Hyperspectral SPECT: Time to revisit the spectral domain for gamma ray imaging?

361 Objectives: While spectral SPECT techniques, such as dual-isotope SPECT imaging has been widely explored for clinical applications [1-2], their success has been limited due to the poor energy resolution of clinical SPECT instrumentation. Over the past few years, the newer generation of room-temperature semiconductor imaging detectors have promised to offer not only an excellent 3-D spatial resolution (sub-250 μm), but also an unpreceded energy resolution (0.5 -1.2 keV between 30-200 keV). This opens the door for SPECT imaging to extend its imaging capability by exploring the spectral domain - in precise and quantitative imaging of the interplay between multiple molecular tracers and exploring the information contained in both primary and Compton scattered gamma rays. In this work we will: (a) demonstrate the performance benefit of using newly developed CZT and CdTe detectors, the 3-D HEXITEC detectors and (b) present results from imaging studies to establish the profound impact that hyperspectral imaging can have in diagnostic and therapeutic SPECT imaging applications. MethodsSpectroscopic gamma ray imaging detector. In this study, we used the HEXITEC detector [3-4] with CZT and CdTe crystals of 1 mm-2.5 mm in thickness. Each detector has an active area of 2 cm × 2 cm stud-bonded to a 2-D readout ASIC with 80 × 80 pixels of 250 μm × 250 μm pitch (Fig.1 A). The detector can be operated in two energy ranges of 2-200 keV and 6 - 600 keV, respectively, and records the precise spatial and spectral information for each gamma ray incident onto the detector surface. Hyperspectral SPECT imaging studies. We will use the HEXITEC detectors coupled with different apertures to acquire list-mode projection data from resolution and human ART phantoms (Fig. 1.B), filled with common diagnostic and therapeutic radioisotopes, such as I-131, Lu-177, Tc-99m, I-123, etc. Preliminary Results A 1-mm CdTe HEXITEC detector is used to record the spectral responses to: (a) a multi-isotope phantom made of two I-125 seeds, an Am-241 disk, a Co-57 point source and a liquid Tc-99m source; (b) a I-131 liquid source in air and inside the trachea hole of a tissue-equivalent male ART phantom, in correspondence to the thyroid region (Fig.1B). The phantom was positioned on a rotational motor stage behind a 1.5 cm-thick tungsten pinhole collimator of 1 mm. The spectrum acquired by a standard single pixel from the multi-isotope phantom is shown in Fig.1C, while the summed energy spectrum from all 80 × 80 pixels is shown in Fig.1D. The photopeak FWHMs of the four radioisotopes are: 1.4% FWHM at 35.5 keV (I-125), 1.1% at 59.5 keV (Am-241), 0.76 % at 122 keV (Co-57) and 0.75% at 140.5 keV (Tc-99m), significantly superior to other solid-state detectors in use in commercial SPECT systems (DSPECT 5.5% FWHM [5] and GE Discovery NM 530c/570c 5.7% [6] at 140 keV). The spectral response to a I-131 liquid source, both in air and inside the ART phantom, are shown in Fig.1E-F, respectively, with consistent 1.06% FWHM at 284 keV and 1.13% at 364 keV. Conclusions Recent CZT and CdTe detectors have shown great promises for hyperspectral SPECT imaging, where one could achieve an excellent spatial resolution along with an unprecedented energy resolution. Future SPECT systems based on these spectral-imaging sensors could significantly improve the quantitative accuracy for multi-isotope SPECT imaging studies, such as triple-isotope Tc-99m/Tl-201/I-123 cardiac SPECT imaging [7], and for quantitative imaging in molecular radiotherapeutic applications [8]. It also offers the potential of deriving extra imaging information from different spectral regions [9-10], such as Compton continuum for deriving the attenuation map without the need for an extra CT scan. The results presented aim to demonstrate the spectral capability of the HEXITEC detector for hyper-spectral SPECT imaging, and to study how the enhanced spatial-spectral capability is translated into a greatly improved imaging performance for future SPECT instrumentation.