Clinical Evaluation of Zero-Echo-Time Attenuation Correction for Brain ¹⁸F-FDG PET/MRI: Comparison with Atlas Attenuation Correction

Abstract

Accurate attenuation correction (AC) on PET/MR is still challenging. The purpose of this study was to evaluate the clinical feasibility of AC based on fast zero-echo-time (ZTE) MRI by comparing it with the default atlas-based AC on a clinical PET/MR scanner. Methods: We recruited 10 patients with malignant diseases not located on the brain. In all patients, a clinically indicated whole-body 18 F-FDG PET/CT scan was acquired. In addition, a head PET/MR scan was obtained voluntarily. For each patient, 2 AC maps were generated from the MR images. One was atlas-AC, derived from T1-weighted liver acquisition with volume acceleration flex images (clinical standard). The other was ZTE-AC, derived from proton-density-weighted ZTE images by applying tissue segmentation and assigning continuous attenuation values to the bone. The AC map generated by PET/CT was used as a silver standard. On the basis of each AC map, PET images were reconstructed from identical raw data on the PET/MR scanner. All PET images were normalized to the SPM5 PET template. After that, these images were qualified visually and quantified in 67 volumes of interest (VOIs; automated anatomic labeling, atlas). Relative differences and absolute relative differences between PET images based on each AC were calculated. 18 F-FDG uptake in all 670 VOIs and generalized merged VOIs were compared using a paired t test. Results: Qualitative analysis shows that ZTE-AC was robust to patient variability. Nevertheless, misclassification of air and bone in mastoid and nasal areas led to the overestimation of PET in the temporal lobe and cerebellum (%diff of ZTE-AC, 2.46% ± 1.19% and 3.31% ± 1.70%, respectively). The j%diffj of all 670 VOIs on ZTE was improved by approximately 25% compared with atlas-AC (ZTE-AC vs. atlas-AC, 1.77% ± 1.41% vs. 2.44% ± 1.63%, P , 0.01). In 2 of 7 generalized VOIs, j%diffj on ZTE-AC was significantly smaller than atlas-AC (ZTE-AC vs. atlas-AC: insula and cingulate, 1.06% ± 0.67% vs. 2.22% ± 1.10%, P , 0.01; central structure, 1.03% ± 0.99% vs. 2.54% ± 1.20%, P , 0.05). Conclusion: The ZTE-AC could provide more accurate AC than clinical atlas-AC by improving the estimation of head-skull attenuation. The misclassification in mastoid and nasal areas must be addressed to prevent the overestimation of PET in regions near the skull base.

PET/MRI: Where might it replace PET/CT?

“…MRI, on the other hand, cannot directly assess tissue density and in particular has difficulty imaging the lung and bones; therefore, creating an accurate attenuation correction map remains elusive . New pulse sequences are being developed to improve attenuation maps, but have not yet become widely validated, broadly available, or FDA approved . Established reconstruction algorithms have been developed in PET/CT, allowing for reproducible quantification as evaluated with specialized phantoms 26,27; however, this remains a challenge on PET/MRI systems because these phantoms are not usable in the PET/MRI environment which requires alternative materials …”

The Effect of Susceptibility Artifacts Related to Metallic Implants on Adjacent-Lesion Assessment in Simultaneous TOF PET/MR

“…Other solutions might include, for example, whole-body AC models with bone tissue and possibly implants (31). MR sequences such as zero echo time allow the imaging of bone tissue and improve the AC maps (32)(33)(34). However, most of these new methods are still under investigation and not yet commercially available.…”

Attenuation Correction of PET/MR Imaging

“…The majority of these approaches rely on UTE or ZTE MR techniques that have very short or zero echo times 23,24,27,30,47,48 . These approaches potentially will provide lung, bone and soft tissue maps allowing for accurate patient specific attenuation correction.…”

“…A linear correlation between inverse log-scaled ZTE and CT signal has been observed for HU between −300 and 1500 HU (corresponding to soft-tissue and bone). Sekine et al employed the ZTE method proposed by Wiseinger et al and evaluated its utility in PET MRAC 27 . Continuous attenuation values were assigned to the bone-tissue using a linear regression between CT and ZTE MR values.…”