Gregory T. Clement

500-Element Ultrasound Phased Array System for Noninvasive Focal Surgery of the Brain: A Preliminary Rabbit Study With Ex Vivo Human Skulls

The aim of this study was to test a prototype MRI‐compatible focused ultrasound phased array system for trans‐skull brain tissue ablation. Rabbit thigh muscle and brain were sonicated with a prototype, hemispherical 500‐element ultrasound phased array operating at frequencies of 700–800 kHz. An ex vivo human skull sample was placed between the array and the animal tissue. The temperature elevation during 20–30‐sec sonications was monitored using MRI thermometry. The induced focal lesions were observed in T2 and contrast‐enhanced T1‐weighted fast spin echo images. Whole brain histology evaluation was performed after the sonications. The results showed that sharp temperature elevations can be produced both in the thigh muscle and in the brain. High‐power sonications (600–1080 W) produced peak temperatures up to 55°C and focal lesions that were consistent with thermal tissue damage. The lesion size was found to increase with increasing peak temperature. The device was then modified to operate in the orientation that will be used in the clinic and successfully tested in phantom experiments. As a conclusion, this study demonstrates that it is possible to create ultrasound‐induced lesions in vivo through a human skull under MRI guidance with this large‐scale phased array. Magn Reson Med 52:100–107, 2004.

A Magnetic Resonance Imaging–Compatible, Large-Scale Array for Trans-Skull Ultrasound Surgery and Therapy

Advances in ultrasound transducer array and amplifier technologies have prompted many intriguing scientific proposals for ultrasound therapy. These include both mildly invasive and noninvasive techniques to be used in ultrasound brain surgery through the skull. In previous work, it was shown how a 500‐element hemisphere‐shaped transducer could correct the wave distortion caused by the skull with a transducer that operates at a frequency near 0.8 MHz. Because the objective for trans‐skull focusing is its ultimate use in a clinical context, a new hemispheric phased‐array system has now been developed with acoustic parameters that are optimized to match the values determined in preliminary studies.

Standing-Wave Suppression for Transcranial Ultrasound by Random Modulation

Low-frequency transcranial ultrasound (¿1 MHz) is being investigated for a number of brain therapies, including stroke, tumor ablation, and localized opening of the blood-brain barrier. However, lower frequencies have been associated with the production of undesired standing waves and cavitation in the brain. Presently, we examine an approach to suppress standing waves during continuous-wave (CW) transcranial application. The investigation uses a small randomization in the frequency content of the signal for suppressing standing waves. The approach is studied in an ex-vivo human skull and a plastic-walled chamber, representing idealized conditions. The approach is compared to single-frequency CW operation as well as to a swept-frequency input. Acoustic field scans demonstrate that the swept-frequency method can suppress standing waves in the plastic chamber and skull by 3.4 and 1.6 times, respectively, compared to single-frequency CW excitation. With random modulation, standing waves were reduced by 5.6 and 2 times, respectively, in the plastic chamber and skull. It is expected that the process may play a critical role in providing a safer application of the ultrasound field in the brain and may have application in other areas where standing waves may be created.

A harmonic cancellation technique for an ultrasound transducer excited by a switched-mode power converter

The aim of this study is to evaluate the feasibility of using harmonic cancellation for a therapeutic ultrasound transducer excited by a switched-mode power converter without an additional output filter. A switching waveform without the 3rd harmonic was created by cascading two switched-mode power inverter modules at which their output waveforms were pi/3 phase-shifted from each other. The waveform and harmonic contents of the acoustic pressure generated by a 1-MHz self-focused piezoelectric transducer with and without harmonic cancellation have been evaluated. Measured results indicated that the acoustic 3rd harmonic-to-fundamental ratio at the focus was small (-48 dB) with harmonic cancellation, compared to that without harmonic cancellation (-20 dB). The measured acoustic levels of the 5th harmonic for both cases with and without harmonic cancellation were also small (-46 dB) compared to the fundamental. This study shows that it is viable to drive a piezoelectric ultrasound transducer using a switched-mode power converter without the requirement of an additional output filter in many high intensity focused ultrasound (HIFU) applications.

A Computer-Controlled Ultrasound Pulser-Receiver System for Transskull Fluid Detection using a Shear Wave Transmission Technique

The purpose of this study was to evaluate the performance of a computer-controlled ultrasound pulser-receiver system incorporating a shear mode technique for transskull fluid detection. The presence of fluid in the sinuses of an ex vivo human skull was examined using a pulse-echo method by transmitting an ultrasound beam through the maxilla bone toward the back wall on the other side of the sinus cavity. The pulser was programmed to generate bipolar pulse trains with 5 cycles at a frequency of 1 MHz, repetition frequency of about 20 Hz, and amplitude of 100 V to drive a 1-MHz piezoelectric transducer. Shear and longitudinal waves in the maxilla bone were produced by adjusting the bone surface incident angle to 45deg and 0deg, respectively. Computer tomography (CT) scans of the skull were performed to verify the ultrasound experiment. Using the shear mode technique, the echo waveform clearly distinguishes the presence of fluid, and the estimated distance of the ultrasound traveled in the sinus is consistent with the measurement from the CT images. Contrarily, using the longitudinal mode, no detectable back wall echo was observed under the same conditions. As a conclusion, this study demonstrated that the proposed pulser-receiver system with the shear mode technique is promising for transskull fluid detecting, such as mucus in a sinus.

Forward planar projection through layered media

A planar forward projection algorithm is combined with ray theory to describe longitudinal propagation through an arbitrary number of randomly oriented isotropic layers. This method first measures the space-time pressure field in a plane, then uses wavevector frequency-domain methods to project the field through layered media and to an arbitrary new plane, not necessarily parallel to the initial plane. The approach is valid for longitudinal propagation through liquid layers and in solids, such as soft tissues, that can be approximated as viscous liquids. The algorithm is verified by propagating the field from a 0.5 MHz planar transducer through a combination of rubber, plastic, and water layers. Hydrophone measurements indicate correlation between measured and simulated fields for angles below the longitudinal critical angles of the layered materials.

A wireless batteryless deep-seated implantable ultrasonic pulser-receiver powered by magnetic coupling

This study tests a deep-seated implantable ultrasonic pulser-receiver powered wirelessly by magnetic coupling. The energy-transmitting coil, which wraps around the body, is driven at 5.7 MHz using 1.2A to generate a magnetic field. A 2 cm receiving coil is positioned at the center of the primary coil for powering the implantable device. A capacitor-diode voltage multiplier is used to step-up the receiving coil voltage from 12.5V to 50V to operate an ultrasonic pulser. FEA magnetic field simulations, bench top and ex-vivo rabbit measurements showed that the magnetic energy absorption in body tissue is negligible, and that the magnetic coupling has low sensitivity to receiving coil placement. The receiving coil and the implantable circuit do not contain ferromagnetic material so an MR-compatible device can be achieved. A 5-MHz ultrasound transducer operating in pulse-echo mode is used to test the circuit. Received echoes are amplified, envelope-detected, frequency-modulated, and transmitted out of the body by radio wave. The modulated echo envelope was received by an external receiver located approximately 10cm away from the primary coil. The study establishes the feasibility of a deep-seated implantable ultrasonic pulser-receiver powered by coplanar magnetic coupling.

Acoustic standing wave suppression using randomized phase-shift-keying excitations

Recent papers have demonstrated that acoustic standing waves can be inhibited by frequency-modulated spread-spectrum excitation. An alternative method is studied here that is designed to be more practical for implementation in phased arrays. The method operates using phase-shift-keying (PSK), which introduces phase shifts into the driving signal to break wave symmetry. Sequential and random binary-PSK (BPSK) and quadrature-PSK (QPSK) excitations are studied in water, using a carrier frequency of 250 kHz and a time segment of 10 cycles. The resulting acoustic field is measured with a transducer inside a plastic-walled chamber and compared with continuous wave excitation. Results indicate that both the random BPSK and QPSK methods can reduce time-averaged spatial intensity variation caused by standing waves by approximately six times.

Automated sonographic evaluation of testicular perfusion

Contrast-enhanced ultrasound (US) imaging is potentially applicable to the investigation of vascular disorders of the testis. We investigated the ability of two automated computer algorithms to analyse contrast-enhanced pulse inversion US data in a rabbit model of unilateral testicular ischaemia and to correctly determine relative testicular perfusion: nonlinear curve fitting of the US backscatter intensity as a function of time; and spectral analysis of the intensity time trace. We compared (i) five metrics based on the algorithmic data to testicular perfusion ratios obtained with radiolabelled microspheres, a reference standard; (ii) qualitative assessment of the US images by two independent readers blinded to the side of the experimental and control testes to the radiolabelled microsphere perfusion ratios; and (iii) results of the algorithmically-derived metrics to the qualitative assessments of the two readers. For the curve fit method, the algorithmically-derived metrics agreed with the reference standard in 54% to 68% of all cases. For the spectral method, the results agreed in 70% of all cases. The two readers agreed with the reference standard in 40% and 35% of all cases, respectively. These results suggest that automated methods of analysis may provide useful information in the assessment of testicular perfusion.

128 Element ultrasound array for transcranial imaging

The medical potential of transcranial ultrasound has been severely limited by strong attenuation and scattering by the skull, which has prevented its use through most regions of the bone. We have been investigating methods to image through these areas by applying various encoding approaches at reduced frequency, thus reducing both absorption and scattering while also permitting the propagation of both lateral and shear modes. Based on a simulation study, a 0.75 MHz center frequency was determined to be sufficiently low for transcranial propagation, while still preserving medically-relevant resolution. A 128 element, PZT 1–3 composite configuration was optimized for bandwidth and steering range at a 10 cm focal distance. Preliminary test results of the completed array are presented here. Array performance is assessed using a standardized calibration phantom. B-Mode and C-mode transcranial images are also acquired in polyvinyl alcohol (PVA) brain phantoms after propagation through preserved ex vivo human calvaria. For comparison, identical images are acquired without the calvaria. Overall, the ability to reconstruct features through the skull has been established, and critical information has been obtained for constructing a second-generation array planned for human subject testing.