The distribution of sufficient thermal dose is then calculated and assumed to correspond to thermally ablated tissue. The temporal resolution of MR thermometry is 1-4 seconds per image, and the spatial resolution is determined by the size of the image voxel which is typically about 2mm x 2mm x 6mm
(40). Therefore, MR-guided HIFU is only suitable for treatments in which the heating occurs slowly, on the order of tens of seconds for a single lesion. Motion artifact due to breathing and heartbeat is also a concern in clinical setting. The only US FDA-approved HIFU device available for clinical therapy utilizes MR thermometry Inhibitors,research,lifescience,medical during treatment of uterine fibroids (39),(41). Ultrasound imaging Inhibitors,research,lifescience,medical used in current clinical devices does not have the capability of performing thermometry, but it provides real-time imaging using the same energy modality as HIFU. This is a significant benefit, because adequate ultrasound imaging of the target suggests that there is no obstruction (e.g., bowel gas or bone) to ultrasound energy reaching the target, and the risk of causing thermal injury to unintended tissue is minimized. One method that Inhibitors,research,lifescience,medical is sometimes used for confirmation of general targeting accuracy is the appearance of a hyperechoic region on the ultrasound
image during treatment. This region has been shown to correspond to the formation of a large boiling bubble at the focus when tissue temperature reaches 100°C, and underestimates the actual size of the thermal lesion since thermal lesions develop at temperatures
below Inhibitors,research,lifescience,medical 100°C (42). Imaging methods to assess HIFU treatment are similar to those used to assess the response to other methods of ablation such as radiofrequency ablation and include contrast enhanced CT and MRI (43). In addition, the use Inhibitors,research,lifescience,medical of microbubble contrast-enhanced sonography is also being examined as a method to evaluate the treatment effect of HIFU (44). These methods all examine the change in vascularity of the treated volume. HIFU of pancreatic tumors Devices Currently, HIFU treatment of pancreatic cancer is widely available in China, with limited availability in South Korea and Europe. There are two US-guided HIFU devices that are commercially available outside of China for treatment of pancreatic tumors, both manufactured in China: The FEP-BY™ HIFU tumor therapy PD184352 (CI-1040) device (Yuande Biomedical Engineering Limited Corporation, Beijing, China, Figure 4) and HAIFU (Chongqing Haifu Technology Co.,) (45). Both devices operate at similar ultrasound frequencies – 0.8 and 1 MHz respectively; both are capable of putting out total acoustic power of about 300 W (corresponding intensity up to 20 000W/cm2). Selleck Metformin B-mode ultrasound is also used in both machines for targeting and image guidance. In addition, a patient with pancreatic tumor was recently treated in Italy using the MR-guided ExAblate™ system (InSightec, Israel) for palliation of pain.