Development and evaluation of an improved quantitative (90)Y bremsstrahlung SPECT method

Abstract

Purpose: Yttrium-90 ((90)Y) is one of the most commonly used radionuclides in targeted radionuclide therapy (TRT). Since it decays with essentially no gamma photon emissions, surrogate radionuclides (e.g., (111)In) or imaging agents (e.g., (99m)Tc MAA) are typically used for treatment planning. It would, however, be useful to image (90)Y directly in order to confirm that the distributions measured with these other radionuclides or agents are the same as for the (90)Y labeled agents. As a result, there has been a great deal of interest in quantitative imaging of (90)Y bremsstrahlung photons using single photon emission computed tomography (SPECT) imaging. The continuous and broad energy distribution of bremsstrahlung photons, however, imposes substantial challenges on accurate quantification of the activity distribution. The aim of this work was to develop and evaluate an improved quantitative (90)Y bremsstrahlung SPECT reconstruction method appropriate for these imaging applications.

Methods: Accurate modeling of image degrading factors such as object attenuation and scatter and the collimator-detector response is essential to obtain quantitatively accurate images. All of the image degrading factors are energy dependent. Thus, the authors separated the modeling of the bremsstrahlung photons into multiple categories and energy ranges. To improve the accuracy, the authors used a bremsstrahlung energy spectrum previously estimated from experimental measurements and incorporated a model of the distance between (90)Y decay location and bremsstrahlung emission location into the SIMIND code used to generate the response functions and kernels used in the model. This improved Monte Carlo bremsstrahlung simulation was validated by comparison to experimentally measured projection data of a (90)Y line source. The authors validated the accuracy of the forward projection model for photons in the various categories and energy ranges using the validated Monte Carlo (MC) simulation method. The forward projection model was incorporated into an iterative ordered subsets-expectation maximization (OS-EM) reconstruction code to allow for quantitative SPECT reconstruction. The resulting code was validated using both a physical phantom experiment with spherical objects in a warm background and a realistic anatomical phantom simulation. In the physical phantom study, the authors evaluated the method in terms of quantitative accuracy of activity estimates in the spheres; in the simulation study, the authors evaluated the accuracy and precision of activity estimates from various organs and compared them to results from a previously proposed method.

Results: The authors demonstrated excellent agreement between the experimental measurement and Monte Carlo simulation. In the XCAT phantom simulation, the proposed method achieved much better accuracy in the modeling (error in photon counts was -1.1 %) compared to a previously proposed method (errors were more than 20 %); the quantitative accuracy of activity estimates was excellent for all organs (errors were from -1.6 % to 11.9 %) and comparable to previously published results for (131)I using the same collimator.

Conclusions: The proposed (90)Y bremsstrahlung SPECT reconstruction method provided very accurate estimates of organ activities, with accuracies approaching those previously observed for (131)I. The method may be useful in verifying organ doses for targeted radionuclide therapy using (90)Y.

Figure 1

Energy spectra (50–2000 keV, 10 keV interval) of ⁹⁰Y bremsstrahlung photons in water simulated using MCNPX (solid line) and estimated from experimental measurement by Rault et al. (dashed line).

Figure 2

Sample distance histograms (bin width was 0.1 mm) displayed for four different energies.

Figure 3

Schematic of forward projection model used in MER method. Blocks marked “emitted photons” correspond to bremsstrahlung photons emitted in the indicated energy range that exited the body without interacting. Blocks marked ”effective scatter source” indicate that ESSE was used to simulate photons having the indicated range of energies after their last scatter event. On each line labeled “Attenuation, CDR” the photons were attenuated using an effective attenuation coefficient appropriate for an appropriate energy spectrum in the energy range indicated in the previous block.

Figure 4

Custom designed rod phantom used as a line source for MC simulation validation and sensitivity measurements.

Figure 5

Elliptical phantom used in physical phantom experiments showing three spheres of different sizes.

Figure 6

(a) Projection of the rod phantom simulated using the improved SIMIND (left) and measured (right). (b) Ten pixel wide horizontal profiles through the middle of the rod phantom images in (a). The profiles were normalized to unit total area and enough photons were simulated so that the noise in the profiles was very small.

Figure 7

(a) Central slice of the reconstructed image obtained after ten iterations (16 subsets per iteration) of OS-EM using the MER (left) and SER (middle) method and the difference image (right) for the physical phantom experiment. (b) Fifteen pixel wide vertical profiles through the center of each sphere in (a).

Figure 8

Percent errors in activity estimates as a function of the number of OS-EM iterations (16 subsets per iteration) for each of the three spheres using the MER method. Percent error = (Estimated Activity − True Activity)/True Activity × 100%.

Figure 9

(a) Anterior projection view estimated using the MER method (left), MC simulated (middle), and the difference image (right) all shown on the same gray scale; (b) Profiles through projections at the position indicated by lines in (a) of MC simulated images and images estimated using the MER and SER methods for the 100–500 and 105–195 keV energy windows, as indicated in the legend.

Figure 10

(a) Sample coronal slices of (left to right) the attenuation map, activity distribution, images reconstructed using the MER method and attenuation compensation alone from data with noise corresponding to 2960 MBq total activity and 30 min imaging time, and images reconstructed using the MER and SER 100–500 keV from data without added noise and the difference image. The reconstructed images were obtained using ten iterations with 16 subsets per iteration and the third and fourth images were filtered using a 3D Butterworth postreconstruction filter with order 8 and cutoff 0.11 pixels⁻¹. (b) Vertical profiles through the center of the liver in the fifth and sixth images in (a).

Figure 11

Mean and standard deviation of percent errors (left) and percent RMSE (right) of the activity estimates for the liver from 50 Poisson noise realizations as a function of the number of OS-EM iterations (16 subsets per iteration) for the MER method and SER method using data acquired in the energy windows indicated in the legend. The error bars in the left figure represent the standard deviations.

Figure 12

SIMIND simulated ⁹⁰Y bremsstrahlung energy spectrum of primary photons after passing through 0, 5, and 20 cm of water, respectively. The spectra were normalized to have the same total area over the energy range 80–1500 keV.