Hybrid ZTE/Dixon MR-based attenuation correction for quantitative uptake estimation of pelvic lesions in PET/MRI

Abstract

Purpose: This study introduces a new hybrid ZTE/Dixon MR-based attenuation correction (MRAC) method including bone density estimation for PET/MRI and quantifies the effects of bone attenuation on metastatic lesion uptake in the pelvis.

Methods: Six patients with pelvic lesions were scanned using fluorodeoxyglucose (18F-FDG) in an integrated time-of-flight (TOF) PET/MRI system. For PET attenuation correction, MR imaging consisted of two-point Dixon and zero echo-time (ZTE) pulse sequences. A continuous-value fat and water pseudoCT was generated from a two-point Dixon MRI. Bone was segmented from the ZTE images and converted to Hounsfield units (HU) using a continuous two-segment piecewise linear model based on ZTE MRI intensity. The HU values were converted to linear attenuation coefficients (LAC) using a bilinear model. The bone voxels of the Dixon-based pseudoCT were replaced by the ZTE-derived bone to produce the hybrid ZTE/Dixon pseudoCT. The three different AC maps (Dixon, hybrid ZTE/Dixon, CTAC) were used to reconstruct PET images using a TOF-ordered subset expectation maximization algorithm with a point-spread function model. Metastatic lesions were separated into two classes, bone lesions and soft tissue lesions, and analyzed. The MRAC methods were compared using a root-mean-squared error (RMSE), where the registered CTAC was taken as ground truth.

Results: The RMSE of the maximum standardized uptake values (SUV_max ) is 11.02% and 7.79% for bone (N = 6) and soft tissue lesions (N = 8), respectively, using Dixon MRAC. The RMSE of SUVmax for these lesions is significantly reduced to 3.28% and 3.94% when using the new hybrid ZTE/Dixon MRAC. Additionally, the RMSE for PET SUVs across the entire pelvis and all patients are 8.76% and 4.18%, for the Dixon and hybrid ZTE/Dixon MRAC methods, respectively.

Conclusion: A hybrid ZTE/Dixon MRAC method was developed and applied to pelvic regions in an integrated TOF PET/MRI, demonstrating improved MRAC. This new method included bone density estimation, through which PET quantification is improved.

Keywords: MRAC; Dixon MRI; TOF-PET/MRI; pelvis; zero echo-time (ZTE) MRI.

Figure 1

Flowchart summarizing the methodology of the paper. Registered CT images are used as ground truth. The CT and pseudoCT images are converted to AC maps with a bilinear model. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 2

Example images of each step of the bone segmentation process are shown in (a). 3‐D rendering of the ZTE‐segmented bone (c) shows strong visual similarity to CT‐segmented bone (b). [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 3

Coregistered ZTE (a) and CT (b) images were used to produce a linear model to convert the normalized ZTE signal intensity to Hounsfield units. Only bone voxels as defined by the bone mask (c) are used in the modeling. Iteratively reweighted least squares fitting was used to produce the continuous two‐segment piecewise linear model (d). The hybridization step combines the Dixon pseudoCT (e) and ZTE‐segmented continuous‐valued bone (f) using the linear model in (d) to produce the hybrid ZTE/Dixon pseudoCT (g). [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 4

Dixon pseudoCT (a, top row) and MRAC (b, top row) and hybrid ZTE/Dixon pseudoCT (a, middle row) and MRAC (b, middle row), vs CT (a, bottom row) and CTAC (c, bottom row) for patient #1. The ZTE‐derived bone from Fig. 2 is added to the Dixon pseudoCT using the model proposed in Fig. 3. The bowel air and arms from the Dixon pseudoCT are copied to CT images to account for differences in air distribution and arm positioning.

Figure 5

Difference images between MR‐based attenuation coefficient maps using Dixon MRAC (a, top row) and hybrid MRAC (a, middle row) and (b) joint histograms comparing to CTAC for patient #1. The underestimation of bone is seen in the difference images and the joint histogram for Dixon MRAC. This underestimation is largely eliminated by the addition of bone information in hybrid ZTE/Dixon MRAC. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 6

Difference images comparing Dixon PET (a, middle row), hybrid PET (a, bottom row) with CTAC PET (a, top row), and joint histograms (b) with CTAC PET for patient #1. PET images from all AC methods are shown in Supplementary Figure S1. Uptake estimation in and around bone is underestimated in Dixon PET and is minimized by the addition of ZTE bone. The joint histograms indicate that Dixon PET is underestimating uptake over the whole image and is largely corrected by the addition of ZTE bone. Uptake in certain regions in the hybrid PET image is overestimated suggesting that bone is overestimated around those regions. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 7

(a) Representative 18F‐FDG‐PET images overlaid on the registered CT images used for reconstruction, where arrows denote the metastatic lesions. (b) Scatter plots of pelvic lesion voxels in all patients between Dixon PET (left) and hybrid ZTE/Dixon PET (right) vs CTAC PET. The slope of the least‐squares line in the scatter plots shows strong agreement of lesion uptake between the hybrid ZTE/Dixon MRAC and CTAC reconstructions. (c) Box plots of the SUVmax for bone (left) and soft tissue lesions (right). With the addition of bone in the hybrid ZTE/Dixon PET, the uptake of malignant lesions is better approximated as can be seen in (b) and (c). [Colour figure can be viewed at wileyonlinelibrary.com]