Design and development of a prototype endocavitary probe for high-intensity focused ultrasound delivery with integrated magnetic resonance imaging.
OBJECTIVE
To integrate a high intensity focused ultrasound (HIFU) transducer with an MR receiver coil for endocavitary MR-guided thermal ablation of localized pelvic lesions.


METHODS
A hollow semicylindrical probe (diameter 3.2 cm) with a rectangular upper surface (7.2 cm x 3.2 cm) was designed to house a HIFU transducer and enable acoustic contact with an intraluminal wall. The probe was distally rounded to ease endocavitary insertion and was proximally tapered to a 1.5-cm diameter cylindrical handle through which the irrigation tubes (for transducer cooling) and electrical connections were passed. MR compatibility of piezoceramic and piezocomposite transducers was assessed using gradient-echo (GRE) sequences. The radiofrequency (RF) tuning of identical 6.5 cm x 2.5 cm rectangular receiver coils on the upper surface of the probe was adjusted to compensate for the presence of the conductive components of the HIFU transducers. A T1-weighted (T1-W) sliding window dual-echo GRE sequence monitored phase changes in the focal zone of each transducer. High-intensity (2400 W/cm(-2)), short duration (<1.5 seconds) exposures produced subtherapeutic temperature rises.


RESULTS
For T1-W images, signal-to-noise ratio (SNR) improved by 40% as a result of quartering the conductive surface of the piezoceramic transducer. A piezocomposite transducer showed a further 28% improvement. SNRs for an endocavitary coil in the focal plane of the HIFU trans-ducer (4 cm from its face) were three times greater than from a phased body array coil. Local shimming improved uniformity of phase images. Phase changes were detected at subtherapeutic exposures.


CONCLUSIONS
We combined a HIFU transducer with an MR receiver coil in an endocavitary probe. SNRs were improved by quartering the conductive surface of the piezoceramic. Further improvement was achieved with a piezocomposite transducer. A phase change was seen on MR images during both subtherapeutic and therapeutic HIFU exposures.
Wharton
Iain P
IP
Therapeutic Ultrasound, Joint Department of Physics, The Institute of Cancer Research, Sutton, Surrey, UK.
Rivens
Ian H
IH
Ter Haar
Gail R
GR
Gilderdale
David J
DJ
Collins
David J
DJ
Hand
Jeff W
JW
Abel
Paul D
PD
deSouza
Nandita M
NM
D000818:Animals; D016477:Artifacts; D002138:Calibration; D017115:Catheter Ablation / Q000295:instrumentation* / Q000592:standards; D002417:Cattle; D004867:Equipment Design / Q000379:methods; D006358:Hot Temperature; D007089:Image Enhancement / Q000379:methods; D008099:Liver; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods; D019047:Phantoms, Imaging; D013696:Temperature; D014159:Transducers; D014464:Ultrasonic Therapy / Q000295:instrumentation* / Q000379:methods