A deep learning classification task for brain navigation in rodents using micro-Doppler ultrasound imaging

Positioning and navigation are essential components of neuroimaging as they improve the quality and reliability of data acquisition, leading to advances in diagnosis, treatment outcomes, and fundamental understanding of the brain. Functional ultrasound imaging is an emerging technology providing high-resolution images of the brain vasculature, allowing for the monitoring of brain activity. However, as the technology is relatively new, there is no standardized tool for inferring the position in the brain from the vascular images. In this study, we present a deep learning-based framework designed to address this challenge. Our approach uses an image classification task coupled with a regression on the resulting probabilities to determine the position of a single image. To evaluate its performance, we conducted experiments using a dataset of 51 rat brain scans. The training positions were extracted at intervals of 375 μm, resulting in a positioning error of 176 μm. Further GradCAM analysis revealed that the predictions were primarily driven by subcortical vascular structures. Finally, we assessed the robustness of our method in a cortical stroke where the brain vasculature is severely impaired. Remarkably, no specific increase in the number of misclassifications was observed, confirming the method's reliability in challenging conditions. Overall, our framework provides accurate and flexible positioning, not relying on a pre-registered reference but rather on conserved vascular patterns.

Brain-wide continuous functional ultrasound imaging for real-time monitoring of hemodynamics during ischemic stroke

Ischemic stroke occurs abruptly causing sudden neurologic deficits, and therefore, very little is known about hemodynamic perturbations in the brain immediately after stroke onset. Here, functional ultrasound imaging was used to monitor variations in relative cerebral blood volume (rCBV) compared to baseline. rCBV levels were analyzed brain-wide and continuously at high spatiotemporal resolution (100 μm, 2 Hz) until 70mins after stroke onset in rats. We compared two stroke models, with either a permanent occlusion of the middle cerebral artery (MCAo) or a tandem occlusion of both the common carotid and middle cerebral arteries (CCAo + MCAo). We observed a typical hemodynamic pattern, including a quick drop of the rCBV after MCAo, followed by spontaneous reperfusion of several brain regions located in the vicinity of the ischemic core. The severity and location of the ischemia were variable within groups. On average, the severity of the ischemia was in good agreement with the lesion volume (24 hrs after stroke) for MCAo group, while larger for the CCAo + MCAo model. For both groups, we observed that infarcts extended to initially non-ischemic regions located rostrally to the ischemic core. These regions strongly colocalize with the origin of transient hemodynamic events associated with spreading depolarizations.

Circuit mechanism for suppression of frontal cortical ignition during NREM sleep

Conscious perception is greatly diminished during sleep, but the underlying circuit mechanism is poorly understood. We show that cortical ignition-a brain process shown to be associated with conscious awareness in humans and non-human primates-is strongly suppressed during non-rapid-eye-movement (NREM) sleep in mice due to reduced cholinergic modulation and rapid inhibition of cortical responses. Brain-wide functional ultrasound imaging and cell-type-specific calcium imaging combined with optogenetics showed that activity propagation from visual to frontal cortex is markedly reduced during NREM sleep due to strong inhibition of frontal pyramidal neurons. Chemogenetic activation and inactivation of basal forebrain cholinergic neurons powerfully increased and decreased visual-to-frontal activity propagation, respectively. Furthermore, although multiple subtypes of dendrite-targeting GABAergic interneurons in the frontal cortex are more active during wakefulness, soma-targeting parvalbumin-expressing interneurons are more active during sleep. Chemogenetic manipulation of parvalbumin interneurons showed that sleep/wake-dependent cortical ignition is strongly modulated by perisomatic inhibition of pyramidal neurons.

Functional ultrasound imaging of stroke in awake rats

Anesthesia is a major confounding factor in preclinical stroke research as stroke rarely occurs in sedated patients. Moreover, anesthesia affects both brain functions and the stroke outcome acting as neurotoxic or protective agents. So far, no approaches were well suited to induce stroke while imaging hemodynamics along with simultaneous large-scale recording of brain functions in awake animals. For this reason, the first critical hours following the stroke insult and associated functional alteration remain poorly understood. Here, we present a strategy to investigate both stroke hemodynamics and stroke-induced functional alterations without the confounding effect of anesthesia, i.e., under awake condition. Functional ultrasound (fUS) imaging was used to continuously monitor variations in cerebral blood volume (CBV) in +65 brain regions/hemispheres for up to 3 hr after stroke onset. The focal cortical ischemia was induced using a chemo-thrombotic agent suited for permanent middle cerebral artery occlusion in awake rats and followed by ipsi- and contralesional whiskers stimulation to investigate on the dynamic of the thalamocortical functions. Early (0-3 hr) and delayed (day 5) fUS recording enabled to characterize the features of the ischemia (location, CBV loss), spreading depolarizations (occurrence, amplitude) and functional alteration of the somatosensory thalamocortical circuits. Post-stroke thalamocortical functions were affected at both early and later time points (0-3 hr and 5 days) after stroke. Overall, our procedure facilitates early, continuous, and chronic assessments of hemodynamics and cerebral functions. When integrated with stroke studies or other pathological analyses, this approach seeks to enhance our comprehension of physiopathologies towards the development of pertinent therapeutic interventions.

Functional Ultrasound Neuroimaging

Functional ultrasound (fUS) is a neuroimaging method that uses ultrasound to track changes in cerebral blood volume as an indirect readout of neuronal activity at high spatiotemporal resolution. fUS is capable of imaging head-fixed or freely behaving rodents and of producing volumetric images of the entire mouse brain. It has been applied to many species, including primates and humans. Now that fUS is reaching maturity, it is being adopted by the neuroscience community. However, the nature of the fUS signal and the different implementations of fUS are not necessarily accessible to nonspecialists. This review aims to introduce these ultrasound concepts to all neuroscientists. We explain the physical basis of the fUS signal and the principles of the method, present the state of the art of its hardware implementation, and give concrete examples of current applications in neuroscience. Finally, we suggest areas for improvement during the next few years.

Neural correlates of blood flow measured by ultrasound

Functional ultrasound imaging (fUSI) is an appealing method for measuring blood flow and thus infer brain activity, but it relies on the physiology of neurovascular coupling and requires extensive signal processing. To establish to what degree fUSI trial-by-trial signals reflect neural activity, we performed simultaneous fUSI and neural recordings with Neuropixels probes in awake mice. fUSI signals strongly correlated with the slow (<0.3 Hz) fluctuations in the local firing rate and were closely predicted by the smoothed firing rate of local neurons, particularly putative inhibitory neurons. The optimal smoothing filter had a width of ∼3 s, matched the hemodynamic response function of awake mice, was invariant across mice and stimulus conditions, and was similar in the cortex and hippocampus. fUSI signals also matched neural firing spatially: firing rates were as highly correlated across hemispheres as fUSI signals. Thus, blood flow measured by ultrasound bears a simple and accurate relationship to neuronal firing.

Quantitative Hemodynamic Measurements in Cortical Vessels Using Functional Ultrasound Imaging

Red blood cell velocity (RBCv), cerebral blood flow (CBF), and volume (CBV) are three key parameters when describing brain hemodynamics. Functional ultrasound imaging is a Doppler-based method allowing for real-time measurement of relative CBV at high spatiotemporal resolution (100 × 110 × 300 μm3, up to 10 Hz) and large scale. Nevertheless, the measure of RBCv and CBF in small cortical vessels with functional ultrasound imaging remains challenging because of their orientation and size, which impairs the ability to perform precise measurements. We designed a directional flow filter to overpass these limitations allowing us to measure RBCv in single vessels using a standard functional ultrasound imaging system without contrast agents (e.g., microbubbles). This method allows to quickly extract the number of vessels in the cortex that was estimated to be approximately 650/cm3 in adult rats, with a 55–45% ratio for penetrating arterioles versus ascending venules. Then, we analyzed the changes in RBCv in these vessels during forepaw stimulation. We observed that ∼40 vessels located in the primary somatosensory forelimb cortex display a significant increase of the RBCv (median ΔRBCv ∼15%, maximal ΔRBCv ∼60%). As expected, we show that RBCv was higher for penetrating arterioles located in the center than in the periphery of the activated area. The proposed approach extends the capabilities of functional ultrasound imaging, which may contribute to a better understanding of the neurovascular coupling at the brain-wide scale.

Whole-brain functional ultrasound imaging in awake head-fixed mice

Most brain functions engage a network of distributed regions. Full investigation of these functions thus requires assessment of whole brains; however, whole-brain functional imaging of behaving animals remains challenging. This protocol describes how to follow brain-wide activity in awake head-fixed mice using functional ultrasound imaging, a method that tracks cerebral blood volume dynamics. We describe how to set up a functional ultrasound imaging system with a provided acquisition software (miniScan), establish a chronic cranial window (timing surgery: ~3-4 h) and image brain-wide activity associated with a stimulus at high resolution (100 × 110 × 300 µm and 10 Hz per brain slice, which takes ~45 min per imaging session). We include codes that enable data to be registered to a reference atlas, production of 3D activity maps, extraction of the activity traces of ~250 brain regions and, finally, combination of data from multiple sessions (timing analysis averages ~2 h). This protocol enables neuroscientists to observe global brain processes in mice.

Optogenetic fUSI for brain-wide mapping of neural activity mediating collicular-dependent behaviors

Neuronal cell types are arranged in brain-wide circuits that guide behavior. In mice, the superior colliculus innervates a set of targets that direct orienting and defensive actions. We combined functional ultrasound imaging (fUSI) with optogenetics to reveal the network of brain regions functionally activated by four collicular cell types. Stimulating each neuronal group triggered different behaviors and activated distinct sets of brain nuclei. This included regions not previously thought to mediate defensive behaviors, for example, the posterior paralaminar nuclei of the thalamus (PPnT), which we show to play a role in suppressing habituation. Neuronal recordings with Neuropixels probes show that (1) patterns of spiking activity and fUSI signals correlate well in space and (2) neurons in downstream nuclei preferentially respond to innately threatening visual stimuli. This work provides insight into the functional organization of the networks governing innate behaviors and demonstrates an experimental approach to explore the whole-brain neuronal activity downstream of targeted cell types.

A Platform for Brain-wide Volumetric Functional Ultrasound Imaging and Analysis of Circuit Dynamics in Awake Mice

Imaging large-scale circuit dynamics is crucial to understanding brain function, but most techniques have a limited depth of field. Here, we describe volumetric functional ultrasound imaging (vfUSI), a platform for brain-wide vfUSI of hemodynamic activity in awake head-fixed mice. We combined a high-frequency 1,024-channel 2D-array transducer with advanced multiplexing and high-performance computing for real-time 3D power Doppler imaging at a high spatiotemporal resolution (220 × 280 × 175 μm³, up to 6 Hz). We developed a standardized software pipeline for registration, segmentation, and temporal analysis in 268 individual brain regions based on the Allen Mouse Common Coordinate Framework. We demonstrated the high sensitivity of vfUSI under multiple experimental conditions, and we successfully imaged stimulus-evoked activity when only a few trials were averaged. We also mapped neural circuits in vivo across the whole brain during optogenetic activation of specific cell types. Moreover, we identified the sequential activation of sensory-motor networks during a grasping water-droplet task.

Evidence from functional ultrasound imaging of enhanced contralesional microvascular response to somatosensory stimulation in acute middle cerebral artery occlusion/reperfusion in rats: A marker of ultra-early network reorganization?

Following middle cerebral artery (MCA) stroke, enhanced contralesional evoked responses have been consistently reported both in man and rodents as part of plastic processes thought to influence motor recovery. How early this marker of large-scale network reorganization develops has however been little addressed, yet has clinical relevance for rehabilitation strategies targeting plasticity. Previous work in mice has reported enhanced contralesional responses to unaffected-side forepaw stimulation as early as 45 min after MCA small branch occlusion. Using functional ultrasound imaging (fUSi) in anesthetized rats subjected to distal temporary MCA occlusion (MCAo), we assessed here (i) whether enhanced contralesional responses also occurred with unaffected-side whisker pad stimulation, and if so, how early after MCAo; and (ii) the time course of this abnormal response during occlusion and after reperfusion. We replicate in a more proximal MCA occlusion model the earlier findings of ultra-early enhanced contralesional evoked responses. In addition, we document this phenomenon within minutes after MCAo, and its persistence throughout the entire 90-min occlusion as well as 90-min reperfusion periods studied. These findings suggest that plastic processes may start within minutes following MCAo in rodents. If replicated in man, they might have implications regarding how early plasticity-enhancing therapies can be initiated after stroke.

Understanding the neurovascular unit at multiple scales: Advantages and limitations of multi-photon and functional ultrasound imaging

Developing efficient brain imaging technologies by combining a high spatiotemporal resolution and a large penetration depth is a key step for better understanding the neurovascular interface that emerges as a main pathway to neurodegeneration in many pathologies such as dementia. This review focuses on the advances in two complementary techniques: multi-photon laser scanning microscopy (MPLSM) and functional ultrasound imaging (fUSi). MPLSM has become the gold standard for in vivo imaging of cellular dynamics and morphology, together with cerebral blood flow. fUSi is an innovative imaging modality based on Doppler ultrasound, capable of recording vascular brain activity over large scales (i.e., tens of cubic millimeters) at unprecedented spatial and temporal resolution for such volumes (up to 10μm pixel size at 10kHz). By merging these two technologies, researchers may have access to a more detailed view of the various processes taking place at the neurovascular interface. MPLSM and fUSi are also good candidates for addressing the major challenge of real-time delivery, monitoring, and in vivo evaluation of drugs in neuronal tissue.

Mapping the dynamics of brain perfusion using functional ultrasound in a rat model of transient middle cerebral artery occlusion

Following middle cerebral artery occlusion, tissue outcome ranges from normal to infarcted depending on depth and duration of hypoperfusion as well as occurrence and efficiency of reperfusion. However, the precise time course of these changes in relation to tissue and behavioral outcome remains unsettled. To address these issues, a three-dimensional wide field-of-view and real-time quantitative functional imaging technique able to map perfusion in the rodent brain would be desirable. Here, we applied functional ultrasound imaging, a novel approach to map relative cerebral blood volume without contrast agent, in a rat model of brief proximal transient middle cerebral artery occlusion to assess perfusion in penetrating arterioles and venules acutely and over six days thanks to a thinned-skull preparation. Functional ultrasound imaging efficiently mapped the acute changes in relative cerebral blood volume during occlusion and following reperfusion with high spatial resolution (100 µm), notably documenting marked focal decreases during occlusion, and was able to chart the fine dynamics of tissue reperfusion (rate: one frame/5 s) in the individual rat. No behavioral and only mild post-mortem immunofluorescence changes were observed. Our study suggests functional ultrasound is a particularly well-adapted imaging technique to study cerebral perfusion in acute experimental stroke longitudinally from the hyper-acute up to the chronic stage in the same subject.

Abstract TMP56: Mapping the Dynamics of Brain Perfusion During and Following Transient Middle Cerebral Artery Occlusion (tMCAo) in the Rat Using Functional Ultrasound (fUS)

Introduction: Following MCAo, tissue outcome varies depending on depth and duration of hypoperfusion and efficiency of reperfusion. However, the precise time-course of these events in relation to tissue and behavioral outcome remains unsettled due to lack of a wide field-of-view quantitative imaging technique able to map perfusion in the rodent brain at high spatiotemporal resolution. Here we used fUS, a novel approach to map cerebral blood volume (CBV) without contrast agent.

Hypothesis: fUS will allow quantitative, near real-time mapping of CVB during and after tMCAo.

Methods: 45min filament tMCAo was induced in adult SD rats; sham rats were also used. fUS was used to map the penetrating arterioles and venules of the ipsi- and contra-lateral motor (M1-2) and somatosensory (S1) cortex in coronal sections across the MCA territory at 80μm resolution. Three-min coronal scans were taken at different levels before, during and immediately after MCAo, and at 3 and 6 days thanks to a thinned-skull preparation. CBV was expressed relative to mirror ROI. In addition, a 1-hr movie (one frame/5s) was taken starting a few mins after reperfusion. Serial Neuroscore and 2 sensorimotor tasks were given over 3w post-MCAo, and then NeuN, IBa1 and GFAP immunofluorescence (IF) at post-mortem.

Results: fUS showed a ∼80% CBV reduction in S1 during occlusion (p<0.001; n=7), with partial (∼60%, p<0.001) return of CBV on reperfusion, followed by a full return at days 3 and 6. As expected for this model, similar but less conspicuous CBV changes prevailed in M1-2. Continuous reperfusion was depicted in 5/7 rats (slope range: 8-25%/hr relative to prior CBV), but not in 2 rats. There were no significant changes in behavior relative to the sham group (n=4), and IF showed no infarction but marked selective neuronal loss (SNL) in the striatum in 5/7 rats and milder cortical SNL in 4/7 rats.

Conclusions: fUS efficiently mapped the acute changes in CBV during occlusion and following reperfusion with high spatio-temporal resolution, allowing the charting of fine tissue reperfusion dynamics in the individual rat. fUS is ideal to longitudinally map real-time cerebral perfusion in experimental stroke from the hyper-acute through to the chronic stage.

Out-of-plane Doppler imaging based on ultrafast plane wave imaging

Retrieving the out-of-plane blood flow velocity vector from two-dimensional transverse acquisitions of large vessels could improve the quantification of flow rate and maximum speed. The in-plane vector flow component can be computed easily using the Doppler frequency shift. The main problem is estimating the angle between the probe imaging plane and the vessel axis to derive the out-of-plane component from in-plane measurements. In this article, we study the case in which the velocity vector can be decomposed on two directions: the out-of-plane direction and the in-plane depth direction. We explore the combination of a technique called intrinsic spectral broadening with ultrafast plane wave imaging to retrieve the out-of-plane component of the flow velocity vector. Using a one-time probe calibration of this intrinsic spectral broadening, out-of-plane angle and flow speed can be recovered easily, thus avoiding approximations of a complex theoretical analysis. For the calibration step, ultrafast plane wave imaging permits a fast calibration procedure for the Doppler intrinsic spectral broadening. In vitro experimental validations are performed on a homogeneous flow phantom and a Poiseuille flow; the absolute speed was retrieved with 6% error. The potential of the technique is demonstrated in vivo on the human carotid artery. Combined with in-plane vector flow approaches, this out-of-plane Doppler imaging method paves the way to threedimensional vector flow imaging using only conventional onedimensional probe technology.

Chronic assessment of cerebral hemodynamics during rat forepaw electrical stimulation using functional ultrasound imaging

Functional ultrasound imaging is a method recently developed to assess brain activity via hemodynamics in rodents. Doppler ultrasound signals allow the measurement of cerebral blood volume (CBV) and red blood cells' (RBCs') velocity in small vessels. However, this technique originally requires performing a large craniotomy that limits its use to acute experiments only. Moreover, a detailed description of the hemodynamic changes that underlie functional ultrasound imaging has not been described but is essential for a better interpretation of neuroimaging data. To overcome the limitation of the craniotomy, we developed a dedicated thinned skull surgery for chronic imaging. This procedure did not induce brain inflammation nor neuronal death as confirmed by immunostaining. We successfully acquired both high-resolution images of the microvasculature and functional movies of the brain hemodynamics on the same animal at 0, 2, and 7 days without loss of quality. Then, we investigated the spatiotemporal evolution of the CBV hemodynamic response function (HRF) in response to sensory-evoked electrical stimulus (1 mA) ranging from 1 (200 μs) to 25 pulses (5s). Our results indicate that CBV HRF parameters such as the peak amplitude, the time to peak, the full width at half-maximum and the spatial extent of the activated area increase with stimulus duration. Functional ultrasound imaging was sensitive enough to detect hemodynamic responses evoked by only a single pulse stimulus. We also observed that the RBC velocity during activation could be separated in two distinct speed ranges with the fastest velocities located in the upper part of the cortex and slower velocities in deeper layers. For the first time, functional ultrasound imaging demonstrates its potential to image brain activity chronically in small animals and offers new insights into the spatiotemporal evolution of cerebral hemodynamics.

Non-invasive ultrasonic surgery of the brain in non-human primates

High-intensity focused ultrasound causes selective tissue necrosis efficiently and safely, namely, in the prostate, liver, and uterine fibroid. Nevertheless, ablation of brain tissue using focused ultrasound remains limited due to strong aberrations induced by the skull. To achieve ultrasonic transcranial brain ablation, such aberrations have to be compensated. In this study, non-invasive therapy was performed on monkeys using adaptive correction of the therapeutic beam and 3D simulations of transcranial wave propagation based on 3D computed tomographic (CT) scan information. The aim of the study was two-fold: induce lesions in a non-human primate brain non-invasively and investigate the potential side effects. Stereotactic targeting was performed on five Macaca fascicularis individuals. Each hemisphere was treated separately with a 15-day interval and animals were sacrificed two days after the last treatment. The ultrasonic dose delivered at the focus was increased from one treatment location to the other to estimate the thermal dose for tissue alteration. Thermal doses in the brain were determined by numerical computations. Treatment efficiency and safety were evaluated histologically. The threshold for tissue damage in the brain was measured to be between 90 and 280 cumulative equivalent minutes at 43 °C. Intravenous injection of corticoids before the treatment limited the side effects.

Functional ultrasound imaging of the brain: theory and basic principles

Hemodynamic changes in the brain are often used as surrogates of neuronal activity to infer the loci of brain activity. A major limitation of conventional Doppler ultrasound for the imaging of these changes is that it is not sensitive enough to detect the blood flow in small vessels where the major part of the hemodynamic response occurs. Here, we present a μDoppler ultrasound method able to detect and map the cerebral blood volume (CBV) over the entire brain with an important increase in sensitivity. This method is based on imaging the brain at an ultrafast frame rate (1 kHz) using compounded plane wave emissions. A theoretical model demonstrates that the gain in sensitivity of the μDoppler method is due to the combination of 1) the high signal-to-noise ratio of the gray scale images, resulting from the synthetic compounding of backscattered echoes; and 2) the extensive signal averaging enabled by the high temporal sampling of ultrafast frame rates. This μDoppler imaging is performed in vivo on trepanned rats without the use of contrast agents. The resulting images reveal detailed maps of the rat brain vascularization with an acquisition time as short as 320 ms per slice. This new method is the basis for a real-time functional ultrasound (fUS) imaging of the brain.

Ultrafast Doppler imaging of blood flow dynamics in the myocardium

Imaging intramyocardial vascular flows in real-time could strongly help to achieve better diagnostic of cardiovascular diseases. To date, no standard imaging modality allows describing accurately myocardial blood flow dynamics with good spatial and temporal resolution. We recently introduced a novel ultrasonic Doppler imaging technique based on compounded plane waves transmissions at ultrafast frame rate. The high sensitivity of this ultrafast Doppler technique permits to image the intramyocardial blood flow and its dynamics. A dedicated demodulation-filtering process is implemented to compensate for the large tissue velocity of the myocardium during the cardiac cycle. A signed power Doppler processing provides the discrimination between arterial and venous flows. Experiments were performed in vivo in a large animal open chest model ( N = 5 sheep) using a conventional ultrasonic probe placed at the surface of the heart. Results show the capability of the technique to image intramyocardial vascular flows in normal physiological conditions with good spatial (200 μm) and temporal resolution (10 ms). Flow dynamics over the cardiac cycle were investigated and the imaging method demonstrated a phase opposition of flow waveforms between arterial and venous flows. Finally, ultrafast Doppler combined with tissue motion compensation was found able to reveal vascular flow disruption in ischemic regions during occlusion of the main diagonal coronary artery.

Imaging of perfusion, angiogenesis, and tissue elasticity after stroke

Blood flow interruption in a cerebral artery causes brain ischemia and induces dramatic changes of perfusion and metabolism in the corresponding territory. We performed in parallel positron emission tomography (PET) with [(15)O]H(2)O, single photon emission computed tomography (SPECT) with [(99m)Tc]hexamethylpropylene-amino-oxime ([(99m)Tc]HMPAO) and ultrasonic ultrafast shear wave imaging (SWI) during, immediately after, and 1, 2, 4, and 7 days after middle cerebral artery occlusion (MCAO) in rats. Positron emission tomography and SPECT showed initial hypoperfusion followed by recovery at immediate reperfusion, hypoperfusion at day 1, and hyperperfusion at days 4 to 7. Hyperperfusion interested the whole brain, including nonischemic areas. Immunohistochemical analysis indicated active angiogenesis at days 2 to 7, strongly suggestive that hyperperfusion was supported by an increase in microvessel density in both brain hemispheres after ischemia. The SWI detected elastic changes of cerebral tissue in the ischemic area as early as day 1 after MCAO appearing as a softening of cerebral tissue whose local internal elasticity decreased continuously from day 1 to 7. Taken together, these results suggest that hyperperfusion after cerebral ischemia is due to formation of neovessels, and indicate that brain softening is an early and continuous process. The SWI is a promising novel imaging method for monitoring the evolution of cerebral ischemia over time in animals.

Aberration correction by time reversal of moving speckle noise

Focusing a wave through heterogeneous media is an important problem in medical ultrasound imaging. In such aberrating media, in the presence of a small number of point reflectors, iterative time reversal is a well-known method able to focus on the strongest reflector. However, in presence of speckle noise generated by many non-resolved scatterers, iterative time reversal alone does not work. In this paper, we propose the use of the echoes coming from moving particles in a flow, such as red blood cells, to generate a virtual point reflector by iterative time reversal. The construction of the virtual point reflector is performed by a coherent addition of independent realizations of speckle coming from moving particles. After focusing on a virtual point reflector, ultrasound images can be locally corrected inside an isoplanatic patch. An application for the correction of power Doppler images is presented. A theoretical analysis shows that this iterative method allows focusing on the point of maximal insonification of the uncorrected beam.

In vivo mapping of brain elasticity in small animals using shear wave imaging

A combination of radiation force and ultrafast ultrasound imaging is used to both generate and track the propagation of a shear wave in the brain whose local speed is directly related to stiffness, characterized by the dynamic shear modulus G*. When performed on trepanated rats, this approach called shear wave imaging (SWI) provides 3-D brain elasticity maps reaching a spatial resolution of 0.7 mm×1 mm×0.4 mm with a good reproducibility (<13%). The dynamic shear modulus of brain tissues exhibits values in the 2-25 kPa range with a mean value of 12 kPa and is quantified for different anatomical regions. The anisotropy of the shear wave propagation is studied and the first in vivo anisotropy map of brain elasticity is provided. The propagation is found to be isotropic in three gray matter regions but highly anisotropic in two white matter regions. The good temporal resolution (~10 ms per acquisition) of SWI also allows a dynamic estimation of brain elasticity to within a single cardiac cycle, showing that brain pulsatility does not transiently modify local elasticity. SWI proves its potential for the study of pathological modifications of brain elasticity both in small animal models and in clinical intra-operative imaging.

Time reversal of speckle noise

Focusing a wave in an unknown inhomogeneous medium is an open problem in wave physics. This work presents an iterative method able to focus in pulse-echo mode in an inhomogeneous medium containing a random distribution of scatterers. By performing a coherent summation of the random echoes backscattered from a set of points surrounding the desired focus, a virtual bright pointlike reflector is generated. A time-reversal method enables an iterative convergence towards the optimal wave field focusing at the location of this virtual scatterer. Thanks to this iterative time-reversal process, it is possible to focus at any arbitrary point in the heterogeneous medium even in the absence of pointlike source. An experimental demonstration is given for the correction of strongly distorted images in the field of medical ultrasound imaging. This concept enables envisioning many other applications in wave physics.

Ultrafast compound Doppler imaging: providing full blood flow characterization

Doppler-based flow analysis methods require acquisition of ultrasound data at high spatio-temporal sampling rates. These rates represent a major technical challenge for ultrasound systems because a compromise between spatial and temporal resolution must be made in conventional approaches. Consequently, ultrasound scanners can either provide full quantitative Doppler information on a limited sample volume (spectral Doppler), or averaged Doppler velocity and/or power estimation on a large region of interest (Doppler flow imaging). In this work, we investigate a different strategy for acquiring Doppler information that can overcome the limitations of the existing Doppler modes by significantly reducing the required acquisition time. This technique is called ultrafast compound Doppler imaging and is based on the following concept: instead of successively insonifying the medium with focused beams, several tilted plane waves are sent into the medium and the backscattered signals are coherently summed to produce high-resolution ultrasound images. We demonstrate that this strategy allows reduction of the acquisition time by a factor of up to of 16 while keeping the same Doppler performance. Depending on the application, different directions to increase performance of Doppler analysis are proposed and the improvement is quantified: the ultrafast compound Doppler method allows faster acquisition frame rates for high-velocity flow imaging, or very high sensitivity for low-flow applications. Full quantitative Doppler flow analysis can be performed on a large region of interest, leading to much more information and improved functionality for the physician. By leveraging the recent emergence of ultrafast parallel beamforming systems, this paper demonstrates that breakthrough performances in flow analysis can be reached using this concept of ultrafast compound Doppler.

MR-guided adaptive focusing of ultrasound

Adaptive focusing of ultrasonic waves under the guidance of a magnetic resonance (MR) system is demonstrated for medical applications. This technique is based on the maximization of the ultrasonic wave intensity at one targeted point in space. The wave intensity is indirectly estimated from the local tissue displacement induced at the chosen focus by the acoustic radiation force of ultrasonic beams. Coded ultrasonic waves are transmitted by an ultrasonic array and an MRI scanner is used to measure the resulting local displacements through a motion-sensitive MR sequence. After the transmission of a set of spatially encoded ultrasonic waves, a non-iterative inversion process is employed to accurately estimate the spatial-temporal aberration induced by the propagation medium and to maximize the acoustical intensity at the target.Both programmable and physical aberrating layers introducing strong distortions (up to 2pi radians) were recovered within acceptable errors (<0.8 rad). This noninvasive technique is shown to accurately correct phase aberrations in a phantom gel with negligible heat deposition and limited acquisition time. These refocusing performances demonstrate a major potential in the field of MR-guided ultrasound therapy in particular for transcranial brain high-intensity focused ultrasound.

Viscoelastic and anisotropic mechanical properties of in vivo muscle tissue assessed by supersonic shear imaging

The in vivo assessment of the biomechanical properties of the skeletal muscle is a complex issue because the muscle is an anisotropic, viscoelastic and dynamic medium. In this article, these mechanical properties are characterized for the brachialis muscle in vivo using a noninvasive ultrasound-based technique. This supersonic shear imaging technique combines an ultra-fast ultrasonic system and the remote generation of transient mechanical forces into tissue via the radiation force of focused ultrasonic beams. Such an ultrasonic radiation force is induced deep within the muscle by a conventional ultrasonic probe and the resulting shear waves are then imaged with the same probe (5 MHz) at an ultra-fast framerate (up to 5000 frames/s). Local tissue velocity maps are obtained with a conventional speckle tracking technique and provide a full movie of the shear wave propagation through the entire muscle. Shear wave group velocities are then estimated using a time of flight algorithm. This approach provides a complete set of quantitative and in vivo parameters describing the muscle's mechanical properties as a function of active voluntary contraction as well as passive extension of healthy volunteers. Anisotropic properties are also estimated by tilting the probe head with respects to the main muscular fibers direction. Finally, the dispersion of the shear waves is studied for these different configurations and shear modulus and shear viscosity are quantitatively assessed assuming the viscoelastic Voigt's model.

Energy-based adaptive focusing of waves: application to noninvasive aberration correction of ultrasonic wavefields

An aberration correction method based on the maximization of the wave intensity at the focus of an emitting array is presented. The potential of this new adaptive focusing technique is investigated for ultrasonic focusing in biological tissues. The acoustic intensity is maximized noninvasively through direct measurement or indirect estimation of the beam energy at the focus for a series of spatially coded emissions. For ultrasonic waves, the acoustic energy at the desired focus can be indirectly estimated from the local displacements induced in tissues by the ultrasonic radiation force of the beam. Based on the measurement of these displacements, this method allows determination of the precise estimation of the phase and amplitude aberrations, and consequently the correction of aberrations along the beam travel path. The proof of concept is first performed experimentally using a large therapeutic array with strong electronic phase aberrations (up to 2pi). Displacements induced by the ultrasonic radiation force at the desired focus are indirectly estimated using the time shift of backscattered echoes recorded on the array. The phase estimation is deduced accurately using a direct inversion algorithm which reduces the standard deviation of the phase distribution from sigma = 1.89 radian before correction to sigma = 0.53 radian following correction. The corrected beam focusing quality is verified using a needle hydrophone. The peak intensity obtained through the aberrator is found to be -7.69 dB below the reference intensity obtained without any aberration. Using the phase correction, a sharp focus is restored through the aberrator with a relative peak intensity of -0.89 dB. The technique is tested experimentally using a linear transmit/receive array through a real aberrating layer. The array is used to automatically correct its beam quality, as it both generates the radiation force with coded excitations and indirectly estimates the acoustic intensity at the focus with speckle tracking. This technique could have important implications in the field of high-intensity focused ultrasound even in complex configurations such as transcranial, transcostal, or deep seated organs.

Ultrafast imaging of ultrasound contrast agents

The disappearance of ultrasound contrast agents after disruption can provide useful information on their environment. However, in vivo acoustical imaging of this transient phenomenon, which has a duration on the order of milliseconds, requires high frame rates that are unattainable by conventional ultrasound scanners. In this article, ultrafast imaging is applied to microbubble tracking using a 128-element linear array and an elastography scanner. Contrast agents flowing in a wall-less tissue phantom are insonified with a high-intensity disruption pulse followed by a series of plane waves emitted at a 5kHz PRF. A collection of compounded images depicting the evolution of microbubbles is obtained after the echoes are beamformed in silico. The backscattering of the microbubbles appears to increase in the first image after disruption (4 ms) and decrease following an exponential decay in the next hundred milliseconds. This microbubble dynamic depends on the length and amplitude of the high-intensity pulse. Furthermore, confined microbubbles are found to differ significantly from their free-flowing counterparts in their dissolution curves. The high temporal resolution provided by ultrafast imaging could help distinguish targeted microbubbles during molecular imaging.

Non-invasive transcranial ultrasound therapy based on a 3D CT scan: protocol validation and in vitro results

A non-invasive protocol for transcranial brain tissue ablation with ultrasound is studied and validated in vitro. The skull induces strong aberrations both in phase and in amplitude, resulting in a severe degradation of the beam shape. Adaptive corrections of the distortions induced by the skull bone are performed using a previous 3D computational tomography scan acquisition (CT) of the skull bone structure. These CT scan data are used as entry parameters in a FDTD (finite differences time domain) simulation of the full wave propagation equation. A numerical computation is used to deduce the impulse response relating the targeted location and the ultrasound therapeutic array, thus providing a virtual time-reversal mirror. This impulse response is then time-reversed and transmitted experimentally by a therapeutic array positioned exactly in the same referential frame as the one used during CT scan acquisitions. In vitro experiments are conducted on monkey and human skull specimens using an array of 300 transmit elements working at a central frequency of 1 MHz. These experiments show a precise refocusing of the ultrasonic beam at the targeted location with a positioning error lower than 0.7 mm. The complete validation of this transcranial adaptive focusing procedure paves the way to in vivo animal and human transcranial HIFU investigations.

Shear wave spectroscopy for in vivo quantification of human soft tissues visco-elasticity

In vivo assessment of dispersion affecting the propagation of visco-elastic waves in soft tissues is key to understand the rheology of human tissues. In this paper, the ability of the supersonic shear imaging (SSI) technique to generate planar shear waves propagating in tissues is fully exploited. First, by strongly limiting shear wave diffraction in the imaging plane, this imaging technique enables to discriminate between the usually concomitant influences of both medium rheological properties and diffraction affecting the shear wave dispersion. Second, transient propagation of these plane shear waves in soft tissues can be measured using echographic images acquired at very high frame. In vitro and in vivo experiments demonstrate that dispersion curves, which characterize the rheological behavior of tissues by measuring the frequency dependence of shear wave speed and attenuation, can be recovered in the 75-600 Hz frequency range. Based on a phase difference algorithm, the dispersion curves are computed in 1 cm2 regions of interest from the acquired propagation movie. In vivo measurements in Biceps Brachii muscle and liver of three healthy volunteers show important differences in the rheological behavior of these different tissues. Liver tissue appears to be much more dispersive with a phase velocity ranging from approximately 1.5 m/s at 75 Hz to approximately 3 m/s at 500 Hz whereas muscle tissue shows an important anisotropy, shear waves propagating longitudinally to the muscular fibers are almost nondispersive while those propagating transversally are very dispersive with a shear wave speed ranging from 0.5 to 2 m/s between 75 and 500 Hz. The estimation of dispersion curves is local and can be performed separately in different regions of the organ. This signal processing approach based on the SSI modality introduces the new concept of in vivo shear wave spectroscopy (SWS) that could become an additional tool for tissue characterization. This paper demonstrates the in vivo ability of this SWS to quantify both local shear elasticity and dispersion in real time.

Coherent plane-wave compounding for very high frame rate ultrasonography and transient elastography

The emergence of ultrafast frame rates in ultrasonic imaging has been recently made possible by the development of new imaging modalities such as transient elastography. Data acquisition rates reaching more than thousands of images per second enable the real-time visualization of shear mechanical waves propagating in biological tissues, which convey information about local viscoelastic properties of tissues. The first proposed approach for reaching such ultrafast frame rates consists of transmitting plane waves into the medium. However, because the beamforming process is then restricted to the receive mode, the echographic images obtained in the ultrafast mode suffer from a low quality in terms of resolution and contrast and affect the robustness of the transient elastography mode. It is here proposed to improve the beamforming process by using a coherent recombination of compounded plane-wave transmissions to recover high-quality echographic images without degrading the high frame rate capabilities. A theoretical model is derived for the comparison between the proposed method and the conventional B-mode imaging in terms of contrast, signal-to-noise ratio, and resolution. Our model predicts that a significantly smaller number of insonifications, 10 times lower, is sufficient to reach an image quality comparable to conventional B-mode. Theoretical predictions are confirmed by in vitro experiments performed in tissue-mimicking phantoms. Such results raise the appeal of coherent compounds for use with standard imaging modes such as B-mode or color flow. Moreover, in the context of transient elastography, ultrafast frame rates can be preserved while increasing the image quality compared with flat insonifications. Improvements on the transient elastography mode are presented and discussed.

Suppression of tissue harmonics for pulse-inversion contrast imaging using time reversal

Pulse-inversion (PI) sequences are sensitive to the nonlinear echoes from microbubbles allowing an improvement in the blood-to-tissue contrast. However, at larger mechanical indices, this contrast is reduced by harmonics produced during nonlinear propagation. A method for tissue harmonics cancellation exploiting time reversal is experimentally implemented using a 128-channel 12-bit emitter receiver. The probe calibration is performed by acquiring the nonlinear echo of a wire in water. These distorted pulses are time-reversed, optimized and used for the PI imaging of a tissue phantom. Compared to normal (straight) pulses, the time-reversed distorted pulses reduced the tissue signal in PI by 11 dB. The second harmonic signals from microbubbles flowing in a wall-less vessel were unaffected by the correction. This technique can thus increase the blood-to-tissue contrast ratio while keeping the pressure and the number of pulses constant.

Quantitative assessment of breast lesion viscoelasticity: initial clinical results using supersonic shear imaging

This paper presents an initial clinical evaluation of in vivo elastography for breast lesion imaging using the concept of supersonic shear imaging. This technique is based on the combination of a radiation force induced in tissue by an ultrasonic beam and an ultrafast imaging sequence capable of catching in real time the propagation of the resulting shear waves. The local shear wave velocity is recovered using a time-offlight technique and enables the 2-D mapping of shear elasticity. This imaging modality is implemented on a conventional linear probe driven by a dedicated ultrafast echographic device. Consequently, it can be performed during a standard echographic examination. The clinical investigation was performed on 15 patients, which corresponded to 15 lesions (4 cases BI-RADS 3, 7 cases BI-RADS 4 and 4 cases BI-RADS 5). The ability of the supersonic shear imaging technique to provide a quantitative and local estimation of the shear modulus of abnormalities with a millimetric resolution is illustrated on several malignant (invasive ductal and lobular carcinoma) and benign cases (fibrocystic changes and viscous cysts). In the investigated cases, malignant lesions were found to be significantly different from benign solid lesions with respect to their elasticity values. Cystic lesions have shown no shear wave propagate at all in the lesion (because shear waves do not propage in liquid). These preliminary clinical results directly demonstrate the clinical feasibility of this new elastography technique in providing quantitative assessment of relative stiffness of breast tissues. This technique of evaluating tissue elasticity gives valuable information that is complementary to the B-mode morphologic information. More extensive studies are necessary to validate the assumption that this new mode potentially helps the physician in both false-positive and false-negative rejection.

Compensating for bone interfaces and respiratory motion in high-intensity focused ultrasound

Bursts of focused ultrasound energy a thousand times more intense than diagnostic ultrasound have become a non-invasive option for treating cancer, from breast to prostate or uterine fibroid, during the last decade. Despite this progress, many issues still need to be addressed. First, the distortions caused by defocusing obstacles, such as the skull or ribs, on the ultrasonic therapeutic beam are still being investigated. Multi-element transducer technology must be used in order to achieve such transcranial or transcostal adaptive focusing. Second, the problem of motion artifacts, a key component in the treatment of abdominal lesions, has been shown significantly to influence the efficacy and treatment time. Though many methods have been proposed for the detection of organ motion, little work has been done to develop a comprehensive solution including motion tracking and feedback correction in real time. This paper is a review of the work achieved by authors in transcranial high-intensity focused ultrasound (HIFU), transcostal HIFU and motion compensated HIFU. For these three issues, the optimal solution can be reached using the same technology of multi-element transducers devices able to work both in transmit and receive modes.

In-vivo non-invasive motion tracking and correction in high intensity focused ultrasound therapy

A method for tracking locally the 3D motion of biological tissues is developed and applied to the correction of motion during high intensity focused ultrasound (HIFU) therapy. The motion estimation technique is based on an accurate ultrasonic speckle tracking method. A pulse-echo sequence is performed for a subset of the transducers of a phased array. For each of these sub-apertures, the displacement is estimated by computing the 1D cross-correlation of the backscattered signals acquired at two consecutive times. The local 3D motion vector is then computed using a inversion algorithm. This technique is experimentally validated in vivo on anesthetized pigs. The 3D motion of liver tissues is tracked in real-time. The technique is combined with HIFU sequences and a real-time feedback correction of the HIFU beam is achieved by adjusting the delays of each channel. The sonications "locked on target" are interleaved with very motion estimation sequences.

Non-invasive transcranial ultrasound therapy guided by CT-scans

Brain therapy using focused ultrasound remains very limited due to the strong aberrations induced by the skull. A technique using time-reversal was validated recently in-vivo on 20 sheep. The principal handicap of this technique is the need of an hydrophone at the focal point for the first step of the time-reversal procedure, which is minimally invasive but lightly traumatizing. A completely noninvasive therapy requires a reliable model of the acoustical properties of the skull in order to simulate this first step. 3-D simulations based on high-resolution CT images of a skull have been successfully performed with a finite differences code developed in our Laboratory. Thanks to the skull porosity, directly extracted from the CT images, we reconstructed acoustic speed, density and absorption maps and performed the computation. Computed wavefronts are in good agreement with experimental wavefronts acquired through the same part of the skull and this technic was validated in-vitro in the laboratory. A stereotactic frame has been designed and built in order to perform non invasive transcranial focusing. Here we will describe all the steps of our new protocol, from the CT-scans to the therapy treatment and the first in vivo results on monkeys will be presented. This protocol is based on protocols already existing in radiotherapy.

Building three-dimensional images using a time-reversal chaotic cavity

The design of two-dimensional (2-D) arrays for three-dimensional (3-D) ultrasonic imaging is a major challenge in medical and nondestructive applications. Thousands of transducers are typically needed for focusing and steering in a 3-D volume. In this article, we propose a different concept allowing us to obtain electronic 3-D focusing with a small number of transducers. The basic idea is to couple a small number of transducers to a chaotic reverberating cavity with one face in contact with the body of the patient. The reverberations of the ultrasonic waves inside the cavity create at each reflection virtual transducers. The cavity acts as an ultrasonic kaleidoscope multiplying the small number of transducers and creating a much larger virtual transducer array. By exploiting time-reversal processing, it is possible to use collectively all the virtual transducers to focus a pulse everywhere in a 3-D volume. The reception process is based on a nonlinear pulse-inversion technique in order to ensure a good contrast. The feasibility of this concept for the building of 3-D images was demonstrated using a prototype relying only on 31 emission transducers and a single reception transducer.

Spatio-temporal coding in complex media for optimum beamforming: the iterative time-reversal approach

Spatio-temporal encoding in transmit and receive modes is of major importance in the development of ultrasound imaging devices. Classically, the assumption of constant sound speed in the medium allows one to restrict the beamforming process to the application of a cylindrical time-delay law on the elements of a multiple-transducer array. Here is proposed an iterative time-reversal method capable of taking into account all the heterogeneities of the medium, concerning density, speed of sound, and absorption variations. It will be shown that this iterative focusing process converges toward a spatio-temporal inverse filter focusing, the first step of the process being a time-reversal focusing on the targeted point. This method can be seen as a calibration process and has been successfully applied to transskull focusing and intraplate echoes suppression. It is leading the way to promising applications such as high-resolution ultrasonic brain imaging and high-resolution focusing through complex reverberating media, in nondestructive testing and telecommunications. This work highlights the advantages of using spatio-temporal coding to focus through complex media. Such codes require the use of fully programmable, multichannel electronics to implement this technique in real time.

Time reversal of electromagnetic waves

We report the first experimental demonstration of time-reversal focusing with electromagnetic waves. An antenna transmits a 1-micros electromagnetic pulse at a central frequency of 2.45 GHz in a high-Q cavity. Another antenna records the strongly reverberated signal. The time-reversed wave is built and transmitted back by the same antenna acting now as a time-reversal mirror. The wave is found to converge to its initial source and is compressed in time. The quality of focusing is determined by the frequency bandwidth and the spectral correlations of the field within the cavity.

Revisiting iterative time reversal processing: application to detection of multiple targets

The iterative time reversal processing represents a high speed and easy way to self-focus on the strongest scatterer in a multitarget medium. However, finding weaker scatterers is a more difficult task that can be solved by computing the eigenvalue and eigenvector decomposition of the time reversal operator, the so-called DORT method. Nevertheless, as it requires the measurement of the complete interelements response matrix and time-consuming computation, the separation of multiple targets may not be achieved in real time. In this study, a new real time technique is proposed for multitarget selective focusing that does not require the experimental acquisition of the time reversal operator. This technique achieves the operator decomposition using a particular sequence of filtered waves propagation instead of computational power. Due to its simplicity of implementation, this iterative process can be achieved in real time. This high speed selective focusing is experimentally demonstrated by detecting targets through a heterogeneous medium and in a speckle environment. A theoretical analysis compares this technique to the DORT formalism.

Real time inverse filter focusing through iterative time reversal

In order to achieve an optimal focusing through heterogeneous media we need to build the inverse filter of the propagation operator. Time reversal is an easy and robust way to achieve such an inverse filter in nondissipative media. However, as soon as losses appear in the medium, time reversal is not equivalent to the inverse filter anymore. Consequently, it does not produce the optimal focusing and beam degradations may appear. In such cases, we showed in previous works that the optimal focusing can be recovered by using the so-called spatiotemporal inverse filter technique. This process requires the presence of a complete set of receivers inside the medium. It allows one to reach the optimal focusing even in extreme situations such as ultrasonic focusing through human skull or audible sound focusing in strongly reverberant rooms. But, this technique is time consuming and implied fastidious numerical calculations. In this paper we propose a new way to process this inverse filter focusing technique in real time and without any calculation. The new process is based on iterative time reversal process. Contrary to the classical inverse filter technique, this iteration does not require any computation and achieves the inverse filter in an experimental way using wave propagation instead of computational power. The convergence from time reversal to inverse filter during the iterative process is theoretically explained. Finally, the feasibility of this iterative technique is experimentally demonstrated for ultrasound applications.

Time-Reversal Acoustics in Biomedical Engineering

Time reversal is a very powerful method for focusing through complex and heterogeneous media and shows very promising results in biomedical applications. In this paper, we review some of the main applications investigated during the past decade. An iterative implementation of the time-reversal process allows tracking gallstones in real time during lithotripsy treatments. In this application domain, a smart exploitation of the reverberations in solid waveguides permits the focusing of high-amplitude ultrasonic shock waves with a small number of transducers. Finally, because time reversal is able to correct the strong distortions induced by the skull bone on ultrasonic propagation, this adaptive focusing technique is very promising for ultrasonic hyperthermia brain therapy.

[Imported myiasis: 7 cases of cutaneous parasitism caused by Dermatobia hominis flie larvas]

Myiasis is the parasitism of organs and tissues of warm-blooded vertebrates by flies larvae. D hominis is a flie geographically restricted to tropical America from Mexico to northern Argentina. The adult flie, which is not hematophagous, needs to put its eggs on the abdominal surface of hematophagous arthropods which serve as carriers of future larvae which are deposited on the skin of the hosts (mammals, birds and accidentally men) when biting. Seven patients (two females) aged 7 to 35 years old, of different nationalities, recalled receiving mosquito bites, after staying in tropical American areas in the previous forty days. They presented furuncle-like lesions in exposed surfaces of the body. These lesions, 2-3 cm long, pruritic and mildly tender, broke and released a serous or serohematic fluid. Through the resulting opening, it was possible to partially observe the larva. Larvae were extracted by manual pressure (4) or surgical incision (3) and identified as D hominis larvae. Diagnosis of dermatobiasis, an imported myiasis, must be based on the characteristics of lesions and the previous residence in endemic areas of America.

Community- and hospital-acquired respiratory syncytial virus infections in Chile

Nasopharyngeal aspirates were obtained on admission from 614 patients younger than 2 years of age who were hospitalized in a ward for acute respiratory infections from June 1988 through October, 1989, in Santiago, Chile. Patients in two rooms were followed during the cold seasons by sampling aspirates every other day during the child's entire hospital stay. Clinical features were recorded daily. Indirect monoclonal immunofluorescent assay and isolation in HEp-2 were used for respiratory syncytial virus (RSV) diagnosis. The mean RSV detection rate was 39% at the time of admission, ranging from 8% in April, 1989, to 62% in July, 1988. During the cold months 43 of 288 (15%) nosocomial RSV cases were detected. Pneumonia and wheezing bronchitis were the principal diagnoses of both groups admitted, whether they were shedding RSV or not. It is concluded that RSV plays a major role in admissions for acute respiratory infections, as well as in nosocomial infections, in Santiago. Because clinical features do not allow one to differentiate viral from bacterial acute respiratory infections, the importance of rapid viral diagnosis is emphasized.