The targeting accuracy of a preclinical MRI‐guided focused ultrasound system

• Published in: Medical physics (Lancaster) 2015-01, Vol.42 (1), p.430-439
• Main Authors: Ellens, N. P. K., Kobelevskiy, I., Chau, A., Waspe, A. C., Staruch, R. M., Chopra, R., Hynynen, K.
• Format: Article
• Language: English
• Subjects: Acoustic analysis; Animals; Biological material, e.g. blood, urine; Haemocytometers; Biomedical instrumentation and transducers, including micro‐electro‐mechanical systems (MEMS); biomedical MRI; biomedical transducers; blood‐brain barrier disruption; Brain; Canrenone; Clinical applications; Digital computing or data processing equipment or methods, specially adapted for specific applications; Equipment Design; Germanium; Image data processing or generation, in general; image registration; Image scanners; Involving electronic [emr] or nuclear [nmr] magnetic resonance, e.g. magnetic resonance imaging; Magnetic Resonance Imaging - instrumentation; Magnetic Resonance Imaging - methods; medical image processing; Medical magnetic resonance imaging; Medical treatment planning; Mice; MRI combatible; MRI‐guided focused ultrasound; Polystyrenes; preclinical; Processes or apparatus for generating mechanical vibrations of infrasonic, sonic or ultrasonic frequency; Rats; Registration; Therapeutic applications; ultrasonic therapy; ultrasonic transducers; Ultrasonography; Ultrasonography - methods; Ultrasound therapy; User-Computer Interface; Water
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.4903950

Purpose: Assess the accuracy, precision, and sources of error using a preclinical MR‐guided focused ultrasound system. Methods: A preclinical focused ultrasound system, described previously [Chopra et al., Med. Phys. 36(5), 1867–1874 (2009)], was tested on a benchtop and with 3T GE, 3T Philips, and 7T Bruker MR scanners for spatial targeting accuracy and precision. Randomly distributed water‐filled holes drilled into a polystyrene plate were imaged using MRI and targeted using treatment planning software. The ultrasound focus of a 72 mm, f‐number 0.8, 1.16 MHz transducer was aimed at the target locations, and 1–2 s continuous‐wave sonications were performed on clear polystyrene plates to create localized spots of melted plastic. The distance between target and observed locations was measured and analyzed. Retrospective analysis of targeting accuracy was performed on preclinical data obtained from other experiments at their institution using the same system. Results: The results suggest that the sources of targeting error under MR guidance can be roughly separated into three components—normally distributed random error; constant shift from inaccuracy in detection of the initial ultrasound focus; and angular misalignment between MR and focused ultrasound (FUS) coordinates. The lower bound on the targeting error was estimated to be 0.25 ± 0.13 mm, while the maximum observed targeting error did not exceed 2 mm. Measures required to reduce errors and improve targeting were developed to reduce the registration and misalignment errors such that maximum error was reduced to 0.36 ± 0.14 mm. Retrospective in vivo analysis indicated that the error was 1.02 ± 0.43 mm, including error extrinsic to the system. Conclusions: The FUS system, as described, is capable of precise and accurate sonications. The largest source of error—misregistration of the coordinate systems of the scanner and ultrasound system—was addressed which reduced the error to 0.36 ± 0.14 mm, sufficient for many preclinical applications.