Claude Cohen-Bacrie

Medical ultrasound imaging using the fully adaptive beamformer

Medical ultrasound beamforming is conventionally done using a classical delay-and-sum operation. This simplest beamforming suffers from drawbacks. Indeed, in phased array imaging, the beamformed radiofrequency signal is often polluted with off-axis energies. We investigate the use of an adaptive beamforming approach widely used in array processing, the fully adaptive beamformer, to reduce the bright off-axis energies contribution. We show that the fully adaptive beamformer cannot be applied to medical ultrasound as it was initially derived since the medical ultrasonic medium produces coherent or highly correlated signals and the algorithm fails to work within this context. Spatial smoothing preprocessing is introduced which allows the fully adaptive beamformer to operate. A complementary preprocessing that uses the received data obtained using consecutive transmission lines further improve the performances. Very promising results for the application of adaptive array processing techniques in medical ultrasound are obtained.

RSNA/QIBA: Shear wave speed as a biomarker for liver fibrosis staging

An interlaboratory study of shear wave speed (SWS) estimation was performed. Commercial shear wave elastography systems from Fibroscan, Philips, Siemens and Supersonic Imagine, as well as several custom laboratory systems, were involved. Fifteen sites were included in the study. CIRS manufactured and donated 11 pairs of custom phantoms designed for the purposes of this investigation. Dynamic mechanical tests of equivalent phantom materials were also performed. The results of this study demonstrate that there is very good agreement among SWS estimation systems, but there are several sources of bias and variance that can be addressed to improve consistency of measurement results.

Time reversal operator decomposition with focused transmission and robustness to speckle noise : Application to microcalcification detection

The decomposition of the time reversal operator (DORT) is a detection and focusing technique using an array of transmit-receive transducers. In the absence of noise and under certain conditions, the eigenvectors of the time reversal operator contain the focal laws to focus ideally on well-resolved scatterers even in the presence of strong aberration. This paper describes a new algorithm, FDORT, which uses focused transmission schemes to acquire the operator. It can be performed from medical scanner data. A mathematical derivation of this algorithm is given and it is compared with the conventional algorithm, both theoretically and with numerical experiments. In the presence of strong speckle signals, the DORT method usually fails. The influence of the speckle noise is explained and a solution based on FDORT is presented, that enables detection of targets in complex media. Finally, an algorithm for microcalcification detection is proposed. In-vivo results show the potential of these techniques.

A novel phase aberration measurement technique derived from the DORT method: comparison with correlation-based method on simulated and in-vivo data

Clinical ultrasound image quality is degraded by tissue velocity inhomogeneities that distort the phase of the traveling ultrasonic wavefronts. degrading image resolution and contrast. This paper investigates an entirely new approach for aberration measurement that uses a modified DORT (French acronym for decomposition of the time reversal operator) method, which uses imaging transmission schemes to acquire the time reversal operator. In this novel scheme, the aberration profile is directly estimated from the unwrapped phase of the first DORT eigenvectcr. We term this method focused DORT (FDORT). We compared the FDORT method with a correlation-based, least-mean-squares (LMS) method using 1D arrays on: simulated data; experimental phantom data with a thin-film rubber aberrator; and in-vivo breast data. Measurements were carried out in regions of speckle and point-like scatterers, and corrections were performed using a near-field phase screen aberration model. In simulation, the residual rms error (between applied and estimated aberrator) was 12.5ns with FDORT and 19.5ns with LMS; the point target brightness improvement was 122% (/spl plusmn/29%) for FDORT and 132% (/spl plusmn/20%) for LMS. The phantom experiments showed 46% (/spl plusmn/25%) and 48% (/spl plusmn/22%) improvements in point target brightness for FDORT and LMS, respectively. In clinical data, microcalcifications were identified and used to estimate the aberrations. FDORT measured an average 65ns rms, 6.5mm FWHM aberrator with an average brightness improvement of 58% (n=3). LMS measured an average 50.2ns rms, 5.7mm FWHM aberrator with an average improvement of 61% (n=3). FDORT is observed to follow wavefront variations, even where LMS does not perform well due to low correlation values. This is particularly evident in the breast data where coherent wavefronts do not extend across the entire aperture. The initial results presented here indicate that FDORT is a useful method to estimate aberration profiles for adaptive imaging. LMS breaks down in the presence of interfering wavefronts from off-axis and multiple strong scatterers. MORT is able to identify this situation since the individual wavefronts are associated with different eigenvectors. We see the potential for FDORT to cope with these conditions and thereby estimate aberrations with greater robustness.

Evaluation of the DORT method for the detection of microcalcifications in the breast

The DORT method (French acronym for diagonalization of the time reversal operator) is derived from the theory of iterative time reversal mirroring. It consists of a singular value decomposition of the time reversal operator obtained through single element transmissions and receptions. The number of eigenvalues relates to the number of bright point scatterers in the medium, and each eigenvector is the transmit signals that focuses on each bright scatterer. However, the signal‐to‐noise ratio (SNR) resulting from a single element transmit is low, which negatively impacts the sensitivity of the DORT method. This work consists of an adaptation of the DORT method to an imaging mode. Focused transmissions in the medium are used and a windowing preprocessing operation on the received signals significantly increases the sensitivity. The more robust behavior of this modified DORT method is tested on Field II simulated data and then on a phantom made of strings of different material embedded in speckle. Data on fres...

Estimation of viscosity from ultrasound measurements of velocity

Blood and plasma viscosity have recently been identified as independent risk factors for atherothrombotic vascular diseases. On the other hand, a monitoring of viscosity parameter is also required for the treatments of some cardiovascular diseases in order to control blood fluidity. This work suggests the use of ultrasound systems to achieve this monitoring in a non invasive manner, and proposes a method to estimate blood viscosity. The estimation procedure has been developed in a more general framework and computes simultaneously viscosity and pressure gradient from the knowledge of blood flow profiles. In a rigid tube, velocity profile shapes of a Newtonian fluid in a laminar flow are related to both viscosity and pressure gradient following Navier-Stokes equation. The idea of this paper is to use inverse problem approaches to estimate parameters of the flow that are not directly observable. Flow profiles are processed as an indirect observation of these quantities. To do so, N samples of velocity values taken along the tube radius are arbitrary chosen. An integration of Navier-Stokes equation over these N values leads to an N equations system. The contribution here is an inversion algorithm that uses a maximum likelihood estimator. The estimation process is tested on a fluid with measured viscosity that is placed in a flow phantom with a controlled flow rate.

In vitro characterization of carotid plaque using a clinical ultrasound imaging system

The goal of this work is to demonstrate the feasibility of objective noninvasive methods for in vivo characterization of carotid plaque content. Using a clinical ultrasonic imager connected to a radiofrequency (RF) signal acquisition system, both RF signals and grayscale (GS) images were obtained from a tissue-mimicking reference phantom and from 26 independent segments of formalin-fixed human carotid endarterectomy specimens. Integrated backscatter (IBS) images of each segment were constructed from an FFT-based analysis on individual RF lines using a sliding Hamming window technique (32 point length, 1 point shift). For 82 matched ROIs, average IBS from parameter images was compared to average GS values normalized with respect to GS in the reference phantom. For 70 of the ROIs, plaque content was assessed by histology in terms of collagen, lipid, hemorrhage and calcification content. The current GS and IBS approaches were not found to be equivalent. The IBS values (mean standard error) for collagen, lipid and hemorrhage were 8.1, 1.1, 5.3 /spl plusmn/ 1.0 and 4.9 /spl plusmn/ 0.7 dB, respectively.

MR‐Guided Ultrasonic Brain Therapy: High Frequency Approach

A novel MR-guided brain therapy device operating at 1MHz has been designed and constructed. The system has been installed and tested in a clinical 1.5 T Philips Achieva MRI. Three dimensional time domain finite differences simulations were used to compute the propagation of the wave field through three human skulls. The simulated phase distortions were used as inputs for transcranial correction and the corresponding pressure fields were scanned in the focal plane. At half of the maximum power (10 W/cm2 on the surface of the transducers), necroses were induced 2 cm deep in turkey breasts placed behind a human skull. In vitro experiments on human skulls show that simulations restore more than 85% of the pressure level through the skull bone when compared to a control correction performed with an implanted hydrophone. Finally, high power experiments are performed though the skull bone and a MR-Thermometry sequence is used to map the temperature rise in a brain phantom every 3 s in two orthogonal planes (focal plane and along the axis of the probe).

Ultrasound process for the determination of the location of a parietal surface in a tissue and of the absolute radius of an artery, and ultrasound apparatus for carrying out such process

An ultrasound process and apparatus for determining the location of an ultrasonic homogeneity discontinuity called parietal surface situated in a tissue from a radio-frequency signal (y(z)) function of digital depths along an excitation line (Z) crossing the parietal surface. The process and apparatus includes filtering the radio-frequency signal in order to provide a tissue signal (e(z)) representative of tissue scatterers, apart from a radio-frequency excitation signal. Also, filtering the tissue signal (e(z)) in order to provide a variance amplitude signal (S(z)) as a function of the digital depths on the excitation line wherefrom the corresponding parietal surface digital depth is determined. This process is used to estimate an artery radius from the digital depths of the arterial walls on the excitation line.

MR-guided ultrasonic brain therapy: High frequency approach

A novel MR-guided brain therapy device operating at 1MHz has been designed and constructed. The system has been installed and tested in a clinical 1.5 T Philips Achieva MRI. Three dimensional time domain finite differences simulations were used to compute the propagation of the wave field through three human skulls. The simulated phase distortions were used as inputs for transcranial correction and the corresponding pressure fields were scanned in the focal plane. At half of the maximum power (10 W/cm2 on the surface of the transducers), necroses were induced 2 cm deep in turkey breasts placed behind a human skull. In vitro experiments on human skulls show that simulations restore more than 85% of the pressure level through the skull bone when compared to a control correction performed with an implanted hydrophone. Finally, high power experiments are performed though the skull bone and a MR-Thermometry sequence is used to map the temperature rise in a brain phantom every 3 s in two orthogonal planes (focal plane and along the axis of the probe).