Pin-hole collimator, parallax, and the localization of mediastinal parathyroid adenoma

A parathyroid adenoma was wrongly localized by pinhole Tc-99m sestamibl scintigraphy to the anterior mediastinum near a lower pole of the thyroid gland. After a failed surgical attempt to resect the adenoma, the patient was re-imaged with a parallel-hole collimator and SPECT. The adenoma was found to be near the heart, anterior to the carina. The pinhole findings were replicated using a phantom and measurements obtained from a CT of the chest. By placing the pinhole collimator at the sternal notch level, the lesion was correctly located deep in the chest. Moving the pinhole collimator cephalad or tilting the collimator toward the feet, maneuvers sometimes needed to clear the chest in large patients, projected the lesion closer to the sternal notch. Scintigraphy with a parallel-hole collimator is recommended and SPECT should be considered when the lesion is located below the sternal notch and when a lesion is not detected, despite the clinical and biochemical diagnosis of hyperparathyroidism.