Purpose

Attenuation correction (AC) in whole-body simultaneous positron emission tomography/magnetic resonance imaging (PET/MRI) is usually performed using a two-point DIXON T1w sequence [1] with reliable results compared to PET/CT demonstrated for more than 2300 patients [2]. However, the DIXON stacks acquired are only usuable for AC because its resolution in axial orientation (4.1 x 2.6 x 3.1 mm³) is not optimal for diagnostic assessment. The present study explores the value and reproducibility of a novel magnetic resonance based attenuation correction (MRAC) using a high-resolution (1.9 x 1.4 x 3.2 mm³) CAIPIRINHA-accelerated T1-weighted Dixon-based prototype (MRAC_caipi) for PET/MRI compared to the clinical standard (MRAC_std).

Materials and Methods

All measurements were performed on a 3 Tesla Siemens Biograph mMR (Erlangen, Germany), capable of whole-body imaging. The study was approved by the institutional review board. A prototype CAIPIRINHA VIBE sequence implementation by Siemens Healthineers (Erlangen, Germany) was used allowing for higher parallel imaging acceleration factors on the MR side along with the CAIPIRINHA delta shift parameters to twist the undersampling pattern in the 2 phase encoding directions. The reconstruction pipeline includes inline Dixon fat/water separation as well as the 4-compartment MR-based attenuation correction segmentation algorithm adapted to higher resolution input images. The prototype sequence also supports inline reconstruction of PET AC images from parallel PET acquisitions. CAIPIRINHA-settings were: acceleration factor PE 2, acceleration factor 3D 2, delta shift PAR 1, ref lines PE 24, ref lines PAR, slice oversampling 40%, and 80 slices per slab. PET data were iteratively (3D) reconstructed using an ordered subsets expectation maximization (OSEM) algorithm with 3 iterations, 21 subsets and Gaussian post-filtering with a full-width at half-maximum 2.The PET raw data of 19 patients from clinical routine with various tracers were reconstructed with MRAC_std and the novel CAIPIRINHA-accelerated MRAC_caipi, both 19 s/bed position. Volume of interests (VOIs) for liver, lung and all voxels of the total image stack were created to calculate standardized uptake values (SUV_mean) followed by inter-method agreement using Passing-Bablok regression and Bland-Altman plot. A voxel-wise SUV comparison per patient was performed for intra-individual correlation between MRAC_std and MRAC_caipi. Difference images of the attenuation maps (MRAC_std-MRAC_caipi) and SUV images were created. A healthy volunteer was scanned repeatedly for 5 times (no tracer injection) in one session over 7 bed-positions for whole-body assessment with both MRAC_std and MRAC_caipi. The image quality of MRAC_caipi-derived sequences was assessed by two blinded board-certified radiologists on a Likert-scale (1-5) including intra-class coefficients for inter-reader agreement

Results

SUV_mean correlations of VOIs were close to linearity with high significance (0.95<Spearman’s rho<1, p<0.0001, respectively). See Figure 1 for SUV_mean comparison of all patients of the complete image stack. Scatter-plots of voxel-wise comparison per patient showed significant linearity (Figure 2, 1.79x10⁸ pixels were evaluated in total). Points of deviation from the center line could be explained by different segmentation of air-containing tissue, lungs, kidneys, metal implants, diaphragm edge or small air bubbles in the gastrointestinal tracts that moved between MRAC acquisitions. The repeated-measurements experiment with one individual demonstrated small differences at the edges of the diaphragm (slightly different level of breath-hold positions in between the acquisitions) and air bubbles in the bowel that moved between the acquisition of MRAC_std and MRAC_caipi. Nasal sinuses and the trachea were better segmented in MRAC_caipi (Figure 3). High-resolution T1w Dixon images were acquired in all cases and could be used for PET-fusion purposes (Figure 4,5). The diagnostic images derived from MRAC_caipi were of high diagnostic quality (4.2±0.8) with 0.92-0.96 intra-class correlation.

Discussion

The novel protoype MRAC_caipi extends the value for attenuation correction in PET/MRI by providing a high-resolution image series in DIXON technique suited for diagnostic assessment. The whole setup is performed in the same amount of time as the clinical standard (19s/bed position). The SUV quantification resulting from the MR-based µmap was shown to be reproducible and robust. While some anatomical parts are better delineated in the new µmap from MRAC_caipi, breathing artifacts may sometimes occur in the thorax that transfer on the µmap. These artifacts need to be checked by technician and physician in the µmap although artifact-free images may be easily acquired in compliant patients. The novel protoype is useful for clinical PET/MRI studies towards time-efficient protocols as a usually separately acquired T1w-sequence may be omitted.

Acknowledgements

No acknowledgement found.