The development of radiation detectors capable of delivering spatial information about gamma-ray interactions was one of the key enabling technologies for nuclear medicine imaging and, eventually, single-photon emission computed tomography (SPECT). The continuous sodium iodide scintillator crystal coupled to an array of photomultiplier tubes, almost universally referred to as the Anger Camera after its inventor, has long been the dominant SPECT detector system. Nevertheless, many alternative materials and configurations have been investigated over the years. Technological advances as well as the emerging importance of specialized applications, such as cardiac and preclinical imaging, have spurred innovation such that alternatives to the Anger Camera are now part of commercial imaging systems. Increased computing power has made it practical to apply advanced signal processing and estimation schemes to make better use of the information contained in the detector signals. In this review we discuss the key performance properties of SPECT detectors and survey developments in both scintillator and semiconductor detectors and their readouts with an eye toward some of the practical issues at least in part responsible for the continuing prevalence of the Anger Camera in the clinic.

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SPECT detectors: the Anger Camera and beyond.

Secondary radiation emission induced by nuclear reactions is correlated to the path of ions in matter. Therefore, such penetrating radiation can be used for in vivo control of hadrontherapy treatments, for which the primary beam is absorbed inside the patient. Among secondary radiations, prompt-gamma rays were proposed for real-time verification of ion range. Such a verification is a desired condition to reduce uncertainties in treatment planning. For more than a decade, efforts have been undertaken worldwide to promote prompt-gamma-based devices to be used in clinical conditions. Dedicated cameras are necessary to overcome the challenges of a broad- and high-energy distribution, a large background, high instantaneous count rates, and compatibility constraints with patient irradiation. Several types of prompt-gamma imaging devices have been proposed, that are either physically-collimated or electronically collimated (Compton cameras). Clinical tests are now undergoing. Meanwhile, other methods than direct prompt-gamma imaging were proposed, that are based on specific counting using either time-of-flight or photon energy measurements. In the present article, we make a review and discuss the state of the art for all techniques using prompt-gamma detection to improve the quality assurance in hadrontherapy.

Prompt-gamma monitoring in hadrontherapy: A review

This paper reviewed the deep learning-based studies for medical imaging synthesis and its clinical application. Specifically, we summarized the recent developments of deep learning-based methods in inter- and intra-modality image synthesis by listing and highlighting the proposed methods, study designs, and reported performances with related clinical applications on representative studies. The challenges among the reviewed studies were then summarized with discussion.

A review on medical imaging synthesis using deep learning and its clinical applications.

Unmanned aerial vehicle for internet of everything: Opportunities and challenges

Positron emission tomography (PET) is the most sensitive of all medical imaging modalities for quantitatively probing biologic processes at the molecular level. However, spatial resolution in PET is significantly inferior to that of other imaging modalities that can provide exquisite images of the anatomy, such as X-ray computed tomography (CT) or magnetic resonance (MR) imaging. It has been one of the outstanding challenges of the last decade to combine PET with these complementary imaging modalities in order to synergistically exploit the benefits of each modality and to enhance the role of PET in pre-clinical research as well as in clinical routine and research. The simple juxtaposition of tomographs around a common axial bed, such as with current PET/CT technology, is very successful in allowing sequential acquisition of PET and anatomical data. However, novel imaging combinations are being considered that would enable simultaneous, or at least concurrent, dual-modality imaging through combined PET/MR or PET/CT. The development of these new integrated instruments creates new bewildering challenges for PET detection systems, which, in addition to the ability to measure annihilation radiation in PET, must satisfy several other critical requirements.

Novel detector technology for clinical PET

To calibrate pinhole MicroSPECT system, two improved methods using only a single point source were proposed. The uniqueness of calibration result by the proposed methods was proved theoretically. And simulation calibration of a dual-head system was carried out to validate the performance of the proposed methods. The simulation results show that reasonable calibration error is given by both proposed methods.

Calibration of pinhole MicroSPECT using a single point source

We use the numerical model to study the control method for the ion sputter flux and energy at the surface of multilayer mirrors in hydrogen plasmas induced by extreme ultraviolet (EUV) radiation. This plasma is generated via photoionization by EUV photons with wavelengths at 13.5 nm and collision ionization by high-energy electrons. An electric field is formed by applying different bias voltages to the cylindrically symmetrical cavity and sample holder, which guides the transfer of charged particles and increases their energy. The evolution of pulsed EUV-induced plasma under the field is described by a two-dimension particle-in-cell model and the Monte Carlo simulation to represent collisions between charged particles and background molecules. The results show that the distribution of the electric field varies during the pulse and point out that the secondary electrons, which gain energy from the varying field and generate more plasma by collisions with hydrogen molecules, are crucial plasma sources in this scheme. We then propose a stable and efficient control method for EUV-induced hydrogen plasma by optimizing the cavity structure as an ellipsoid and treating the surface of the cavity in contact with the plasma.

Research on efficient and stable control of EUV-induced hydrogen plasma

Monolithic scintillator detector have been shown to provide better energy resolution and sensitivity without degrading the spatial resolution [1], But 3D position information extraction is much more complicated compared to discrete scintillator detector. 3D maximum likelihood-based approach and artificial neural network (ANN) method are widely used to extract position information [2]. However both methods need reference or training data sets measured from experiments. An analytical model to characterize the optical photon transportation process in the crystal can eliminate the measurement procedure and provide better positioning algorithm.

An analytical model of optical photon transportation for monolithic PET detector

We developed SPECT imaging capability on a LabPETTM animal scanner. The add-on SPECT uses a multiplepinhole collimator and a step-and-shoot helical scan scheme. The objective of this work was to determine I) the effective imaging volume of a given scan protocol, and 2) the helical scan parameters, that is, the angular and axial step sizes, for achieving uniform high resolution images throughout the imaging volume. Three rotational step sizes, {1°, 2°, 3°}, and three axial step sizes, {0.3 mm, 0.6 mm, 1.2 mm}, were combined to form 9 helical scan protocols. These step sizes were derived based on the reference values analytically calculated using the spatial resolution of the SPECT system and the Nyquist sampling theorem. Two numerical indices, the sampling completeness percentage (SCP), and the directional resolution (DR), were designed to quantify the projection sampling and resolution performance of the helical protocols. Scanning of a digital sphere-grid phantom centered in the SPECT system's FOV was simulated for each of the 9 helical protocols. Aggregated RMS resolution values derived from the image of distributed spheres over the FOV were used to assess the resolution performance of the scan protocols, and for validating the predictive value of the SCP and DR indices, which were calculated over the matching grid. The RMS results showed that the effective image volume of a helical scan where sampling is adequate was defined by the start and end pinhole planes. The results also showed that using a small axial step size (about half the value derived from Nyquist theorem) achieved best imaging resolution, while the angular step size was less critical. The RMS images correlated well with that of SCP and DR. We conclude the following for designing a helical scan protocol with a multiple pinhole collimator. First, the effective imaging volume should be confined between the start and end planes of the pinholes' movement range. Second, using an axial step size that is half of what is calculated from the Nyquist theorem is desirable for optimal resolution performance. In addition, SCP and DR are useful indices for assessing scan protocols analytically.

https://ieeexplore.ieee.org/iel7/6531926/6551044/06551872.pdf

Parameter optimization and effective imaging volume determination of helical scan for a pinhole animal SPECT

Purpose Single-photon emission computed tomography (SPECT) is an important tool for myocardial perfusion imaging (MPI). Mechanical collimators cause the resolution-sensitivity trade-off in the existing cardiac SPECT systems, which hinders fast cardiac scan capability. In this work, we propose a novel collimator-less cardiac SPECT system with interspaced mosaic-patterned scintillators, aiming to significantly improve sensitivity and reduce scan time without trading-off image resolution. Methods We propose to assemble a collimator-less cardiac SPECT with 7 mosaic-patterned detector modules forming a half-ring geometry. The detector module consists of 10 blocks, each of which is assembled with 768 sparsely distributed scintillators with a size of 1.68 mm × 1.68 mm × 20 mm, forming a mosaic pattern in the trans-axial direction. Each scintillator bar contains 5 GAGG(Ce) scintillators and 5 optical-guide elements, forming a mosaic pattern in the axial direction. In the Monte Carlo simulations, the in-plane resolution and axial resolution are evaluated using a hot-rod phantom and 5 disk phantoms, respectively. We simulate a cardiac phantom that is placed in a water-filled cylinder and evaluate the image performance with different data acquisition time. We perform image reconstruction with the expectation–maximization algorithm using system matrices derived from the simulation of a uniform cylindrical source filling the field-of-view (FOV). Besides, a 2-D prototype system is designed to demonstrate the feasibility of the collimator-less imaging concept. Results In the simulation system, the sensitivity is 16.31% ± 8.85% in a 180 mm (Φ) × 100  mm (L) FOV. The 6-mm rods in the hot rod phantom and the 5-mm disks in the disk phantom are clearly separable. Satisfactory MPI image quality is achieved in the cardiac phantom study with an acquisition time of 30 s. In prototype experiments, the point sources with an 8 mm center-to-center distance are clearly separable at different positions across the FOV. Conclusion The study reveals a promising approach to high-sensitivity SPECT imaging without a heavy-metal collimator. In cardiac imaging, this approach opens the way to a very fast cardiac scan with good resolution. Further works are ongoing to build a practical 3-D imaging system based on the existing design.

https://www.frontiersin.org/articles/10.3389/fmed.2023.1145351/pdf

Tianyu Ma

Papers

Calibration of pinhole MicroSPECT using a single point source

Research on efficient and stable control of EUV-induced hydrogen plasma

An analytical model of optical photon transportation for monolithic PET detector

Parameter optimization and effective imaging volume determination of helical scan for a pinhole animal SPECT

High-sensitivity cardiac SPECT system design with collimator-less interspaced mosaic-patterned scintillators