Molecular imaging refers to the use of non-invasive imaging techniques to detect signals that originate from molecules, often in the form of an injected tracer, and observe their interaction with a specific cellular target in vivo. Differences in the underlying physical principles of these measurement techniques determine the sensitivity, specificity and length of possible observation of the signal, characteristics that have to be traded off according to the biological question under study. Here, we describe the specific characteristics of single photon emission computed tomography (SPECT) relative to other molecular imaging technologies. SPECT is based on the tracer principle and external radiation detection. It is capable of measuring the biodistribution of minute (<10(-10) molar) concentrations of radio-labelled biomolecules in vivo with sub-millimetre resolution and quantifying the molecular kinetic processes in which they participate. Like some other imaging techniques, SPECT was originally developed for human use and was subsequently adapted for imaging small laboratory animals at high spatial resolution for basic and translational research. Its unique capabilities include (i) the ability to image endogenous ligands such as peptides and antibodies due to the relative ease of labelling these molecules with technetium or iodine, (ii) the ability to measure relatively slow kinetic processes (compared with positron emission tomography, for example) due to the long half-life of the commonly used isotopes and (iii) the ability to probe two or more molecular pathways simultaneously by detecting isotopes with different emission energies. In this paper, we review the technology developments and design tradeoffs that led to the current state-of-the-art in SPECT small animal scanning and describe the position SPECT occupies within the matrix of molecular imaging technologies.

IntroductionAn ideal organ‐specific insert phantom should be able to simulate the anatomical features with appropriate appearances in the resultant computed tomography (CT) images. This study investigated a 3D printing technology to develop a novel and cost‐effective cardiac insert phantom derived from volumetric CT image datasets of anthropomorphic chest phantom.MethodsCardiac insert volumes were segmented from CT image datasets, derived from an anthropomorphic chest phantom of Lungman N‐01 (Kyoto Kagaku, Japan). These segmented datasets were converted to a virtual 3D‐isosurface of heart‐shaped shell, while two other removable inserts were included using computer‐aided design (CAD) software program. This newly designed cardiac insert phantom was later printed by using a fused deposition modelling (FDM) process via a Creatbot DM Plus 3D printer. Then, several selected filling materials, such as contrast media, oil, water and jelly, were loaded into designated spaces in the 3D‐printed phantom. The 3D‐printed cardiac insert phantom was positioned within the anthropomorphic chest phantom and 30 repeated CT acquisitions performed using a multi‐detector scanner at 120‐kVp tube potential. Attenuation (Hounsfield Unit, HU) values were measured and compared to the image datasets of real‐patient and Catphan® 500 phantom.ResultsThe output of the 3D‐printed cardiac insert phantom was a solid acrylic plastic material, which was strong, light in weight and cost‐effective. HU values of the filling materials were comparable to the image datasets of real‐patient and Catphan® 500 phantom.ConclusionsA novel and cost‐effective cardiac insert phantom for anthropomorphic chest phantom was developed using volumetric CT image datasets with a 3D printer. Hence, this suggested the printing methodology could be applied to generate other phantoms for CT imaging studies.

In a previous simulation study, we demonstrated the feasibility of using coded apertures together with pixelated detectors for small animal SPECT. In this paper, we further explore the potential of this approach with a prototype detector and simulated multipinhole apertures. We also investigated the effect of multiplexing due to overlapped projections on convergence properties, image signal-to-noise ratio (SNR) and spatial resolution. The detector comprises a 48 44 array of NaI(Tl) crystals, each 1 mm 1 mm 5 mm on a 1.25-mm pitch. The crystal array is directly coupled to a Hamamatsu R3941 8 cm position sensitive photomultiplier tube. Multipinhole apertures were simulated by performing repeated SPECT acquisitions of the same object with a single tungsten pinhole translated to different positions in the aperture plane. Image reconstruction is based on a three-dimensional ray driven projector which is an extension of a method described for single pinhole SPECT with a displaced center of rotation. Image estimates are updated using the maximum likelihood expectation maximization (ML-EM) algorithm. The effect of multiplexing was to slow convergence and reduce the achievable SNR by approximately 15% compared with nonmultiplexed data (but the result may be achieved in a fraction of the time). The reconstructed resolution obtained with a resolution phantom was 1.5-mm full width at half maximum and there was no appreciable difference between the resolution of multiplexed and nonmultiplexed data. These results encourage us to develop a prototype coded aperture system for high sensitivity, high resolution small animal SPECT.Index Terms-Coded aperature, iterature reconstruction, single photon emission computed tomography (SPECT), small animal imaging.

Respiratory motion degrades the detection and quantification capabilities of PET/CT imaging. Moreover, mismatch between a fast helical CT image and a time-averaged PET image due to respiratory motion results in additional attenuation correction artifacts and inaccurate localization. Current motion compensation approaches typically have 3 limitations: the mismatch among respiration-gated PET images and the CT attenuation correction (CTAC) map can introduce artifacts in the gated PET reconstructions that can subsequently affect the accuracy of the motion estimation; sinogram-based correction approaches do not correct for intragate motion due to intracycle and intercycle breathing variations; and the mismatch between the PET motion compensation reference gate and the CT image can cause an additional CT-mismatch artifact. In this study, we established a motion correction framework to address these limitations. In the proposed framework, the combined emission-transmission reconstruction algorithm was used for phase-matched gated PET reconstructions to facilitate the motion model building. An event-by-event nonrigid respiratory motion compensation method with correlations between internal organ motion and external respiratory signals was used to correct both intracycle and intercycle breathing variations. The PET reference gate was automatically determined by a newly proposed CT-matching algorithm. We applied the new framework to 13 human datasets with 3 different radiotracers and 323 lesions and compared its performance with CTAC and non-attenuation correction (NAC) approaches. Validation using 4-dimensional CT was performed for one lung cancer dataset. For the 10 F-FDG studies, the proposed method outperformed ( < 0.006) both the CTAC and the NAC methods in terms of region-of-interest-based SUV, SUV, and SUV ratio improvements over no motion correction (SUV: 19.9% vs. 14.0% vs. 13.2%; SUV: 15.5% vs. 10.8% vs. 10.6%; SUV ratio: 24.1% vs. 17.6% vs. 16.2%, for the proposed, CTAC, and NAC methods, respectively). The proposed method increased SUV ratios over no motion correction for 94.4% of lesions, compared with 84.8% and 86.4% using the CTAC and NAC methods, respectively. For the 2 F-fluoropropyl-(+)-dihydrotetrabenazine studies, the proposed method reduced the CT-mismatch artifacts in the lower lung where the CTAC approach failed and maintained the quantification accuracy of bone marrow where the NAC approach failed. For theF-FMISO study, the proposed method outperformed both the CTAC and the NAC methods in terms of motion estimation accuracy at 2 lung lesion locations. The proposed PET/CT respiratory event-by-event motion-correction framework with motion information derived from matched attenuation-corrected PET data provides image quality superior to that of the CTAC and NAC methods for multiple tracers.

The aim of this systematic review is to investigate the national diagnostic reference level (NDRL) methods for positron emission tomography/computed tomography (PET/CT) and single photon emission tomography/computed tomography (SPECT/CT) procedures. A search strategy was based on the preferred, reporting items for systematic review and meta-analysis (PRISMA). Relevant articles retrieved from Medline, Scopus, Web of Science, Embase, Cinahl, and Google Scholar published up to October 2017. The search yielded 1057 articles. Fourteen articles were included in the review after a screening process. Relevant information from the selected articles were summarised and analysed. Discrepancies were found between the methodologies utilised to establish and report both PET/CT and SPECT/CT NDRLs, e.g. patient sampling and administered activity. Further research should focus on reporting more NDRLs for hybrid PET/CT and SPECT/CT examinations, and establish a robust NDRL standard for the CT portion associated with PET/CT and SPECT/CT examinations. This review provides updated NDRL reommndations to deliver more comparable international radation doses for administered activity and CT dose across PET/CT and SPECT/CT clinics.