The analytic solution has five parameters: signal (true activity within the sphere), background level, FWHM of the PSF, and the radius as well as the (cold) wall thickness of the spherical inserts.
Hot spheres of 10, 20, 30, and 40 mm diameter and a 40-mm hot sphere with a 25-mm solid (cold) centre were inserted into the warm liver background region with a sphere:background ratio of 4 1 and a clinically relevant total activity of 1340 MBq 90Y.
The contrast recovery coefficients obtained with a sphere phantom would depend on sphere and background activities, sphere to background contrasts, and the number of total acquired counts [23].
The 18F solution concentration used to fill the spheres and background was adjusted such that for the 3.75:1 contrast ratio scan; the background 18F solution concentration would be 143 nCi/mL or 5.29 kBq/mL at the start of the PET scan.
where Cmeas and Ctrue are the measured and true spheres to background specific activity ratios, respectively.
The ratio of spheres to background activity concentrations was very high (30 1).
The activity concentration ratio between the spheres and background was equal to 25. a AC-OSEM.
ROIs were placed in the slice intersecting the spheres center (background, lung insert, biggest and smallest hot spheres).
The phantom spheres and background were filled with FDG activity (ratio 10 1), and the scan duration was 10 min.
The same image quality phantom preparations were used for both system, to avoid different activity ratios between spheres and background, which may occur when preparing a phantom twice.
The threshold level which gave the most accurate estimation of volume (compared to the true sphere volume) was determined for 111In, and 68Ga for all spheres and background activity levels.
