CHAPTER ONE
Tools of the Trade - Interventional Radiology

Fei Sun

Endoluminal Therapy and Diagnosis Department, Jesús Usón Minimally Invasive Surgery Centre, Caceres, Spain

Interventional radiology refers to image-guided interventions characterized by minimal invasiveness. Technically, these interventions employ various tools introduced via percutaneous access or through natural orifices, navigating to the target organ, and finally deploying specific devices or delivering drugs for therapeutic purpose. Image-guided interventions rely heavily on instrumentation. Ensuring the proper selection of a particular instrument and making rational combinations of the tools and devices available is key to improving technical success, shortening procedure times, and avoiding potential complications. Knowledge of basic instrumentation is of great importance to the clinical practice of interventional radiology. Some of the more commonly used devices will be introduced in this chapter.

Digital Fluoroscopy and Digital Subtraction Angiography

Digital fluoroscopy, also called digital radiography, is a computer-based digital image-processing technique by which real time radiographic images are projected on an image-intensifying fluorescent screen, and in turn converted or digitized for storage or reproduction through an image processor. Compared with conventional fluoroscopy, digital fluoroscopy has several advantages including: post processing that may greatly enhance contrast resolution, high speed image acquisition up to 30 frames/second, and digital image distribution and archiving.

In veterinary hospitals, the standard C-arm digital fluoroscopy system (Figure 1.1) is commonly located in an angiography suite or surgical operating room. It is a mobile and self-contained unit requiring no connections to other equipment. The image intensifier of the C-arm unit normally comes in 23 or 30 cm (9 inch or 12 inch) sizes and can provide a sufficient field of view for interventional procedures in small animals. Advanced hardware and software upgrades may allow the traditional C-arm unit to meet the requirements for more contemporary clinical applications, including cardiovascular, neurovascular, and urological interventions. More advanced digital flat-panel detectors attached to the ceilings or floors are becoming more popular among larger referral veterinary centers.

Figure 1.1 Mobile C-arm fluoroscopy system (BV Pulsera, Philips Medical Systems). (A) C-arm stand and imaging system; (B) mobile view station.

Used with permission from Usón J, Sun F, Crisóstomo V, et al. (2010) Manual de técnicas endoluminales y radiología intervencionista en veterinaria. Jesús Usón Minimally Invasive Surgery Centre, Caceres, Spain.

Currently, digital subtraction angiography (DSA) has become an indispensable tool in angiography and endovascular interventions. DSA refers specifically to techniques by which an initial no-contrast mask image is electronically subtracted from subsequent serial images following injection of contrast medium into the target vessels. After subtraction, the static anatomic structure common to both images is removed; the remaining blood vessels containing contrast medium are opacified. DSA (Figure 1.2) substantially improves the contrast resolution of angiography; however, any slight motion of the structures inside the field of view during the image acquisition may induce remarkable artifacts greatly compromising the image quality. Accordingly, temporarily controlled apnea by suspension of mechanical ventilation is often recommended for abdominal angiography when performing DSA.

Figure 1.2 Selective digital subtraction angiography of the azygos vein (Lateral view). (A) mask image showing the background structures before injection of contrast medium; (B) The live or contrast image including azygos vein and surrounding anatomic structure; (C) digital subtraction angiogram of the azygos vein with background subtracted.

Roadmapping (also called trace subtract fluoroscopy) is the fluoroscopic equivalent of DSA. It is widely used to guide and facilitate endovascular manipulation of the catheter and guide wire. During the procedure, a desired background angiogram, with or without subtraction, is obtained. With the patient remaining perfectly still, the background angiogram is used as a mask to perform subtraction fluoroscopy (roadmapping) in the same field of view. In contrast to DSA, the contrast-filled vessel in roadmapping will appear white, as opposed to black; images of a catheter and guide wire and their motion are visualized superimposed on the background mask image (Figure 1.3). Roadmapping may improve safety during catheter and guide wire manipulations, reduce radiation exposure and procedure times, and minimize contrast use.

Figure 1.3 Roadmapping fluoroscopy as a guidance in manipulation of selective catheterization in the right common carotid artery. 1 - brachiocephalic trunk, 2 - left common carotid artery, 3 - right subclavian artery, 4 - angiographic catheter, 5 - guide wire, 6 - right common carotid artery.

Used with permission from Usón J, Sun F, Crisóstomo V, et al. (2010) Manual de técnicas endoluminales y radiología intervencionista en veterinaria. Jesús Usón Minimally Invasive Surgery Centre, Caceres, Spain.

Interventional radiology procedures may involve significant radiation exposure and associated risks for both staff and patients. Radiation protection is one of the main concerns in interventional radiology. For the operators and assistants, wearing an appropriate lead apron, lead glasses and thyroid collar is essential; maintaining maximal distance from the radiation source whenever possible, is also important. Techniques to minimize radiation exposure include the use of low frame rate pulsed fluoroscopy, lower dose exposure (higher kV, lower mA) and the option of last-image-hold, use of the collimator when necessary, maximizing the source-to-patient distance, minimizing the air gap between the patient and the image intensifier/digital flat panel, and limiting the use of electronic magnifications.

Access Needles

Percutaneous access needles are thin-walled with relative large lumens to allow passage of the guide wire. The gauge system is used for sizing the outer diameter of access needles; higher gauge means thinner needle. Commonly used vascular access needles range from 18 to 21 gauge (G). For non-vascular access, a fine needle of 21 G is frequently used in order to minimize the damage to target organs. The lumens of needles may vary in size even if they are of the same gauge in outer diameter. Generally a 18 G needle allows for passage of a 0.038" and 0.035" guide wire; a 19 G needle, however, accommodates 0.035" but not 0.038" guide wires. Before puncturing, it is important to ensure that the guide wire to be used can pass through the needle.

The traditional vascular access needle, also called a Seldinger needle, consists of two parts. The outer metallic cannula has a blunt tip into which a pointed inner stylet is placed (Figure 1.4). Standard Seldinger needles are 18 G and 7 to 8 cm long. The use of Seldinger needles involves the double-wall puncture technique to achieve vascular access. Both walls of the target vessel are punctured with the needle assembly. After removal of the inner stylet, the needle cannula is retracted slowly until its blunt tip is back within the lumen of the vessel identified by blood return. A guide wire is inserted through the needle into the target vessel and vascular access can be subsequently obtained. Currently, the Seldinger needle is used less frequently due to the complexity of its two-part design and the concern of potential complications from double-wall puncture such as bleeding and damage to more proximal vessels during access.

Figure 1.4 Seldinger needle (18 G) consists of an outer cannula and a pointed inner stylet.

Used with permission from Usón J, Sun F, Crisóstomo V, et al. (2010) Manual de técnicas endoluminales y radiología intervencionista en veterinaria. Jesús Usón Minimally Invasive Surgery Centre, Caceres, Spain.

Instead, a single-wall puncture technique is more frequently used. The single-wall needle has only one part, a metallic cannula with a sharply beveled tip without an inner stylet (Figure 1.5). This needle is designed for puncturing the more superficial wall of the target vessel. When the sharp needle tip pierces the wall and enters the vessel lumen, the blood return is identified. When puncturing a small artery, however, it is more difficult to position the needle tip totally within the arterial lumen by using the single-wall technique. If the beveled needle tip is partially placed in the lumen, insertion of the guide wire into the lumen is difficult or impossible so that meticulous repositioning of the needle is required. Either a double-wall technique is used or preferably a micropuncture set.

Figure 1.5 Single-wall puncture needle (19 G) is also called a one-part needle.

Used with permission from Usón...