Abstract
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Introduction: Peripheral Vascular Disease (PVD) affects approximately 8 million Americans and an estimated 10% of the worldwide population, with increasing prevalence in older individuals [1]. PVD has significant health implications, resulting in progressive limb ischemia that can lead to life-altering claudication, non-healing ulcers, limb amputation, and, in severe cases, death [2]. While the early diagnosis of PVD is crucial for effective intervention and reversibility, current diagnostic tools are limited in the ability to assess molecular and microvascular changes that underlie the disease [3,4]. This underscores the need for advanced imaging modalities to provide comprehensive assessment of molecular and physiological changes in the lower extremities in patients with PVD.
SPECT stands out among molecular imaging technologies for intrinsic multi-functional capabilities, enabling the simultaneous imaging and quantification of specific SPECT radiotracers targeting diverse molecular and physiological processes, including inflammation, angiogenesis, and thrombosis, enable the early-stage detection of PVD [4,5].
To address the need for optimal systems for imaging lower extremity PVD, we have developed the Dynamic Extremity SPECT (DE-SPECT) system (Fig.1A), a dedicated clinical SPECT imaging system that offers multi-functional targeted in vivo imaging of PVD [8]. It is equipped with a motor-driven interchangeable aperture system (Fig.1B) for high-sensitivity and high-resolution (HS-HR) imaging. Initial configuration and the preliminary experimental assessment of the system are provided.
Methods: DE-SPECT system comprises 48 3-D imaging spectrometers arranged across 6 detector panels (Fig. 1A). Each spectrometer has 2 × 2 CZT crystals of 2.2 cm × 2.2 cm × 1.0 cm in size, ensuring optimal sensitivity within a broad energy range of 50-600 keV. The CZT detectors offer excellent spatial and energy resolutions [6,7] (Fig. 1D-E). They capture the 5-dimensional photon fluence, incorporating 3D position (x, y, z), time (t), and energy (E) information in list mode.
Notably, the DE-SPECT system features a dual-field-of-view (FOV) collimator (Fig. 1B), enabling on-the-fly switching between a wide FOV (28 cm diameter) imaging configuration and a focused (16 cm diameter) HS-HR FOV configuration—accomplished without disturbing the patient for multi-parametric imaging. A computer-controlled pneumatic linear actuator is used to axially slide the tungsten plate enabling rapid interchange (~2 s) of the aperture sets.
Results: The preliminary assessment of the CZT sensors shows: (a) a precise 3-D spatial resolution of <0.75 mm FWHM in 3 dimensions (Fig.1D), and (b) an unmatched spectroscopic performance (<2 keV at 140 keV, 3 keV at 200 keV, 4.5 keV at 450 keV) with the ability to detect multiple simultaneous gamma-ray interactions (Fig.1E). We also conducted a preliminary imaging study with 1 CZT sensor coupled to 1 pinhole. The imaging result (Fig. 1F) of an IQ phantom filled with Tc-99m solution demonstrates the capability of the system to perform non-invasive simultaneous SPECT imaging of multiple tracers and to evaluate various physiologic indices that define the pathophysiology of PVD, such as skeletal muscle perfusion, angiogenesis, and atherosclerosis. Finally, we will present the fully assembled system along with preliminary imaging acquisitions.
Conclusions: The development of the DE-SPECT system signifies a groundbreaking shift in radionuclide-based emission tomography, moving beyond single-functional imaging to embrace multi-functional spectral imaging [3]. This unique system enables in vivo gamma-ray spectrometry on a voxel-by-voxel basis, providing concentrations of various radiotracers and therapeutic radionuclides within user-defined regions. Notably, this capability enhances precision in diagnostic imaging and positions the DE-SPECT system as an attractive tool for characterization of PVD and molecularly-guided interventions, opening new horizons in nuclear medicine.
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