Frederick W. Kremkau, MD, PhD*; Ana M. Bircher, MD**; Philippe Jeanty, MD, PhD**.

*    Professor & Director, Center for Medical Ultrasound (http://www.wfubmc.edu/ultrasound), Wake Forest University School of Medicine, Winston-Salem, NC 27157-1039, Tel.: 336-716-0312; Fax: 336-716-2447; USA.
**   Inner Vision Women"s Ultrasound, Nashville, Tennessee, USA.

Case report

Figures 1, 2: Figure 1 shows a color-Doppler image of a 19-week fetal thorax. Note the interesting multiple flow reversals suggested by the alternating orange and blue regions in the descending aorta. Do these color reversals indicate flow reversals? They are separated by black confirming that the Doppler baseline has been crossed in each case. Therefore, there is no aliasing occurring in the descending aorta. But if these were flow reversals, the fact that there are 12 of them would indicate that it took about 4 seconds to generate the image, which of course is an unreasonable result. In fact, the indication at the top of the image is that the frame rate is 33 Hz (30 ms frame period).
The white arrow in Figure 2 shows the direction of flow from the heart through the aorta.

Figure 3: Figure 3 shows how the color-Doppler would present without the intervening bony processes. The sector-type image produced by the convex array yields a varying Doppler angle across the image. The green scan line is at 90 degrees to the flow direction and yields the expected black (baseline) region in the vessel. Lines to the right of that are oriented upstream to the flow with the expected positive (orange) Doppler shifts. Lines to the left of the perpendicular detect downstream flow and negative Doppler shifts (blue).

Figures 4, 5: But in the presence of the bony processes, how are the multiple reversals explained? Apparently the intervening bony processes cause refraction of the sound beam due to their higher sound speed (compared to soft tissue). The solid line in Figure 4 shows a refracted sound beam path. The beam then encounters downstream flow and a negative Doppler shift. This is shown in blue on the unrefracted scan line (dashed line). Figure 5 shows the refracted beam from the next bone. It encounters upstream flow and a positive Doppler shift which is shown as orange along the (dashed) scan line. Because refraction happens for all the intervening bony processes, there is an alternating of the positive and negative Doppler shift regions along the vessel. The right side of each bone refracts the beam to the right while the left side refracts it to the left. It is impossible to predict the exact path of each pulse because of the complicated anatomy (two bony layers and intervening soft tissue), but the refraction hypothesis fits much of the image quite well. It is possible that diffraction effects (primary due to the interruption of portions of the unfocused beam at the bony locations) contribute to the altered pulse paths also. Because refraction and/or diffraction alter the pulse paths by only a few degrees, it is necessary for the flow to be nearly perpendicular to the scan lines for the alternating Doppler shift regions to occur. Thus, this is an unusual presentation requiring a specific geometric arrangement to occur.