The Semicolon Sign: Dopamine Transporter Imaging Artifact from Head Tilt

Dopamine transporter (DAT) imaging is a valuable tool to aid in the diagnosis of Parkinson disease and other Parkinsonian syndromes. DAT imaging is special among clinical nuclear medicine scans in that the already small caudate and putamen are presented in multiple thin axial cuts. Because the imaged basal ganglia are small, slight differences in head tilt may result in a significant artifact that we have termed the semicolon sign. The semicolon sign occurs when forward head tilt creates select images that show the caudate nuclei separate from the putamen. This gives the false impression that DAT activity in the putamen is decreased or absent. To avoid falsely attributing this artifact to loss of putaminal activity, it is imperative that the interpreting physician first recognize the artifact and then mentally integrate all provided images to identify normal activity in the putamen on subsequent levels. Furthermore, quantitative software packages for automated DAT scan interpretation are now available. If images demonstrating the semicolon sign are used for automated interpretation, loss of activity in the putamen may be falsely calculated, thereby contributing to erroneous results. Quality control measures are essential to ensure that technologists correctly position each patient's head to minimize head tilt artifact on DAT scan images. A protocol to obtain optimal head positioning is presented.