High-Resolution Positronium Lifetime Tomography at Clinical Activity Levels on the PennPET Explorer

Abstract

The objective of this study is to demonstrate high-resolution positronium lifetime tomography using a 3-dimensional phantom with multiple radioisotopes at activity levels relevant to human imaging. Methods: A cylinder phantom was constructed with a 6-mm thick polycarbonate slab inserted at its center. The phantom was filled with radioactive solutions and scanned in the PennPET Explorer scanner, a long-axial-field-of-view scanner with high sensitivity. Four scans were conducted with 3 different radioisotopes at varying activity levels: ⁸²Rb (240 MBq), ⁶⁸Ga (110 MBq), and ⁴⁴Sc (7.4 and 40.7 MBq). The average positron lifetime images were reconstructed with correction for random events. Radial and axial resolutions were measured from lifetime profiles across a hole on the slab and through the slab, respectively. Results: The polycarbonate slab was resolvable in all reconstructed positron lifetime images and showed a lifetime longer than that in water. The lifetime images from ⁸²Rb and ⁶⁸Ga were noisy because of their low prompt γ yields and high random fractions, whereas noise was substantially reduced in the ⁴⁴Sc images. The average lifetime estimates were consistent across the 4 scans. The estimated radial and axial resolutions were 3.7 ± 1.8 mm and 3.9 ± 0.4 mm, respectively. Conclusion: This study successfully demonstrated the feasibility of high-resolution positronium lifetime tomography using ⁸²Rb, ⁶⁸Ga, and ⁴⁴Sc on the PennPET Explorer.

Keywords: PennPET; SIMPLE; polycarbonate; positronium lifetime tomography.

Graphical abstract

FIGURE 1.

(A) PennPET EXPLORER PET/CT scanner. (B) Cylinder phantom. (C) 6-mm-thick polycarbonate insert. (D) Schematic illustration of phantom with dimension and orientation.

FIGURE 2.

Left: energy spectra of ⁸²Rb, ⁶⁸Ga, and ⁴⁴Sc scans with background radiation from ¹⁷⁶Lu subtracted. Thirty second data of each scan are used for plot. Their activities were scaled to 40 MBq. Corresponding contribution from ¹⁷⁶Lu radiation is plotted for comparison. Right: true lifetime spectra of ⁸²Rb, ⁶⁸Ga, and ⁴⁴Sc scans with type I–III randoms subtracted. A.U. = arbitrary unit (normalized by maximum).

FIGURE 3.

x–y and y–z slices (scanner coordinate) of reconstructed activity and lifetime images with and without CT-guided smoothing. For lifetime reconstruction, 2 subsets and 2 iterations were used for ⁸²Rb and ⁶⁸Ga images, whereas 2 subsets and 4 iterations were used for ⁴⁴Sc images. Activity images were reconstructed from triple coincidences using 8 iterations and 2 subsets.

FIGURE 4.

Plots of mean lifetime value in water (A) and slab (B) ROIs against background SD by varying iteration number with 2 subsets. Dashed lines represent results of kernel method. Eight iterations are shown for all scans.

FIGURE 5.

Measured and fitted lifetime profiles. Radial profile (A) traverses through 20-mm hole, and axial profile (B) across 6-mm thick slab. Radial and axial full width at half maximum values were determined to be 3.7 ± 1.8 mm and 3.9 ± 0.4 mm, respectively.