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PubMedDate not supplied

Determination of the optimal imaging parameters of the RODEO pulse sequence by computer simulation.

A computer program has been developed for evaluating the NMR signal response of various imaging parameters and its efficiency in fat suppression of the RODEO (ROtating Delivery of Excitation Off-resonance) pulse sequence. Both spoiled and refocused RODEO pulse sequences have been considered. By numerically solving the Bloch equation modified for a three-dimensional volume of spins, for realistic RF excitations and Lorentzian distribution of the frequency spectrum for both fat and water, the program permits the imaging contrast and fat suppression of the RODEO pulse sequences to be assessed quantitatively. We have found that excellent fat suppression can be achieved by choosing appropriate imaging parameters. Imaging contrast for different tissues can be enhanced by using longer repetition time (TR) in the spoiled scheme. The complex pattern of NMR signal response and imaging contrast has been observed in the refocused scheme.

Adipose Tissue↗
PubMed1991

T2-weighted spin-echo pulse sequence with variable repetition and echo times for reduction of MR image acquisition time.

Use of intraacquisition modification of pulse-sequence parameters to reduce acquisition time for conventional T2-weighted spin-echo images was evaluated. With this technique (variable-rate spin-echo pulse sequence), the repetition time and echo time (TR msec/TE msec) were reduced during imaging as a function of the phase-encoding view. To maintain T2-based contrast, TR and TE for the low-spatial-frequency views were left at their prescribed values (eg, 2,000/80). TR and TE for the high-spatial-frequency views were progressively reduced during imaging (eg, to 1,000/20). Acquisition time was reduced by as much as 25%. In one pulse sequence, the duration of multisection imaging nominally performed at TR 2,000 and with 256 phase-encoding views was reduced from 9 minutes 30 seconds to 6 minutes 30 seconds. In all sequences, edges and small structures were enhanced, and T2 contrast was somewhat decreased in high spatial frequencies. Filtering of the raw data before reconstruction can suppress these effects and provide a net increase in contrast-to-noise ratio.

Brain↗
PubMed1994

[The use of new turbo spin-echo pulse sequences with and without fat suppression in the diagnosis and staging of prostatic carcinoma].

MR studies using turbo spin-echo pulse sequences (TSE) were performed on 27 patients with histologically confirmed prostate cancer. A prospective study was conducted in 15 patients who underwent radical prostatectomy. Turbo SE pulse sequences generate strongly T2-weighted images of excellent quality with reduction of measurement time by a factor of 4-6. A comparison with standard T2-weighted spin-echo pulse sequences indicated an objective improvement in the contrast of pathological structures (p < 0.001). With respect to differentiation of stages T2 and T3 (TNM) during pathohistological correlation, sensitivity of 80%, specificity of 40% and an accuracy rate of 67% were obtained. Sensitivity of 71% and specificity of 75% were obtained when decoding cases of infiltration into the seminal vesicles, with an accuracy rate of 73%.

Adipose Tissue↗
PubMed2002

NMR spin locking of proton magnetization under a frequency-switched Lee-Goldburg pulse sequence.

The spin dynamics of NMR spin locking of proton magnetization under a frequency-switched Lee-Goldburg (FSLG) pulse sequence is investigated for a better understanding of the line-narrowing mechanism in PISEMA experiments. For the sample of oriented 15N(1,3,5,7)-labeled gramicidin A in hydrated DMPC bilayers, it is found that the spin-lattice relaxation time T(1rho)(H) in the tilted rotating frame is about five times shorter when the 1H magnetization is spin locked at the magic angle by the FSLG sequence compared to the simple Lee-Goldburg sequence. It is believed that the rapid phase alternation of the effective fields during the FSLG cycles results in averaging of the spin lock field so that the spin lock becomes less efficient. A FSLG supercycle has been suggested here to slow the phase alternation. It has been demonstrated experimentally that a modified PISEMA pulse sequence with such supercycles gives rise to about 30% line narrowing in the dipolar dimension in the PISEMA spectra compared to a standard PISEMA pulse sequence.

Gramicidin↗
PubMed2004

Hypercapnic normalization of BOLD fMRI: comparison across field strengths and pulse sequences.

The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal response to neural stimulation is influenced by many factors that are unrelated to the stimulus. These factors are physiological, such as the resting venous cerebral blood volume (CBV(v)) and vessel size, as well as experimental, such as pulse sequence and static magnetic field strength (B(0)). Thus, it is difficult to compare task-induced fMRI signals across subjects, field strengths, and pulse sequences. This problem can be overcome by normalizing the neural activity-induced BOLD fMRI response by a global hypercapnia-induced BOLD signal. To demonstrate the effectiveness of the BOLD normalization approach, gradient-echo BOLD fMRI at 1.5, 4, and 7 T and spin-echo BOLD fMRI at 4 T were performed in human subjects. For neural stimulation, subjects performed sequential finger movements at 2 Hz, while for global stimulation, subjects breathed a 5% CO(2) gas mixture. Under all conditions, voxels containing primarily large veins and those containing primarily active tissue (i.e., capillaries and small veins) showed distinguishable behavior after hypercapnic normalization. This allowed functional activity to be more accurately localized and quantified based on changes in venous blood oxygenation alone. The normalized BOLD signal induced by the motor task was consistent across different magnetic fields and pulse sequences, and corresponded well with cerebral blood flow measurements. Our data suggest that the hypercapnic normalization approach can improve the spatial specificity and interpretation of BOLD signals, allowing comparison of BOLD signals across subjects, field strengths, and pulse sequences. A theoretical framework for this method is provided.

Adult↗
PubMed2000

Compound radiofrequency-driven recoupling pulse sequences for efficient magnetization transfer by homonuclear dipolar interaction under magic-angle spinning conditions

The maximum of the transferred magnetization in rotating powdered solids under the radiofrequency-driven recoupling (RFDR) pulse sequence is enhanced by reducing the orientation dependence of the effective recoupled homonuclear dipolar interaction. The compound RFDR (CRFDR) pulse sequence for this enhancement consists of RFDR pulse units (tau(i)-pi-tau(R)-pi-1171;tau(i)) with different tau(i), where tau(R) is the sample rotation period, tau(i) and 1171;tau(i) (=tau(R) - tau(i)) are delays, and pi is a 180 degrees pulse. The delay tau(i) modifies the zero-quantum spin operators and the sample rotation-angle dependence of the recoupled dipolar Hamiltonian. The CRFDR pulse sequences were optimized for mixing by varying tau(i). Numerical simulation for the two-spin system only with a dipolar interaction and isotropic chemical shifts indicates that the transfer efficiency of CRFDR averaged over the powder is about 70%, which is 30% higher than the efficiency of the RFDR pulse over a broad range of about 1/tau(R) in resonance frequency difference. The CRFDR sequences need about 60% longer mixing times to maximize the transferred magnetizaion in comparison with the original RFDR sequence. Chemical shift anisotropy, the other dipolar interactions, and relaxation generally reduce the enhancement by CRFDR. Experiments for fully (13)C-labeled alanine, however, show that the maximum of the magnetization transferred with CRFDR from the carboxyl to alpha carbon is about 15% greater than that with RFDR. Copyright 2000 Academic Press.

Journal Article↗
PubMed1989

Detection of hepatic metastases with MR imaging: spin-echo vs phase-contrast pulse sequences at 0.6 T.

The purpose of this study was to compare the sensitivity of T1-weighted and T2-weighted spin-echo (SE) pulse sequences with T2-weighted phase-contrast (PC) imaging techniques for the detection of hepatic metastases. Pulse-sequences performance was evaluated in 52 consecutive patients with 88 hepatic metastases who underwent MR imaging at 0.6 T. Lesion-liver contrast-to-noise ratios (CNR) on SE 260/14 (-12.4 +/- 6.7) and PC 2350/60 (+10.8 +/- 4.2) images were significantly (p less than .05) greater than on SE 2350/60 (+ 7.8 +/- 3.9), SE 2350/120 (+8.1 +/- 4.8), SE 2350/180 (+7.9 +/- 4.5), and PC 2350/30 (+4.6 +/- 2.9) images. Sensitivity for detection of 88 individual metastases was comparable on SE 260/14 (78 of 88 patients) and PC 2350/60 (81 of 88 patients) images and was significantly (p less than .05) greater than on in-phase T2-weighted SE images (TE = 60, 70 of 88 patients; TE = 120, 69 of 88 patients; TE = 180, 65 of 88 patients). Histologic analysis of tumor-free liver showed fatty change in 11 of 13 specimens available for pathologic evaluation. In all 11 of those patients, PC images increased tumor-liver contrast in comparison with the in-phase SE images. This analysis suggests that for detection of hepatic metastases at midfield strengths, the T1-weighted, short TR/short TE (SE 260/14) and the T2-weighted, phase-contrast (PC 2350/60) pulse sequences offer comparable performance.

Adult↗
PubMed1995

Optimization of MR pulse sequences for Bayesian image segmentation.

A method for optimizing MR imaging pulse sequence parameters in a statistical framework is presented. Parameters are defined to be optimal when the resulting scalar images yield optimal image segmentations using Bayesian pixel classification. Thus, Bayes risk is used as the objective function to minimize. Approximations are made to give a tractable solution in a four-step procedure. A sample calculation is carried out to determine the optimal TR and flip angle for scalar SPGR imaging of the brain. Overall, this paper gives a new approach to optimize MRI pulse sequences for the specific objective of improved image segmentation.

Bayes Theorem↗
PubMed2000

[The value of STIR pulse sequence MR imaging in patients with pulmonary carcinoma].

OBJECTIVE: To evaluate the value of short TI inversion-recovery (STIR) MR imaging technique in diagnosing pulmonary carcinoma. METHODS: One hundred and eight patients with pathologically confirmed pulmonary carcinoma were examined by STIR pulse sequence and the results were compared with these by SE pulse sequence. RESULTS: STIR MR imaging was superior to conventional SE pulse sequences in significantly increasing the detection rate of lesions, improving images along the edge of the lesion, tumor invasion to adjacent structures, and demonstrating lymph node enlargement. CONCLUSION: STIR, with its sensitivity, is particularly useful to reveal metastatic lymph nodes and invasion to the pleura and chest wall.

Adult↗
PubMed1994

Modified Jeener solid-echo pulse sequences for the measurement of the proton dipolar spin-lattice relaxation time (T1D) of tissue solid-like macromolecular components.

Modified Jeener solid-echo pulse sequences are proposed for the measurement of the proton dipolar spin-lattice relaxation time, T1D, of motionally restricted (solid-like) components in the presence of mobile molecular species, such as encountered in biological tissue. A phase-cycled composite-pulse sequence was used for detection of the dipolar signal and cancellation of the Zeeman signal. A homospoil gradient pulse was added to the Jeener echo pulse sequence to enhance dephasing of the transverse magnetization components of mobile species, thereby aiding in elimination of the Zeeman signal during dipolar signal acquisition. A modified Jeener echo sequence incorporating water suppression is also proposed as a means to further depress the Zeeman signal arising from mobile components. The modified Jeener echo sequences were successfully used for the measurement of proton T1D values of solid 2,6-dimethylphenol and Sephadex gels of differing degrees of cross linking and hydration.

Cross-Linking Reagents↗
PubMed1997

NQR transient nutation and rotary echoes in the effective field of multiple-pulse sequences.

We present the results of the experimental investigations of the transient processes preceding the establishment of quasistationary states in multiple-pulse nuclear quadrupole resonance. It is shown that the inversion of the phase of radio frequency pulses in the pulse sequence or an extra pulse produces the echo signal in the effective field of the multiple-pulse sequence (the echo on the envelope of echo signals). The train of the echo signals in the effective field is also obtained. The application of this technique for the investigation of the dipole-dipole interactions in spin systems with large inhomogeneous broadening is discussed.

Magnetic Resonance Spectroscopy↗
PubMedDate not supplied

High resolution 3D imaging of the inner ear with a modified fast spin-echo pulse sequence.

A fast spin-echo pulse sequence is described that produces high resolution images of the inner ear without susceptibility artifacts. It uses thin overlapping slices that can be reformatted in multiple projections to provide a view of the 3D geometry of inner ear structures, such as the cochlea, vestibule, semicircular canals, and internal auditory canal. It has proven useful in screening adults for the presence of acoustic schwannoma and in identifying structural congenital lesions in children with sensorineural hearing loss.

Adult↗
PubMed1991

Longitudinal spin-order-based pulse sequence for lactate editing.

A new pulse sequence which edits proton spectra of lactate with full signal return and gives good suppression of water and fat signals is described. This sequence exploits longitudinal spin-order from lactate to edit lactate from fat. Experimental results from phantoms and excised pig heart are presented.

Animals↗
PubMedDate not supplied

Correction of CSF motion artifact on MR images of the brain and spine by pulse sequence modification: clinical evaluation.

A modification of the standard spin-echo pulse sequence designed to suppress motion artifacts was clinically evaluated on T2-weighted MR images of the cervicocranial region. A retrospective study involving 40 patients, half of whom were examined with a standard T2-weighted multislice spin-echo sequence and half of whom were examined with a gradient waveform modification of the same sequence, uniformly demonstrated restoration of CSF signal intensity on images obtained with the gradient modified sequence. The cervical subarachnoid spaces, cisterna magna, medullary cistern, pontine cistern, fourth ventricle, and aqueduct were more consistently and brightly represented. However, the phase-encoding artifacts arising from CSF motion were not significantly reduced by using the gradient waveform modified pulse sequence. Digital subtraction of an image obtained with the standard sequence from an image of the same slice with the gradient modified sequence provides a direct image representation of CSF flow.

Brain↗
PubMedDate not supplied

A mathematical model for signal from spins flowing during the application of spin echo pulse sequences.

Models are presented for both laminar and plug flow that predict the signal from spins flowing during the application of slice-selective spin echo pulse sequences. The models permit calculation of the total signal from a cylindrical vessel lying perpendicular to the slice and incorporate the effect of the physical displacement of the spins between successive excitations. This time-of-flight effect gives a signal which is composed of contributions from a finite number of spin populations, with each population signal weighted by the fractional volume of that spin population within the cylindrical vessel segment. The signal and fractional volume from each spin population are derived analytically for ten different spin echo pulse sequences. The models for plug and laminar flow have important application for predicting and interpreting flow effects observed in clinical images. They are shown to be useful for selecting pairs of pulse sequences that can be used to obtain digitally subtracted MR images which provide optimum contrast for flowing blood with essentially complete suppression of stationary anatomy. These models provide a means for quantitatively comparing the expected signal from flowing spins for the many techniques presently being investigated for MR angiography.

Blood Circulation↗
PubMedDate not supplied

The use of in vitro magnetic resonance tissue studies to optimise pulse sequences in the imaging of intracranial haemorrhage.

The choice of appropriate MR pulse sequences to highlight a particular pathology to best advantage is not always straightforward. In this study of intracranial haemorrhage, tissue relaxation times measured in vitro were entered into a computer program which calculated the signal intensity of each tissue (brain, blood, CSF, and bloody CSF) for all possible echo (TE) and repeat (TR) times. Analysis of graph plots of the results enabled the selection of pulse sequences which gave optimal separation of the signal intensities of intracranial haemorrhage from those of normal intracranial contents. The sequences thus chosen were used successfully in the imaging of patients with intracranial haemorrhage.

Cerebral Hemorrhage↗
PubMed2002

Superparamagnetic iron oxide-mediated hepatic signal intensity change in patients with and without cirrhosis: pulse sequence effects and Kupffer cell function.

PURPOSE: To analyze superparamagnetic iron oxide (SPIO)-mediated hepatic signal intensity change in cirrhotic and noncirrhotic liver and to investigate the relationship between pulse sequence effects in SPIO-enhanced magnetic resonance (MR) imaging for hepatic cirrhosis. MATERIALS AND METHODS: Twelve patients with and 12 patients without cirrhosis underwent T2-weighted fast spin-echo, T2*-weighted gradient-echo (GRE), and T1-weighted GRE MR imaging before and twice (early and late phase) after SPIO administration. To assess the effect of SPIO, postcontrast relative signal-to-noise ratio (SNR) changes were statistically analyzed with repeated measurements analysis of variance for each pulse sequence. RESULTS: No interaction was shown between groups and data time points for any pulse sequence. There was no significant difference in mean hepatic relative SNR change on T2-weighted fast spin-echo images between the cirrhotic group and noncirrhotic group (-38.6% and -40.7%, early phase; -42.2% and -49.6%, late phase, respectively). For GRE images, statistically significant differences in mean hepatic relative SNR change were found between the cirrhotic group and noncirrhotic group (-14.2% and -44.5%, early phase; -28.5% and -56.4%, late phase on T2*-weighted GRE images (P <.001); 31.8% and 12.9%, early phase; 23.8% and 2.2%, late phase on T1-weighted GRE images (P <.05), respectively. CONCLUSION: Decreased overall phagocytic activity in cirrhotic liver is more likely due to Kupffer cell dysfunction than to Kupffer cell depletion, since magnetic susceptibility effects on T2*-weighted GRE images depend on intracellular SPIO cluster size.

Adult↗
PubMed1991

Three-dimensional gadolinium-enhanced MR imaging of the breast: pulse sequence with fat suppression and magnetization transfer contrast. Work in progress.

A pulse sequence with magnetization transfer contrast and fat suppression was used in three-dimensional magnetic resonance imaging of the breast. Two healthy volunteers, one person with silicone implants, and 12 patients with clinical and/or mammographic findings suspicious for malignancy were evaluated prior to and following infusion of gadopentetate dimeglumine. Imaging time was approximately 7 minutes for each set of data (128 sections). Final voxel dimensions ranged from 1.4 x 0.8 x 0.8 mm to 1.6 x 0.9 x 0.9 mm. All carcinomas, including ductal and lobular types, were enhanced before and after infusion of contrast medium. Multifocal carcinoma and inflammatory carcinoma could be clearly visualized. Enhancement was not evident in patients with fat necrosis (n = 1) or scar (n = 1). Fibrocystic changes in one patient were visible as areas of increased signal intensity on preinfusion images. Resolution and contrast of MR images obtained with this pulse sequence appeared to be improved over that achieved with conventional breast MR imaging techniques. This method has the potential to supplement conventional diagnostic methods in the evaluation of breast disease.

Adult↗