Magnetic resonance image intensity ratio, a normalized measure to enable interpatient comparability of left atrial fibrosis
Introduction
Catheter ablation is frequently performed for the treatment of atrial fibrillation (AF).1 Prior studies have revealed that late gadolinium enhancement (LGE) on magnetic resonance imaging (MRI) can noninvasively estimate the extent of left atrial (LA) scar before and after ablation2, 3 and that AF recurrence after ablation is associated with the extent of preablation LGE as a surrogate of LA scar.4 In a recent issue of HeartRhythm, we provided evidence to support the utility of T1 mapping for the quantification of diffuse atrial scar.5 Nonetheless, LGE-MRI remains necessary for the identification of focal scar. An intrinsic limitation of LGE, however, is that MRI signal intensity is measured in “arbitrary units” with variable magnitude and scale across examinations. Although LA wall image intensity on LGE-MRI primarily varies as a function of gadolinium retention in fibrotic regions, it is also affected by parameters such as surface coil proximity, contrast dose, delay time of image acquisition after contrast injection, patient hematocrit, glomerular filtration rate, and body mass index.6, 7 Normalization of the image intensity may decrease the variability of measurement with regard to the latter covariates. The objectives of this study were as follows: (1) to develop the image intensity ratio (IIR) as a quantitative and normalized measure of LA fibrosis, (2) to validate the IIR against invasive bipolar electrogram voltage amplitude measures, and (3) to establish IIR thresholds for the identification of abnormal LA myocardium and dense scar corresponding to bipolar voltage amplitudes <0.5 mV8 and <0.1 mV,9 respectively.
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Methods
The protocol was reviewed and approved by our institutional review board, and all participants provided written informed consent. Seventy-five consecutive patients referred for AF ablation underwent preprocedural MRI between November 2011 and December 2012.
Patient characteristics
We enrolled 75 patients with a mean age of 62.4 ± 8.3 years (range 44–73 years). Table 1 summarizes the baseline characteristics of patients. Forty-two (56%) patients had paroxysmal AF, and 43 (57.3%) were undergoing their first ablation procedure. Fifty-six (74.7%) patients were men. Eleven (14.7%) had congestive heart failure, 36 (48%) had hypertension, 5 (6.7%) had diabetes mellitus, and 4 (5.3%) had a history of transient ischemic attack or stroke. The median CHA2DS2-VASc score was 1
Discussion
The main finding of this study is that the IIR, a normalized measure of atrial myocardial LGE, is strongly associated with EAM bipolar voltage measures. To the extent that voltage amplitude correlates with atrial scar,8, 9 determination of the IIR can be used to noninvasively assess the presence and extent of atrial scar. Compared with the estimation of scar based on image intensity in typical arbitrary units with varying magnitude and scale, the IIR is a normalized measure with attributes that
Conclusions
The IIR is closely correlated with local intracardiac bipolar LA voltage measures. This measure may provide utility for interpatient and longitudinal intrapatient comparisons of LA scar burden and may ultimately improve patient selection and procedural guidance for AF ablation.
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Dr Nazarian is on the MRI Advisory Panel for Medtronic and is a scientific advisor to and principal investigator for research funding to Johns Hopkins University from Biosense Webster.
The study was funded by a Biosense Webster grant and National Institutes of Health grants K23HL089333 and R01HL116280 to Dr Nazarian, the Dr. Francis P. Chiaramonte Foundation, and the Norbert and Louise Grunwald Cardiac Arrhythmia Research Fund. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.