Francois Guy Gerard Marie Vignon

Capon beamforming in medical ultrasound imaging with focused beams

Medical ultrasound imaging is conventionally done by insonifying the imaged medium with focused beams. The backscattered echoes are beamformed using delay-and-sum operations that cannot completely eliminate the contribution of signals backscattered by structures off the imaging beam to the beamsum. It leads to images with limited resolution and contrast. This paper presents an adaptation of the Capon beam- former algorithm to ultrasound medical imaging with focused beams. The strategy is to apply data-dependent weight functions to the imaging aperture. These weights act as lateral spatial filters that filter out off-axis signals. The weights are computed for each point in the imaged medium, from the statistical analysis of the signals backscattered by that point to the different elements of the imaging probe when insonifying it with different focused beams. Phantom and in vivo images are presented to illustrate the benefits of the Capon algorithm over the conventional delay-and-sum approach. On heart sector images, the clutter in the heart chambers is decreased. The endocardium border is better defined. On abdominal linear array images, significant contrast and resolution enhancement are observed.

Microbubble cavitation imaging

Ultrasound cavitation of microbubble contrast agents has a potential for therapeutic applications such as sonothrombolysis (STL) in acute ischemic stroke. For safety, efficacy, and reproducibility of treatment, it is critical to evaluate the cavitation state (moderate oscillations, stable cavitation, and inertial cavitation) and activity level in and around a treatment area. Acoustic passive cavitation detectors (PCDs) have been used to this end but do not provide spatial information. This paper presents a prototype of a 2-D cavitation imager capable of producing images of the dominant cavitation state and activity level in a region of interest. Similar to PCDs, the cavitation imaging described here is based on the spectral analysis of the acoustic signal radiated by the cavitating microbubbles: ultraharmonics of the excitation frequency indicate stable cavitation, whereas elevated noise bands indicate inertial cavitation; the absence of both indicates moderate oscillations. The prototype system is a modified commercially available ultrasound scanner with a sector imaging probe. The lateral resolution of the system is 1.5 mm at a focal depth of 3 cm, and the axial resolution is 3 cm for a therapy pulse length of 20 μs. The maximum frame rate of the prototype is 2 Hz. The system has been used for assessing and mapping the relative importance of the different cavitation states of a microbubble contrast agent. In vitro (tissue-mimicking flow phantom) and in vivo (heart, liver, and brain of two swine) results for cavitation states and their changes as a function of acoustic amplitude are presented.

A non-disruptive technology for robust 3d tool tracking for ultrasound-guided interventions

In the past decade ultrasound (US) has become the preferred modality for a number of interventional procedures, offering excellent soft tissue visualization. The main limitation however is limited visualization of surgical tools. A new method is proposed for robust 3D tracking and US image enhancement of surgical tools under US guidance. Small US sensors are mounted on existing surgical tools. As the imager emits acoustic energy, the electrical signal from the sensor is analyzed to reconstruct its 3D coordinates. These coordinates can then be used for 3D surgical navigation, similar to current day tracking systems. A system with real-time 3D tool tracking and image enhancement was implemented on a commercial ultrasound scanner and 3D probe. Extensive water tank experiments with a tracked 0.2mm sensor show robust performance in a wide range of imaging conditions and tool position/orientations. The 3D tracking accuracy was 0.36 +/- 0.16mm throughout the imaging volume of 55 degrees x 27 degrees x 150mm. Additionally, the tool was successfully tracked inside a beating heart phantom. This paper proposes an image enhancement and tool tracking technology with sub-mm accuracy for US-guided interventions. The technology is non-disruptive, both in terms of existing clinical workflow and commercial considerations, showing promise for large scale clinical impact.

Effects of Attenuation and Thrombus Age on the Success of Ultrasound and Microbubble-Mediated Thrombus Dissolution

The purpose of this study was to examine the effects of applied mechanical index, incident angle, attenuation and thrombus age on the ability of 2-D vs. 3-D diagnostic ultrasound and microbubbles to dissolve thrombi. A total of 180 occlusive porcine arterial thrombi of varying age (3 or 6 h) were examined in a flow system. A tissue-mimicking phantom of varying thickness (5 to 10 cm) was placed over the thrombosed vessel and the 2-D or 3-D diagnostic transducer aligned with the thrombosed vessel using a positioning system. Diluted lipid-encapsulated microbubbles were infused during ultrasound application. Percent thrombus dissolution (%TD) was calculated by comparison of clot mass before and after treatment. Both 2-D and 3-D-guided ultrasound increased %TD compared with microbubbles alone, but %TD achieved with 6-h-old thrombi was significantly less than 3-h-old thrombi. Thrombus dissolution was achieved at 10 cm tissue-mimicking depths, even without inertial cavitation. In conclusion, diagnostic 2-D or 3-D ultrasound can dissolve thrombi with intravenous nontargeted microbubbles, even at tissue attenuation distances of up to 10 cm. This treatment modality is less effective, however, for older aged thrombi.

Diagnostic Ultrasound Induced Inertial Cavitation to Non-Invasively Restore Coronary and Microvascular Flow in Acute Myocardial Infarction

Ultrasound induced cavitation has been explored as a method of dissolving intravascular and microvascular thrombi in acute myocardial infarction. The purpose of this study was to determine the type of cavitation required for success, and whether longer pulse duration therapeutic impulses (sustaining the duration of cavitation) could restore both microvascular and epicardial flow with this technique. Accordingly, in 36 hyperlipidemic atherosclerotic pigs, thrombotic occlusions were induced in the mid-left anterior descending artery. Pigs were then randomized to either a) ½ dose tissue plasminogen activator (0.5 mg/kg) alone; or same dose plasminogen activator and an intravenous microbubble infusion with either b) guided high mechanical index short pulse (2.0 MI; 5 usec) therapeutic ultrasound impulses; or c) guided 1.0 mechanical index long pulse (20 usec) impulses. Passive cavitation detectors indicated the high mechanical index impulses (both long and short pulse duration) induced inertial cavitation within the microvasculature. Epicardial recanalization rates following randomized treatments were highest in pigs treated with the long pulse duration therapeutic impulses (83% versus 59% for short pulse, and 49% for tissue plasminogen activator alone; p<0.05). Even without epicardial recanalization, however, early microvascular recovery occurred with both short and long pulse therapeutic impulses (p<0.005 compared to tissue plasminogen activator alone), and wall thickening improved within the risk area only in pigs treated with ultrasound and microbubbles. We conclude that although short pulse duration guided therapeutic impulses from a diagnostic transducer transiently improve microvascular flow, long pulse duration therapeutic impulses produce sustained epicardial and microvascular re-flow in acute myocardial infarction.

The Stripe Artifact in Transcranial Ultrasound Imaging

Objective. Transcranial images are affected by a “stripe artifact” (also known as a “streak artifact”): two dark stripes stem radially from the apex to the base of the scan. The stripes limit the effective field of view even on patients with good temporal windows. This study investigated the angle dependency of ultrasound transmission through the skull to elucidate this artifact. Methods. In vivo transcranial images were obtained to illustrate the artifact. In vitro hydrophone measurements were performed in water to evaluate transcranial wavefronts at different incidence angles of the ultrasound beam. Both a thin acrylic plate, as a simple bone model, and a human temporal bone sample were used. Results. The imaging wavefront splits into two after crossing the solid layer (acrylic model or skull sample) at an oblique angle. An early‐arrival wavefront originates from the direct longitudinal wave transmission through water‐bone interfaces, while a late‐arrival wavefront results from longitudinal‐to‐transverse mode conversion at the water‐bone interface, propagation of the transverse wave through the skull, and transverse‐to‐longitudinal conversion at the bone‐water interface. At normal incidence, only the direct wavefront (without mode conversion) is observed. As the incidence angle increases, the additional “mode conversion” wavefront appears. The amplitude of the transcranial wavefront decreases and reaches a minimum at an incidence angle of about 27°. Beyond that critical angle, only the mode conversion wavefront is transmitted. Conclusions. The stripes are a consequence of the angle‐dependent ultrasound transmission and mode conversion at fluid‐solid interfaces such as between the skull and the surrounding fluidlike soft tissues.

Improvements in cerebral blood flow and recanalization rates with transcranial diagnostic ultrasound and intravenous microbubbles after acute cerebral emboli.

ObjectivesIntravenous microbubbles (MBs) and transcutaneous ultrasound have been used to recanalize intra-arterial thrombi without the use of tissue plasminogen activator. In the setting of acute ischemic stroke, it was our objective to determine whether skull attenuation would limit the ability of ultrasound alone to induce the type and level of cavitation required to dissolve thrombi and improve cerebral blood flow (CBF) in acute ischemic stroke. Materials and MethodsIn 40 pigs, bilateral internal carotid artery occlusions were created with 4-hour-old thrombi. Pigs were then randomized to high–mechanical index (MI = 2.4) short-pulse (5 microseconds) transcranial ultrasound (TUS) alone or a systemic MB infusion (3% Definity) with customized cavitation detection and imaging system transmitting either high-MI (2.4) short pulses (5 microseconds) or intermediate-MI (1.7) long pulses (20 microseconds). Angiographic recanalization rates of both internal carotids were compared in 24 of the pigs (8 per group), and quantitative analysis of CBF with perfusion magnetic resonance imaging was measured before, immediately after, and at 24 hours using T2* intensity versus time curves in 16 pigs. ResultsComplete angiographic recanalization was achieved in 100% (8/8) of pigs treated with image-guided high-MI TUS and MBs, but in only 4 of 8 treated with high-MI TUS alone or 3 of 8 pigs treated with image-guided intermediate-MI TUS and MBs (both P < 0.05). Ipsilateral and contralateral CBF improved at 24 hours only after 2.4-MI 5-microsecond pulse treatments in the presence of MB (P < 0.005). There was no evidence of microvascular or macrovascular hemorrhage with any treatment. ConclusionsGuided high-MI impulses from an ultrasound imaging system produce sustained improvements in ipsilateral and contralateral CBF after acute cerebral emboli.

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.

Determination of temporal bone Isoplanatic Patch sizes for transcranial phase aberration correction

Phase aberration is a leading cause of transcranial ultrasound image degradation. In order to realign aberrated wavefronts, a delay map corresponding to the aberration can be computed from signals backscattered from a region of interest (ROI) in the medium, and used to correct the beamforming delays. However, such a map is only effective for correcting the aberration in a limited area called the isoplanatic patch (IP) around the ROI. This fundamentally limits the effectiveness of transcranial aberration correction to restore image quality. In this paper, IP sizes are measured in vitro for aberration correction with an X7-2 2D array (Philips Healthcare, Andover, MA) through 12 ex vivo human temporal bone samples. The angular IP size is found to be 36deg plusmn 18deg. An in vivo experiment confirms that the IP is limited angularly (~30deg) but large in depth (~15 cm). Small IP sizes and high refocusing effectiveness within the IP are correlated with high gradients in the measured phase aberration maps. This study indicates that phase aberration correction with a single delay map is only effective for transcranial ultrasound applications with a small angular field of view.

Real-Time Two-Dimensional Imaging of Microbubble Cavitation

Ultrasound cavitation of microbubble contrast agents has a potentialfor therapeutic applications, including sonothrombolysis in acute ischemic stroke. For safety, efficacy, and reproducibility of treatment, it is critical to evaluate the cavitation state (e.g. stable versus inertial forms of cavitation) and intensity in and around a treatment area. Acoustic Passive Cavitation Detectors (PCDs) have been used but lack spatial information. This paper presents a prototype ofa 2D cavitation imager capable of producing images of the dominantcavitation state and intensity in a region of interest at a frame rate of 0.6Hz. The system is based on a commercial ultrasound scannerand imaging probe (iE33 scanner with S5-1 probe, Philips). Cavitation imaging is based on the spectral analysis of acoustic signal radiated by the cavitating microbubbles: ultraharmonics of the excitationfrequency indicate stable cavitation, while noise bands indicate inertial cavitation. The system demonstrates the capability to robustly identify stable and inertial cavitation thresholds of Definity microbubbles (Lantheus) in a vessel phantom through 3 ex-vivo human temporal bones, as well as to spatially discriminate the location of cavitation activities.