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Terbium-149 PET/CT: First Quantitative Imaging with a Clinical Long-Axial Field-of-View Scanner

Lorenzo Mercolli, Pascal V. Grundler, Anzhelika N. Moiseeva, Lars Eggimann, Saverio Braccini, Nicholas P. van der Meulen

Abstract

Introduction: Terbium-149 ($^{149}$Tb) is a promising radionuclide for targeted $α$ therapy that has a non-zero branching ratio (BR) for positron decay. However, its relatively low positron branching fraction and multiple prompt $γ$ emissions may challenge quantitative imaging. This study evaluates, for the first time, the imaging performance and quantitative accuracy of $^{149}$Tb using a clinical long axial field-of-view (LAFOV) PET/CT system. Methods: Quantitative accuracy of $^{149}$Tb was assessed with a NEMA IEC body phantom, which was filled with about 45 MBq $^{149}$Tb and a sphere-to-background ration of 10:1. The phantom was scanned for 20 min and shorter scan times and lower activities were simulated. Recovery coefficients, coefficient of variation, and lung residual error were evaluated for different reconstruction settings and compared to the EARL standard 2 for $^{18}$F. Results: High-quality PET images of $^{149}$Tb were obtained, even with a simulated total activity of 4.5 MBq. The 20 min and full activity scan yielded a mean recovery coefficient $RC_\textit{mean}$ of $0.55$, $0.69$, $0.73$, $0.76$, $0.79$, and $0.81$ for the six phantom spheres. Despite the low count statistics, the coefficient of variation stays mostly below $15\,\%$. Relative scatter correction combined with prompt $γ$ modeling provided robust quantification. Conclusion: $^{149}$Tb can be imaged using a commercial LAFOV PET/CT with a quantitative accuracy comparable to the EARL standard 2 for $^{18}$F. These findings demonstrate the feasibility of PET-based treatment verification and dosimetry for targeted $α$ therapy with $^{149}$Tb.

Terbium-149 PET/CT: First Quantitative Imaging with a Clinical Long-Axial Field-of-View Scanner

Abstract

Introduction: Terbium-149 (Tb) is a promising radionuclide for targeted therapy that has a non-zero branching ratio (BR) for positron decay. However, its relatively low positron branching fraction and multiple prompt emissions may challenge quantitative imaging. This study evaluates, for the first time, the imaging performance and quantitative accuracy of Tb using a clinical long axial field-of-view (LAFOV) PET/CT system. Methods: Quantitative accuracy of Tb was assessed with a NEMA IEC body phantom, which was filled with about 45 MBq Tb and a sphere-to-background ration of 10:1. The phantom was scanned for 20 min and shorter scan times and lower activities were simulated. Recovery coefficients, coefficient of variation, and lung residual error were evaluated for different reconstruction settings and compared to the EARL standard 2 for F. Results: High-quality PET images of Tb were obtained, even with a simulated total activity of 4.5 MBq. The 20 min and full activity scan yielded a mean recovery coefficient of , , , , , and for the six phantom spheres. Despite the low count statistics, the coefficient of variation stays mostly below . Relative scatter correction combined with prompt modeling provided robust quantification. Conclusion: Tb can be imaged using a commercial LAFOV PET/CT with a quantitative accuracy comparable to the EARL standard 2 for F. These findings demonstrate the feasibility of PET-based treatment verification and dosimetry for targeted therapy with Tb.

Paper Structure

This paper contains 5 sections, 2 equations, 5 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Materials and methods
  3. Results
  4. Discussion
  5. Conclusions

Figures (5)

  • Figure 1: Top left panel: probability of positron annihilation in water for point sources of different radionuclides. Top right panel: HPGe spectrum of the stock solution sample with the details of the [149]Tb peak at 164.98 ± 0.02keV and [149]Gd peak at 149.730 ± 0.010keV as well as for [145]Eu at 893.73 ± 0.03keV. Bottom left panel: CT image of the NEMA phantom centered in the axial direction. Bottom center panel: CT image with the off-axis NEMA phantom with visible spatial distortions. Bottom right panel: undistorted off-axis CT image of the NEMA phantom.
  • Figure 2: Central slices of the phantom image for different iterations, scan times and scatter correction.
  • Figure 3: [149]Tb recovery for 3 iterations and different scan times and scatter corrections in comparison with the EARL standard 2 for [18]F. Top and center rows: recovery coefficients for on-axis phantom with standard (top) and PR-specific (center) PSF correction. Center row: recovery coefficients for on-axis phantom with Bottom row: recovery coefficients for on/off-axis phantom and with a corrected AC CT.
  • Figure 4: Top and center row: convergence of $RC_{mean}$ for increasing number of iterations as a function $\mathrm{COV}$ for different scan times and scatter correction. Bottom row: lung error and $\mathrm{COV}$ as a function of the number of iterations for different scan times and scatter correction.
  • Figure 5: Top row: Emission sinogram (solid line) together with total correction, p$\gamma$ correction and absolute scatter sinograms for the on- (left) and off-axis (center) measurements at $0^\circ$ view angle (arbitrary unit on y axis). Bottom row: Same sinograms but at $45^\circ$ view angle. Right column: Ratio sinograms with original and modified AC CT for p$\gamma$ (dash-dotted line) and relative scatter.