We have read with great interest the recent article entitled “Intraoperative ultrasound and tissue elastography measurements do not predict the size of hepatic microwave ablations” by Correa-Gallego et al. . It concludes that ultrasound strain elastography underestimates the area of ablation zones and is thus unable to predict the size of ablation zones in hepatic microwave ablations . We think that there are three points of discussion and special interest about this matter.
First, a critical issue associated with quasistatic ultrasound strain elastography-based hepatic ablation monitoring lies in the stress decay in deep tissues. This may cause that the stress is not enough to produce sufficient deformation of the tissue, making the ultrasound elastographic strain image unable to reflect the true size of ablation zones. To address this issue, open ablation and ablation electrode displacement elastography were exploited. Van Vledder et al. investigated the ability of quasistatic elastography strain imaging to monitor open radiofrequency (RF) ablation of hepatic malignancies in vivo and observed a favorable size correlation between strain images and postablation computed tomography (CT). Kolokythas et al. evaluated the technical feasibility of RF electrode displacement elastography as a tool for assessing the size and shape of the RF ablation zone in a patient with a liver metastasis in vivo, showing that strain images obtained from the patient correlated well with the nonenhancing area of ablation on contrast-enhanced CT. Second, it is challenging to ensure that the ultrasound imaging plane is exactly the same as the CT imaging plane. When the two imaging planes are not identical, the comparison of the two modalities will be unfair. One possible alternative is to compare three-dimensional (3D) ablation zones, such as the volume of 3D ablation zones. Third, ablations induce the formation of gas bubbles in the ablation zone because ablation heating approximates the tissue temperature close to the boiling point. The gas bubble–related information can cause artifacts in ultrasound strain elastography of ablation zones . One possible strategy is to integrate gas bubble–based ablation monitoring techniques such as ultrasound Nakagami imaging with ultrasound strain elastography. In conclusion, we argue that ultrasound strain elastography may be able to monitor hepatic ablations when stress decay, imaging plane, and gas bubble are considered.
References
1. Correa-Gallego C., Karkar A.M., Monette S., et. al.: Intraoperative ultrasound and tissue elastography measurements do not predict the size of hepatic microwave ablations. Acad Radiol 2014; 21: pp. 72-78.
2. Van Vledder M.G., Boctor E.M., Assumpcao L.R., et. al.: Intra-operative ultrasound elasticity imaging formonitoring of hepatic tumour thermal ablation. HPB (Oxford) 2010; 12: pp. 717-723.
3. Kolokythas O., Gauthier T., Fernandez A.T., et. al.: Ultrasound-based elastography: a novel approach to assess radio frequency ablation of liver masses performed with expandable ablation probes: a feasibility study. J Ultrasound Med 2008; 27: pp. 935-946.
4. Varghese T., Techavipoo U., Zagzebski J.A., et. al.: Impact of gas bubbles generated during interstitial ablation on elastographic depiction of in vitro thermal lesions. J Ultrasound Med 2004; 23: pp. 535-544.
5. Wang C.Y., Geng X., Yeh T.S., et. al.: Monitoring radiofrequency ablation with ultrasound Nakagami imaging. Med Phys 2013; 40: pp. 072901.