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At least 1,207 records · Page 67Linked to original sources
PubMed1997

Fast inversion-recovery MR: the effect of hybrid RARE readout on the null points of fat and cerebrospinal fluid.

PURPOSE: To evaluate the effect of the hybrid RARE (rapid acquisition with relaxation enhancement) readout, commonly coupled to inversion-recovery pulse sequences, on the null inversiton time (TI) of fluid and fat using both phantoms and human volunteers. METHODS: Two phantoms, simulating fat (phantom A) and cerebrospinal fluid (phantom B), respectively, were imaged using a fast inversion-recovery sequence that coupled an inversion-recovery preparation pulse to a hybrid RARE readout. At repetition times (TRs) ranging from 700 to 20,000, the TI necessary to null the signal from each phantom (null TI) was determined for an echo train length of 4, 6, 8, 10, 12, 14, 16, 18, and 20, respectively. Plots of null TI versus echo train length at different TRs were generated for both phantoms. Fast inversion-recovery MR imaging of the cervical spine and brain was performed in healthy volunteers. At a fixed TR and TI, the adequacy of signal suppression from bone marrow and cerebrospinal fluid was assessed as a function of echo train length. RESULTS: There was a gradual decrease of null TI for both phantoms with echo train length. This decrease persisted at longer TRs for phantom B (T1 = 3175 +/- 70 milliseconds) than for phantom A (T1 = 218 +/- 5 milliseconds). In the human volunteers, there was a gradual loss of suppression of signal from bone marrow and cerebrospinal fluid, with changes in the hybrid RARE readout. CONCLUSION: To optimize specific tissue suppression, radiologists implementing fast inversion-recovery MR imaging should be aware of the effects of the hybrid RARE readout on null TI.

Adipose Tissue↗
PubMed1988

Nd:YAG laser shock waves in artificial eyes.

Nd:YAG laser surgery is performed by inducing optical breakdown in the interior of the eye. The optical breakdown is accompanied by shock waves which expand throughout the entire eye. In our experiments, we measured the amplitudes of shock waves at the walls of artificial eyes caused by single and multiple pulses of optical energy originating from an Nd:YAG laser at a wavelength of 1.064 micron. We have shown that the effects observed are not caused by mechanical resonance. We believe that similar results can be expected for human eyes. The peak pulse amplitudes in artificial eyes of different length were found to increase linearly as a function of the energy transferred into the eye. The peak amplitudes of pulse sequences were found to be only half of those for single pulses of comparable energy. In laser surgery, such sequences can be preferable if they achieve the same effect.

Energy Transfer↗
PubMed1990

Brain-stem auditory-evoked responses elicited by maximum length sequences: effect of simultaneous masking noise.

The effects of masking noise on wave V of the brain-stem auditory-evoked response (BAER) obtained to pseudorandom pulse sequences are evaluated in two experiments. In the first experiment, the level of broadband noise was covaried with minimum pulse interval (rate) using maximum length sequence analysis (MLSA). Both increasing noise level and decreasing minimum pulse interval decrease wave V amplitude and increase wave V latency. A nonadditivity of rate and noise level was observed such that, at the shortest interpulse intervals, simultaneous background noise produced virtually no latency change and minimal amplitude change, for the noise levels tested. In a second experiment, high-pass masking was performed to assess the feasibility of derived-band techniques using maximum length sequence analysis (MLSA) and to compare the frequency regions responsible for the BAER using MLSA versus conventional averaging. Results of experiment 2 showed that reliable responses across high-pass masker cutoff frequency could be obtained in normal-hearing listeners. The frequency specificity of the MLSA-based responses was nearly identical to that obtained by conventional averaging, although both amplitude and latency of wave V were affected by the high-pass masker cutoff and minimum pulse interval values. These studies suggest that the neuronal populations and frequency regions responsible for the BAER are virtually the same for MLSA and conventional averaging.

Adult↗
PubMed1997

Multiple nonlinear stimulated echoes.

Three-pulse sequences in the presence of magnetic field gradients at high magnetic fields produce multiple nonlinear stimulated echoes (NOSE) at times ntau1 after the third pulse, where n is an integer and tau1 the interval between the first two pulses. These phenomena are due to the demagnetizing field produced by the spatial modulation of the nuclear magnetization arising in the sample after the first two pulses. The theory is presented and compared with experiments. The dependence of the NOSE amplitudes on the flip angles and on the pulse intervals is described. Implications for multidimensional NMR experiments based on sequences of three or more pulses in the presence of field gradients are discussed.

Dimethylpolysiloxanes↗
PubMedDate not supplied

Three-dimensional monitoring of small temperature changes for therapeutic hyperthermia using MR.

Radiofrequency hyperthermia of deep-seated pelvic tumors requires noninvasive monitoring of temperature distributions in patients. Methods of MR thermography were reported to be a promising tool in solving this problem. However, to be truly useful for monitoring hyperthermia treatments, MR thermography should be able to cover the entire pelvis in acquisition times no longer than for a breath-hold (< or = 15 seconds) and to resolve small temperature differences (< 1 degrees C). Three methods exploiting the temperature dependence of spin-lattice relaxation time (T1), of self-diffusion coefficient (D), and of chemical shift of proton resonance frequency (PRF) were applied in phantom experiments; the pulse sequences were the T1-weighted gradient echo, the pulsed diffusion gradient spin echo made faster through the keyhole technique, and the gradient echo with the phase reconstruction, respectively. The high planar resolution was compromised, and instead, coarse and more isotropic voxels were used. Experiments were performed in two consecutive steps, thus imitating a possible scenario for monitoring hyperthermia. In the first step, calibration curves were recorded, which were then used in the second step to obtain maps of temperature changes. The results show a clear superiority of the PRF method, followed by the D and the T1 methods. The uncertainty of temperature changes predicted both from calibration curves and from maps was less than 1 degrees C only with the PRF and the D-based methods.

Calibration↗
PubMed2002

Pathway selection by pulsed field gradients.

A method for pathway selection in a multiple spin-echo pulse sequence applying crusher gradients before and after each pi pulse, to dephase unwanted pathways, is described. This method selects the only pathway that would contribute to the measurable signal if the pi pulses were perfect (1,-1,1,-1, em leader ). Good pathway selection is essential in pulse programming, especially when the CPMG condition is not met. An interactive applet was also developed to deal with these calculations.

Algorithms↗
PubMed1985

Application of multipulse NMR to observe 13C-labeled metabolites in biological systems.

Limitations in resolution and sensitivity of 13C NMR spectroscopy have reduced the information obtainable from intact biological systems. With the aim of increasing the information from in vivo 13C NMR two multipulse NMR techniques, the DEPT pulse sequence and the gated spin-echo sequence, were used to obtain edited 13C NMR spectra from different 13C-labeled mammalian tissues. This allowed the separation of the 13C NMR signals from the tissues into subspectra containing either CH, CH2, or CH3 signals, thereby increasing the information obtainable from these spectra. Comparing the two techniques, the DEPT sequence gives more accurate editing than the gated spin-echo sequence but suffers from the difficulty of determining 1H pulse angles in vivo.

Acetates↗
PubMed1998

Simple two-pulse detection scheme in pulsed EPR for studying low-frequency nuclear coherences

A microwave two-pulse sequence with a weak and long 180 degrees first pulse and a hard 90 degrees second pulse is employed to detect nuclear coherences in pulsed EPR. The coherences created by the first pulse are transferred after an evolution period T into an observable FID by the second pulse. The free induction is measured at some fixed delay after the second pulse; it is modulated when T is varied. As the second pulse may be switched on immediately after the first pulse, the nuclear coherences may be detected immediately as they start to freely oscillate, without loss of information within the instrumental dead time. The method is demonstrated for a sample of the radical cation of 15N-labeled bacteriochlorophyll a. Copyright 1998 Academic Press.

Journal Article↗
PubMed2004

NMR measurements of diffusion in concentrated samples: avoiding problems with radiation damping.

Pulsed field gradient spin echo NMR is generally the method of choice for diffusion measurements on liquid samples. With modern high field instruments, however, severe problems can arise when it is applied to samples with very high proton concentrations because of the presence of radiation damping. The problems may be greatly reduced by a suitable choice of experimental parameters, in particular the use of modified stimulated echo pulse sequences with a reduced flip angle for the first pulse.

Journal Article↗
PubMed1990

Magnetic resonance imaging of the thoracic cavity using a paused 3DFT acquisition technique.

A new pulse sequence designed for magnetic resonance imaging of the entire thoracic cavity is described. This sequence, called 3DPAUSE, is a rapid three-dimensional Fourier transform (3DFT) sequences with periodic pauses for breathing and additional rf pulses after each pause to restore the magnetization to steady-state before data acquisition resumes. Cardiac motion artifacts are effectively removed by signal averaging. Respiratory motion artifacts are removed by breath hold. Image artifacts caused by an inadequate number of pauses or by inappropriate placement of the pauses within a scan are shown, and ways to avoid these artifacts are discussed. 3DPAUSE provides the ability to acquire three-dimensional arrays in the thoracic cavity with minimal artifacts from respiratory and cardiac motions in a clinically reasonable time.

Animals↗
PubMed1992

Partial angle inversion recovery (PAIR) MR imaging: spin-echo and snapshot implementation.

The effects of varying the inversion or excitation RF pulse flip angles on image contrast and imaging time have been investigated in IR imaging theoretically, with phantoms and with normal volunteers. Signal intensity in an IR pulse sequence as a function of excitation, inversion and refocusing pulse flip angles was calculated from the solution to the Bloch equations and was utilized to determine the contrast behavior of a lesion/liver model. Theoretical and experimental results were consistent with each other. With the TI chosen to suppress the fat signal, optimization of the excitation pulse flip angle results in an increase in lesion/liver contrast or allows reduction in imaging time which, in turn, can be traded for an increased number of averages. This, in normal volunteers, improved spleen/liver contrast-to-noise ratio (9.0 vs. 5.7, n = 8, p less than 0.01) and suppressed respiratory ghosts by 33% (p less than 0.01). Reducing or increasing the inversion pulse from 180 degrees results in shorter TI needed to null the signal from the tissue of interest. Although this decreases the contrast-to-noise ratio, it can substantially increase the number of sections which can be imaged per given TR in conventional IR imaging or during breathold in the snapshot IR (turboFLASH) technique. Thus, the optimization of RF pulses is useful in obtaining faster IR images, increasing the contrast and/or increasing the number of imaging planes.

Abdomen↗
PubMed1990

Magnetic resonance imaging of the triangular fibrocartilage complex.

Magnetic resonance imaging of seven fresh human cadaveric specimens was used to evaluate the integrity of the triangular fibrocartilage complex of the wrist. A variety of imaging parameters were systematically investigated, including T1-weighted images with and without contrast, long repetition times, short echo time images, and T2-weighted sequences. A variety of imaging planes were also evaluated. Wrist arthrography, dissection, and frozen coronal sections were done to substantiate our interpretations of the magnetic resonance images. T2-weighted images in the coronal plane proved to be of the greatest diagnostic value because the synovial fluid of the joint spaces serves as an excellent endogenous contrast agent. Long repetition time, short echo time sequences could be simultaneously obtained with T2-weighted sequences with the use of a multi-echo pulse sequence to provide an excellent diagnostic package in the future. On the basis of our investigation, the triangular fibrocartilage complex can be consistently and accurately evaluated with magnetic resonance imaging. As magnetic resonance imaging technology improves, wrist probes and suitable magnets should become available that will make the evaluation of triangular fibrocartilage complex abnormalities with magnetic resonance imaging clinically useful.

Cartilage, Articular↗
PubMed2005

Single molecule modulation spectroscopy of conjugated polymers.

This paper describes a new single molecule spectroscopy approach for the investigation of triplet-triplet and singlet-triplet interactions in conjugated polymers. The technique involves the irradiation of isolated single, mulitchromophoric, conjugated polymer molecules by a repetitive sequence of variable-intensity microsecond time scale excitation pulses. The fluorescence intensity is synchronously time-averaged for thousands of cycles of the pulse sequence to yield a high signal-to-noise fluorescence transient on the microsecond time scale. The transient can be analyzed with kinetic models to obtain quantitative information about the kinetics of triplet-triplet exciton annihilation and the quenching of singlet excitons by triplet excitons in conjugated polymers.

Journal Article↗
PubMed2003

Frequency-locked, injection-seeded, pulsed narrowband optical parametric generator.

A frequency-locked, injection-seeded, pulsed optical parametric generator (OPG) has been developed for short-range infrared differential absorption lidar (DIAL) applications. The periodically poled lithium niobate OPG is pumped by a passively Q-switched Nd:YAG microlaser and is seeded by a distributed feedback (DFB) diode laser. The OPG is designed for DIAL measurement of a narrow R-branch transition of methane at 3.2704 microm. The output of the OPG is a two-pulse sequence with a 100-micros temporal separation between the pulses, where the first pulse is absorbed by methane and the second pulse is not absorbed. The first pulse is actively locked to the methane absorption feature by use of the derivative of the transmission spectrum through a reference cell. Although the device was not optimized for output power, the 3.27-microm OPG output energies of the first and second pulses are 5.5 and 5.9 microJ, respectively, producing 21 mW when operated at 1818 Hz.

Journal Article↗
PubMed2003

Efficient 5QMAS NMR of spin-5/2 nuclei: use of fast amplitude-modulated radio-frequency pulses and cogwheel phase cycling.

We report here an efficient multiple-quantum magic-angle spinning (MQMAS) pulse sequence involving fast amplitude-modulated (FAM) radio-frequency pulses for excitation and conversion of five-quantum (5Q) coherences of spin-5/2 nuclei. The use of a FAM-I type pulse train for the conversion of 5Q into 1Q coherences proves to be easier to implement experimentally than the earlier suggested use of a FAM-II type sequence [J. Magn. Reson. 154 (2002) 280], while delivering at least equal signal enhancement. Results of numerical simulations and experimental 27Al 5QMAS spectra of aluminium acetylacetonate for different excitation and conversion schemes are compared to substantiate these claims. We also demonstrate the feasibility of acquiring 5QMAS spectra of spin-5/2 systems using cogwheel phase cycling [J. Magn. Reson. 155 (2002) 300] to select the desired coherence pathways. A cogwheel phase cycle of only 57 steps is shown to be as effective as the minimum conventional nested 77-step phase cycle.

Journal Article↗
PubMed1992

Factors influencing contrast in fast spin-echo MR imaging.

Multi-echo pulse sequences for producing T2-weighted images in much reduced imaging times have recently been developed for routine clinical use. A number of recent articles have described the contrast obtained with fast spin-echo (FSE) sequences and have generally indicated that they depict tissues very similarly to conventional spin-echo (SE) imaging. There are, however, some important differences in contrast between some tissues in FSE images. This work presents a detailed study of the contrast obtained with FSE imaging sequences and examines the image sequence and tissue parameters which influence contrast. The use of multiple refocusing pulses produces several subtle effects not seen in conventional SE imaging sequences, and in this study the precise nature and extent of such effects are described. The relative contributions to image contrast of magnetization transfer, the decoupling of J-modulation effects, the production of stimulated echoes and direct saturation effects, of diffusion and of the effects of the differential attenuation of different spatial frequencies, are each quantified. The mechanisms responsible for the brighter fat signal seen in FSE images, as well as the loss of signal from some other tissues, are explained. Computer simulations, phantom experiments, and clinical images are all used to support the conclusions.

Brain↗
PubMed2003

Quantitative imaging of magnetization transfer using an inversion recovery sequence.

A new imaging method has been developed for quantitatively measuring magnetization transfer (MT). It uses a simple inversion recovery sequence, although one with very short (milliseconds) inversion times, and thus can be implemented on clinical imaging systems with little modification to existing pulse sequences. The sequence requires an inversion pulse with a length much longer than T(2m) (typically 10 micros) and much shorter than T(2f) (typically tens of ms) and 1/k(mf) (typically tens of ms), where T(2m) and T(2f) are the transverse relaxation times of the immobile macromolecular and free water protons, respectively, and k(mf) is the rate of MT between these populations. The resultant NMR signal is sensitive to MT when this inversion pulse affects the mobile and immobile proton pools to different degrees and by appropriate analysis of the signals obtained for different inversion times, quantitative information can be derived on the macromolecular content and exchange rates within the sample. The method has been used in conjunction with echo planar imaging to produce maps of the spatial distribution of the macromolecular content and MT rate in cross-linked bovine serum albumin. Comparisons between this method and other quantitative MT techniques are discussed.

Echo-Planar Imaging↗
PubMed1999

Multiple-quantum relaxation in the magic-angle-spinning NMR of 13C spin pairs.

We determine the decay rate constants of zero-, double- and single-quantum coherence for 13C spin pairs in magic-angle-spinning solid-state NMR. The double-quantum coherence is excited by a C7 pulse sequence and converted into zero-quantum coherence by a frequency-selective pair of pi/2 pulses. The zero-quantum coherence is reconverted into observable magnetization by a second pair of pi/2 pulses followed by a second C7 sequence. In a magnetically dilute system where the 13C-13C distance is 0.296 nm, the relaxation rate constants are consistent with a model of uncorrelated random fields at the two labeled 13C sites. In a fully-labelled system with a short 13C-13C distance of 0.153 nm, the measured rate constants are inconsistent with the uncorrelated random field model.

Carbon Isotopes↗