More than 50% of the natural progression of digestive tumors leads to liver metastases. At present, the best strategy for LM is surgical resection. However, not all patients are eligible to undergo resection owing to poor liver function, multiple lesions, unique lesion location, advanced age, contraindication of general anesthesia, and subjective rejection or refusal to be operated upon8. During the past 30 years, image-guided ablation techniques such as MA, radiofrequency ablation (RFA) and cryoablation have shown rapid development to become the first-line local treatment protocol for LM9,10. An increasing number of basic and clinical studies have shown that MA can generate, deposit, and transmit local energy more quickly than conventional RFA, which means that MA may have wider clinical indications and better clinical outcomes11,12. In theory, the preferable image-guidance modality should provide convenient, fast, accurate, and efficient guidance, along with precise and instant evaluation of the lesion and adequate ablation with sufficient boundary and minimal damage to normal liver tissue13. At present, mainstream ablation-guidance modalities such as CT or ultrasound have their own advantages and disadvantages. Ultrasound had no ionizing radiation exposure and provides real-time monitoring with any-angle imaging function, but it is limited by low tissue resolution and sound window affected by gas or bone. While contrast-enhanced ultrasound can better display the characteristics of tissue enhancement after ablation, the bubbles produced by local high temperature after ablation can lead to high echogenicity, thereby causing overestimation of the ablated area14,15. Multi-slice spiral CT had the advantages of fast scanning and wide adaptability, but it involves exposure to ionizing radiation and shows poor instant assessment of the ablation boundary.
MR possesses many advantages such as very high soft-tissue resolution, multi-parameter imaging, and free ionizing radiation exposure. There are very few clinical studies describing MR-guided MA, as most of them are carried out under 0.5–1.0T open low-field magnets15–21 (Table 5). Theoretically, soft tissue resolution is higher in field strengths >1.5 T than in open low field strength, which means more technical advantages in detection, monitoring and evaluation for LM and MA results. In 2000, Chen et al. reported five patients with prostate tumor who underwent 1.5T MR-guided MA. Although the reported technical success was 100%, this method did not have wide clinical application owing to technical limitations such as the microwave generator needing to be arranged outside the MR room and the long-distance coaxial cable producing additional noise15. The microwave ablation machine used in our study can be placed within 5 gauss line, and there is no obvious interference on the image, we believe that can be more widely used in the future.
Table 5
Clinical studies published on MR-guided Microwave ablation
| Authors | Year | Pts number | MR-type | Study design | Study objective | Tumor type | Microwave equipment | Results |
| Chen JC, et al.15 | 2000 | 5 pts | 1.5T whole-body MR(GE) | Rs | Safety and feasibility | Prostate cancer | Urowave®, Dornier 915 MHz Germany | Technical success100% |
| Morikawa S, et al.16 | 2002 | 30 pts | 0.5 T open MR (GE) | Rs | Safety and feasibility | liver metastases | Microtaze®, OT-110M, 2450 MHz, Japan | Satisfactory results |
| Morikawa S, et al.17 | 2004 | 33 pts | 0.5 T open MR (GE) | Rs | Respiratory triggering for ablation | Liver tumor | Microtaze®, OT-110M, 2450 MHz, Japan | Feasibility of Respiratory triggering for ablation under general anesthesia |
| Abe H, e t al.18 | 2005 | 8 pts | 0.5 T open MR(Philips) | Rs | Safety and feasibility | liver metastases | Microtaze®, OT-110M, 2450 MHz, Japan | Technical success100% |
| Murakami K, et al.19 | 2015 | 6 pts | 0.5 T open MR (GE) | Rs | Feasibility for MR guided laparoscopic ablation | Liver tumors | Microtaze®, HSD20M, 2450 MHz, Japan | Effective treatment for tumor ablation avoiding adjacent organs |
| Hoffmann R, et al.20 | 2016 | 11pts | 1.5T wide-bore MR(Siemens) | Ps | Safety an effectiveness | Liver tumors | Medwaves AvecireTM, 928 MHz, USA | Near real-time MR guidance for ablation |
| Lin Z, et al.21 | 2019 | 35 pts | 1.5T whole body MR (GE) | Rs | Safety and feasibility | Liver tumors | Vision®, MTC-3CA-Ⅱ, 2450 MHz, China | Technical efficacy 100% |
| Current study | 2021 | 39 pts | 3.0T wide-bore MR (Siements) | Ps | Safety and feasibility | liver metastases | ECO-100E®, 2450MHz, China | Technical success100% |
| Pts: Patients; RS: Retrospective study; Ps: Perspective study |
To our best knowledge, there are only few reports that have described the 3.0T wide-bore MR-guided MA for LM. This pilot study showed that the technical success was 100%, and the complete ablation rates for lesions measuring ≦2.5 and >2.5 cm were 100% and 92.5%, respectively, without severe complications. Liver function showed a transient increase and return to baseline by day 30 post MA, which meant that the tissue damage was temporary. This was likely expected, because we used the same ablation process here as in previous studies, with only small modification to the guidance tool15–21. Furthermore, the MR-based unique signal change can be used as an objective, reliable, and rapid assessment tool given the high signal on T1WI, low signal on T2WI, and sharp contrast with the primary tumors in the ablation zone. The 3.0T high field strength resulted in high soft-tissue resolution, thus capturing the signal changes of the boundary after ablation for instant precise evaluation. Such typical changes are mainly due to the characteristics of water decrease in tissues. Sheng et al.22 and Lee et al.23 both agreed that the “target sign” unenhanced T1WI scan was enough to evaluate the ablation extent and identify the residual tumor. On the other hand, CT can only distinguish between high and low density. Ablation leads to low tissue density, while ablation edge tissue hemorrhage and edema lead to local density increase that in turn affect accurate evaluation of the edge24. The formation of air bubbles during ultrasound-guided ablation results in a blurred boundary. Thus, MR-guided ablation plays a very important role in the ablation of very small lesions and lesions at specific locations25,26. It has been reported that poor display microwave applicator tip (length, 1.6 cm) on T1WI may be related to the ceramic structure of the tip, especially for oblique non-coplanar puncture, but the depth and shape of the applicator tip can be observed better after scanning along the long axis of the microwave applicator27.
There are still some unaddressed disadvantages of MR-guidance, such as long scanning time, being unsuited for patients with poor breath-holding coordination, high cost of MR-compatible applicator adding to the treatment costs, a closed magnet system that offers very limited procedural convenience, and being contraindicated in patients with artificial pacemaker, metal implants, and claustrophobia. The limited follow-up duration and patient selection led to an unclear clinical outcome in this pilot study. Further randomized controlled trials are need in future to explore the clinical significance of MR guidance.
This pilot study showed that the 3.0T wide-bore MR-guided MA for LM is safe and effective. We believe that with adequate medical resources reallocation to make wide-bore MR and MR-compatible microwave materials more accessible, interventional MR has a strong potential for wide and successful clinical application.