Case report: The “yin–yang” sign—seen with spiral computed tomography for pulmonary embolus

The diagnostic evaluation of pulmonary embolism has recently been modified to include contrast-enchanced spiral computed tomography of the chest. CT has found acceptance as not only an imaging modality for diagnosing pulmonary embolus, but is also useful in depicting alternate diagnoses to explain acute shortness of breath. This article describes a unique pattern of contrast enhancement observed during a routine spiral contrast-enhanced CT to evaluate suspected pulmonary embolism. The appearance has been named the “yin-yang” sign due to the alternating areas of contrast enhancement seen in the cardiac chambers on sagittal and coronaly reformatted images. This article describes the appearance of the sign and attempts to explain how intravenous contrast assumes this configuration with rapid spiral acquisition of images. The importance of recognizing this normal variant of contrast enhancement which may appear on routine examinations for pulmonary embolus is also discussed.