Svein-Erik Måsøy

SURF imaging: In vivo demonstration of an ultrasound contrast agent detection technique

A dual-band method for ultrasound contrast agent detection is demonstrated in vivo in an animal experiment using pigs. The method is named Second-order UltRasound Field Imaging, abbreviated SURF Imaging. It relies on simultaneously transmitting two ultrasound pulses with a large separation in frequency. Here, a low-frequency pulse of 0.9 MHz is combined with a high-frequency pulse of 7.5 MHz. The low-frequency pulse is used to manipulate the properties of the contrast agent, and the high- frequency pulse is used for high-resolution contrast detection and imaging. An annular array capable of transmitting the low- and high-frequency pulses simultaneously was constructed and fitted to a mechanically scanned probe used in a GE Vingmed System 5 ultrasound scanner. The scanner was modified and adapted for the dual-band transmit technique. In-house software was written for post-processing of recorded IQ-data. Contrast-processed B-mode images of pig kidneys after bolus injections of 1 mL of Sonovuereg are presented. The images display contrast detection with contrast-to-tissue ratios ranging from 15-40 dB. The results demonstrate the potential of SURF Imaging as an ultrasound contrast detection technique for clinically high ultrasound frequencies. This may allow ultrasound contrast imaging to be available for a wide range of applications.

Radial Modulation Imaging of Microbubble Contrast Agents at High Frequency

In this paper, radial modulation imaging of microbubbles is investigated at high frequency. A modulation pulse frequency of 3.7 MHz with an amplitude ranging from 0 to 250 kPa, and a 1.3-MPa 20-MHz broadband imaging pulse were used. Radial modulation effects were observed on a population of flowing microbubbles and quantified using a Doppler-type processing technique. Artifact signals related to the generation of harmonics by bubbles strongly resonating at the modulation frequency were observed. The bubble response to simultaneous modulation and imaging excitations was simulated for different combinations of bubble sizes and modulation amplitudes. Simulation results confirm the hypothesis that the generation of harmonics of the modulation frequency can be detected by the imaging transducer. Simulations indicate that the modulation frequency should be chosen lower than the resonant frequency of the biggest bubbles present in the population. The simulation also suggests that a 10% variation of bubble diameter induced by the modulation excitation is sufficient for radial modulation imaging. In conclusion, the effects of radial modulation are detectable at a high frequency. Therefore, radial modulation imaging has potential for high-resolution imaging of microbubbles in the microvasculature.

Aberration in nonlinear acoustic wave propagation

Theory and simulations are presented indicating that imaging at the second-harmonic frequency does not solve the problem of ultrasonic wave aberration. The non-linearity of acoustic wave propagation in biological tissue is routinely exploited in medical imaging because the improved contrast resolution leads to better image quality in many applications. The major sources of acoustic noise in ultrasound images are aberration and multiple reflections between the transducer and tissue structures (reverberations), both of which are the result of spatial variations in the acoustic properties of the tissue. These variations mainly occur close to the body surface, i.e., the body wall. As a result, the nonlinearly generated, second harmonic is believed to alleviate both reverberation and aberration because it is assumed that the second harmonic is mainly generated after the body wall. However, in the case of aberration, the second harmonic is generated by an aberrated source. Thus the second harmonic experiences considerable aberration at all depths, originating from this source. The results in this paper show that the second harmonic experiences similar aberration as its generating source, the first harmonic.

Abersim: A simulation program for 3D nonlinear acoustic wave propagation for arbitrary pulses and arbitrary transducer geometries

Abersim, a simulation package for 3D nonlinear acoustic wave propagation of wide-band pulses from arbitrary transducers, is presented. This study validates the 3D diffraction and attenuation produced by Abersim, by comparing them to those obtained using Field II. For the presented test cases, the axial pressure amplitude and the RMS values deviate less than 0.7 dB, both for axis-symmetric and array transducers. Abersim has a MATLAB programming interface, and is freely available under the GPL license.

Utilizing dual frequency band transmit pulse complexes in medical ultrasound imaging

A method of acoustic imaging that potentially can improve the diagnostic capabilities of medical ultrasound is presented. The method, given the name SURF (Second order UltRasound Field) imaging, is achieved by processing the received signals from transmitted dual frequency band pulse complexes with at least partly overlapping high frequency (HF) and low frequency (LF) pulses. The transmitted HF pulses are used for image reconstruction, whereas the transmitted LF pulses are used to manipulate the elastic properties of the medium observed by the HF imaging pulses. The present paper discusses fundamental concepts in relation to the use of dual frequency band pulse complexes for medical ultrasound imaging.

Transmit beams adapted to reverberation noise suppression using dual-frequency SURF imaging

A method that uses dual-frequency pulse complexes of widely separated frequency bands to suppress noise caused by multiple scattering or multiple reflections in medical ultrasound imaging is presented. The excitation pulse complexes are transmitted to generate a second order ultrasound field (SURF) imaging synthetic transmit beam. This beam has reduced amplitude near the transducer, which illustrates the multiple scattering suppression ability of the imaging method. Field simulations solving a nonlinear wave equation are used to calculate SURF imaging beams, which are compared with beams for pulse inversion (PI) and fundamental imaging. In addition, a combined SURF and PI beam generation is described and compared with the beams mentioned above. A quality ratio, relating the energy within the near-field to that within the imaging region, is defined and used to score the multiple scattering and multiple reflection suppression abilities when imaging with the different beams. The realized combined SURF-PI beam scores highest, followed by SURF, PI (that score equally well), and the fundamental. The amplitude in the imaging region and therefore also the SNR is highest for the fundamental followed by SURF, PI, and SURF-PI. The work hence indicates that when substituting PI for SURF, one may trade increased SNR into use of increased imaging frequencies without loss of multiple scattering and multiple reflection noise suppression.

Estimation of ultrasound wave aberration with signals from random scatterers

A method for estimating waveform aberration from random scatterers in medical ultrasound imaging has been derived and its properties investigated using two-dimensional simulations. The method uses a weighted and modified cross-spectrum in order to estimate arrival time and amplitude fluctuations from received signals. The arrival time and amplitude fluctuations were used in a time delay, and a time delay and amplitude aberration correction filter, for evaluation of the retransmitted aberration corrected signal. Different types of aberration have been used in this study. First, aberration was concentrated on the plane of the transmitting/receiving array. Second, aberration was generated with a distributed aberrator. Both conditions emulated aberration from the human abdominal wall. Results show that for the concentrated aberrator, arrival time and amplitude fluctuations were estimated in close agreement with reference values. The reference values were obtained from simulations with a point source in the focal point of the array. Correction of the transmitted signal with a time delay, and a time delay and amplitude filter produced approximately equal correction as with point source estimates. For the distributed aberrator, the estimator performance degraded significantly. Arrival time and amplitude fluctuations deviated from reference values, leading to a limited correction of the retransmitted signal.

Iteration of transmit-beam aberration correction in medical ultrasound imaging

Simulations of iterative transmit-beam aberration correction using a time-delay and amplitude filter have been performed to study the convergence of such a process. Aberration in medical ultrasonic imaging is usually modeled by arrival-time and amplitude fluctuations concentrated on the transducer array. This is an approximation of the physical aberration process, and may be applied to correct the transmitted signal using a time-delay and amplitude filter. Estimation of such a filter has proven difficult in the presence of severe aberration. Presented here is an iterative approach, whereby a filter estimate is applied to correct the transmit-beam. This beam induces acoustic backscatter better suited for arrival-time and amplitude estimation, thus facilitating an improved filter estimate. Two correlation-based methods for estimating arrival-time and amplitude fluctuations in received echoes from random scatterers were employed. Aberration was introduced using eight models emulating aberration produced by the human abdominal wall. Results show that only a few iterations are needed to obtain corrected transmit-beam profiles comparable to those of an ideal aberration correction filter. Furthermore, a previously developed focusing criterion is found to quantify the convergence accurately.

Effect of Ultrasound Parameters on the Release of Liposomal Calcein

The ultrasound exposure parameters that maximize drug release from dierucoyl-phosphatidylcholine (DEPC)-based liposomes were studied using two transducers operating at 300 kHz and 1 MHz. Fluorescent calcein was used as a model drug, and the release from liposomes in solution was measured using a spectrophotometer. The release of calcein was more efficient at 300 kHz than at 1 MHz, with thresholds of peak negative pressures of 0.9 MPa and 1.9 MPa, respectively. Above this threshold, the release increased with increasing peak negative pressure, mechanical index (MI), and duty cycle. The amount of drug released followed first-order kinetics and increased with exposure time to a maximal release. To increase the release further, the MI had to be increased. The results demonstrate that the MI and the overall exposure time are the major parameters that determine the drugs release. The drugs release is probably due to mechanical (cavitation) rather than thermal effects, and that was also confirmed by the detection of hydroxide radicals.

Sensitivity of minimum variance beamforming to tissue aberrations

Minimum variance beamformers adapt to the received data. In the case of perfect data, this beamformer can produce images with better point and edge definitions than conventional delay-and-sum beamformers. But in medical ultrasound imaging, the quality of the data is often corrupted by aberration. As higher resolution often is associated with less robustness, a study using a 1D phase aberrator was done to find how the minimum variance beamformer performs with realistic phase aberrations. We found that with proper subaperture smoothing and regularization, the minimum variance beamformer will give better or similar performance compared to the delay-and-sum beamformer for aberrations typically found in the human body.