The solution, developed at Amsterdam University Medical Center and now demonstrated across three major European centres, is called contrast tagging. The concept is deceptively simple. During CT hepatic arteriography, or CTHA, contrast medium is injected directly into the hepatic artery through a small catheter threaded from the groin. At the precise moment when the tumour lights up with maximal enhancement, the radiologist switches on the ablation energy. The heat traps the iodinated contrast inside the tumour, effectively freezing a bright, hyperdense snapshot of the cancer’s exact boundaries into the post-ablation scan. When the unenhanced CT is performed immediately afterwards, the tumour appears as a clearly visible bright region sitting inside the darker ablation zone, allowing the physician to see, at a glance, whether the margins are adequate.

The technical choreography behind the technique is worth appreciating. Patients are brought to the angiography suite, where a 5 French sheath is placed in the right common femoral artery and a catheter is advanced to the common hepatic artery. The patient then moves to the CT suite, where a dual-phase CTHA is acquired 7 and 22 seconds after injecting 40 millilitres of a 1:1 iodinated contrast and saline solution at 4 millilitres per second. These two time points capture late arterial and early portal venous phases, revealing when the tumour reaches peak enhancement. The microwave antenna, in this series an Emprint HP system from Medtronic-Covidien, is positioned within the tumour under CT fluoroscopy, and energy delivery is triggered at the predetermined moment of maximal tumour perfusion. CT fluoroscopy confirms that contrast is being retained, that the lesion is shrinking as expected, and that gas formation around the zone is proceeding normally.

The pictorial essay, led by Domenico Santangelo and Susan van der Lei with senior author Martijn Meijerink, presents eleven clinical cases drawn from Amsterdam UMC, University Hospitals of Strasbourg, and the Royal Marsden Hospital in London. The cases span an impressive range of disease: hepatocellular carcinoma, colorectal liver metastases, a pancreatic neuroendocrine tumour metastasis in a 29-year-old woman with MEN-1 syndrome, and a breast cancer liver metastasis. Tumour sizes ranged from 12 to 27 millimetres, and despite variations in enhancement patterns, energy parameters, and antenna positioning, tagging consistently rendered the original tumour boundaries visible on the immediate post-ablation scan. In several cases, the tagged tumour could be seen nested perfectly within the ablation zone, giving radiologists direct visual confirmation of a complete kill with sufficient circumferential margin.

Why does the margin matter so much? Decades of outcome data show that the single most influential factor in preventing local tumour progression is the peri-ablation safety margin, defined as the distance between the tumour’s edge and the boundary of the ablation zone. Traditional teaching held that a circumferential margin of at least 5 millimetres was required, though more recent work using three-dimensional volumetric assessment suggests margins as small as 3 millimetres may suffice in specific settings. The problem is that measuring this margin has always depended on software that fuses pre-ablation and post-ablation images, a process that is notoriously fragile. Differences in breathing mode, patient positioning, tissue shrinkage, thermal injury, and perilesional oedema can all deform the liver and throw off the registration. Hydrodissection, in which fluid is injected to push vulnerable organs like the kidney or colon away from the ablation zone, distorts anatomy even further.

This is where tagging shows its most compelling advantage: it is immune to anatomical distortion. Because the contrast signature marks the tumour itself on the same scan that shows the ablation zone, the relationship between the two is preserved regardless of how much the surrounding liver has shifted or deformed. In one illustrative case, an 80-year-old man with a hepatocellular carcinoma in segment VI underwent hydrodissection with a 5:1 saline-contrast mixture to displace his right kidney. Despite the expected anatomical alteration, the tagged tumour remained clearly visible, and the minimal ablation margin could be assessed reliably without depending on co-registration integrity. Notably, the tagging in this case appeared less pronounced, which the authors attribute to the lower iodine concentration of the contrast agent used, 270 milligrams per millilitre compared with 300 or 350 in other cases.

The essay is refreshingly honest about the technique’s pitfalls. In one case, a bilobed hepatocellular carcinoma was only partially ablated in a first pass; because no second contrast bolus was injected before treating the anterior component, that portion retained no contrast and remained invisible on the post-ablation scan. Tagging, the authors stress, depends on real-time contrast uptake immediately before energy delivery, so any tumour component not enhanced at the moment of ablation will not be tagged. Another case showed that small satellite nodules several millimetres from the main lesion failed to tag, likely because they sat in the passive conduction heating zone, farther from the antenna tip. The authors also demonstrate that tagging is transient: in one patient, the hyperdense rim marking the original tumour faded progressively on scans performed three days and three weeks later, meaning margin assessment must happen immediately after the procedure.

Perhaps most striking is the technique’s success with tumours that are hard to see in the first place. Two cases involved hypovascular colorectal metastases in patients heavily pretreated with chemotherapy, lesions so faint on early-phase CTHA that they were barely perceptible. Even so, subtle peripheral enhancement captured at the 22-second mark was enough to tag the tumour borders, and the tagged region on the post-ablation scan matched the lesion’s morphology on the pre-procedural MRI almost perfectly. In another case, tagging succeeded in a tumour adjacent to a region previously treated with stereotactic body radiotherapy, a scenario where altered perfusion typically complicates image interpretation. A research bolus-tracking experiment, acquiring single-slice scans every 2 seconds after injection, identified peak enhancement at 10 seconds in one patient, supporting the hypothesis that individualized timing maximizes contrast retention and tagging quality.

The technique also compares favourably with the established alternative of tagging tumours with ethiodized oil, or Lipiodol. While Lipiodol accumulates durably in tumours and persists after ablation, it reduces tumour perfusion in ways that can make ablation zones less predictable, and it carries risks of systemic oil embolism, hepatotoxicity, and diffuse non-tumour retention when injected lobarly. The contrast tagging approach requires no additional time or cost when integrated into an existing CTHA-guided ablation workflow, and the authors report it demanded a near-zero learning curve at centres already performing CTHA guidance. No tagging-specific complications were recorded, and no catheter dislocations or groin hematomas occurred in the cohort. The estimated radiation exposure, a CTDI of 77 milligray across the typical 6 to 8 CT acquisitions, remains within the expected range for these complex procedures.

Looking forward, the authors argue that tagging could dovetail naturally with artificial intelligence and deep learning platforms that automate lesion segmentation, registration, and margin quantification. A persistent hyperdense landmark corresponding to the original tumour gives fusion algorithms a reliable anchor in the immediate post-ablation scan, potentially transforming margin assessment from a subjective art into an objective, reproducible measurement. The authors are careful to note that confirmation software remains essential, since it is still unclear whether tagging is perfectly specific to tumour tissue or may also involve adjacent enhancing parenchyma near the antenna tip. Prospective studies are now needed to validate the technique’s impact on local tumour control and to clarify the biological mechanism of contrast retention. If those trials succeed, contrast tagging during CTHA-guided ablation could become a standard component of image-guided liver cancer treatment, giving physicians something they have never had before: a clear, immediate, and honest picture of whether the tumour is truly gone.

Subject of Research: Contrast tagging during CT hepatic arteriography-guided thermal ablation of liver tumours

Article Title: The role of CT hepatic angiography with contrast tagging in liver tumour ablation – an international pictorial essay

Article References: Santangelo, D., van der Lei, S., Johnston, E. W., Cazzato, R. L., De Cobelli, F., & Meijerink, M. R. (2025). The role of CT hepatic angiography with contrast tagging in liver tumour ablation – an international pictorial essay. CVIR Oncology, 1(1), Article 12. https://doi.org/10.1007/s44343-025-00012-4

Image Credits: AI Generated

DOI: 10.1007/s44343-025-00012-4

Keywords: liver tumour ablation, CT hepatic arteriography, contrast tagging, microwave ablation, safety margins, minimal ablation margin, interventional radiology, hepatocellular carcinoma, colorectal liver metastases, image fusion, local tumour progression, CVIR Oncology