Physiologic mechanisms underlying the delayed delta sign

The negative or empty delta sign is considered to be nearly pathognomonic of superior sagittal sinus thrombosis on contrast-enhanced CT scans. We describe a visually similar sign, seen in five (10%) of 50 patients who had delayed cranial CT scans performed at intervals greater than 30 min after injection of contrast material. This new sign (which we call the delayed delta sign) closely mimics the classic negative delta sign and may thus represent a potential visual pitfall leading to an erroneous diagnosis of sagittal sinus thrombosis if only postcontrast images are viewed. The physiologic mechanisms underlying the appearance of the delayed delta sign are discussed.