Selecting the Right Vascular Treatment for Patients with Peripheral Arterial Disease

Selecting the Right Vascular Treatment for Patients with Peripheral Arterial Disease

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Dr. Andrew Dueck
Associate Scientist, Physical Sciences, Schulich Heart Research Program, Sunnybrook Research Institute
Vascular Surgeon, Head of Vascular Surgery, Sunnybrook Health Sciences Centre
Assistant Professor, Division of Vascular Surgery, University of Toronto

By: Mikaeel Valli
Photo By: Mikaeel Valli

Peripheral arterial disease (PAD) is a common circulatory problem in which plaque build-up in the arteries leads to narrowing of the vessels, commonly in the legs. This arterial narrowing process, also known as atherosclerosis, hinders sustainable blood supply to the limbs and impedes their function. In severe cases, patients’ legs could face amputation as the last treatment resort. PAD affects 800,000 Canadians, with the most common symptom being claudication—that is, leg pain when engaging in simple physical activities (such as walking) which improves with rest. However, 50% of patients are asymptomatic, known as ‘silent’ PAD, that leaves tens of thousands of Canadians at greater risk for preventable cardiovascular disease including heart attacks and strokes.

Fortunately, PAD treatments are available which greatly improve patients’ quality of life and significantly preserves and protects their legs from amputation. When conservative measures including medication and supervised exercise are unable to improve the circulation to the limbs, revascularization becomes necessary. Bypass surgery is an approach that involves attaching a graft that bypasses the blocked artery. Conversely, the endovascular technique is a commonly used approach that opens the arterial blockage using a catheter or a thin guidewire. This method is preferred over bypass surgery because it is minimally invasive, making it less risky with fewer treatment complications. However, vascular surgeons are faced with a dilemma: which PAD patients are an appropriate candidate for endovascular treatment?

The IMS Magazine had the pleasure of interviewing Dr. Andrew Dueck, a vascular surgeon and associate scientist at Sunnybrook Health Sciences Centre in Toronto. He recalled the many incidences of variability in the amount of time it takes to insert catheters into his patients, as well as multiple occasions where insertion was not possible at all. He explained, “20% technical failure rate occurs for endovascular treatment, and most immediate failure is due to the inability of the guidewire to cross the plaque build-up.” Dr. Dueck emphasized that this variation depends on the plaque being hard or soft, where hard plaque that is especially calcified greatly impacts the guidewires ability to penetrate and thereby affecting the efficacy of the treatment.

Currently, the planning of revascularization is determined through several imaging modalities. However, these imaging techniques lack the accuracy and detail required to provide surgeons with a representative picture of the lesions in the vessels, especially relating to their level of calcification. As a result, Dr. Dueck explained that surgeons cannot make fully informed decisions about revascularization strategies. The gold-standard is X-ray with digital subtraction angiography, which can detect only heavily calcified vessel walls with 60-80% sensitivity. “The drawback is that it cannot differentiate the morphology of the calcification, which is a crucial component that impacts endovascular outcome,” Dr. Dueck explained.

Computed tomography (CT) angiography is another method that can characterize the calcification in medium-sized vessels. The downside, Dr. Dueck explained, “is that it cannot accurately evaluate the tibial vessels of the legs because of the calcium blooming, which is an imaging artefact that shows the calcification to appear larger and therefore makes the occlusion appear bigger than it actually is.” In addition to this being misleading, Dr. Dueck elaborated that “the angiogram does not show the full extent of the lesions in especially distal vessels.” Duplex ultrasound is the third imaging modality that can provide a view of the vessel wall and provide physiological flow information. However, the calcification makes it challenging as it causes acoustic shadowing, creating image artefacts and obscuring the view of the vessels. “There needs to be a better way,” Dr. Dueck declared, “to plan and predict whether endovascular surgery is the right treatment for patients with PAD.” Therefore, Dr. Dueck has teamed up with Dr. Trisha Roy, a vascular surgery resident, and Dr. Grahm Wright, a scientist in cardiovascular imaging at Sunnybrook, to find a suitable solution.

Through funding from CIHR as well as the Heart & Stroke Lewer Centre of Excellence/Toronto Academic Cardiac Vascular Collaborative, the team is exploring whether magnetic resonance imaging (MRI) could be an effective alternative tool for vascular surgeons. Dr. Roy, the main lead of this project, experimented with two MRI techniques with specific sequences: flow independent MR angiogram using 3D steady-state free precession sequence (SSFP), and 3D ultrashort echo time (UTE) using a prototype 3D cones sequence. “SSFP angiograms allow the visualization of the lesion and blood vessel anatomy while the UTE advantageously allows the view of calcium and dense collagen,” Dr. Roy explained. Unlike X-ray angiogram, Dr. Roy described that these MRI techniques provide “a very good soft-tissue contrast and permit a detailed picture of blocked vessels, especially downstream.”

The trio ran a pilot study in collaboration with Dr. Hou-Jen Chen at Sunnybrook, using the two MRI techniques to see if they can effectively identify which lesions will be more difficult to cross with a guidewire. Dr. Roy enthusiastically stated that “lesions identified as hard by MRI correlated with longer guidewire travel time, an average of 14 minutes. In contrast, lesions defined as soft by the MRI took about 2 minutes to cross.” She continued to explain that the images were able to detect hidden passages within the arteries that were not entirely blocked (called ‘occult patencies’) and non-calcified hard lesions that were not visible on X-ray angiography. “This pilot study showed that MRI poses to be a potentially useful and effective tool in the planning of peripheral endovascular interventions, or to decide if conventional surgery is a better option, as we now have the prior knowledge of where the lesion is and what kind of lesion is deposited in the arteries,” Dr. Roy said.

The tricky part of using these two MRI techniques is that they are complicated for clinicians to interpret compared to conventional CT scans. Dr. Dueck cautioned that if the generated images are not simplified or easy to read and use, translating the use of MRI to clinical practice will be hindered, and MRI will remain a research tool. Therefore, the team collaborated with Dr. Michael Schumaker, who developed software equipped with segmentation algorithms and is capable of automated image processing. Dr. Roy explained, “this automated software colour codes the MRI to make it easier to read; for instance, purple represents dense collagen while blue is for calcium.” The team has filed a patent for this software and are looking forward to optimizing its function as a user-friendly clinical tool for guiding all vascular surgeons in selecting the appropriate patients for endovascular treatment.

“The next step going forward is to validate the use of these MRI techniques and software on a larger patient scale at St. Michael’s and Toronto General Hospital,” Dr. Dueck explained. However, before doing that, he clarified, “we are looking to hire an MRI software engineer that can make the specific MR sequences compatible with other brands of MRI machines as currently, it is only compatible with the G.E. brand.”

In conclusion, Dr. Dueck and Dr. Roy believe that the use of MRI will permit better revascularization planning and patient selection, allowing for a higher success rate of endovascular treatment. The MRI based tools have the potential to reduce treatment complications and ultimately, improve the overall quality of life for PAD patients.