Abstract
A challenge in multicenter trials that use quantitative positron emission tomography (PET) imaging is the often unknown variability in PET image values, typically measured as standardized uptake values, introduced by intersite differences in global and resolution-dependent biases. We present a method for the simultaneous monitoring of scanner calibration and reconstructed image resolution on a per-scan basis using a PET/computed tomography (CT) “pocket” phantom. We use simulation and phantom studies to optimize the design and construction of the PET/CT pocket phantom (120 × 30 × 30 mm). We then evaluate the performance of the PET/CT pocket phantom and accompanying software used alongside an anthropomorphic phantom when known variations in global bias (±20%, ±40%) and resolution (3-, 6-, and 12-mm postreconstruction filters) are introduced. The resulting prototype PET/CT pocket phantom design uses 3 long-lived sources (15-mm diameter) containing germanium-68 and a CT contrast agent in an epoxy matrix. Activity concentrations varied from 30 to 190 kBq/mL. The pocket phantom software can accurately estimate global bias and can detect changes in resolution in measured phantom images. The pocket phantom is small enough to be scanned with patients and can potentially be used on a per-scan basis for quality assurance for clinical trials and quantitative PET imaging in general. Further studies are being performed to evaluate its performance under variations in clinical conditions that occur in practice.
Highlights
In oncology clinical trials and clinical practice, estimation of standardized uptake values (SUVs) of malignant lesions in positron emission tomography (PET) images can be used to assess response to therapy [1,2,3,4,5,6]
We evaluate the performance of the PET/computed tomography (CT) pocket phantom in practice when imaged alongside an anthropomorphic phantom and propose a method for the correction of SUVs in biased images
Expressed as a percentage of the range midpoint, ranges of mean regions of interest (ROIs) signal were reduced from 80% in uncorrected images to Ͻ5% for corrected ones, indicating that the pocket phantom system successfully compensated for the simulated scanner miscalibration in our test image set
Summary
In oncology clinical trials and clinical practice, estimation of standardized uptake values (SUVs) of malignant lesions in positron emission tomography (PET) images can be used to assess response to therapy [1,2,3,4,5,6]. ROI measurements of activity in the centers of the largest spheres in the urethane fillable testbed phantom, which were not subject to partial volume effects, showed that this bias was real and not a failure of the algorithm This prevents us from computing scanner calibration bias directly from the known radiotracer concentration. To correct this problem in our solid prototypes and future work, we have proposed and tested the use of a calibration prescan (see Section Pocket Phantom Data Rescaling.) where the algorithm is precalibrated to compensate for biases in the pocket phantom signal from physical effects such as attenuation and scatter correction.
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