Contribution of Computed Tomographic Angiography to Pretreatment Planning of Radio-embolization of Liver Tumors

Arterial Supply of the Liver and its Importance in TARE

The complex embryonal development of the hepatobiliary system predisposes this area to having a highly variable arterial anatomy (4). A classic arrangement with two hepatic arteries arising from the proper hepatic artery, which is a branch of the common hepatic artery arising from the truncus coeliacus, occurs in 50-61% of individuals (5-7). In other cases, the arteries are replaced, or accessory arteries are found (Figure 1). According to Michels' classification, 10 types of arterial variants exist (Table I) (8). The arteries that supply the surrounding organs can also originate from hepatic arteries. For example, the right gastric artery, accessory left hepatic artery, lower diaphragmatic artery and umbilical artery may be branches of the left hepatic artery (9). An examination of liver segment IV should be focused on the possible presence of the middle hepatic artery, which may be a branch of the right hepatic artery or arise directly from the proper hepatic artery as a trifurcation with the end branches of the right or left hepatic artery passing on the other side. Moreover, mapping the distance of normal arterial branches from the point of microsphere application is also important, especially for the gastroduodenal and cystic arteries (10). Specifically, up to 17% of tumors located on the liver surface have a parasitic arterial supply (Table II) (11).

Non-target radiopharmaceutical deposition may cause serious complications, such as cholecystitis, gastrointestinal ulceration or bleeding, gastritis, duodenitis, pancreatitis, hepatitis, supra-umbilical dermatitis and radiation pneumonitis (12-14). Therefore, all patients undergo protective embolization of the gastroduodenal artery, left gastric artery, cystic artery or other arteries which may allow the non-target deposition of microspheres (15, 16). To ensure the redistribution of blood in the liver, individual branches supplying liver tissue may be embolized in some patients. Other adverse factors that increase the probability of embolization in non-target areas are stenoses and the slowing of flow in the hepatic arteries. Proper catheter positioning based on the knowledge of the liver arterial architecture is essential in order to perform the evaluation of liver perfusion and assessment of hepatopulmonary fraction deposition of microspheres before TARE. Hepatopulmonary shunting is evaluated with single photon-emission computed tomography/computed tomography (SPECT/CT) using technetium-99 m-labelled human albumin macro-aggregate in order to assess the potential radiation injury, occurring when the arteriovenous shunting exceeds more than one-tenth of the applied activity.

Imaging of portal venous system is also very important in the planning of TARE. Portal vein thrombosis with the absence of hepatopetal flow increases the likelihood of ischemic involvement of the liver and is a contraindication for TARE using resin microspheres. The procedure with glass micro-spheres can be successfully performed in this condition (17).

Technique of CT Angiography (CTA)

CT images of the liver are obtained after the injection of contrast medium (CM) in the arterial, venous and, optionally, equilibrium phases. The volume of CM should correspond to 500-600 mgI per kilogram of body weight, and the iodine flux should be in the range of 1.6-2.0 g/s. CM injection is followed by a saline flush to ensure high-quality visualization of both the arterial and venous system, the liver parenchyma, and possible focal lesions (19). In order to increase the contrast between the vessels and structures of the gastrointestinal tract, the per-oral application of water or mannitol water solution is advisable. Moreover, if the examination is performed only to map the liver artery, the volume of CM can be reduced by approximately 20% (20).

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Figure 1.

Computed tomography angiography (CTA) of the liver showing the intrahepatic segmentation of the hepatic artery. A trifurcation of the hepatic artery (arrow) is visible. The supplementary artery supplies the gall bladder and part of the metastasis. A: CTA, volume-rendered 3D reconstruction. B: CTA, maximum intensity projection of hepatic arteries. C: Catheter-based perfusion image from cone-beam CT. D: Axial CT image of liver metastasis before transarterial radio-embolisation (TARE). E: Axial CT image 6 weeks after TARE. F: Axial CT image 6 months after TARE, with visible size reduction and decrease in density representing necrosis.

The early arterial phase (15-20 s after the start of CM injection or immediately after reaching a threshold of 100 HU in the aorta) is recommended for arterial imaging. At this stage, the concentration of CM is sufficient in the arteries but not in the portal vein and hypervascular lesions, which are not yet enhanced. Therefore, the signal-to-noise ratio is higher between the vessels and surrounding structures, which simplifies the visualization of small arteries. However, liver tumors should be imaged in the late arterial phase (approximately 35-40 s after the start of the contrast medium injection or 15-20 s after reaching a threshold of 100 HU in the aorta). At this phase, the portal vein and hypervascular liver lesions are enhanced, which results in a lower signal-to-noise ratio. However, in a study to compare the benefits of early and late arterial phase examinations of liver arterial variants, Van den Hoven et al. showed that liver arterial imaging did not produce significantly worse results in the late arterial phase than the early arterial phase (21).

In non-obese patients, the low-voltage (80-100 kV) technique is preferred in because it increases the CM contrast and consequently increases the resolution of blood vessels and hypervascular focal lesions at a lower radiation dose (22). Depending on the voltage drop, the current must be increased to optimize image quality, and the ratio of contrast to noise is not significantly reduced under these conditions (23). To optimize the exposure parameters, automatic kVp setting and automatic off-line and on-line current modulation systems are currently used. Alternatively, image noise and radiation load can also be reduced using iterative reconstructions.

Scanning on multi-detector devices is performed using submillimetric collimation (typically 0.5-0.75 mm), which ensures an isotropic data field that allows multiplanar reformations and three-dimensional reconstructions in any plane without a loss of resolution. To increase spatial resolution, some authors recommend the use of the edge-enhancement reconstruction algorithm (24). Thin axial sections and multiplanar reformations or maximum intensity projection reconstructions are used for evaluation (25).

The Importance of CTA

CTA provides sufficient time and spatial resolution for replacing diagnostic angiography in clinical practice. Optimally performed examinations can detect arterial branches with a diameter of 1 mm, and visualizing the hepatic arteries with proximal widths between 2-3 mm is usually sufficient (26, 27). Comparative studies of the accuracy of CTA and other methods (DSA, cone-beam CT) indicate that DSA is more accurate when detecting liver arterial anomalies, especially in the area of the right hepatic artery (detectable at 65% CTA). However, the authors of these studies also agree that CTA accurately assesses the supply of the fourth hepatic segment (up to 36% higher detecability than with DSA) (21, 28, 29). Similarly, good results were obtained in the detection of the falciform artery – Burgman et al. detected this artery in 4.5 as many patients when using CTA compared with DSA (52.3 vs. 11.9%) (29). MRI, alone or as a part of PET/MRI, is another commonly used method for detection, differential diagnosis and staging of liver tumors. As with CT, liver artery anatomy can be evaluated in images obtained in the arterial phase. However, this method is less accurate than CTA; one study showed that the reliability of MRI was lower than that of CTA, especially in the area of the left hepatic artery (18).

Despite the limitations associated with CTA, it can be considered a suitable method for planning the catheterization procedure. Specifically, it accelerates and consequently reduces the radiation load and the amount of CM required, which is beneficial, especially for patients with impaired renal function (10, 21, 28).