DOI: 10.1148/rg.282075032
RadioGraphics 2008;28:529-548
© RSNA, 2008
Multidetector CT Angiography of Infrainguinal Arterial Bypass1
Jorge E. Lopera, MD,
Clayton K. Trimmer, DO,
Shellie G. Josephs, MD,
Matthew E. Anderson, MD,
Scott Schuber, MD,
Ruizong Li, MD,
Bart Dolmatch, MD, and
Boulos Toursarkissian, MD
1 From the Department of Radiology, University of Texas Southwestern Medical Center, Dallas (J.E.L., C.K.T., S.G.J., M.E.A., S.S., R.L., B.D.), and the Department of Vascular Surgery, University of Texas Health Science Center at San Antonio (B.T.). Presented as an education exhibit at the 2006 RSNA Annual Meeting. Received February 26, 2007; revision requested June 11 and received July 26; accepted August 27. All authors have no financial relationships to disclose.
Address correspondence to J.E.L., Department of Radiology, University of Texas Health Science Center, 7703 Floyd Curl Dr, San Antonio, TX 78229 (e-mail: Lopera{at}uthscsa.edu).
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Abstract
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Infrainguinal arterial bypass (IGAB) surgery is commonly performed in patients with claudication, critical limb ischemia, or other arterial problems in the lower extremities. An IGAB is constructed from different materials depending on the anatomy of the lesion and the availability of an autogenous vein. The ideal material for IGAB is the greater saphenous vein, especially for distal below-knee bypass. In patients with no available autogenous vein, IGAB can be performed by using different prosthetic materials or biologic grafts. After the surgery, periodic surveillance is performed with duplex ultrasonography and clinical assessment of peripheral pulses and ankle-brachial indexes. If complications are detected, further work-up is performed with conventional arteriography, multidetector computed tomographic (CT) angiography, or magnetic resonance angiography. CT angiography has become a powerful tool for assessing the potential early and late complications of IGAB and for planning further therapy in a fast, reliable, and noninvasive manner.
© RSNA, 2008
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Introduction
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Infrainguinal arterial bypass (IGAB) is an established surgical procedure for the treatment of complications of peripheral vascular disease, aneurysms, and lower extremity trauma. Limb salvage rates depend on the type of conduit and qualities of the inflow and distal runoff. Better results are reported with native vein than synthetic material. 
Periodic surveillance of the bypass is Periodic surveillance of the bypass is critical to detect correctable lesions in a timely fashion and thereby maintain graft patency. Duplex ultrasonography (US) is the primary imaging method of surveillance (1,2). When abnormalities are detected with US, computed tomographic (CT) angiography or magnetic resonance (MR) angiography are commonly used to further plan the treatment of early and late complications after IGAB surgery, replacing conventional angiography in most cases (3,4). A basic understanding of the different types of IGAB and adjunctive surgical techniques and the potential early and late complications of the different types of bypasses is essential for adequate interpretation of the CT angiography results.
The aim of this article is to review the surgical classification and basic surgical techniques of IGAB. In addition, we discuss the various types of materials used for creation of IGAB and their results. We also review the normal CT angiography findings of the different types of IGAB and of the early and late complications after IGAB.
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Types of IGAB
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IGABs are usually classified according to the location of the anastomoses. The most common type of bypass is the one created between the common femoral artery (CFA) and the popliteal artery, also known as a femoropopliteal bypass. This type is further classified as an above- or below-knee bypass, depending on the segment of popliteal artery used (Fig 1). More distal bypasses between the CFA and distal crural vessels are known as femorodistal bypasses (Fig 2).
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Figure 1a. Normal above-knee femoropopliteal bypass. The bypass was created with a reversed greater saphenous vein (GSV) for a long-segment occlusion of the superficial femoral artery (SFA). (a–c) Axial contrast-enhanced CT images show a proximal bypass originating from the right CFA (arrow in a), running in the medial thigh in a typical location over the sartorius muscle (arrow in b), then diving into the popliteal fossa (arrow in c) before joining the popliteal artery. Note the occluded stent in the left SFA (arrowhead in b). (d) Coronal volume-rendered semitransparent background CT angiogram shows the bypass from the right CFA to the proximal popliteal artery (arrows). The left SFA is occluded; note the metallic stent in the proximal segment (arrowhead).
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Figure 1b. Normal above-knee femoropopliteal bypass. The bypass was created with a reversed greater saphenous vein (GSV) for a long-segment occlusion of the superficial femoral artery (SFA). (a–c) Axial contrast-enhanced CT images show a proximal bypass originating from the right CFA (arrow in a), running in the medial thigh in a typical location over the sartorius muscle (arrow in b), then diving into the popliteal fossa (arrow in c) before joining the popliteal artery. Note the occluded stent in the left SFA (arrowhead in b). (d) Coronal volume-rendered semitransparent background CT angiogram shows the bypass from the right CFA to the proximal popliteal artery (arrows). The left SFA is occluded; note the metallic stent in the proximal segment (arrowhead).
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Figure 1c. Normal above-knee femoropopliteal bypass. The bypass was created with a reversed greater saphenous vein (GSV) for a long-segment occlusion of the superficial femoral artery (SFA). (a–c) Axial contrast-enhanced CT images show a proximal bypass originating from the right CFA (arrow in a), running in the medial thigh in a typical location over the sartorius muscle (arrow in b), then diving into the popliteal fossa (arrow in c) before joining the popliteal artery. Note the occluded stent in the left SFA (arrowhead in b). (d) Coronal volume-rendered semitransparent background CT angiogram shows the bypass from the right CFA to the proximal popliteal artery (arrows). The left SFA is occluded; note the metallic stent in the proximal segment (arrowhead).
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Figure 1d. Normal above-knee femoropopliteal bypass. The bypass was created with a reversed greater saphenous vein (GSV) for a long-segment occlusion of the superficial femoral artery (SFA). (a–c) Axial contrast-enhanced CT images show a proximal bypass originating from the right CFA (arrow in a), running in the medial thigh in a typical location over the sartorius muscle (arrow in b), then diving into the popliteal fossa (arrow in c) before joining the popliteal artery. Note the occluded stent in the left SFA (arrowhead in b). (d) Coronal volume-rendered semitransparent background CT angiogram shows the bypass from the right CFA to the proximal popliteal artery (arrows). The left SFA is occluded; note the metallic stent in the proximal segment (arrowhead).
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Figure 2a. Normal femorodistal bypass. Coronal volume-rendered CT angiograms, displayed from superior (a) to inferior (c), show a long GSV bypass (straight arrows) extending from the proximal SFA to the dorsalis pedis artery (arrowhead in c). There is a mild stenosis at the proximal anastomosis (curved arrow in a). Diffuse calcifications are seen in the native arteries.
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Figure 2b. Normal femorodistal bypass. Coronal volume-rendered CT angiograms, displayed from superior (a) to inferior (c), show a long GSV bypass (straight arrows) extending from the proximal SFA to the dorsalis pedis artery (arrowhead in c). There is a mild stenosis at the proximal anastomosis (curved arrow in a). Diffuse calcifications are seen in the native arteries.
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Figure 2c. Normal femorodistal bypass. Coronal volume-rendered CT angiograms, displayed from superior (a) to inferior (c), show a long GSV bypass (straight arrows) extending from the proximal SFA to the dorsalis pedis artery (arrowhead in c). There is a mild stenosis at the proximal anastomosis (curved arrow in a). Diffuse calcifications are seen in the native arteries.
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The type of material and technique used for the bypass are also commonly used to further describe the different bypass types. IGAB can be created with autogenous vein by using the greater saphenous vein (GSV) in a reversed or nonreversed fashion or with an in situ technique. When the GSV is not available, some surgeons may elect to use the lesser saphenous or arm veins. If multiple pieces of vein are put together, this is known as a spliced conduit. Prosthetic bypasses include polytetrafluoroethylene (PTFE) and polyester (Dacron) bypasses. Biologic grafts include human umbilical vein, cadaveric cryopreserved vein, and arterial homografts.
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Basic Surgical Approach
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Most bypasses are performed from the CFA extending into the popliteal artery. Usually, the anterior aspect of the distal CFA is exposed in the groin and used for the anastomosis. In cases where vein length is not sufficient, the anastomosis may be created to the proximal SFA or profunda femoris artery. If the origin or proximal portion of the profunda femoris artery is diseased, surgical profundoplasty or endarterectomy is commonly performed. In some more distal bypasses, the popliteal artery may be used as the inflow vessel; such bypasses are known as popliteal-distal bypasses.
Usually, the bypass is tunneled in the medial aspect of the thigh. A lateral approach in the thigh is used in patients with prior surgical reconstructions or with sepsis of the soft tissues. Some surgeons will routinely tunnel bypasses going to the anterior tibial artery laterally in the thigh. Some bypasses may be tunneled superficially under the skin or deep along the native vessels (so-called anatomic tunneling). For prosthetic femoropopliteal bypasses, a subsartorius tunnel, either deep or superficial to the deep muscular fascia, is created from the groin into the popliteal space.
For above-knee femoropopliteal bypass, a popliteal approach is used through a medial incision with retraction of the sartorius muscle. For below-knee femoropopliteal bypass, a medial calf incision is performed posterior to the medial femoral condyle. The tunnel is then extended between the heads of the gastrocnemius muscle into the popliteal fossa (5). The ideal bypass will be as short as possible and anastomosed to a disease-free distal arterial segment. Even in cases in which the distal popliteal artery is occluded, a femoropopliteal bypass to an isolated segment of the popliteal artery can be successfully performed (6,7).
Distal bypasses are performed only when a femoropopliteal bypass is not possible. Ideally, the tibial arteries should run continually into the foot, but successful vein bypasses to isolated tibial segments have also been performed (7). The origin of the anterior tibial artery is very difficult to reach from a medial approach through the popliteal fossa. Therefore, a lateral calf approach is often performed. A lateral calf approach with resection of a segment of fibula is frequently used to approach the distal peroneal artery. For distal peroneal or anterior tibial bypass, a tunnel can be created from the medial calf through the interosseous membrane in order to reach the vessel that has been exposed through a lateral incision. Alternatively, for the distal third of the anterior tibial artery or the dorsalis pedis artery, the vein can be routed over the anterior surface of the tibia in a subcutaneous plane. As mentioned earlier, a bypass to the anterior tibial artery anywhere along its course may be tunneled in a superficial subcutaneous lateral plane from the CFA or proximal SFA.
The posterior tibial artery, as well as the proximal and mid peroneal arteries, are most easily accessed by using a medial incision; the same incision created for the vein dissection is deepened to the appropriate plane. The peroneal artery is likely the one most difficult to access because of its depth. More distal bypasses could be performed to the dorsalis pedis or the plantar arch. When insufficient length of saphenous vein is available for conventional femorotibial bypass, a shorter bypass from a patent below-knee popliteal artery can also be performed (8).
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Types of IGAB Grafts
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Autogenous Vein Grafts
Reversed GSV.—
The ideal vascular conduit is the GSV bypass. The GSV is harvested through short longitudinal skin incisions in the medial thigh and calf. The tributaries are divided and ligated, and the vein is removed. The vein is gently flushed and stored in chilled solution. Either the proximal or distal anastomosis is created first. Care must be taken to avoid any twists when the vein is brought through the tunnel. For below-knee femoropopliteal bypass tunneled deeply, it is important to make sure the vein is passed between the two heads of the gastrocnemius muscle and not through one of the muscle bellies, since this often results in extrinsic compression and occlusion.
Nonreversed GSV.—
Either the ipsilateral or contralateral GSV is used. The vein is harvested as in the reversed technique, and the valves are lysed with a valvulotome. The vein is used without being reversed. This technique is often used when there is a size mismatch between the groin end of the vein and the outflow vessel to be bypassed. A nonreversed vein can be used for extraanatomic tunneling in the anterolateral thigh or can be replaced in its harvest bed or any other anatomic plane.
In Situ GSV.—
In the in-situ technique, the GSV is exposed through a long longitudinal incision or via skip incisions. The vein is not removed from its bed. The saphenofemoral junction is transected, and the common femoral vein venotomy is closed. The proximal anastomosis is created first, in contrast to the reversed vein bypass. This allows distention of the vein with arterial blood. The valves are then lysed from proximal to distal with a valvulotome. The valvulotome is introduced from the distal end of the vein or from one of the many side branches, depending on its type. These patent side branches or fistulas are then ligated. Perforating veins that are not ligated can divert a significant portion of the arterial flow, decreasing the flow to the distal bypass or potentially creating venous hypertension with limb swelling. Fistulas into the superficial branches frequently thrombose spontaneously and rarely create problems, but they can persist if large (9). The distal anastomosis is usually created in the distal portion of the popliteal artery just above the origin of the anterior tibial artery. This location provides a gentler angle than the above-knee anastomosis (9).
Advantages of the in situ technique include a better size match between the artery and the vein at the anastomoses and an increased vein utilization rate (>90%). However, the in situ operation is technically more demanding, requiring elimination of all venous valves and ligation of the branches. In situ vein bypass seems to be the best conduit for long distal bypasses with small-diameter veins. While a 3.0-mm GSV caliber is required for optimal results with the reversed vein technique, smaller veins are usually satisfactory with the in situ technique, with successful use of veins as small as 2.0–2.5 mm in caliber (9–11). Some surgeons use angioscopy for direct valve lysis, localization of arteriovenous fistulas, and direct identification of intraluminal disease in the veins. Although the incisions are shorter, resulting in a lower rate of wound complications, angioscopy is not widely used. This is due in part to the long learning curve of the technique (12). Fundamentally, the main difference between the in situ and nonreversed vein techniques is that the vein is not taken out of its bed in the in-situ technique.
Alternative Autogenous Veins.—
Autogenous vein is the material of choice for distal bypass, especially for below-knee reconstructions. In 20%–30% of patients, the GSV is unavailable, of inadequate caliber, or of poor quality (10). Furthermore, among patients requiring repeat surgery and revision procedures, a suitable vein is not available in 40%–50% (13). In these cases, the lesser saphenous vein, accessory saphenous vein, or arm veins could be used in a reversed or nonreversed fashion. Also, the superficial femoral vein and popliteal vein have been used for IGAB.
Other Biologic Grafts
Biologic grafts, including the human umbilical vein, cadaveric cryopreserved vein, and arterial homografts, have the advantages of being readily available. However, biologic grafts are not commonly used because they are very expensive and prone to aneurysmal degeneration. The use of biologic grafts is usually limited to limb salvage in cases of previous infections or when no other conduit is available. Although the umbilical vein has shown better patency than PTFE for femoropopliteal bypasses in some studies (14), the majority of studies have shown that these grafts do not perform any better than prosthetic grafts. Their main use seems to be in infected fields when no autogenous vein is available.
Prosthetic Grafts
Tubular expanded PTFE is the material of choice in the absence of suitable autogenous vein. It is available with external support rings or coils (Fig 3), which are created to decrease mechanical external compression and avoid the risk of kinking when placed across the knee joint (15).
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Figure 3a. PTFE bypass graft. Coronal anterior (a) and magnified posterior (b) volume-rendered CT angiograms show a ringed PTFE femoropopliteal bypass with a cuff of vein (arrow) at the distal anastomosis with the popliteal artery. Note the corrugated appearance of the rings of the PTFE graft.
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Figure 3b. PTFE bypass graft. Coronal anterior (a) and magnified posterior (b) volume-rendered CT angiograms show a ringed PTFE femoropopliteal bypass with a cuff of vein (arrow) at the distal anastomosis with the popliteal artery. Note the corrugated appearance of the rings of the PTFE graft.
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Polyester fabrics (Dacron) are usually used in large-caliber vessels, with excellent durability and patency. Polyester grafts tend to perform poorly in small-caliber vessels. However, according to recent studies, patency rates in above-knee bypass are comparable to those of PTFE (16), with some reports showing higher patency rates (17). Such grafts are usually in the 6–8-mm-diameter range.
Prosthetic grafts are not the ideal material for below-knee and tibial IGAB, with an average patency rate of 14% for infrapopliteal reconstructions created with PTFE (8). In selected patients with patent tibial vessels and no other available surgical option but amputation, acceptable results are sometimes reported, with limb salvage rates of 38% at 4 years (18).
Advantages of prosthetic grafts include that they are easier to use, require less surgical time, and allow more limited incisions and dissection. Some authorities believe that prosthetic grafts are the material of choice for above-knee bypass, especially in the high-risk elderly patient population, with the vein preserved for repeat operations (15,19). Although the topic is still controversial in the surgery literature, most authors favor vein over prosthetics for above-knee femoropopliteal bypass, owing to higher patency and limb salvage rates with autogenous vein than with either human umbilical vein or PTFE (14).
Disadvantages of prosthetic grafts include less compliance, which may result in a higher rate of intimal hyperplasia and acceleration of distal atherosclerosis. Prosthetic grafts have a higher frequency of thrombosis, anastomotic aneurysms, intimal hyperplasia, and structural deterioration than autogenous vein bypasses, as well as a higher rate of graft infections and greater need for repeat surgery (15). Moreover, failure of prosthetic grafts often leads to threatened extremities due to distal embolization, whereas failed vein bypasses more often merely return the patients to their premorbid condition.
Adjunctive techniques may improve the patency of prosthetic grafts. The patency of prosthetic IGABs is affected by aggressive intimal hyperplasia, usually at the distal anastomosis. A collar or patch of vein may be interposed at the distal anastomosis between the PTFE graft and the artery to reduce intimal hyperplasia related to poor compliance of the prosthetic graft (Fig 3) (20,21). There are many varieties available (Miller cuff, Taylor patch, St Mary’s hood). When faced with the need to place a prosthetic graft to a tibial vessel, many surgeons may choose to place an adjunctive arteriovenous fistula from the graft to the adjoining tibial vein; such an arteriovenous fistula increases the flow in the bypass graft and may enhance long-term patency (22,23). Other adjunctive techniques include the use of very long hooded anastomoses to the distal vessels, often with a vein patch incorporated at the anastomosis. Recently, ringed PTFE grafts bonded to heparin were introduced for use in difficult distal bypasses. Long-term anticoagulation may improve infragenicular graft patency by two to three times (24).
Composite Grafts
Composite grafts were designed to prevent the problems of synthetic materials in below-knee reconstructions. Typically, the proximal segment is made of polyester or PTFE, while the distal segment uses available portions of autogenous vein (Figs 4, 5). As an alternative, two or more segments of available autogenous vein are anastomosed together to form a long sequential vein bypass (spliced vein) (Fig 6). In tibial bypass, composite grafts provide better patency rates than do grafts of all-prosthetic material (25–27). These complex vascular reconstructions require additional surgical time and are usually reserved for patients with critical limb ischemia for limb salvage purposes only; they are not indicated for palliation of claudication (8).
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Figure 4a. Types of composite grafts. (a) In a direct composite graft, prosthetic material and a vein are joined by end-to-end anastomosis. (b) In a sequential composite graft, the prosthetic material is anastomosed to an isolated segment of the popliteal artery; the vein is then anastomosed to the prosthetic material in an end-to-side fashion and extends distally to a crural vessel.
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Figure 4b. Types of composite grafts. (a) In a direct composite graft, prosthetic material and a vein are joined by end-to-end anastomosis. (b) In a sequential composite graft, the prosthetic material is anastomosed to an isolated segment of the popliteal artery; the vein is then anastomosed to the prosthetic material in an end-to-side fashion and extends distally to a crural vessel.
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Figure 5a. Use of a sequential composite graft. The patient had an aortic-bifemoral bypass with an occluded left limb and an occluded femoral-femoral bypass. A left axillofemoral bypass was created with a composite graft to treat critical left lower extremity ischemia. Anterior (a) and posterior (b) volume-rendered CT angiograms show a proximal axillopopliteal PTFE bypass (straight arrows in a). A vein graft (curved arrow) extends from the PTFE bypass to the posterior tibial artery. The posterior tibial artery is very small and retrogradely fills the tibioperoneal trunk. There is significant venous contamination secondary to hyperemia.
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Figure 5b. Use of a sequential composite graft. The patient had an aortic-bifemoral bypass with an occluded left limb and an occluded femoral-femoral bypass. A left axillofemoral bypass was created with a composite graft to treat critical left lower extremity ischemia. Anterior (a) and posterior (b) volume-rendered CT angiograms show a proximal axillopopliteal PTFE bypass (straight arrows in a). A vein graft (curved arrow) extends from the PTFE bypass to the posterior tibial artery. The posterior tibial artery is very small and retrogradely fills the tibioperoneal trunk. There is significant venous contamination secondary to hyperemia.
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Figure 6a. Use of a spliced vein sequential graft. Three segments of cephalic and basilic veins were anastomosed together to form a longer bypass. Owing to inadequate length of the vein, the bypass was placed from the mid-distal SFA to the anterior tibial artery. (a) Sagittal volume-rendered CT angiogram shows the long vein bypass (arrows). An area of moderate stenosis is seen at one of the anastomoses (arrowhead). (b) Magnified posterior volume-rendered CT angiogram shows the stenosis (arrow). The stenosis has been stable at follow-up duplex US.
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Figure 6b. Use of a spliced vein sequential graft. Three segments of cephalic and basilic veins were anastomosed together to form a longer bypass. Owing to inadequate length of the vein, the bypass was placed from the mid-distal SFA to the anterior tibial artery. (a) Sagittal volume-rendered CT angiogram shows the long vein bypass (arrows). An area of moderate stenosis is seen at one of the anastomoses (arrowhead). (b) Magnified posterior volume-rendered CT angiogram shows the stenosis (arrow). The stenosis has been stable at follow-up duplex US.
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Bypass Surveillance
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Reversed and in situ GSV bypasses have similar patency rates, superior to those of arm or lesser saphenous vein bypasses and significantly higher than those of prosthetic bypass. Autogenous vein bypass has 5-year primary patency rates of 63%–75% and secondary patency rates of 80%–83% (8,28,29), 10-year primary patency rates of 55%–60% and secondary patency rates of 70%–76%, and limb salvage rates of 84%–90% (30). Surveillance of IGAB starts immediately after the bypass is completed and takes the form of intraoperative angiography, Duplex US, and sometimes angioscopy. Intraluminal disease and technical defects are corrected immediately. Early identification of stenoses by means of periodic surveillance is critical so that lesions can be corrected before graft thrombosis occurs. Lesions that cause significant stenosis of a graft are simpler to treat than those in an already thrombosed graft. Revision of isolated lesions improves long-term patency to 80% at 5 years; similar interventions in recently thrombosed grafts have lower patency rates of 20%–30% (8,13,31).
Duplex US is the primary imaging method for bypass surveillance (1). It has 97%–100% sensitivity for detection of grafts that are at risk of thrombosis within 3–6 months when there is a decrease in the peak systolic blood flow velocity and a negative predictive value of 98%–100% for graft thrombosis in the ensuing 6 months when the overall graft velocity exceeds 40–45 cm/sec (2). When complications are detected with US, CT angiography is commonly used to further plan the treatment of complications after IGAB surgery. CT angiography has been shown to be an accurate and reliable technique for assessment of IGAB patency and detection of graft-related complications, including stenosis, aneurysmal changes, and arteriovenous fistulas (3).
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CT Angiography Technique
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We have almost completely replaced conventional angiography with CT angiography for the study of IGAB, since it is safer, less expensive, and quicker and allows anatomic definition of the vascular structures as well as the soft tissues. CT angiography also avoids unnecessary puncture and catheterization of a bypass graft and the potential complications of conventional angiography (32–34). Adequate visualization of the infrapopliteal circulation is especially important in noninvasive evaluation of IGAB. In early studies performed with four-section CT scanners, limited diagnostic value was noted in the infrapopliteal segment due to limited resolution; with the widespread use of 16-section and recently 64-section CT scanners, the resolution of the infrapopliteal vessels has increased significantly (32), which has also resulted in increased utility of CT angiography for evaluation of even the most distal IGAB. Willmann et al (3) reported sensitivity and specificity values of more than 95% when four–detector row CT angiography was compared with duplex US and conventional angiography for diagnosis of arterial bypass graft–related complications.
In a recent systematic review of the literature, Sun (32) described pooled sensitivity, specificity, and accuracy rates of 92%, 91%, and 91%, respectively, for all arterial levels (aortoiliac to infrapopliteal). A significant difference was found in the sensitivity of CT angiography in peripheral vascular disease between four–detector row and 16–detector row CT scanners. In comparison with conventional angiography, 16–detector row CT angiography has shown 96% sensitivity and 97% specificity in the detection of hemodynamically significant stenosis in all aortoiliac and peripheral vascular segments (34).
Occasionally, we have found CT angiography to be inadequate for evaluation of IGAB, mainly due to poor opacification of the distal circulation secondary to poor timing of the contrast material injection. Only when there are unresolved questions about vessel caliber or patency do we proceed to conventional angiography, as is occasionally the case for evaluation of distal outflow bypass grafts and for patients with extensive calcifications of the arterial walls (35). Limitations of CT angiography include the use of ionizing radiation, which may be an important issue in the long-term follow-up of patients with IGAB; the use of potentially nephrotoxic contrast agents; and the occasional limited visualization of the distal vessels due to severe calcifications, metal artifacts, or poor contrast material bolus timing. At our institution, CT angiography is preferred over MR angiography for evaluation of IGAB. With MR angiography, metallic clips may simulate graft stenosis, while they tend to cause little artifact with CT angiography. MR angiography also has limited spatial resolution. Finally, it is easier for the vascular surgeon to obtain CT angiography data and interpret the images.
CT Angiography Protocol
We currently use the Aorta and Long Leg Protocol on 16-section scanners (LightSpeed; GE Healthcare, Milwaukee, Wis). After acquisition of an initial scout image, the scanning range is planned to encompass the entire vascular system, from the diaphragm to the level of the toes. For optimal intraluminal contrast enhancement, the delay time between the start of contrast material administration and the start of scanning is obtained for each patient individually by using a bolus-tracking technique (SmartPrep; GE Healthcare) with the threshold set around 100 HU at about the celiac abdominal aorta. Each patient receives 150 mL of iohexol (Omnipaque; Amersham Health, Princeton, NJ) intravenously, administered with a dual injector at 4 mL/sec for 75 mL then 2 mL/sec for 75 mL, followed by a bolus of 70 mL of normal saline at 2 mL/sec. Data are acquired craniocaudally by using 1.25-mm-thick sections at 1-mm intervals. The tube voltage setting is 120 kV, and the tube current is 200 mAs. All abdominal scans are obtained during breath holds.
Delayed Images
Delayed images (90-second delay from the initial time of injection) are obtained in patients with poor cardiac output or slow distal runoff. Delayed scanning is especially important in patients with dilated distal arteries, in whom contrast medium tends to get diluted and stagnant with poor opacification distally (Fig 7). Although delayed images are not obtained routinely, the technologists performing the examination should be instructed to obtain delayed images when poor opacification of the distal arteries is seen after the initial scan.
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Figure 7a. Use of delayed images in a young patient with bilateral bypasses for popliteal artery occlusion due to entrapment. (a) Frontal volume-rendered CT angiogram shows that proximal enhancement is adequate. However, contrast material has become stagnant in the proximal dilated portions of the bypasses (arrows) without distal visualization. The study was repeated with delayed images. (b) Frontal CT angiogram shows the right bypass extending from the distal SFA to the posterior tibial artery. There is dilatation of the proximal portion of this bypass (curved arrow). The left bypass extends from the distal SFA to the distal popliteal artery. There is an aneurysm of the proximal segment of this bypass (straight arrow).
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Figure 7b. Use of delayed images in a young patient with bilateral bypasses for popliteal artery occlusion due to entrapment. (a) Frontal volume-rendered CT angiogram shows that proximal enhancement is adequate. However, contrast material has become stagnant in the proximal dilated portions of the bypasses (arrows) without distal visualization. The study was repeated with delayed images. (b) Frontal CT angiogram shows the right bypass extending from the distal SFA to the posterior tibial artery. There is dilatation of the proximal portion of this bypass (curved arrow). The left bypass extends from the distal SFA to the distal popliteal artery. There is an aneurysm of the proximal segment of this bypass (straight arrow).
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Image Processing and Interpretation
At our institution, we work with the Vitrea workstation (Vital Images, Minnetonka, Minn). Three-dimensional reconstructions are used with volume rendering techniques (Fig 8) and occasionally maximum intensity projection images. In addition, we use the vessel analysis software to grade areas of possible stenosis. Although most of the image interpretation is based on the axial images, three-dimensional reconstructions are extremely helpful for depicting subtle stenosis in any plane and understanding the bypass anatomy, especially when there are variations of the standard surgical technique (Fig 9). The Table lists the most important findings to consider in CT angiographic evaluation of IGAB.
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Figure 8a. Commonly used reformation techniques. Posterior volume-rendered CT angiograms, created with the "show all" (a), subtracted (b), and semitransparent (c) techniques, show an SFA–anterior tibial bypass.
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Figure 8b. Commonly used reformation techniques. Posterior volume-rendered CT angiograms, created with the "show all" (a), subtracted (b), and semitransparent (c) techniques, show an SFA–anterior tibial bypass.
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Figure 8c. Commonly used reformation techniques. Posterior volume-rendered CT angiograms, created with the "show all" (a), subtracted (b), and semitransparent (c) techniques, show an SFA–anterior tibial bypass.
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Figure 9a. Obturator bypass for treatment of a bleeding pseudoaneurysm of the proximal SFA in a patient with a history of an open wound in the right groin after dissection and radiation therapy for a recurrent melanoma. A PTFE graft was placed from the right external iliac artery to the distal SFA through the obturator foramen. (a) Axial contrast-enhanced CT image shows the graft coursing through the obturator foramen (arrow). (b, c) Anterior (b) and posterior (c) volume-rendered CT angiograms show the graft (arrows) extending from the external iliac artery to the distal SFA through the obturator foramen.
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Figure 9b. Obturator bypass for treatment of a bleeding pseudoaneurysm of the proximal SFA in a patient with a history of an open wound in the right groin after dissection and radiation therapy for a recurrent melanoma. A PTFE graft was placed from the right external iliac artery to the distal SFA through the obturator foramen. (a) Axial contrast-enhanced CT image shows the graft coursing through the obturator foramen (arrow). (b, c) Anterior (b) and posterior (c) volume-rendered CT angiograms show the graft (arrows) extending from the external iliac artery to the distal SFA through the obturator foramen.
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Figure 9c. Obturator bypass for treatment of a bleeding pseudoaneurysm of the proximal SFA in a patient with a history of an open wound in the right groin after dissection and radiation therapy for a recurrent melanoma. A PTFE graft was placed from the right external iliac artery to the distal SFA through the obturator foramen. (a) Axial contrast-enhanced CT image shows the graft coursing through the obturator foramen (arrow). (b, c) Anterior (b) and posterior (c) volume-rendered CT angiograms show the graft (arrows) extending from the external iliac artery to the distal SFA through the obturator foramen.
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Complications of IGAB
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Problems with Vein Bypass
Technical errors are more common with autogenous vein bypass than with prosthetic grafts. Repeat surgery in the early postoperative period (<30 days) is usually due to residual competent valve cusps (4%–12%) (Fig 10) and missed arteriovenous fistulas. Other technical errors include vein injury, anastomotic stenosis, valvulotome injury, intimal flaps, small-caliber vein, and kinks (36). Problems between 1 and 24 months after the vein bypass are related to intimal hyperplasia, which occurs at anastomotic sites, areas of vein repair, and sites of valve incisions (Figs 11, 12). After 24 months, changes are usually due to progression of atherosclerosis with inflow and outflow problems (11). Aneurysmal degeneration is rare with saphenous vein grafts (Fig 7). Other potential problems with failing IGAB can also be detected with CT angiography (Figs 13, 14). CT angiography could also potentially help triage between endovascular and surgical repair of failing IGAB, allowing selection of patients with short focal stenosis for endovascular treatment (Fig 15) and patients with long segment stenosis for surgical repair (Fig 11).
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Figure 10a. Retained valve cusp in a patient who underwent right aortofemoral bypass with a deep vein and right femoropopliteal bypass with an arm vein. Follow-up duplex US showed a significant stenosis in the midportion of the femoropopliteal bypass. Frontal (a) and lateral (b) volume-rendered CT angiograms show the significant focal stenosis in the midbody of the bypass (arrow). The stenosis was caused by a retained valve cusp. Revision with an interposition graft was performed.
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Figure 10b. Retained valve cusp in a patient who underwent right aortofemoral bypass with a deep vein and right femoropopliteal bypass with an arm vein. Follow-up duplex US showed a significant stenosis in the midportion of the femoropopliteal bypass. Frontal (a) and lateral (b) volume-rendered CT angiograms show the significant focal stenosis in the midbody of the bypass (arrow). The stenosis was caused by a retained valve cusp. Revision with an interposition graft was performed.
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Figure 11a. Bypass stenoses in a patient with bilateral femoropopliteal bypasses created from GSV. Duplex US findings were suggestive of a stenosis at the distal end of the right bypass. (a) Posterior volume-rendered CT angiogram of the right bypass shows a relatively long segment stenosis (arrows) and a high-grade stenosis at the distal anastomosis (arrowhead). The distal stenosis was repaired with a jump graft created from basilic vein. Repeat duplex US showed high velocities at the proximal anastomosis of the jump graft. (b) Posterior volume-rendered CT angiogram shows the jump graft bypass (curved arrow), which originates from a segment of narrowed vein proximally (straight arrow). A second jump graft created from an arm vein was placed to bypass the proximal stenosis.
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Figure 11b. Bypass stenoses in a patient with bilateral femoropopliteal bypasses created from GSV. Duplex US findings were suggestive of a stenosis at the distal end of the right bypass. (a) Posterior volume-rendered CT angiogram of the right bypass shows a relatively long segment stenosis (arrows) and a high-grade stenosis at the distal anastomosis (arrowhead). The distal stenosis was repaired with a jump graft created from basilic vein. Repeat duplex US showed high velocities at the proximal anastomosis of the jump graft. (b) Posterior volume-rendered CT angiogram shows the jump graft bypass (curved arrow), which originates from a segment of narrowed vein proximally (straight arrow). A second jump graft created from an arm vein was placed to bypass the proximal stenosis.
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Figure 12a. Bypass stenosis in a patient with severe bilateral claudication and a vein graft in the right lower extremity. Frontal (a) and posterior (b) volume-rendered CT angiograms show a long vein graft (arrows in a) extending from the right CFA, which has a small proximal segment, to the anterior tibial artery, where a severe distal stenosis is seen (arrowhead). Note the long segment occlusion of the left SFA with a single anterior tibial artery runoff.
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Figure 12b. Bypass stenosis in a patient with severe bilateral claudication and a vein graft in the right lower extremity. Frontal (a) and posterior (b) volume-rendered CT angiograms show a long vein graft (arrows in a) extending from the right CFA, which has a small proximal segment, to the anterior tibial artery, where a severe distal stenosis is seen (arrowhead). Note the long segment occlusion of the left SFA with a single anterior tibial artery runoff.
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Figure 13a. Bypass stenoses in a patient who underwent multiple IGAB procedures in the left lower extremity. The most recent bypass became thrombosed and was revised with thrombolysis and a metallic stent placed in the distal segment. However, the patient continued to have critical limb ischemia. (a) Frontal volume-rendered CT angiogram shows a left femoropopliteal bypass that is occluded near the distal anastomosis (arrowhead). There is a sequential vein bypass from the synthetic graft to the posterior tibial artery (arrows). (b) Posterior volume-rendered CT angiogram shows stenoses at the origin and in the proximal segment of the sequential bypass (arrowheads); a metallic stent is seen in the distal segment (arrow). (c) Frontal maximum intensity projection CT angiogram and axial images obtained for vessel analysis (displayed from proximal to distal relative to the stent) show a significant stenosis at the proximal end of the stent (arrowheads); the stenosis was caused by a crush injury to the stent. Below-knee amputation was elected for definitive therapy.
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Figure 13b. Bypass stenoses in a patient who underwent multiple IGAB procedures in the left lower extremity. The most recent bypass became thrombosed and was revised with thrombolysis and a metallic stent placed in the distal segment. However, the patient continued to have critical limb ischemia. (a) Frontal volume-rendered CT angiogram shows a left femoropopliteal bypass that is occluded near the distal anastomosis (arrowhead). There is a sequential vein bypass from the synthetic graft to the posterior tibial artery (arrows). (b) Posterior volume-rendered CT angiogram shows stenoses at the origin and in the proximal segment of the sequential bypass (arrowheads); a metallic stent is seen in the distal segment (arrow). (c) Frontal maximum intensity projection CT angiogram and axial images obtained for vessel analysis (displayed from proximal to distal relative to the stent) show a significant stenosis at the proximal end of the stent (arrowheads); the stenosis was caused by a crush injury to the stent. Below-knee amputation was elected for definitive therapy.
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Figure 13c. Bypass stenoses in a patient who underwent multiple IGAB procedures in the left lower extremity. The most recent bypass became thrombosed and was revised with thrombolysis and a metallic stent placed in the distal segment. However, the patient continued to have critical limb ischemia. (a) Frontal volume-rendered CT angiogram shows a left femoropopliteal bypass that is occluded near the distal anastomosis (arrowhead). There is a sequential vein bypass from the synthetic graft to the posterior tibial artery (arrows). (b) Posterior volume-rendered CT angiogram shows stenoses at the origin and in the proximal segment of the sequential bypass (arrowheads); a metallic stent is seen in the distal segment (arrow). (c) Frontal maximum intensity projection CT angiogram and axial images obtained for vessel analysis (displayed from proximal to distal relative to the stent) show a significant stenosis at the proximal end of the stent (arrowheads); the stenosis was caused by a crush injury to the stent. Below-knee amputation was elected for definitive therapy.
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Figure 14a. Bilateral popliteal artery aneurysms repaired with IGAB. (a) Axial contrast-enhanced CT image shows that the right bypass is patent (arrow); the right popliteal aneurysm continues to perfuse (arrowhead). (b) Posterior volume-rendered CT angiogram shows the patent bypass from the right SFA to the peroneal artery (arrows) with an aneurysmal dilatation at the distal anastomosis (arrowhead). Severe streaking artifacts are seen owing to a total left knee replacement. Note the continuous filling of the right popliteal aneurysm (A) from the SFA. The patient underwent reexclusion of the aneurysm with ligation of the distal SFA and popliteal artery. A new bypass was created with GSV. (c) Axial contrast-enhanced CT image obtained after repeat surgery shows successful thrombosis of the aneurysm (arrow). (d) Volume-rendered CT angiogram shows the patent bypass, which has a nonvisualized segment (arrowhead) due to the metallic artifact. The popliteal aneurysm is thrombosed.
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Figure 14b. Bilateral popliteal artery aneurysms repaired with IGAB. (a) Axial contrast-enhanced CT image shows that the right bypass is patent (arrow); the right popliteal aneurysm continues to perfuse (arrowhead). (b) Posterior volume-rendered CT angiogram shows the patent bypass from the right SFA to the peroneal artery (arrows) with an aneurysmal dilatation at the distal anastomosis (arrowhead). Severe streaking artifacts are seen owing to a total left knee replacement. Note the continuous filling of the right popliteal aneurysm (A) from the SFA. The patient underwent reexclusion of the aneurysm with ligation of the distal SFA and popliteal artery. A new bypass was created with GSV. (c) Axial contrast-enhanced CT image obtained after repeat surgery shows successful thrombosis of the aneurysm (arrow). (d) Volume-rendered CT angiogram shows the patent bypass, which has a nonvisualized segment (arrowhead) due to the metallic artifact. The popliteal aneurysm is thrombosed.
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Figure 14c. Bilateral popliteal artery aneurysms repaired with IGAB. (a) Axial contrast-enhanced CT image shows that the right bypass is patent (arrow); the right popliteal aneurysm continues to perfuse (arrowhead). (b) Posterior volume-rendered CT angiogram shows the patent bypass from the right SFA to the peroneal artery (arrows) with an aneurysmal dilatation at the distal anastomosis (arrowhead). Severe streaking artifacts are seen owing to a total left knee replacement. Note the continuous filling of the right popliteal aneurysm (A) from the SFA. The patient underwent reexclusion of the aneurysm with ligation of the distal SFA and popliteal artery. A new bypass was created with GSV. (c) Axial contrast-enhanced CT image obtained after repeat surgery shows successful thrombosis of the aneurysm (arrow). (d) Volume-rendered CT angiogram shows the patent bypass, which has a nonvisualized segment (arrowhead) due to the metallic artifact. The popliteal aneurysm is thrombosed.
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Figure 14d. Bilateral popliteal artery aneurysms repaired with IGAB. (a) Axial contrast-enhanced CT image shows that the right bypass is patent (arrow); the right popliteal aneurysm continues to perfuse (arrowhead). (b) Posterior volume-rendered CT angiogram shows the patent bypass from the right SFA to the peroneal artery (arrows) with an aneurysmal dilatation at the distal anastomosis (arrowhead). Severe streaking artifacts are seen owing to a total left knee replacement. Note the continuous filling of the right popliteal aneurysm (A) from the SFA. The patient underwent reexclusion of the aneurysm with ligation of the distal SFA and popliteal artery. A new bypass was created with GSV. (c) Axial contrast-enhanced CT image obtained after repeat surgery shows successful thrombosis of the aneurysm (arrow). (d) Volume-rendered CT angiogram shows the patent bypass, which has a nonvisualized segment (arrowhead) due to the metallic artifact. The popliteal aneurysm is thrombosed.
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Figure 15a. Bypass stenoses repaired with endovascular treatment in a patient with a recurrent nonhealing ulcer of the right lower extremity. Duplex US showed a high-grade stenosis at the proximal anastomosis of a GSV bypass. (a) Anterior volume-rendered CT angiogram shows the high-grade stenosis at the proximal anastomosis (straight arrow) and a second stenosis at the distal anastomosis (curved arrow). (b) Digital subtraction angiogram (anteroposterior projection) shows the proximal (straight arrow) and distal (curved arrow) stenoses. Both lesions were successfully treated with balloon angioplasty.
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Figure 15b. Bypass stenoses repaired with endovascular treatment in a patient with a recurrent nonhealing ulcer of the right lower extremity. Duplex US showed a high-grade stenosis at the proximal anastomosis of a GSV bypass. (a) Anterior volume-rendered CT angiogram shows the high-grade stenosis at the proximal anastomosis (straight arrow) and a second stenosis at the distal anastomosis (curved arrow). (b) Digital subtraction angiogram (anteroposterior projection) shows the proximal (straight arrow) and distal (curved arrow) stenoses. Both lesions were successfully treated with balloon angioplasty.
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Problems with Prosthetic Grafts
PTFE bypass for the femoropopliteal portion has a 5-year primary patency of 57% and secondary patency of 73% for patients with claudication, and 48% primary patency and 54% secondary patency for patients with critical ischemia (37), with a significantly lower patency for infrapopliteal reconstruction. Occlusion of prosthetic grafts has a prevalence of 2%–7% in the first 30 days. Causes of graft thrombosis include technical defects, hypercoagulable disorders, poor runoff, hypotension, intimal hyperplasia, progression of proximal or distal atherosclerotic disease, or lesions within the graft itself (5). While previously occluded vein bypasses are many times not visualized at CT angiography, the walls of the occluded PTFE bypass will be commonly seen as a hyperattenuating circle. Also, the external support rings of PTFE grafts are made of metal and will be seen as a spiral structure around the bypass (Fig 16).
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Figure 16a. PTFE grafts in a patient who underwent multiple IGAB procedures in the right lower extremity. (a, b) Axial contrast-enhanced CT images (a obtained at a higher level than b) show the hyperattenuating wall of an occluded PTFE graft (arrow). The high wall attenuation is due to the high fluorine content of the material. A second PTFE bypass is patent (arrowhead). (c) Frontal volume-rendered CT angiogram shows the patent femoropopliteal bypass (straight arrows); note the spiral configuration of the rings of the occluded PTFE bypass (arrowheads). Significant inflow stenosis of the right external iliac artery is also seen (curved arrow).
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Figure 16b. PTFE grafts in a patient who underwent multiple IGAB procedures in the right lower extremity. (a, b) Axial contrast-enhanced CT images (a obtained at a higher level than b) show the hyperattenuating wall of an occluded PTFE graft (arrow). The high wall attenuation is due to the high fluorine content of the material. A second PTFE bypass is patent (arrowhead). (c) Frontal volume-rendered CT angiogram shows the patent femoropopliteal bypass (straight arrows); note the spiral configuration of the rings of the occluded PTFE bypass (arrowheads). Significant inflow stenosis of the right external iliac artery is also seen (curved arrow).
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Figure 16c. PTFE grafts in a patient who underwent multiple IGAB procedures in the right lower extremity. (a, b) Axial contrast-enhanced CT images (a obtained at a higher level than b) show the hyperattenuating wall of an occluded PTFE graft (arrow). The high wall attenuation is due to the high fluorine content of the material. A second PTFE bypass is patent (arrowhead). (c) Frontal volume-rendered CT angiogram shows the patent femoropopliteal bypass (straight arrows); note the spiral configuration of the rings of the occluded PTFE bypass (arrowheads). Significant inflow stenosis of the right external iliac artery is also seen (curved arrow).
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Local Complications
Local complications of IGAB include hemorrhage, graft thrombosis, and infection. Early bleeding is usually related to suture line breakage or a poorly ligated arterial or venous branch. It requires prompt surgical exploration (Fig 17). Late bleeding is usually related to underlying infection of the anastomotic line and usually requires ligation or excision of the graft (Fig 18).
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Figure 17a. Early bleeding in a patient who underwent placement of an aortic-bifemoral graft and femoropopliteal bypasses. (a) Axial contrast-enhanced CT image shows a large expanding hematoma in the left groin with active extravasation of contrast material (arrow). (b) Axial contrast-enhanced CT image obtained inferior to a shows partial thrombosis of the left femoropopliteal bypass (arrow). (c) Frontal volume-rendered CT angiogram shows the contrast material extravasation (arrowhead); the partial thrombosis of the left femoropopliteal bypass is seen as a stenosis only on the volume-rendered image (arrow). The patient required emergent surgical exploration.
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Figure 17b. Early bleeding in a patient who underwent placement of an aortic-bifemoral graft and femoropopliteal bypasses. (a) Axial contrast-enhanced CT image shows a large expanding hematoma in the left groin with active extravasation of contrast material (arrow). (b) Axial contrast-enhanced CT image obtained inferior to a shows partial thrombosis of the left femoropopliteal bypass (arrow). (c) Frontal volume-rendered CT angiogram shows the contrast material extravasation (arrowhead); the partial thrombosis of the left femoropopliteal bypass is seen as a stenosis only on the volume-rendered image (arrow). The patient required emergent surgical exploration.
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Figure 17c. Early bleeding in a patient who underwent placement of an aortic-bifemoral graft and femoropopliteal bypasses. (a) Axial contrast-enhanced CT image shows a large expanding hematoma in the left groin with active extravasation of contrast material (arrow). (b) Axial contrast-enhanced CT image obtained inferior to a shows partial thrombosis of the left femoropopliteal bypass (arrow). (c) Frontal volume-rendered CT angiogram shows the contrast material extravasation (arrowhead); the partial thrombosis of the left femoropopliteal bypass is seen as a stenosis only on the volume-rendered image (arrow). The patient required emergent surgical exploration.
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Figure 18a. Late bleeding in a patient who presented with a painful popliteal mass with erythema and induration 1 week after repair of a left popliteal artery aneurysm with a GSV bypass. His white blood cell count was elevated. (a) Axial contrast-enhanced CT image shows a large pseudoaneurysm (arrowhead) near the GSV bypass. (b) Posterior volume-rendered CT angiogram shows the proximal and distal anastomoses of the bypass (arrows) with the pseudoaneurysm in the midportion of the bypass (arrowhead). The infected pseudoaneurysm was resected and bypassed with femoropopliteal vein.
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Figure 18b. Late bleeding in a patient who presented with a painful popliteal mass with erythema and induration 1 week after repair of a left popliteal artery aneurysm with a GSV bypass. His white blood cell count was elevated. (a) Axial contrast-enhanced CT image shows a large pseudoaneurysm (arrowhead) near the GSV bypass. (b) Posterior volume-rendered CT angiogram shows the proximal and distal anastomoses of the bypass (arrows) with the pseudoaneurysm in the midportion of the bypass (arrowhead). The infected pseudoaneurysm was resected and bypassed with femoropopliteal vein.
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The reported prevalence of graft infection for femoropopliteal bypass is 0.9%–4.6% (38). Risk factors include emergency procedures, bypass in a subcutaneous tunnel, and a prosthetic graft anastomosed to the femoral artery. Hematomas, lymphoceles, and wound sepsis are predisposing conditions.
Early graft infection (<3 months) is commonly caused by hospital-acquired bacteria (Staphylococcus aureus and gram-negative bacteria) and manifests as aggressive infections with sepsis, fever, leukocytosis, and bacteremia. If the infection is not treated early, anastomotic bleeding can occur (most often with Pseudomonas aeruginosa and other endotoxin-producing gram-negative bacteria). The amputation rates and early mortality rates are high in this group of patients (39). Recently, methicillin-resistant S aureus has emerged as an aggressive and devastating organism in graft infections. Aortic infection is almost always fatal. Infrainguinal infections are associated with a higher prevalence of death from sepsis, limb loss, and overall worst prognosis (40).
Late graft infection (>3 months) is typically caused by low-virulence microorganisms (Staphylococcus epidermidis and other coagulase-negative staphylococci). These organisms are normal skin flora but have the tendency to colonize the biomaterials and grow in the biofilm adherent to the prosthesis surface. Clinically, this indolent infection manifests as healing complications, a perigraft cavity with fluid, a cutaneous sinus tract, or an anastomotic aneurysm without systemic signs or symptoms of infection. It is characterized by absence of graft incorporation with the tissues and perigraft fluid rich in leukocytes. It is typically seen in patients who undergo multiple repeat surgeries for graft thrombosis or false aneurysms. Cultures of perigraft fluid are often negative. Aspirates can be subjected to ultrasonic oscillation (sonication) and tissue grinding and cultured in liquid media (instead of agar), with the cultures kept for a minimum of 14 days (38,40,41).
CT is the imaging modality of choice for diagnosis of IGAB infection with an overall sensitivity and specificity of 94% and 85%, respectively (42). CT findings of infection include loss of normal tissue planes, perigraft fluid collections, and false aneurysms. Other signs include graft occlusion, adjacent osteomyelitis, and wound dehiscence (Fig 19). Perigraft fluid is rare more than 1 month after the initial operation and if it persists beyond 3 months is highly suspicious for infection. The presence of gas is normal in the early postoperative period and should resolve. Any gas after 1 month is almost pathognomonic of graft infection (Fig 20).
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Figure 19a. Bypass graft infection. Axial contrast-enhanced CT image (a) and volume-rendered CT image (b) show an exposed bypass graft (arrow) with wound dehiscence in the left groin.
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Figure 19b. Bypass graft infection. Axial contrast-enhanced CT image (a) and volume-rendered CT image (b) show an exposed bypass graft (arrow) with wound dehiscence in the left groin.
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Figure 20a. Bypass graft infection. Axial contrast-enhanced CT images show a patent left femoropopliteal bypass with an extensive fluid collection (arrows in b) containing air bubbles (arrowhead in a). The infection was caused by methicillin-resistant S aureus.
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Figure 20b. Bypass graft infection. Axial contrast-enhanced CT images show a patent left femoropopliteal bypass with an extensive fluid collection (arrows in b) containing air bubbles (arrowhead in a). The infection was caused by methicillin-resistant S aureus.
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CT-guided aspiration of fluid and drainage is useful for early and late infections. Conventional arteriography was considered a routine procedure for development of a treatment strategy in patients with infected bypass (38). CT angiography has replaced conventional arteriography in evaluation and treatment planning of infected IGAB. CT angiography accurately demonstrates the extent of the infectious process in relationship to the vessels for placement of vascular clamps, provides accurate vascular mapping with identification of anastomotic aneurysms, and allows planning of additional bypasses based on the status of the inflow and runoff vessels.
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Conclusions
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CT angiography is a valuable imaging test for studying the different types of IGAB. Understanding the different materials and surgical techniques used in creation of surgical bypasses will aid in interpretation of the CT angiographic findings. CT angiography is also a powerful tool for detecting early and late complications of IGAB and has replaced conventional arteriography as the imaging modality of choice to better delineate any potential problems detected with duplex US. CT angiography is playing an increasingly important role in the periodic surveillance and maintenance of IGABs.
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Footnotes
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Abbreviations: CFA = common femoral artery, GSV = greater saphenous vein, IGAB = infrainguinal arterial bypass, PTFE = polytetrafluoroethylene, SFA = superficial femoral artery
See the commentary by Matalon following this article.
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References
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T. A. S. Matalon
Invited Commentary
RadioGraphics,
March 1, 2008;
28(2):
549 - 549.
[Full Text]
[PDF]
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Abstract
Introduction
