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At least 937 records · Page 52Linked to original sources
PubMed2005

Pulse oximetry as a potential screening tool for lower extremity arterial disease in asymptomatic patients with diabetes mellitus.

BACKGROUND: Lower extremity arterial disease (LEAD) is common and underdiagnosed in patients with diabetes mellitus and is associated with higher total mortality. METHODS: We compared the accuracy of pulse oximetry, the ankle-brachial index (ABI), and the combination of the two to diagnose LEAD in consecutive outpatients with type 2 diabetes who had no symptoms of LEAD, in a primary care setting. Exclusions were age younger than 40 years, known LEAD, or typical symptoms of LEAD. Fifty-seven patients were enrolled. All patients had (1) ABI measurement; (2) pulse oximetry to measure Sao2 of their index fingers and big toes in the supine position and at 12-in elevation; and (3) Doppler waveform analysis of the lower extremity arteries. The ABI was considered abnormal if it was less than 0.9. Pulse oximetry of the toes was considered abnormal if the Sao2 was more than 2% lower from the finger or on 12-in elevation of the foot. The combination was considered positive if either the ABI or pulse oximetry was positive for LEAD and negative if both were negative. We defined LEAD as monophasic waveforms on waveform analysis. RESULTS: Of our patients, 31% had LEAD. Pulse oximetry had a sensitivity of 77% (95% confidence interval [CI], 61%-88%) and a specificity of 97% (95% CI, 91%-99%); ABI had a sensitivity of 63% (95% CI, 46%-77%) and a specificity of 97% (95% CI, 91%-99%). Positive likelihood ratios were 30 (95% CI, 7.6-121) for pulse oximetry and 24.8 (95% CI, 6.2-99.8) for ABI; negative likelihood ratios were 0.23 (95% CI, 0.12-0.43) for pulse oximetry and 0.38 (95% CI, 0.25-0.59) for ABI. For the combination, sensitivity was 86% (95% CI, 71%-94%) and specificity was 92% (95% CI, 84%-96%). CONCLUSIONS: Pulse oximetry of the toes seems as accurate as ABI to screen for LEAD in patients with type 2 diabetes. Combination of the two tests increases sensitivity.

Adult↗
PubMed1996

Interactions among paired-pulse facilitation and post-tetanic and long-term potentiation in the mossy fiber-CA3 pathway in rat hippocampus.

Long-term potentiation (LTP) observed at the synapses of mossy fiber-CA3 (MF-CA3) pathway differs from that observed at the Schaffer collateral-CA1 pathway (SC-CA1), in being independent of N-methyl-D-aspartate (NMDA) receptors. The induction and expression mechanisms of MF-CA3 LTP remain to be determined. We have compared the occurrence and magnitude of LTP with that of two other indicators of presynaptic plasticity, post-tetanic potentiation (PTP) and paired-pulse facilitation in control brain slices from young rats and in slices treated with phorbol-12, 13-diacetate (PDAc), a protein kinase C activator. Paired-pulse facilitation is a potentiation of the second of two responses at intervals of tens of milliseconds and is due to a presynaptic calcium increase. Tetanic stimulation of mossy fibers induced LTP is area CA3 in only 64% of slices. In those slices that showed LTP, the size of the PTP was significantly greater than in those slices that did not, and the degree of correlation between LTP and PTP amplitude overall was r = 0.7. The degree of paired-pulse facilitation before tetanic stimulation was also positively correlated to the occurrence and magnitude of LTP and PTP after tetanic stimulation. The correlation coefficient between PTP and PPF was 0.749 for all slices studied, while that between LTP and PPF was 0.835 overall. Application of PDAc potentiated synaptic transmission and abolished paired-pulse facilitation (control ratio of second to first response, 2.1; after PDAc ratio 0.8) and LTP. PTP was absent at the control stimulus intensity in PDAc, but was apparent if the stimulus intensity was reduced to give a response of the same amplitude as before administration of PDAc. Stable LTP was also accompanied by a marked decrease in paired-pulse facilitation. These data suggest that MF-CA3 LTP, PTP and paired-pulse facilitation share common mechanisms and are all at least primarily of presynaptic origin. The occurrence of large paired-pulse facilitation or PTP is a predictor of a preparation which will show LTP. It is likely that presynaptic [Ca2+]i is an essential factor in LTP, PTP and paired-pulse facilitation, as well as the potentiation induced by application of PDAc, but the factors which determine whether or not [Ca2+]i rises following these various stimuli are not clear from the techniques used in these investigations.

Animals↗
PubMed2000

Origin of retinal pigment epithelium cell damage by pulsed laser irradiance in the nanosecond to microsecond time regimen.

BACKGROUND AND OBJECTIVE: Selective photodamage of the retinal pigment epithelium (RPE) is a new technique to treat a variety of retinal diseases without causing adverse effects to surrounding tissues such as the neural retina including the photoreceptors and the choroid. In this study, the mechanism of cell damage after laser irradiation was investigated. STUDY DESIGN/MATERIALS AND METHODS: Single porcine RPE-melanosomes and RPE cells were irradiated with a Nd:YLF laser (wavelength lambda = 527 nm, adjustable pulse duration tau = 250 nsec-3 microsec) and a Nd:YAG laser (lambda = 532 nm, tau = 8 nsec). Fast flash photography was applied to observe vaporization at melanosomes in suspension. A fluorescence viability assay was used to probe the cells vitality. RESULTS: The threshold radiant exposures for vaporization around individual melanosomes and for ED50 cell damage are similar at 8-nsec pulse duration. Both thresholds increase with pulse duration; however, the ED50 cell damage radiant exposure is 40% lower at 3 microsec. Temperature calculations to model the onset of vaporization around the melanosomes are in good agreement with the experimental results when assuming a surface temperature of 150 degrees C to initiate vaporization and a homogeneous melanosome absorption coefficient of 8,000 cm(-1). Increasing the number of pulses delivered to RPE cells at a repetition rate of 500 Hz, the ED50 value decreases for all pulse durations. However, the behavior does not obey scaling laws such as the N 1/4 equation. CONCLUSION: The origin of RPE cell damage for single pulse irradiation up to pulse durations of 3 microsec can be described by a damage mechanism in which microbubbles around the melanosomes cause a rupture of the cell structure. The threshold radiant exposure for RPE damage decreases with increasing number of pulses applied.

Animals↗
PubMed2006

Respiratory changes in the pulse-oximetry waveform associated with pericardial tamponade.

BACKGROUND: Timely identification of hemodynamic compromise in patients with acute pericardial effusion and tamponade is critical in patient management. Respiratory variability in pulse-oximetry waveforms has been correlated with pulsus paradoxus, but has not been reported with cardiac tamponade in adult patients. HYPOTHESIS: This study describes changes in respiratory variability in pulse-oximetry waveform pre and post pericardiocentesis in patients with hemodynamically significant pericardial effusions. METHODS: A single-center, catheterization laboratory hemodynamic database was reviewed for all patients who underwent pericardiocentesis for clinically suspected tamponade and had continuous digital pulse-oximetry, electrocardiographic, and respiration waveforms recorded during the procedure. Phasic respiratory changes in pulse-oximetry waveform amplitude (maxima-minima) were expressed as an expiratory/inspiratory ratio and compared pre and post pericardiocentesis. RESULTS: The study population consisted of 12 patients (6 men:6 women, age 60 +/- 10 years) with pericardial effusion documented by echocardiography on the day of pericardiocentesis. Phasic respiratory variability in the pulse-oximetry waveform was evident in all patients prior to aspiration (respiratory ratio = 1.9 +/- 0.5). Following pericardiocentesis (aspirated volume: 650 +/- 300 ml), the respiratory ratio decreased in all patients (1.2 +/- 0. 1, p = 0.001). Receiver operator characteristic curve analysis suggests that pulse-oximetry respiratory ratios > or = 1.5 should raise suspicion of hemodynamic compromise in high-risk populations. CONCLUSIONS: Pulse-oximetry is a commonly used tool for monitoring critically ill patients. The present study suggests that increased respiratory variability in the pulse-oximetry waveform should raise suspicion for hemodynamic compromise in patients at risk for pericardial effusion.

Acute Disease↗
PubMed1988

Effect of a light pulse during the dark on photoperiodic regulation of the rate of thyroxine-induced, spontaneous, and prolactin-inhibited metamorphosis in Rana pipiens tadpoles.

Since Rana pipiens tadpoles injected with thyroxine (T4) early in the dark develop more slowly than those injected in the light, we studied the effect of giving a light pulse of 1 hr early in the dark. Tadpoles injected under a 7.5-W red light bulb in a darkened room with 0.2 microgram T4 daily at 2200 hr went through metamorphosis faster on a 12L:3D:1L:8D cycle with a light pulse after injection than on a 12L:12D cycle without a light pulse, and even faster on a 12L:1.5D:1L:9.5D cycle with a light pulse before the injection. Thus a 1-hr light pulse counteracted the metamorphic delay resulting from administration of T4 in the dark, and set in motion the conditions that resulted in a more rapid response to an injection of T4. However, a 1-hr light pulse in the early dark had no effect on growth and development of older or younger untreated tadpoles or those constantly immersed in 30 micrograms/liter T4. Larvae on 21L:3D with T4 injection in the dark and on 12L:3D:1L:8D with T4 injection at 0700 hr just before the start of the main light phase progressed faster than 12L:3D:1L:8D with injection at 2200 hr in the dark before only a 1-hr light pulse. Thus the length of the light phase immediately after T4 injection was significant. There was no difference on 12L:12D and 12L:3D:1L:8D cycles in the effectiveness of daily injections of 10 micrograms prolactin (PRL) in the early dark at 2200 hr in promoting tail growth or antagonizing tail resorption induced by T4 immersion. Under these conditions, PRL utilization did not appear to be inhibited by the light pulse.

Animals↗
PubMed2004

New FOCI pulses with reduced radiofrequency power requirements.

PURPOSE: To propose new frequency offset corrected inversion (FOCI) pulses with significantly reduced radiofrequency (RF) power deposition for spin echo imaging by incorporating the variable-rate selective excitation (VERSE) schemes into the pulse design. MATERIALS AND METHODS: Two schemes are proposed to design the new FOCI pulses with dramatically reduced peak RF power requirements. In scheme A, the time-dilation function is derived from a predefined adiabaticity factor modulation function. In scheme B, the time-dilation function is predefined, while the adiabaticity factor is conserved. RESULTS: The new FOCI pulses are shown to be able to operate at reduced specific absorption rate (SAR), specifically at the same peak RF power as that of a five- or seven-lobe sinc inversion pulse of the same duration. Using the new FOCI pulse, significant gain in sensitivity was observed in in vivo spin-echo echo-planar imaging, which was attributed to the improved refocusing slice profile. CONCLUSION: The new FOCI pulses can replace the 180 degrees five- or seven-lobe sinc pulses in spin-echo imaging with the same peak RF power requirement and significantly improved slice profile.

Animals↗
PubMed1992

Comparison of asymmetrical and symmetrical pulse waveforms in electromagnetic stimulation.

Pulsing electromagnetic field (PEMF) stimulation is a noninvasive therapeutic modality that has been successfully used to stimulate healing of surgically resistant human bone fracture nonunions. Asymmetry of the stimulus pulse waveform was thought to be necessary for therapeutic effectiveness, but asymmetrical pulses require significant electrical energy that constrains clinical delivery systems to suboptimal designs. Development of low-energy consuming stimuli will enable clinical device improvements and may provide additional information about the interaction of electromagnetic fields with tissues. The objectives of this study were (a) to determine if asymmetry of the stimulus pulse waveform is needed for efficacy and (b) to determine if symmetrical pulse waveform stimuli also can produce a beneficial therapeutic response. The rabbit fibular osteotomy model was used to answer these questions and to identify which components of the clinically used asymmetrical PEMF produce the therapeutic response. The results suggest that asymmetry is not necessary and that a narrow pulse width, symmetrical square wave signal can also stimulate stiffness increases in this model. The data also suggest that the high-amplitude, narrow-pulse portion of the asymmetrical PEMF is the principal component of the signal pulse that is responsible for the clinical therapeutic effect.

Animals↗
PubMed2000

Pulsed electromagnetic field stimulation of MG63 osteoblast-like cells affects differentiation and local factor production.

Pulsed electromagnetic field stimulation has been used to promote the healing of chronic nonunions and fractures with delayed healing, but relatively little is known about its effects on osteogenic cells or the mechanisms involved. The purpose of this study was to examine the response of osteoblast-like cells to a pulsed electromagnetic field signal used clinically and to determine if the signal modulates the production of autocrine factors associated with differentiation. Confluent cultures of MG63 human osteoblast-like cells were placed between Helmholtz coils and exposed to a pulsed electromagnetic signal consisting of a burst of 20 pulses repeating at 15 Hz for 8 hours per day for 1, 2, or 4 days. Controls were cultured under identical conditions, but no signal was applied. Treated and control cultures were alternated between two comparable incubators and, therefore, between active coils; measurement of the temperature of the incubators and the culture medium indicated that application of the signal did not generate heat above the level found in the control incubator or culture medium. The pulsed electromagnetic signal caused a reduction in cell proliferation on the basis of cell number and [3H]thymidine incorporation. Cellular alkaline phosphatase-specific activity increased in the cultures exposed to the signal, with maximum effects at day 1. In contrast, enzyme activity in the cell-layer lysates, which included alkaline phosphatase-enriched extracellular matrix vesicles, continued to increase with the time of exposure to the signal. After 1 and 2 days of exposure, collagen synthesis and osteocalcin production were greater than in the control cultures. Prostaglandin E2 in the treated cultures was significantly reduced at 1 and 2 days, whereas transforming growth factor-beta1 was increased; at 4 days of treatment, however, the levels of both local factors were similar to those in the controls. The results indicate enhanced differentiation as the net effect of pulsed electromagnetic fields on osteoblasts, as evidenced by decreased proliferation and increased alkaline phosphatase-specific activity, osteocalcin synthesis, and collagen production. Pulsed electromagnetic field stimulation appears to promote the production of matrix vesicles on the basis of higher levels of alkaline phosphatase at 4 days in the cell layers than in the isolated cells, commensurate with osteogenic differentiation in response to transforming growth factor-beta1. The results indicate that osteoblasts are sensitive to pulsed electromagnetic field stimulation, which alters cell activity through changes in local factor production.

Alkaline Phosphatase↗
PubMed2003

Reduction in lateral thermal damage using heat-conducting templates: a comparison of continuous wave and pulsed CO2 lasers.

BACKGROUND AND OBJECTIVES: The advantages of the continuous wave (c.w.) CO(2) laser are offset by the delay in laser wound healing secondary to thermal damage. We have developed novel heat-conducting templates to reduce laser thermal damage. Because shortened pulse durations also decrease thermal damage, we tested the effectiveness of heat-conducting templates with a c.w. CO(2) clinical laser and a short-pulsed CO(2) laser to determine the best method and mechanism to minimize thermal damage. STUDY DESIGN/MATERIALS AND METHODS: Comparison of 0.2-second shuttered c.w. and 5-microsecond pulsed CO(2) lasers were made by doing incisions on 150 tissue samples from reduction mammoplasties and abdominoplasties. Copper, aluminum, glass, and Plexiglass heat-conducting templates were tested against no template (air) with both lasers. Histological samples were evaluated using computerized morphometrics analysis. RESULTS: Statistically significant reductions in lateral thermal damage were seen with the copper (50%) and aluminum (39%) templates used with the c.w. CO(2) laser. Only the copper template (39%) significantly reduced thermal damage when used with the pulsed CO(2) laser. Less thermal damage was seen using the pulsed CO(2) laser compared to the c.w. CO(2) laser with each template. CONCLUSIONS: Heat-conducting templates significantly reduced the amount of lateral thermal damage when used with the c.w. CO(2) laser (copper and aluminum) and short-pulsed CO(2) laser (copper). The c.w. CO(2) laser with the copper template compared favorably to the short-pulsed CO(2) laser without a template. Therefore, both heat conductive templates and short-pulse structure provide successful methods for reducing lateral thermal damage, and a combination of the two appears to provide optimal results.

Aluminum↗
PubMed2001

Er:YAG laser skin resurfacing using repetitive long-pulse exposure and cryogen spray cooling: II. Theoretical analysis.

BACKGROUND AND OBJECTIVE: To analyze the effects of laser pulse duration and cryogen spray cooling (CSC) on epidermal damage and depth of collagen coagulation in skin resurfacing with repetitive Er:YAG laser irradiation. STUDY DESIGN/MATERIALS AND METHODS: Evolution of temperature field in skin is calculated using a simple one-dimensional model of sub-ablative pulsed laser exposure and CSC. The model is solved numerically for laser pulse durations of 150 and 600 microsec, and 6 msec cryogen spurts delivered just prior to ("pre-cooling"), or during and after ("post-cooling") the 600 microsec laser pulse. RESULTS: The model indicates a minimal influence of pulse duration on the extent of thermal effect in dermis, but less epidermal damage with 600 microsec pulses as compared to 150 microsec at the same pulse fluence. Application of pre- or post-cooling reduces the peak surface temperature after laser exposure and accelerates its relaxation toward the base temperature to a different degree. However, the temperature profile in skin after 50 msec is in either example very similar to that after a lower-energy laser pulse without CSC. CONCLUSIONS: When applied in combination with repetitive Er:YAG laser exposure, CSC strongly affects the amount of heat available for dermal coagulation. As a result, CSC may not provide spatially selective epidermal protection in Er:YAG laser skin resurfacing.

Burns↗
PubMed1990

Dependence of the XeCl laser cut rate of plaque on the degree of calcification, laser fluence, and optical pulse duration.

A XeCl laser with an optical pulse duration of 35 ns was used to determine the cut depth per laser pulse of postmortem human aorta as a function of laser fluence for four main categories of plaque development. The data indicate that the cut depth per pulse progressively decreases as the degree of calcification increases even at very high (100 mJ/mm2) laser fluences. A comparison was made between the XeCl laser cut rate data obtained using the 35-ns duration laser pulses to data obtained using 200-ns duration pulses for each of the four plaque types. As the degree of tissue calcification increased higher XeCl laser fluences were required for the long pulse case to achieve the same cut depth per pulse as that observed using the shorter pulse duration.

Angioplasty, Laser↗
PubMed2005

Hollow-waveguide-based nanosecond, near-infrared pulsed laser ablation of tissue.

BACKGROUND AND OBJECTIVE: Short-pulse solid-state lasers have recently received much attention as new coherent light sources for medical applications, but steady transmission of their high-energy output pulses through a solid quartz fiber is difficult because of the onset of laser-induced breakdown. We previously demonstrated that hollow waveguides could be used to deliver nanosecond laser pulses for tissue ablation. The aim of this study was to determine the optimum laser pulse energy and range of defocused distance for obtaining a deep and sharp ablation channel in myocardial tissue with laser pulses transmitted through a hollow waveguide. STUDY DESIGN/MATERIALS AND METHODS: Cyclic-olefin-polymer-coated silver hollow waveguides of 1 mm in inner diameter and 1 m in length were used. A vacuum-cored scheme was applied to the waveguides to suppress laser-induced air breakdown. Porcine myocardial tissue was irradiated with 300 laser pulses that were delivered through the waveguide in vitro at various laser energy levels and defocused distances, and depths and diameters of channels were measured. Histological analysis of the ablated tissues was also performed. RESULTS: At an ablation energy of approximately 60 mJ/pulse, deep (>4.5 mm) and sharp (depth-to-diameter ratio of > 6) channels were created in tissue in the range of defocused distances of -4 approximately + 0.5 mm. Under these conditions, waveguide bending did not cause a remarkable change in ablation characteristics. Histological analysis of ablated tissue showed limited thermal damage but suggested a certain extent of mechanical effects in the tissue. CONCLUSION: With near-infrared, nanosecond laser pulses delivered through a cyclic-olefin-polymer-coated silver hollow waveguide, efficient and sharp ablation of myocardial tissue can be achieved, suggesting the usefulness of the hollow waveguide as a new flexible delivery system for high-intensity laser pulses.

Animals↗
PubMed2005

Improved signal to noise in high-resolution magic angle spinning total correlation spectroscopy studies of prostate tissues using rotor-synchronized adiabatic pulses.

A rotor-synchronized WURST-8 adiabatic pulse scheme was compared to the conventional MLEV-17 hard pulse scheme for isotropic mixing in total correlation spectroscopy (TOCSY) studies of intact human prostate tissues under high-resolution magic angle spinning (HR-MAS) conditions. Both mixing schemes were extremely sensitive to the rotational resonance condition and dramatic reductions in signal to noise were observed when pulse durations deviated from 1/(spin rate). A significant increase in cross-peak intensities was observed using rotor-synchronized WURST-8 adiabatic pulses versus those observed using the rotor-synchronized MLEV-17 hard pulse scheme in both solution and tissue. In tissue, absolute signal intensities ranged from 1.5x to 10.5x greater (average: 4.75x) when WURST-8 was used in place of MLEV-17. Moreover, the difference was so dramatic that several metabolite cross peaks observed using WURST-8 pulses were not observed using MLEV-17 pulses, including cross peaks corresponding to many of the choline- and ethanolamine-containing metabolites. Due to the complex modulation of TOCSY cross peaks for multiply coupled spins and the shorter T(2) relaxation times of tissue metabolites, maximum cross-peak intensities occurred at shorter mixing times than predicted by theory. In summary, a WURST-8 adiabatic mixing scheme produced significantly greater absolute cross-peak signal intensities than MLEV-17 hard pulse mixing, and maximum cross-peak intensity versus mixing time must be established for specific spin systems and T(2) relaxation times.

Humans↗
PubMed2005

Exchange-influenced T2rho contrast in human brain images measured with adiabatic radio frequency pulses.

Transverse relaxation in the rotating frame (T(2rho)) is the dominant relaxation mechanism during an adiabatic Carr-Purcell (CP) spin-echo pulse sequence when no delays are used between pulses in the CP train. The exchange-induced and dipolar interaction contributions (T(2rho,ex) and T(2rho,dd)) depend on the modulation functions of the adiabatic pulses used. In this work adiabatic pulses having different modulation functions were utilized to generate T(2rho) contrast in images of the human occipital lobe at magnetic field of 4 T. T(2rho) time constants were measured using an adiabatic CP pulse sequence followed by an imaging readout. For these measurements, adiabatic full passage pulses of the hyperbolic secant HSn (n = 1 or 4) family having significantly different amplitude-and frequency-modulation functions were used with no time delays between pulses. A dynamic averaging (DA) mechanism (e.g., chemical exchange and diffusion in the locally different magnetic susceptibilities) alone was insufficient to fully describe differences in brain tissue water proton T(2rho) time constants. Measurements of the apparent relaxation time constants (T(2) (dagger)) of brain tissue water as a function of the time between centers of pulses (tau(cp)) at 4 and 7 T permitted separation of the DA contribution from that of dipolar relaxation. The methods presented assess T(2rho) relaxation influenced by DA in tissue and provide a means to generate T(2rho) contrast in MRI.

Brain Mapping↗
PubMed2006

Improved gradient-echo 3D magnetic resonance imaging using pseudo-echoes created by frequency-swept pulses.

Frequency-swept pulses are not typically employed to excite spins in NMR. When used for selective excitation in MRI, such pulses do not produce a proper echo because the phase of the transverse magnetization varies in a quadratic manner across the slice or slab. Previously, frequency-swept pulses such as the chirp pulse have been shown to offer an approach to reduce the peak radiofrequency power required for excitation. It has also been shown that chirp excitation produces a unique type of echo (dubbed "pseudo-echo" here) and images can be generated from the resultant pseudo-echoes using a quadratic reconstruction method (J.G. Pipe, Magn Reson Med 1995;33:24-33). The present work describes a general theory and methods for exciting spins with other types of frequency-swept pulses (HSn pulses), which offer the advantage of delivering better excitation profiles than the chirp pulse. Here, pseudo-echoes are produced with HSn pulses in conventional gradient-echo 3D MRI, and high-quality images are reconstructed using standard fast Fourier transformation. An optional apodization procedure using a sliding window function is also introduced. When the dynamic range of the analog-to-digital converter is limiting, signal-to-noise ratio of pseudo-echo imaging is superior to that obtained with standard excitations.

Brain Mapping↗
PubMed1994

Multiple-quantum filters of spin-3/2 with pulses of arbitrary flip angle.

The influence of inhomogeneous RF fields on the double- and triple-quantum filtering of spin-3/2 nuclei in the presence of biexponential relaxation is analyzed. In this analysis, spherical tensor operators have been used for density-matrix calculations. In the presence of inhomogeneous RF fields, it is shown that the three-pulse triple-quantum filter (TQF) without a refocusing 2 theta pulse (with delta omega = 0) is on average about 100% more sensitive than the corresponding double-quantum filter (DQF), and in the case of four-pulse DQF and TQF with a refocusing 2 theta pulse (with delta omega not equal to 0), two relaxation coefficients f(1)11(tau) and f(1)33(tau) also contribute to the observed DQ and TQ coherences. It is also shown that the three-pulse filters are more sensitive than the corresponding four-pulse filters. In both three- and four-pulse cases, when used with surface coils, these filters act as depth pulses and thus yield spatial localization. The experimental results obtained with homogeneous RF coils are in excellent agreement with the theoretical results.

Equipment Design↗
PubMed1998

Evolution strategy optimization for selective pulses in NMR

We present a first set of improved selective pulses, obtained with a numerical technique similar to the one proposed by Geen and Freeman. The novelty is essentially a robust and efficient "evolution strategy" which consistently leads, in a matter of minutes, to "solutions" better than those published so far. The other two ingredients are a "cost function," which includes contributions from peak and average radiofrequency power, and some understanding of the peculiar requirements of each type of pulse. For example, good solutions for self-refocusing pulses and "negative phase excitation pulses" (which yield a maximum signal well after the end of the pulse) are found, as may have been predicted, among amplitude modulated pulses with 270 degrees tip angles. Emphasis is given to the search for solutions with low RF power for selective excitation, saturation, and inversion pulses. Experimental verification of accuracy and power requirements of the pulses has been performed with a 4.7 T Sisco imager. Copyright 1998 Academic Press.

Journal Article↗
PubMed2001

T(2) selectivity: comparison between different kinds of RF pulses.

Techniques allowing characterization of specific T(2) species are used to perform MR imaging of structures whose signal intensity is poor on conventional methodologies. T(2)-selective pulses can be used to discriminate signals from tissues with different T(2) values. In case of soliton pulses, the magnetization from all the T(2) species is inverted, with the exception of specific T(2) structures which have both the longitudinal and the transverse magnetization nulled on resonance. Solitons have the drawback of being very long even if the selected T(2) value is in the order of tens of milliseconds. The aim of this paper is to examine whether it is possible to use pulses shorter then solitons keeping their T(2) selectivity, that is keeping null longitudinal magnetization, while accepting some residual transverse magnetization for structures with the selected T(2). Different kinds of inversion pulses were generated, all to select the same T(2) species. Pulses performance was analyzed by studying the dependence of the residual transverse magnetization on resonance from the length of the pulse. An exponential relationship between transverse magnetization and pulse length was found, which was not dependent on the pulse type.

Journal Article↗