Quantitative evaluation of scintillation camera imaging characteristics of isotopes used in liver radioembolization

Abstract

Background: Scintillation camera imaging is used for treatment planning and post-treatment dosimetry in liver radioembolization (RE). In yttrium-90 (90Y) RE, scintigraphic images of technetium-99m (99mTc) are used for treatment planning, while 90Y Bremsstrahlung images are used for post-treatment dosimetry. In holmium-166 (166Ho) RE, scintigraphic images of 166Ho can be used for both treatment planning and post-treatment dosimetry. The aim of this study is to quantitatively evaluate and compare the imaging characteristics of these three isotopes, in order that imaging protocols can be optimized and RE studies with varying isotopes can be compared.

Methodology/principal findings: Phantom experiments were performed in line with NEMA guidelines to assess the spatial resolution, sensitivity, count rate linearity, and contrast recovery of 99mTc, 90Y and 166Ho. In addition, Monte Carlo simulations were performed to obtain detailed information about the history of detected photons. The results showed that the use of a broad energy window and the high-energy collimator gave optimal combination of sensitivity, spatial resolution, and primary photon fraction for 90Y Bremsstrahlung imaging, although differences with the medium-energy collimator were small. For 166Ho, the high-energy collimator also slightly outperformed the medium-energy collimator. In comparison with 99mTc, the image quality of both 90Y and 166Ho is degraded by a lower spatial resolution, a lower sensitivity, and larger scatter and collimator penetration fractions.

Conclusions/significance: The quantitative evaluation of the scintillation camera characteristics presented in this study helps to optimize acquisition parameters and supports future analysis of clinical comparisons between RE studies.

Figure 1. Schematic overview of the spatial resolution measurement of the line-source centered in 20 cm PMMA.

Shown are the camera, including the collimator (A), a stack of 20 PMMA slabs of size 40×40×1 cm (B), the location of the line-source (C), and the patient bed (D). The line-source to collimator-face distance is 11 cm.

Figure 2. Overview of the hot sphere and background ROI.

The slice through the center of the spheres of the contrast recovery phantom filled with ^99mTc is shown. Overlaid are the locations of the lung insert (central red ROI), the hot sphere ROI (peripheral red ROI), and 11 of the 55 background ROI of the largest sphere (green ROI).

Figure 3. Intrinsic and system count rate linearity curves.

(A) The observed count rate is plotted as a function of ideal count rate. Sorensen's count rate model for paralyzable cameras (solid line) is fitted to the intrinsic count rate measurements (data points). The ideal camera count rate response, without dead-time effects, is plotted by the dashed line. (B) Shown are the system count rate curves (solid lines), composed of the intrinsic count rate linearity curve and the system sensitivity measurements. The slope of the ideal system camera count rate response (dashed lines) corresponds to the system sensitivity.

Figure 4. FBP reconstructed images of the contrast recovery phantom.

The slices through the center of the spheres of the contrast recovery phantom are shown for ^99mTc and VXGP (A), ⁹⁰Y 120–250 keV and MEGP (B), ⁹⁰Y 120–250 keV and HEGP (C), ⁹⁰Y 50–250 keV and MEGP (D), ⁹⁰Y 50–250 keV and HEGP (E), ¹⁶⁶Ho and MEGP (F), and ¹⁶⁶Ho and HEGP (G). All images were linearly window-leveled from 0 to 4 times the average background intensity.

Figure 5. Contrast recovery as a function of sphere diameter.

QH is the recovery of sphere-to-background contrast in the measurement, as compared to the true contrast in the phantom.

Figure 6. Measured and simulated LSF of the line-source centered in 20 cm PMMA.

(A) LSF of the ^99mTc line-source and VXGP collimator, (B) ¹⁶⁶Ho and HEGP, (C) ⁹⁰Y 120–250 keV and HEGP, and (D) ⁹⁰Y 50–250 keV and MEGP. Data is plotted on semi-logarithmic scale, showing good agreement between the measurements (data points) and simulations (blue solid line). The intensity is normalized to the total number of counts in the ROI. Contributions of primary, scattered, and penetrated photons are shown in green, light blue, and red, respectively.