Temporal Contrast Research Articles

Contrast Gradient-Based Blood Velocimetry With Computed Tomography: Theory, Simulations, and Proof of Principle in a Dynamic Flow Phantom.

The aim of this study was to introduce a new theoretical framework describing the relationship between the blood velocity, computed tomography (CT) acquisition velocity, and iodine contrast enhancement in CT images, and give a proof of principle of contrast gradient-based blood velocimetry with CT. The time-averaged blood velocity (v(blood)) inside an artery along the axis of rotation (z axis) is described as the mathematical division of a temporal (Hounsfield unit/second) and spatial (Hounsfield unit/centimeter) iodine contrast gradient. From this new theoretical framework, multiple strategies for calculating the time-averaged blood velocity from existing clinical CT scan protocols are derived, and contrast gradient-based blood velocimetry was introduced as a new method that can calculate v(blood) directly from contrast agent gradients and the changes therein. Exemplarily, the behavior of this new method was simulated for image acquisition with an adaptive 4-dimensional spiral mode consisting of repeated spiral acquisitions with alternating scan direction. In a dynamic flow phantom with flow velocities between 5.1 and 21.2 cm/s, the same acquisition mode was used to validate the simulations and give a proof of principle of contrast gradient-based blood velocimetry in a straight cylinder of 2.5 cm diameter, representing the aorta. In general, scanning with the direction of blood flow results in decreased and scanning against the flow in increased temporal contrast agent gradients. Velocity quantification becomes better for low blood and high acquisition speeds because the deviation of the measured contrast agent gradient from the temporal gradient will increase. In the dynamic flow phantom, a modulation of the enhancement curve, and thus alternation of the contrast agent gradients, can be observed for the adaptive 4-dimensional spiral mode and is in agreement with the simulations. The measured flow velocities in the downslopes of the enhancement curves were in good agreement with the expected values, although the accuracy and precision worsened with increasing flow velocities. The new theoretical framework increases the understanding of the relationship between the blood velocity, CT acquisition velocity, and iodine contrast enhancement in CT images, and it interconnects existing blood velocimetry methods with research on transluminary attenuation gradients. With these new insights, novel strategies for CT blood velocimetry, such as the contrast gradient-based method presented in this article, may be developed.

Accelerating ions with high-energy short laser pulses from submicrometer thick targets

Using the example of the PHELIX high-energy short pulse laser we discuss the technical preconditions to investigate ion acceleration with submicrometer thick targets. We show how the temporal contrast of this system was improved to prevent pre-ionization of such targets on the nanosecond timescale. Furthermore the influence of typical fluctuations or uncertainties of the on-target intensity on ion acceleration experiments is discussed. We report how these uncertainties were reduced by improving the assessment and control of the on-shot intensity and by optimizing the positioning of the target into the focal plane. Finally we report on experimental results showing maximum proton energies in excess of 85 MeV for ion acceleration via the target normal sheath acceleration mechanism using target thicknesses on the order of one micrometer.

Generation of high-contrast, joule-level pulses based on Nd:glass chirped pulse amplification laser

We demonstrate a high-contrast, joule-level Nd:glass laser system operating at 0.5 Hz repetition rate based on a double chirped pulse amplification (CPA) scheme. By injecting high-contrast, high-energy seed pulses into the Nd:glass CPA stage, the pulse energy is amplified to 1.9 J through two optical parametric CPA stages and two Nd:glass amplifiers. The temporal contrast of compressed pulse is measured down to the level of$10^{-8}$at tens of ps, and$10^{-10}$near 200 ps before the main pulse, respectively.

The generation of amplified spontaneous emission in high-power CPA laser systems.

An analytical model is presented describing the temporal intensity contrast determined by amplified spontaneous emission in high‐intensity laser systems which are based on the principle of chirped pulse amplification. The model describes both the generation and the amplification of the amplified spontaneous emission for each type of laser amplifier. This model is applied to different solid state laser materials which can support the amplification of pulse durations ≤350 fs . The results are compared to intensity and fluence thresholds, e.g. determined by damage thresholds of a certain target material to be used in high‐intensity applications. This allows determining if additional means for contrast improvement, e.g. plasma mirrors, are required for a certain type of laser system and application. Using this model, the requirements for an optimized high‐contrast front‐end design are derived regarding the necessary contrast improvement and the amplified “clean” output energy for a desired focussed peak intensity. Finally, the model is compared to measurements at three different high‐intensity laser systems based on Ti:Sapphire and Yb:glass. These measurements show an excellent agreement with the model.

Generation of 25-TW Femtosecond Laser Pulses at 515 nm with Extremely High Temporal Contrast

We report on the frequency doubling of femtosecond laser pulses at 1030 nm center wavelength generated from the fully diode-pumped laser system POLARIS. The newly generated pulses at a center wavelength of 515 nm have a pulse energy of 3 J with a pulse duration of 120 fs. On the basis of initially ultra-high contrast seed pulses we expect a temporal intensity contrast better 10 17 200 ps before the peak of the main pulse. We analyzed the temporal intensity contrast from milliseconds to femtoseconds with a dynamic range covering more than 20 orders of magnitude. The pulses were focussed with a f/2-focussing parabola resulting in a peak intensity exceeding 10 20 W / cm 2 . The peak power and intensity are to the best of our knowledge the highest values for 515 nm-laser-pulses achieved so far.

Optimized 4D time-of-flight MR angiography using saturation pulse.

To assess arterial visibility on 4D time-of-flight (4D-TOF) by temporal magnetization transfer contrast pulse (t-MTC) and temporal tilted optimized nonsaturating excitation (t-TONE). 3D-TOF magnetic resonance angiography (MRA) is used for the noninvasive assessment of the intracranial arteries. However, it does not provide temporal information for diagnosing hemodynamics. To noninvasively obtain more detailed hemodynamics-related information, we developed a novel time-resolved MRA without the arterial spin labeling technique, termed 4D-TOF MRA using saturation pulse. On a 3.0T MRI, three techniques were compared to optimize the visibility of the arteries above the circle of Willis; 1) simple 4D-TOF, 2) 4D-TOF with t-MTC, and 3) 4D-TOF with t-MTC and t-TONE. Eight healthy volunteers were scanned with these three sequences. The contrast changes between the background tissue and the arteries in temporal phases were assessed and compared quantitatively and qualitatively. The contrast between the background and the arteries for 4D-TOF with t-MTC and t-TONE was significantly higher than those for the other methods in delayed phases (P < 0.001).The qualitative assessment showed that 4D-TOF with t-MTC and with t-MTC and t-TONE provided better visualization of the intracranial artery than simple 4D-TOF (P < 0.001). 4D-TOF with t-TONE and t-MTC enabled observation of the intracranial hemodynamics. Optimized 4D-TOF provides high-quality images without image subtraction, and good visibility of the intracranial arteries even in the prolonged observation time. J. Magn. Reson. Imaging 2016;43:1320-1326.

Is High Temporal Resolution Achievable for Paediatric Cardiac Acquisitions during Several Heart Beats? Illustration with Cardiac Phase Contrast Cine-MRI.

BackgroundDuring paediatric cardiac Cine-MRI, data acquired during cycles of different lengths must be combined. Most of the time, Feinstein’s model is used to project multiple cardiac cycles of variable lengths into a mean cycle.ObjectiveTo assess the effect of Feinstein projection on temporal resolution of Cine-MRI.Methods1/The temporal errors during Feinstein’s projection were computed in 306 cardiac cycles fully characterized by tissue Doppler imaging with 6-phase analysis (from a population of 7 children and young adults). 2/The effects of these temporal errors on tissue velocities were assessed by simulating typical tissue phase mapping acquisitions and reconstructions. 3/Myocardial velocities curves, extracted from high-resolution phase-contrast cine images, were compared for the 6 volunteers with lowest and highest heart rate variability, within a population of 36 young adults.Results1/The mean of temporal misalignments was 30 ms over the cardiac cycle but reached 60 ms during early diastole. 2/During phase contrast MRI simulation, early diastole velocity peaks were diminished by 6.1 cm/s leading to virtual disappearance of isovolumic relaxation peaks. 3/The smoothing and erasing of isovolumic relaxation peaks was confirmed on tissue phase mapping velocity curves, between subjects with low and high heart rate variability (p = 0.05).ConclusionsFeinstein cardiac model creates temporal misalignments that impair high temporal resolution phase contrast cine imaging when beat-to-beat heart rate is changing.

Highly efficient nonlinear ellipse rotation in bulk SrTiO3 crystal and its applications

We investigated the highly efficient, nonlinear ellipse rotation (NER) effect in bulk SrTiO3 crystal, which provides practical application as a pulse cleaner for ultrashort pulses. A highly stable, ultrafast NER effect was realized by using a bulk SrTiO3 crystal with a fast response time, strong optical Kerr effect, high damage threshold, and wide transmission wavelength range. Transmissivity higher than 30% was obtained at a level of several mJ, in which the temporal contrast can be enhanced by about three orders of magnitude. This provides a simple and feasible technology for improving the temporal contrast for the ultrahigh power laser system.

Monocular and binocular mechanisms mediating flicker adaptation

Flicker adaptation reduces subsequent temporal contrast sensitivity. Recent studies show that this adaptation likely results from neural changes in the magnocellular visual pathway, but whether this adaptation occurs at a monocular or a binocular level, or both, is unclear. Here, two experiments address this question. The first experiment exploits the observation that flicker adaptation is stronger at higher than lower temporal frequencies. Observers’ two eyes adapted to 3Hz flicker with an incremental pulse at 1/4 duty cycle, either in-phase or out-of-phase in the two eyes. At the binocular level, the flicker rate was 6Hz in the out-of-phase condition if the two eyes’ pulse trains sum. Similar sensitivity reduction was found in both phase conditions, as expected for independent monocular adapting mechanisms. The second experiment tested for interocular transfer of adaptation between eyes. Results showed that (1) flicker adaptation was strongest with adapting and test fields in only the same eye, (2) adaptation can be partially transferred interocularly with adaptation in only the opposite eye, and (3) adaptation was weakened when both eyes were adapted simultaneously at different contrasts, compared to test-eye adaptation alone. Taken together, the findings are consistent with mechanisms of flicker adaptation at both the monocular and binocular level.

Monitoring capillary blood flow using laser speckle contrast analysis with spatial and temporal statistics

Laser Speckle Contrast Imaging (LSCI) is a simple and powerful technique that is used for full-field imaging of blood flow. The technique analyzes fluctuations in a dynamic speckle pattern to detect the movement of particles. In this article Laser speckle contrast analysis based on spatial and temporal correlations (sLASCA and tLASCA) were used to provide a velocity map of the area of interest in real time without the need for scanning. Spatial and temporal laser speckle contrast analysis have been shown to be a viable real-time method for detecting variations in blood flow. The results of some initial experiments on volunteers are presented. Both the techniques namely sLASCA and tLASCA are able to pick up changes in blood flow, which may arise due to various physiological conditions. Further, more blood flow reflected due to less skin thickness was sensed by the tLASCA technique which can be put to good use in the follow-up of the treatment of certain diseases which may cause decreased blood flow. Thus, both the techniques can be used to compare differences in blood flow values at region in normal subjects and in diseased subjects.

Science and applications of the coherent amplifying network (CAN) laser

Accelerators are today in every walk of science and life [1]. High intensity lasers drive frontiers of contemporary science. Lasers in the TW and PW regime have the potential to replace conventional accelerators with the distinct advantage to be dramatically shorter by a factor of a thousand or more [2]. For instance, electrons are accelerated to few GeV over only few centimeters, representing three to fours orders of magnitude higher accelerating gradients than traditional RF-based accelerators can offer. The approach was proposed in 1979 [3], where a strong laser pulse [4], moving in a plasma creates a wake in which electrons are trapped and violently accelerated. In addition, under ultra high intensity, high energy protons over 100 MeV have been demonstrated as well as high energy radiation greater that MeV [5]. Key to laser-driven accelerators, ion, X-Ray, or Gamma-Ray production are ultra high peak power lasers at the petawatt level [6]. However, current petawatt laser exhibits low repetition rates (state-of-the-art is about 1 Hz) due to thermo-optical character in their gain medium, resulting in low average powers in the 50 W range. In addition, a rather poor wall-plug efficiency (electrical power to optical power) of 10−3 % avoids any scaling perspectives. Hence, state-of-the-art high peak power laser system cannot pretend to be tomorrow's replacement to conventional RF Technology –a new class of ultrafast lasers is urgently needed. Under the ICFA-ICUIL [2] initiative, laser experts in the field of particle acceleration and high intensity lasers defined target parameters of a future laser system should deliver to pave the way for a new kind of accelerator technology revolutionizing fundamental science and applications. The following laser parameters are envisaged for what could be a future linear e–e+ collider: peak power in the PW regime, defined by a 10's of Joules of pulse energy and an ultrashort pulse duration below 50 fs, in combination with an unparalleled average power exceeding 100 kW even exceeding the megawatt level, implying repetition rates of >10 kHz. These extreme parameters should be contained in a beam of excellent spatial quality, featuring outstanding temporal stability and temporal contrast. An excellent wall-plug efficiency of >30% is an essential condition that such average powers are realized in a cost effective, economic and compact way. Overall, any known laser technology known today faces severe issues, with current performance orders of magnitude below these target parameters. Inspired by these ground-breaking challenges under the Eu leadership, the International Coherent Amplification Network (ICAN) group was formed. It combines the complementary expertise of science authorities in the field of high performance fiber amplifiers, theoretical and applied optics of optical systems and finally ultra high intensity lasers. ICAN aspired to study the fundamentals of interferometric amplification i.e. spatially separated amplification followed by coherent addition, of ultrashort laser pulses as the underlying concept of a breakthrough in laser physics. In detail ICAN has studied: 1) average/peak power and efficiency limits of coherently combined ultrafast laser systems 2) synchronization, spatial and temporal recombination of a large number of fibers amplifiers 3) temporal and spatial beam quality, combining efficiency of coherent addition, amplitude and phase stability as a function of the number of fibers and their individual performance 4) reduction of pulse duration and manipulation of pulse shape.

A Dynamic Vision Sensor With 1% Temporal Contrast Sensitivity and In-Pixel Asynchronous Delta Modulator for Event Encoding

A dynamic vision sensor (DVS) encodes temporal contrast (TC) of light intensity into address-events that are asynchronously transmitted for subsequent processing. This paper describes a DVS with improved TC sensitivity and event encoding. To enhance the TC sensitivity, each pixel employs a common-gate photoreceptor for low output noise and a capacitively-coupled programmable gain amplifier for continuous-time signal amplification without sacrificing the intra-scene dynamic range. A proposed in-pixel asynchronous delta modulator (ADM) better preserves signal integrity in event encoding compared with self-timed reset (STR) used in previous DVSs. A 60 $\times$ 30 prototype sensor array with a 31.2 $~\mu\hbox{m}$ pixel pitch was fabricated in a 1P6M 0.18 $~\mu\hbox{m}$ CMOS technology. It consumes 720 $~\mu\hbox{W}$ at a 100k event/s output rate. Measurements show that a 1% TC sensitivity with a 35% relative standard deviation is achieved and that the in-pixel ADM is up to 3.5 times less susceptible to signal loss than STR in terms of event number. These improvements can facilitate the application of DVSs in areas like optical neuroimaging which is demonstrated in a simulated experiment.

OPCPA front end and contrast optimization for the OMEGA EP kilojoule, picosecond laser

OMEGA EP is a large-scale laser system that combines optical parametric amplification and solid-state laser amplification on two beamlines to deliver high-intensity, high-energy optical pulses. The temporal contrast of the output pulse is limited by the front-end parametric fluorescence and other features that are specific to parametric amplification. The impact of the two-crystal parametric preamplifier, pump-intensity noise, and pump-signal timing is experimentally studied. The implementation of a parametric amplifier pumped by a short pump pulse before stretching, further amplification, and recompression to enhance the temporal contrast of the high-energy short pulse is described.

Temporal-contrast measurements of a white-light-seeded noncollinear optical parametric amplifier

Ultra-intense optical parametric chirped-pulse systems require front ends with broad bandwidth and high temporal contrast. Temporal cross-correlation measurements of a white-light–seeded noncollinear optical parametric amplifier (NOPA) show that its prepulse contrast exceeds the 120 dB dynamic range of the broadband NOPA-based cross-correlator.

Monocular and binocular mechanisms mediating flicker adaptation

Flicker adaptation reduces subsequent temporal contrast sensitivity. Recent studies show that this adaptation likely results from neural changes in the magnocellular visual pathway, but whether this adaptation occurs at a monocular or a binocular level, or both, is unclear. Here, two experiments address this question. The first experiment exploits the observation that flicker adaptation is stronger at higher than lower temporal frequencies. Observers' two eyes adapted to 3Hz flicker with an incremental pulse at 1/4 duty cycle, either in-phase or out-of-phase in the two eyes. At the binocular level, the flicker rate was 6Hz in the out-of-phase condition if the two eyes' pulse trains sum. Similar sensitivity reduction was found in both phase conditions, as expected for independent monocular adapting mechanisms. The second experiment tested for interocular transfer of adaptation between eyes. Results showed that (1) flicker adaptation was strongest with adapting and test fields in only the same eye, (2) adaptation can be partially transferred interocularly with adaptation in only the opposite eye, and (3) adaptation was weakened when both eyes were adapted simultaneously at different contrasts, compared to test-eye adaptation alone. Taken together, the findings are consistent with mechanisms of flicker adaptation at both the monocular and binocular level.

Quantum analysis of optical parametric fluorescence in the optical parametric amplification process

The temporal contrast of the ultra-intense laser pulse can be degraded by optical parametric fluorescence (OPF) in high-gain and pulse-pumped optical parametric amplification (OPA). However, to the best of our knowledge, no theory is proposed to describe the quantum noise in such conditions from the first principle. In this paper, we propose a theory based on the quantization of energy flux and a linearization method for investigating both lasers and the OPF in high-gain and pulse-pumped OPA. Following the proposal, the amplification of laser is consistent with classical nonlinear optics. Moreover, an analytical formula of OPF energy is obtained under undepleted and quasi-monochromatic pump conditions. A simplified formula is also obtained under the assumption of small spectral width and acceptant solid angle. Furthermore, a formula of the OPF duration is obtained with a Gaussian pump pulse. Excellent consistency is achieved between previous experiments and our theory.

The Effect of Cataract on Early Stage Glaucoma Detection Using Spatial and Temporal Contrast Sensitivity Tests.

BackgroundTo investigate the effect of cataract on the ability of spatial and temporal contrast sensitivity tests used to detect early glaucoma.MethodsTwenty-seven glaucoma subjects with early cataract (mean age 60 ±10.2 years) which constituted the test group were recruited together with twenty-seven controls (cataract only) matched for age and cataract type from a primary eye care setting. Contrast sensitivity to flickering gratings at 20 Hz and stationary gratings with and without glare, were measured for 0.5, 1.5 and 3 cycles per degree (cpd) in central vision. Perimetry and structural measurements with the Heidelberg Retinal Tomograph (HRT) were also performed.ResultsAfter considering the effect of cataract, contrast sensitivity to stationary gratings was reduced in the test group compared with controls with a statistically significant mean difference of 0.2 log units independent of spatial frequency. The flicker test showed a significant difference between test and control group at 1.5 and 3 cpd (p = 0.019 and p = 0.011 respectively). The percentage of glaucoma patients who could not see the temporal modulation was much higher compared with their cataract only counterparts. A significant correlation was found between the reduction of contrast sensitivity caused by glare and the Glaucoma Probability Score (GPS) as measured with the HRT (p<0.005).ConclusionsThese findings indicate that both spatial and temporal contrast sensitivity tests are suitable for distinguishing between vision loss as a consequence of glaucoma and vision loss caused by cataract only. The correlation between glare factor and GPS suggests that there may be an increase in intraocular stray light in glaucoma.

Enhancement of temporal contrast of high-power laser pulses in an anisotropic medium with cubic nonlinearity

We consider the methods for enhancing the temporal contrast of super-high-power laser pulses, based on the conversion of radiation polarisation in a medium with cubic nonlinearity. For a medium with weak birefringence and isotropic nonlinearity, we propose a new scheme to enhance the temporal contrast. For a medium with anisotropic nonlinearity, the efficiency of the temporal contrast optimisation is shown to depend not only on the spatial orientation of the crystal and -integral, but also on the type of the crystal lattice symmetry.

Comprehensive description of the Orion laser facility

The Orion laser facility at the atomic weapons establishment (AWE) in the UK has been operational since April 2013, fielding experiments that require both its long and short pulse capability. This paper provides a full description of the facility in terms of laser performance, target systems and diagnostics currently available. Inevitably, this is a snapshot of current capability—the available diagnostics and the laser capability are evolving continuously. The laser systems consist of ten beams, optimised around 1 ns pulse duration, which each provide a nominal 500 J at a wavelength of 351 nm. There are also two short pulse beams, which each provide 500 J in 0.5 ps at 1054 nm. There are options for frequency doubling one short pulse beam to enhance the pulse temporal contrast. More recently, further contrast enhancement, based on optical parametric amplification (OPA) in the front end with a pump pulse duration of a few ps, has been installed. An extensive suite of diagnostics are available for users, probing the optical emission, x-rays and particles produced in laser-target interactions. Optical probe diagnostics are also available. A description of the diagnostics is provided.

High-speed incoming infrared target detection by fusion of spatial and temporal detectors.

This paper presents a method for detecting high-speed incoming targets by the fusion of spatial and temporal detectors to achieve a high detection rate for an active protection system (APS). The incoming targets have different image velocities according to the target-camera geometry. Therefore, single-target detector-based approaches, such as a 1D temporal filter, 2D spatial filter and 3D matched filter, cannot provide a high detection rate with moderate false alarms. The target speed variation was analyzed according to the incoming angle and target velocity. The speed of the distant target at the firing time is almost stationary and increases slowly. The speed varying targets are detected stably by fusing the spatial and temporal filters. The stationary target detector is activated by an almost zero temporal contrast filter (TCF) and identifies targets using a spatial filter called the modified mean subtraction filter (M-MSF). A small motion (sub-pixel velocity) target detector is activated by a small TCF value and finds targets using the same spatial filter. A large motion (pixel-velocity) target detector works when the TCF value is high. The final target detection is terminated by fusing the three detectors based on the threat priority. The experimental results of the various target sequences show that the proposed fusion-based target detector produces the highest detection rate with an acceptable false alarm rate.