Original ArticleEffect of PET-CT misalignment on the quantitative accuracy of cardiac 15O-water PET
Introduction
Cardiac positron emission tomography (PET) is increasingly being used in the clinical evaluation of coronary artery disease.1 Quantification of myocardial blood flow (MBF) has been shown superior over qualitative evaluations and 15O-water PET is considered the gold standard for non-invasive quantification of MBF.1, 2, 3, 4, 5, 6 For accurate estimation of tracer uptake, essential for quantification, a proper attenuation correction is required. PET/CT scanners make use of a CT scan directly before or after the PET scan to perform this correction. Misalignment between PET and CT acquisitions can induce artifacts in PET images and has been reported to be a common problem occurring in 50%-67% of cases.7,8 Misalignment can be induced by respiratory and patient body motion. CT scans are typically short and represent a ‘snap-shot’ of the respiratory cycle whereas the PET scan is averaged over many cycles, potentially leading to a misalignment, mainly in the cranio-caudal direction.9 In one study, an average motion of the heart of 8 mm during the PET acquisition was attributed directly to respiratory motion.10 in addition, patient body motion ranging between 5 and 10 mm commonly occurs.11
These motions frequently lead to a misalignment between PET and CT and induce errors in attenuation correction. In a qualitative analysis using 82Rb, false positive perfusion defects occurred and were induced by superimposed lung tissue on the anterior and lateral myocardial wall.8,12 In quantitative analysis of 82Rb, MBF was also shown to be mostly affected in the same regions but the effect extended over the whole myocardium.13
For all PET tracers except 15O-water, MBF is determined based on the uptake rate of the tracer, representing transmural MBF (MBFt). With 15O-water, however, MBF is determined primarily based on its washout rate rather than its uptake rate, representing MBF in perfusable tissue only, although 15O-water uptake rate resembles transmural MBF similarly as for the other tracers.14 In scarred regions, quantification of both MBFt and MBF when using 15O-water, adds information on scar burden in addition to ischemic burden in the perfusable and viable tissue. Since errors in attenuation correction due to misalignment affect the amplitude, but not the shape, of tissue time-activity curves, we hypothesize that misalignment may have less effects on washout-based MBF estimates for 15O-water than uptake-based MBFt that is used for other tracers.
The impact of misalignment on the quantitative accuracy of cardiac 15O-water PET has not, to the best of our knowledge, been evaluated yet in detail. Hence, the purpose of the present study was to investigate the influence of misalignment between PET and CT on the quantitative accuracy of washout-based MBF and uptake-based MBFt using 15O-water PET/CT.
Section snippets
Patients
Stress data from 10 patients referred for routine clinical assessment of MBF with 15O-water PET was used in the present work. Misalignment between PET and CT was ruled out by visual inspection (excluding 4 out of 14) and no obvious dynamic patient motion during the PET scan could be seen. Since only anonymized images were used and the present work was purely an image processing study, this study did not require ethics permission according to the Swedish Law on Medical Research in Humans.
Data acquisition
The
Results
Global stress MBF ranged between 1.0 and 4.2 mL/g/min for all patients. For 5 patients, the results from 20 mm left-lateral misalignment (X + 20) were excluded since the resulting artifacts in PTF made the myocardial wall impossible to segment. Washout-based MBF was only to a minor extent affected by misalignments but larger effects were seen in uptake-based MBFt. In any of the three coronary regions, 5 misalignments showed a significant difference for MBF and 9 misalignments for MBFt. On the
Discussion
Misalignment between PET and CT is a common problem in cardiac PET. In this study the influence of misalignment on the quantitative accuracy of MBF measurements based on 15O-water PET was investigated. Using clinical 15O-water PET/CT data, misalignment at different amplitudes in the cranio-caudal, anterior, and lateral directions was simulated.
When using 15O-water, MBF is calculated from the washout rate which should not be affected by attenuation correction. Actually, quantification of MBF
Conclusion
Misalignment between PET and CT has no significant effect on the quantitative accuracy of MBF from 15O-water PET, in all but the most extreme cases. Washout-based estimates of MBF were accurate and within 1.7% ± 1.4% and were much more robust than uptake-based estimates considering their sensitivities to misalignments. This implies that it is likely that misalignment is less of a problem for 15O-water than for other perfusion tracers. When only MBF is of interest, correction for misalignment
New knowledge gained
For assessment of ischemic burden using 15O-water PET and clearance-based MBF, image co-registration between PET and CT is not necessary.
Acknowledgements
The authors want to thank the staff at Uppsala PET Centre, especially Lars Lindsjö, Tomas Nyberg, and Hampus Romelin, for assistance with patient scanning, data handling, and production of 15O-water.
Disclosures
ML, HJH and JS are working co-founders of Medtrace Pharma A/S, Copenhagen, Denmark. No other potential conflicts of interest relevant to this article exist.
Funding
Open access funding provided by Uppsala University.
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