Original ArticleThe performance of phase analysis of gated SPECT myocardial perfusion imaging in the presence of perfusion defects: A simulation study
Introduction
Cardiac resynchronization therapy (CRT) is a FDA-approved therapy indicated for patients with end-stage heart failure (HF), severe left ventricular (LV) dysfunction (LV ejection fraction ≤ 35%), and prolonged QRS duration (≥120 ms).1, 2, 3, 4 Although CRT has been shown to improve the quality of life of recipients, there are still significant percentages (20-40%) of patients who do not respond favorably to CRT.2, 3, 4 A few of the emerging studies have suggested that LV mechanical dyssynchrony is an important predictor of response for patients undergoing CRT.5,6 As a result, many techniques have been developed to utilize common medical imaging modalities, such as echocardiography via tissue Doppler imaging (TDI)7 or strain imaging,8 magnetic resonance imaging,9 and gated blood-pool single photon emission computed tomography (SPECT) to measure LV mechanical dyssynchrony.10,11 Phase analysis is a technique that allows gated SPECT myocardial perfusion imaging (MPI) to measure LV mechanical dyssynchrony.12 Moreover, it can integrate dyssynchrony assessment with myocardial scar assessment using data acquired from a single acquisition of gated SPECT MPI. Studies have shown that both regional dyssynchrony and regional scar are important for optimizing LV lead placement, which is an important factor related to CRT response.13,14
Phase analysis is a count-based method that measures mechanical dyssynchrony using the linear relationship between variation of regional maximum counts and myocardial wall thickening during a cardiac cycle. A study by Galt et al.15 has shown that regional maximum counts are linearly proportional to myocardial wall thickening based on the partial volume effect. By fitting a continuous first-harmonic Fourier curve to the discrete regional maximum counts during the cardiac cycle, one can approximate the phase angle of the curve to represent onset of mechanical contraction of the region. Once a phase angle is obtained from each region, a phase distribution is formed for the assessment of LV (dys)synchrony. Phase analysis has been validated against TDI,16,17 and has been shown to predict patient response to CRT18 and identify optimal positions for CRT LV lead placement.14
Although phase analysis has shown promising clinical results, a question has been raised regarding its accuracy to measure phase information in regions of severe perfusion defect, where the true uptake counts are low. In such regions, signal-to-noise ratio (SNR) is low, i.e., the signal from true counts is not distinguishable from the erroneous counts from noise. Perfusion defects are common in patients suffering from HF with more than 60% of patients having ischemic etiology. A previous study by Cooke et al.19 has evaluated the accuracy of the count-based methodology for calculating systolic wall thickening (SWT) amplitude in regions of low count densities. That study showed that Fast Fourier Transform-derived SWT can be measured in regions of hypoperfusion as low as 5% of normal uptake. However, that study did not investigate the accuracy of the count-based methodology for calculating phase in relation to low count densities. The objective of this study is to evaluate the performance of phase analysis under varying degrees of hypoperfusion in synchronous and dyssynchronous ventricles using a digital phantom.
Section snippets
XCAT Phantom and Simulation
This study utilized the XCAT (eXtended Cardiac Torso) digital human phantom to simulate gated SPECT MPI data. The XCAT phantom uses non-uniform rational basis splines (NURBS) to mathematically define geometrical surfaces of the human anatomy.20 NURBS is commonly used in computer graphics because it is a mathematical model that defines continuous surfaces and curves with great precision. Thus, the XCAT phantom is capable of simulating activity and attenuation distribution with high spatial and
Results
Table 1 shows the phases of the 35 B-spline points with normal and abnormal perfusion uptakes. The phases with perfusion defects of 50%-10% did not significantly differ from those with normal uptake. However, once perfusion defect reached a severity of 5% of normal uptake, it resulted in significantly different phases from those with normal uptake (P < .05). The lowest SNR for the 10% uptake activity was determined to be 12.0. Once the SNR dropped below 12.0, the noise significantly affected
Discussion
This study used the XCAT digital phantom and a modified phase analysis algorithm to characterize the performance of phase analysis in perfusion defects. The phase analysis algorithm accurately measured phases and phase shifts in perfusion defects with as low as 10% of normal perfusion uptake. Such defect level corresponded to a SNR of 12.0 or greater within the defect region. For the dataset with perfusion defects with 5% of normal uptake activity, which corresponded to a regional SNR below
Conclusion
Phase analysis is capable of measuring phases in regions with abnormal perfusion uptake as low as 10% of the perfusion uptake in the normal regions, which corresponded to a regional SNR of 12.0 or greater. In 42 consecutive patients with myocardial infarction >20% of the left ventricle, only two patients had a SNR within the perfusion defects below that threshold.
Acknowledgments
This study was supported in part by an NIH grant (1R01HL094438, PI: Ji Chen, PhD). The authors thank Dr William P. Segars, Duke University, for his contribution to the modified XCAT phantom.
Conflict of interest
Dr Faber, Dr Garcia, and Dr Chen receive royalties from the sale of the Emory Cardiac Toolbox with SyncTool. The terms of this arrangement have been reviewed and approved by Emory University in accordance with its conflict-of-interest practice.
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