A Quantitative Evaluation of Joint Activity and Attenuation Reconstruction in TOF PET/MR Brain Imaging

Abstract

Time-of-flight (TOF) PET data provide an effective means for attenuation correction (AC) when no (or incomplete or inaccurate) attenuation information is available. Since MR scanners provide little information on photon attenuation of different tissue types, AC in hybrid PET/MR scanners has always been challenging. In this contribution, we aim at validating the activity reconstructions of the maximum-likelihood ordered-subsets activity and attenuation (OSAA) reconstruction algorithm on a patient brain data set. We present a quantitative comparison of joint reconstructions with the current clinical gold standard-ordered-subsets expectation maximizationusing CT-based AC in PET/CT, as well as the current state of the art in PET/MR, that is, zero time echo (ZTE)-based AC. Methods: The TOF PET emission data were initially used in a preprocessing stage to estimate crystal maps of efficiencies, timing offsets, and timing resolutions. Applying these additional corrections during reconstructions, OSAA, ZTE-based, and the vendor-provided atlas-based AC techniques were analyzed and compared with CT-based AC. In our initial study, we used the CT-based estimate of the expected scatter and later used the ZTE-based and OSAA attenuation estimates to compute the expected scatter contribution of the data during reconstructions. In all reconstructions, a maximum-likelihood scaling of the single-scatter simulation estimate to the emission data was used for scatter correction. The reconstruction results were analyzed in the 86 segmented regions of interest of the Hammers atlas. Results: Our quantitative analysis showed that, in practice, a tracer activity difference of 10.5% (±2.1%) and 10.1% (±2.3%) could be expected for the state-of-the-art ZTE-based and OSAA AC methods, respectively, in PET/MR compared with the clinical gold standard in PET/CT. Conclusion: Joint activity and attenuation estimation methods can provide an effective solution to the challenging AC problem for brain studies in hybrid TOF PET/MR scanners. With an accurate TOF-based (timing offsets and timing resolutions) calibration, and similar to the results of the state-of-the-art method in PET/MR, regional errors of joint TOF PET reconstructions are within a few percentage points.

Magnetic Resonance-Based Attenuation Correction and Scatter Correction in Neurological Positron Emission Tomography/Magnetic Resonance Imaging—Current Status With Emerging Applications

“…Data-driven methodologies such as MLAA benefit from accurate data corrections. Recently, it was shown that improved scatter correction is helpful for increasing the visual and quantitative accuracy of PET and could also result in improved attenuation correction with data-driven methodologies using PET and MRI [45]. Furthermore, accurate scatter correction methods could be useful in improving the quantitative accuracy of dynamic PET data with low count statistics [150], which is often the case in neuroimaging research.…”

“…A scale-corrected MLACF has recently been shown to provide images that quantitatively and visually correspond to CTAC-reconstructed PET images in 57 patients [131]. Benoit MLAA method was compared with state-of-the-art MRAC and CTAC, producing errors within a few percent [45]. The authors suggested that MLAA might be useful in patients where metal implants or other imaging challenges hinder the use of traditional segmentation-based methods.…”

A Review of Deep-Learning-Based Approaches for Attenuation Correction in Positron Emission Tomography

“…However, the knowledge of the total prior activity is necessary for MLACF to determine the constant scaling, given that no image-domain prior can be applied. In addition, a scatter estimate of the emission measurement should be assumed in the joint estimation algorithms [178].…”

PET-enabled Dual-energy CT: A Proof-of-Concept Simulation Study

“…A seminal theoretical work later proved that TOF data determine µ up to a constant [8,9]. Since then, the MLAA method has received a wide range of interests (e.g., [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]). It is worth noting that previous attention of MLAA reconstruction was focused on PET attenuation correction.…”

“…Previous attention on this topic has been given to achieve transmission-less PET imaging by excluding the X-ray CT component (e.g., [11][12][13][14][15][16]). Existing studies were also primarily aimed to improve the aspect of attenuation correction for PET activity image reconstruction for PET/CT (e.g., [17][18][19][20]) or PET/MR (e.g., [21][22][23][24][25][26]). The γ-ray attenuation image itself did not receive much attention and no work explored it for dual-energy or multienergy CT spectral imaging, which however is the focus of this paper.…”