Detector Ring Research Articles

Simulation of the expected performance of INSERT: A new multi-modality SPECT/MRI system for preclinical and clinical imaging

A new multi-modality imaging tool is under development in the framework of the INSERT (INtegrated SPECT/MRI for Enhanced Stratification in Radio-chemo Therapy) project, supported by the European Community. The final goal is to develop a custom SPECT apparatus, that can be used as an insert for commercially available MRI systems such as 3T MRI with 59cm bore diameter. INSERT is expected to offer more effective and earlier diagnosis with potentially better outcome in survival for the treatment of brain tumors, primarily glioma. Two SPECT prototypes will be developed, one dedicated to preclinical imaging, the second one dedicated to clinical imaging.The basic building block of the SPECT detector ring is a small 5cm×5cm gamma camera, based on the well-established Anger architecture with a continuous scintillator readout by an array of silicon photodetectors. Silicon Drift Detectors (SDDs) and Silicon PhotoMultipliers (SiPM) are being considered as possible scintillator readout, considering that the detector choice plays a predominant role for the final performance of the system, such as energy and spatial resolution, as well as the useful field of view of the camera. Both solutions are therefore under study to evaluate their performances in terms of field of view (FOV), spatial and energy resolution. Preliminary simulations for both the preclinical and clinical systems have been carried out to evaluate resolution and sensitivity.

Development of a single-ring OpenPET prototype

One of the challenging applications of PET is implementing it for in-beam PET, which is an in situ monitoring method for charged particle therapy. For this purpose, we have previously proposed an open-type PET scanner, OpenPET. The original OpenPET had a physically opened field-of-view (FOV) between two detector rings through which irradiation beams pass. This dual-ring OpenPET (DROP) had a wide axial FOV including the gap. This geometry was not necessarily the most efficient for application to in-beam PET in which only a limited FOV around the irradiation field is required. Therefore, we have proposed a new single-ring OpenPET (SROP) geometry which can provide an accessible and observable open space with higher sensitivity and a reduced number of detectors than the DROP. The proposed geometry was a cylinder shape with its ends cut at a slant, in which the shape of each cut end became an ellipse. In this work, we developed and evaluated a small prototype of the SROP geometry for proof-of-concept. The SROP prototype was designed with 2 ellipse-shaped detector rings of 16 depth-of-interaction (DOI) detectors each. The DOI detectors consisted of 1024 GSOZ scintillator crystals which were arranged in 4 layers of 16×16 arrays, coupled to a 64-channel FP-PMT. Each ellipse-shaped detector ring had a major axis of 281.6mm and a minor axis of 207.5mm. For the slant mode, the rings were placed at a 45-deg slant from the axial direction and for the non-slant mode (used as a reference) they were at 90deg from the axial direction with no gap. The system sensitivity measured from a 22Na point source was 5.0% for the slant mode. The average spatial resolutions of major and minor axis directions were calculated as 3.8mm FWHM and 4.9mm FWHM, respectively for the slant mode. This difference resulted from the ellipsoidal ring geometry and the spatial resolution of the minor axis direction degraded by the parallax error. Comparison between the slant mode and the non-slant mode showed no remarkable difference in terms of the sensitivity distribution and spatial resolution performance. Therefore, we concluded that the SROP geometry has a good potential as an open geometry especially suitable for in-beam imaging.

Spatial resolution limits for the isotropic-3D PET detector X’tal cube

Positron emission tomography (PET) has become a popular imaging method in metabolism, neuroscience, and molecular imaging. For dedicated human brain and small animal PET scanners, high spatial resolution is needed to visualize small objects. To improve the spatial resolution, we are developing the X’tal cube, which is our new PET detector to achieve isotropic 3D positioning detectability. We have shown that the X’tal cube can achieve 1mm3 uniform crystal identification performance with the Anger-type calculation even at the block edges. We plan to develop the X’tal cube with even smaller 3D grids for sub-millimeter crystal identification. In this work, we investigate spatial resolution of a PET scanner based on the X’tal cube using Monte Carlo simulations for predicting resolution performance in smaller 3D grids. For spatial resolution evaluation, a point source emitting 511keV photons was simulated by GATE for all physical processes involved in emission and interaction of positrons. We simulated two types of animal PET scanners. The first PET scanner had a detector ring 14.6cm in diameter composed of 18 detectors. The second PET scanner had a detector ring 7.8cm in diameter composed of 12 detectors. After the GATE simulations, we converted the interacting 3D position information to digitalized positions for realistic segmented crystals. We simulated several X’tal cubes with cubic crystals from (0.5mm)3 to (2mm)3 in size. Also, for evaluating the effect of DOI resolution, we simulated several X’tal cubes with crystal thickness from (0.5mm)3 to (9mm)3. We showed that sub-millimeter spatial resolution was possible using cubic crystals smaller than (1.0mm)3 even with the assumed physical processes. Also, the weighted average spatial resolutions of both PET scanners with (0.5mm)3 cubic crystals were 0.53mm (14.6cm ring diameter) and 0.48mm (7.8cm ring diameter). For the 7.8cm ring diameter, spatial resolution with 0.5×0.5×1.0mm3 crystals was improved 39% relative to the (1mm)3 cubic crystals. On the other hand, spatial resolution with (0.5mm)3 cubic crystals was improved 47% relative to the (1mm)3 cubic crystals. The X’tal cube promises better spatial resolution for the 3D crystal block with isotropic resolution.

Towards Software-Based Real-Time Singles and Coincidence Processing of Digital PET Detector Raw Data

This paper presents a software-based singles and coincidence processing (SCP) architecture for a digital PET/MR system that is based on SiPM detectors with local digitization coupled to preclinical crystal arrays. Compared with traditional PET systems, our system outputs detector raw data of the individual detector elements via optical Gigabit Ethernet interfaces instead of singles or coincidences. The raw data contains the digitized timestamps, energies, and identifiers of triggered SiPM pixels (hits). Although this approach requires a high bandwidth for the detector data transmission system, the availability of detector raw data offers unique opportunities to employ more accurate and computationally complex, iterative algorithms, which can lead to PET images with higher quality and accuracy. In this paper, we evaluate a parallel software-based SCP for three different crystal position estimation approaches with regard to its real-time capabilities. The SCP receives detector raw data as input and outputs list-mode coincidence data. The investigated PET system features ten singles processing units (SPU), each equipped with two PET detector stacks and a Gigabit Ethernet interface to a data acquisition and processing server (Dell Poweredge R910 equipped with 4× Intel Xeon X7560@2.27 GHz CPUs and 256 GByte DDR3-RAM), allowing lossless real-time acquisition of the entire raw data stream. Using the detector raw data of three previously stored measurements, our results show that the throughput (in Mhits/s) of a center-of-gravity (COG)-based parallel SCP is nearly 4× higher on average than the estimated detector raw data output that is generated from an activity of 37 MBq in the iso-center of the detector ring. Under the same conditions, an iterative maximum-likelihood (ML)-based parallel SCP leads to a 6× higher throughput on average, while a Gaussian-based parallel SCP also results in a 13× higher throughput on average. Compared with a serial processing approach, the parallel implementations show speedups of up to 38× on average for the ML-based, 39× on average for Gaussian-based, and up to 34× on average for the COG-based parallelized SCP for the three previously-stored measurements.

A novel orientation measurement using optical sensor for spherical motor

The measurement of spherical rotor orientation is crucial to the close-loop control of spherical motors. This paper presents a novel method for the measuring of three-degree-of-freedom (DOF) rotor orientation of spherical motors using optical sensors. The spatial orientation of spherical rotor is output in the form of ZXZ Euler angles. Firstly, the structure of the measuring system composed of optical sensors and the patterns on the rotor surface are presented, and the operational principle of recognizing intersection points between the optical ring detectors and the latitude/longitude on the rotor surface is illustrated. The analytical model of input-output characteristic is established for the measuring system of three-DOF rotor orientation. Afterwards, the effect of parameters of the optical ring detectors on the linearity, sensitivity, resolving power and measuring range of the measuring system is analyzed using the analytical model. Finally, the feasibility of the measurement is validated through experiments of prototype measuring system. The analysis is expected to be a basis for the design parameter optimization of the orientation measuring system of a PM spherical motor.

Improved dead-time correction for PET scanners: application to small-animal PET

Pile-up and dead-time are two main causes of nonlinearity in the response of a PET scanner as a function of activity in the field of view (FOV). For a given scanner and acquisition system, pile-up effects depend on the material and size of the object being imaged and on the distribution of activity inside and outside the FOV, because these factors change the singles-to-coincidences ratio (SCR). Thus, it is difficult to devise an accurate correction that would be valid for any acquisition. In this work, we demonstrate a linear relationship between SCR and effective dead-time, which measures the effects of both dead-time (losses) and pile-up (gains and losses). This relationship allows us to propose a simple method to accurately estimate dead-time and pile-up corrections using only two calibration acquisitions with, respectively, a high and low SCR. The method has been tested with simulations and experimental data for two different scanner geometries: a scanner with large area detectors and no pile-up rejection, and a scanner composed of two full rings of smaller detectors. Our results show that the SCR correction method is accurate within 7%, even for high activities in the FOV, and avoids the bias of the standard single-parameter method.

PET-based geometrical calibration of a pinhole SPECT add-on for an animal PET scanner

We developed SPECT imaging capability on an animal PET scanner to provide a cost effective option for animal SPECT imaging. The SPECT add-on sub-system was enabled by mechanically integrating a multiple-pinhole collimator in the PET detector ring. This study introduces a method to calibrate the geometrical parameters of the SPECT add-on using the PET imaging capability of the scanner. The proposed PET imaging-based calibration method consists of two steps: (1) paint the pinhole apertures of the collimator with a positron emitting radioactive solution; and (2) image the collimator inside the scanner in PET mode. The geometrical parameters of the multi-pinhole SPECT add-on can then be derived directly from a set of PET images by simple linear calculation and used in defining the SPECT system. The method was compared to our implementation of a SPECT calibration approach with model-based fitting of SPECT projection data. The procedure for carrying out the PET imaging-based calibration method is simpler and faster than that of our implementation of the SPECT model-based calibration method. Since it does not require model fitting, the uniqueness of the calibration result is warranted. Better quality SPECT images were reconstructed using the PET-derived calibration parameters rather than our implementation of the SPECT model-based calibration parameters. We conclude that the proposed PET imaging-based calibration method provides a highly effective means for enabling SPECT imaging on a PET scanner.

Initial Evaluation of LabPET/SPECT Dual Modality Animal Imaging System

We developed SPECT imaging capability on a LabPET animal scanner to provide an integrated PET/SPECT dual-modality imaging system. The add-on SPECT sub-system was enabled by 1) mechanically integrating a multiple-pinhole collimator in the PET detector ring, and 2) configuring the detectors to acquire singles events in the 120-160 keV range. We report on the design parameters, data acquisition protocols and initial performance assessment of this cost-effective SPECT imaging solution. Phantom and animal images demonstrating the relevance of the system for various imaging tasks in preclinical research are presented.

PET/MRI: a new era in multimodality molecular imaging

Hybrid PET/MR imaging has long been considered an impossible combination due to the difficulty of ensuring the functioning of PET detectors within the magnetic field of an MR scanner, and the artifacts from the electronics of the PET scanner, which can affect MR images. Recent advances in technology have brought to the market hybrid devices in which the two modalities are set side by side along a single bed or in which solid-state detector rings are integrated into the MRI gantry. While the clinical and added value of such hybrid devices is still to be demonstrated, the ability to acquire perfectly registered images from both modalities in a single imaging session offers tangible advantages to patients needing to undergo both examinations as part of their clinical workup. Such cases include patients with head and neck cancers, prostate cancers, patients undergoing staging, and monitoring of extensive and high-risk breast cancers and gynecological cancers, and pediatric oncology cases where repeated studies are required and the reduction of radiation exposure by replacing the CT component of PET/CT scans with MRI is particularly important. It is anticipated that wider adoption of PET/MR imaging will occur once the added value and improvement in diagnostic accuracy have been demonstrated in prospective clinical studies. The introduction of new PET tracers is also likely to lead to wider clinical use of PET/MR. One example is the recent introduction of 18F-labeled NaF for bone metastases that makes PET in combination with MR imaging ideal for the characterization of bone lesions. The development of other new tracers may extend the use of PET/MR to domains other than oncology, such as cardiology and neurology.

Development of a flexible optical fiber based high resolution integrated PET/MRI system

The simultaneous measurement of PET and magnetic resonance imaging (MRI) is an emerging field for molecular imaging research. Although optical fiber based PET∕MRI systems have advantages on less interference between PET and MRI, there is a drawback in reducing the scintillation light due to the fiber. To reduce the problem, the authors newly developed flexible optical fiber bundle based block detectors and employed them for a high resolution integrated PET∕MRI system. The flexible optical fiber bundle used 0.5 mm diameter, 80 cm long double clad fibers which have dual 12 mm × 24 mm rectangular inputs and a single 24 mm × 24 mm rectangular output. In the input surface, LGSO scintillators of 0.025 mol.% (decay time: ∼31 ns: 0.9 mm × 1.3 mm × 5 mm) and 0.75 mol.% (decay time: ∼46 ns: 0.9 mm × 1.3 mm × 6 mm) were optically coupled in depth direction to form depth-of-interaction detector, arranged in 11 × 13 matrix and optically coupled to the fiber bundle. The two inputs of the bundle are bent for 90°, bound to one, and are optically coupled to a Hamamatsu 1-in. square position sensitive photomultiplier tube. Light loss due to the fiber bundle could be reduced and the performance of the block detectors was improved. Eight optical fiber based block detectors (16 LGSO blocks) were arranged in a 56 mm diameter ring to form a PET system. Spatial resolution and sensitivity were 1.2 mm full-width at half-maximum and 1.2% at the central field-of-view, respectively. Sensitivity change was less than 1% for 2 °C temperature changes. This PET system was integrated with a 0.3 T permanent magnet MRI system which has 17 cm diameter hole at the yoke area for insertion of the PET detector ring. There was no observable interference between PET and MRI. Simultaneous imaging of PET and MRI was successfully performed for small animal studies. The authors confirmed that the developed high resolution PET∕MRI system is promising for molecular imaging research.

Design and inital evaluation of a 4-layer DOI-PET system: the jPET-D4.

PET scanners are an important component in functional brain imaging devices. Several PET scanners have been developed during the last two decades for brain research studies. We are developing a high-performance brain PET scanner, jPET-D4. This scanner is designed to achieve not only high spatial resolution but also high sensitivity using four-layered depth-of-interaction (DOI) detectors. The scanner has five block detector rings with the ring diameter of 390 mm and each block detector ring consists of 24 DOI detectors. In previous work, we have demonstrated that 3 mm FWHM uniform spatial resolution within the field-of-view could be realized. In this paper, we describe the jPET-D4 system and evaluate its performance. The average energy resolution for 120 DOI detectors is optimized to 16 % +/- 1.0 %. The scatter fraction for this system is 40 % with an energy window of 400-600 keV. The sensitivity for the point source is 102 kcps/MBq (10.2 %) with a 400 keV LLD. Maximum noise equivalent count rate (NECR) is 154 kcps at 11 kBq/ml with a 10 ns coincidence time window. We evaluated scatter fraction and NECR following procedures based on NEMA NU2-1994. These first evaluation measurements indicate the jPET-D4 has good performance.

Engineering and Performance (NEMA and Animal) of a Lower-Cost Higher-Resolution Animal PET/CT Scanner Using Photomultiplier-Quadrant-Sharing Detectors

The dedicated murine PET (MuPET) scanner is a high-resolution, high-sensitivity, and low-cost preclinical PET camera designed and manufactured at our laboratory. In this article, we report its performance according to the NU 4-2008 standards of the National Electrical Manufacturers Association (NEMA). We also report the results of additional phantom and mouse studies. The MuPET scanner, which is integrated with a CT camera, is based on the photomultiplier-quadrant-sharing concept and comprises 180 blocks of 13 × 13 lutetium yttrium oxyorthosilicate crystals (1.24 × 1.4 × 9.5 mm(3)) and 210 low-cost 19-mm photomultipliers. The camera has 78 detector rings, with an 11.6-cm axial field of view and a ring diameter of 16.6 cm. We measured the energy resolution, scatter fraction, sensitivity, spatial resolution, and counting rate performance of the scanner. In addition, we scanned the NEMA image-quality phantom, Micro Deluxe and Ultra-Micro Hot Spot phantoms, and 2 healthy mice. The system average energy resolution was 14% at 511 keV. The average spatial resolution at the center of the field of view was about 1.2 mm, improving to 0.8 mm and remaining below 1.2 mm in the central 6-cm field of view when a resolution-recovery method was used. The absolute sensitivity of the camera was 6.38% for an energy window of 350-650 keV and a coincidence timing window of 3.4 ns. The system scatter fraction was 11.9% for the NEMA mouselike phantom and 28% for the ratlike phantom. The maximum noise-equivalent counting rate was 1,100 at 57 MBq for the mouselike phantom and 352 kcps at 65 MBq for the ratlike phantom. The 1-mm fillable rod was clearly observable using the NEMA image-quality phantom. The images of the Ultra-Micro Hot Spot phantom also showed the 1-mm hot rods. In the mouse studies, both the left and right ventricle walls were clearly observable, as were the Harderian glands. The MuPET camera has excellent resolution, sensitivity, counting rate, and imaging performance. The data show it is a powerful scanner for preclinical animal study and pharmaceutical development.

A single-ring OpenPET enabling PET imaging during radiotherapy

We develop an OpenPET system which can provide an accessible open space to the patient during PET scanning. Our first-generation OpenPET geometry which we called dual-ring OpenPET consisted of two separated detector rings and it could extend its axial field of view (FOV) therefore enabling imaging the gap region in addition to the in-ring region. However, applications such as dose verification by in-beam PET measurement during particle therapy and real-time tumor tracking by PET require sensitivity focused onto the gap rather than on the wide FOV. In this paper, we propose a second-generation OpenPET geometry, single-ring OpenPET, which can provide an accessible and observable open space with higher sensitivity and a reduced number of detectors than the earlier one. The proposed geometry has a cylinder shape cut at a slant angle, in which the shape of each cut end becomes an ellipse. We provided a theoretical analysis for sensitivity of the proposed geometry, compared with the dual-ring OpenPET and a geometry where the conventional PET was positioned at a slant angle against the patient bed to form an accessible open space, which we called a slant PET. The central sensitivity depends on the solid angle of these geometries. As a result, we found that the single-ring OpenPET has a sensitivity 1.2 times higher than the dual-ring OpenPET and 1.3 times higher than the slant PET when designed for a 600 mm bed width with 300 mm accessible open space and about 200 detector blocks, each with a front area of 2500 mm2. In addition, numerical simulation was carried out to show the imaging property of the proposed geometry realized with the ellipsoidal rings and these results indicate that the depth-of-interaction detector can provide uniform resolution even when the detectors are arranged in an ellipsoidal ring.

Investigation of X-ray Fluorescence Computed Tomography (XFCT) and K-Edge Imaging

This work provides a comprehensive Monte Carlo study of X-ray fluorescence computed tomography (XFCT) and K-edge imaging system, including the system design, the influence of various imaging components, the sensitivity and resolution under various conditions. We modified the widely used EGSnrc/DOSXYZnrc code to simulate XFCT images of two acrylic phantoms loaded with various concentrations of gold nanoparticles and Cisplatin for a number of XFCT geometries. In particular, reconstructed signal as a function of the width of the detector ring, its angular coverage and energy resolution were studied. We found that XFCT imaging sensitivity of the modeled systems consisting of a conventional X-ray tube and a full 2-cm-wide energy-resolving detector ring was 0.061% and 0.042% for gold nanoparticles and Cisplatin, respectively, for a dose of ∼ 10 cGy. Contrast-to-noise ratio (CNR) of XFCT images of the simulated acrylic phantoms was higher than that of transmission K-edge images for contrast concentrations below 0.4%.

WE-C-217BCD-09: Compressed Sensing in PET Imaging with Partial Detector Rings.

Investigate the possibility of reducing the number of PET detector elements per ring (introduce gaps) while maintaining image quality by employing compressed sensing techniques. A uniform Ge-68 phantom was imaged on a D-RX PET/CT scanner twice; once with all detectors operational (baseline) and once with 8 equidistant detector blocks turned off (partially sampled [PS], 11% detectors off. The resulting PS sinogram was then decomposed in two different components, each sparsely represented in a specific transform domain. An iterative optimization technique was then used to recover the PS sinogram based on the solution of a combination of underdetermined system of equations and block- coordinated relaxations. In addition, the total variation is minimized for the first component to direct it into a piece-wise smooth model. Finally the two components were summed to obtain the sinogram which was used to compare with the original PS sinogram. Comparison was done only for existing sinogram pixel values. This process was repeated iteratively until a RMS error of 5% or a total of 100 iterations were reached. For each iteration update, the values of the pixels corresponding to the missing detectors were obtained from the previous iteration while the remaining pixel values were extracted from the baseline sinogram. The resultant corrected sinograms where then reconstructed using OSEM and FBP and the corresponding images for full and PS sinograms were compared using mean and max activity concentration in a ROI placed centrally over the phantom. For OSEM (FBP) reconstruction, the mean and max activity concentration difference were -0.06% (-0.02%) and 3.7% (5.5%) respectively when compared to baseline. Compressed sensing seems to have the ability to recover PS PET data. Such an approach can potentially be used to generate PET images with accurate quantitation while reducing number of detectors/ring.

TH-A-213CD-01: Compton Scatter in X-Ray Fluorescence CT Imaging

Purpose: To design an x-ray fluorescence CT (XFCT) imaging system for high-sensitivity molecular imaging of gold and platinum probes by means of minimizing Compton scatter contamination using Monte Carlo (MC) simulations. Methods: XFCT and conventional CT images of 5-cm diameter phantoms containing solutions of gold and platinum were simulated in a modified version of the EGSnrc/DOSXYZnrc MC code. In our MC model, fluorescence x-rays excited by heavily filtered 110 kVp x-ray pencil beam were detected by photon-counting detectors arranged in a number of geometries with the aim to vary the contribution of Compton scatter. XFCT images were reconstructed based on sinograms generated using the net number of fluorescence x-rays with simple back-projection. The extraction of the net number of fluorescence x-rays and XFCT image quality as a function of MC-estimated Compton scatter for a range of scattering angles was investigated. A number of detector arrangements (angular distributions and thicknesses) as well as their energy resolution were studied. Results: We found that the imaging sensitivity significantly increases when forward Compton scattered photons are excluded from data acquisition. For 2-cm wide detectors with 1 % energy resolution and a dose of ∼5 cGy, the imaging sensitivity for a full detector ring was 0.086% and 0.060% for gold and platinum, respectively, and it increased to 0.057% and 0.037% when x-rays scattered by less than 90° and 45° degrees were excluded from data acquisition. Moreover, we observed a decrease in sensitivity with decreasing the detector width and its energy resolution. Conclusions: XFCT data acquisition geometry should be carefully chosen for each combination of excitation x-ray spectrum, detector energy resolution, and imaging probe to minimize the contribution of Compton scatter in order to maximize XFCT imaging sensitivity. Based on our MC simulations, the design of an x-ray fluorescence CT scanner will be proposed.

WE‐A‐213CD‐01: Molecular Imaging of the Breast

Molecular imaging of the breast using either SPECT or PET based radiopharmaceuticals is a relatively new area of focus for nuclear medicine and has grown in importance over the last few years with the development of specialized small field of view gamma cameras and small PET systems designed to provide high resolution functional images of the breast. Molecular Breast Imaging (MBI) / Breast Specific Gamma imaging (BSGI): MBI and BSGI are currently performed with Tc‐99m sestamibi and have shown a high sensitivity for the detection of small breast lesions, with reported sensitivities of ∼90%. Sensitivity is lowest for tumors < 5 mm in size. This talk will review the technology behind both MBI and BSGI and the primary clinical applications. It will also review the potential use of MBI as a screening technique. Recent studies have shown that it may be considerably more sensitive than mammography in women with dense breast tissue. A significant issue with MBI / BSGI is radiation burden and this talk will review efforts to increase the sensitivity of these technologies to allow a reduction in the administered dose of radiopharmaceutical, and thereby achieve a comparable radiation burden to mammography. Positron Emission Mammography (PEM): PEM is currently performed on a dual detector system incorporating 2 scanning arrays of BGO crystals providing limited angle tomographic imaging. Studies using F‐18 FDG have shown PEM to have comparable sensitivity to MRI for the detection of breast cancer. It is a useful tool in the preoperative assessment of disease extent and in evaluating response to neoadjuvant therapy. As with single photon systems, sensitivity is lowest for tumors < 5 mm in size. Current systems employ doses of 10 mCi F‐18 FDG. Recent technical developments have seen the emergence of systems using small ring detectors or rotating planar detectors that provide full tomographic views of the breast. Clinical studies are expected to show improvements in spatial resolution and a reduction in the administered dose of F‐18 required. COI: The author and Mayo Clinic receive royalties from patents licensed to Gamma Medica Ideas, a manufacturer of MBI systems. Learning Objective:1) Understand the different technologies (PEM,MBI, BSGI) and detector configurations now available for molecular imaging of the breast. 2) Understand the potential applications for molecular imaging of the breast in both the diagnostic and screening environments. 3) Compare the strengths and weakness of molecular imaging techniques with other breast imaging modalities. 4) Appreciate the potential future applications of this technology in the understanding of breast disease. 5) Demonstrate understanding of the radiation dose implications of current radiopharmaceuticals and how the dose can be reduced with new technologies and radiopharmaceuticals.

LISST-100 response to large particles

Particles in the marine environment vary in size from sub-micron colloids to flocculated aggregates of the order of millimetres. The LISST-100 (laser in-situ scattering and transmissometer) range of instruments (Sequoia Scientific Inc.) determine the distribution of particle sizes in-situ using laser diffraction, but are limited to specific size ranges governed by the instrument configuration. Using numerical predictions of scattering and a novel observational system to combine digital holography and the LISST-100 type-c, here we examine the response of the LISST to particles larger than the intended size range of the instrument. For spheres greater than the type-c instrument limit of 500μm, both theory and observations indicate that the inversion of the recorded scattering into particle size distributions produces volume distributions that peak at varying sizes between 250 and 400μm. This is caused by the principal peaks in scattering moving off the inside of the ring detectors, leading to the remaining peaks being interpreted as the principal peaks. The aliasing of larger particles as a distribution of smaller particles is also applicable to the type-b configuration of the LISST, only occurring at the 250μm size limit instead of 500μm. When extending the Junge particle size distribution up to varying maximum sizes, numerical tests predict an increase in volume concentration of up to 45%. For power-law distributions with gradients less than that of a Junge distribution, the contamination from large particles becomes increasingly influential over the concentration and shape of the inverted size distribution.

Simulations of a micro-PET system based on liquid xenon

The imaging performance of a high-resolution preclinical micro-positron emission tomography (micro-PET) system employing liquid xenon (LXe) as the gamma-ray detection medium was simulated. The arrangement comprises a ring of detectors consisting of trapezoidal LXe time projection ionization chambers and two arrays of large area avalanche photodiodes for the measurement of ionization charge and scintillation light. A key feature of the LXePET system is the ability to identify individual photon interactions with high energy resolution and high spatial resolution in three dimensions and determine the correct interaction sequence using Compton reconstruction algorithms. The simulated LXePET imaging performance was evaluated by computing the noise equivalent count rate, the sensitivity and point spread function for a point source according to the NEMA-NU4 standard. The image quality was studied with a micro-Derenzo phantom. Results of these simulation studies included noise equivalent count rate peaking at 1326 kcps at 188 MBq (705 kcps at 184 MBq) for an energy window of 450–600 keV and a coincidence window of 1 ns for mouse (rat) phantoms. The absolute sensitivity at the center of the field of view was 12.6%. Radial, tangential and axial resolutions of 22Na point sources reconstructed with a list-mode maximum likelihood expectation maximization algorithm were ⩽0.8 mm (full-width at half-maximum) throughout the field of view. Hot-rod inserts of <0.8 mm diameter were resolvable in the transaxial image of a micro-Derenzo phantom. The simulations show that a LXe system would provide new capabilities for significantly enhancing PET images.

System design of a small OpenPET prototype with 4-layer DOI detectors

We have proposed an OpenPET geometry which consists of two axially separated detector rings. The open gap is suitable for in-beam PET. We have developed the small prototype of the OpenPET especially for a proof of concept of in-beam imaging. This paper presents an overview of the main features implemented in this prototype. We also evaluated the detector performance. This prototype was designed with 2 detector rings having 8 depth-of-interaction detectors. Each detector consisted of 784 Lu(2x)Gd(2(1-x))SiO₅:Ce (LGSO) which were arranged in a 4-layer design, coupled to a position-sensitive photomultiplier tube (PS-PMT). The size of the LGSO array was smaller than the sensitive area of the PS-PMT, so that we could obtain sufficient LGSO identification. Peripheral LGSOs near the open gap directly detect the gamma rays on the side face in the OpenPET geometry. Output signals of two detectors stacked axially were projected onto one 2-dimensional position histogram for reduction of the scale of a coincidence processor. Front-end circuits were separated from the detector head by 1.2-m coaxial cables for the protection of electronic circuits from radiation damage. The detectors had sufficient crystal identification capability. Cross talk between the combined two detectors could be ignored. The timing and energy resolutions were 3.0 ns and 14%, respectively. The coincidence window was set 20 ns, because the timing histogram showed that not only the main peak, but also two small shifted peaks were caused by the coaxial cable. However, the detector offers the promise of sufficient performance, because random coincidences are at a nearly undetectable level for in-beam PET experiments.