Quantitative Accuracy of Clinical ^99mTc SPECT/CT Using Ordered-Subset Expectation Maximization with 3-Dimensional Resolution Recovery, Attenuation, and Scatter Correction

Abstract

We present a calibration method of a clinical SPECT/CT device for quantitative 99m Tc SPECT. We use a commercially available reconstruction package including ordered-subset expectation maximization (OSEM) with depth-dependent 3-dimensional resolution recovery (OSEM-3D), CT-based attenuation correction, and scatter correction. We validated the method in phantom studies and applied it to images from patients injected with 99m Tc-diphosponate. Methods: The following 3 steps were performed to derive absolute quantitative values from SPECT reconstructed images. In step 1, we used simulations to characterize the SPECT/CT system and derive emission recovery values for various imaging parameter settings. We simulated spheres of varying diameters and focused on the dependencies of activity estimation errors on structure size and position, pixel size, count density, and reconstruction parameters. In step 2, we cross-calibrated our clinical SPECT/CT system with the well counter using a large cylinder phantom. This step provided the mapping from image counts to kBq/mL. And in step 3, correction factors from steps 1 and 2 were applied to reconstructed images. We used a cylinder phantom with variable-sized spheres for verification of the method. For in vivo validation, SPECT/CT datasets from 16 patients undergoing 99m Tc-diphosponate SPECT/CT examinations of the pelvis including the bladder were acquired. The radioactivity concentration in the patients' urine served as the gold standard. Mean quantitative accuracy and SEs were calculated. Results: In the phantom experiments, the mean accuracy in quantifying radioactivity concentration in absolute terms was within 3.6% (SE, 8.0%), with a 95% confidence interval between 219.4% and 112.2%. In the patient studies, the mean accuracy was within 1.1% (SE, 8.4%), with a 95% confidence interval between 215.4% and 117.5%. Conclusion: Current commercially available SPECT/CT technology using OSEM-3D reconstruction, scatter correction, and CT-based attenuation correction allows quantification of 99m Tc radioactivity concentration in absolute terms within 3.6% in phantoms and 1.1% in patients with a focus on the bladder. This opens up the opportunity of SPECT quantitation entering the routine clinical arena. Still, the imprecision caused by unavoidable measurement errors is a dominant factor for absolute quantitation in a clinical setup.

SPECT/CT activity quantification in 99mTc-MAA acquisitions.

“…Our results show that activity quantification in 99m Tc phantom acquisitions with hot inserts can be achieved with an accuracy of about 7%. This value is comparable with the accuracy measured by Zeintl et al in [1]. Since activity quantification is affected by partial volume effects, the recovery coefficients for different source to background ratios were determined with an accuracy within 5%.…”

“…In this work we tested the quantitative performance of our SPECT/CT scanner using the procedure suggested in [1]. Our results show that activity quantification in 99m Tc phantom acquisitions with hot inserts can be achieved with an accuracy of about 7%.…”

“…The aim of the work was the scanner calibration and the absolute activity quantification (with 99m Tc) both in phantom and in vivo acquistions. To perform the study, we used the methodology proposed by Zeintl et al [1]. The procedure consists of the calibration of the scanner which was achieved by phantom studies.…”

“…Materials and methods: SPECT/CT calibration was performed using the methodology proposed by Zeintl et al [1]. A calibration factor converting cps (counts per second) to activity was determined using a uniform radioactive solution in a cylindrical water phantom.…”

Practical reconstruction protocol for quantitative 90Y bremsstrahlung SPECT/CT

“…31 Acrylic slabs of various thicknesses (2, 5, 9, 12, and 16 cm) were placed right below the phantom to introduce attenuation and scatter. For the 5 cmthick acrylic slab, the distance between the collimator and the phantom was varied to 15,20,25,30, and 35 cm. Planar static images were acquired for each imaging condition.…”