Objective: There are reports on physiologic and pathologic mediastinal uptake on radioiodine imaging in patients with thyroid carcinoma. The most commonly reported physiologic causes of mediastinal uptake are esophageal retention of salivary secretion and uptake by hyperplastic or normal thymus gland. In this study, we evaluated physiologic cardiac blood pool (CBP) activity and its associated findings on radioiodine imaging.
Methods: Cardiac blood pool activity was evaluated in 186 postablation and 32 posttherapy scans. After oral radioiodine administration, imaging was initiated on day 7. Thyroglobulin, anti-thyroglobulin and TSH blood level determination was carried out in all patients. Whole-body scans were inspected with regard to residual thyroid uptake, esophageal uptake, CBP activity, hepatic activity and metastatic iodine uptake. Uptake in the residual thyroid tissue was graded visually as mild (m), moderate (M) and significant (S). Whenever CBP activity was detected on visual inspection, its intensity was graded as mild (1), moderate (2), and significant (3).
Results: Cardiac blood pool activity was detected in 61 postablation scans (33 %). Residual thyroid uptake was observed in all patients with CBP. A significant correlation existed between the intensity of uptake and the presence of CBP (p < 0.05). The mean Tg in patients with CBP was significantly higher than those without CBP (p < 0.05). Cardiac blood pool activity was not observed in any posttherapy scans and in those scans with distant metastases in the postablation group. Linear or irregular activity pattern thought to be due to esophagus was detected in 4 % of postablation scans.
Conclusions: The results have revealed CBP to be common on postablation scans. Its presence correlated with the intensity of residual thyroid uptake and had a significant association with increased thyroglobulin levels. Its absence on posttherapy scans despite increased Tg levels suggested that this activity is due to labeled thyroid hormones released by the residual thyroid tissue. Its presence might imply a high level of blood radiation dose.