The true lumen is easily recognized in the ascending aorta, but in the descending thoracic aorta at times it may be difficult. It is usually smaller in size and blood flow demonstrates increased velocity by pulsed Doppler and turbulent flow by color flow Doppler. Outward movement of the intimal flap during early systole can be used to identify the true lumen. It may be difficult to recognize the true lumen when there is poor runoff downstream from peripheral vascular disease resulting in low velocity flow in the true lumen. Again, as mentioned above, meticulous incremental examination of the aorta from the root to the diaphragm should be carried out using a multiplane TEE transducer. Care should be taken to image all walls of the aorta. Use of ultrasound contrast may aid in the correct identification of the true lumen as long as there is differential flow between true and false lumen.
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