Motion-corrected simultaneous cardiac positron emission tomography and coronary MR angiography with high acquisition efficiency

Abstract

Purpose: Develop a framework for efficient free-breathing simultaneous whole-heart coronary magnetic resonance angiography (CMRA) and cardiac positron emission tomography (PET) on a 3 Tesla PET-MR system.

Methods: An acquisition that enables nonrigid motion correction of both CMRA and PET has been developed. The proposed method estimates translational motion from low-resolution 2D MR image navigators acquired at each heartbeat and 3D nonrigid respiratory motion between different respiratory bins from the CMRA data itself. Estimated motion is used for correcting the CMRA as well as the emission and attenuation PET data sets to the same respiratory position. The CMRA approach was studied in 10 healthy subjects and compared for both left and right coronary arteries (LCA, RCA) against a reference scan with diaphragmatic navigator gating and tracking. The PET-CMRA approach was tested in 5 oncology patients with ¹⁸ F-FDG myocardial uptake. PET images were compared against uncorrected and gated PET reconstructions.

Results: For the healthy subjects, no statistically significant differences in vessel length and sharpness (P > 0.01) were observed between the proposed approach and the reference acquisition with navigator gating and tracking, although data acquisition was significantly shorter. The proposed approach improved CMRA vessel sharpness by 37.9% and 49.1% (LCA, RCA) and vessel length by 48.0% and 36.7% (LCA, RCA) in comparison with no motion correction for all the subjects. Motion-corrected PET images showed improved sharpness of the myocardium compared to uncorrected reconstructions and reduced noise compared to gated reconstructions.

Conclusion: Feasibility of a new respiratory motion-compensated simultaneous cardiac PET-CMRA acquisition has been demonstrated. Magn Reson Med 79:339-350, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Keywords: cardiac PET-MR; coronary MRA; myocardial PET; nonrigid motion correction.

Figure 1

Simultaneous cardiac PET‐MR acquisition scheme. Before the CMRA‐PET acquisition, a μ‐map is acquired for attenuation correction of the PET data and a 2D CINE image is acquired to define a subject‐dependent trigger delay and acquisition window for the CMRA acquisition. 3D CMRA data are acquired using a golden‐step Cartesian spiral profile order sampling trajectory (one spiral interleaf per heartbeat), and a low‐resolution 2D image navigator (2D iNAV) is acquired at each cardiac cycle using spatially encoded low flip angle start‐up echoes at the beginning of the CMRA acquisition. T₂ preparation (T₂prep) and fat saturation (FatSat) pulses are applied before CMRA acquisition to improve contrast between the coronary arteries and the surrounding tissues. List‐mode PET data are acquired during the whole CMRA acquisition.

Figure 2

Motion‐corrected PET‐MR reconstruction scheme. The foot‐head translational motion estimated from the iNAVs, acquired at each cardiac cycle, is used to bin the acquired MR and PET data into different respiratory windows. Reconstructed MR images at each respiratory position are used to estimate 3D nonrigid motion fields that are then used to correct both PET (emission and attenuation) and CMRA data.

Figure 3

Reformatted images for 3 representative healthy subjects (rows) showing NMC, translational motion corrected (TC), translational plus nonrigid motion‐corrected (TC+GMD), and gated and tracked (Gated) images. Improvements in the visualization of the distal part of the RCA and LAD can be observed when applying TC and TC+GMD in comparison to NMC. Because the motion is tracked near the apex of the heart (Fig 1, in red over iNAV), TC produces a loss in definition of the proximal LAD for subject 1. The TC+GMD approach produces images of quality comparable to the Gated images.

Figure 4

Vessel length along the RCA (a) and LAD (b) arteries for 10 healthy subjects for NMC, TC, and TC+GMD. Each measure is normalised to the length observed in the corresponding Gated image. *Statistically significant difference with P < 0.01 compared to the Gated images.

Figure 5

Vessel sharpness of the RCA and LAD for 10 healthy subjects for NMC, TC, TC+GMD, and Gated. (a,c) Vessel sharpness for the first 4 cm (a) and full length (c) of the RCA. (b,d) Vessel sharpness for the first 4 cm (b) and full length (d) of the LAD. *Statistically significant difference with P < 0.01 compared to the Gated images.

Figure 6

Reformatted images for 3 representative oncology patients (rows) showing NMC, TC, and TC+GMD images. Extreme motion prevents visualizing the LAD in patient 2. Improvements in the visualization of the vessels are observed when applying TC, and further improvements are observed with TC+GMD in all cases.

Figure 7

Image metrics for the RCA and LAD arteries for 5 oncology patients for NMC, TC, and TC+GMD. (a,d) Vessel length along the RCA (a) and LAD (d). (b,c) Vessel sharpness for the first 4 cm (b) and full length (c) of the RCA. (e,f) Vessel sharpness for the first 4 cm (e) and full length (f) of the LAD.

Figure 8

Coronal slice for 5 oncology patients (rows) showing NMC, Gated, and MC PET images, alongside with profiles across the myocardium. MC improves the sharpness of the myocardium compared to NMC and reduces noise compared to Gated.

Figure 9

Mean and CV of the SUV for NMC, Gated, and MC PET images in a spherical ROI within the myocardium of 10‐mm diameter. Each reconstructed image was smoothed after reconstruction with a 4‐mm Gaussian filter, and both the unsmoothed and smoothed images were analyzed. For all patients, MC outperforms Gated and NMC in terms of noise (ie, less CV) and outperforms NMC in terms of mean SUV, suggesting an increased sharpness.

Figure 10

Example coronal, sagittal, and transverse views of fused motion compensated cardiac PET‐CMRA images for (a) patient 3 and (b) patient 5.