The preclinical SPECT scanner presented in this work has been designed on the basis of an imaging system previously engineered by our group 5. The detectors, named High-Resolution Imaging System (HiRIS2), have been developed using several Monte Carlo (MC) simulations to define the mechanical structure and compose the preparatory technical drawings 8.
Each SPECT head, shown in Figure 1, is made up of a Hamamatsu H13700 Flat Panel PSPMT (Hamamatsu Photonics K.K., JP) coupled with an array of 28×28 CRY018 (Crytur spol. s.r.o., CZ) scintillation crystals with a size of 1.4 mm × 1.4 mm × 6.0 mm. The scintillator elements are separated by a reflective layer of white epoxy of 200 μm, obtaining a resulting pitch of 1.6 mm, and a field of view (FOV) of about 45 mm × 45 mm. Finally, a 36 mm parallel square hole collimator of pure tungsten with 200 μm thick septa and hole size of 1.4 mm × 1.4 mm, is matched with the scintillation crystals array. To better exploit the H13700 features, we have designed a dedicated resistive chain readout made up of a single miniaturized board connected directly on the PSPMT, which is read by a dedicated compact 4-channel ADC board (125 MSamples/s for each channel). A field-programmable gate array (FPGA) reads the ADC output and implements the digital pulse processing. Moreover, the FPGA is responsible for managing data transfer through a USB 2.0 transceiver that ensures a transfer data rate up to 8 Mbyte/s. The connection is master/slave, where the PC (master) sends a request to a detector (slave), and the detector responds to that request.
The HiRIS2 detector has overall dimensions of 69 mm × 69 mm × 165 mm, and a total weight of about 2 kg.
The HiRIS2 SPECT system, depicted in Figure 2, consists of two detector heads integrated within a rotating mechanical system. The detectors are respectively positioned at a distance of 5 cm and at 180° from each other, they are therefore assembled in opposition. The 360° full rotation of the gantry, the animal bed, and the SSR movements are provided by NEMA 23 stepper motors each controlled by a 1067_0B driver (Phidgets Inc., US) connected to a PC via USB.
According to the SSR method, the scanner allows managing the lateral shifts in the trans-axial plane of both detectors, while shifts along the rotation axis are carried out moving the animal bed. Since the crystal dimension is 1.6 mm by side, all the SSR shifts must be submultiples of this value, for instance, to perform a 2-step SSR along the lateral and/or axial directions, displacements of 0.8 mm must be carried out (half a pixel) 8. Specifically, the sub-pixel movements may also be restricted to the tangential or axial direction only, depending on the ongoing study, thus limiting the number of projections required. For simplicity, we will use the notation N-step to indicate the number of steps performed by the SSR along the desired direction i.e., axial or trans-axial (e.g. 4-step trans-axial SSR). In the case that the SSR shifts are carried out in both directions, the direction will not be specified, e.g. 9-step SSR means 9 images acquired on a grid of 3 shifts along the axial and 3 along the trans-axial direction.
The prototype software was developed in C++ with Visual Studio 2019, using the Qt ver. 5.15 framework. It provides an automatic scanning procedure that performs image acquisitions through a sequence of successive angular steps around the rotation axis. Moreover, to implement the SSR required movements, for each angular step the detector heads are shifted by a number of displacements accordingly with the method. Finally, for SPECT image reconstruction the STIR software ver. 4.0.1 was used, which is an Open-Source consolidated and validated package, being widely used also as a reference in several scientific studies 9-11. Then image analysis was carried out using Mango - Multi-image Analysis GUI (Research Imaging Institute, UTHSCSA) 12 and AMIDE (Amide's a Medical Image Data Examiner) 13 image processing software.
2.1. Mechanical assessment
In order to fine-tune the SPECT scanner, the rotational axis of the gantry was assessed through the use of a mechanical centering tool aligned with the system. This tool holds a glass capillary positioned along the axis of rotation. The capillary was filled with a solution containing 99mTc-pertechnetate. This way, the position of the rotation axis was verified on the acquired images (Figure 3).
2.2. Phantoms evaluation
The characterization of the sole HiRIS2 detector has been performed in our previous work by using complex patterns allowing the assessment of the spatial linearity and the overall imaging capability 5,6. In this study, the HiRIS2 SPECT system performance was determined experimentally using a custom-designed polymethyl methacrylate (PMMA) phantom, and a glass capillary, both filled with a solution of 99mTcO4- (𝛾-ray 140.5 keV). This Derenzo-like phantom was used to assess the spatial resolution. It consists of a block of PMMA with a thickness of 20 mm that contains several cylindrical holes grouped into three sectors according to their diameter, which is 1.6 mm, 1.8 mm, and 2.2 mm respectively. In addition, for each sector, the distance between the centres of two adjacent cylinders is twice their diameter. The activity of 99mTcO4- used for these measurements was 37 MBq. Two acquisitions were performed: a 4-step planar SSR, and a 2-step trans-axial SSR acquisition. In the former, the phantom was positioned at contact with the surface of one detector, and aligned orthogonally with it. Meanwhile, regarding the SPECT acquisition, the phantom was positioned nearby the centre of the system FOV, aligned with the axial direction orbiting the detectors over 360° from the phantom in 24 steps. The 1 mm glass capillary filled with a 99mTcO4- solution, positioned along the axis of rotation, was used to assess the SPECT system spatial linearity, the SPECT spatial resolution, and the centering of the rotation axis (Figure 3B). Specifically, the SPECT spatial resolution was measured in terms of full-width-at-half-maximum (FWHM) of the point spread function (PSF) obtained at the corresponding source’s positions. All SPECT images were obtained in step-and-shoot mode by collecting 48 projection views in 24 steps over a 360◦ arc. Generally, each projection view was acquired for a time of 1 min, then the full standard SPECT scan required 24 min, while the 2-step SSR acquisition has needed 48 min. Finally, the tomographic images have been reconstructed using the Software for Tomographic Image Reconstruction (STIR) framework with the order subsets expectation maximization (OSEM) algorithm 14.
2.3. Animal studies
The HiRIS2 SPECT system was evaluated in a proof-of-principle animal study, in accordance with international and local guidelines. In these studies, different clinical radiopharmaceuticals radiolabelled with 99mTc and 123I (𝛾-ray 159 keV) were administered.
99mTc-pertechnetate (18.5 MBq in 50 µl), a well-known anion, used for clinical imaging of thyroid function, was injected under isofluorane anesthesia in the tail vein of a female, 6 weeks-old Balb/c mouse. SPECT images without SSR were obtained at 1 h after injection by collecting 48 projections (24 steps) over an arc of 360° focusing on the thyroid. Each acquisition lasted 1 min per step, for a total of 24 minutes. Subsequently, the 2-step trans-axial SSR was carried out, performing 24 steps. The total scan time was 48 minutes.
2.3.2 Technetium-99m-methyl diphosphonate
Technetium-99m-methyl diphosphonate (99mTc-MPD, 7.4 MBq in 50 µl) for bone imaging was injected under isofluorane anesthesia in the tail vein of a female, 6 weeks-old Balb/c mouse. Planar and 16-step whole body SSR images were acquired at 4 h post-injection according to the kinetics of the tracer to obtain the maximum radiopharmaceutical concentration in the bone.
2.3.3 123I-ioflupane (DaTSCAN®)
123I-ioflupane (DaTSCAN®, 21.1 MBq in 100 µl) was injected under isofluorane anesthesia in the tail vein of a female, 6 weeks old Balb/c mouse for brain dopamine active transporter (DAT) imaging (Figure 4). The SPECT scan was performed at 1 h after injection and then a 2-step trans-axial SSR SPECT acquisition was carried out for a total acquisition time of 48 min (1 min acquisition time for each step).