Abstract
The use of computed tomography (CT) images to correct for photon attenuation in positron emission tomography (PET) produces unbiased patient images, but it is not optimal for synthetic materials. For test objects made from epoxy, image bias and artifacts have been observed in well-calibrated PET/CT scanners. An epoxy used in commercially available sources was infused with long-lived 68Ge/68Ga nuclide and measured on several PET/CT scanners as well as on older PET scanners that measured attenuation with 511-keV photons. Bias in attenuation maps and PET images of phantoms was measured as imaging parameters and methods varied. Changes were made to the PET reconstruction to show the influence of CT-based attenuation correction. Additional attenuation measurements were made with a new epoxy intended for use in radiology and radiation treatment whose photonic properties mimic water. PET images of solid phantoms were biased by between 3% and 24% across variations in CT X-ray energy and scanner manufacturer. Modification of the reconstruction software reduced bias, but object-dependent changes were required to generate accurate attenuation maps. The water-mimicking epoxy formulation showed behavior similar to water in limited testing. For some solid phantoms, transformation of CT data to attenuation maps is a major source of PET image bias. The transformation can be modified to accommodate synthetic materials, but our data suggest that the problem may also be addressed by using epoxy formulations that are more compatible with PET/CT imaging.
Highlights
With proper calibration, positron emission tomography (PET) accurately quantifies the concentration of radiolabeled molecules in patients noninvasively and with excellent sensitivity
We have investigated signal bias in solid phantoms made from an epoxy that is used in commercially available sources
While the PET image bias does change with computed tomography (CT) values, as shown in Table 1, X-ray energy cannot be varied arbitrarily and no user-selectable setting led to unbiased PET images
Summary
Positron emission tomography (PET) accurately quantifies the concentration of radiolabeled molecules in patients noninvasively and with excellent sensitivity. Biomarkers computed from these measured concentrations have proven utility in managing the treatment of certain cancers [1,2,3,4]. Poor calibration or nonoptimal processing of the data leads to biased images [5,6,7]. This bias may reduce PET’s prognostic value for patients and researchers [8]. Scattered photons affect raw PET data because PET’s coincidence detection, which does not use physical collimation, cannot distinguish between scattered and unscattered photons for small deflections and misplaces them in the raw projection data
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