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PubMed1998

Volumetric analysis of white matter, gray matter, and CSF using fractional volume analysis.

Quantitative cerebral tissue volumes may be useful for an objective assessment of pathological changes in brain. Accurate determination of tissue volumes is complicated, however, by the partial volume averaging (PVA) effect. We have, therefore, developed a new pulse sequence that minimizes the PVA through the use of inversion-recovery (IR) and double inversion-recovery (DIR) techniques. This pulse sequence simultaneously acquires four different sets of images to provide the necessary information for volumetric analysis and reduces potential spatial misregistration of images due to patient motion. The image sets acquired from the proposed pulse sequence are 1) gray matter visible, 2) white matter visible, 3) FLAIR, and 4) fast spin-echo proton-density weighted images. An algorithm has been implemented to correct for differential T1-weighting and for tissue quantitation.

Artifacts↗
PubMed1997

Recent and future advances in high-speed imaging.

Long acquisition times have long been a major drawback of magnetic resonance imaging (MRI) and have limited its use for all those organ systems with various types of movement, such as respiration, pulsation, and peristalsis. Recent advances in scanner hard- and software, most notably improvements in gradient and radio frequency coil design, in amplifier technology as well as in pulse sequence development, have created new fields of application for MRI. In this review article we give an overview of the development of pulse sequences from the spin echo technique through gradient echo techniques to the fastest imaging technique thus far developed, echo planar imaging (EPI). A variety of clinical applications for the different pulse sequences is included, along with a discussion on the advantages and drawbacks of each technique. The review ends with a discussion of possible future advances in the field of high-speed MR imaging.

Brain Diseases↗
PubMed2003

Retrograde flow in the left inferior petrosal sinus and blood steal of the cavernous sinus associated with central vein stenosis: MR angiographic findings.

BACKGROUND AND PURPOSE: We attempted to identify the cause of abnormal venous flow seen during arterial MR angiography in the inferior petrosal sinus by use of in three female patients (aged 51, 48, and 70 years, respectively). METHODS: Arterial 3D time-of-flight MR angiography was performed with a tilted optimized nonsaturating excitation pulse sequence (TR/TE, 31/7; flip angle, 20 degrees; section thickness, 65 mm; effective thickness, 1 mm; number of sections, 1 to 2); no magnetization transfer pulse sequence was used. Contrast-enhanced 3D MR angiography of the neck was performed with a 3D fast low-angle shot pulse sequence (TR/TE, 4.6/1.8; flip angle, 40 to 45 degrees; section thickness, 80 mm; intersection gap, 1.5 mm; acquisition matrix, 180 x 256; acquisition time, 27 s) on a system with a whole-body coil. RESULTS: In all three patients, 3D time-of-flight MR angiography revealed abnormal vascular signal originating from the left cavernous sinus, continuing through the inferior petrosal sinus, and ending in the proximal internal jugular vein at the jugular bulb level. Abnormal vascular signal at the jugular bulb, sluggish flow and flow-related enhancement in the left internal jugular vein, and signal void in the contralateral jugular vein were noted. Contrast-enhanced delayed-phase MR angiography showed stenosis in the left brachiocephalic vein in all patients. CONCLUSION: High signal intensity noted at the inferior petrosal sinus resulted from retrograde flow. Retrograde flow was due to blood stealing from the internal jugular vein toward the cavernous sinus because of venous stenosis in the brachiocephalic vein.

Aged↗
PubMed2002

Comparison of accuracy and interreader agreement in side-by-side versus independent evaluations of MR imaging of the medial collateral ligament of the elbow.

RATIONALE AND OBJECTIVES: The authors compared independent and side-by-side evaluation of magnetic resonance (MR) images of the medial collateral ligament (MCL) of the elbow, with regard to sensitivity, specificity, and interreader agreement MATERIALS AND METHODS: Six MR imaging sequences were used to image the MCLs in 28 cadaveric specimens, eight with surgically created lesions. Two reading methods were used. For independent evaluation, the images were first evaluated independently and rated on a five-point scale by two musculoskeletal radiologists experienced in interpreting MR images and blinded to the MCL integrity. The images were then reevaluated on the same scale by both readers after at least 2 weeks, with images from all six sequences shown side by side. For each MR sequence and reading method, the sensitivity and specificity were estimated nonparametrically, and differences were tested with the McNemar test. Interreader agreement was assessed with a K statistic, and differences were tested with Z and chi2 tests after adjustment for the dependence structure between correlated K statistics. RESULTS: For all sequences, side-by-side evaluation generally yielded higher specificity than independent evaluation, as well as better agreement between readers. CONCLUSION: Observer performance is superior when multiple MR imaging pulse sequences are reviewed simultaneously rather than independently and separately. Side-by-side review of different MR pulse sequences enabled higher accuracy and lower interreader variability for evaluation of the elbow MCL. These findings have implications for the design of studies to optimize MR imaging protocols by using multiple pulse sequences and multiple readers.

Aged↗
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↗
PubMed2002

Homonuclear zero-quantum recoupling in fast magic-angle spinning nuclear magnetic resonance.

Solid-state magic-angle-spinning NMR pulse sequences which implement zero-quantum homonuclear dipolar recoupling are designed with the assistance of symmetry theory. The pulse sequences are compensated on a short time scale by the use of composite pulses and on a longer time scale by the use of supercycles. (13)C dipolar recoupling is demonstrated in powdered organic solids at high spinning frequencies. The new sequences are compared to existing pulse sequences by means of numerical simulations. Experimental two-dimensional magnetization exchange spectra are shown for [U-(13)C]-L-tyrosine.

Alanine↗
PubMed1986

Leo J. Rigler lecture. MR imaging of the liver.

Recent technical and clinical advances in MR of the liver are reviewed with special reference to the role of MR as a primary screening technique for detection of space-occupying lesions, especially metastases. The major current problem in upper abdominal MR imaging is physiologic motions, and this appears to have been effectively solved by newly introduced pulse-sequence and timing-parameter strategies. Short-TR/TE spin-echo sequences with extensive signal averaging and heavy T1-weighting produce images with exceptional anatomic detail and liver-cancer contrast differences. With this sequence superior sensitivity for liver-cancer detection has been shown in quantitative signal-difference to noise comparisons with other pulse sequences and in clinical comparisons with CT. MR discovered 14% more individual metastases and 3% more patients with liver cancer than CT in a blinded comparative study of 142 patients undergoing both exams. MR also showed greater specificity (98%) than CT (91%) in distinguishing patients without liver metastases. Differentiation of hemangioma from metastases was possible with greater than 90% specificity by using heavily T2-weighted sequences. Use of a fast-scan, gradient-recalled echo technique can also produce good-quality, multislice, T1-weighted studies of the liver in 20 sec--a breath-hold. MR contrast agents (such as gadolinium-DTPA and reticuloendothelial-system-specific, superparamagnetic ferrite-iron-oxide particles) offer further promise for enhanced sensitivity for liver-cancer detection. When optimal pulse sequences are employed, MR can now be appropriate as a primary screening method for detecting liver neoplasms.

Adult↗
PubMed2004

Measuring protein self-diffusion in protein-protein mixtures using a pulsed gradient spin-echo technique with WATERGATE and isotope filtering.

Here we report a modified pulsed gradient spin-echo (PGSTE) pulse sequence to measure diffusion coefficients. This approach incorporates WATERGATE combined with isotopic filtering into a standard PGSTE experiment. Doing this eliminates much of the disadvantages from the combination of diffusion encoding and heteronuclear selection intervals and allows for facile modification of the diffusion pulse sequence with flexibility of the time period between RF pulses. The new diffusion pulse sequence is demonstrated using an 15N-labeled peptide and an 15N-labeled protein in a mixture with a protein of similar size.

Algorithms↗
PubMed1985

Magnetic resonance imaging of soft-tissue tumors: comparison with computed tomography.

Twenty-seven patients with soft-tissue tumors were examined with a Picker 0.15-tesla resistive magnet and by computed tomography (CT). In all but one patient, MRI was better than or equal to CT in defining the anatomic extent of the tumor. We could determine whether major vascular structures were engulfed by the tumor in 80% of the MRI examinations but only in 62% of the CT scans. MRI and CT were equally effective in determining the presence or absence of bony invasion. The MRI images of all the tumors showed increased signal intensity relative to normal muscle when spin-echo (SE) pulse sequences with long repeat times were used (SE: echo time [TE], 60 ms; repetition time [TR], 2,000 ms). When T1 weighted pulse sequences were used (SE: TE, 30 ms; TR, 500 ms or inversion recovery: inversion time, 500 ms; TE, 40 ms; TR, 2,000 ms) the malignant tumors showed decreased signal intensity compared to normal muscle. Only lipomas showed high signal intensity on both T1 and T2 weighted pulse sequences.

Adolescent↗
PubMed2001

Characteristics of acoustic noise in echo-planar imaging.

Characteristics of the acoustic noise generated by magnetic resonance imagers of different systems and performance levels were studied when operating in echo-planar imaging (EPI) sequence. Continuous equivalent A-weighted sound pressure levels (Leq) and peak impulse sound pressure levels (Lpeak) during EPI were measured in 12 clinical super-conducting MRI systems (0.5-1.5 T). Sound pressure levels and frequency spectra of EPI were compared with those of nine different pulse sequences. EPI sound pressure levels differed among institutions (Leq = 94.2 +/- 2.7 dBA. Lpeak = 109.1 +/- 3.5 dB), but these were within permissible noise exposure levels. Sound pressure levels during EPI were not significantly different from those during other pulse sequences. However, compared to other pulse sequences. EPI had a significantly greater proportion of acoustic noise in the high octave-frequency band. Single-shot EPI had relatively higher frequency noise and greater Leq than multishot EPI, but the difference in Leq decreased when the number of slices in multishot EPI was increased.

Echo-Planar Imaging↗
PubMed1998

[Imaging and differentiation of atherosclerotic plaque with magnetic resonance tomography].

Magnetic resonance (MR) imaging studies have allowed the imaging of an atheroma, its size, shape, and lipid contents. The aim of our study was to characterize atherosclerotic lesions using a 0.5 T magnet, to delineate plaque components, and to compare MR results with histology. Thirty necropsy specimens of human iliac arteries were studied. Magnetic resonance imaging studies were carried out on a 0.5 Tesla superconducting magnet using a 5 cm surface coil. The position for the coronal MR planes was oriented by an external marker. The matrix size was 256 x 256, 4 NEX, and the FOV was 45 mm. The pulse sequences used included SE 520/29 and SE 2200/28 and 90. Signal intensity (SI) of fibrous plaques increased significantly from 28.3 +/- 3.8 to 49.1 +/- 8.2 (p < or = 0.0001) and decreased at SE 2200/90 to 24.1 +/- 6.8 (p < or = 0.0001). However, lipid plaque components showed no significant change in SI between T1-weighted pulse-sequences (28.2 +/- 5.4) and T2-weighted pulse-sequences (25.5 +/- 5.9). Only at SE 2200/90 SI of lipid plaques decreased significantly (11.8 +/- 3.9, p < or = 0.0001). As compared to histology, MR has shown a high sensitivity and specificity in the detection of fibrous and lipid plaque components. In conclusions, our study demonstrated that MR is highly effective in the characterization of atherosclerotic lesions.

Arteriosclerosis↗
PubMed2004

The set of triple-resonance sequences with a multiple quantum coherence evolution period.

The new pulse sequence building block that relies on evolution of heteronuclear multiple quantum coherences is proposed. The particular chemical shifts are obtained in multiple quadrature, using linear combinations of frequencies taken from spectra measured at different quantum levels. The pulse sequences designed in this way consist of small number of RF-pulses, are as short as possible, and could be applied for determination of coupling constants. The examples presented involve 2D correlations HNCO, HNCA, HN(CO)CA, and H(N)COCA via heteronuclear zero and double coherences, as well as 2D HNCOCA technique with simultaneous evolution of triple and three distinct single quantum coherences. Applications of the new sequences are presented for 13C,15N-labeled ubiquitin.

Carbon Isotopes↗
PubMedDate not supplied

A method for reducing motion induced errors in T2-weighted magnetic resonance imaging.

It is known that motion of an object imaged by magnetic resonance imaging leads to errors and artifacts in the image. This effect is especially important when a pulse sequence with long echo time (TE) and long TR is used to provide T2-weighted images. We have modified the usual spin warp imaging pulse sequence in a way which greatly reduces the severity of such errors. The method was tested on both a moving phantom and a living rabbit. The pulsing of the scanner was gated in synchrony to the motion of the phantom; two pulses per mechanical cycle were used to create a special situation which served to verify the validity of a theoretical analysis of the loss of signal with echo time. Under such conditions it was shown that the conventional method led to a progressive loss of signal with increasing TE while the new method gave a pure exponential decay of signal in accordance with the known T2 of the sample. When either the phantom or the animal was scanned without gating, the new pulse sequence showed far less artifacts attributable to motion. The potential application of the technique to improved quantitative imaging of the abdomen in clinical situations is discussed.

Abdomen↗
PubMed1997

A new diffusion SSFP imaging technique.

In this paper a new diffusion sensitive steady-state free precession (SSFP) pulse sequence with a reduced sensitivity to physiological brain motion is presented. The signal attenuation due to diffusion in this SSFP sequence is derived theoretically and confirmed experimentally with a phantom. It is shown that for brain tissue this signal attenuation is approximately independent of T1 and T2, but depends only on the pulse sequence used, i.e., the timing and the size of the RF and the gradient pulses. On this basis the diffusion constant can be calculated for any region in the image. Diffusion sensitive images of the brain obtained with our pulse sequence are presented and shown to be superior over an image obtained with a "conventional" diffusion sensitive SSFP sequence.

Brain↗
PubMed2002

Isotropic resolution diffusion tensor imaging with whole brain acquisition in a clinically acceptable time.

Our objective was to develop a diffusion tensor MR imaging pulse sequence that allows whole brain coverage with isotropic resolution within a clinically acceptable time. A single-shot, cardiac-gated MR pulse sequence, optimized for measuring the diffusion tensor in human brain, was developed to provide whole-brain coverage with isotropic (2.5 x 2.5 x 2.5 mm) spatial resolution, within a total imaging time of approximately 15 min. The diffusion tensor was computed for each voxel in the whole volume and the data processed for visualization in three orthogonal planes. Anisotropy data were further visualized using a maximum-intensity projection algorithm. Finally, reconstruction of fiber-tract trajectories i.e., "tractography" was performed. Images obtained with this pulse sequence provide clear delineation of individual white matter tracts, from the most superior cortical regions down to the cerebellum and brain stem. Because the data are acquired with isotropic resolution, they can be reformatted in any plane and the sequence can therefore be used, in general, for macroscopic neurological or psychiatric neuroimaging investigations. The 3D visualization afforded by maximum intensity projection imaging and tractography provided easy visualization of individual white matter fasciculi, which may be important sites of neuropathological degeneration or abnormal brain development. This study has shown that it is possible to obtain robust, high quality diffusion tensor MR data at 1.5 Tesla with isotropic resolution (2.5 x 2.5 x 2.5 mm) from the whole brain within a sufficiently short imaging time that it may be incorporated into clinical imaging protocols.

Adult↗
PubMed2005

Drug eluting coronary stent: in vitro evaluation of magnet resonance safety at 3 Tesla.

PURPOSE: To evaluate MR safety at 3 Tesla for a drug eluting coronary stent. METHODS: A drug eluting coronary stent (Endeavor, cobalt alloy, Medtronic Vascular, Santa Rosa, CA) was evaluated for magnetic field interactions, heating, and artifacts at 3 Tesla. MRI-related heating was assessed with the stent in a gelled saline-filled phantom using a transmit/received RF body coil with a whole body averaged SAR of 2.0 W/kg. Artifacts were characterized using T1-weighted, spin echo, and gradient echo pulse sequences. RESULTS: The stent exhibited minor magnetic field interactions that will not cause migration. Heating was not substantial (+0.5 degrees C). Artifacts may create a problem if the area of interest is in the same area or close to the stent (e.g., for a T1-weighted, spin echo pulse sequence, within approximately 16 mm; for a gradient echo pulse sequence, within approximately 23 mm). Conclusion. The findings indicated that it would be safe for a patient with this cobalt alloy-based, drug-eluting coronary stent to undergo MRI at 3 Tesla or less. Importantly, because of the relative lack of magnetic field interactions, MRI may be performed immediately after implantation.

Artifacts↗
PubMed2000

Modified spectral editing methods for (13)C CP/MAS experiments in solids.

The spectral editing approach of Zilm and coworkers utilizes polarization, polarization inversion, and spin depolarization methods for enhancing or suppressing NMR spectral lines in solids. The proposed pulse sequences allow nonprotonated C, CH, CH(2), and CH(3) types of carbon resonances to be separated from one another and identified accordingly. The former method tentatively separates the nonprotonated C and CH(3) peaks with a cutoff shift of 35 ppm. This shift is a reasonable demarcation shift for a preponderance of organic molecules, but exceptions do exist that could constitute a serious drawback in a few instances. The new approach separates the nonprotonated C and CH(3) carbon peaks unequivocally using modified pulse sequences similar to those of Zilm. Further, both the CH only and CH(2) only spectra, respectively, can be acquired directly from combining so called (+) and (-) sequences using different spectral delay periods and pulse parameters. The (+) and the (-) pulse sequences produce signals for the nonprotonated and methyl carbons that have essentially the same amplitude but opposite phases. These spectra, combined with the previously reported CH(3) and nonprontonated C only spectra, offer a complete spectral editing technique for solid samples. Examples of these spectral editing methods are provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-methylpodocarpate and 10-deacetylbaccatin III.

Magnetic Resonance Spectroscopy↗
PubMed1987

Ingested manganese chloride as a contrast agent for magnetic resonance imaging.

Solutions of manganese chloride were force-fed to Sprague-Dawley rats. Magnetic resonance (MR) imaging was performed on (a) syringes containing different concentrations of manganese chloride, (b) rats after force feeding and (c) livers excised after sacrifice of the force-fed rats. Imaging was done with a 0.15-T resistive magnet. Multiple pulse sequences were used and T1 values were calculated. The signal intensity and T1 value obtained from a solution depended on the manganese concentration and the pulse sequence employed. At higher concentrations, no signal was produced due to extreme T2 shortening. Absorbed manganese affected the signal intensities and T1 values of the rats' livers. By appropriate selection of manganese concentration and pulse sequence, ingested manganese can serve as a combined gastrointestinal and hepatic MR contrast agent.

Animals↗