Vascular time-activity variation in patients undergoing ¹²³I-MIBG myocardial scintigraphy: implications for quantification of cardiac and mediastinal uptake

Abstract

Purpose: For the quantification of cardiac (123)I-metaiodobenzylguanidine (MIBG) uptake, the mediastinum is commonly used as a reference region reflecting nonspecific background activity. However, variations in the quantity of vascular structures in the mediastinum and the rate of renal clearance of (123)I-MIBG from the blood pool may contribute to increased interindividual variation in uptake. This study examined the relationship between changes in heart (H) and mediastinal (M) counts and the change in vascular (123)I-MIBG activity, including the effect of renal function.

Methods: Fifty-one subjects with ischemic heart disease underwent early (15 min) and late (4 h) anterior planar images of the chest following injection of (123)I-MIBG. Vascular (123)I-MIBG activity was determined from venous blood samples obtained at 2 min, 15 min, 35 min, and 4 h post-injection. From the vascular clearance curve of each subject, the mean blood counts/min per ml at the time of each acquisition and the slope of the clearance curve were determined. Renal function was expressed as the estimated creatinine clearance (e-CC) and the estimated glomerular filtration rate (e-GFR). Relations between H and M region of interest (ROI) counts/pixel, vascular activity, and renal function were then examined using linear regression.

Results: Changes in ROI activity ratios between early and late planar images could not be explained by blood activity, the slope of the vascular clearance curves, or estimates of renal function. At most 3% of the variation in image counts could be explained by changes in vascular activity (p = 0.104). The e-CC and e-GFR could at best explain approximately 1.5% of the variation in the slopes of the vascular clearance curve (p = 0.194).

Conclusion: The change in measured H and M counts between early and late planar (123)I-MIBG images is unrelated to intravascular levels of the radiopharmaceutical. This suggests that changes in M counts are primarily due to decrease in soft tissue activity and scatter from the adjacent lungs.

Fig. 1

The upper panels show an example of the planar ¹²³I-MIBG images of a male subject (age 58 years, NYHA III, LVEF 36%) with a history of a previous myocardial infarction. The upper panel on the left displays the image at 15 min p.i. (early) and the upper panel on the right shows the image at 4 h p.i. (late). The lower panels display the positioning of the ROIs on the early (lower left) and late (lower right) planar images. The positioning of the mediastinal ROI was standardized in relation to the lung apex and the midline between the lungs. In this example the early H/M was 1.54, the late H/M 1.40, and myocardial WO 9%

Fig. 2

In this typical example of a blood activity clearance curve, there is a clear distinction between a more accelerated phase and a slower phase. The slopes of both the faster (S_f) and slower (S_s) phases were calculated as illustrated in the figure

Fig. 3

Scatter plot showing the blood activity in relation to time for all subjects