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GRAFT SURVEILLANCE AND PREOPERATIVE VEIN MAPPING FOR BYPASS SURGERY
requires correcting, such as a graft stenosis, inflow stenosis or run-off occlusion. Conversely, some grafts develop a local aneurysm that may become so large that a segment of graft has to be replaced.
PRACTICAL CONSIDERATIONS FOR
SCANNING BYPASS GRAFTS
The objective of the scan is to detect any possible graft defects that could compromise flow and lead to graft occlusion. This usually involves some assess- ment of the inflow and outflow arteries above and below the graft. No special preparation is required for the examination, and the vast majority of graft scans can be completed within half an hour. The majority of bypass scans are performed with the patient lying supine or semi-supine. When scanning vein grafts, the leg should be externally rotated and the knee gently flexed and supported. It is some- times necessary to roll the patient over to one side in order to scan the posterior lower thigh, popliteal fossa or upper posterior calf if the graft is anasto- mosed to the popliteal artery. Positions for scan- ning the tibial arteries are discussed in Chapter 9. The scanner should be configured for a graft scan, or in the absence of a specific preset, a lower limb arterial investigation. Adjustment of the controls is f r e q u e n t l y n e c e s s a r y, e s p e c i a l l y i f t h e r e i s l o w - volume flow in the graft (see Ch. 7).
Before beginning the scan, it is important to know the position and type of graft that is to be examined. The examination request card or opera- tion notes should indicate this information. A potentially confusing situation can occur if a pre- vious graft has been performed, which has since occluded. An old thrombosed graft might be mis- takenly identified as the new graft, which would then be reported as occluded. A combination of 5 and 10 MHz, or broad-band equivalent, flat linear array transducers are most suited for graft surveillance in the thigh and calf. A 3.5 MHz probe is required for imaging grafts above the inguinal ligament or for grafts that have been tunnelled very deep in the thigh.
SCANNING TECHNIQUES
In situ vein graft
The main body of an in situ vein graft remains superficial in the leg and runs along the medial
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aspect of the thigh (Fig. 14.5). It is often easier to locate the graft in the upper medial thigh using a transverse imaging plane and then to follow the graft up to the proximal anastomosis (Fig. 14.5A).
The transducer is rotated into a longitudinal scan plane at the proximal anastomosis (Fig. 14.5B). Ideally, a minimum 5 cm length of the inflow artery above the graft origin should be examined to exclude any disease. For instance, damped waveforms at this level are likely to indicate significant inflow disease. The proximal anastomosis should be carefully inter- rogated using color flow imaging and spectral Doppler for any signs of stenosis.
The graft is then carefully followed in longitudi- nal section along the thigh (Fig. 14.5C) with the color pulse repetition frequency (PRF) optimized to use the full color scale to demonstrate any flow disturbances. A 10MHz (or broad-band equiva- lent) transducer provides the best image of the main body of the graft. Frequent spectral Doppler measurements should be made along the length of the graft, looking for waveform changes. It is often difficult to obtain good Doppler angles when scanning superficial vein grafts, and gentle ‘heel-toeing’ of the transducer may be required. However, it is important not to apply too much pressure with the transducer, as this can cause compression of superficial in situ vein grafts, giv- ing the impression of a stenosis, especially if the graft passes over a bony surface. A wedge of ultra- sound gel can help if there is a specific region that needs close examination.
The distal portion of many in situ vein grafts runs deep to join a native artery at the distal anas- tomosis (Fig. 14.5D). This is especially true for grafts joined to the popliteal or peroneal arteries. It is often necessary to use a 5 MHz transducer in this region. The distal anastomosis should be scru- tinized very closely with color flow imaging and spectral Doppler. Grafts that are anastomosed to the anterior tibial artery are commonly tunnelled through the interosseous membrane (Fig. 14.6). The graft is imaged on the medial or posteromedial aspect of the calf, where it is seen to drop away very sharply and disappear through the membrane. The graft can then be relocated by scanning over the anterolateral aspect of the calf, where it will be seen to rise toward the transducer, and followed dis- tally to locate the anastomosis. There should be a