Quantitative Evaluation of Scintillation Camera Imaging Characteristics of Isotopes Used in Liver Radioembolization

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.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.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

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

<p>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.</p

FBP reconstructed images of the contrast recovery phantom.

<p>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.</p

Monte Carlo simulated primary, scatter and collimator penetration fractions for the experimental set-up with the line source centered in 20 cm PMMA.

<p>Monte Carlo simulated primary, scatter and collimator penetration fractions for the experimental set-up with the line source centered in 20 cm PMMA.</p

Collimator characteristics.

<p>All collimators have hexagonal hole shapes.</p

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

<p>(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.</p

Sensitivity in counts per second per unit activity.

<p>Sensitivity in counts per second per unit activity.</p

Radioisotope characteristics and measurement settings.

a<p>E_βmax represents the maximum energy and abundance of the beta particles.</p>b<p>E_γ represents the gamma photon energy and abundance.</p>c<p>E_win represents the lower and upper limits of the energy window.</p

Spatial resolution given as the FWHM (FWTM) in mm.

<p>S01D02 corresponds to the measurement with 1 cm of scatter material and line-source to collimator distance of 2 cm, S01D06 to the measurement with 1 cm of scatter material and line-source to collimator distance of 6 cm, etc.</p