Latest Update on CLI Imaging Modalities

Introduction

J.A. Mustapha, MD

Like many avenues in medicine, there is always some form of agreement and disagreement on how to perform a procedure or diagnosis. Most of the time it tends to be that the diagnosis can be obtained from multiple modalities. In critical limb ischemia (CLI), there are many ways of making the diagnosis. Clinical diagnosis is usually the first step, but a majority of the time we need secondary and tertiary imaging modalities to study the underlying cause of the clinical CLI. Arterial disease can be imaged in many ways and in this issue, we are honored to have Dr. Constantino Peña from Miami, who will be sharing with us the different types of imaging currently available and the feasibility of each.

J.A. Mustapha, MD: In your practice, after you see a patient and you make the clinical diagnosis of CLI, what is your next step in terms of imaging modalities?

Constantino Peña, MD:  Arterial vascular testing is our initial modality to evaluate these patients. Most of the time, these are obtained before the patient is seen in the office, especially if they are being referred. These examinations include segmental limb pressures, plethysmography (PPG), and duplex and Doppler evaluation. If the patient is able to ambulate safely, segmental pressures at the ankle level are obtained after exercise. This examination not only distinguishes vascular from nonvascular causes of disease, but can quantitate the level and severity of disease, and can be used to monitor the effects of treatment and/or progression of disease. It should not be used to confirm or identify patients with CLI, as these should be defined clinically.   

Dr. Mustapha: What have you observed of the evolution of the imaging diagnostic approach to CLI over the last decade?

Dr. Peña: Over the last decade, the role for morphologic imaging with computed tomographic angiography (CTA) and magnetic resonance angiography (MRA) has increased significantly. These examinations, even though not ideal, add significant and valuable information in patients that are being considered for revascularization. These imaging studies help in planning treatment, and should decrease treatment and procedure time. 

Dr. Mustapha: What are the pros and cons of arterial duplex in the CLI patient?

Dr. Peña: Arterial duplex evaluation in CLI patients can be limited due to vessel calcification. It is helpful in the assessment of inflow disease, particularly aortoiliac disease, especially when an antegrade puncture is contemplated. Arterial duplex examination can also quantitate the severity of a stenosis while directly assessing the nature and character of a diseased vessel wall. Arterial duplex may also be limited, as it may not be helpful in all patient body types, particularly for assessment of the aortoiliac segments.

Dr. Mustapha: What are the pros and cons of CTA for evaluating the superficial femoral artery (SFA)/popliteal, and for evaluating the pedal and tibial arteries?

Dr. Peña: CTA examinations unfortunately utilize iodinated contrast material, which may worsen renal insufficiency, particularly in high-risk patients with CLI.  With CTA examinations of the lower extremity, the diameter of the superficial femoral and popliteal arteries allows for identification of greater than 50% stenosis even in the setting of severe calcification. However, in the pedal and tibial vessels, there are challenges. The size of the vessel and the amount of contrast material necessary to sufficiently opacify the vessels during the CTA acquisition may make it difficult to assess the size of the patent lumen. This is made even more difficult with the density of adjacent vessel calcification (blooming).  

Dr. Mustapha: What are the pros and cons of MRA for evaluating the SFA/popliteal, and for evaluating the pedal and tibial arteries?

Dr. Peña: MRA examinations are ideal for the evaluation of the tibial and pedal vessels that are typically involved in a large percentage of CLI patients. There are numerous techniques to perform these studies; however, typically, contrast-enhanced studies are the most commonly utilized. Unfortunately, gadolinium chelates (contrast used in MR) are contraindicated in patients with severe renal insufficiency due to the risk of nephrogenic systemic fibrosis (NSF). The ability to image the contrast flowing within the vessel with MRA without any significant effect from vessel calcification highlights the ability of MRA to distinguish between patent and occluded vessels. Additionally, time-resolved imaging allows for a temporal evaluation of the vessel, demonstrating its enhancement over time.  

Dr. Mustapha: CO₂ angiography has been mentioned as a possible alternative to contrast in patients with chronic kidney disease. Do you believe CO₂ is as effective as contrast in the aortoiliac, the SFA/popliteal, and the tibial/pedal circulations?

Dr. Peña: We have used a considerable amount of CO₂ gas as an angiographic contrast agent over the years. This is a great imaging alternative. Unfortunately, it becomes limited in the distal tibial and pedal vessels due to the size of the vessels and usual distance from the catheter. Typically we can perform a lower extremity evaluation with CO₂ to the level of the proximal tibial vessels and may need to supplement with several milliliters of iodinated contrast to evaluate the foot. 

Dr. Mustapha: What new imaging modalities are on the horizon as potential tools for assessing the infrainguinal circulation in CLI patients?

Dr. Peña: In the future, I hope that we will continue to improve non-contrast MRA sequences to not only assess patency of vessels, but also the amount of tissue perfusion in both a quantitative, as well as qualitative, manner. Unfortunately, because of the slow or minimal flow in the distal tibial vessels, the use of contrast-enhanced sequences in the future will likely continue. The use of small iron particles as a contrast agent may fill a critical role in these patients. Presently, this is an off-label indication for the use of these iron particles and further study will be necessary. These agents could help patients in which gadolinium is contraindicated.

Dr. Mustapha: Out of all the mentioned imaging modalities above, which exposes the CLI patient to the highest radiation and the highest contrast dose?

Dr. Peña: There is always a trade-off between the benefits and potential risks. CTA exposes patients not only to the risk of iodinated contrast material, but also to ionizing radiation. In the CLI patient population, more of the concern is related to risk of contrast-induced nephropathy and its associated mortality than the risk of radiation. In this patient population, the morbidity and mortality associated with CLI should justify the radiation risk. I do feel that we must always consider radiation dose and attempt to minimize it as necessary using the ALARA principle.   

Dr. Mustapha: Can every center reproduce the same imaging clarity using the above tools, or do variations exist between centers? Can one assume that with proper training and higher volumes, all centers can then provide similar imaging qualities?

Dr. Peña: Unfortunately, there are many factors that affect the quality and clarity of imaging. These factors include patient factors (body habitus, cardiac output, extent and asymmetry of disease) as well as technical factors (protocol, contrast type, machine type). There does exist a variation in the quality of studies from center to center; however, this may be related to the number of cases each center performs, as well as to the patient factors just discussed. Centralizing the studies to a certain scanner or certain location, and increasing in the number studies performed in that location, with appropriate feedback and education, is a solution to increasing quality.

Dr. Mustapha: What is your advice to the CLI specialist on when and which imaging modality to use during the pre-invasive phase of the CLI patient evaluation?

Dr. Peña: Morphologic imaging with CTA or MRA is critical for planning and therefore, safer treatments and improved outcomes. I would advise CLI specialists to consider these examinations when they are strongly considering the decision to intervene, especially in patients that they are treating for the first time. These examinations can help determine proper access sites, necessary tools and devices, and risk assessment prior to treatment, and should play a significant role in the treatment and healing of the patient’s wound. 

Dr. J.A. Mustapha can be contacted at jihad.mustapha@metrogr.org.
Dr. Constantino S. Peña can be contacted at tinopena@msn.com.