SPECT Imaging

As a diagnostic nuclear medicine technique, single photon emission computed tomography (SPECT) provides true three-dimensional information using gamma rays that can be freely reformatted or manipulated as needed. SPECT requires the delivery of a gamma-emitting radioisotope into the patient, which can be a simple soluble dissolved ion, but in most cases, the labeled radioisotope is attached to a specific ligand, forming a radioligand that can bind to certain specific tissues. Compared to PET, SPECT is less expensive because they can use radioisotopes that are longer-lived and more readily available.

Radionuclides for SPECT Imaging

Current non-invasive techniques used for bioimaging include magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), and single photon emission computed tomography (SPECT). Among them, PET and SPECT are highly sensitive and can visualize information quantitatively. Unlike PET, SPECT can image multiple processes simultaneously by using the corresponding detection windows of different radionuclides having different gamma-ray energies. In addition, despite the higher sensitivity and resolution of PET, SPECT is more readily available and less expensive. SPECT can also greatly improve resolution and sensitivity when combined with CT. Common radionuclides used for SPECT include technetium-99m (^99mTc), indium-111 (¹¹¹In), and iodine-123 (¹²³I).

• ^99mTc for SPECT Imaging
  As the most used radionuclide for SPECT imaging, ^99mTc is readily available from commercial ⁹⁹Mo/^99mTc generators. ^99mTc has latent chemical properties for radiolabeling and attractive physical properties, including an appropriate half-life (6.02 h) and γ-ray (140 keV), with benefits for both effective imaging and radiation safety.
• ¹¹¹In for SPECT Imaging
  Another attractive radionuclide for SPECT applications, ¹¹¹In is efficiently produced by cyclotron and emits γ-rays at 173 and 247 keV with a long half-life (2.8 days).
• ¹²³I for SPECT Imaging
  As a low-energy γ-ray (35 keV), ¹²³I is not suitable for clinical SPECT imaging, but is useful for radioimmunoassay test, implantation therapy and preclinical studies due to its long half-life (60.1 days).

Dendrimer-based SPECT Imaging Agents

Bioimaging has revolutionized modern medicine, and nanotechnology can further improve the specificity and sensitivity of imaging. Medical imaging plays an increasingly important role in modern medicine, facilitating the acquisition of accurate information for disease diagnosis and monitoring treatment. Nanotechnology can further improve the sensitivity and specificity of molecular imaging through enhanced penetration and retention (EPR) effects, enabling better imaging and diagnosis. As a result, numerous nanosystems have been explored for development for PET and SPECT imaging. As a nanomaterial, dendrimers are surrounded by repeating branching units and, more importantly, confer the ability to multi-functionalize dendrimers as the number of functional groups on the surface of the dendrimer increases. Dendrimers can be used not only for drug delivery and gene delivery, but also for molecular imaging techniques. After appropriate surface modification, dendrimers have good biocompatibility, non-immunogenicity, and abundant peripheral functional groups.

Schematic illustration of the synthesis and applications of radiolabeled-dendrimer based nanodevices: surface functionalization and interior encapsulation. (Xiao T, et al., 2020)

• ^99mTc-based SPECT Imaging
  Dendrimers can be easily labeled as ^99mTc when conjugated with DTPA, which can act as a chelator for ^99mTc with high radiochemical yield and stability. One of the dendrimers radiolabeled with ^99mTc has enabled the development of various dual model imaging applications.
• ¹¹¹In-based SPECT Imaging
  Like ^99mTc, ¹¹¹In can undergo chelation by DTPA ligands. Dendrimers modified by targeting ligands can greatly improve targeting when radiolabeled by ¹¹¹In. Contrast agents constructed with dendrimers have tremendous advantages for SPECT imaging applications, including overcoming the disadvantages that small molecule iodinated, as well as improving the fluorescence quantum yield for optical imaging.
• ¹²⁵I-based SPECT Imaging
  There are also numerous biodistribution studies on ¹²⁵I-labeled dendrimers for SPECT, which have shown that this imaging agent is more persistent in the vasculature and has higher tumor uptake, while having lower levels in the lung, liver, and spleen.

How We Can Help

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Reference

1. Xiao T.; et al. PAMAM Dendrimer-Based Nanodevices for Nuclear Medicine Applications. Macromol Biosci. 2020, 20: e1900282.