Loss Of Temporal Resolution Research Articles

Quantitative image reconstruction for microscopy with correct sub-pixel photon statistics.

The diffraction of light forces a lower limit on spatial resolution available with conventional optical microscopes. However, sophisticated computational techniques in conjunction with inhomogeneous illumination have facilitated image acquisition without this limitation. Typical image reconstruction algorithms for such superresolution microscopy techniques, including structured illumination microscopy (SIM), involve combining multiple diffraction-limited raw images in Fourier space via minimization of some cost function. This leads to undesired artifacts including loss of correct noise properties for the photons collected by the camera and loss of temporal resolution. Here, we propose a Bayesian framework performing reconstructions in real space while incorporating all sources of noise such as camera electronics and incoming photon Poisson statistics. Furthermore, this framework inherently allows corrections for inhomogeneous illumination incident on a biological sample. As a result, superresolution reconstructions can now be performed with fewer images otherwise needed by existing reconstruction methods while maintaining Poisson statistics for collected photons at the sub-pixel level. We benchmark our framework using both simulated and experimentally generated images.

Concurrent CBF and BOLD fMRI with dual-echo spiral simultaneous multi-slice acquisitions at 7T

Measurement of cerebral blood flow (CBF) using the Arterial Spin Labeling (ASL) technique is a desirable fMRI approach due to the higher specificity of CBF to the site of neural activation. However, ASL has inherent limitations, such as a low signal-to-noise ratio (SNR) and low coverage/resolution due to the limited readout window following the labeling. Recently, ASL has been implemented at ultra-high field (UHF) strengths in an attempt to mitigate the SNR challenges. Even though ASL intrinsically allows concurrent acquisition of CBF and BOLD contrasts, a compromise in the echo time (TE) for either of the contrasts is inevitable with single-echo acquisitions. Long durations of the Cartesian EPI readout do not allow for multi-echo acquisitions for resolutions ≤2 mm where both contrasts can be acquired at their optimal TE at UHF. With its higher acquisition efficiency, single-shot spiral imaging provides a promising alternative to EPI, and with a dual-echo, out-in trajectory allows both CBF and BOLD contrasts to be acquired at their respective optimal TE. In this work, we implemented a dual-echo spiral out-in ASL sequence with simultaneous multi-slice (SMS) readout for increased coverage, and validated its application to fMRI with a visuomotor paradigm. Conventional Cartesian EPI acquisitions with matched parameters served as a reference. The dual-echo spiral ASL acquisitions resulted in robust CBF and BOLD activations maps. The absolute and relative CBF changes measured with the dual-echo spiral readout were in agreement with previous reports in the literature as well as the reference Cartesian acquisitions. The BOLD response amplitude was higher compared to the Cartesian acquisitions, attributable to a more optimal TE of the second echo. In conclusion, dual-echo spiral out-in SMS acquisition shows promise for concurrent acquisitions of BOLD and non-BOLD contrasts that require a short TE, with no loss in temporal resolution.

Thermodynamics at Very Long Time and Space Scales.

Any observation, and hence concept, is limited by the time and length scale of the observer and his instruments. Originally, we lived on a timescale of minutes and a length scale of meters, give or take an order of magnitude or two. Therefore, we devloped laboratory sized concepts, like volume, pressure, and temperature of continuous media. The past 150 years we managed to observe on the molecular scale and similarly nanoseconds timescale, leading to atomic physics that requires new concepts. In this paper, we are moving in the opposite direction, to extremely large time and length scales. We call this regime “slow time”. Here, we explore which laboratory concepts still apply in slow time and which new ones may emerge. E.g., we find that temperature no longer exists and that a new component of entropy emerges from long time averaging of other quantities. Just as finite-time thermodynamics developed from the small additional constraint of a finite process duration, here we add a small new condition, the very long timescale that results in a loss of temporal resolution, and again look for new structure.

Ultrafast Power Doppler Imaging Using Frame-Multiply-and-Sum-Based Nonlinear Compounding.

Ultrafast power Doppler imaging based on coherent compounding (UPDI-CC) has become a promising technique for microvascular imaging due to its high sensitivity to slow blood flows. However, since this method utilizes a limited number of plane-wave or diverging-wave transmissions for high-frame-rate imaging, it suffers from degraded image quality because of the low contrast resolution. In this article, an ultrafast power Doppler imaging method based on a nonlinear compounding framework, called frame-multiply-and-sum (UPDI-FMAS), is proposed to improve contrast resolution. In UPDI-FMAS, unlike conventional channel-domain delay-multiply-and-sum (DMAS) beamforming, the signal coherence is estimated based on autocorrelation function over plane-wave angle frames. To avoid phase distortion of blood flow signals during the autocorrelation process, clutter filtering is preferentially applied to individual beamformed plane-wave data set. Therefore, only coherent blood flow signals are emphasized, while incoherent background noise is suppressed. The performance of the UPDI-FMAS was evaluated with simulation, phantom, and in vivo studies. For the simulation and phantom studies with a constant laminar flow, the UPDI-FMAS showed improvements in the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) to those of UPDI-CC, i.e., over 10 and 7 dB for 13 plane waves, respectively, and the performances were improved as the number of plane waves increased. Moreover, the enhancement of the image quality due to the increased SNR and CNR in UPDI-FMAS was more clearly depicted with the in vivo study, in which a human kidney and a tumor-bearing mouse were evaluated. These results indicate that the FMAS compounding can improve the image quality of UPDI for microvascular imaging without loss of temporal resolution.

Split-bolus single-pass CT for vascular complications in acute pancreatitis: assessment of radiation dose and multi-phasic contrast enhancement compared to single-bolus multi-pass CT

To assess vascular contrast enhancement and radiation dose of split-bolus single-pass computed tomography (CT) compared to single-bolus multi-pass CT for acquiring a multi-phasic study. Patients who underwent CT for acute pancreatitis were included retrospectively. Thirty consecutive patients scanned with a split-bolus protocol were compared to 30 consecutive patients scanned with a single-bolus protocol. Data were collected on attenuation measurements (aorta, portal vein and spleen) and images were assessed for subjective vascular enhancement quality and splenic homogeneity. Radiation dose was measured by dose-length product (DLP). There was no significant difference in the aortic (p=0.88) or portal vein (p=0.35) attenuation values between the two groups. The percentage of examinations reaching the target aortic and portal attenuation in the split-bolus group were 96.7% and 93.3%, and in the single-bolus group were 96.7% and 85.7%, respectively. The mean DLP was 492 mGy.cm for the single-bolus group and 940 mGy.cm for the split-bolus group (p < 0.0001). Subjective assessment revealed higher rates of splenic heterogeneity in the split-bolus group. In acute pancreatitis, split-bolus imaging can produce arterial and venous enhancement comparable to a multi-pass technique with a significant reduction in radiation dose. Loss of temporal resolution and increased splenic heterogeneity are the main disadvantages. The low prevalence of pseudoaneurysms favours the lower-dose imaging technique.

Psi-PIV: a novel framework to study unsteady microfluidic flow

In microscopic particle image velocimetry (micro-PIV), correlation averaging over multiple frames is often required, leading to a loss in temporal resolution, therefore limiting the measurement accuracy for unsteady flows. Here, we present a new PIV method suitable to study steady and unsteady laminar flows between parallel plates (i.e., Hele-Shaw flow), which is a common flow configuration in microfluidic applications. Our method reduces the effective seeding density and yields similar if not higher signal-to-noise ratio (SNR) compared to conventional micro-PIV. We call this algorithm Psi-PIV. Psi-PIV requires a much smaller number of frames to reach the same SNR compared to the widely used correlation averaging method. This leads to a significant improvement of the temporal resolution. The Psi-PIV algorithm is used in an experimental investigation of steady and unsteady flows in a Hele-Shaw cell. Our experiment shows that Psi-PIV reduces the number of required frames by 8 times and 30 times compared to the frames required by conventional PIV for steady and unsteady laminar flow, respectively. In this study, PIV and Psi-PIV use a single-pass cross-correlation to present the underlying difference between the two approaches.Graphic abstract

Fast Multi-Focus Ultrasound Image Recovery Using Generative Adversarial Networks

In conventional ultrasound (US) imaging, it is common to transmit several focused beams at multiple locations to generate a multi-focus image with constant lateral resolution throughout the image. However, this method comes at the expense of a loss in temporal resolution, which is important in applications requiring both high-frame rate and constant lateral resolution. Moreover, relative motions of the target with respect to the probe often exist due to hand tremors or biological motions, causing blurring artifacts in the multi-focus image. This article introduces a novel approach for multi-focus US image recovery based on Generative Adversarial Network (GAN) without a reduction in the frame-rate. Herein, a mapping function between the single-focus US image and multi-focus version for having a constant lateral resolution everywhere is estimated through different GANs. We use adversarial loss functions in addition to Mean Square Error (MSE) to generate more realistic ultrasound images. Moreover, we use the boundary seeking method for improving the stability of training, which is currently the main challenge in using GANs. Experiments on simulated and real phantoms as well as on ex vivo data are performed. Results confirm that having both adversarial loss function and boundary seeking training provides better results in terms of the mean opinion score test. Furthermore, the proposed method enhances the resolution and contrast indexes without sacrificing the frame-rate. As for the comparison with other approaches which are not based on NNs, the proposed approach gives similar results while requiring neither channel data nor computationally expensive algorithms.

4D flow imaging with UNFOLD in a reduced FOV.

Two-dimensional selective excitation (2DRF) allows shortening 4Dflow scan times by reducing the FOV, but the longer 2DRF pulse duration decreases the temporal resolution, yielding underestimated peak flow values. Multiple k-space lines per cardiac phase, nl ≥ 2, are commonly applied in 4Dflow MRI to shorten the inherent long scan times. We demonstrate that 2DRF 4D flow with nl ≥ 2 can be easily combined with UNFOLD (UNaliasing by Fourier-encoding the Overlaps using the temporaL Dimension), a technique that allows regaining nominally the temporal resolution of the respective acquisition with nl = 1, to assure peak flow quantification. Two different 2DRF pulses with spiral k-space trajectories were designed and integrated into a 4Dflow sequence. Flow phantom experiments and 7 healthy control 4Dflow in vivo measurements, with and without UNFOLD reconstructions, were compared with conventional reconstruction and 1D slab-selective excitation(1DRF) by evaluating time-resolved flow curves, peak flow, peak velocity, blood flow volume per cardiac cycle, and spatial aliasing. Applying UNFOLD to 4Dflow imaging with 2DRF and reduced FOV increased the quantified in vivo peak flow values significantly by 3.7% ± 2.3% to 5.2% ± 2.4% (P < .05). Accordingly, the peak flow underestimation of 2DRF scans compared with conventional 1DRF scans decreased with UNFOLD. Finally, 2DRF combined with UNFOLD accelerated the 4Dflow acquisition 3.5 ± 1.4 fold by reducing the FOV and increasing the effective temporal resolution by 6.7% compared with conventional 1D selective excitation, with 2 k-space lines per cardiac phase. Two-dimensional selective excitation combined with UNFOLD allows limiting the FOV to shorten 4Dflow scan times and compensates for the loss in temporal resolution with 2DRF (Δt = 64.8 ms) compared with 1DRF (Δt = 43.2 ms), yielding an effective resolution of Δteff = 40.5 ms to enhance peak flow quantification.

On Aethalometer measurement uncertainties and an instrument correction factor for the Arctic

Abstract. Several types of filter-based instruments are used to estimate aerosol light absorption coefficients. Two significant results are presented based on Aethalometer measurements at six Arctic stations from 2012 to 2014. First, an alternative method of post-processing the Aethalometer data is presented, which reduces measurement noise and lowers the detection limit of the instrument more effectively than boxcar averaging. The biggest benefit of this approach can be achieved if instrument drift is minimised. Moreover, by using an attenuation threshold criterion for data post-processing, the relative uncertainty from the electronic noise of the instrument is kept constant. This approach results in a time series with a variable collection time (Δt) but with a constant relative uncertainty with regard to electronic noise in the instrument. An additional advantage of this method is that the detection limit of the instrument will be lowered at small aerosol concentrations at the expense of temporal resolution, whereas there is little to no loss in temporal resolution at high aerosol concentrations ( &gt; 2.1–6.7 Mm−1 as measured by the Aethalometers). At high aerosol concentrations, minimising the detection limit of the instrument is less critical. Additionally, utilising co-located filter-based absorption photometers, a correction factor is presented for the Arctic that can be used in Aethalometer corrections available in literature. The correction factor of 3.45 was calculated for low-elevation Arctic stations. This correction factor harmonises Aethalometer attenuation coefficients with light absorption coefficients as measured by the co-located light absorption photometers. Using one correction factor for Arctic Aethalometers has the advantage that measurements between stations become more inter-comparable.

Single-shot dual-wavelength interferometric microscopy.

Interferometric microscopy (IM) can provide complex field information of the biological samples with high spatial and temporal resolution with virtually no photodamage. Measuring wavelength-dependent information in particular has a wide range of applications from cell and tissue refractometry to the cellular biophysical measurements. IM measurements at multiple wavelengths are typically associated with a loss in temporal resolution, field of view, stability, sensitivity, and may involve using expensive equipment such as tunable filters or spatial light modulators. Here, we present a novel and simple design for an interferometric microscope that provides single-shot off-axis interferometric measurements at two wavelengths by encoding the two spectral images at two orthogonal spatial frequencies that allows clean separation of information in the Fourier space with no resolution loss. We demonstrated accurate simultaneous quantification of polystyrene bead refractive indices at two wavelengths.

Autocorrelation analysis of high resolution weighing lysimeter time series as a basis for determination of precipitation

AbstractThe determination of precipitation (P) is still a challenge, but central for quantifying soil water and element balances. Time series of mass changes (ΔM) from high precision weighing lysimeter may be used to estimate P if deep drainage rates are determined independently. High temporal resolution, however, is accompanied by problems such as correlated data and noise. The objective was to analyze the temporal autocorrelation (AC) in ΔM time series and to identify temporal resolutions for determining uncorrelated P rates. Minute‐based time series of ΔM are analyzed; the data have been recorded at the UMS Science Lysimeters that are located in Dedelow (northeast Germany) as part of the TERENO SoilCan lysimeter network. Periods in 2012 and 2013 were selected in which the wind speed was below 6 ms−1. Data noise‐correction was carried out by using a moving average before the ΔM values cumulated over 60, 30, and 10 min intervals were compared. On a monthly basis, the temporal AC lengths for ΔM were larger in spring (68 min), autumn (62 min), and winter (76 min) than in the summer (23 min). These AC lengths reflected mainly the effect of differences in P‐rates and ‐duration between lower‐intensity rainfall and shorter summer storms. The monthly sums of P based on the 60‐min interval were up to 20% lower than those obtained by using the 10‐min intervals. For P‐values obtained by summing up the ΔM over periods shorter than the autocorrelation length, oscillated fluctuations in ΔM did not cancel out within an interval. The temporal autocorrelation in the highly‐resolved lysimeter data limited the evaluation of ΔM time series. Compared to rain gauge data, the P‐rates obtained from the weighing lysimeters were generally higher. However, this difference decreased when increasing the time interval for cumulating mass changes. Cumulated positive ΔM values based on time intervals larger than the AC length (e.g., 60 min) provided an optimal approximation of the quantity of P, but on the expense of a loss in temporal resolution limited by the AC lengths. Smoothing could reduce noise in the original lysimeter data; however, not the validity problems that are related to the temporal AC.

Radial simultaneous multi-slice CAIPI for ungated myocardial perfusion

ObjectiveSimultaneous multi-slice (SMS) imaging is a slice acceleration technique that acquires multiple slices in the same time as a single slice. Radial controlled aliasing in parallel imaging results in higher acceleration (radial CAIPIRINHA or CAIPI) is a promising SMS method with less severe slice aliasing artifacts as compared to its Cartesian counterpart. Here we use radial CAIPI with data undersampling and constrained reconstruction to improve the utility of ungated cardiac perfusion acquisitions. We test the proposed framework with a traditional saturation recovery fast low-angle shot (turboFLASH) sequence and also without saturation recovery as a steady-state spoiled gradient echo (SPGR) sequence on animal and human studies. MethodsSimulations and phantom studies were performed for both the turboFLASH and the SPGR radial CAIPI methods. Ungated undersampled golden ratio radial CAIPI data with saturation recovery were acquired in 8 dogs and 2 human subjects. The CAIPI data without saturation pulses were acquired in 4 human subjects. For both methods, slice acceleration factors of two and three were used. A new spatio-temporal reconstruction using total variation and patch-based low rank constraints was used to jointly reconstruct the multi-slice multi-coil images. ResultsPhantom scans and computer simulations showed that ungated SPGR generally provides better contrast to noise ratio (CNR) than the saturation recovery sequence if the saturation recovery time is less than 100ms. Both of the ungated radial CAIPI methods demonstrated promising image quality in terms of preserving dynamics of the contrast agent and maintaining anatomical structures, even with three slices acquired simultaneously. ConclusionUngated simultaneous multi-slice acquisitions with either a saturation recovery turboFLASH sequence or a steady-state gradient echo SPGR sequence are feasible and provide increased slice coverage without loss of temporal resolution. Compared with a sensitivity encoding (SENSE) SMS reconstruction, the constrained reconstruction method provides better image quality for undersampled radial CAIPI data.

Predicting long-term temperature increase for time-dependent SAR levels with a single short-term temperature response.

Present a novel method for rapid prediction of temperature in vivo for a series of pulse sequences with differing levels and distributions of specific energy absorption rate (SAR). After the temperature response to a brief period of heating is characterized, a rapid estimate of temperature during a series of periods at different heating levels is made using a linear heat equation and impulse-response (IR) concepts. Here the initial characterization and long-term prediction for a complete spine exam are made with the Pennes' bioheat equation where, at first, core body temperature is allowed to increase and local perfusion is not. Then corrections through time allowing variation in local perfusion are introduced. The fast IR-based method predicted maximum temperature increase within 1% of that with a full finite difference simulation, but required less than 3.5% of the computation time. Even higher accelerations are possible depending on the time step size chosen, with loss in temporal resolution. Correction for temperature-dependent perfusion requires negligible additional time and can be adjusted to be more or less conservative than the corresponding finite difference simulation. With appropriate methods, it is possible to rapidly predict temperature increase throughout the body for actual MR examinations.

Ultrastable AFM: Improved Stability, Precision, and Bandwidth for Bio-AFM

Enhancing the short-term force precision and temporal resolution of atomic force microscopy (AFM) while maintaining excellent long-term stability would result in improved performance across multiple AFM modalities, including single molecule force spectroscopy. However, instrumental drift in AFM remains a critical issue that limits the precision and duration of experiments. Previously, we developed an active optical stabilization technique to improve tip-sample stability at ambient conditions. However, force drift also occurs via uncontrolled deflection of the zero-force position of the cantilever. We found that the primary source of force drift in liquid for a popular class of soft cantilevers is their gold coating, even though they are coated on both sides to minimize drift. While removing the gold led to ∼10-fold reduction in reflected light, we nonetheless achieved a 10-fold improvement in force stability of bioAFM, with a sub-pN force precision over a broad bandwidth (0.01-20 Hz) just 30 minutes after loading. We subsequently extended AFM's sub-pN bandwidth by a factor of ∼50 to span five decades of bandwidth (Δf ≈ 0.01-1,000 Hz by developing an efficient process to modify a short (L = 40 μm) commercially available cantilever (BioLever Mini) with a focused ion beam (FIB). Measurements of mechanically stretching individual proteins showed improved force precision coupled with state-of-the-art force stability and no significant loss in temporal resolution compared to stiffer, unmodified cantilevers. Ongoing work in our lab extends this concept down to ultrashort cantilevers (L =10 μm) along with instrumental development to detect these cantilevers in a commercial AFM. Such cantilevers enable sensitive detection of protein unfolding and refolding with ∼1 μs time resolution. Importantly, these cantilevers were robust and were reused for SFMS over multiple days. Hence, we expect these responsive, yet stable, cantilevers to broadly benefit diverse AFM-based studies.

Landscape Disturbance and Lake Response: Temporal and Spatial Perspectives

Lake sediments are natural archives that record the response of a lake to both in-lake processes and catchment disturbance (over a range of timescales). The response (or lack of) of a lake to external forcing is a function of the severity of the disturbance (and its areal extent) but is also strongly mediated by catchment characteristics, such as slope and hydrological connectivity. Many studies of recent lake disturbance have focussed on anthropogenic disruption of geochemical cycles, e.g. acidification and eutrophication, which because of their “ecological” power appear to over-ride catchment filters. Lake sediments also record the variability of natural disturbance regimes themselves (fire, volcanic ash falls, species invasions, and climate) and the consequent lake response but this aspect has received much less attention. The possibility of using lake sediments to address long-term (102 – 103 yr) interactions between e.g., climate, catchment ontogeny and disturbance events (and their synergies) has not been fully explored, largely because of an over-riding emphasis on climate as a direct driver of ecological change.Traditionally, limnology has also focussed on a few key sites (sentinels) but in response to the development of landscape ecology has progressively embraced a regional approach to understanding how lakes respond to external forcing (climate) and disturbance (“lakes in the landscape”). Although the interaction of a lake with its catchment is implicit, i.e. via hydrological and nutrient loading, palaeolimnologists rarely take an explicitly spatial view of this interaction nor have they considered spatial location (i.e. response of a number of lakes within a lake district). Arguably, the inherent spatial variability of terrestrial disturbance has been ignored by palaeolimnologists, largely as a result of focussing on one core from a single lake.This paper reviews the impact of terrestrial disturbance on lakes but also argues for explicit consideration of space and location in determining the resultant temporal variability of the ecological response. The importance of within-lake spatial heterogeneity is also high-lighted (i.e. the major contribution of the littoral zone to both diversity and production). However, any attempt to determine spatially replicated (i.e. at a regional scale), holistic (i.e. whole lake) responses to disturbance will encounter considerable problems associated with dating, loss of temporal resolution and among site comparison. Despite this, it is clear that recent methodological developments in the area of biomarkers, compound specific stable isotopes coupled with progress in dating (age-models), ecological modelling and statistical analysis offer the possibility of undertaking regionally-replicated studies of lake response to natural disturbance, thereby contributing and expanding our understanding of ecosystem dynamics at a range of spatial and temporal scales.

Improved Single Molecule Force Spectroscopy Using Micromachined Cantilevers

Enhancing the short-term force precision of atomic force microscopy (AFM) while maintaining excellent long-term force stability would result in improved performance across multiple AFM modalities, including single molecule force spectroscopy (SMFS). SMFS is a powerful method to probe the nanometer-scale dynamics and energetics of biomolecules (DNA, RNA, and proteins). The folding and unfolding rates of such macromolecules are sensitive to sub-pN changes in force. Recently, we demonstrated sub-pN stability over a broad bandwidth (Δf = 0.01-16 Hz) by removing the gold coating from a 100 μm long cantilever. However, this stability came at the cost of increased short-term force noise, decreased temporal response, and poor sensitivity. Here, we avoided these compromises while retaining excellent force stability by modifying a short (L = 40 μm) cantilever with a focused ion beam. Our process led to a ∼10-fold reduction in both a cantilever's stiffness and its hydrodynamic drag near a surface. We also preserved the benefits of a highly reflective cantilever while mitigating gold-coating induced long-term drift. As a result, we extended AFM's sub-pN bandwidth by a factor of ∼50 to span five decades of bandwidth (Δf ≈ 0.01-1000 Hz). Measurements of mechanically stretching individual proteins showed improved force precision coupled with state-of-the-art force stability and no significant loss in temporal resolution compared to the stiffer, unmodified cantilever. Finally, these cantilevers were robust and were reused for SFMS over multiple days. Hence, we expect these responsive, yet stable, cantilevers to broadly benefit diverse AFM-based studies.

Fat‐water separation in dynamic objects using an UNFOLD‐like temporal processing

To separate fat and water signals in dynamic imaging. Because important features may be embedded in fat, and because fat may take part in disease processes, separating fat and water signals may be of great importance in a number of clinical applications. This work aims to achieve such separation at nearly no loss in temporal resolution compared to usual, nonseparated acquisitions. In contrast, the well-known 3-point Dixon method may cause as much as a 3-fold reduction in temporal resolution. The proposed approach involves modulating the echo time TE from frame to frame, to force fat signals to behave in a conspicuous manner through time, so they can be readily identified and separated from water signals. The strategy is inspired from the "unaliasing by Fourier encoding the overlaps in the temporal direction" (UNFOLD) method, although UNFOLD involves changes in the sampling function rather than TE, and aims at suppressing aliased material rather than fat. The method was implemented at 1.5 T and 3 T, on cardiac cine and multiframe steady-state free precession sequences. In addition to phantom results, in vivo results from volunteers are presented. Good separation of fat and water signals was achieved in all cases.

Enhanced Background Rejection in Thick Tissue with Differential-Aberration Two-Photon Microscopy

When a two-photon excited fluorescence (TPEF) microscope is used to image deep inside tissue, out-of-focus background can arise from both ballistic and nonballistic excitation. We propose a solution to largely reject TPEF background in thick tissue. Our technique is based on differential-aberration imaging with a deformable mirror. By introducing extraneous aberrations in the excitation beam path, we preferentially quench in-focus TPEF signal while leaving out-of-focus TPEF background largely unchanged. A simple subtraction of an aberrated, from an unaberrated, TPEF image then removes background while preserving signal. Our differential aberration (DA) technique is simple, robust, and can readily be implemented with standard TPEF microscopes with essentially no loss in temporal resolution when using a line-by-line DA protocol. We analyze the performance of various induced aberration patterns, and demonstrate the effectiveness of DA-TPEF by imaging GFP-labeled sensory neurons in a mouse olfactory bulb and CA1 pyramidal cells in a hippocampus slice.

Poor readers of chinese respond slower than good readers in phonological, rapid naming, and interval timing tasks

The role of information processing deficits in poor readers of nonalphabetic scripts such as Chinese is not well documented. Here, we examined perceptual processing in good and poor readers of Chinese. Specifically, two groups of third grade children comprising 20 "good readers" and 19 "poor readers," drawn from a larger pool of 254 students, were tested using an auditory version of the Stop Reaction Time (Stop-RT) interval timing task, a phoneme perception task, and several measures of phonological awareness and orthographic processing. Mean Stop RTs for good readers were significantly faster than those for poor readers, and good readers were also faster in a test of rapid digit naming, but good and poor readers did not differ on measures of phoneme onset awareness or orthographic processing. Although good and poor readers did not differ in overall categorical perception of /pa5/ and /pha5/ stimuli, the good readers responded significantly faster than the poor readers. Taken together, the group differences obtained here are consistent with a slowing of information processing and behavioral output in poor readers of Chinese, rather than a loss of temporal resolution of perceptual processing.

Blood flow velocity profiles in the aortic annulus: A 3-dimensional freehand color flow Doppler imaging study

Background: The use of a single sample volume in Doppler measurements of the velocity time integral (VTI) in the aortic annulus may introduce errors in calculations of stroke volumes, shunts, regurgitant fractions, and aortic valve area. To study the blood flow velocity distribution and assess this potential error, we used a dynamic 3-dimensional color flow Doppler imaging method. Methods and Results: Seventeen healthy volunteers were studied. The ultrasound data were captured from 10 to 20 heartbeats at a high frame rate (mean 57 frames per second) while freely tilting the transducer in the apical position. A magnetic position-sensor system recorded the spatial position and orientation of the probe. The raw digital ultrasound data were analyzed off-line with no loss of temporal resolution. Blood flow velocities were integrated across a spherical surface that tracked the aortic annulus during systole. The ratios of the systolic maximum to the systolic mean VTI ranged from 1.2 to 1.5 (mean 1.4). At the time of systolic peak flow, the ratios of the maximum to the mean velocity ranged from 1.1 to 2.0 (mean 1.5). The location of the maximum velocities and VTI showed individual variation. Conclusion: The blood flow velocity profile was nonuniform. By using a single sample volume in Doppler measurements of the VTI in the aortic annulus, errors ranging from 20% to 50% may be introduced in calculations of stroke volumes. (J Am Soc Echocardiogr 2002;15:328-33.)