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Original Research |
1 Department of Cardiovascular and Interventional Radiology, Medical University
of Vienna, Währinger Gürtel, 18-20, Vienna 1090, Austria.
2 Present address: Department of Radiology, Stanford University Medical Center,
Stanford, CA.
Received February 15, 2007;
accepted after revision May 20, 2007.
Supported by Fonds zur Förderung der wissenschaftlichen Forschung
(FWF, Austria) under the contract no. L291-N04.
Abstract
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MATERIALS AND METHODS. Fifty-eight patients with stage IIb PAOD underwent CT angiography of the abdominal aorta and runoff vessels for further treatment planning. Treatment reports, discharge summaries, and follow-up examinations were reviewed to determine the number of treatments correctly planned on the basis of CT angiographic findings.
RESULTS. On the basis of CT angiographic findings, endovascular treatment was indicated for 18 patients, surgical revascularization for nine patients, and a combined endovascular and surgical approach for two patients. Conservative treatment was indicated for 29 patients. On the basis of successful revascularization, the correctness of the treatment decision was confirmed in all but one patient (n = 28). The treatment plan was modified for one patient referred for surgical revascularization. In that patient, stenosis of the common femoral artery had been overlooked on CT angiography. Patients for whom conservative management was indicated on the basis of CT angiographic findings (n = 29) had a mean follow-up period of 501 days without needing revascularization treatment. This result was defined as indirect confirmation of the accuracy of the decision made with CT angiography.
CONCLUSION. The findings on MDCT angiography led to correct treatment recommendations for patients with claudication. Thus, CT angiography should be used in the management of PAOD.
Keywords: claudication • CT angiography • peripheral arterial occlusive disease • treatment decision • vascular imaging
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Subjects
During the study period, 58 consecutively enrolled patients with known or
clinically suspected PAOD in Fontaine stage IIb were referred to undergo CT
angiography of the abdominal aorta and runoff vessels. Clinical diagnosis and
indication for imaging were determined by referring specialists from the
departments of vascular surgery and vascular medicine at our university
hospital.
The study population (n = 58) consisted of 42 men (72%) and 16 women (28%). The mean age of the study group was 65 ± 9 (SD) years (range, 49–84 years). The mean age of the men was 62 ± 9 years (range, 49–81 years) and of the women was 69 ± 7 years (range, 56–84 years). Twenty-six patients had unilateral symptoms, and 32 (55%) had bilateral symptoms. Thirty-five (60%) of the patients had hypertension or hyperlipidemia. Twenty-five (43%) of the patients had coronary heart disease, 20 (35%) had diabetes mellitus, and 16 (28%) were smokers at the time of the examination. Ten (17%) of the patients had cardiac arrhythmias, and two (3%) had a history of stroke.
CT Angiography
All peripheral CT angiograms were obtained on a 16-MDCT scanner (Somatom
Sensation 16, Siemens Medical Solutions).
CT angiography scanning parameters—All patients underwent CT angiography from the level of the renal arteries through the feet. A detector configuration of 16 x 0.75 mm was combined with a table increment (table translation per 360° of gantry rotation) of 14 mm (28 mm/s). This protocol allowed mean coverage of 1,224 ± 72 mm (range, 1,060–1,371 mm) in approximately 40–45 seconds. Sections 2 mm thick were reconstructed at 1-mm intervals with a medium filter and a mean image field of view of 310 ± 27 mm (range, 260–374 mm).
Scan timing—An automated bolus-triggering technique (CARE Bolus, Siemens Medical Solutions) was used in all patients. Nonincremental images were obtained at the level of the abdominal aorta 8 seconds after the start of the injection of contrast medium. The CT angiographic acquisitions were initiated at a trigger threshold of 150 H within the aorta.
CT angiography contrast injection parameters—Automated injections of contrast medium were performed with a programmable double-barrel power injector (Injektron CT2 revision CT2A113, Medtron). Iomeprol (Iomeron 400, Bracco Austria), a nonionic iodinated contrast medium, was used at a concentration of 400 mg I/mL for all CT studies. Biphasic injections of contrast medium followed by a saline flush were used in all patients [12]. The first phase (duration, 5–6 seconds) consisted of injection of 25 mL of the contrast agent at a flow rate of 4.5 mL/s and was followed by a second phase in which the agent was injected at a flow rate of 2.3 mL/s. The duration of the second phase was 10 seconds shorter than the scanning time, ranging from 25 to 35 seconds [13]. The corresponding total volume of contrast medium was 83–105 mL (mean, 94 mL). Contrast injection was immediately followed by a flush of 40 mL of normal saline solution injected at a flow rate of 2.3 mL/s.
CT angiography image postprocessing—Custom-made peripheral CT angiography visualization tools were used to network CT data sets to a workstation [14]. The following sets of images were generated: maximum intensity projections (MIPs), multipath curved planar reformations (CPRs), and two variants of CPRs. The variants were 3-mm projected CPRs (thick CPRs) and unit-thickness stretched CPRs with a virtual gauging catheter (thin CPRs) [15]. Each set of images was generated over a viewing range of 180° in 9° intervals. Images were made in DICOM format to maintain CT attenuation information and were networked to our hospital PACS.
Postprocessing was conducted by one of three radiologic technologists experienced in image processing. Postprocessing tasks included semiautomated bone editing for MIPs and semiautomated vessel tracking to compute a 3D branching tree of arterial centerlines [16]. The centerline tree began in the abdominal aorta and had six end points: the dorsalis pedis, common plantar, and distal peroneal arteries of each leg. Postprocessing tasks took 15–45 minutes depending on the complexity of the disease and the amount of arterial calcification.
CT angiography image interpretation—Peripheral CT angiograms were routinely interpreted on a PACS viewing workstation (Impax, Agfa HealthCare) where transverse source images, MIPs, multipath CPRs, thick CPRs, and thin CPRs are available for interactive review. Selection of postprocessed series or transverse source images was at the discretion of the radiologist and depended on the manifestations of the disease, notably the presence and extent of arterial calcification. When the relevant vasculature was not obscured by calcifications, MIP images were used for image interpretation. Multipath CPRs depicted simultaneous longitudinal cross sections through the conducting arteries, and thus interpretation was possible even in the presence of calcification. Series of thick CPRs and thin CPRs were reviewed only when relevant arterial segments were not clearly displayed in the multipath CPR series or when length measurements were desired.
Peripheral CT angiograms were initially interpreted and the reports dictated by a trainee in the final year of radiology residency. The images then were reviewed by a staff radiologist with more than 5 years of experience in vascular imaging. Detected vessel lesions were classified into categories according to TransAtlantic Inter-Society Consensus [11] guidelines. The guidelines define four categories of aortoiliac and femoropopliteal lesions, depending on the degree, number, and length of stenoses. Recommendations 36 and 37 state that an endovascular procedure is the treatment of choice of patients with class A lesions (most benign) and that surgery is the treatment of choice of patients with class D lesions (most severe). For patients with class B lesions, endovascular treatment is preferred, whereas surgery is preferred for patients at good risk with class C lesions.
The patient's comorbid conditions, fully informed patient preference, and the local operator's long-term success rates must be considered in treatment recommendations for class B and class C lesions. There is no TransAtlantic Inter-Society Consensus guideline on infrainguinal vessels, because almost no randomized trials have been conducted to compare endovascular therapy with bypass surgery for lesions of these vessels. According to Inter-Society Consensus recommendations 34 and 35, patients with short infrainguinal lesions and intermittent claudication should undergo endovascular treatment, whereas patients with longer infrainguinal lesions and chronic limb ischemia should undergo surgical treatment [11].
Treatment Decisions
Treatment decisions were made in consensus during an interdisciplinary
vascular conference, which is routinely held two times per week at our
institution. The conference is attended by interventional radiologists,
vascular surgeons, and vascular medicine physicians. During these conferences,
the treatment plan was fixed for every patient on the basis of the patient's
history, clinical symptoms, and CT angiographic findings. Treatment decisions
were based on the TransAtlantic Inter-Society Consensus
[11] guidelines.
Data Acquisition
For study purposes, a chart review was performed for every patient.
Disease-related information collected included reports from CT angiography and
other imaging methods, including digital subtraction angiography, MR
angiography, and sonography, if available. Treatment reports (including
reports of both surgical and endovascular treatments), discharge summaries,
patient histories, and follow-up reports until June 2006 were reviewed.
Statistical Analysis
The acquired data were entered in a database (Office Excel 2003 SP1,
Microsoft) and exported to a statistics program (SPSS 12.0 for Windows, SPSS).
The mean, minimum, and maximum ages of all patients were calculated, as was
the sex distribution. A summary of each patient's history was created. The
primary location of symptoms of PAOD classified as stage IIb for each patient
was obtained from the report of the clinical examination by the referring
physician. The distribution of inpatients and outpatients was recorded
retrospectively. The findings described in each CT angiography report were
classified individually according to the treatment recommendations of the
TransAtlantic Inter-Society Consensus
[11]. Depending on the
treatment group, different parameters were selected to confirm the treatment
decision. For patients who underwent medical treatment, the follow-up period
and treatment-free interval were calculated. For patients referred for
endovascular revascularization, the findings on CT angiography were correlated
with those of digital subtraction angiography performed during treatment. For
patients referred for surgical revascularization, CT angiographic findings
were compared with the findings in the treatment report and with the findings
from preoperative digital subtraction angiography, if available.
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Endovascular Treatment Indicated on the Basis of CT Angiographic Findings
In the group of patients referred for endovascular treatment after CT
angiography (n = 18), 10 percutaneous transluminal angioplasties
(PTAs) and eight stent implantations were initially planned. In one patient, a
stenotic lesion in the right superficial femoral artery could not be passed
during the first angiographic session. However, stent placement through an
antegrade approach was successful in a second treatment session. In another
patient, dissection during the first angiographic session necessitated stent
placement at a second session. No conversion to surgery was recorded. In
summary, nine PTAs and 10 stent implantations were performed successfully; one
PTA could not be performed as initially planned.
In all patients in whom endovascular treatment was indicated on the basis of CT angiographic results (n = 18), the diagnostic angiograms obtained during intervention confirmed the CT angiographic findings (Fig. 1A, 1B, 1C, 1D, 1E, 1F). Treatment was performed as planned in all but two patients. In those two patients, additional stent implantations were necessary because of complications during intervention. The CT angiographic findings before intervention were confirmed on digital subtraction angiography even in these patients, and the decision for endovascular treatment was confirmed afterward by successful completion of the procedure.
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No Revascularization Recommended on the Basis of CT Angiographic Findings
After CT angiography, conservative management was chosen for 29 patients.
In these patients, several lesions were detected and classified according to
TransAtlantic Inter-Society Consensus guidelines (14 class A lesions, one
class B lesion, one class C lesion, and 16 class D lesions). Two failing
grafts also were detected. The patients with class A and class B lesions
(n = 14) did not undergo endovascular treatment because the clinical
situation required only conservative treatment. These patients had a mean
follow-up period of 501 ± 356 days (range, 80–1,058 days) without
endovascular or surgical revascularization or additional vascular imaging.
This outcome indirectly confirmed the clinical decision for conservative
treatment. Patients with class C and those with class D lesions (n =
13) and failing grafts (n = 2) did not undergo surgical treatment
because of contraindications to surgery. Six of these patients had severe
coronary artery disease, one patient had tumor cachexia, and two patients did
not have a distal outflow vessel.
Follow-Up
In the entire follow-up period (until June 2006), patients underwent
additional examinations independent of the findings on baseline CT angiography
(several sonographic investigations, 23 MR and 31 CT angiographic
examinations, one digital subtraction angiographic examination). These
additional examinations were performed as follow-up studies after treatment or
because of clinical progression. Eighteen of the 29 patients treated after
initial CT angiography had no additional findings, a result that confirmed the
diagnosis rendered on CT angiography. The other 11 patients had clinical
progression of disease and underwent several treatments (five bypass graft,
two bifemoral grafts, one patch, five PTAs, and eight stent placements). In
one patient, a femoropopliteal bypass graft had to be removed because of
inflammation of the graft. For patients who had no indications for treatment
on initial CT angiography (n = 29), the mean follow-up period between
the initial and additional imaging examinations was 501 days. Continuation of
conservative treatment of 20 of these patients after the additional
examination was considered confirmation of the decision made after initial CT
angiography. Eight patients in whom clinical progression of PAOD was
associated with findings on additional examinations were referred for several
treatments (three bypass grafts, one patch, five PTAs, and seven stent
placements). One patient in whom no distal outflow vessel was detected on
additional MR angiography or digital subtraction angiography underwent
amputation at the thigh level.
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For appropriate treatment decisions and planning, clinical examination and ankle–brachial index are not sufficient. According to the therapeutic recommendations of the TransAtlantic Inter-Society Consensus [11], the choice of either surgical or endovascular treatment depends on localization and length of lesions. In addition, for the optimal therapeutic strategy, not only the target lesion but also the entire peripheral vascular tree, including the inflow and outflow, should be visualized to detect lesions that can interfere with the outcome of the planned revascularization procedure. Finally, a baseline examination is needed for appropriate treatment follow-up. To obtain all this important information, visualization of the peripheral arteries before treatment is mandatory.
The current imaging standard of reference for complete delineation of the peripheral vasculature is digital subtraction angiography because it has high spatial and temporal resolution. Digital subtraction angiography, however, is hampered by invasiveness and by exposure of the investigator and patient to ionizing radiation [20]. Digital subtraction angiography also is a time- and cost-consuming procedure, and at some institutions, at least one night of hospitalization is mandatory. In the United States, a recovery period of at least 4 hours is necessary. Finally, digital subtraction angiography results only in luminograms, and thus information about plaque constituents and vessel surroundings cannot be acquired [21].
A noninvasive alternative to digital subtraction angiography in evaluation of PAOD is in high demand. Sonography does not depict complete inflow (including the iliac arteries) or outflow (including the pedal arteries) for routine purposes because acquisition of a complete duplex angiogram is extremely time-consuming and investigator-dependent. Furthermore, there is a risk of missing lesions in patients with multiple stenoses. Finally, for postoperative surveillance of lower extremity venous bypass grafts, sonography often does not depict the actual site of stenosis.
Two promising noninvasive methods for evaluation of PAOD are MR angiography and CT angiography. Both techniques have developed rapidly over the last few years. The important innovations in MR angiography have been gadolinium enhancement, multistation protocols, and moving-bed imaging [22–25]. CT angiography has improved vastly with the clinical availability of MDCT scanners [7, 26, 27]. Studies comparing MR and CT angiography with digital subtraction angiography with regard to diagnostic accuracy in the detection of vascular lesions in patients with PAOD have shown high sensitivity and specificity of both MR angiography [22, 25, 28–31] and CT angiography [2–6, 32–34]. Willmann et al. [35] concluded that the advantages of CT angiography over MR angiography are higher image resolution for better evaluation of the small vessels in the calves and higher patient acceptance due to a shorter examination time. The disadvantages of CT angiography are use of ionizing radiation and potentially nephrotoxic contrast agents. Although the clinical use of CT angiography is hampered by the complex, time-consuming reconstruction needed for digital subtraction angiography–like presentation of peripheral CT angiograms [35], CT angiography is still more cost-effective than digital subtraction angiography in the evaluation PAOD [9].
Several studies have shown high diagnostic confidence in CT angiography in comparison with digital subtraction angiography with regard to stenosis detection and quantification [2–8] and the clinical utility of CT angiography in the evaluation of PAOD [9, 10]. However, the reliability of CT angiography for treatment decisions according to TransAtlantic Inter-Society Consensus [11] classifications has not been previously shown, to our knowledge. Thus, the present study was focused on the reliability of peripheral CT angiography in a routine clinical setting according to Inter-Society Consensus [11] guidelines with regard to treatment planning and follow-up. Our goal was to assess whether peripheral CT angiography can be used for the triage of patients with stage II PAOD to undergo conservative, endovascular, or surgical treatment. All CT data were compared in a retrospective analysis of the reports of surgical and radiologic treatment to confirm not only whether the treatment was safely planned but also whether the decision was correct for specific patients.
The results of this analysis of treatment decisions based on findings on CT angiographic examinations performed in a clinical setting indicate that CT angiography can provide all the information needed for the triage and clinical care of patients with stage IIb PAOD. That all endovascular treatments were completed as planned emphasizes the accuracy of decisions based on CT angiographic findings. Treatment (endovascular, surgical, or conservative treatment) was selected at a multidisciplinary conference that included interventional radiologists, vascular surgeons, and angiologists who reviewed the CT angiograms of all patients. Even in the group of surgical patients, the treatment decision made after CT angiography was completely confirmed for all but one patient. That one patient needed an additional patch indicates the importance of correct presurgical assessment. Such an overlooked inflow lesion can negatively influence the function of a subsequent bypass graft.
Because stage IIb PAOD is the entry point for interventional or surgical treatment, only patients with disease in this clinical stage were selected. Twenty-nine patients, however, underwent no treatment after CT angiography. Patients with TransAtlantic Inter-Society Consensus class C or D lesions did not undergo surgical revascularization because they had contraindications to surgery. Lesions considered class A or B were not managed with endovascular therapy if the individual quality of life of the patient was not significantly decreased as the result of PAOD. In all of these patients, no additional imaging or intervention was performed within a mean time of more than 17 months. With regard to the progressive nature of PAOD, this interval without the need for intervention confirms the correctness of the conservative approach to the treatment of these patients. Therefore, the appropriate treatment option can be determined on the basis of CT angiographic findings.
The prototype workstation, which was established at our institution by an interdisciplinary working group of radiologists, technicians, and computer scientists, allows use of peripheral CT angiography in daily routine. Visualization algorithms [15, 16, 36, 37] were developed to allow a short reconstruction time of 15–45 minutes per patient and fast but exact interpretation of angiograms by the responsible radiologist. Among the study patients, only one relevant lesion was missed during routine use, showing high clinical confidence in this method. Multipath CPR [38] can display centered paths through all vessels of both extremities within the same images. Rotation around the sagittal axis is possible and centered in every path, allowing exact visualization of all vessels in a 360° range. This feature is important for the clinical use of peripheral CT angiography because in the detection of stenoses on CT angiograms, diagnosis based only on MIPs without axial images has insufficient accuracy. Rieker et al. [32, 33] measured a decrease to a range of 74–100% for specificity and 36–89% for sensitivity when axial source data were not used for diagnosis. However, diagnosis based on axial source slices, usually more than 1,300 slices per patient, is time-consuming. Without additional reliable reconstruction algorithms, peripheral CT angiography can hardly be used for large numbers of patients in a routine clinical setting. In our multidisciplinary project, peripheral CT angiograms have been reconstructed in this way since the beginning of 2003, allowing examination of approximately 200 patients in the first year and of more than 600 patients overall.
In conclusion, the results of the present study indicate that CT angiography can be a viable and reliable noninvasive imaging method for the evaluation and triage of patients with stage II PAOD. CT angiographic findings are a highly accurate basis for treatment decisions and planning.
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