Tools of the Trade

Imaging Tools

Image guidance is fundamental to performing all procedures in vascular and interventional radiology. The majority of procedures are performed in the Angio-Interventional Suite using high resolution fluoroscopy with digital and digital subtraction imaging. The Angio-Interventional Suite differs from a conventional angiography room in both live image quality and versatility. Most interventional fluoroscopy units are mounted on a double yoke that allows rotation around both the longitudinal and lateral axes of the patient. The imaging chain supports several levels of magnification fluoroscopy in addition to trace image or digital road mapping that provides a superimposed digital subtraction image over live fluoroscopy. This technology permits the operator to see small vessels on the monitor during selective and super-selective catheterization.

Interventional procedures are also performed under computed tomographic and sonographic guidance. These are predominately drainage and biopsy procedures, although occasionally combined modality studies such as CT arterial portography are performed. CT guidance has the disadvantage of not being a live or real time imaging modality. Consequently, interventions performed under CT guidance require that the patient be brought in and out of the scanning gantry multiple times during a procedure to monitor progress, while the actual placement and manipulation of needles, guide wires, and catheters is done blindly.

Sonography is an excellent modality for localizing fluid collections in the abdomen, pelvis and pleural space. Sonography also allows real time imaging of the needle placement for percutaneous drainage of such collections. Special needles with a burnished finish having high echo reflectivity are now available for sonographically guided procedures. Sonography is also useful for biopsy of larger liver lesions and for kidney in the work-up of medical renal disease, but much less satisfactory for biopsy of small lesions and deep structures such as the pancreas and para-aortic lymph nodes.

There is some current literature describing magnetic resonance imaging (MRI) as a means for image guided biopsy using special nonmagnetic needles. However, at the present time, MRI lacks sufficient spatial resolution for many lesions and, as with CT, requires the patient to be brought in and out of the scanning gantry multiple times during the procedure. Within the next five to ten years, MRI units will become available with an open C-shaped gantry and specialized scanning sequences to produce near real time images. It remains to be seen if so-called "MR Fluoroscopy" will add significantly to the armamentarium of interventional radiology.

For Good Measure

As with most disciplines, medicine is steeped in tradition. Many of the traditions in medicine are invaluable, even essential, to our concept of medical practice. Others hang like an albatross around our necks. Nowhere is this more true than in the outrageous conglomeration of measurement systems adopted by medicine over the years. Unfortunately, Vascular and Interventional Radiology has been unable to avoid these outdated conventions, despite its position as a new subspecialty. Therefore, a quick review of some of these systems is in order.

Needles are measured by the Birmingham or Stub's gauge, wherein the larger the gauge number, the smaller the needle diameter. Although gauge may have had some physical meaning at one time, comparison of gauge number to diameter or cross-section yields erratic results. It is more likely that gauge numbers represent an inverse rank ordering of sizes. The English and metric equivalents of gauge size are given in the table on the left. Of course, the outside diameter of a needle tells one nothing about the needle lumen size. Catheters diameters are measured in French (Fr.) size, where 3 Fr. is equal to 1 millimeter. Thus, a 5 Fr. catheter is 12/3 mm in diameter. However, the size of the catheter lumen varies with the thickness of the catheter material. Guide wires are in English units of 1000th of an inch, and must be matched to a needle or catheter luminal diameter. Vascular sheaths are measured in French, but the measurement refers to the lumen (i.e., inside diameter), not the overall diameter of the sheath. Obviously, keeping track of which needles, wires, catheters and sheaths are compatible is a difficult task at best. To add to the confusion, there are length discrepancies and material incompatibilities that must also be considered when selecting devices for a particular case.

Catheters and Guide Wires

In the early days of angiography, radiologists made their own catheters and guide wires. Today, there are literally dozens of companies producing a dizzying array of devices for interventional radiology. Advances in biotechnology, metallurgy, and plastics have and continue to yield new materials such as nitinol, PET, PEMT, modified polyurethanes and hydrophilic gel coating, to name just a few. New materials and new manufacturing techniques combine to make guide wires and catheters with exceptional torque control, tracking ability, and biocompatibility. Diagnostic catheters have gone from the work horse 8 Fr. "overcooked spaghetti" of the past to 4 and 5 Fr. metal braided diagnostic catheters with excellent torque control and radio-opacity, and microcatheters as small as 2 Fr. to negotiate the most tortuous small vessels in the head, abdomen and extremities. Every aspect of interventional radiology is and continues to be impacted by new devices and technology brought to the market on an almost daily basis.

Most diagnostic catheters can be thought of as miniaturized versions of a garden hose. However, many of the catheters used for interventional techniques are highly specialized devices. Angioplasty balloons are dual lumen catheters with a semi-circumferential outer lumen for inflation and deflation of the balloon. Micro-balloon catheters have a single lumen with a side port to the balloon chamber. A special guide wire with a tip occluding bead is used to block the end hole and force fluid through the side port during balloon inflation and deflation. Atherectomy catheters utilize a balloon to position a cutting window against an atherosclerotic plaque. The plaque is then excised with a rotating motor driven blade. The excised material is packed into a chamber at the front of the catheter for removal. Infusion catheters for thrombolytic therapy have multiple side holes or slits to distribute the lytic agent throughout the thrombus. In most cases a tip occluding guide wire prevents escape of the agent through the end hole. Some infusion catheters have a slit valve at the catheter tip that allows passage of a guide wire but closes completely during infusion. Other devices under study or soon to be released include thrombectomy catheters that homogenize and liquefy thrombus through the use of a pneumatic impeller or hydraulic fluid vortices created at the catheter tip. These devices may significantly reduce or eliminate the need for thrombolytic agents in many cases, and make catheter directed treatment of acute thrombosis possible in many patients for whom thrombolytic therapy is otherwise contraindicated. Specialized catheters are also used for delivery of endovascular prostheses (stents). The WallStent device is a self expanding metallic stent mounted on a catheter beneath a constraining membrane. This 7 Fr. device is positioned across a lesion and the membrane is rolled off to allow the stent to expand. Unconstrained, these stents will achieve a diameter of up to 12mm, more than 5 times the diameter of the delivery system. This is just a sampling of a few of the catheter-based devices that are or soon will be available for the treatment of vascular diseases in interventional radiology. Devices for embolotherapy, the urinary tract, the biliary system and other organs also proliferate. Many of these will be mentioned in subsequent sections.

Intravascular Contrast Media

A complete discussion of the various intravascular contrast agents is beyond the scope of this discussion. Nevertheless, all physicians who use or request examinations using intravascular contrast media should have a basic knowledge of these agents. Currently, there are two broad categories of contrast agents available for intravascular use. Chemically, most of these agents are composed of monomers or dimers of substituted benzene rings containing 3 iodine atoms. All agents approved for intravascular administration may be used both for intraarterial (arteriography) and intravenous (venography, IVP, CT) injections, and for opacification of hollow organs and body cavities.

Ionic media have been available for many years. These agents have been shown to be relatively safe with significant adverse events occurring in fewer than 7% of cases. Toxic reactions resulting in a fatal outcome occur in 1/40,000 IV injections. Intraarterial injection of ionic contrast material produces significant burning pain which is most likely related to the high osmolality of these agents. Furthermore, ionic media are intrinsically toxic to many tissues. This effect is most apparent in the CNS and in cases of contrast extravasation into subcutaneous tissues. Because of their toxicity, these agents cannot be used for intrathecal injections.

Nonionic contrast agents became available in the United States in the mid 1980's. These agents have a markedly improved safety profile, and significantly less tissue toxicity. Some of the nonionic agents are approved for intrathecal use. These agents have much lower osmolality than ionic contrast (600 vs. 1400 to 1900 mosm.) and produce significantly less discomfort on arterial injection in most patients. The newest nonionic agent, iodixanol, is a dimer having 6 iodine atoms per molecule. Iodixanol is formulated to be isotonic to blood. In clinical studies, iodixanol was found to have a safety profile similar to the nonionic monomers, and superior to ionic dimers. Because it produces minimal discomfort on arterial injection and subjects the patient to a significantly decreased osmotic load, iodixanol may become the contrast agent of choice for arteriography and interventional vascular procedures. Iodixanol received FDA approval in July, 1996.

All of the available iodinated contrast agents (ionic and nonionic) are potentially nephrotoxic. Nephrotoxicity is most apparent in patients with preexisting renal disease, diabetes, dehydration, and in patients with multiple myeloma. While there is no statistically significant difference in the incidence of nephrotoxicity between ionic and nonionic agents, it appears that the renal injury may be less, and recovery more complete with the nonionic agents.

The most striking difference between the two classes of agents is in the incidence of adverse reactions following intravenous injection. This difference has been shown in several large randomized trials which, in aggregate, have included over 500,000 subjects. These studies indicate a six fold reduction in the incidence of moderate and severe reactions with nonionic contrast. Moreover, analysis of various recognized risk factors for adverse reactions showed the most significant risk factor to be the choice of contrast agent. It is assumed that this greater margin of safety would also apply to intraarterial use. However, for reasons which are as yet unclear, adverse reactions from intraarterial injections of either ionic or nonionic contrast media are extremely rare, and statistically significant differences would be very difficult to uncover.

The other very important difference between these two types of agents is cost. Ionic contrast media cost from $.06 to $.10 per ml in the U.S., and approximately four times that amount in Europe and Asia. The U.S. price for nonionic contrast has decreased significantly in the last few years. Currently, nonionic monomers are priced at about $ .32 to $ .50 per ml, still slightly more than in foreign markets. This amounts to an overall three to five fold increase in cost for nonionic contrast over ionic media, a substantially narrower margin than the ten to fifteen fold difference that existed only a few years ago. Currently, nearly 90% of all iodinated contrast administered by radiologists and cardiologists is nonionic. The impact of this on the U.S. health care budget is estimated to be $1-1.5 billion per year.

Efforts to reduce the overall health care expenditure for contrast media have centered on the selective use of nonionic contrast in "high risk" patients, while "low risk" patients receive ionic media. These stratification schemes attempt to define various risk factors such as diabetes, cardiovascular disease, renal disease, age, asthma, allergy, previous contrast reaction, etc. However, all such schemes are based on the false premise that individual risk factors outweigh the overall risks intrinsic to the contrast agents themselves. This is simply not borne out by results of the Katayama and Palmer studies which showed that "high risk" patients receiving nonionic contrast are at lower risk than "low risk" patients receiving ionic media. Legitimate concern for patient safety and comfort should be the primary determinant of which class of agent is used. Cost savings achieved by technological advances that reduce the amount of contrast required for a procedure are laudable. Savings achieved at the expense of patient safety, particularly in the litigious atmosphere of contemporary American medical practice, should and must be eschewed.

There is one other agent which is suitable for angiographic studies but has received very little attention. Carbon dioxide (CO2) can be injected intraarterially and imaged using digital subtraction techniques. The cost of medical grade CO2 is negligible, and it has been shown to be safe, nontoxic, and a useful alternative to conventional contrast agents in evaluating peripheral vascular disease. Following intraarterial injection, CO2 is rapidly absorbed by blood, transported to the lungs, and exhaled. In animal experiments, CO2 given by continuous intraarterial infusion appeared to induce no adverse physiological response. Nevertheless, CO2 is probably not suitable for angiography of end arteries such as the renal or cerebral vessels because of the lack of collateral circulation. One recent publication implicated CO2 as probably responsible for multiple micro-infarctions in a kidney subjected to selective CO2 angiography. The major problems associated with CO2 angiography are related to its buoyancy and difficulty in directing the gas bolus to the area of interest. In addition, there is currently no good injector system for CO2 delivery. However, prototype systems are currently under development and clinically useful systems may be available within a few years.