The calibration of targeting errors for an ultrasound-guided high-intensity focused ultrasound system

Accurate targeting is one indispensable feature of image-guided high-intensity focused ultrasound (HIFU) systems for treatment safety and efficacy. In our previously developed ultrasound-guided phased-array HIFU system, a rotatable imaging probe was mounted into the central hole of applicator for targeting and monitoring. Two-dimensional image sequence of different imaging planes can be obtained by rotation of the probe. The misalignment between the spots predetermined in the image and the spots sonicated in the tissue can result in the ablation of normal tissue outside the targeting volume, and thus targeting error is unavoidable. An acrylic plate internally placing two flat-head bolts was constructed to measure and calibrate the targeting error. The imaging planes were switched from -90 degrees to 90 degrees with a 30 degrees step, and the targeting errors were measured 12 times for each of these planes before and after calibration. The targeting errors in other imaging planes could be estimated by linear interpolation using the measured errors in the nearest two imaging planes. The coordinates of the spots to be sonicated were corrected in consideration of the targeting errors in the selected imaging plane. After calibration, the mean targeting errors were reduced to 0.30 similar to 0.68 mm from 0.86 similar to 1.74 mm. Besides, in the ex vivo experiment the needle-thermocouple tip was used as the target which could be identified in the image. The temperature rise measured by the thermocouple during sonication was in accordance with the theoretical result. In conclusion, the calibration of targeting errors is effective for our system, and the targeting accuracy is also capable to ensure safe sonication.