Interventional Radiology
Interventional radiology (IR) involves the use of fluoroscopy to gain access to different structures in order to deliver materials for therapeutic purposes. These minimally invasive techniques are currently being investigated in veterinary medicine and have the potential to provide alternatives for our patients in whom conventional therapies are declined, not indicated, or associated with excessive morbidity or mortality. IR can be applied to any body system in patients of all sizes and therefore will likely have very broad applications in our veterinary population.
Following the description of percutaneous arterial catheterization by Seldinger in the 1950’s, angiography developed into a widely-utilized and essential diagnostic tool in human medicine. Technological advances have since helped transform this diagnostic modality into a sub-specialization with enormous therapeutic potential. Interventional radiology (IR) involves the use of contemporary imaging techniques such as fluoroscopy and specialized equipment such as catheters, guidewires, and stents for therapeutic purposes. The field has grown considerably over the past 20 years and IR techniques are considered the gold standard to treat a variety of conditions in humans.
Despite applications in veterinary medicine, IR techniques have not been widely adopted. Examples in the veterinary literature include coil embolization to treat patent ductus arteriosus, stenting for tracheal collapse, coil embolization of the carotid and maxillary arteries for treatment of hemorrhage from guttural pouch mycosis in horses, and coil embolization of canine intrahepatic portosystemic shunts.
This article will include a general introduction to IR including advantages and disadvantages, and a brief review of some of the necessary equipment involved. Next, a categorical approach will be used to briefly describe different procedures currently being performed in veterinary patients. Potential future applications will be discussed. Despite overlap with interventional cardiology, these techniques are beyond the scope of this article and will not be discussed.
Advantages and Disadvantages
The use of IR techniques in veterinary patients offers a number of advantages to more traditional therapies. These procedures are minimally invasive and can therefore lead to reduced peri-operative morbidity and mortality, shorter anesthesia times and shorter hospital stays. Some less equipment-intensive procedures can result in reduced costs as well. In addition, some techniques such as chemoembolization of tumors or palliative stenting for malignant obstructions offer treatment options for patients with various conditions that may not be amenable to standard therapies or when the standard-of-care treatments are associated with excessive morbidity, cost, or poor outcome.
The primary disadvantages of IR include the required technical expertise that is not part of traditional veterinary training, the specialized equipment necessary (fluoroscopy with or without digital subtraction capabilities), and the initial capital investment necessary to provide a suitable inventory of catheters, guidewires, balloons, stents and coils.
Equipment and Technique
As most of these procedures are minimally-invasive (performed through natural orifices or small holes in the skin), traditional sterile operating rooms are not required, but recommended. Most of these procedures are performed in clean angiography suites. The entry sites receive a traditional sterile scrub, and operators wear full lead gowns, lead thyroid shields, caps, gowns, and masks. The radiation exposure during these procedures can be minimized by following well delineated ALARA (“As Low As Reasonably Achievable”) guidelines. The operator should review radiation safety protocols that discuss minimizing exposure time, proper collimation, source to image distance, and shielding.
For many of the more commonly performed IR procedures, a traditional fluoroscopy unit is sufficient. A C-arm fluoroscopy unit has the advantage of mobility of the image intensifier permitting multiple tangential views without moving the patient. Occasionally, ultrasonography is useful for percutaneous needle access into vessels or other structures. Digital subtraction angiography (DSA) and “road-mapping” are specialized, computer software, imaging techniques based on fluoroscopy that allow high resolution images to be obtained with minimal use of contrast agent which is often a concern in our relatively small veterinary patients. DSA is required for super-selective angiograms of small caliber vessels and those vessels in the head (or where there is substantial overlying bone that makes visualization difficult).
Before performing these procedures, the operator should be familiar with basic IR equipment including guidewires, dilators, vascular sheaths/introducers, angiography catheters, stents, and embolic agents. There is an abundance of information available through various medical textbooks although little information currently exists in the veterinary literature.
Endoscopy, in combination with fluoroscopy, can be used for various minimally invasive interventions as well. This modality is termed interventional endoscopy (IE). Some examples include endoscopic ureteral stenting, percutaneous nephrolithotomy (PCNL), cystoscopic-laser ablation (CLA) of ureteral ectopia, and endoscopic retrograde cholangiopancreatography (ERCP) with biliary stenting. These endoscopic procedures are facilitated by the use of fluoroscopy to aid in the guidance of an assortment of guidewires, catheters and stents. The various endoscopes used include, but are not limited to, the following types: rigid cystoscopes (ranging in size from 1.9 mm to 7mm diameter), flexible ureteroscopes (ranging in size from 2.7-3.4 mm diameter), nephroscopes (5.3-7.3 mm), and side-view duodenoscopes (7.5-11 mm diameter).