable collimator during boron neutron capture therapy (BNCT). Coincidence events from a tumor region that included boron particles were identified by a PET scanner before BNCT; subsequently, the prompt gamma ray events from the same tumor region were collected after exposure to an external neutron beam through an insertable collimator on the PET detector. Five tumor regions that contained boron particles and were located in the water phantom and in the BNCT system with the PET module were simulated with Monte Carlo simulation code. The acquired images were quantitatively analyzed. Based on the receiver operating characteristic (ROC) curves in the five boron regions, A, B, C, D, and E, the PET and single-photon images were 10.2%, 11.7%, 8.2% (center region), 12.6%, and 10.5%, respectively. We were able to acquire simultaneously PET and single prompt photon images for tumor regions monitoring by using an insertable collimator without any additional isotopes.

A prototype MR insertable brain PET using tileable GAPD arrays.

PURPOSE: The aim of this study was to develop a prototype magnetic resonance (MR)-compatible positron emission tomography (PET) that can be inserted into a MR imager and that allows simultaneous PET and MR imaging of the human brain. This paper reports the initial results of the authors' prototype brain PET system operating within a 3-T magnetic resonance imaging (MRI) system using newly developed Geiger-mode avalanche photodiode (GAPD)-based PET detectors, long flexible flat cables, position decoder circuit with high multiplexing ratio, and digital signal processing with field programmable gate array-based analog to digital converter boards. METHODS: A brain PET with 72 detector modules arranged in a ring was constructed and mounted in a 3-T MRI. Each PET module was composed of cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled to a tileable GAPD. The GAPD output charge signals were transferred to preamplifiers using 3 m long flat cables. The LYSO and GAPD were located inside the MR bore and all electronics were positioned outside the MR bore. The PET detector performance was investigated both outside and inside the MRI, and MR image quality was evaluated with and without the PET system. RESULTS: The performance of the PET detector when operated inside the MRI during MR image acquisition showed no significant change in energy resolution and count rates, except for a slight degradation in timing resolution with an increase from 4.2 to 4.6 ns. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI. Rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. The activity distribution patterns between the white and gray matter in the Hoffman brain phantom were well imaged. The hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences were observed without any noticeable artifacts, although MR image quality requires some improvement. CONCLUSIONS: These results demonstrate that the simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.

Development of brain PET using GAPD arrays.

PURPOSE: In recent times, there has been great interest in the use of Geiger-mode avalanche photodiodes (GAPDs) as scintillator readout in positron emission tomography (PET) detectors because of their advantages, such as high gain, compact size, low power consumption, and magnetic field insensitivity. The purpose of this study was to develop a novel PET system based on GAPD arrays for brain imaging. METHODS: The PET consisted of 72 detector modules arranged in a ring of 330 mm diameter. Each PET module was composed of a 4 × 4 matrix of 3 × 3 × 20 mm(3) cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled with a 4 × 4 array three-side tileable GAPD. The signals from each PET module were fed into preamplifiers using a 3 m long flat cable and then sent to a position decoder circuit (PDC), which output a digital address and an analog pulse of the interacted channel among 64 preamplifier signals transmitted from four PET detector modules. The PDC outputs were fed into field programmable gate array (FPGA)-embedded data acquisition (DAQ) boards. The analog signal was then digitized, and arrival time and energy of the signal were calculated and stored. RESULTS: The energy and coincidence timing resolutions measured for 511 keV gamma rays were 18.4 ± 3.1% and 2.6 ns, respectively. The transaxial spatial resolution and sensitivity in the center of field of view (FOV) were 3.1 mm and 0.32% cps/Bq, respectively. The rods down to a diameter of 2.5 mm were resolved in a hot-rod phantom image, and activity distribution patterns between the white and gray matters in the Hoffman brain phantom were well imaged. CONCLUSIONS: Experimental results indicate that a PET system can be developed using GAPD arrays and the GAPD-based PET system can provide high-quality PET imaging.

A feasibility study of photosensor charge signal transmission to preamplifier using long cable for development of hybrid PET-MRI.

PURPOSE: A new positron emission tomography (PET) detector signal processing method, the charge signal transmission approach, is proposed for the development of a hybrid PET-magnetic resonance imaging (MRI). A number of experiments were performed to demonstrate that the Geiger-mode avalanche photodiode (GAPD) charge output could be transmitted to a preamplifier using a long cable without degrading the PET signal performance. METHODS: A PET module consisted of LYSO and a GAPD with a 4 x 4 array. The GAPD output was transmitted to the preamplifier through flexible flat cables. The effect of the cable length on the PET performance was examined using seven different lengths ranging from 10 to 300 cm outside and inside the 7 T animal MRI. Four parameters (rise time, fall time, amplitude, and area of the preamplifier output) were measured as a function of the cable length using a 10 GS/s oscilloscope and three parameters (photopeak position, energy resolution, and time resolution) were measured using a 100 MS/s DAQ unit. The effect of the cable length on the MR phantom images was investigated. In addition, the effect of the PET module configuration on its temperature stability was assessed by acquiring the energy and time spectra. RESULTS: There were no significant changes in the PET module performance as a function of the cable length, both outside and inside MRI. The performance changes in energy information, such as the amplitude, area, photopeak position, and energy resolution, were <3% with cable lengths ranging from 10 to 300 cm and the change in the time resolution was <6%. There were no obvious artifacts or changes in the line profile in the MR phantom images. Moreover, no manifest changes in the photopeak position and coincidence counting rate were observed in the PET modules employing the charge signal transmission approach, whereas considerable degradation of the PET module performance was observed in the voltage signal transmission approach. CONCLUSIONS: This study demonstrated that it is feasible to design a hybrid PET-MRI using the charge signal transmission approach, which is expected to have more advantages than other approaches.

Development of a dual modality imaging system: a combined gamma camera and optical imager.

Several groups have reported the development of dual modality Gamma camera/optical imagers, which are useful tools for investigating biological processes in experimental animals. While previously reported dual modality imaging instrumentation usually employed a separated gamma camera and optical imager, we designed a detector using a position sensitive photomultiplier tube (PSPMT) that is capable of imaging both gamma rays and optical photons for combined gamma camera and optical imager. The proposed system consists of a parallel-hole collimator, an array-type crystal and a PSPMT. The top surface of the collimator and array crystals is left open to allow optical photons to reach the PSPMT. Pulse height spectra and planar images were obtained using a Tc-99m source and a green LED to estimate gamma and optical imaging performances. When both gamma rays and optical photon signals were detected, the signal interferences caused by each other signal were evaluated. A mouse phantom and an ICR mouse containing a gamma ray and optical photon source were imaged to assess the imaging capabilities of the system. The sensitivity, energy resolution and spatial resolution of the gamma image acquired using Tc-99m were 1.1 cps/kBq, 26% and 2.1 mm, respectively. The spatial resolution of the optical image acquired with an LED was 3.5 mm. Signal-to-signal interference due to the optical photon signal in the gamma pulse height spectrum was negligible. However, the pulse height spectrum of the optical photon signal was found to be affected by the gamma signal, and was obtained between signals generated by gamma rays with a correction using a veto gate. Gamma ray and optical photon images of the mouse phantom and ICR mouse were successfully obtained using the single detector. The experimental results indicated that both optical photon and gamma ray imaging are feasible using a detector based on the proposed PSPMT.

Performance improvement of small gamma camera using NaI(Tl) plate and position sensitive photo-multiplier tubes.

The purpose of this study was to improve the performance of a small gamma camera, utilizing a NaI(Tl) plate and a 5" position sensitive PMT. We attempted to build a NaI(Tl) plate crystal system which retained all its advantages, while at the same time integrating some of the advantages inherent in an array-type scintillation crystal system. Flood images were obtained with a lead hole mask, and position mapping was performed by detecting hole positions in the flood image. Energy calibration was performed using the energy spectra obtained from each hole position. Flood correction was performed using a uniformity correction table containing the relative efficiency of each image element. The spatial resolution was improved about 16% after correction at the centre field of view. Resolution deterioration at the outer field of view (OFOV) was considerably ameliorated, from 6.7 mm to 3.2 mm after correction. The sensitivity at the OFOV was also increased after correction, from 0.7 cps microCi(-1) to 2.0 cps microCi(-1). The correction also improved uniformity, from 5.2% to 2.1%, and linearity, from 0.5 mm to 0 mm. The results of this study indicate that the revised correction method can be employed to considerably improve the performance of a small gamma camera using a NaI(Tl) plate-type crystal. This method also provides high spatial resolution and linearity, like array-type crystals do, while retaining the specific advantages of plate-type crystals.