SPECT/CT activity quantification in 99mTc-MAA acquisitions.

Abstract

Aim of this work is the calibration of a SPECT/CT scanner (SYMBIA T2-SIEMENS) for activity quantification in 99m Tc-macroaggregated albumin (99m Tc-MAA) clinical acquisitions used for radiation dosimetry before radioembolization. Materials and methods: SPECT/CT calibration was performed using the methodology proposed by Zeintl et al [1]. A calibration factor converting cps (counts per second) to activity was determined using a uniform radioactive solution in a cylindrical water phantom. Recovery coefficients, background variability and contrast were obtained from acquisitions of an IEC NEMA torso-phantom with hot sphere inserts. 99m Tc was used to prepare the radioactive solutions and different sphere-to-background ratio (RSB). The phantom was acquired using the 99m Tc-MAA clinical protocol. Images were reconstructed with the proprietary OSEM 3D algorithm (FLASH 3D, Siemens Healthcare), scatter and attenuation correction. The phantom results were validated on 99m Tc-MAA in vivo patient acquisitions. Results: The calibration factor for 99m Tc was 464.8 cps/MBq. Recovery coefficients vary with sphere volumes from 90% to 20%. Hot sphere contrast and activity quantification improve with higher iterations and subsets while the sphere to background ratio and the background variability increase. A balance between a satisfactory activity recovery and a low background variability (<12%) was obtained with 8 iterations, 8 subsets and no filtering. Hot spheres activity was determined with an accuracy ≤7%. Activity in liver from 99m Tc-MAA patient acquisitions was 8% (mean value) which was less than the administrated activity to the patients. This underestimation was attributed to some extrahepatic shunt. Conclusions: Activity recovery using the tested scanner can be achieved within an accuracy of 7%. With a simple scanner calibration procedure, the parameters of the clinical 99m Tc protocols can be optimized improving image quality and activity recovery according to the clinical requirements.