with interventional techniques, which may result in more efficient and accurate IO procedures in the future (Ladd et al. 2000; Hellinger and Rubin 2006; Thornton and Grist 2006; Kos et al. 2008).
Ultrasound has many applications in IR. Many of the disease processes that may require IO treatments can be diagnosed by ultrasonography. In dogs, hepatocellular carcinoma, a tumor commonly treated by embolization or chemoembolization and radiofrequency ablation (RFA) in humans (Okusaka et al. 2009; Hiraoka et al. 2010), is easily identified by abdominal ultrasound (Liptak et al. 2004b). In one study of dogs with hepatocellular carcinoma, diagnosis of a hepatic mass was made with ultrasound in 93.5% of cases (Liptak et al. 2004b; Dernell et al. 2004). Other tumors that may require an IO treatment, such as urethral, colonic, and thyroid neoplasia, can also be evaluated by ultrasound (Hume et al. 2006; Weisse et al. 2006; Barber 2007). In a recent case series of dogs with masses obstructing hepatic venous outflow, masses were often identified by ultrasonography (Schlisksup et al. 2009). In addition to the diagnostic utility of ultrasound, this modality is regularly employed in humans to aid in obtaining vascular access (to initiate the Seldinger technique) during the performance of IR procedures (Longo et al. 1994; Dodd et al. 1996; Ahmad et al. 2008; Arthurs et al. 2008). Ultrasound can also be used to perform procedures, including the placement of drainage catheters, percutaneous biopsies, percutaneous ethanol injection (PEI) of hepatic neoplasia, and RFA (Longo et al. 1994; Dodd et al. 1996; Solbiati 1998).
Contrast Agents
Contrast agents are required components of most intravascular (IV) procedures and many stenting procedures. The predominant contrast agents used for angiography include iodinated agents (fluoroscopy, CT), gadolinium‐based agents (MRI), and carbon dioxide (CT) (Ehrmann et al. 1994; Moresco et al. 2000; Spinosa et al. 2000, 2001; Brown et al. 2003; Namasivayam et al. 2006; Bui et al. 2007). Iodinated contrast agents are available in both ionic and nonionic forms; nonionic agents are less osmolar than their ionic counterparts (Singh and Daftary 2008). Severe reactions are reported to occur with similar incidence among all iodinated contrast agents, but mild and moderate contrast reactions occur more commonly with the use of higher osmolality iodinated contrast agents (Singh and Daftary 2008). Nephrotoxicity is a major potential complication associated with the use of iodinated contrast agents and has become the third most common cause of acute renal failure in humans (Akgun et al. 2006). The most commonly used iodinated contrast agents are the nonionic monomers such as iohexol, iopromide, iopamidol, and ioversol (Dickinson and Kam 2008).
Gadolinium‐based contrast agents and CO2 are used most commonly in patients who have had a previous adverse reaction to an iodinated contrast agent and in those patients with an increased risk for development of nephrotoxicity (Moresco et al. 2000; Spinosa et al. 2000, 2001; Dickinson and Kam 2008), although some studies have reported nephrotoxicity in association with gadolinium contrast usage (Akgun et al. 2006; Ergün et al. 2006). Agents such as gadopentetate dimeglumine, gadodiamide, gadoteridol, and gadoversetamide are the most readily available gadolinium‐based contrast agents (Akgun et al. 2006) and are used when previous CO2 usage has resulted in a suboptimal study due to bowel gas artifacts or as a supplement to CO2 angiography (Spinosa et al. 2000, 2001). Gadolinium‐based contrast agents produce less detailed contrast studies as compared with iodinated agents and are therefore less useful for angiography during IR procedures (Spinosa et al. 2000). When using gadolinium‐based contrast agents, digital subtraction angiography is recommended to compensate for the less detailed study that is otherwise obtained (Spinosa et al. 2000).
To outline a hollow viscus such as the esophagus, urethra, and colon, substances such as barium and iodinated contrast agents have been used in veterinary patients (Hume et al. 2006; Weisse et al. 2006; Farese et al. 2008). In a recent study of esophageal tumors in dogs, barium sulfate was found to be useful in identifying mass location (Farese et al. 2008). In dogs, iodinated contrast agents have been used to evaluate urethral obstructions prior to urethral stenting (Weisse et al. 2006). Additionally, iodinated contrast agents have been used prior to colonic and esophageal stenting to delineate obstructions (Hume et al. 2006; Hansen et al. 2012).
Instrumentation and Implants
Access Needles
Traditional hypodermic needles or over‐the‐needle catheters (Figure 3.1) can be used to puncture vessels when obtaining vascular access using the Seldinger technique (Seldinger 1953). The size of the access needle used determines the wire size that can be introduced through the needle and into the vessel. The standard venous access needle is an 18‐gauge needle, which accepts guidewires up to 0.038 inches in diameter (Braun 1997). Needles that are 21‐ to 22‐gauge are considered to be micropuncture needles and allow for introduction of guidewires up to 0.018 inches in diameter (Braun 1997; Valji 2006).
Figure 3.1 Interventional oncology instrumentation. From left to right: (a) 18‐gauge over‐the‐needle catheter (left), 22‐gauge over‐the‐needle catheter (right). (b) 0.035‐inch hydrophilic guidewire. (c) Dilator and vascular access sheath. (d) Catheter with angled‐tip.
Guidewires
Selection of a particular guidewire (Figure 3.1) is dictated by the size of access needle that has been placed, the technique to be performed, and the vessel(s) to be selected. Most guidewires are available in three standard lengths: 150, 180, and 260 cm (Braun 1997). Alternative lengths of 60, 125, and 145 cm have been reported, but these are not readily available (Valji 2006; Kipling et al. 2009). The standard diameters of most guidewires are 0.035 and 0.038 inches. Smaller gauge wires generally ranging from 0.010 to 0.018 inches are used when microcatheters and smaller (micropuncture) vascular access needles are used (Braun 1997; Valji 2006; Kipling et al. 2009).
There are a few primary principles that must be adhered to when using guidewires. First, many guidewires contain a hydrophilic coating made of polytetrafluoroethylene that requires priming with saline to allow for smooth passage through the lumen that has been selected (Braun 1997; Kipling et al. 2009). When sufficiently wet, the guidewire should pass easily through a catheter and allow an increased ability to perform vascular selection (Braun 1997; Kipling et al. 2009). It is essential that the guidewire remains wet during the procedure to improve the function of the guidewire (Kipling et al. 2009). Second, the length of the selected guidewire should be at least twice the length of the catheter that is being used (Braun 1997). Third, if a guidewire is not passing easily through a vascular access needle, the needle may need to be repositioned. The wire should not be forced, as the needle may be subintimal or against a sidewall (Valji 2006). Lastly, a torque device can be placed on the end of a guidewire (approximately 5–10 cm from a catheter hub that has been introduced over the guidewire) to better manipulate and steer the guidewire (Kipling et al. 2009). These torque devices can be invaluable when passing a guidewire into vessels that are difficult to access and when crossing stenotic regions.
Guidewires are also used for nonvascular stenting procedures (Hume et al. 2006; Weisse et al. 2006; Culp et al. 2007; Kipling et al. 2009; Hansen et al. 2012). Stents that are placed through malignant obstructions are introduced over a guidewire, and the stent delivery system tapers down to the guidewire to allow for easier placement. In companion animals, 0.035‐inch hydrophilic guidewires have been used to facilitate stent placement for tracheal, urethral, esophageal, and colonic obstructions (Hume et al. 2006; Weisse et al. 2006; Culp et al. 2007, 2011; Hansen et al. 2012).
Sheaths
The use of IV sheaths (Figure