Transcranial ultrasound angiography (T USA): a new approach for contrast specific imaging of intracranial arteries

Continuous transcranial ultrasound in large vessel stroke: Image guidance for high-intensity focused sonothrombolysis

BACKGROUND AND PURPOSE

Sonothrombolysis is a potential adjunctive therapy for large vessel occlusion (LVO) stroke. Bedside ultrasound image-guided high-intensity focused ultrasound (HIFU) therapy could deliver higher energy therapeutic ultrasound to the thrombus with higher precision than what was previously accomplished in human trials. The aim is to test the feasibility of diagnostic transcranial contrast-enhanced ultrasound (CEUS) to image the occlusion site and continuously maintain the guidance image on-target for a sufficient exposure time for HIFU to be effective during LVO stroke evaluation and treatment.

METHODS

This prospective, single center, observational cohort study included adult patients, presenting within 6 hours of stroke symptom onset, with LVO identified on computed tomography angiography (CTA). A hand-held CEUS imaging study was initiated following CTA and lasted up to 30 minutes. The primary outcome is the proportion of patients where a guidance CEUS image of the occlusion was achieved.

RESULTS

A CEUS image of the occluded artery was obtained in 32/35 of the included patients. The median total imaging time was 23 minutes (interquartile range 15-30). Patients undergoing thrombectomy had a lower total imaging time (17 vs. 29.5 minutes, p = .002). When imaging was successful, on-target image was maintained for only 58% (standard deviation 23.8%) of total imaging time. No complications related to CEUS were observed.

CONCLUSIONS

This feasibility study explored the use of diagnostic transcranial CEUS for continuous imaging of occlusion sites in LVO strokes. Challenges in maintaining target image during HIFU were identified, highlighting the need for technical advances for clinical application.

Transcranial activation and imaging of low boiling point phase-change contrast agents through the temporal bone using an ultrafast interframe activation ultrasound sequence

PURPOSE

As a cavitation enhancer, low boiling point phase-change contrast agents (PCCA) offer potential for ultrasound-mediated drug delivery in applications including intracerebral hemorrhage or brain tumors. In addition to introducing cavitation, ultrasound imaging also has the ability to provide guidance and monitoring of the therapeutic process by localizing delivery events. However, the highly attenuating skull poses a challenge for achieving an image with useful contrast. In this study, the feasibility of transcranial activation and imaging of low boiling point PCCAs through the human temporal bone was investigated by using a low frequency ultrafast interframe activation ultrasound (UIAU) imaging sequence with singular value decomposition-based denoising.

METHODS

Lipid-shelled PCCAs filled with decafluorobutane were activated and imaged at 37°C in tissue-mimicking phantoms both without and with an ex vivo human skull using the new UIAU sequence and a low frequency diagnostic transducer array at frequencies from 1.5 to 3.5 MHz. A singular value decomposition-based denoising filter was developed and used to further enhance transcranial image contrast. The contrast-to-tissue ratio (CTR) and contrast enhancement (CE) of UIAU was quantitatively evaluated and compared with the amplitude modulation pulse inversion (AMPI) and vaporization detection imaging (VDI) approaches.

RESULTS

Image results demonstrate enhanced contrast in the phantom channel with suppressed background when the low boiling point PCCA was activated both without and with an ex vivo human skull using the UIAU sequence. Quantitative results show that without the skull, low frequency UIAU imaging provided significantly higher image contrast (CTR ≥ 18.56 dB and CE ≥ 18.66 dB) than that of AMPI and VDI (P < 0.05). Transcranial imaging results indicated that the CE of UIAU (≥18.80 dB) was significantly higher than AMPI for free-field activation pressures of 5 and 6 MPa. The CE of UIAU is also significantly higher than that of VDI when PCCAs were activated at 2.5 MHz and 3 MHz (P < 0.05). The CTR (23.30 [20.07-25.56] dB) of denoised UIAU increased by 12.58 dB relative to the non-denoised case and was significantly higher than that of AMPI at an activation pressure of 4 MPa (P < 0.05).

CONCLUSIONS

Results indicate that low boiling point PCCAs can be activated and imaged at low frequencies including imaging through the temporal bone using the UIAU sequence. The UIAU imaging approach provides higher contrast than AMPI and VDI, especially at lower activation pressures with additional removal of electronic noise.

Research progress in ultrasound use for the diagnosis and treatment of cerebrovascular diseases

Cerebrovascular diseases pose a serious threat to human survival and quality of life and represent a major cause of human death and disability. Recently, the incidence of cerebrovascular diseases has increased yearly. Rapid and accurate diagnosis and evaluation of cerebrovascular diseases are of great importance to reduce the incidence, morbidity and mortality of cerebrovascular diseases. With the rapid development of medical ultrasound, the clinical relationship between ultrasound imaging technology and the diagnosis and treatment of cerebrovascular diseases has become increasingly close. Ultrasound techniques such as transcranial acoustic angiography, doppler energy imaging, three-dimensional craniocerebral imaging and ultrasound thrombolysis are novel and valuable techniques in the study of cerebrovascular diseases. In this review, we introduce some of the new ultrasound techniques from both published studies and ongoing trials that have been confirmed to be convenient and effective methods. However, additional evidence from future studies will be required before some of these techniques can be widely applied or recommended as alternatives.

Increased Excitability Induced in the Primary Motor Cortex by Transcranial Ultrasound Stimulation

Background: Transcranial Ultrasound Stimulation (tUS) is an emerging technique that uses ultrasonic waves to noninvasively modulate brain activity. As with other forms of non-invasive brain stimulation (NIBS), tUS may be useful for altering cortical excitability and neuroplasticity for a variety of research and clinical applications. The effects of tUS on cortical excitability are still unclear, and further complications arise from the wide parameter space offered by various types of devices, transducer arrangements, and stimulation protocols. Diagnostic ultrasound imaging devices are safe, commonly available systems that may be useful for tUS. However, the feasibility of modifying brain activity with diagnostic tUS is currently unknown. Objective: We aimed to examine the effects of a commercial diagnostic tUS device using an imaging protocol on cortical excitability. We hypothesized that imaging tUS applied to motor cortex could induce changes in cortical excitability as measured using a transcranial magnetic stimulation (TMS) motor evoked potential (MEP) paradigm. Methods: Forty-three subjects were assigned to receive either verum (n = 21) or sham (n = 22) diagnostic tUS in a single-blind design. Baseline motor cortex excitability was measured using MEPs elicited by TMS. Diagnostic tUS was subsequently administered to the same cortical area for 2 min, immediately followed by repeated post-stimulation MEPs recorded up to 16 min post-stimulation. Results: Verum tUS increased excitability in the motor cortex (from baseline) by 33.7% immediately following tUS (p = 0.009), and 32.4% (p = 0.047) 6 min later, with excitability no longer significantly different from baseline by 11 min post-stimulation. By contrast, subjects receiving sham tUS showed no significant changes in MEP amplitude. Conclusion: These findings demonstrate that tUS delivered via a commercially available diagnostic imaging ultrasound system transiently increases excitability in the motor cortex as measured by MEPs. Diagnostic tUS devices are currently used for internal imaging in many health care settings, and the present results suggest that these same devices may also offer a promising tool for noninvasively modulating activity in the central nervous system. Further studies exploring the use of diagnostic imaging devices for neuromodulation are warranted.

Simultaneous bilateral real-time 3-d transcranial ultrasound imaging at 1 MHz through poor acoustic windows

Ultrasound imaging has been proposed as a rapid, portable alternative imaging modality to examine stroke patients in pre-hospital or emergency room settings. However, in performing transcranial ultrasound examinations, 8%-29% of patients in a general population may present with window failure, in which case it is not possible to acquire clinically useful sonographic information through the temporal bone acoustic window. In this work, we describe the technical considerations, design and fabrication of low-frequency (1.2 MHz), large aperture (25.3 mm) sparse matrix array transducers for 3-D imaging in the event of window failure. These transducers are integrated into a system for real-time 3-D bilateral transcranial imaging-the ultrasound brain helmet-and color flow imaging capabilities at 1.2 MHz are directly compared with arrays operating at 1.8 MHz in a flow phantom with attenuation comparable to the in vivo case. Contrast-enhanced imaging allowed visualization of arteries of the Circle of Willis in 5 of 5 subjects and 8 of 10 sides of the head despite probe placement outside of the acoustic window. Results suggest that this type of transducer may allow acquisition of useful images either in individuals with poor windows or outside of the temporal acoustic window in the field.

Pitch-catch phase aberration correction of multiple isoplanatic patches for 3-D transcranial ultrasound imaging

Having previously presented the ultrasound brain helmet, a system for simultaneous 3-D ultrasound imaging via both temporal bone acoustic windows, the scanning geometry of this system is utilized to allow each matrix array to serve as a correction source for the opposing array. Aberration is estimated using cross-correlation of RF channel signals, followed by least mean squares solution of the resulting overdetermined system. Delay maps are updated and real-time 3-D scanning resumes. A first attempt is made at using multiple arrival time maps to correct multiple unique aberrators within a single transcranial imaging volume, i.e., several isoplanatic patches. This adaptive imaging technique, which uses steered unfocused waves transmitted by the opposing, or beacon, array, updates the transmit and receive delays of 5 isoplanatic patches within a 64° x 64° volume. In phantom experiments, color flow voxels above a common threshold have also increased by an average of 92%, whereas color flow variance decreased by an average of 10%. This approach has been applied to both temporal acoustic windows of two human subjects, yielding increases in echo brightness in 5 isoplanatic patches with a mean value of 24.3 ± 9.1%, suggesting that such a technique may be beneficial in the future for performing noninvasive 3-D color flow imaging of cerebrovascular disease, including stroke.

Large improvement of the electrical impedance of imaging and high-intensity focused ultrasound (HIFU) phased arrays using multilayer piezoelectric ceramics coupled in lateral mode

With a change in phased-array configuration from one dimension to two, the electrical impedance of the array elements is substantially increased because of their decreased width (w)-to-thickness (t) ratio. The most common way to compensate for this impedance increase is to employ electrical matching circuits at a high cost of fabrication complexity and effort. In this paper, we introduce a multilayer lateral-mode coupling method for phased-array construction. The direct comparison showed that the electrical impedance of a single-layer transducer driven in thickness mode is 1/(n²(1/(w/t))²) times that of an n-layer lateral mode transducer. A large reduction of the electrical impedance showed the impact and benefit of the lateral-mode coupling method. A one-dimensional linear 32-element 770-kHz imaging array and a 42-element 1.45-MHz high-intensity focused ultrasound (HIFU) phased array were fabricated. The averaged electrical impedances of each element were measured to be 58 Ω at the maximum phase angle of -1.2° for the imaging array and 105 Ω at 0° for the HIFU array. The imaging array had a center frequency of 770 kHz with an averaged -6-dB bandwidth of approximately 52%. For the HIFU array, the averaged maximum surface acoustic intensity was measured to be 32.8 W/cm² before failure.

Evaluation of regional hepatic perfusion (RHP) by contrast-enhanced ultrasound in patients with cirrhosis

BACKGROUND & AIMS

Ultrasonographic contrast agents allow the assessment of myocardial and renal perfusion through the analysis of refill kinetics after microbubbles rupture. This study evaluated the feasibility of contrast-enhanced ultrasonographic (CEUS) estimations of regional hepatic perfusion in patients with cirrhosis, and its correlation with clinical and hemodynamic parameters.

METHODS

Fifty-five patients with cirrhosis undergoing hepatic vein catheterization were included. Hepatic perfusion was studied by CEUS (using Contrast Coherent Imaging) during a continuous i.v. infusion of microbubbles (SonoVue®); after their rupture (high insonation power), tissue refill was digitally recorded and time-intensity curves were electronically calculated on a region of interest of the right hepatic lobe. Regional hepatic perfusion (RHP) was calculated as microbubbles velocity×microbubble concentration. During hepatic vein catheterization, we measured hepatic blood flow by indocyanine green (ICG) infusion, hepatic venous pressure gradient (HVPG), and cardiac output (Swan-Ganz catheter).

RESULTS

RHP was higher in patients than in healthy controls (5.1±3.7 vs. 3.4±0.7, p=0.003), and correlated with MELD (R=0.403, p=0.002), Child-Pugh score (R=0.348, p=0.009), and HVPG (R=0.279, p=0.041). RHP inversely correlated with ICG extraction (R=-0.346, p=0.039), ICG intrinsic clearance (R=-0.327, p=0.050), and ICG clearance (R=0.517, p=0.001), and directly correlated with hyperdynamic syndrome markers (cardiac index R=0.422, p=0.003; mean arterial pressure R=-0.405, p=0.004; systemic vascular resistance R=-0.496, p=0.001).

CONCLUSIONS

RHP increases in patients with cirrhosis and correlates with the degree of liver failure and hyperdynamic syndrome. RHP increases along with liver functional reserve decrease, suggesting that RHP increase occurs mainly through anatomical/functional shunts. RHP by CEUS is a feasible novel, objective, quantitative, non-invasive tool, potentially useful for the estimation of hepatic perfusion in patients with cirrhosis.

The ultrasound brain helmet: feasibility study of multiple simultaneous 3D scans of cerebral vasculature

We describe early stage experiments to test the feasibility of an ultrasound brain helmet to produce multiple simultaneous real-time three-dimensional (3D) scans of the cerebral vasculature from temporal and suboccipital acoustic windows of the skull. The transducer hardware and software of the Volumetrics Medical Imaging (Durham, NC, USA) real-time 3D scanner were modified to support dual 2.5 MHz matrix arrays of 256 transmit elements and 128 receive elements which produce two simultaneous 64 degrees pyramidal scans. The real-time display format consists of two coronal B-mode images merged into a 128 degrees sector, two simultaneous parasagittal images merged into a 128 degrees x 64 degrees C-mode plane and a simultaneous 64 degrees axial image. Real-time 3D color Doppler scans from a skull phantom with latex blood vessel were obtained after contrast agent injection as a proof of concept. The long-term goal is to produce real-time 3D ultrasound images of the cerebral vasculature from a portable unit capable of internet transmission thus enabling interactive 3D imaging, remote diagnosis and earlier therapeutic intervention. We are motivated by the urgency for rapid diagnosis of stroke due to the short time window of effective therapeutic intervention.

Three-dimensional reconstruction of fine vascularity in ultrasound breast imaging using contrast-enhanced spatial compounding: in vitro analyses

RATIONALE AND OBJECTIVES

Ultrasound image quality can be improved by imaging an object (here: the female breast) from different viewing angles in one image plane. With this technique, which is commonly referred to as spatial compounding, a more isotropic resolution is achieved while speckle noise and further artifacts are reduced. We present results obtained from a combination of spatial compounding with contrast-enhanced ultrasound imaging in three dimensions to reduce contrast specific artifacts (depth dependency, shadowing, speckle) and reconstruct vascular structures.

MATERIALS AND METHODS

We used a conventional ultrasound scanner and a custom made mechanical system to rotate an ultrasound curved array probe around an object (360 degrees, 36 transducer positions). For 10 parallel image planes, ultrasound compound images were generated of a flow-mimicking phantom consecutively supplied with water and contrast agent. These compound images were combined to form a volume dataset and postprocessed to obtain a sonographic subtraction angiography.

RESULTS

Image quality was significantly improved by spatial compounding for the native (ie, without contrast agent), and, in particular, for the contrast-enhanced case. After subtracting the native images from the contrast-enhanced ones, only structures supplied with contrast agent remain. This technique yields much better results for compound images than for conventional ultrasound images because speckle noise and an anisotropic resolution affect the latter.

CONCLUSIONS

With the presented approach contrast specific artifacts can be eliminated efficiently, and a subtraction angiography can be computed. A speckle reduced three-dimensional reconstruction of submillimeter vessel structures was achieved for the first time. In the future, this technique can be applied in vivo to image the vascularity of cancer in the female breast.

Transcranial sound field characterization

In the scope of therapeutic ultrasound applications in the adult brain, such as sonothrombolysis in stroke, a better understanding of the intracranial acoustic properties during insonation through the temporal bone is warranted. Innovative ultrasound imaging techniques, like transcranial duplex sonography, may open new avenues to apply ultrasound for therapeutic purposes and to visually monitor the effect using the same device. The aim was to study the transcranial sound field aberrations and the changes of acoustic parameters, using a high-end duplex machine. Six cadaver skulls were insonated through the temporal bone window, using a diagnostic duplex ultrasound device. The measurements were done in a water tank, using a needle hydrophone to assess and compute acoustic parameters, such as peak intensity, peak-to-peak, peak-positive, peak-negative acoustic pressure, beam area etc. in a 2-D plane. It could be shown that the absorption and wavefront distortion effects of the temporal bone are variable among different skulls. Because of signal absorption of the bone, the mechanical index of the incident ultrasound wave drops by a factor > or =10 in most cases. However, the beam area might be increased by a factor of almost 4, because of phase aberration (i.e., defocusing). (

Real-time 3-d intracranial ultrasound with an endoscopic matrix array transducer

A transducer originally designed for transesophageal echocardiography (TEE) was adapted for real-time volumetric endoscopic imaging of the brain. The transducer consists of a 36 x 36 array with an interelement spacing of 0.18 mm. There are 504 transmitting and 252 receive channels placed in a regular pattern in the array. The operating frequency is 4.5 MHz with a -6 dB bandwidth of 30%. The transducer is fabricated on a 10-layer flexible circuit from Microconnex (Snoqualmie, WA, USA). The purpose of this study is to evaluate the clinical feasibility of real-time 3-D intracranial ultrasound with this device. The Volumetrics Medical Imaging (Durham, NC, USA) 3-D scanner was used to obtain images in a canine model. A transcalvarial acoustic window was created under general anesthesia in the animal laboratory by placing a 10-mm burr hole in the high parietal calvarium of a 50-kg canine subject. The burr-hole was placed in a left parasagittal location to avoid the sagittal sinus, and the transducer was placed against the intact dura mater for ultrasound imaging. Images of the lateral ventricles were produced, including real-time 3-D guidance of a needle puncture of one ventricle. In a second canine subject, contrast-enhanced 3-D Doppler color flow images were made of the cerebral vessels including the complete Circle of Willis. Clinical applications may include real-time 3-D guidance of cerebrospinal fluid extraction from the lateral ventricles and bedside evaluation of critically ill patients where computed tomography and magnetic resonance imaging techniques are unavailable.

Intraoperative ultrasound using phase inversion harmonic imaging: first experiences

OBJECTIVE

To study the feasibility of intraoperative ultrasound using the phase inversion harmonic imaging (PIHI) technique.

METHODS

Eight patients with intracranial middle cerebral artery aneurysms and five patients with arteriovenous malformations were studied after written informed consent. A first ultrasound study was performed through the intact dura mater after cranial trepanation to assess the pathology, its feeding artery, and downstream segments. A second ultrasound study was performed immediately after intervention to monitor the success of the procedure. All patients were studied using a Siemens Sonoline Antares ultrasound machine (Siemens Medical Solutions USA, Inc., Malvern, PA) before and after intravenous administration of an ultrasound contrast agent (Optison; GE Healthcare, Milwaukee, WI). Other than conventional brightness mode, PIHI is sensitive to the nonlinear acoustic response of tissue, and especially to ultrasound contrast agent microbubbles. The latter enables contrast-specific vascular imaging.

RESULTS

PIHI provided anatomically detailed information. In combination with an ultrasound contrast agent, angiography-like views of the vascular pathologies, including their surrounding vessels, could be obtained. Flow velocities in afferent and downstream vascular segments, as well as inside the pathology, could be assessed. Flow dynamics inside the aneurysm sac or the arteriovenous malformation could be studied in real-time. Postintervention, contrast-enhanced PIHI could be used to immediately monitor the success of the surgical procedure.

CONCLUSION

PIHI enables intraoperative visualization and morphological assessment of neurovascular pathologies, such as middle cerebral artery aneurysms or arteriovenous malformations. In combination with an ultrasound contrast agent, the flow dynamics of these lesions can be displayed in real-time.

Microbubble ultrasound contrast agents: an update

Microbubble contrast agents for ultrasound (US) have gained increasing interest in recent years, and contrast-enhanced US (CEUS) is a rapidly evolving field with applications now extending far beyond the initial improvements achieved in Doppler US. This has been achieved as a result of the safe profile and the increased stability of microbubbles persisting in the bloodstream for several minutes, and also by the availability of specialized contrast-specific US techniques, which allow a definite improvement in the contrast resolution and suppression of signal from stationary tissues. CEUS with low transmit power allows real-time scanning with the possibility of prolonged organ insonation. Several reports have described the effectiveness of microbubble contrast agents in many clinical applications and particularly in the liver, spleen, and kidneys. CEUS allows the assessment of the macrovasculature and microvasculature in different parenchymas, the identification and characterization of hepatic and splenic lesions, the depiction of septal enhancement in cystic renal masses, and the quantification of organ perfusion by the quantitative analysis of the echo-signal intensity. Other fields of application include the assessment of abdominal organs after traumas and the assessment of vesico-ureteral reflux in children. Finally, tumor-targeted microbubbles make possible the depiction of specific biologic processes.

Intraoperative brain ultrasound: a new approach to study flow dynamics in intracranial aneurysms

The aim was to evaluate the potential of contrast-enhanced ultrasound to visualize the hemodynamics in intracranial aneurysms during neurosurgical intervention and to quantify the ultrasound data using digital particle image velocimetry (DPIV) technique. Aneurysms were scanned through the intact dura mater, preclipping and again postclipping after closure of the dura. After intravenous injection of Optison, angio-like views of the vascular tree surrounding the aneurysm, including the aneurysm sac, were obtained. Single ultrasound contrast agent microbubbles could be visualized in the aneurysm sac and the flow dynamics could be assessed in vivo. Spatial and temporal distributions of the velocity in the aneurysm and in the parent vessels were measured with DPIV using the backscattered signals from the microbubbles. Subsequently, the fluid stresses, vorticity, circulation, etc., were calculated from the velocity fields. We demonstrate in this paper that intraoperative contrast-enhanced ultrasound can be used to quantify the flow dynamics within an aneurysm.