Imaging Guidelines during Percutaneous Liver Ablation to Optimize Outcomes and Patient Safety

Abstract

Image-guided ablation procedures have become a mainstay in cancer therapy. Typically performed from a percutaneous approach, thermal-based ablation procedures rely heavily on imaging guidance both prior to and during the procedure itself. Advances in imaging as they relate to ablation procedures are as important to successful treatments as advancements in the ablation technology itself. Imaging as it relates to procedural planning, targeting and monitoring, and assessment of procedural endpoint is the focus of this article.

Percutaneous image-guided ablation of hepatic tumors is now a routine treatment option for patients with both primary and secondary tumors. Percutaneous ablation offers a safe and effective option compared with traditional surgical techniques in the appropriate patient. ^{1 2} These patients are increasingly referred to interventional oncologists for treatment following multidisciplinary tumor board discussions. An independent review of the patient's preprocedure imaging should be performed by the treating physician to ensure the lesion(s) is amenable to ablation and assess for sites of extrahepatic disease. Some authors recommend obtaining a triple-phase computed tomography (CT) to ensure the lesion is visible on the noncontrast (NCCT) phase, as this is similar to what would be seen during the ablation procedure. ³

Selecting the proper imaging modality for treatment is integral to planning the procedure to ensure adequate tumor visibility for targeting and monitoring the ablation zone. Multiple modalities, including ultrasound, CT, and magnetic resonance imaging (MRI), are available. The choice of imaging modality will vary based on equipment availability, target lesion site, lesion visibility, and operator's preference. The purpose of this article is to review the general approach to imaging use during an ablation procedure. This is divided into three phases: the planning phase (determining the route and location of probe placement), the intraprocedural phase (placement of the probes and ablation), and the post-ablation phase (assessment of margins and evaluation for early complications).

Procedure Planning

The first step in every ablation procedure is determining the appropriate imaging modality that will be used to complete the procedure. B-mode ultrasound is commonly used to target the lesion and monitor the ablation zone. The benefits of ultrasound guidance for ablation include that it is widely available, allows for real-time visualization, and does not expose the patient to radiation. Limitations of ultrasound can include difficulty visualizing the lesion, imaging planes that do not correlate to standard orthogonal planes used in preprocedure imaging, and decreased lesion visibility during ablation secondary to gas formation. Ultrasound with microbubble contrast has demonstrated improved accuracy for this imaging modality. ⁴ More recently, improvements in image fusion techniques combining real-time ultrasound with preprocedure CT/MRI allow for better visualization of small tumors and more accurate assessment of ablation zones. ⁵

In the authors' institution, CT is the preferred modality because it provides the operator with a complete assessment of the target lesion and its spatial relationship to adjacent structures. CT also provides superior visibility when monitoring ablation progress. Ultrasound and CT are often used in conjunction; in such cases, ultrasound may be used for initial probe placement, followed by CT to confirm probe positioning and track the ablation.

At the authors' institution, almost all ablations are performed with the patient under general anesthesia and fully paralyzed to allow for reproducible breath holds. A multiphasic CT scan is performed. The images are reconstructed in axial, sagittal, and coronal planes. The images are reviewed by the treating physician in the control room and the final treatment plan is constructed.

Sometimes, the target lesion may be small or difficult to visualize on non-contrast CT. If the lesion is visible on contrast-enhanced CT (CECT), a treatment plan can be made, and the placement of ablation probes can be performed via landmarks. Some CT scanner units are loaded with software that allows for real-time multiplanar reformatting, which allows the target lesion and entry point to be marked in the planning phase. The same target and path can then be seen on subsequent imaging. This technique is very useful for small or difficult-to-see lesions, as they can be marked on a planning CECT sequence and then targeted on subsequent NCCT images. As mentioned earlier, reproducible breath holds under general anesthesia are critical when using this technique to avoid misregistration artifacts. Software with real-time multiplanar reformatting capabilities can be useful in planning off-axis approaches for difficult-to-reach lesions ( Fig. 1 ).

Fig. 1.

Images from percutaneous ablation of a colorectal liver metastasis. ( a ) Axial post-contrast CT images demonstrate a 1.2-cm mass in the posterior hepatic dome (white arrow), a challenging location for ablation due to the surrounding lung. These preprocedural images are used to define the ablation target. ( b, c ) Multiplanar reformatting during the case allows for off-axis planning and placement of an ablation probe from a caudal approach to avoid pleural transgression. Though the lesion is not seen without contrast, the position of the previously defined target is carried forward through subsequent imaging, allowing for accurate targeting even on noncontrast imaging. The white arrowhead points to the entry point, while the black arrow points to the defined target.

CT hepatic arteriography (CTHA) is an adjunctive technique that has several benefits for both the planning and treatment of small or difficult-to-see lesions. ⁶ A hybrid angiography suite with access to both fluoroscopy and CT imaging can be used in such cases. A catheter is placed within the hepatic artery (common, proper, right, or left depending on the tumor location) for intermittent CECT scans. Small aliquots of contrast are then injected as needed. This technique requires a lower volume of contrast over repeat administrations, which may be beneficial for patients with underlying renal disease. The main disadvantage to this technique is the requirement of an arterial puncture, which carries its own risks; however, with the use of modern techniques, the rate of complications requiring treatment is <1%. ⁷ The technique is discussed in more detail in the next section.

Positron emission tomography (PET) imaging utilizing fluorine 18 fluorodeoxyglucose (FDG) is widely performed for staging and follow-up imaging of oncology patients. The use of PET guidance is uncommon secondary to cost and equipment availability. Nonetheless, if the target lesion cannot be adequately visualized on conventional imaging modalities, FDG PET can be used for successful targeting and treatment. A study by Ryan et al describes a split dose FDG PET technique that provides adequate target visualization while also limiting radiation exposure to the treatment team. ⁸

Targeting and Monitoring

There are multiple methods available on modern CT scanners for probe guidance including traditional helical CT, helical CT in a “biopsy” mode using a short z-axis, intermittent CT fluoroscopy (iCTF), and continuous CT fluoroscopy (cCTF). The term CT fluoroscopy is used inconsistently in the literature to refer to both intermittent and continuous methods. For the purposes of this article, iCTF refers to intermittent acquisitions controlled by the operator via a foot pedal. cCTF refers to real-time image guidance during needle manipulation by the operator. This distinction is important because there are significant differences in radiation dose to the patient and operator with these two techniques. Early studies comparing cCTF to helical CT for image-guided biopsies demonstrated significantly increased radiation dose to the patient with the use of cCTF. ⁹ Furthermore, it is well documented that there is a significantly increased dose to the operator when using cCTF, specifically to the hands and eyes. ^{10 11} Multiple techniques to reduce operator exposure have been described, including the use of needle holders, adequate shielding, and eye protection. ^{10 12 13 14} Multiple studies on iCTF-guided biopsies have demonstrated a significant reduction in radiation dose to the patient and a reduction in procedure times when compared with helical CT. These studies demonstrated no increased rate of adverse events. ^{15 16 17}

Most operators at the authors' institution use iCTF with a remote foot pedal behind a leaded shield by the CT gantry in the procedure room. Operators also use personal protective equipment, including lead aprons and lead glasses. In these conditions, the dose to the operator is nearly zero. The ablation probe(s) are placed sequentially with imaging guidance. In cases where the lesion is not visible on NCCT, the probes can be placed using anatomic landmarks, CT intervention package guidance, repeat intravenous contrast injection, or CTHA.

For CTHA, the authors often use a power injection rate of 1 to 2 mL/second for a total of 12 to 24 mL and perform a helical scan through the liver. The X-ray delay for the scan is calculated by subtracting the total scan time from the total injection time (e.g., for injection time of 12 seconds and scan time of 7 seconds, the X-ray delay would be 5 seconds). Scan parameters can be adjusted as needed to optimize imaging depending on the clinical scenario or operator's preference. This technique provides opacification of both the lesion and hepatic arteries to allow for improved lesion visibility and treatment accuracy. ^{18 19} If the lesion is not well visualized on this early arterial phase of imaging, a late arterial phase can also be performed with a delay of ∼16 seconds. ⁶ An added safety benefit is that hepatic arterial branches are clearly opacified and can be avoided with intermittent contrast injections while advancing the ablation probes. CTHA and noncontrast iCTF are used in conjunction during these cases ( Fig. 2a, b ).

Fig. 2.

CT hepatic arteriography images from a case of percutaneous ablation of metastatic melanoma to the liver. ( a ) Contrast injection from a microcatheter in the proper hepatic artery demonstrates enhancement of a segment 7 metastasis (white arrow). Several hepatic artery branches are also opacified (white arrowheads), and a path to avoid these vessels can be planned. (b ) CTHA is performed intermittently to more accurately visualize and target the lesion (long black arrow) as the probe (short black arrow) is advanced. ( c ) CTHA is performed during the ablation to track the borders of the ablation cavity, seen here as a hyperemic border around an area of nonenhancement (black arrowheads). A prior ablation cavity is seen in segment 8 (dotted white circle).

After all ablation probes are placed, a noncontrast helical CT through the whole liver is performed. The images are reconstructed in multiple planes and reviewed by the operator to ensure appropriate needle placement. Lesions that were visible on NCCT prior to probe placement are sometimes difficult to see after placement due to streak artifacts from the probe. In this case, one can use anatomic landmarks, rigid registration software built into the scanner, or commercial ablation confirmation software to confirm probe positioning. If contrast is needed, it can be administered intravenously (IV) or intra-arterially (IA).

The ablation is started once probe placement is confirmed. Helical CT scans are repeated every 2 to 3 minutes during the ablation to monitor the growth of the ablation zone and assess for immediate complications that would necessitate early termination of the procedure. During a microwave or radiofrequency ablation, foci of air within the ablation cavity may be used as a surrogate marker of the ablation margin, though the true ablation margin may extend beyond the foci. One benefit of CTHA is that intra-arterial injections can be performed throughout the ablation to track the ablation zone more clearly, ensuring both appropriate tumor coverage and critical structure avoidance ( Fig. 2c ). If cryoablation is being performed, the ablation zone can reliably be tracked by the water density ice ball on CT. A final helical CT is performed at ablation completion to allow measurements of the probe tract if tract ablation will be performed.

Assessment of Ablation End Point

An imaging assessment of the ablation zone should be performed immediately following the procedure. B-mode ultrasound cannot reliably distinguish between necrotic and viable tumor tissue. ¹⁸ We recommend performing an immediate CECT using identical parameters to the planning CECT performed at the beginning of the procedure. These images can be readily compared with the planning CT to assess the ablation zone and margins. Modern imaging suites offer fusion software that can overlay the scans for comparison. It is important to note that with microwave ablation there can be significant tissue contraction which will affect the fusion registration. Commercial party ablation confirmation systems are also available and provide three-dimensional visualization of the ablation zone overlayed with the target lesion and desired margin ( Fig. 3 ). These systems often use elastic registration, which can help reduce the misregistration artifact due to liver contraction. It is helpful to perform imaging through the entire liver for both probe placement and postablation scans to optimize elastic registration protocols.

Fig. 3.

( a ) A 2-cm segment 7 hepatic metastasis (white arrow) seen on axial contrast-enhanced CT prior to ablation. ( b ) Use of ablation confirmation software (Neuwave Microwave Ablation System; Johnson and Johnson Med, Irvine, CA) is seen in axial (left), sagittal (top right), coronal (middle right), and 3D reformatted (bottom right) images. In each image, the innermost border represents the target lesion, the middle border represents a 7-mm margin around the target lesion, and the outermost border represents the ablation cavity. Ablation confirmation software can be used in this way to confirm appropriate ablation margins and minimize the risk of local recurrence.

The imaging appearance of the ablation zone on noncontrast images is often hyperdense to the adjacent parenchyma secondary to coagulative necrosis. Contrast-enhanced images will demonstrate a hypodense, nonenhancing ablation zone. A thin, peripheral rim of enhancement may be visible surrounding the ablation zone which represents transient hyperemia. ²⁰ The ablation zone should encompass the entirety of the target tumor and a margin of at least 5 to 10 mm, depending on the modality of ablation confirmation that is being used. ^{21 22} The images should be assessed for evidence of complications that would require immediate intervention such as active hemorrhage and pneumothorax. ²³

In cases where the ablation zone is inadequate (<5 mm) and the intent is curative, the patient may be retreated in the same setting. The margin in question can be targeted using the imaging techniques described earlier and subsequently ablated. Based on operator's preference, a CECT or NCCT is then repeated to reassess margins and confirm adequate treatment before the patient leaves the procedure suite.

Conclusion

The selection of the appropriate imaging modalities for an ablation procedure is important to ensure successful treatment. The choice of modality will vary based on geographic location, visibility of the target lesion, and the preference of the operator. The techniques described in this article should help achieve excellent lesion visualization that is reproducible throughout all phases of the procedure.