Improving quantitative dosimetry in (177)Lu-DOTATATE SPECT by energy window-based scatter corrections

Abstract

Purpose: Patient-specific dosimetry of lutetium-177 ((177)Lu)-DOTATATE treatment in neuroendocrine tumours is important, because uptake differs across patients. Single photon emission computer tomography (SPECT)-based dosimetry requires a conversion factor between the obtained counts and the activity, which depends on the collimator type, the utilized energy windows and the applied scatter correction techniques. In this study, energy window subtraction-based scatter correction methods are compared experimentally and quantitatively.

Materials and methods: (177)Lu SPECT images of a phantom with known activity concentration ratio between the uniform background and filled hollow spheres were acquired for three different collimators: low-energy high resolution (LEHR), low-energy general purpose (LEGP) and medium-energy general purpose (MEGP). Counts were collected in several energy windows, and scatter correction was performed by applying different methods such as effective scatter source estimation (ESSE), triple-energy and dual-energy window, double-photopeak window and downscatter correction. The intensity ratio between the spheres and the background was measured and corrected for the partial volume effect and used to compare the performance of the methods.

Results: Low-energy collimators combined with 208 keV energy windows give rise to artefacts. For the 113 keV energy window, large differences were observed in the ratios for the spheres. For MEGP collimators with the ESSE correction technique, the measured ratio was close to the real ratio, and the differences between spheres were small.

Conclusion: For quantitative (177)Lu imaging MEGP collimators are advised. Both energy peaks can be utilized when the ESSE correction technique is applied. The difference between the calculated and the real ratio is less than 10% for both energy windows.

Fig. 1

The 10-l acylic phantom (dimensions 24.1×30.5×24.1 cm) with six spheres of inner diameters 10, 13, 17, 22, 28 and 37 mm. The phantom is described in NEMA NU2-2007, section 7 ‘Image quality accuracy of attenuation and scatter corrections’. Image courtesy of PTW (Physikalisch-Technische Werkstätten).

Fig. 2

The acquired energy windows, see Table 2. DEW, dual-energy window; DS, downscatter; DPW, dual-photopeak window; TEW, triple-energy window.

Fig. 3

Two transaxial slices (medium-energy general purpose collimators and 208 keV energy window), reconstructed and both attenuation and scatter corrected with Astonish. (a) Low-dose computer tomography slice through the centre of the (visible) walls of the spheres and the volume of interest (VOIs) in different colours. (b) Single photon emission computer tomography (SPECT) of the same slice as in (a) with the VOIs shown in different colours. (c) SPECT slice in the phantom far away from the spheres illustrating the VOI for the background. The banana-like VOI outside the phantom is used for determination of the weight for certain energy window correction methods. Colour scales are linear with highest intensity at the top.

Fig. 4

Six single photon emission computer tomography reconstructions [low-energy general purpose collimators for 113 keV (a, b, and c) and medium-energy general purpose for 208 keV (d, e, and f)] illustrating the effect of scatter and downscatter correction. (a, d) Slice through the centre of the spheres with attenuation correction only. (b, e) Same as (a) and (d) but with scatter correction with the dual-energy window method (weight factor of 0.5). (c, f) Same as (a) and (d) but with combined scatter and downscatter correction with the triple-energy window method with ‘wide’ energy windows. Colour scales are linear with highest intensity at the top.

Fig. 5

Reconstructions of the three collimators [low-energy high resolution (a, d), low-energy general purpose (b, e) and medium-energy general purpose (c, f)] for 113 keV (a, b, c) and 208 keV (d, e, f) energy windows. All slices are reconstructed with attenuation correction and the effective scatter source estimation method. Colour scales are linear with highest intensity at the top. Notice the low image quality for the 208 keV energy window with low-energy collimators due to septal penetration.

Fig. 6

Results of the low-energy high resolution collimator for 113 keV. DEW, dual-energy window; DS, downscatter; ESSE, effective scatter source estimation; TEW, triple-energy window.

Fig. 7

Results of the low-energy general purpose collimator for 113 keV. DEW, dual-energy window; DS, downscatter; ESSE, effective scatter source estimation; TEW, triple-energy window.

Fig. 8

Results of the medium-energy general purpose collimator for 113 keV. DEW, dual-energy window; DS, downscatter; ESSE, effective scatter source estimation; TEW, triple-energy window.

Fig. 9

Results of the medium-energy general purpose collimator for 208 keV. DEW, dual-energy window; DS, downscatter; ESSE, effective scatter source estimation; TEW, triple-energy window.