Individual CT Phases

Pre-contrast phase is essential to help distinguish thyroid tissue from parathyroid tissue by equating high density to iodine content. This is particularly important for distinguishing subcapsular parathyroid glands, but it is also critical in identifying ectopic rests of thyroid tissue. Arterial phase is performed during contrast infusion that starts 20–45 seconds from the start of a rapid 3 or 4 cc/second infusion of contrast followed by a saline flush. A parathyroid adenoma or enlarged parathyroid gland is bright on arterial phase and is picked out as a bright spot on scrolling through the images. The focus of attention is medial to the carotid arteries, since parathyroid glands almost always lie medial to or towards medial aspect of the normal carotid arteries. Venous phase is usually started 60–90 seconds following the start of the infusion or 30 seconds after the arterial phase, in different protocols. Some protocols use both 60 and 90 second runs (3 post contrast phases) which may be helpful when rate of opacification is not optimal. The parathyroid gland or adenoma washes out in the venous phase, distinguishing it from enhancing nodes, which usually have no washout or increase in enhancement.