A Guide To New Advances In Vascular Imaging

Given recent technological advances with vascular imaging modalities, successful limb salvage may be an option for patients previously resigned to amputation. This author details how various imaging options, including multi-channel computerized tomography angiography, can be beneficial for high-risk podiatric patients.

   Primary amputations still occur worldwide at an alarming rate. Worldwide, it is estimated that there is a diabetic foot amputation every 30 seconds.1

   Therefore, an opportunity exists with the contemporary non-invasive vascular imaging to better identify the vascular and chronic limb ischemia (CLI) patient who would be a candidate for limb salvage revascularization and make an impact on the high incidence of primary amputation.

   In a recent analysis of 417 CLI patients in the United States, 67 percent had a primary amputation as their initial CLI treatment and only 49 percent had any diagnostic vascular evaluation. Of those, only 34 percent underwent an ankle-brachial index (ABI) study and 16 percent had angiography.2

   There have been recent improvements in noninvasive vascular imaging including magnetic resonance angiography (MRA), multi-channel computerized tomography angiography (CTA) and the safety of traditional angiography. Given all those improvements, I believe no CLI patient should be scheduled for an amputation without at least noninvasive vascular imaging and preferably limb salvage angiography beforehand.

   Indeed, the healthcare industry’s next challenge should be to change this “pathway to amputation” to a “pathway of revascularization,” especially with the dramatic improvement in infrapopliteal diagnostic and treatment options over the last three to four years.

What You Should Know About Multidetector CTA

   Conventional diagnostic angiography (CDA) and digital subtraction angiography (DSA) remain the clinical “gold standards” for vascular imaging but both have multiple limitations. Foremost among these limitations are the invasive nature of the procedures with the attendant morbidity associated with catheterization.

   Physicians have advocated the use of MRA to address the limitations of DSA but MRA also possesses significant limitations, notably limited temporal and spatial resolution. Magnetic resonance angiography has also recently been associated with renal failure and systemic nephrogenic sclerosis, is very expensive and is not widely available.3

   Multidetector CTA is an emerging, non-invasive modality, which enables one to assess vasculature including coronary artery anatomy. In comparison to conventional angiography or DSA, multi-detector CTA is less expensive, less invasive and faster. The technology requires fewer medical professionals and potentially reduces radiation exposure and contrast use. Other advantages of CTA include the ability to view vessels in multiple tomographic planes and create 3-D reconstructions.

   Since computerized tomography angiography techniques were originally described in 1992, CT scanners have advanced from single-detector systems to powerful multi-detector systems capable of rapidly acquiring 16 to 64 channels of data with higher spatial resolution. Current multidetector CT scanners obtain very high-resolution images and allow coverage of more than 120 cm with a single scan. The multidetector CT now permits scan times of less than a second, scan thickness of less than 1 mm and the acquisition of more than 4,000 images per examination with a single venous injection.

   Optimized contrast enhancement, improved 3-D volumetric data analysis and sophisticated computer software and workstations are just some of the advantages of the current technology. These advantages are the reasons why several sources are predicting that multidetector CTA will “replace 80 to 90 percent of all conventional diagnostic angiography.”4 Multidetector CTA has already demonstrated comparability and even diagnostic superiority over conventional diagnostic angiography in several vascular applications. These applications include the aorta, carotid, renal, iliac and infrainguinal disease.

   Typically, one administers 80 to 100 mL of a low osmolar contrast agent intravenously through a 20-gauge plastic venous cannula in an antecubital vein at an injection rate of 4 to 6 mL/second. The physician determines contrast dose, acquisition parameters, coverage speed and circulating time by protocols based on the clinical information requested, the imaging modality used and patient-specific variables (age, body size, cardiac output, renal function, etc.).

   Using currently available technology, it is possible for the treating physician to obtain images from the clavicles or abdomen down to the knees, or from the abdomen and pelvis to the patient’s feet, all in less than 30 seconds.

Taking An ‘Endovascular First’ Approach To Aortoiliac Occlusive Disease

   Well over a decade ago, our practice adapted an “endovascular first” approach towards aortoiliac occlusive disease (AIOD) revascularization. Acceptable long-term results in iliac PTA/stenting have substantiated this policy. The diagnostic accuracy of digital subtraction angiography is adversely affected by vascular calcification and DSA may fail to detect aneurysmal disease. When it comes to digital subtraction angiography, there is often poor visualization of vessels distal to a more proximal stenosis or occlusion.

   Native common femoral artery (CFA) disease is not uncommon. Frequently, patients will present with “stick site injuries” from multiple prior procedures. Avoiding a “diagnostic stick” with DSA has significant clinical implications. We have found multi-detector CTA to be particularly useful in avoiding “sticking into disease” and in planning our endovascular treatments.

   Peripheral vascular disease (PVD) patients often have limited vascular access (prior grafts, procedures, heavy common femoral artery calcification, etc.) and are known to be at higher risk for vascular access site complications. Multidetector CTA has allowed our institution to greatly decrease access complications by utilizing the iliofemoral and common femoral artery imaging information to plan our access site management strategies. This is a little appreciated but important advantage of multidetector CTA in managing the PVD patient.

   Several recent studies have shown the sensitivity and specificity of multidetector CTA in detecting significant AIOD to be greater than 96 percent. Rubin and colleagues recently reported 100 percent concordance between multidetector CTA and DSA in AIOD, using 2.5 mm slices to cover 120 mm in less than 45 to 60 seconds of acquisition time.5 Researchers have shown that multidetector CTA is superior to DSA in evaluating vascular trauma, dissections and popliteal aneurysms.

   Several asymptomatic significant iliac and popliteal aneurysms greater than 5.0 cm are diagnosed each month when patients undergo diagnostic imaging for suspected AIOD.5-6 Recent reports have shown a reduction in contrast use and a fourfold reduction in radiation exposure when researchers compared multidetector CTA and DSA in diagnosing aortoiliac occlusive disease.7

What The Research Reveals About Multidetector CTA And Infrainguinal Disease

   The superficial femoral arteries and crural vessels can be among the most calcified vessels in the body. Accordingly, they are challenging in regard to accurate assessment of the degree of stenosis. High incidences of vessel occlusions, asymmetric disease and vascular calcifications are characteristic of infrainguinal disease.

   Known advantages of multidetector CTA relative to DSA or digital ultrasound include improved accuracy in vascular occlusions, calcifications and patients with asymmetrical disease. No published data comparing DSA or DU with multidetector CTA in infrainguinal vessels are available. However, several promising image enhancement processing techniques are available, especially when it comes to infrapopliteal vessels.

   Curved planar reformation and semitransparent volume rendering with automated measurements are new 3-D imaging modalities that improve the accuracy of multidetctor CTA in highly calcified vessels.8 The editing of bony structures (osseous segmentation) is now available at the workstation through the use of automatic region growing imaging techniques. Segmentation of the tibia, fibula and tarsal osseous structures can have significant clinical implications in achieving limb salvage. Maximum intensity projection offers a 3-D workstation that allows maximal contrast opacification and vessel interrogation.

   Utilizing multidetector CTA due to the improved arterial opacification distal to an occluded segment, Rubin and colleagues identified 26 additional infrainguinal arterial segments that were not identified with DSA.5 Identification of “distal targets” could steer patients toward tibial bypass or endovascular revascularization. Otherwise, these patients may only be offered amputation.

   This is especially the case when one considers the six- to 12-month limb salvage rates of more than 90 percent reported by Laird and co-workers who utilized the Excimer laser (Spectranetics) in the multi-center LACI trial.9 Our group reported similar results in a “LACI-equivalent” report.10-11

   Magnetic resonance angiography is considered the gold standard in imaging pedal vessels. However, it is less reliable in calcified vessels. In addition to being time-consuming, MRA is not widely available and is affected by retrograde flow artifacts, stent artifacts and pacemakers. We have found infrainguinal multidetector CTA invaluable in the diagnosis, treatment, planning and follow-up of our infrainguinal interventions, especially in infrapopliteal vessels in which DU is rarely helpful.

   Sixty-four-channel multidetector CTA has the potential to “miss” infrainguinal PVD as the speed of the scanner may simply be too fast to acquire timely images secondary to the delay imposed by lower extremity occlusive disease. No validation data exists regarding the role of 64-channel multidetector CTA in the diagnosis and treatment of infrainguinal occlusive disease. However, different imaging acquisition settings are likely necessary to optimize imaging in infrainguinal and especially infrapopliteal arteries.

   This is particularly important in light of recent CLI data suggesting that an estimated 220,000 to 240,000 amputations are performed yearly in the U.S. and Europe collectively, and that less than 50 percent have digital subtraction angiography before undergoing a primary amputation.12-13 Recent reports demonstrate that endovascular treatments for treating CLI have more than 90 percent limb salvage rates at six and 12 months. These reports justify the emergence of noninvasive, diagnostic infrainguinal and infrapopliteal imaging techniques.9,11,14

   Our group recently reported the first experience utilizing a novel 64-channel multidetector CTA “limb salvage” imaging protocol designed to optimize 64-slice infrainguinal imaging.15-16 Our current protocol includes the use of 75 mL of iopamidol (Isovue 370, Bracco Diagnostics), a 10-second scan delay and an injection rate of 4 cc/sec with a 40 cc normal saline bolus chase. We moved the automated trigger position down from the level of the diaphragm to 1 cm above the iliac bifurcation and increased the Hounsfield units from 180 H to 250 H.

   The resolution quality and post-processing time improved in comparison to the standard protocols, and strongly correlated with DSA in the 60 patients treated for CLI with endovascular revascularization.14-15 Our group is initiating further imaging optimization protocols at this time.

Inside Insights On Emerging Technologies

   Researchers are currently evaluating several emerging technologies and their capabilities for assessing wound healing and evaluating pedal microcirculatory function pre- and post-peripheral vascular intervention (PVI).

   The OxyVu (Hypermed) uses medical hyperspectral imaging, a camera-based diagnostic tool that quantifies hyperspectral tissue oxygenation in diabetic foot ulcer wound healing. This has the potential to offer an objective assessment of pedal microcirculatory disease and function.

   The SensiLase System (Väsamed) is a laser Doppler exam, which combines skin perfusion pressure (SPP) and pulse volume recording (PVR). This diagnostic tool enables one to access the progression of wound healing, the status of capillary circulation, and help predict the level of amputation.

   Both technologies may show promise as objective physiologic assessments of pedal microcirculatory function and may help define the clinical role of distal embolization associated with PVI. There are several clinical trials being organized at this time.

In Conclusion

   The PV-CTA is rapidly becoming a valuable noninvasive tool in the “PAD-CLI tool box” as it has the potential to identify PAD and CLI patients facing amputations who are today viable candidates for limb salvage revascularization. The PV-CTA is as valuable a tool as the contemporary balloons, lasers, stents and plaque-removing atherectomy devices.

   In our hands, PV-CTA has significantly improved our overall outcomes by facilitating the pre- and peri-procedural management of the CLI patient. We believe no patient today should undergo an amputation without a vascular consult and at least a PV-CTA.

Dr. Allie is the Chief of Cardiothoracic and Endovascular Surgery at the Cardiovascular Institute of the South in Lafayette, La.
Dr. Allie thanks Kelly M. Tilbe, NCMA for her assistance with the preparation of this article.

References:

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