Guillaume Matte

Broadband reduction of the second harmonic distortion during nonlinear ultrasound wave propagation.

Ultrasound contrast harmonic imaging and detection techniques are hampered by the harmonic distortion of the ultrasound wave caused by the nonlinearities of the medium. To increase the discrimination between the tissue and ultrasound contrast agents at higher harmonics, we investigate a tissue harmonic suppression technique. The main attention of the research is the signal that is introduced at the source and is constructed out of several discrete frequency components from the second harmonic band. Therefore, this method was coined as the multiple component second harmonic reduction signal or multiple component SHRS. By adjusting the amplitude and phase of discrete components and simultaneously propagating multiple component SHRS with the imaging signal, the nonlinear distortion of the ultrasound waveform is considerably reduced. Using the numerical simulation, the optimal parameters for multiple component SRHS were deduced. The simulations results were corroborated in the water tank experiments and showed 40 dB reduction with respect to the fundamental, covering up to 75% of the entire second harmonic band. In the other series of experiments with the clinically used contrast agent, the uniform increase in agent-to-tissue ratio of 7.4 dB over a relatively large region of imaging was observed. The use of the proposed method in the everyday clinical practice can improve discrimination between the tissue and the contrast agent in harmonic imaging.

Optimization of a phased-array transducer for multiple harmonic imaging in medical applications: frequency and topology

Second-harmonic imaging is currently one of the standards in commercial echographic systems for diagnosis, because of its high spatial resolution and low sensitivity to clutter and near-field artifacts. The use of nonlinear phenomena mirrors is a great set of solutions to improve echographic image resolution. To further enhance the resolution and image quality, the combination of the 3rd to 5th harmonics - dubbed the superharmonics - could be used. However, this requires a bandwidth exceeding that of conventional transducers. A promising solution features a phased-array design with interleaved low- and high-frequency elements for transmission and reception, respectively. Because the amplitude of the backscattered higher harmonics at the transducer surface is relatively low, it is highly desirable to increase the sensitivity in reception. Therefore, we investigated the optimization of the number of elements in the receiving aperture as well as their arrangement (topology). A variety of configurations was considered, including one transmit element for each receive element (1/2) up to one transmit for 7 receive elements (1/8). The topologies are assessed based on the ratio of the harmonic peak pressures in the main and grating lobes. Further, the higher harmonic level is maximized by optimization of the center frequency of the transmitted pulse. The achievable SNR for a specific application is a compromise between the frequency-dependent attenuation and nonlinearity at a required penetration depth. To calculate the SNR of the complete imaging chain, we use an approach analogous to the sonar equation used in underwater acoustics. The generated harmonic pressure fields caused by nonlinear wave propagation were modeled with the iterative nonlinear contrast source (INCS) method, the KZK, or the Burgers equation. The optimal topology for superharmonic imaging was an interleaved design with 1 transmit element per 6 receive elements. It improves the SNR by ~5 dB compared with the interleaved (1/2) design reported in literature. The optimal transmit frequency for superharmonic echocardiography was found to be 1.0 to 1.2 MHz. For superharmonic abdominal imaging this frequency was found to be 1.7 to 1.9 MHz. For 2nd-harmonic echocardiography, the optimal transmit frequency of 1.8 MHz reported in the literature was corroborated with our simulation results.

A new frequency compounding technique for super harmonic imaging

Second harmonic imaging is currently the de-facto standard in commercial echographic systems for diagnosis because of its improved resolution and contrast to tissue ratio. An emerging technique called super harmonic imaging is based on a combination of multiple frequency components generated during the propagation of sound in tissue. This combination of third to fifth harmonic has the potential to further enhance resolution and image quality of echographic pictures. In order to fulfill the bandwidth requirements of super harmonic imaging, a special interleaved phased array transducer has been developed. Currently, the achievable bandwidth for phased array elements used in transmission is close to 80%, which involves that generated harmonics will be separated by gaps in the frequency domain. That will introduce specific artifacts visible as ripples in the echo image. We propose a two-pulse technique that reduces the ripple artifacts and recovers the axial resolution. Method. This technique consists in firing two lines with a 15% frequency shift for the second firing. Summing the echoes of those two lines will result in filling the gaps in the frequency band of the distorted signal. The optimal choice for the frequency of the second pulse can be derived analytically. Standard detection methods applied to this two-pulse technique will strongly minimize artifacts encountered with envelope detection on super harmonic signals. Results. Theoretical calculations show an improvement in axial resolution by a factor of 2.7 at the -15 dB level compared to second harmonic imaging. For a fair comparison, the super harmonic signal will be compared with the lowest frequency component of its spectrum, which is the third harmonic. A shortening of the pulse is visible in the enclosed figure where axial point spread function simulations of third harmonic and super harmonic pulses are compared. Experimentally, this method shortens the pulse by a factor 2.3 at the -15 dB level compared to second harmonic, and 1.9 compared to third harmonic. Conclusion. Super harmonic imaging quality can be further improved by frequency compounding techniques such as the two-pulse method described here.

Dual-pulse frequency compounded superharmonic imaging

Tissue second-harmonic imaging is currently the default mode in commercial diagnostic ultrasound systems. A new modality, superharmonic imaging (SHI), combines the third through fifth harmonics originating from nonlinear wave propagation through tissue. SHI could further improve the resolution and quality of echographic images. The superharmonics have gaps between the harmonics because the transducer has a limited bandwidth of about 70% to 80%. This causes ghost reflection artifacts in the superharmonic echo image. In this work, a new dual-pulse frequency compounding (DPFC) method to eliminate these artifacts is introduced. In the DPFC SHI method, each trace is constructed by summing two firings with slightly different center frequencies. The feasibility of the method was established using a single-element transducer. Its acoustic field was modeled in KZK simulations and compared with the corresponding measurements obtained with a hydrophone apparatus. Subsequently, the method was implemented on and optimized for a setup consisting of an interleaved phased-array transducer (44 elements at 1 MHz and 44 elements at 3.7 MHz, optimized for echocardiography) and a programmable ultrasound system. DPFC SHI effectively suppresses the ghost reflection artifacts associated with imaging using multiple harmonics. Moreover, compared with the single-pulse third harmonic, DPFC SHI improved the axial resolution by 3.1 and 1.6 times at the -6-dB and -20-dB levels, respectively. Hence, DPFC offers the possibility of generating harmonic images of a higher quality at a cost of a moderate frame rate reduction.

Dual pulse frequency compounded super harmonic imaging for phased array transducers

Second harmonic imaging is currently the standard in commercial echographic systems. A new modality, super harmonic imaging (SHI), is based on combining the 3rd to 5th harmonic generated during sound propagation in tissue. This emerging modality could further enhance resolution and quality of echographic images. To meet the bandwidth requirement for SHI an interleaved phased array was developed. Array elements used in transmission generally have bandwidths of ∼ 80% leading to gaps between harmonics in the spectral domain. This causes ripple artifacts in the echo image. Last year we introduced a new dual pulse frequency compounding method to reduce these artifacts and showed initial single element results [1]. In this work we implement and optimize the dual pulse method for an interleaved array on an ultrasound system and research its imaging characteristics, i.e. point spread functions (PSF). In the dual pulse SHI method each trace is constructed by the summing of two firings, the second slightly frequency shifted compared to the first. To study the dual pulse methods performance an interleaved array (44 1 MHz and 44 3.7 MHz elements, optimized for echocardiography) was used in combination with a fully programmable ultrasound system. Initial estimates for the frequencies of the first and second pulses as well as the pulse duration were optimized experimentally. Our findings confirm that the transfer functions of both transducer and system have to be taken into account to determine the optimal transmission frequencies for the dual pulse SHI method. Moreover, a trade off exists between dual pulse signal length and peak intensity. The optimal results with the dual pulse technique were achieved using a transmission length of 2.5 cycles and transmission frequencies of 0.87 MHz and 1.12 MHz. The lateral beam widths of the optimal dual pulse signal are 1.2 times smaller at the −6 dB level and equal at the −20 dB level compared to the third harmonic. The axial beam widths of the optimal dual pulse signal are 3.1 times smaller at the −6 dB level and 1.6 times smaller at the −20 dB level compared to the third harmonic. Not only does dual pulse method solve the ripple artifacts associated with imaging using multiple harmonic bands, dual pulse SHI has markedly improved axial and lateral resolutions compared to the third harmonic at higher than second harmonic intensities.

Estimating acoustic peak pressure generated by ultrasound transducers from harmonic distortion level measurement.

Pressure amplitude measurement is important for general research on ultrasound. Because it requires high accuracy, it is usually done using a hydrophone calibrated by an accredited laboratory. In this paper, a method is proposed for estimating the pressure amplitude in the ultrasound field using an uncalibrated single-element transducer and Khokhlov-Zabolotskaya-Kuznetsov simulations of the ultrasound field. The accuracy of the method is shown to be better than 20% for slightly focused and nonfocused transducers. Extending the method to a pulse-echo setup enables pressure measurement of a transducer without the need for an extra transducer or hydrophone.

Low-frequency source for very long-range underwater communication

Long range underwater acoustic communication achievement is a decisive milestone for very long cruising AUVs deployment (>1,000 km). This paper describes a new kind of acoustic source designed and manufactured by iXBlue, the Janus-Hammer Bell (JHB) as shown in Figure 1, which was recently used in the at-sea experiments for long-range communication. The modem center frequency is 500 Hz, and its bandwidth 100 Hz. It provides a 200 dB (ref. 1 μPa @ 1 m) flat signal level spectrum with full immersion capability and is designed to be battery operated. This patented underwater acoustic source was exploited in a “passive time reversal” process. This method newly achieved long range communication at a rate of 100 bit/s at 1,000 km.

A NEW TRANSESOPHAGEAL PROBE FOR NEWBORNS

Current transesophageal probes are designed for adults and are used both in the operating theatre for monitoring as well as in the outpatient clinic for patients with specific indications, like obesity, artificial valves, etc. For newborns (<5 kg), transesophageal echocardiography (TEE) is not possible because the current probes are too big for introducing them into the esophagus. There is a clear need for a small probe in newborns that are scheduled for complicated cardiac surgery and catheterization. We present the design and realization of a small TEE phased array probe with a tube diameter of 5.2mm and head size of only 8.2-7 mm. The number of elements is 48 and the center frequency of the probe is 7.5 MHz. A separate clinical evaluation study was carried out in 42 patients (Scohy et al. 2007).

Superharmonic imaging based on chirps

In medical ultrasound harmonic images of biological tissue are commonly obtained by analyzing the reflected echoes from the 2nd harmonic band. A new modality dubbed super-harmonic imaging (SHI) targets a combination of the 3rd–5th harmonics. SHI is expected to yield enhanced spatial resolution and thus to increase the quality of echographic images. On the other hand, those images obtained using short imaging pulses are susceptible to so-called multiple axial reflection artifacts, stemming from the troughs in between harmonics in the frequency domain. The recently proposed dual-pulse frequency compounding method suppresses these artifacts but reduces the frame rate by a factor of 2. In this work we research the feasibility of employing a chirp protocol to perform SHI without compromising the frame rate. The chirp protocol was implemented using an interleaved phased array transducer (44 elements tuned at 1 MHz, 44 elements at 3.7 MHz) in combination with a fully programmable ultrasound system. The transducer was mounted in the side of a water-filled tank. Linear chirps with a center frequency of 1 MHz and a bandwidth of 40% were used as excitation pulses. Radio frequency traces were recorded at the focal plane along the lateral axis using a hydrophone, filtered over the superharmonic band and convolved with a decoding signal to obtain point spread functions (PSFs). The decoding signal was acquired by simulating the emitted beam using the KZK method for a rectangular aperture. The decoded superharmonic chirp had an SNR of 35–40 dB. Comparing to a the 3rd harmonic produced by a 2.5 cycle 1 MHz Gaussian apodized sine burst transmission the lateral beam width of the superharmonic chirp signal is 0.8 and 0.9 times that of the 3rd harmonic at the −6 dB and −20 dB levels respectively. Regarding the axial beam width, the superharmonic chirp signal has 0.9 and 0.8 times the axial beam width of the 3rd harmonic at the −6 dB and −20 dB levels respectively. The superharmonic chirp PSF is virtually free from imaging artifacts. Based on the SNR measurements the chirp protocol yields a sufficient dynamic range. The PSF has increased spatial resolution in comparison with the 3rd harmonic. The first in-vitro images show promise, but the decoding pulse requires improvement.

A study of phased array transducer topology for superharmonic imaging

Since its introduction in the 90s, tissue 2nd harmonic imaging has become the standard in medical ultrasound. Recently, superharmonic imaging (SHI) was introduced. It targets the combination of the 3rd till 5th harmonics. SHI offers increased spatial resolution, lower sidelobes and less artifacts compared to 2nd harmonic imaging. However, a system for SHI has to deal with the lower energy content of the higher harmonics. The broad bandwidth (−6 dB > 130%) required for SHI prompts for an unconventional phased array design. One of the solutions divides the transmit and receive parts into separate acoustic stacks. Such a design reduces the surface area available for reception. Firstly, we investigate the blockwise and interleaved distribution (topology) of the transmit and receive elements in terms of beam characteristics. Secondly, we research the optimal ratio between transmit and receive elements to increase the area dedicated to receiving while retaining a high quality beam. The latter was assessed using the grating lobe to main beam ratio. The acoustic fields were computed using a combination of numerical methods. FIELD II was used to determine the locations and the peak pressure in the fundamental main and grating lobes. The pressure levels of the harmonics in the main and grating lobes were calculated using the INCS method and Burgers equation, respectively. 3 cycle Gaussian apodized sine bursts were used in transmission. The MI was 1.5 at the transmit frequency of 1.2 MHz — optimal for cardiac SHI. The interleaved topology produces the best defined beam (straight and with low sidelobe levels) compared to the blockwise topologies. Consequently, the main to grating lobe ratios for the different harmonic components were calculated for the interleaved topologies only. The 1/2 till 1/7 interleaved topologies provided enough dynamic range (40 dB) for SHI with 1/7 maximizing the surface area for reception. This increases the SNR by 5 dB.