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Impact of motion on simultaneously acquired PET/MRI of myocardial infarcted heart.1Lawson Health Research Institute, London, ON, Canada, 2Medical Biophysics, Western University, London, ON, Canada, 3Siemens Healthcare Limited, Oakville, ON, Canada, 4MyHealth Centre, Arva, ON, Canada
Synopsis
This study investigates the impact of motion in myocardial PET/MRI data by registering pre-mortem images to post-mortem images from a simultaneously acquired PET/MRI scan. After a registration in MRI, PET data is transformed and analyzed using the Patlak model. There are significant differences and correlation between pre- and post-mortem PET images. There are discrepancies in different segments of heart, but none were found to be significantly different. These results suggest that the level of inflammation in the heart might be misrepresented. Further comparison with motion corrected images will provide more concrete discrepancy due to motion for myocardial PET/MRI imaging.
Introduction
A myocardial infarction (MI) is an adverse health episode that may leads to other potential health risks such as recurrent heart failure.1 A sustained pro-inflammatory response is associated with detrimental ventricular remodeling post-MI and plays a significant role in heart failure.2 Therefore, it is essential to quantify the progression of post-MI inflammation to optimize therapeutic interventions and improve prognosis post-MI. Both MRI and PET are complementary imaging modalities to determine the presence and severity of injuries post-MI. MRI provides soft tissue contrast of heart muscles. Thus, it is excellent for detecting viability and functionality of tissue with the help of contrast agents.3 Furthermore, 18FDG PET, can quantify molecular uptake of glucose in both tissue and activated macrophages associated with the inflammatory response.4 However, despite the usefulness of these imaging modalities, heart and lung motion produce considerable motion artifacts during imaging. These motion artifacts can lead to poor image quality for further analysis of MI.5 In this study, we investigated the impact of motion in dynamic 18FDG-PET images of a canine MI model by registering pre- and post-mortem images acquired with simultaneous PET/MRI.Methods
The canine MI model has been approved by the Animal Care Committee of Western University. The animal was induced by permanently placing a snare ligature around the left anterior descending coronary artery during left thoracotomy. After 5 days MI was induced, animals were imaged. Cardiac PET/MRI was acquired simultaneously pre- and post-mortem in a Biograph mMR (Siemens Healthcare). For MRI, 3D T1-weighted images were acquired with a 3D inversion recovery sequence (ECG-triggered with navigator echo for respiratory gating, 0.625 x 0.625 x 0.900 mm3, TE = 1.18 ms, TR = 419.29 ms, inversion time = 280 ms). List-mode PET was acquired throughout a constant infusion of 18FDG (0.17MBq/min/kg) for 150 mins pre-mortem. In order to suppress normal myocardial uptake of 18FDG, heparin (2000 units) and lipid (Intralipid, 0.25ml/min/kg) were infused at 40 mins and 50 mins respectively into the 18FDG infusion. Pre-mortem MRI 3D T1-weighted images were manually registered to post-mortem MRI 3D T1-weighted images using 3D Slicer (version 4.10.2) with the ThinPlate landmark transformation. Then, same transformation matrix was applied to pre-mortem dynamic PET images. Dynamic PET images (pre- and post-mortem) were then analyzed using Carimas (version 2.10, Turku PET center, Finland). Net Influx rate (Ki) of 18FDG in the heart were calculated using the Patlak model6 at three time points; (before the suppression, during the suppression and after the suppression.Results
Two animals have been analyzed. A showcase of MRI manual registration is shown in figure 1. Overall, higher Ki has been observed after the transformation as shown in figure 2 a) & b). There is a significant difference (p ~ 0.034) between Ki derived from original and transformed PET. Furthermore, original and transformed PET show significant correlation (p < 0.001) with correlation coefficient = 0.959 as shown in figure 2 c). Ki appears to be higher in all 4 segments of heart before and during the lipid infusion in figure 3, however, no significant changes are noted.Discussion
There are many structural differences between pre- and post-mortem cardiac MRI images (figure 1). These differences are driven by the motion of heart and breathing which post-mortem images lack there of. In MRI, these images can be registered by well-defined structures with smooth soft tissue contrast, but it is difficult to do so in PET. Simultaneous PET/MRI imaging helps to circumvent this issue with inherently co-registered PET and MRI images. Studies have shown that uptake of 18FDG in macrophages can be an indication of myocardial inflammation.4 A high correlation between original and transformed images (figure 2 c) ) indicates transformed Ki represents similar trends as the original. At the same time, transformed Ki is significantly different from the original Ki in which may imply different levels of inflammation in the heart. Also, these Ki differences are much more apparent before and during the lipid infusion, suggesting that MRI-guided registration of dynamic PET data can alter the estimated level of inflammation at different time points. Although the registration of pre-mortem to post-mortem images is not perfect, our results demonstrate the impact of motion on estimated Ki derived from dynamic 18FDG-PET data. Further comparison to motion corrected images are planned in the future and will inform an automated MRI-guided motion correction strategy for simultaneous cardiac PET/MRI.Conclusion
This study has successfully registered simultaneously acquired pre-mortem and post-mortem cardiac PET/MRI data. There is a significant difference in PET Ki with high correlation between pre-mortem and post-mortem images. There were also different Ki values dependant on the lipid infusion. Further comparison with motion corrected images will establish the standard that can be used to conclude the impact of motion in PET/MRI data.Acknowledgements
We would like to thank Heather Biernaski for simultaneous PET/MRI acquisitions. Heeseung Lim is supported through a MITACS post doctoral fellowship program that is funded in part by Siemens Healthcare. Benjamin Wilk. is supported by an Ontario Graduate scholarship and a Lawson Internal Research Fund. This work was supported in part by Ontario Research Fund RE7-021 and Canadian Foundation for Innovation no. 11358.References
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2. Wilk B, Wisenberg G, Dharmakumar R, Thiessen JD, Goldhawk DE, Prato FS. Hybrid PET/MR imaging in myocardial inflammation post-myocardial infarction. J Nucl Cardiol. 2019.
3. Pereira RS, Prato FS, Sykes J, Wisenberg G. Assessment of myocardial viability using MRI during a constant infusion of Gd‐DTPA: further studies at early and late periods of reperfusion. Magnetic Resonance in Medicine: An Official Journal of the International Society for Magnetic Resonance in Medicine. 1999;42(1):60-68.
4. Prato FS, Butler J, Sykes J, et al. Can the inflammatory response be evaluated using 18F-FDG within zones of microvascular obstruction after myocardial infarction? Journal of Nuclear Medicine. 2015;56(2):299-304.
5. Fürst S, Grimm R, Hong I, et al. Motion correction strategies for integrated PET/MR. Journal of nuclear medicine. 2015;56(2):261-269.
6. Patlak CS, Blasberg RG, Fenstermacher JD. Graphical evaluation of blood-to-brain transfer constants from multiple-time uptake data. Journal of Cerebral Blood Flow & Metabolism. 1983;3(1):1-7..
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