In this study, we demonstrated for the first time the feasibility for identification of left atrium 123I-mIBG discrete uptake areas (DUA) in healthy individuals using co-registered images from 123I-mIBG solid-state SPECT and a corresponding CT. Also, we evaluated imaging patterns of atrial and ventricular cardiac sympathetic innervation and their variation over time in a small cohort of healthy individuals.
Originally developed as a radionuclide for imaging adrenal tumors, 123I-mIBG has emerged as a promising tool for the prediction of heart failure progression , arrhythmic events , and even prognosis of AF recurrence after interventional treatment . More recently, imaging with dedicated cardiac SPECT cameras using solid-state CZT detectors has been shown to identify discrete areas of sympathetic activity that correlate with GPs identified invasively using high frequency stimulation (HFS) .
Global and regional sympathetic cardiac innervation
Late H/M ratio determination using the planar images from Anger SPECT cameras (A-SPECT) is a well-standardized and reproducible parameter in assessment of global sympathetic cardiac innervation and its prognostic value is widely recognized. However, previous studies using A-SPECT cameras have shown that the acquisition protocol used significantly affects quantification of late 123I-mIBG H/M ratio. In particular the collimators used and the stopping power of the detector material may have an impact on the quantification of the 123I-mIBG H/M ratio .
Also, it has been noted that there are a growing number of nuclear medicine sites that are using the new generation of cardiac-centered CZT-based gamma cameras for routine clinical practice . That suggests the importance of developing further clinical evidence regarding the assessment of 123I-mIBG cardiac uptake features using such dedicated cardiac SPECT cameras. It has been demonstrated that these cameras have a better count detection sensitivity and improved energy resolution, enabling reductions in acquisition times and injected radiopharmaceutical doses [5, 6]. However, there are only a small number of studies evaluating cardiac sympathetic innervation imaging with these new generation detectors [21–23] and only a few studies have compared H/M ratio determined using CZT acquisition versus that determined using A-SPECT [13, 14]. Bellevre et al. demonstrated that determination of the late H/M ratio of 123I-MIBG uptake using a parallel-collimator CZT camera (D-SPECT, Spectrum Dynamics Medical, Caesarea, Israel) was feasible, reliable and equivalent to H/M ratio using A-SPECT in patients with heart failure . It has also been noted that H/M ratio values from D-SPECT were significantly higher than those from A-SPECT, however after applying a correction factor, there was no significant difference between A-SPECT H/M ratio and corrected D-SPECT H/M ratio. In contrast, Blaire et al. using a multi-pinhole CZT camera (Discovery NM 530c, GE Healthcare) found almost perfect concordance between H/M ratio derived from transaxial SPECT images and H/M ratio from planar ones however concordance between H/M ratio from re-projected images and planar ones was only moderate. Blaire et al. considered that was related to the multi-pinhole collimation, which is responsible for a truncation artifact that interferes with the mean counts of the myocardial ROI .
In our study we used the methodology of H/M ratio calculation proposed Bellevre et al. . Thus, quantitative characteristics of global sympathetic activity (H/M ratio, WR) from our study are comparable to those in a recent study of healthy adults  that used planar images for calculation and are lower than reported in ADMIRE-HF study controls participants that used SPECT-derived data . This fact may sustain the methodology of H/M ratio determination using planogram images and a correction factor as the data source for the cardiac dedicated parallel-collimator CZT camera (D-SPECT, Spectrum Dynamics Medical, Caesarea, Israel) used in this study.
A recent study of healthy adults (n = 15) with mean age 54.6 ± 5.4 reported heterogeneous patterns of regional 123I-mIBG uptake predominantly affecting the LV apex, base and inferior wall . This is in concordance with earlier observational studies that reported the effects of age and sex on myocardial 123-mIBG uptake [26, 27]. The recent ADMIRE-HF study, however, failed to show an influence of age on 123I-mIBG uptake in older healthy controls . In our study, we also observed a slight decreasing of 123I-mIBG regional myocardial uptake comprised mostly of mild uptake reduction in LV apex and LV inferior wall segments. The low median age of our participants (31 years [26;41]) makes, however, the abovementioned age-related denervation an unlikely contributor to our findings. Our study subjects also did not have hypertension, diabetes mellitus or any other disease known to affect the autonomic nervous system. Decreased regional myocardial 123I-mIBG uptake has previously been reported in athletes, in the setting of both normal heart rate and sinus bradycardia . In this cohort, inferior, apical and septal defects were all demonstrated, with a significant reduction in percentage regional 123I-mIBG uptake in the inferior region for athletes with sinus bradycardia. Additionally, a normal SPECT database accumulated by the Japanese Society of Nuclear Medicine (JSNM) working group evaluating normal values and standardization of parameters in nuclear cardiology found patterns of lower count distribution of 123I-mIBG, particularly in the inferior region , as we have shown in the cohort of healthy individuals studied here. Hence, the slight decreases in 123I-mIBG regional myocardial uptake in our study subjects can be interpreted as a heterogeneous pattern of regional myocardial 123I-mIBG uptake and indicate normal physiological variation.
Co-registered 123I-mIBG SPECT/CT data
According to the results of a recent study, pre-operative non-invasive imaging of 123I-mIBG DUAs may be helpful in targeting locations for LAGP ablation as a part of interventional AF treatment. This may potentially increase the efficacy of the ablation procedure without touching viable myocardium during LAGP detection using HFS . Hence, knowledge of the distribution and characteristics of DUAs in humans without cardiovascular disease may be helpful for future studies of image-guided AF ablation procedures.
Our study of healthy individuals demonstrates the presence of discrete uptake of 123I-mIBG, a radiolabeled analog of sympathetic neuronal transmitter norepinephrine, around the left atrium. This fact allows us to assume the presence of sympathetic activity areas in sites that correspond to known anatomical clusters of LAGP. The Oklahoma research group have localized and named the major sites around the left atrium that contain autonomic neurons based on response to HFS, thereby facilitating improved communication among clinical electrophysiologists. These are the anterior right GP, the superior left GP, the right and left inferior GP and the ligament of Marshal . Several studies have shown that LAGPs contain both sympathetic and parasympathetic elements, as well as a variety of neuropeptides and neuromodulators [31, 32]. Hence, physiological 123I-mIBG uptake in typical anatomical sites serves as a marker for LAGP location. For ethical reasons, we did not proceed to confirm the DUAs identified in our normal patient cohort invasively with HFS, but this has been done in patients with paroxysmal AF .
In comparison with a recent study of AF patients (n = 21)  we found a much smaller number of DUAs in healthy individuals (4 [3-4.5] vs 1 [1;1]). However, it should be noted that discrete 123I-mIBG uptake could indicate only the presence of local sympathetic neurotransmission functional activity, but not the presence of anatomical (or material) structures. The relationship between left atrial sympathetic activity and atrial fibrillation requires further investigation but our data could suggest that healthy individuals seem to have fewer functionally active GPs than has been reported for patients with AF.
In our study, we demonstrate that DUAs corresponding to the described anatomical LAGP sites are more frequently reproduced in follow-up studies (10/16 (63%)) than DUAs located in other LA areas, corroborating the idea that these sites reflect true LAGPs. It seems that in general, the only slight variations in reproducibility of DUAs imaging, hypothetically could be related with functional changes in CANS activity.