Activity painting: PET images of freely defined activity distributions applying a novel phantom technique

Abstract

The aim of this work was to develop a novel phantom that supports the construction of highly reproducible phantoms with arbitrary activity distributions for PET imaging. It could offer a methodology for answering questions related to texture measurements in PET imaging. The basic idea is to move a point source on a 3-D trajectory in the field of view, while continuously acquiring data. The reconstruction results in a 3-D activity concentration map according to the pathway of the point source. A 22Na calibration point source was attached to a high precision robotic arm system, where the 3-D movement was software controlled. 3-D activity distributions of a homogeneous cube, a sphere, a spherical shell and a heart shape were simulated. These distributions were used to measure uniformity and to characterize reproducibility. Two potential applications using the lesion simulation method are presented: evaluation in changes of textural properties related to the position in the PET field of view; scanner comparison based on visual and quantitative evaluation of texture features. A lesion with volume of 50x50x50 mm3 can be simulated during approximately 1 hour. The reproducibility of the movement was found to be >99%. The coefficients of variation of the voxels within a simulated homogeneous cube was 2.34%. Based on 5 consecutive and independent measurements of a 36 mm diameter hot sphere, the coefficient of variation of the mean activity concentration was 0.68%. We obtained up to 18% differences within the values of investigated textural indexes, when measuring a lesion in different radial positions of the PET field of view. In comparison of two different human PET scanners the percentage differences between heterogeneity parameters were in the range of 5–55%. After harmonizing the voxel sizes this range reduced to 2–16%. The general activity distributions provided by the two different vendor show high similarity visually. For the demonstration of the flexibility of this method, the same pattern was also simulated on a small animal PET scanner giving similar results, both quantitatively and visually. 3-D motion of a point source in the PET field of view is capable to create an irregular shaped activity distribution with high reproducibility.

Physical imaging phantoms for simulation of tumor heterogeneity in PET, CT, and MRI: An overview of existing designs

“…Only the robotic arm for activity painting proposed by Forgacs et al is not suitable for cross modality imaging. 24 Despite all of the reported efforts on the development of heterogeneity phantoms, there appears to be no standardized object that is dedicated for cross modality imaging (PET, CT, MRI). Obviously, such objects would support a single standard for cross modality imaging and, thus, contribute to wide adoption in the imaging community and help facilitate imaging standards.…”

“…Therefore, most of the existing MRI phantoms are composed of tissue-mimicking materials, such as water, fat, and agarose gel. [24][25][26] In terms of phantom building techniques and materials, the design criteria for quantitative MRI phantoms are well-characterized MRI properties and stability of the used materials. It is also recommended avoiding fillable compartments to ensure traceability and consistency of the phantom characteristics.…”

“…Recently, a different method to mimic heterogeneous PET activity distributions was proposed. 24 It consists of a robotic arm able to produce precise and reproducible movements along a 3D plane. A calibrated sealed 22Na point source was attached to one of the sides of the arm, and by controlled movements, different activity distributions patterns were drawn (See Fig.…”

“…However, if the relaxation times are too short, as it is the case in most solid materials, MRI systems will not produce sufficiently high signals for detection. Therefore, most of the existing MRI phantoms are composed of tissue‐mimicking materials, such as water, fat, and agarose gel . In terms of phantom building techniques and materials, the design criteria for quantitative MRI phantoms are well‐characterized MRI properties and stability of the used materials.…”

“…Recently, a different method to mimic heterogeneous PET activity distributions was proposed . It consists of a robotic arm able to produce precise and reproducible movements along a 3D plane.…”