Ramon Quido Erkamp

Mapping skull attenuation for optimal probe placement in transcranial ultrasound applications

It takes skill and time to place an ultrasound probe on the optimal acoustic window for transcranial insonification. This hinders efficient ultrasound imaging and therapy of the brain. This paper presents two approaches for automatically identifying the best transtemporal window in order to facilitate clinical workflow. A mechanically translating matrix imaging probe is used in conjunction with a point source on the contralateral temple. Optimizing the probe position results in improved image sensitivity and limited aberration.

A novel hands-free carotid ultrasound detects low-flow cardiac output in a swine model of pulseless electrical activity arrest

OBJECTIVE To determine if a hands-free, noninvasive Doppler ultrasound device can reliably detect low-flow cardiac output by measuring carotid artery blood flow velocities. We compared the ability of observers to detect carotid artery flow velocity differences between pseudo-pulseless electrical activity (PEA) and true-PEA cardiac arrest. METHODS Five swine were instrumented with aortic (Ao) and right atrial pressure-transducing catheters. The Doppler ultrasound device was adhered to the neck over the carotid artery. Continuous electrocardiogram, pressure readings, and Doppler signal were recorded. Each swine underwent multiple episodes of fibrillation and resuscitation. Episodes of true-PEA and pseudo-PEA were retrospectively identified from all resuscitation attempts by examination of electrocardiogram and Ao waveforms. The sensitivity and specificity of the device to detect pseudo-PEA was obtained using observers blinded to Ao waveform recordings. RESULTS There was good interobserver reliability related to identification of pseudo- and true-PEA (κ = 0.873). The observers blinded to Ao waveform recordings agreed on 8 of the 9 episodes of pseudo-PEA, whereas 4 false positives of 26 true-PEA events were reported (sensitivity, 0.89; specificity, 0.85). The Doppler device was able to detect carotid flow velocity over a wide range of Ao blood pressures. CONCLUSIONS This hands-free, noninvasive Doppler ultrasound device can reliably differentiate pseudo-PEA from true-PEA during resuscitation from cardiac arrest, detecting pressure gradient changes of less than 5 mm Hg through to normotension. This device distinguishes conditions of no cardiac output from low cardiac output and may have applications for use during resuscitation from various etiologies of arrest and shock.

Detection and display of acoustic window for guiding and training cardiac ultrasound users

Successful ultrasound data collection strongly relies on the skills of the operator. Among different scans, echocardiography is especially challenging as the heart is surrounded by ribs and lung tissue. Less experienced users might acquire compromised images because of suboptimal hand-eye coordination and less awareness of artifacts. Clearly, there is a need for a tool that can guide and train less experienced users to position the probe optimally. We propose to help users with hand-eye coordination by displaying lines overlaid on B-mode images. The lines indicate the edges of blockages (e.g., ribs) and are updated in real time according to movement of the probe relative to the blockages. They provide information about how probe positioning can be improved. To distinguish between blockage and acoustic window, we use coherence, an indicator of channel data similarity after applying focusing delays. Specialized beamforming was developed to estimate coherence. Image processing is applied to coherence maps to detect unblocked beams and the angle of the lines for display. We built a demonstrator based on a Philips iE33 scanner, from which beamsummed RF data and video output are transferred to a workstation for processing. The detected lines are overlaid on B-mode images and fed back to the scanner display to provide users real-time guidance. Using such information in addition to B-mode images, users will be able to quickly find a suitable acoustic window for optimal image quality, and improve their skill.

Using redundancy of round-trip ultrasound signal for non-continuous arrays: Application to gap and blockage compensation

In ultrasound imaging, an array of elements is used to image a medium. If part of the array is blocked by an obstacle, or if the array is made from several sub-arrays separated by a gap, grating lobes appear and the image is degraded. The grating lobes are caused by missing spatial frequencies, corresponding to the blocked or non-existing elements. However, in an active imaging system, where elements are used both for transmitting and receiving, the round trip signal is redundant: different pairs of transmit and receive elements carry similar information. It is shown here that, if the gaps are smaller than the active sub-apertures, this redundancy can be used to compensate for the missing signals and recover full resolution. Three algorithms are proposed: one is based on a synthetic aperture method, a second one uses dual-apodization beamforming, and the third one is a radio frequency (RF) data based deconvolution. The algorithms are evaluated on simulated and experimental data sets. An application could be imaging through ribs with a large aperture.

A New Sensor Technology for 2D Ultrasound-Guided Needle Tracking

2D Ultrasound (US) is becoming the preferred modality for image-guided interventions due to its low cost and portability. However, the main limitation is the limited visibility of surgical tools. We present a new sensor technology that can easily be embedded on needles that are used for US-guided interventions. Two different types of materials are proposed to be used as sensor--co-polymer and PZT. The co-polymer technology is particularly attractive due to its plasticity, allowing very thin depositions (10-20 μm) on a variety of needle shapes. Both sensors receive acoustic energy and convert it to an electrical signal. The precise location of the needle can then be estimated from this signal, to provide real-time feedback to the clinician. We evaluated the feasibility of this new technology using (i) a 4DOF robot in a water tank; (ii) extensive ex vivo experiments; and (iii) in vivo studies. Quantitative robotic studies indicated that the co-polymer is more robust and stable when compared to PZT. In quantitative experiments, the technology achieved a tracking accuracy of 0.14 ± 0.03mm, significantly superior to competing technologies. The technology also proved success in near-real clinical studies on tissue data. This sensor technology is non-disruptive of existing clinical workflows, highly accurate, and is cost-effective. Initial clinician feedback shows great potential for large scale clinical impact.

Beamforming techniques for ultrasound microcalcification detection

Evaluation of microcalcifications helps early detection of breast cancer and classification of benign and malignant masses. Though ultrasound is a proven adjunct to mammography, especially in dense breast tissue, its sensitivity to microcalcifications is usually poor. To enable early ultrasound-based cancer detection in dense breast, the sensitivity has to be improved. Here we report beamforming techniques for imaging microcalcifications. The strategy is to involve channel-data-based parameters that favor point/sub-resolution targets, which microcalcifications resemble acoustically. Accordingly, we have devised algorithms based on coherence factor and dominance of the first eigenvalue of covariance matrices. The two parameters are used to derive a binary microcalcification map. The initial results based on in vivo and phantom data show potential of the proposed techniques for microcalcification detection.

Analysis of signal coherence in ultrasound beamforming

Coherence factor weighting is used to suppress sidelobes, reduce artifacts, and improve the contrast and spatial resolution. The method provides a less hardware demanding solution with acceptable results. We conduct an analysis on coherence factor (CF) and a two-aperture coherence (TAC) estimation for the purpose of general understanding and fast estimation. A simulation was implemented based on linear description of ultrasound system and a simplified lateral scatterer distribution at the region of interest to analyze differences. Characters of CF and TAC for a point, homogeneous speckle, and a point and a cyst within homogeneous speckle environment under narrow and broad band were simulated.