Pulse Contrast Enhancement Research Articles

Turbo spin-echo-based enhanced acceleration-selective arterial spin labeling without electrocardiography or peripheral pulse unit triggering and contrast enhancement for lower extremity MRA

PurposeLower extremity magnetic resonance angiography (MRA) without electrocardiography (ECG) or peripheral pulse unit (PPU) triggering and contrast enhancement is beneficial for diagnosing peripheral arterial disease (PAD) while avoiding synchronization failure and nephrogenic systemic fibrosis. This study aimed to compare the diagnostic performance of turbo spin-echo-based enhanced acceleration-selective arterial spin labeling (eAccASL) (TSE-Acc) of the lower extremities with that of turbo field-echo-based eAccASL (TFE-Acc) and triggered angiography non-contrast enhanced (TRANCE). MethodsNine healthy volunteers and a patient with PAD were examined on a 3.0 Tesla magnetic resonance imaging (MRI) system. The artery-to-muscle signal intensity ratio (SIR) and contrast-to-noise ratio (CNR) were calculated. The arterial visibility (1: poor, 4: excellent) and artifact contamination (1: severe, 4: no) were independently assessed by two radiologists. Phase-contrast MRI and digital subtraction angiography were referenced in a patient with PAD. Friedman's test and a post-hoc test according to the Bonferroni-adjusted Wilcoxon signed-rank test were used for the SIR, CNR, and visual assessment. p < 0.05 was considered statistically significant. ResultsNo significant differences in nearly all the SIRs were observed among the three MRA methods. Higher CNRs were observed with TSE-Acc than those with TFE-Acc (anterior tibial artery, p = 0.014; peroneal artery, p = 0.029; and posterior tibial artery, p = 0.014) in distal arterial segments; however, no significant differences were observed upon comparison with TRANCE (all p > 0.05). The arterial visibility scores exhibited similar trends as the CNRs. The artifact contamination scores with TSE-Acc were significantly lower (but within an acceptable level) compared to those with TFE-Acc. In the patient with PAD, the sluggish peripheral arteries were better visualized using TSE-Acc than those using TFE-Acc, and the collateral and stenosis arteries were better visualized using TSE-Acc than those using TRANCE. ConclusionPeripheral arterial visualization was better with TSE-Acc than that with TFE-Acc in lower extremity MRA without ECG or PPU triggering and contrast enhancement, which was comparable with TRANCE as the reference standard. Furthermore, TSE-Acc may propose satisfactory diagnostic performance for diagnosing PAD in patients with arrhythmia and chronic kidney disease.

A simple method for pulse contrast enhancement via self-focusing

Abstract Here we report on a simple-to-implement and cost-effective approach for laser pulse contrast enhancement, based on the ${\chi}^{(3)}$ nonlinear self-focusing effect. An intentionally induced and gently controlled self-focusing in a thin glass transforms the time-dependent intensity into variation in beam divergence. Followed by a spatial discriminating filter, only the strongly focused fraction traverses the setup, at the expense of efficiency. A numerical model, accounting for the pulse and material parameters via a Gaussian ABCD matrix, provides an estimate for the instantaneous beam waist and transmission efficiency, which enables us to evaluate the resulting contrast enhancement. The estimated contrast enhancement spans between 0.5 and 2.5 orders of magnitude, in conjunction with approximately 25%–90% estimated efficiency, depending on the pulse parameters. In a preliminary experiment we demonstrated the effect with 10s-μJ sub GW regime with approximately 40 $\%$ efficiency and a contrast improvement of more than or equal to 20 dB.

Electro-optically modulated lasers: pulse energy and contrast enhancement derived from a theoretical spectrum

In this Letter, we derive analytically the complex optical spectrum of a pulsed laser source obtained when a frequency comb generated by phase modulation is input into a synchronized intensity modulator. We then show how this derivation of the spectrum may help to achieve unprecedented, to the best of our knowledge, accuracy during the experimental spectrum correction step usually carried out with an optical spectrum processor. In numerical examples, for a given average power, we present up to a 75% increase in peak power and an enhancement of the extinction ratio by at least 3 orders of magnitude. This method also enables large-factor rate-multiplications of these versatile coherent sources using the Talbot effect with negligible signal degradation.

Pulse contrast enhancement via non-collinear sum-frequency generation with the signal and idler of an optical parametric amplifier

We outline an approach for improving the temporal contrast of a high-intensity laser system by &amp;gt;8 orders of magnitude using noncollinear sum-frequency generation with the signal and idler of an optical parametric amplifier. We demonstrate the effectiveness of this technique by cleaning pulses from a millijoule-level chirped-pulse amplification system to provide &amp;gt;1012 intensity contrast relative to all prepulses and amplified spontaneous emission &amp;gt;5 ps prior to the main pulse. The output maintains percent-level energy stability on the time scales of a typical user experiment at our facility, highlighting the method's reliability and operational efficiency. After temporal cleansing, the pulses are stretched in time before seeding two multipass, Ti:sapphire-based amplifiers. After re-compression, the 1 J, 40 fs (25 terawatt) laser pulses maintain a &amp;gt;1010 intensity contrast &amp;gt;30 ps prior to the main pulse. This technique is both energy-scalable and appropriate for preparing seed pulses for a terawatt- or petawatt-level chirped-pulse amplification laser system.

Laser contrast enhancement by non-collinear optical Kerr effect of CS2

A pulse contrast enhancement method for lasers with a low initial contrast is presented based on non-collinear optical Kerr effect of CS2. The optical Kerr gate is used as a temporal filter to achieve a “cleaner” laser pulse. A pulse contrast enhancement factor of 104 is expected, with a maximum transmittance of the optical Kerr gate reaching 26%. Measurement of the pulse duration and profile has proved that this method introduces slight changes in the pulse.

Numerical study on pulse contrast enhancement in a short-pulse-pumped optical parametric amplifier

We investigate pulse cleaning behaviors in short-pulse-pumped optical parametric amplifiers (OPA). We theoretically study the contrast enhancement of amplified signal pulse and generated idler pulse, and reveal their dependence on the parametric gain in both the regimes of small signal and saturated amplifications. The signal contrast enhancement is nearly equal to the parametric gain, while the idler contrast is approximately equal to the product of the contrasts of the pump and signal pulses in a low gain OPA and increases with the gain. The effects of group-velocity mismatch and group-velocity dispersion on the contrast enhancement are also investigated. The results presented in this paper are of value for pulse cleaning.

Picosecond pulse contrast enhancement by use of polarization rotation in crystals with the second-order nonlinearity

We report on the implementation of intensity-dependent filter for picosecond pulse temporal cleaning based on an effect of pulse polarization rotation during the unbalanced phase matched second harmonic generation. The pulse contrast enhancement by more than two orders of magnitude and filter transmission of ∼25% has been demonstrated at pulse intensities of <1 GW/cm2. A simple and reliable all-solid-state filter scheme was integrated in a two-stage picosecond Nd:YAG regenerative amplifier seeded with pJ-energy femtosecond pulses of the Yb:KGW oscillator, and allowed us to reduce the ASE intensity level down to 1.5×10−8 with reference to the intensity of main 100 ps and 100 μJ pulse.

Lymph node imaging in breast cancer lymph node metastasis

Research in lymph node imaging has made great progress in recent years. With the increasing in-depth researches and clinical applications of MRI, PET and molecular imaging, the sensitivity, specificity and accuracy of detecting lymph node metastases have also been improved. Incorporating techniques such as various pulse sequences, contrast enhancement and lymph-node contrast agents has made MRI a superior method for detecting lymph node metastasis of breast cancer in clinical settings. Key words: Breast neoplasms; Neoplasm metastasis; Radiography

利用光纤中的非线性偏振旋转效应提高激光脉冲对比度

利用光纤中的非线性偏振旋转效应提高激光脉冲对比度

Highly efficient nonlinear filter for femtosecond pulse contrast enhancement and pulse shortening

We propose a highly efficient scheme for temporal filters devoted to femtosecond pulse contrast enhancement. The filter is based on cross-polarized wave generation with a spatially suger-Gaussian-shaped beam. In a single nonlinear crystal scheme the energy conversion to the cross-polarized pulse can reach 28%. We demonstrate that the process enables a significant spectral broadening. For an efficiency of 23% the pulse shortening is estimated to 2.2, leading to an intensity transmission of the nonlinear filter of 50%.

Nonlinear polarization rotation of elliptical light in cubic crystals, with application to cross-polarized wave generation

We have investigated theoretically and experimentally the nonlinear propagation of intense elliptically polarized light pulses along a fourfold axis of the cubic crystal BaF2. Third-order nonlinear optical processes generate a cross-polarized wave, an effect that presents significant possibilities for application in femtosecond pulse contrast enhancement. The experimental setup consists of an input linear polarized light that passes through a cubic crystal sandwiched between two crossed quarter-wave plates. The exit orthogonal polarization-state production amount is measured at the output of an analyzer. When the light impinging on the sample is elliptically polarized with a quarter-wave plate at 22.5 deg, the achieved efficiency reaches 15%. It is more than twice that of a conventional polarization filter based on nonlinear ellipse rotation in an isotropic medium. This device is compared with previously reported polarization filtering [J. Opt. Soc. Am. B21, 1659 (2004)], in which a linearly polarized light produced a perpendicular field component. The theoretical model describes in detail the obtained dependencies and allows the different nonlinear processes that contribute to the generation of a cross-polarized wave to be distinguished. Possible applications are discussed.

Pulse contrast enhancement of high-energy pulses by use of a gas-filled hollow waveguide

Using nonlinear ellipse rotation in a gas-filled hollow waveguide, we have increased the pulse contrast of a microjoule femtosecond laser pulse by several orders of magnitude. This scheme offers a number of advantages over competing techniques, including a high degree of tunability that allows for a broad range of input pulse parameters, higher throughput, greater stability, and an output pulse with high spatial quality that is compressible to a quarter of the original temporal width.

Saturable absorbers for multiband operation of a high-gain short-pulse CO_2-laser chain

It is shown that a high-gain CO2-laser chain made of rapidly excited high-pressure amplifiers can be operated without interface isolators. Saturable absorbers are then used only to prevent amplification of front-end leakage and are required to absorb solely at the working frequencies. CH3OH, CH3OD, CH3F, CCl2F2, and HCOOH have been characterized in view of pulse contrast enhancement around 9.4 μm in a multiline multiband laser chain. The optimum pressure, the saturating fluence, the small-signal absorption coefficient, and its ratio over the saturated absorption coefficient are given for many CO2-laser lines.

A passive nonresonant technique for pulse contrast enhancement and gain isolation

A passive aperture-scalable device is described which provides high contrast enhancement of a laser pulse over baseline optical noise while maintaining efficient net transmission of the principal laser pulse. The technique, which is based upon the nonresonant phenomenon of self-induced ellipse rotation, is capable of providing effective gain isolation for virtually any high-power laser system. The constrast enhancement capabilities of the device are demonstrated experimentally using a single ultrashort laser pulse obtained from a cavity-dumped passively mode-locked Nd : glass laser. In addition, a discussion is given concerning the ability of the device to temporally compress or stretch the optical pulse.