Intrahepatic collateral vessels can supply arterial flow to tumors across segments or lobes. Selective embolization of intrahepatic branches to redistribute intrahepatic flow patterns to the tumors has been shown to be effective and safe (Karunanithy et al. 2011; Bilbao et al. 2010). During evaluation of intrahepatic arterial tumor supply, multiple feeding vessels may be identified, often in close proximity to hepatico-enteric or hepatico-splanchnic vessels. Although, pre-emptive coil embolization of the hepatico-enteric vessels imparts a high degree of safety, some vessels may be too small or angulated to allow this skeletonization. Embolizing one or more intrahepatic feeding branches can reduce the number of sites of administration of radioactive microspheres, and can facilitate administration distal to recognized hepatico-enteric vessels. For instance, a disadvantageous segment IV artery with small ductal artery branches can be coil embolized so that arterial supply to that segment is taken over by branches of the LHA or RHA or both (Fig. 3). Alternatively, coil embolization of a segment VIII artery supplying the lateral edge of a left lobe tumor can reduce treatment to only the LHA, sparing the remainder of the right lobe.

In a study of 24 patients, 11 of whom had Michels Type I anatomy, single photon emission computed tomography (SPECT) combined with computed tomography (CT) performed after administration of technetium macroaggregated albumin (^99mTC-MAA) showed uptake in the redistributed areas in all 11 Michels Type I patients (Bilbao et al. 2010). Branches embolized include segment IV, segment VIII, and LHA. In a separate study, 11 patients underwent embolization of the anterior division RHA, RHA, segment IV, or LHA to redistribute flow for administration of radioactive microspheres (Karunanithy et al. 2011). Post-treatment PET showed a statistically significant decrease in standardized uptake values (SUV). These studies demonstrate that embolization of intrahepatic branches from either the left lobe or right lobe can successfully redistribute flow to simplify and increase the safety of treatment. It is important to note that both studies performed embolization with coils only. More distal embolization using particles would lodge in the tumor at the arteriolar level (Lee et al. 2008). This could theoretically prevent radioactive microspheres from reaching the tumor.

A similar technique for management of variant hepatic arteries can be used. The goal of consolidation is to create a simpler and safer arterial anatomy for the administration of radioactive microspheres. Since many variant hepatic arteries arise off of branches that also supply the gastrointestinal tract, non-target radioembolization is an increased risk (Riaz et al. 2009). For instance, administration in the rLHA or aLHA originating from the LGA can result in reflux into esophageal and gastric branches just proximal to the course of the variant artery in the fissure of the ligamentum venosum. Likewise, the rRHA or aRHA frequently gives off small branches to the duodenum, and arises from the main SMA, which supplies nearly the entire bowel (Fig. 4). One early study on radioembolization safety consolidated variant hepatic arteries with resultant reconstitution of flow by intrahepatic collaterals and is important because it demonstrated that consolidation is able to limit toxicity, although treatment efficacy was not fully assessed (Andrews et al. 1994). Consolidation by coil embolization of variant or redundant arteries provides a way to achieve distribution of microspheres to the targeted tumors while minimizing non-target deposition complication risk to the patient.

In evaluating for any evidence of variant anatomy, all prior cross-sectional imaging, either contrast enhanced computed tomography (CT) or magnetic resonance (MR) imaging, should be closely studied. Thin-section arterial phase breath-held imaging is the most useful, if available, and coronal and sagittal reformatted images may help to confirm existence, origin, and course of variant vessels. Identified anatomic variants should be compared to the intrahepatic tumor distribution to predict dominant arterial supply to the targeted regions. Scrutiny of cross-sectional imaging will guide and possibly even expedite the preparatory angiography prior to radioembolization treatment.

During preparatory phase angiography, abdominal aortography is performed with injection of contrast medium at up to 15 cc/sec for 30 cc with the flush catheter at the level of mid to lower thorax (T7–T9) to identify variant hepatic and parasitized extrahepatic arteries, including those too small to be detected by CT or MRI, and to establish a baseline for future comparison. Next, all arteries of interest including normal and variant and parasitized vessels should undergo catheter selection and selective digital subtraction angiography (DSA). When available, C-arm cone beam CT (CACT) should also be performed for volumetric definition of subtended arterial territory. If no variant or parasitized arteries are identified, DSA and CACT should be performed while injecting contrast medium into the PHA or CHA. If any territories and especially if any tumors within the liver do not enhance, the search for additional arterial inflow should be renewed (Fig. 5). Selective catheterization and injection of the mesenteric vessels are performed as needed to confirm anatomy of the SMA, CHA, PHA, GDA, LHA, and RHA. Because of the high incidence of hepatofugal branches arising from the LHA, including the RGA, accessory LGA, left inferior phrenic artery, and falciform artery, some authors recommend power-injected angiography of each LHA (Lewandowski et al. 2007).