The myocardial sympathetic nervous system is activated in patients with heart failure (HF) and this activation has been shown to be associated with increased mortality [¹]. Cardiac sympathetic hyperactivity can be scintigraphically visualized by ¹²³I-metaiodobenzylguanidine (MIBG), a radiolabeled analogue of noradrenalin. This noninvasive technique has been demonstrated to be a powerful prognostic marker in HF patients [²–⁴].

In myocardial ¹²³I-MIBG imaging, the most commonly used quantitative parameters are the heart to mediastinum (H/M) ratio and washout (WO) determined from planar images. The H/M is a measure of specific to nonspecific uptake, while the WO is a measure of neuronal integrity. A basic assumption associated with use of the mediastinum as a reference for H/M and WO calculations is that the counts in this region represent nonspecific binding of the radioligand.

One potential confounder for assessment of nonspecific ¹²³I-MIBG background activity is residual tracer in the blood. Since the mediastinum contains a relatively large volume of vascular structures, intravascular ¹²³I-MIBG activity may contribute to total mediastinal activity which may influence the quantification of the H/M ratio. Since the clearance rate of ¹²³I-MIBG from the blood is largely dependent on renal function [⁵, ⁶], and renal dysfunction is often present in HF patients, differences in the rate of vascular clearance may also contribute to increased interindividual variation in uptake quantification [⁷, ⁸].

The objective of this study was to assess the magnitude of the influence of residual vascular ¹²³I-MIBG activity on image quantification by examining the relationship between changes in vascular activity and changes in heart (H) and mediastinal (M) activity between early and late planar ¹²³I-MIBG images.

As part of a prospective multicenter trial [⁹], 51 subjects (48 male, 3 female; mean age 65.2) with ischemic heart disease (history of ≥ 1 myocardial infarction) underwent myocardial ¹²³I-MIBG imaging. The study protocol conformed to the ethical guidelines of the 1975 Declaration of Helsinki as reflected in a priori approval by each institution's human research committee. All subjects signed informed consent before performance of any study procedure.

Image analysis

An experienced nuclear medicine technologist processed all the planar images to determine the H and M count densities. The heart region of interest (ROI) was drawn manually to include both ventricles and any atrial activity that was clearly visible. A 7 × 7 pixel square mediastinal ROI was drawn in the upper mediastinum, using the apices of the lungs as anatomic landmarks (Fig. 1). Activity per ROI (mean counts/pixel) was corrected for decay to the time of injection and expressed as activity in the myocardium and mediastinum at 15 min and 4 h p.i. (H_e, H_l, H_l/H_e, M_e, M_l, and M_l/M_e). In addition, commonly used semiquantitative ¹²³I-MIBG myocardial parameters were assessed to better describe the clinical condition of the subjects. Early (15 min p.i.) and late (4 h p.i.) H/M ratios were calculated as the ratios of the mean counts per pixel in the two ROIs. All images and ROIs were reviewed by three independent readers, and a single aggregate H/M was derived for each image, either the value accepted by at least two readers, or if this criterion was not satisfied, the average H/M for all readers [⁹]. In addition, myocardial WO was calculated as:

[Formula ID: Equa]
[\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \left\{ {\frac{{\left( {early{ }H/M - late{ }H/M} \right)}}{{early{ }H/M}}} \right\} \times 100\% $$\end{document}]

Determination of renal function

Serum concentrations of creatinine were determined from blood samples obtained as part of screening evaluations performed within 7 days prior to ¹²³I-MIBG imaging. Analyses were performed at a central laboratory, with reference ranges of 75–111 μmol/l for men and 53–106 μmol/l for women.

Renal function was estimated using two methods. Estimated creatinine clearance (e-CC) was calculated (in ml/min) using the Cockcroft-Gault equation [¹⁰]:

[Formula ID: Equb]
[\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ e - CC = \frac{{\left( {140 - \left[ {age\;\left( {years} \right)} \right]} \right) \times \left[ {weight\;\left( {kg} \right)} \right]}}{{\left[ {serum{ }creatinine{ }\left( {\mu mol/L} \right)} \right]}} \times \left( {1.04\;for\;females\;and\;1.23\;for\;males} \right) $$\end{document}]

Estimated glomerular filtration rate (e-GFR) was calculated using the abbreviated Modification of Diet in Renal Disease (MDRD) equation [¹¹]:

[Formula ID: Equc]
[\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ e - GFR = 32788 \times {\left[ {serum\;creatinine\;\left( {\mu mol/L} \right)} \right]^{{ - 1.154\;}}} \times \;{\left[ {age\;\left( {years} \right)} \right]^{{ - 0.203}}} \times \left[ {0.742\;for\;females} \right] \times \left[ {1.212\;for\;blacks} \right] $$\end{document}]

e-GFR was expressed per 1.73 m² of body surface area (ml/min per 1.73 m²). According to the guidelines for identification, management, and referral of adults with chronic kidney disease, patients were stratified as having impaired kidney function [e-CC or e-GFR < 60 ml/min(per 1.73 m²)] or normal function {e-CC or e-GFR [≥ 60 ml/min(per 1.73 m²)]} [¹²].

Demographic and cardiac medical history information for the 51 subjects included in the study is summarized in Table 1. The majority of patients was male, had New York Heart Association (NYHA) class II HF, and had left ventricular ejection fraction (LVEF) < 40%. Eight patients had no history of HF. The majority of subjects were on a combination of beta-adrenergic receptor blockers and angiotensin-converting enzyme (ACE) inhibitors or angiotensin II (AT-II) receptor blockers (Table 1).

Estimates of renal function showed a substantial variation (Table 1). In 26 (51%) patients creatinine levels were above the normal limit (25 men and 1 woman). Calculated e-CC was < 60 ml/min in 19 (38%) patients and the calculated e-GFR was < 60 ml/min per 1.73 m² in 24 (47%) patients.

There was a considerable range in the initial (i.e., fast) part of the ¹²³I-MIBG vascular clearance curves (Fig. 3). Despite this individual variation, the mean S_f was more than 60 times steeper compared to the mean S_s (Table 2). Mean intravascular activity (cpm/ml) decreased approximately 65% between 15 min and 4 h p.i. (Table 3). The mean scintigraphically determined myocardial and mediastinal activity (counts/min) decreased approximately 37 and 33%, respectively, between 15 min and 4 h p.i. (Table 3).

Variation in any of the scintigraphic parameters (M_e, M_l, M_l/M_e, H_e, H_l, and H_l/H_e) could not be explained by intravascular activity (Table 4). At most the counts in the blood at 15 min (V_e) could explain approximately 3% of the variation in the mediastinal counts at 15 min (M_e) (p = 0.120). Variation of the scintigraphic parameters could also not be explained by the slope of the clearance curves (Table 5). The slope of the clearance curves could at best explain up to 3% of the variation in the mediastinal counts (M_e vs S_f and M_l vs S_s, p = 0.105 and p = 0.100, respectively).

The variability in the change of intravascular activity (S_f, S_s, and S_f/S_s) could not be explained by either estimate of renal function (e-CC or e-GFR) (Table 6). The e-CC could at best explain approximately 1.5% of the variation in the fast slope of the vascular clearance curve (e-CC vs S_f, p = 0.194).

Most in vivo scintigraphic quantification of regional neurotransporter activity or receptor density involves comparison of uptake in a target ROI to that in a reference region of nonspecific uptake/binding. For assessment of sympathetic myocardial activity with ¹²³I-MIBG scintigraphy, use of the mediastinum as a reference region is based on the assumption that there is a negligible amount of specific uptake of ¹²³I-MIBG in this region. In addition, presence of ¹²³I-MIBG in the blood pool is also assumed to be insignificant. The present results demonstrate that changes in blood pool/vascular ¹²³I-MIBG activity do not significantly correlate with changes in heart and mediastinum activity between early and late planar images. This is consistent with rapid clearance of ¹²³I-MIBG from the blood and uptake into organs (such as the heart) by means of the norepinephrine transporter.

As intravascular activity appears to play no role in the variation of the counts measured in the mediastinum, those counts likely reflect a combination of mediastinal tissue activity and scatter from ¹²³I-MIBG in adjacent organs (e.g., liver, lungs) [¹³]. This shows that for routine quantification, the mediastinum is acceptable as a reference region. However, these findings do not confirm that the mediastinum represents a background region with only nonspecific uptake/binding. To test this hypothesis would require a study where the specific uptake of ¹²³I-MIBG via the presynaptic norepinephrine transporter (i.e., uptake 1) was blocked and compared to a similar study without blocking. To our knowledge this type of study has not been performed in humans.

In subjects with normal renal function, intravenously administered ¹²³I-MIBG is almost exclusively excreted via the kidneys within 24 h after injection, with approximately 35% of administered ¹²³I-MIBG already excreted by 6 h [⁵, ⁶]. There are complex interactions between sympathetic regulation of renal function and cardiac function. For example, increased sympathetic activity reduces the filtration fraction [¹⁴, ¹⁵]. In addition, renal dysfunction is often present in HF patients, which may further increase the interindividual variation of blood pool clearance [⁷, ⁸]. Moreover, as a reduced GFR is associated with a reduced blood clearance of ¹²³I-MIBG, the excretion of ¹²³I-MIBG is not only dependent on filtration but also by tubular secretion [¹⁶]. In the present study, although approximately 40% of the patients had decreased renal function, differences in the rate of renal excretion did not contribute to variability in the mediastinal and myocardial ¹²³I-MIBG uptake. Therefore, within the typical time frame of ¹²³I-MIBG cardiac imaging (up to 4 h after injection), ¹²³I-MIBG mediastinal and myocardial activity determinations appear independent of the rate of blood clearance via the kidneys.

Radioactivity as measured in the blood samples may be in part explained by ¹²³I not bound to MIBG and this may have influenced our results. However, according to specifications of the manufacturer the radiochemical purity of the ¹²³I-MIBG used prior to injection is always more than 97% and in clinical practice the fraction of free ¹²³I does not exceed 2%. Secondly, after intravenous administration, ¹²³I-MIBG in humans is very stable and not subject to in vivo deiodination [¹⁷]. In addition, the clearance rate of both ¹²³I-MIBG and free ¹²³I from the blood pool is rapid and is almost exclusively dependent on renal excretion. Therefore, the impact of free ¹²³I on the present results should be negligible.

In conclusion, in patients with ischemic heart disease, due to rapid clearance of ¹²³I-MIBG from the blood, the heart and mediastinal count densities measured on planar ¹²³I-MIBG images at 15 min and 4 h are not affected by residual vascular activity. In addition, this observation holds for the broad range of renal function commonly encountered in heart disease patients. Therefore, no correction for blood pool activity or renal function is needed for the calculation of myocardial WO. However, the clinical appropriateness of using mediastinal activity as a background correction in such calculations, which depend on the assumption that there is no specific uptake/binding in this region, remains to be demonstrated.

Conflicts of interest Arnold F. Jacobson is employed by GE Healthcare. None of the other authors have a conflict of interest to disclose. Grant support: none of the authors has been supported by a grant.

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