Phantom imaging
Three phantom measurement series were performed using the GE Discovery MI PET scanner (GE Healthcare, General Electric, Boston, MA, USA) with a digital 3-ring detector and a reported sensitivity of 7.3 cps/kBq [13]. Total activity in the field of view was approximately 35 MBq. The absolute activities were measured in the same dose calibrator that is used for periodic calibration of the PET system (ISOMED 2010, MED Dresden GmbH, Germany). To achieve SBR of 2:1, 4:1, and 8:1, the six spheres of a NEMA IEC PET Body Phantom (manufacturer, PTW Freiburg, Germany; sphere diameter, 10, 13, 17, 22, 28, and 37 mm) were filled with 7.5, 12.5, or 24.4 kBq/ml 18F-fluoride, respectively (decay-corrected to the start of the first imaging). The background was filled with either 3.9, 3.1, or 3.1 kBq/ml (true SBR, 1.94:1, 4.06:1, and 7.87:1). Imaging of each phantom was performed for five times every 30 min for 3 min each with the sphere plane aligned to the isocenter of the single bed position. CT data of the phantom obtained for each scan were used for attenuation and scatter correction.
Image reconstruction
PET raw data were reconstructed using Q.Clear with a β value of 150, 300, or 450, respectively (Q.Clear150, Q.Clear300, Q.Clear450). OSEM+TOF reconstruction (GE VUE Point FX) was performed as OSEM+TOF4/16 (4 iterations, 16 subsets, Gaussian in-plane filter of 2.0 mm), OSEM+TOF2/17 (2 iterations, 17 subsets, 2.0 mm filter), and OSEM+TOF2/8 (2 it, 8 ss; 6.4 mm). OSEM+PSF+TOF (hereafter referred to as PSF+TOF; GE VUE Point FX with SharpIR) was reconstructed as PSF+TOF4/16 (4 it, 16 ss, 2.0 mm), as PSF+TOF2/17 (2 it, 17 ss, 2.0 mm) and PSF+TOF4/8 (4 it, 8 ss, 6.4 mm). OSEM+TOF and PSF+TOF reconstructions always included a “standard” z-axis filter. In all 9 reconstruction settings, field of view was 70 cm (matrix size, 256 × 256; voxel size, 2.73 × 2.73 × 2.78 mm3).
Image assessment
Measurement of activity concentrations (AC; kBq/ml) was performed with dedicated software (ROVER, version 3.0.34, ABX advanced biochemical compounds GmbH, Radeberg, Germany). The mean background AC and its standard deviation (SD) were defined with a spherical background volume of interest (VOI; volume, 30 ml). Maximum AC of each sphere was obtained. Using the ACCURATE tool (version v23102018, Ronald Boellaard, Amsterdam UMC, Amsterdam, The Netherlands), the peak AC of each sphere was defined as the average AC in a spherical VOI of 1.2 cm in diameter that was positioned to generate the highest peak AC of this sphere. Peak and maximum CR (CRpeak, CRmax) were defined as the ratio of measured peak/maximum AC of the spheres to the true decay-corrected AC.
SNR was defined as follows:
$$ \mathrm{SNR}=\frac{\ \mathrm{background}\ \mathrm{mean}\ \mathrm{AC}}{\mathrm{background}\ \mathrm{AC}\ \mathrm{standard}\ \mathrm{deviation}} $$
The spatial resolution was assessed as the full width at half maximum (FWHM) of the PSF in the reconstructed images. PSF was modeled by a 3D Gaussian, and FWHM was determined by applying the method described in detail by Hofheinz et al. [14]. This method is based on fitting the analytic solution for the radial activity profile of a homogeneous sphere convolved with a 3D Gaussian to the reconstructed data. In this process, the full 3D vicinity of each sphere is evaluated by transforming the data to spherical coordinates relative to the respective sphere's center (Fig. 1 and Fig. 2). The analytic solution has five parameters: signal (true activity within the sphere), background level, FWHM of the PSF, the sphere radius, and the wall thickness of the spherical inserts. Sphere radius and wall thickness were fixed to their known values. The remaining three parameters were determined by non-linear least-squares fits. With this method the spatial resolution can be determined at finite background as well as for extended objects, and, therefore, allows to study size and contrast dependence of the resolution. Note that this method assumes a Gaussian PSF, which is never exactly the case. However, the method still leads to a reasonable approximation of the spatial resolution as long as the slope at the object boundary (signal decline) is modeled correctly by the fit function.
Inner volumes (25.6, 10.7, 5.4, 2.6, 1.2, and 0.58 ml), inner radii (18.3, 13.7, 10.9, 8.5, 6.6, and 5.2 mm), and wall thickness (range, 0.9 to 1.3 mm) of the spheres were determined based on weighting of the sphere inserts after filling with water (corrected for the volume of the tube mounting) combined with caliper measurements of the outer diameters.
Patient example
A patient example (Fig. 3) was included by way of illustration depicting a male patient (body weight, 63 kg; body mass index [BMI], 21.8 kg/m2) who underwent FDG-PET/CT for early response assessment for Hodgkin’s lymphoma using the same PET scanner. Injection of 250 MBq 18F-FDG was followed by acquisition in the supine position from base of the skull to proximal femora 62 min post-injection (decay-corrected injected activity at scan start, 2.7 MBq/kg; acquisition time, 3 min per bed position). Low-dose-CT was acquired for attenuation correction. PET raw data were reconstructed with the 9 reconstruction algorithms detailed above. SNR in the liver for each algorithm was calculated as the ratio of SUVmean to SUV standard deviation in a spherical VOI in the right liver lobe with an identical location in each dataset (volume, 19.2 ml).
Statistical analysis
Statistical analysis was performed using SPSS 22 (IBM Corporation, Armonk, NY, USA). Descriptive parameters were expressed as mean and SD. Differences in spatial resolution, CRpeak, CRmax, and SNR between reconstruction methods were compared using the Wilcoxon test. Differences in these measures between SBR were compared with either Kruskal-Wallis test or Mann-Whitney U test. Statistical significance was generally assumed at p < 0.05.