NEMA image quality phantom measurements and attenuation correction in integrated PET/MR hybrid imaging.

Susanne Ziegler,Bjoern W Jakoby,Harald H Quick,Daniel H Paulus,Harald Braun

doi:10.1186/s40658-015-0122-3

Abstract

BackgroundIn integrated PET/MR hybrid imaging the evaluation of PET performance characteristics according to the NEMA standard NU 2–2007 is challenging because of incomplete MR-based attenuation correction (AC) for phantom imaging. In this study, a strategy for CT-based AC of the NEMA image quality (IQ) phantom is assessed. The method is systematically evaluated in NEMA IQ phantom measurements on an integrated PET/MR system.MethodsNEMA IQ measurements were performed on the integrated 3.0 Tesla PET/MR hybrid system (Biograph mMR, Siemens Healthcare). AC of the NEMA IQ phantom was realized by an MR-based and by a CT-based method. The suggested CT-based AC uses a template μ-map of the NEMA IQ phantom and a phantom holder for exact repositioning of the phantom on the systems patient table. The PET image quality parameters contrast recovery, background variability, and signal-to-noise ratio (SNR) were determined and compared for both phantom AC methods. Reconstruction parameters of an iterative 3D OP-OSEM reconstruction were optimized for highest lesion SNR in NEMA IQ phantom imaging.ResultsUsing a CT-based NEMA IQ phantom μ-map on the PET/MR system is straightforward and allowed performing accurate NEMA IQ measurements on the hybrid system. MR-based AC was determined to be insufficient for PET quantification in the tested NEMA IQ phantom because only photon attenuation caused by the MR-visible phantom filling but not the phantom housing is considered. Using the suggested CT-based AC, the highest SNR in this phantom experiment for small lesions (<= 13 mm) was obtained with 3 iterations, 21 subsets and 4 mm Gaussian filtering.ConclusionThis study suggests CT-based AC for the NEMA IQ phantom when performing PET NEMA IQ measurements on an integrated PET/MR hybrid system. The superiority of CT-based AC for this phantom is demonstrated by comparison to measurements using MR-based AC. Furthermore, optimized PET image reconstruction parameters are provided for the highest lesion SNR in NEMA IQ phantom measurements.

Highlights

In integrated PET/MR hybrid imaging the evaluation of PET performance characteristics according to the National Electrical Manufacturers Association (NEMA) standard NU 2–2007 is challenging because of incomplete MR-based attenuation correction (AC) for phantom imaging
Optimized PET image reconstruction parameters are provided for the highest lesion signal-to-noise ratio (SNR) in NEMA image quality (IQ) phantom measurements
Regarding the four smallest spheres filled with tracer activity (“radioactive spheres”) the performance values using MR-based AC are lower than the expected values for the underlying PET component when comparing as a general orientation to results reported in the literature for the Biograph mMR system [2]

Summary

Introduction

In integrated PET/MR hybrid imaging the evaluation of PET performance characteristics according to the NEMA standard NU 2–2007 is challenging because of incomplete MR-based attenuation correction (AC) for phantom imaging. For PET scanners in particular, the National Electrical Manufacturers Association (NEMA) has defined a standard to assess the performance of the tomographic system [3]. Such image quality control measurements need to be conducted in PET/MR hybrid imaging for PET performance measurements when introducing a new system [1, 2] or, on a regular basis, when monitoring quality of a specific PET system over time. Dose optimization studies that have been reported for PET/MR hybrid imaging rely on NEMA IQ phantom measurements [9] All these studies have in common that they build on accurate methods for attenuation correction (AC) of the phantoms involved

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Discussion

Conclusion