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PubMed1997

Dynamic shimming for multi-slice magnetic resonance imaging.

Dynamic shimming has been implemented in three pulse sequences on a commercial GE Signa 1.5-T imaging system. Multi-slice field maps are acquired before the imaging sequence, and linear shim terms and center frequencies are calculated for each slice. During the imaging scan, the linear shim terms and center frequency are set before each pulse sequence repetition according to the current slice. Acquisition of multi-slice field maps and calculation of shim terms and center frequency for each slice are accomplished in a matter of seconds. Pulse sequences require only minimal modification to add dynamic shimming capability. Results are shown for a fat saturation spin-echo sequence, a single-shot echo-planar gradient-recalled echo sequence, and a spiral acquisition gradient-recalled echo sequence. In all cases, dynamic shimming with shim currents and center frequency optimized for each slice is shown to give better results than constant shim currents and a single center frequency optimized for the entire volume of interest.

Abdomen↗

PubMed1997

Measurement of AC magnetic field distribution using magnetic resonance imaging.

Electric currents are applied to body in numerous applications in medicine such as electrical impedance tomography, cardiac defibrillation, electrocautery, and physiotherapy. If the magnetic field within a region is measured, the currents generating these fields can be calculated using the curl operator. In this study, magnetic fields generated within a phantom by currents passing through an external wire is measured using a magnetic resonance imaging (MRI) system. A pulse sequence that is originally designed for mapping static magnetic field inhomogeneity is adapted. AC current in the form of a burst sine wave is applied synchronously with the pulse sequence. The frequency of the applied current is in the audio range with an amplitude of 175-mA rms. It is shown that each voxel value of sequential images obtained by the proposed pulse sequence is modulated similar to a single tone broadband frequency modulated (FM) waveform with the ac magnetic field strength determining the modulation index. An algorithm is developed to calculate the ac magnetic field intensity at each voxel using the frequency spectrum of the voxel signal. Experimental results show that the proposed algorithm can be used to calculate ac magnetic field distribution within a conducting sample that is placed in an MRI system.

Algorithms↗

PubMed1998

Complete Dipolar Decoupling of 13C and Its Use in Two-Dimensional Double-Quantum Solid-State NMR for Determining Polymer Conformations

A multiple-pulse technique for complete dipolar decoupling of directly bonded 13C-labeled sites is described. It achieves significant spectral simplifications in a recently introduced two-dimensional double-quantum solid-state NMR experiment for determining torsion angles. Both homonuclear and heteronuclear dipolar couplings are removed by combining a 13C multiple-pulse sequence with continuous-wave irradiation on the protons. The 13C sequence has a fundamental 10-pulse cycle which is a significantly modified magic-sandwich-echo sequence. The crucial heteronuclear decoupling is achieved by breaking the 360 degrees "inner" pulses in the magic sandwich into 90 degrees pulses and spacing them by 1H 360 degrees pulse lengths. Spectral artifacts typical of multiple-pulse sequences are eliminated by phase shifts between cycles. In contrast to many other multiple-pulse decoupling sequences, the long window in the cycle is the dwell time and can be longer than the inverse dipolar coupling, which makes the sequence practical for direct detection even with long pulse ring-down times. A modification of the sequence to scale the chemical shift and increase the effective spectral width is also presented. The 1D and double-quantum 2D experiments are demonstrated on polyethylene with 4% 13C-13C spin pairs. The potential of this approach for distinguishing segmental conformations is illustrated by spectral simulations of the two-dimensional ridge patterns that correlate double-quantum and single-quantum chemical-shift anisotropies. Copyright 1998 Academic Press.

Journal Article↗

PubMed1994

Single breath-hold pulmonary magnetic resonance angiography. Optimization and comparison of three imaging strategies.

RATIONALE AND OBJECTIVES: Ultrafast gradient-recalled-echo techniques for obtaining high-quality pulmonary magnetic resonance angiograms within a single breath-hold were optimized. METHODS: Fourteen subjects were imaged with both the body coil and a phased-array surface coil, using three gradient-recalled-echo pulse sequences: 1) two-dimensional sequential; 2) two-dimensional interleaved; and 3) volumetric acquisitions. Image quality was assessed with varied flip angle, receiver bandwidth, slice thickness/number, and matrix size. Cardiac compensation diminished ghost artifacts in the interleaved sequence. Individual sagittal sections and maximum intensity projections were reviewed. RESULTS: Pulmonary magnetic resonance angiograms acquired with volumetric and two-dimensional interleaved gradient-recalled-echo pulse sequences benefit greatest from intravenous gadolinium and result in greater pulmonary arterial visualization than traditional time-of-flight techniques. Phased-array coils result in improved vessel detection. CONCLUSIONS: High-quality breath-held pulmonary magnetic resonance angiography can be obtained with an intravenous contrast-enhanced gradient-recalled-echo acquisition; however, image quality is dependent on the pulse sequence.

Adult↗

PubMed2001

Double-quantum-filtered rotational-resonance MAS NMR in the presence of large chemical shielding anisotropies.

Double-quantum filtration under rotational resonance MAS NMR conditions where the chemical shielding anisotropies involved exceed the differences in isotropic chemical shielding is considered by means of numerical simulations and (13)C MAS NMR experiments. The responses of two different pulse sequences, suitable for double-quantum filtration specifically under rotational resonance conditions, to large chemical shielding anisotropies are compared. In the presence of large chemical shielding anisotropies a very recently introduced pulse sequence (T. Karlsson, M. Edén, H. Luthman, and M. H. Levitt, J. Magn. Reson. 145, 95-107, 2000) suffers losses in double-quantum-filtration efficiencies. The double-quantum-filtration efficiency of another pulse sequence (N. C. Nielsen, F. Creuzet, R. G. Griffin, and M. H. Levitt, J. Chem. Phys. 96, 5668-5677, 1992) is less afflicted by the presence of large chemical shielding anisotropies. Both sequences deliver double-quantum-filtered lineshapes that sensitively reflect chemical shielding tensor orientations. It is further shown that double-quantum-filtered rotational-resonance lineshapes of spin systems composed of more than two spins offer a suitable experimental approach for determining chemical shielding tensor orientations for cases where conventional rotational-resonance experiments are not applicable due to the presence of additional background resonances.

Anisotropy↗

PubMedDate not supplied

Rapid NMR cardiography with a half-echo M-mode method.

A real-time NMR cardiac profiling pulse sequence has been developed that incorporates two-dimensional (2D) selective excitation and a half-echo readout. The time resolution has been improved by a factor of two relative to the previous flow-compensated, full-echo version. The technique produces a 2D plot of "beam"-axis position versus time, analogous to M-mode echocardiography. In human subjects, details of valve leaflet motion, intracardiac flow, wall motion, and wall thickening may be observed along optimal lines of sight selected interactively. The pulse sequence uses a low-tip-angle 2D selective-excitation pulse derived from a spiral k-space trajectory to excite a narrow cylinder of magnetization, followed by a half-echo readout gradient oriented along the axis of the cylinder. One-dimensional Fourier transformation of the acquired signal results in a magnetization profile along the length of the cylinder, or beam. The pulse sequence is effectively flow compensated without any additional gradient lobes, because the rapid oscillation in the gradient wave forms of the 2D excitation pulse produces relatively small net gradient moments, and the shortened readout gradient has minimal first-order moment relative to center echo. The signal from moving blood can alternatively be velocity encoded by the addition of bipolar gradients along any of the three axes, producing Doppler-like traces of intracardiac blood flow.

Echocardiography↗

PubMed1994

A system for producing and monitoring in vitro calcium pulses similar to those observed in vivo.

We have used a computer-controlled automatic pipettor system in conjunction with a fluorescence spectrometer to produce Ca2+ pulses and pulse sequences within a cylindrical fluorescence cuvette. These pulses or pulse sequences are similar to those observed in the cytosol of many types of cells in vivo and show good reproducibility. Their intensity, duration, shape, and periodicity can be controlled and determined at will. Pulses of other ions or of transmembrane potential can be produced by the same apparatus. As an example of the use of this apparatus, initial studies on uptake of pulses of Ca2+ by isolated liver mitochondria are described in which controls are used to verify that initial rapid mitochondrial Ca2+ uptake is actual net uptake, as opposed to binding or exchange.

Animals↗

PubMed1986

[The contrast to noise ratio as a measure of tissue contrast in nuclear magnetic resonance tomography].

The contrast-to-noise ratio, known from the theory of imaging methods, is applied to magnetic resonance imaging. This ratio is proportional to the product of relative image contrast and the signal-to-noise ratio. After predetermining all intrinsic (T1 and T2 times, proton density) and extrinsic (pulse sequence, layer thickness, number of averagings) parameters, the signal-to-noise and contrast-to-noise ratio can be computed to a proportionality constant for all possible pulse sequences. Studies in six voluntary test subjects and 11 patients with a 0.35 T-unit showed that the measured values for the signal-to-noise ratio and the contrast-to-noise ratio are in good agreement with the computed values. The contrast between gray and white brain matter was used as an example for this. The method is applied in order to gain a better understanding of tissue contrast. However, by using diagrams it can be particularly useful for determining the optimal pulse sequences for answering particular clinical questions. It should thus be possible to enhance the effectiveness of magnetic resonance imaging considerably.

Brain↗

PubMedDate not supplied

Pulmonary arteriovenous malformations: diagnosis by gradient-refocused MR imaging.

Six known or suspected pulmonary arteriovenous malformations (AVMs) in four patients were evaluated with magnetic resonance (MR) imaging at 1.5 T. All lesions were imaged using a gradient-refocused echo pulse sequence with a 25/13 ms [repetition (TR)/echo (TE) times] and a 30 degrees flip angle, as well as with a cardiac-gated spin echo short TR/TE pulse sequence technique. Five of the lesions were vascular in nature based on their signal intensity characteristics, and one nonvascular lesion was a carcinoid tumor. On the spin echo images, the AVMs showed a central signal intensity void with a peripheral rim of intermediate signal intensity that was detectable for lesions greater than or equal to 1.5 cm in size. Smaller lesions were more difficult to distinguish from the surrounding air-filled lung, which normally generates no appreciable signal on MR images. The AVMs demonstrated uniform high signal intensity on the gradient echo pulse sequence and were more conspicuous, irrespective of size. With a single breath-hold scan, the vascular nature of the lesion could be rapidly confirmed with an acquisition time of 13 s. In three patients, the cine MR gradient echo images showed a pulsatile quality to the signal intensity in the lesion over the cardiac cycle similar to that within adjacent pulmonary vessels. The results of this study show a potential role for gradient echo MR imaging as a rapid, noninvasive method to evaluate the vascular nature of an atypical pulmonary nodule.

Adult↗

PubMedDate not supplied

Visualization of brain iron by mid-field MR.

Brain iron was visualized on a mid-field (0.5 T) scanner using a spin-echo pulse sequence. Methemoglobin was hyperintense on T1- and T2-weighted images. Deoxyhemoglobin, hemosiderin, and ferritin were seen as decreased intensity on T2-weighted images. The spin-echo pulse sequences were improved for identification of deoxyhemoglobin, hemosiderin, and ferritin by prolonging the TR to 3000 msec and the TE to 80-120 msec. Phase-encoding artifacts at the level of the sylvian fissures caused increased noise, obscuring the brain iron in the lentiform nuclei with the TE of 120 msec. This artifact was substantially reduced or eliminated by lowering the TE to 80 msec, changing the phase-encoding gradient to the Y axis, or using additional pulsing in the slice and read gradients. Use of either the improved spin-echo or gradient-echo pulse sequences on a mid-field MR scanner provides improved evaluation of brain iron.

Brain Chemistry↗

PubMed2002

Dynamic contrast-enhanced myocardial perfusion imaging using saturation-prepared TrueFISP.

PURPOSE: To develop and test a saturation-recovery TrueFISP (SR-TrueFISP) pulse sequence for first-pass myocardial perfusion imaging. MATERIALS AND METHODS: First-pass magnetic resonance imaging (MRI) of Gd-DTPA (2 mL) kinetics in the heart was performed using an SR-TrueFISP pulse sequence (TR/TE/alpha = 2.6 msec/1.4 msec/55 degrees ) with saturation preparation TD = 30 msec before the TrueFISP readout. Measurements were also performed with a conventional saturation-recovery TurboFLASH (SRTF) pulse sequence for comparison. RESULTS: SR-TrueFISP images were of excellent quality and demonstrated contrast agent wash-in more clearly than SRTF images. The signal increase in myocardium was higher in SR-TrueFISP than in SRTF data. Precontrast SNR and peak CNR were not significantly different between both sequences despite 57% improved spatial resolution for SR-TrueFISP. CONCLUSION: SR-TrueFISP first-pass MRI of myocardial perfusion leads to a substantial improvement of image quality and spatial resolution. It is well suited for first-pass myocardial perfusion studies at cardiovascular MR systems with improved gradient hardware.

Adult↗

PubMed1987

On periodic responses generated by a degenerate analog neuron model with periodic inputs.

Periodic responses to periodically varying stimulating pulse sequences are mathematically described for a degenerate analog neuron model. The model used was derived by Yoshizawa et al. (1982) in their investigation of the state transition of an electronic model using a tunnel diode in a degenerate case. Periodic responses to constant pulse sequences for the model were described by them. In this paper, it is shown that periodic responses to periodically varying pulse sequences for the model are identical with those which the author previously described for a discrete neuron model.

Mathematics↗

PubMed2006

[MRI of arthritis with the USPIO SH U 555 C: optimization of T1 enhancement].

PURPOSE: To optimize contrast agent dose and pulse sequence parameters in order to achieve a maximal T1 enhancement in arthritic knee joints with ultra small superparamagnetic iron oxides (USPIO)-enhanced MRI. MATERIALS AND METHODS: Antigen-mediated arthritis was induced in the right knee of nine Sprague Dawley rats. The arthritic knee joint as well as the contralateral normal knee were investigated in a 2 Tesla MR scanner before as well as in short intervals up to 2 h after USPIO injection, using T1-weighted gradient echo (GE) sequences. Three rats each received intravenous injections of the new USPIO SHU 555 C (SH U 555 C, Schering AG, Berlin) at doses of 40, 100 and 200 micromol Fe/kg. Pulse sequence parameters of the GE-sequence were optimized by varying flip angles (alpha) and echo times (TE). Changes in signal intensities (SI) of the arthritic knee and contralateral normal knee were quantified as DeltaSI (%) = /([SIpost - SIpre] / SIpre) x 100 %/ and compared with histopathology. RESULTS: Histology of the arthritic knees demonstrated a marked inflammatory proliferation of the synovium. The USPIO SH U 555 C caused a significant increase in signal intensity of the arthritic joints on T1-weighted MR images (p < 0.05). This effect was optimized using a flip angle of 60-70 degrees, a minimal TE and a dose of 200 micromol Fe/kg. Visually the contralateral normal knee did not show any USPIO enhancement. CONCLUSION: Inflammation can be depicted with marked T1 enhancement by the USPIO SH U 555 C using high contrast agent doses and optimized MR pulse sequence parameters.

Animals↗

PubMedDate not supplied

Remember true FISP? A high SNR, near 1-second imaging method for T2-like contrast in interventional MRI at .2 T.

Clinical requirements for interventional MRI (I-MRI) monitoring of needle placement or thermal ablation demand rapid (near-real-time) image acquisition rates, high spatial resolution, and T2 weighting. Experimental analysis performed earlier suggests that many sequences used for either rapid scanning or T2 weighting at high fields fail to meet both the speed (conventional spin echo [SE], turbo SE) or contrast (ie, fast low-angle shot [FLASH], fast imaging with steady state precession [FISP]) requirements when used at .2 T. In this work, we revisited a number of pulse sequences advocated primarily for higher field applications requiring T2 weighting and found that refocused steady state coherent pulse sequences, aka, true FISP sequences, performed superiorly in achieving both speed and T2 contrast requirements for I-MRI at .2 T. This work focuses on our experience with this new/old technique in the I-MRI setting and describes how one can take advantage of the low field strength and modest inhomogeneity of .2 T (and similar) systems to design pulse sequences that balance TE, TR (and hence T2 dephasing), and resonant offset frequency effects to provide images with the desired contrast and minimal artifactual field inhomogeneity "banding." At high flip angles (approximately 90 degrees ), reasonably short TEs (approximately 5 msec) and short TRs (approximately 10 msec), we have used this method in our last 25 I-MRI procedures (biopsies and/or radiofrequency [RF] thermal ablations) and found these sequences to be extremely useful in both needle localization phases of I-MRI biopsy procedures, RF thermal ablation electrode guidance, and posttherapy imaging assessment. Design methods and clinical I-MRI cases are presented that highlight these points.

Adult↗

PubMed1987

Fundamentals of magnetic resonance imaging. Council on Scientific Affairs.

Medical imaging methods traditionally have depicted variations in one or two simple physical variables in tissues, eg, physical density, atomic number, acoustic velocity, and radioactivity concentration. Magnetic resonance images reveal differences in several variables, with the prominent variables reflecting complex energy transfer mechanisms that occur at the atomic and nuclear levels. Furthermore, the relative contributions of these variables to the image are readily altered by changing the pulse sequence and the pulsing times within the sequence. These changes dramatically affect the image and its characterization of normal and abnormal anatomy. Hence, magnetic resonance images and their contributions to diagnostic medicine can be properly appreciated only if one has some understanding of the procedures by which they are produced.

Contrast Media↗

PubMed1994

Blood flow in major cerebral arteries measured by phase-contrast cine MR.

PURPOSE: To measure mean blood flow in individual cerebral arteries (carotid, basilar, anterior cerebral, middle cerebral, and posterior cerebral) using a cine phase contrast MR pulse sequence. METHODS: Ten healthy volunteers (22 to 38 years of age) were studied. The cine phase-contrast section was positioned perpendicular to the vessel of interest using oblique scanning planes. This pulse sequence used a velocity encoding range of 60 to 250 cm/sec. From the velocity and area measurements on the cine images, mean blood flow was calculated in milliliters per minute and milliliters per cardiac cycle. In the same subjects, transcranial Doppler measurements of blood velocity in these same vessels were also obtained. RESULTS: There was no difference in blood flow in the paired cerebral arteries. Carotid arteries had mean blood flow in the range of 4.8 +/- 0.4 ml/cycle, the basilar artery 2.4 +/- 0.2 ml/cycle, the middle cerebral artery 1.8 +/- 0.2 ml/cycle, the distal anterior cerebral artery 0.6 +/- 0.1 ml/cycle, and the posterior cerebral artery 0.8 +/- 0.1 ml/cycle. Overall, there was poor correlation between MR-measured and transcranial Doppler-measured peak velocity. CONCLUSION: Although careful attention to technical detail is required, mean blood flow measurements in individual cerebral vessels is feasible using a cine phase-contrast MR pulse sequence.

Adult↗

PubMed1987

Magnetic resonance imaging versus computed tomography in the evaluation of soft tissue tumors of the extremities.

Twenty patients with extremity soft tissue tumors were prospectively evaluated with magnetic resonance imaging (MRI) and computed tomography (CT) scans with subsequent anatomic correlation of surgical findings. MRI and CT had a similar percentage of accuracy in assessing tumor relationship with major neurovascular (80% and 70%, respectively) and skeletal (80% and 75%, respectively) structures. MRI was significantly better than CT in displaying contrast between tumor and muscle when using the T2 weighted spin echo (SE) (p2 less than 0.002) and inversion recovery (IR) (p2 less than 0.005) pulse sequences. MRI and CT were comparable in demonstrating contrast between tumor and fat. The contrast between tumor and vessel was better displayed by MRI compared with CT when using the T1 weighted SE (p2 less than 0.001) and T2 weighted SE (p2 less than 0.001) pulse sequences. T1 and T2 values were measured on fresh tumor and normal tissue samples and were used to predict relative contrast on different MRI pulse sequences using isosignal contour plots. MRI appears to offer several advantages over CT in the evaluation of extremity soft tissue tumors.

Arm↗

PubMed1986

Dynamic spin-echo MRI of liver cancer using Gadolinium-DTPA: animal investigation.

An animal model of liver cancer was used to demonstrate that with a fast MRI technique, Gadolinium-DTPA increases tumor-liver contrast. A spin-echo pulse sequence with short repetition (TR) and echo-delay (TE) times (TR 250/TE 15/Excitations 1) has a scan time of 0.6 min, which allows early dynamic postcontrast infusion imaging. This is necessary to capture peak compartmental differences when an extracellular contrast agent such as Gadolinium-DTPA is used. This short TR/short TE pulse sequence also increases T1-dependent tissue contrast over the traditional (inversion recovery or spin echo) T1-weighted pulse sequences. Our studies suggest a significant potential for improved detection of liver metastases with Gadolinium-DTPA-enhanced liver MRI.

Adenocarcinoma↗