Page 331 - Libro 2

20 — The Renal Vasculature
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Because branches of the celiac and superior mesen- teric arteries may course in proximity to the main or accessory renal arteries, it is important to recognize the mesenteric arterial flow patterns. To achieve this goal, representative Doppler spectral waveforms should be obtained from the proximal celiac and superior mes- enteric arteries. A secondary benefit may be identifica- tion of flow-limiting mesenteric artery stenosis. This is most likely to occur in patients where atherosclerotic plaque is noted along the aortic walls in the region of the mesenteric artery origins.
A cross-sectional image of the aorta is obtained at the level of the superior mesenteric artery (SMA). Just inferior to the SMA, the left renal vein is identi- fied as it crosses either anterior to the aorta or in a retroaortic position (refer to Fig. 20-1). Optimized B-mode and color flow imaging should be used to determine the presence of renal vein thrombosis or entrapment of the renal vein by small bowel or the SMA. Extrinsic compression of the vein may result in “nutcracker” or mesenteric compression syndrome.
The renal arteries normally lie immediately inferior to the left renal vein. Visualization of the proximal seg- ments of these vessels may be facilitated by moving the transducer upright so that it is perpendicular to the patient’s abdominal wall (Fig. 20-8). This posi- tion allows compression of the left renal vein. While remaining upright, the transducer is then angled slightly to the right or left to create a sagittal image of the renal artery (Fig. 20-9). Color flow imaging may facilitate the identification of the vessels and optimal visualization of the renal artery origins and accessory renal arteries. From this scan plane, the renal arteries can most often be visualized from the ostium to the mid-segment. To rule out orificial re- nal artery stenosis, the Doppler sample volume is swept slowly from the aortic lumen through the re- nal ostium. Demonstration of the change in spectral
Figure 20-8 This figure demonstrates appropriate positioning of the ultrasound transducer for the acquisition of images and Doppler spectral waveforms from the ostium and proximal renal artery.
Figure 20-9 Sagittal color flow image of the right renal artery.
waveform pattern from high-resistance, low-diastolic aortic flow to low-resistance, high-diastolic renal artery flow allows for the recognition of abnormal flow patterns and ostial lesions (Fig. 20-10). There- after, Doppler spectral waveforms are obtained con- tinuously throughout all visualized segments, and representative signals are then recorded taking care to ensure that all waveforms are obtained at appro- priate angles of insonation.
To interrogate the mid-to-distal segment of the renal arteries and parenchymal flow, the patient is moved to the lateral decubitus or prone position. With the patient lying in the decubitus position, a transverse image of the kidney can be obtained through an in- tercostal window using a coronal plane from the pa- tient’s flank. The right renal artery is relatively easy to follow from the renal hilum to its origin at the aor- tic wall. The left renal artery may be more difficult to
Figure 20-10 Doppler spectral waveforms demonstrating the transition from the high-resistance (low-diastolic flow) aortic signal to the low-resistance (high-diastolic flow) signal obtained as the sample volume is slowly swept from the aortic lumen into the renal artery ostium. Because the renal arteries often originate from the aortic wall at angles between 70° and 90°, it may be difficult to have a correct angle when collecting the transition signals.