Image-guided, percutaneous thermal tumor ablation using radiofrequency (RF), laser or microwave energy is rapidly gaining acceptance as a treatment option for many tumors in the liver, lung, kidney and bones, with other areas under investigation [1
]. Tumor ablation requires imaging for all aspects of the treatment: tumor diagnosis, localization and targeting, applicator guidance, treatment monitoring and imaging follow-up to assess procedural success. While advances in imaging technologies have improved all of these areas, very little progress has been made to improve practical intra-procedural treatment monitoring.
Current monitoring options include interstitial temperature probes, ultrasound imaging, and MRI. Interstitial temperature probes provide fast and accurate data, but very poor spatial resolution so are typically only used to prevent complications or ensure adequate coverage of a specific point [8
]. Ultrasound imaging can also provide realtime feedback about treatment progress, but is obscured by bubbles created during high-temperature thermal ablation and can be used only to approximate the ablation zone geometry [9
]. New reconstruction techniques may alleviate some of these problems, but are not yet widely available or clinically validated [11
]. Finally, MRI temperature mapping provides fair spatial and temporal resolution, but few interventional MRI systems are available, scan-time is expensive, temperature imaging is relatively slow for volumetric data acquisition, and MRI-compatible ablation equipment is not widespread [14
One alternative to ultrasound and MRI is computed tomography (CT). Contrast between ablated tissue and normal parenchyma is relatively low, so post-ablation imaging is typically performed using contrast-enhancement [16
]. Contrast-enhanced (CE) scans are performed by injecting iodinated contrast material intravenously, then acquiring CT images when the contrast material reaches the target (eg, liver). Iodinated contrast causes signal enhancement, but cannot penetrate into coagulated ablation zones. Therefore, CECT can be used to improve visualization of ablation zones. However, CT uses ionizing radiation (x-rays) and repetitive scans using diagnostic radiation levels would potentially increase the risk of malignancies in the long-term [17
Recently, an image processing technique known as HYPR has been described to increase the signal-to-noise ratio (SNR) in a series of images, while emphasizing changes that occur between images [18
]. The boost in SNR allows lower radiation and contrast doses to be used, enabling the use of periodic CT for treatment monitoring.
Here we present a method for monitoring thermal ablations that uses periodic CECT with HYPR processing that does not increase accumulated radiation dose or contrast load beyond a typical diagnostic CECT. The goals of this preliminary study were to determine whether periodic CECT could be used to monitor treatment progress, and determine the benefit provided by HYPR imaging.