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At least 127 records · Page 7Linked to original sources
PubMed1991

A modified pulse sequence for in vivo diffusion imaging with reduced motion artifacts.

A modified spin-echo diffusion imaging pulse sequence incorporating bipolar gradients that is less sensitive to macroscopic motion-induced artifacts is presented. Expressions for apparent diffusion coefficient in the presence of microcirculation and macroscopic motions and contrast-to-noise ratio in the diffusion map are derived. Diffusion coefficients of liquid phantoms have been measured using a 1.5-T GE Signa system. Experiments performed to demonstrate the immunity to motion-induced errors are reported. Brain images from human volunteer have also been included to show the potential clinical application of the proposed pulse sequence.

Brain↗
PubMed2003

Magnetic resonance imaging of the liver with ultrashort TE (UTE) pulse sequences.

PURPOSE: To assess the feasibility of imaging the liver in volunteers and patients with ultrashort echo time (UTE) pulse sequences. MATERIALS AND METHODS: Seven normal controls as well as 12 patients with biopsy-proven generalized liver disease and three patients with focal disease were examined using pulse sequences with initial TEs of 0.08 msec followed by three later echoes, with or without frequency-based fat suppression. T(2)* values were calculated from regions of interest in the liver. RESULTS: Good image quality was obtained in each subject. There was a highly significant difference in the mean T(2)* values between the normal controls and patients with generalized liver disease (P = 0.001). T(2)* was significantly decreased in hemochromatosis (P = 0.002) and increased in cirrhosis (P = 0.04), compared with controls. T(2)* also correlated with functional status assessed by Child's grade (P = 0.001). A hepatocellular carcinoma showed reduced short T(2) components in the region of thermal ablation and evidence of a subcapsular hematoma which were not apparent with conventional imaging. CONCLUSIONS: Imaging of the liver with UTE sequences showed good image quality and tolerance of abdominal motion. T(2)* was specifically correlated with the presence of hemochromatosis, cirrhosis, and functional grade. Imaging of short T(2) relaxation components may provide useful information in disease.

Adult↗
PubMedDate not supplied

A new pulse sequence for determining T1 and T2 simultaneously.

Determination of the relaxation times T1 and T2 which are important for tissue characterization generally requires the use of different pulse sequences in magnetic resonance imaging. In this study, a new pulse sequence which facilitates simultaneous determination of the T1 and T2 times is presented. Determination takes place in this case pixel by pixel from the measured images. The measuring time corresponds in this case approximately to that of a normal spin-echo sequence with long repetition time and two data acquisitions. The functional dependence of the accuracy of the T1 and T2 determination upon external errors, e.g., angle of rotation errors, is discussed. The tissue contrast behavior of the individual echoes is shown and its dependence on pulse parameters is explained.

Biophysical Phenomena↗
PubMed1998

Fast imaging of phosphocreatine using a RARE pulse sequence.

A technique is described for acquiring phosphocreatine (PCr) images of skeletal muscle using a rapid acquisition with relaxation enhancement (RARE) pulse sequence. All of the phosphorus metabolites other than PCr are forced to dephase within the first few echoes, whereas the Carr-Purcell Meiboom-Gill (CPMG) pulse sequence maintains a high PCr signal long enough to acquire 64 echoes in a single shot. Axial PCr images of a human forearm with a signal-to-noise ratio of 9 were acquired in 2 min. The effect of the refocusing pulse section profile on the ratio of desired to undesired metabolite signal is demonstrated.

Forearm↗
PubMed2004

Fast multi-planar gradient echo MR imaging: impact of variation in pulse sequence parameters on image quality and artifacts.

The purpose of this investigation was to quantitatively evaluate the practical impact of alteration of key imaging parameters on image quality and artifacts in fast multi-planar gradient echo (GRE) pulse sequences. These include multi-planar GRASS (MPGR) and fast multi-planar spoiled GRASS (FMPSPGR). We developed a composite phantom with different T(1) and T(2) values comprising the range of common biological tissues, which was also subjected to periodic motion in order to evaluate motion effects. Magnetic resonance imaging was performed on a GE Signa 1.5-T system. Experimental variations in key parameters included excitation flip angle (FL), echo time (TE), repetition time (TR), and receive bandwidth (BW). Quantitative analysis consisted of signal-to-noise-ratio (SNR) and contrast (CN), image nonuniformity (NU), full-width-at-half-maximum (FWHM) (i.e., blurring or geometric distortion), and ghosting ratio (GR). We found that flip angle, TE, and TR play particularly critical roles in determining image signal, homogeneity, and ghosting artifact with these sequences. Optimum clinical application of these pulse sequences requires careful attention to these imaging parameters and to their complex interactions.

Artifacts↗
PubMed2002

Liver: T2-weighted MR imaging with breath-hold fast-recovery optimized fast spin-echo compared with breath-hold half-Fourier and non-breath-hold respiratory-triggered fast spin-echo pulse sequences.

At liver magnetic resonance (MR) imaging in 38 patients, a breath-hold T2-weighted fast spin-echo (SE) pulse sequence optimized with fast recovery was compared with a conventional respiratory-triggered fast SE sequence and a breath-hold single-shot fast SE sequence. Mean signal-to-noise ratios for liver and contrast-to-noise ratios for hepatic lesions were higher with the breath-hold fast-recovery fast SE sequence than with the respiratory-triggered fast SE sequence (P <.05). Breath-hold fast-recovery images displayed better lesion clarity than did single-shot fast SE images (P <.05) and fewer image artifacts than did respiratory-triggered fast SE images (P <.05). The ability to determine lesion size and the overall image quality was best with the breath-hold fast-recovery sequence (P <.05). These results may justify use of the breath-hold fast-recovery fast SE pulse sequence for first-line T2-weighted MR imaging of the liver.

Adult↗
PubMed1987

[Definitions of pulse sequences and parameter weightings in the NMR tomogram].

Due to the different possibilities of image creation in MR tomography there is no clear terminology of pulse sequences and MR images. This paper tries to define the designation of pulse sequence parameters in a practical way and to specify the term "parameter weighting of MR images". Starting with general definitions, special definitions for CNS and liver are elaborated.

Central Nervous System↗
PubMed1995

Magnetic resonance imaging in partial epilepsy: additional abnormalities shown with the fluid attenuated inversion recovery (FLAIR) pulse sequence.

Thirty six patients with a history of partial epilepsy had MRI of the brain performed with conventional T1 and T2 weighted pulse sequences as well as the fluid attenuated inversion recovery (FLAIR) sequence. Abnormalities were found in 20 cases (56%), in whom there were 25 lesions or groups of lesions. Twenty four of these lesions were more conspicuous with the FLAIR sequence than with any of the conventional sequences. In 11 of these 20 cases, lesions thought to be of aetiological importance were only seen with the FLAIR sequence. In eight this was a solitary lesion. In the other three, an additional and apparently significant lesion (or lesions) was only seen with the FLAIR sequence when another lesion had been identified with both conventional and FLAIR sequences. The 11 additional lesions or groups of lesions were seen in the hippocampus, amygdala, cortex, or subcortical and periventricular regions. No lesion was found with any pulse sequence in 16 (44%) of the original group of 36 patients. In the eight cases where a lesion was seen only with the FLAIR sequence, localisation was concordant with the electroclinical features. Two of the eight patients with solitary lesions seen only on the FLAIR sequence underwent surgery, after which there was pathological confirmation of the abnormality identified with imaging. In one patient with a congenital cavernoma, the primary lesion was best seen with a contrast enhanced T1 weighted spin echo sequence. In this selected series, the FLAIR sequence increased the yield of MRI examinations of the brain by 30%.

Adolescent↗
PubMed2004

A new pulse sequence to visualize slow flow.

The purpose of this paper is to present a new pulse sequence for visualizing slow flow. The new sequence consists of an initial Stejskal-Tanner flow sensitization part followed by a DEFT pulse and a spoiler gradient. A single-shot TSE readout train is then applied to sample the NMR signal. The sequence was initially tested using a simple flow phantom. To verify potential clinical use, both flow-sensitive MRCP and cerebrospinal fluid (CSF) images were produced. The phantom study proved the sequence sensitivity to flow in the range 0-1 cm/s. bVE-factors 1.5, 3, 6 and 12 were chosen. Within this flow velocity range, the signal dropped as predicted theoretically. This indicates that the method can be used to quantify flow. All anatomical features seen in a standard MRCP sequence were identified and the methods sensitivity to CSF flow was demonstrated by sagital images of the head. A new pulse sequence sensitive to slow flow has been developed.

Cerebral Ventricles↗
PubMed1980

Electrical stimulation of the callus formation by means of bipolar rectangular pulse sequences.

The clinical application of the electrical stimulation, lasting several weeks, for the callus formation is reported in 11 patients. Bipolar rectangular pulse sequences were used for the stimulation at a frequency of 1 Hz and a current intensity of +/-20mu amp. The electrical stimulation was successfully employed after distraction osteotomies with a KDA-apparatus in shortening of the leg provoked by different causes or in the treatment of pseudarthroses. The realignment of the newly formed callus and the osseous consolidation are stimulated and speeded up by the bipolar rectangular pulse sequences as it is also shown in the light of the roentgenograms of a case.

Adult↗
PubMed2004

A pulse sequence for directly measuring the anharmonicities of coupled vibrations: Two-quantum two-dimensional infrared spectroscopy.

We have experimentally demonstrated a pulse sequence for the acquisition of heterodyned two-dimensional infrared (2D IR) spectra that correlates the overtone and combination bands to the fundamental frequencies. The spectra are generated by Fourier transforming the time domain signal that is allowed to evolve during one- and two-quantum coherence times. In this manner, the overtone and combination bands appear along the two-quantum axis, resulting in a direct determination of the diagonal and off-diagonal anharmonicities. To demonstrate this pulse sequence, we have collected two-quantum 2D IR spectra of a ruthenium dicarbonyl complex, extracted the diagonal and off-diagonal anharmonicities, and simulated the spectra using an exciton model. Several polarization conditions are presented that suppress the diagonal or cross peaks and we have used them to improve the accuracy of the measurement.

Journal Article↗
PubMed1986

A modified saturation-recovery approximation for multiple spin-echo pulse sequences.

In equations describing multiple spin-echo pulse sequences, the T1 dependence is often approximated by a saturation-recovery expression, which is valid only when the repetition time TR is much greater than the time required for the echoes. A slight modification of this approximation greatly increases its accuracy for small TR values.

Humans↗
PubMedDate not supplied

Toward an automated analysis system for nuclear magnetic resonance imaging. I. Efficient pulse sequences for simultaneous T1-T2 imaging.

Simultaneous T1 and T2 images have been obtained using multiple-echo self-normalizing sequences. Differential T1 and T2 discrimination for two of these sequences, double saturation recovery and inversion-saturation recovery, have been explored in detail. The inversion-saturation recovery sequence exhibits T1 discrimination comparable to, and in many cases better than, that of optimal T1 weighted conventional pulse sequences, but is poor for T2 imaging. The double saturation recovery sequence yields comparable tissue discrimination for T1 and T2 imaging, but is poor compared to optimal T1 weighted and T2 weighted conventional pulse sequences. Simultaneous T1 and T2 images represent quantitative maps of intrinsic tissue properties, and hence form a natural basis for an automated image analysis and tissue classification system.

Brain↗
PubMed2003

Real-time imaging of two-dimensional cardiac strain using a harmonic phase magnetic resonance imaging (HARP-MRI) pulse sequence.

The harmonic phase (HARP) method provides automatic and rapid analysis of tagged magnetic resonance (MR) images for quantification and visualization of myocardial strain. In this article, the development and implementation of a pulse sequence that acquires HARP images in real time are described. In this pulse sequence, a CINE sequence of images with 1-1 spatial modulation of magnetization (SPAMM) tags are acquired during each cardiac cycle, alternating between vertical and horizontal tags in successive heartbeats. An incrementing train of imaging RF flip angles is used to compensate for the decay of the harmonic peaks due to both T(1) relaxation and the applied imaging pulses. The magnitude images displaying coarse anatomy are automatically reconstructed and displayed in real time after each heartbeat. HARP strain images are generated offline at a rate of four images per second; real-time processing should be possible with faster algorithms or computers. A comparison of myocardial contractility in non-breath-hold and breath-hold experiments in normal humans is presented.

Adult↗
PubMed1992

Selective saturation and inversion of multiple resonances in high-resolution solid-state 13C experiments using slow spinning CP/MAS and tailored DANTE pulse sequences.

Taking advantage of the long 13C T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in 13C CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90 degrees DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of this method, including the simplification of the measurement of the principal values of the 13C chemical shift tensor under slow MAS conditions, are described. The determination of the aromaticity of coal using a relatively slow MAS rate is also described.

Anisoles↗
PubMed1993

Fast multiplanar spoiled gradient-recalled imaging of the liver: pulse sequence optimization and comparison with spin-echo MR imaging.

OBJECTIVE: The purpose of this study was to optimize a new rapid-acquisition MR pulse sequence, called fast multiplanar spoiled gradient-recalled (FMPSPGR) imaging, for breath-hold imaging of the liver and to compare unenhanced and contrast-enhanced FMPSPGR with standard spin-echo imaging in detecting liver tumors. MATERIALS AND METHODS: The pulse sequence was optimized at 1.5 T with a healthy volunteer. Various scanning parameters were evaluated, and liver-spleen signal difference/noise measurements were used to estimate lesion contrast-to-noise ratios. We examined 24 patients with hepatic masses using the optimized sequence with spin-echo T1-weighted and T2-weighted imaging as well as unenhanced and gadopentetate dimeglumine-enhanced FMPSPGR imaging. The contrast-to-noise ratio for the hepatic tumors was determined for each sequence. Three radiologists who did not know the biopsy or test results reviewed all images for lesion conspicuity, lesion tissue specificity, and overall image quality. RESULTS: A comparison of unenhanced FMPSPGR images with spin-echo T1-weighted images showed a 40% improvement in mean contrast-to-noise ratio and a 70% improvement in liver signal-to-noise ratio for the FMPSPGR images. A comparison of gadopentetate dimeglumine-enhanced FMPSPGR images with spin-echo T1- and T2-weighted images showed a superior contrast-to-noise ratio for the enhanced FMPSPGR images in 17 (68%) of 25 hepatic lesions, which included all hepatic cysts (n = 3) and all hepatomas (n = 6), and in six of 12 patients with other liver tumors. The results of contrast-to-noise ratio for four patients with hemangiomas were mixed. For the remaining eight lesions, the contrast-to-noise ratio for spin-echo T1- and T2-weighted images predominated in three and five cases, respectively. Contrast-enhanced FMPSPGR images revealed a 40% and 300% increase in contrast-to-noise ratio compared with T2- and T1-weighted images, respectively. All three radiologists preferred the contrast-enhanced FMPSPGR images for overall image quality. For lesion conspicuity and specificity, however, the three radiologists differed, with a preference for the FMPSPGR images in 52%, 80%, and 40% of cases for lesion conspicuity and in 68%, 40%, and 60% of cases for lesion specificity. CONCLUSION: FMPSPGR is a new, ultrafast MR sequence that provides T1-weighted images of the liver during suspended respiration. Contrast-to-noise ratio and liver signal-to-noise ratio are significantly improved over those on conventional spin-echo T1-weighted images. The combination of breath-hold FMPSPGR with gadopentetate dimeglumine is an excellent technique that can be used to rapidly evaluate the liver with superior overall image quality. Contrast-to-noise ratios are generally superior to T2-weighted spin-echo images, making this technique a useful adjunct to conventional spin-echo MR imaging.

Adult↗
PubMed1989

Magnetic resonance imaging: comparison of four pulse sequences in assessing primary bone tumours.

A prospective magnetic resonance imaging (MRI) study was carried out in 13 patients (19 examinations) with primary bone tumours to assess the relative value of each of four pulse sequences in showing the extent and nature of the lesion. The four pulse sequences used were a T1-weighted spin-echo (SE544/44), a T2-weighted spin echo (SE1500/80), a short TI inversion recovery (STIR) (IR500/100/44), and a partial saturation (PS) (PS500/22) with field echo data collection. For soft tissue disease the combination of PS and STIR gave better definition of the boundary of the tumour than the more conventional T1 and T2-weighted spin echo sequences. For the demonstration of bone cortex, periosteal change and calcification, T1 and T2-weighted spin echo sequences were better. However, for calcified tissues, plain radiographs were better than either MRI combination. On the assumption that plain films will be available in all cases, PS and STIR sequences could therefore be substituted for T1 and T2-weighted spin echo sequences allowing an increase in soft tissue detectability for lesions in both red and yellow marrow.

Adolescent↗
PubMed1998

An adiabatic multiple spin-echo pulse sequence: removal of systematic errors due to pulse imperfections and off-resonance effects.

Application of AFP (adiabatic fast passage) pulses for removal of systematic errors associated with multiple spin-echo sequences is demonstrated. The adiabatic fast passage pulses facilitate minimization of cumulative pulse errors for all three components of magnetization. It is also shown that off-resonance effects present in conventional CPMG sequences which degrade image quality in magnetic resonance imaging and introduce systematic errors in measured T2 relaxation time peak amplitudes can be suppressed by introduction of AFP pulses without any degradation of overall signal intensity. The technique has been tested on the 15N spin-spin relaxation time measurements of a 110 amino acid domain of the F-actin cross-linking protein.

Actins↗