Interventional nuclear imaging for SIRT - a phantom study

Objectives Selective Internal Radiotherapy (SIRT) of the liver requires imaging with 99mTc-labeled MAA prior to the delivery of the therapeutic 90Y-labeled microspheres. Both are injected intra-hepatic following angiographic catheterization. SPECT/CT is used to assess its distribution in the patient. We suggest replacing the current workflow, requiring two individual catheterizations and SPECT/CT acquisitions each in the nuclear medicine clinic, by a one-step procedure, performed entirely in the angiography suite using a hand-held gamma camera system (freehand SPECT). The combination of freehand SPECT with interventional C-arm CT would provide the accumulation pattern of both types of radioactive labeled microspheres to consecutively perform diagnostic and therapeutic procedures in a single interventional procedure. This preliminary study evaluates the feasibility of freehand SPECT on an abdominal phantom.

Methods Freehand SPECT imaging was performed with a hand-held gamma-camera using a 40x40mm CZT-detector (declipseSPECT, Surgiceye, Munich, Germany). The 3D-SPECT images were reconstructed (26x26x28cm3 volume of interest, 5mm voxel size, 20 Iterations) from projection data measured with the optically tracked gamma camera. Imaging was performed with a water filled IEC body phantom with spheres (diameter between 10mm and 37mm). Different phantom setups simulated positive contrasts inside the whole phantom using all spheres (scenario A) or the two largest spheres (diameter 28mm and 37mm, scenario B) positioned in a lateral segment of the phantom. For scenario A all spheres were filled with 99mTc (113MBq/mL) without background activity. For scenario B the two largest spheres were filled with activity (99mTc, 100MBq/mL) and with activity inside the background (99mTc, ratio=8:1, B-1). Additionally, scenario B was examined with 90Y in the two largest spheres (2.8MBq/mL) without background activity (B-2). Images were acquired on different realistic (acquisition from the front and sides of the phantom respecting imaging paradigm during the angiographic procedure) and unrealistic (acquisition also from transverse sides and back side of the phantom) trajectories with scanning times below 15 minutes. The image quality is assessed based on the detectability of lesions.

Results In scenario A, all 6 99mTc filled spheres were detectable by using unrealistic scanning trajectories only. In imaging scenario B, both spheres can be distinguished with realistic and unrealistic acquisition trajectories using 99mTc (B-1) and 90Y (B-2), respectively. Activity pattern located in a section of the phantom, comparable to the locations of microspheres accumulation inside the liver during radioembolization procedures, were detectable by scanning conditions conform to requirements from angiographic procedure.

Conclusions This phantom study demonstrated the feasibility of SPECT imaging in parallel to interventional-angiographic procedure by using a hand-held gamma camera. Ongoing work investigates the image quality with more complex and realistic phantoms. Patient studies are also planned.