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At least 1,099 records · Page 61Linked to original sources
PubMed1990

MR imaging of the various stages of normal myelination during the first year of life.

The normal process of myelination of the brain mainly occurs during the first year of life. This process as known from histology can be visualized by MRI. Because of the very long T1 and T2 of immature brain tissue it is necessary to use adjusted pulse sequences with a long TR in order to obtain sufficient tissue contrast. With long TR SE images five stages can be recognized in the process of normal myelination and brain maturation. During the first month of life long TR short TE SE images show what are believed to be myelinated structures by correlation with published histological studies with a high signal intensity, unmyelinated white matter with a low signal intensity and gray matter with an intermediate signal intensity. The signal intensity of unmyelinated and myelinated white matter is reversed on long TR long TE SE images. In the course of a few weeks the signal intensity of unmyelinated white matter becomes high and the signal intensity of myelinated white matter becomes low also on long TR short TE SE images. These changes are believed to be caused by a loss of water and a change in chemical composition of brain tissue just prior to the onset of a wave of myelination. With progression of myelination the signal intensity of white matter changes from high to intermediate to low. These changes result in stages of isointensity, first in the central parts of the brain, later in the peripheral parts. At the end of the first year the adult contrast pattern is reached in all parts of the brain. IR images are also able to depict the progress of myelination, but appear to be less sensitive to subtle changes in the degree of myelination. The precise normal values for the five stages depend on the magnetic field strength and the pulse sequences used.

Brain↗
PubMed1999

A study of dipolar interactions and dynamic processes of water molecules in tendon by 1H and 2H homonuclear and heteronuclear multiple-quantum-filtered NMR spectroscopy.

The effect of proton exchange on the measurement of 1H-1H, 1H-2H, and 2H-2H residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pulse sequences based on heteronuclear and homonuclear multiple-quantum filtering (MQF). Derivation of theoretical expressions for these techniques allowed evaluation of the 1H-1H and 1H-2H residual dipolar interactions and the proton exchange rate at a temperature of 24 degrees C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24 degrees C were 300 and 50 Hz and 3000 s-1, respectively. The results for the residual dipolar interactions were verified by repeating the above measurements at a temperature of 1.5 degrees C, where the spectra of the H2O molecules were well resolved, so that the 1H-1H dipolar interaction could be determined directly from the observed splitting. Analysis of the MQF experiments at 1.5 degrees C, where the proton exchange was in the intermediate regime for the 1H-2H dipolar interaction, confirmed the result obtained at 24 degrees C for this interaction. A strong dependence of the intensities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smaller than the proton exchange rate. Dependence of the relaxation times and signal intensities of the MQF experiments on the orientation of the tendon with respect to the magnetic field was observed and analyzed. One of the results of the theoretical analysis is that, in the fast exchange regime, the signal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T2) depend on the orientation as the square of the second-rank Legendre polynomial.

Achilles Tendon↗
PubMed2006

Multispectral quantitative magnetic resonance imaging of brain iron stores: a theoretical perspective.

OBJECTIVES: To review published magnetic resonance imaging (MRI) iron quantification techniques in the context of quantitative MRI and MR relaxation theories. To analyze comparatively and as a function of age the simultaneous measurements of the proton density (PD), the relaxation times (T1 and T2), and the longitudinal to transverse relaxation times ratio (T1/T2) of brain regions known to accumulate iron preferentially. METHODS: Twenty-seven human subjects were scanned with the mixed turbo spin echo pulse sequence, which is multispectral in PD, T1, and T2. Quantitative MRI (Q-MRI) maps of PD, T1, T2, and T1/T2 were generated, and region of interest measurements were performed in 5 brain regions, namely, frontal white matter (WM), genu of corpus callosum, caudate nucleus, putamen, and globus pallidus. RESULTS: Relaxation time measurements are consistent with results of others and provide further confirmation to our basic understanding of the relaxation effects of iron stores in the brain. Specifically, we found that the iron-rich globus pallidus exhibits enhanced T1 and T2 relaxation relative the iron poorer gray matter tissues (caudate nucleus and putamen) and also relative to the WM matter tissues (frontal WM and genu of the corpus callosum). We also observe that under riding this hypothesis-because we do not have independent confirmation-that iron caused relaxation enhancement, are the normal brain aging patterns, which suggest that the brain tissues become wetter with increasing age. Also noted is the virtual removal of age dependence observed for the T1/T2 ratio of WM tissues, further suggesting that this ratio may become of clinical significance in the diagnosis of neoplastic processes as well as for quantifying iron in tissue. CONCLUSIONS: The theoretical underpinnings of published brain iron Q-MRI techniques have been reviewed. We also examined MR relaxation theory essentials in relation to H-proton relaxation phenomena in diamagnetic tissues as well as theoretical extensions to describe relaxation effects in tissues containing iron deposits with a focus on ferritin. Also reported are in vivo Q-MRI results of 27 human brains obtained with a multispectral technique that uses the mixed turbo spin echo pulse sequence and a model conforming Q-MRI algorithms.

Brain↗
PubMed2001

Temperature quantitation and mapping of frozen tissue.

A method was developed for quantitating the temperature within frozen tissue with the magnetic resonance (MR) parameter R2*. The pulse sequence uses half-pulse excitation and a short spiral readout to achieve echo times as short as 0.2 msec. Fiber-optic temperature sensors were inserted into bovine liver tissue. The tissue was frozen at one end while being held warm at the other end. Once steady state was reached, the parameter R2* was measured. A linear dependence of R2* on temperature was demonstrated. R2* is independent of freeze number and of the orientation of the temperature gradient with respect to the main magnetic field. Feasibility in a canine prostate during cryosurgery is demonstrated. J. Magn. Reson. Imaging 2001;13:99-104.

Animals↗
PubMed1989

The application of spinors to pulse synthesis and analysis.

We show how spinors may be used to solve many problems in pulse synthesis and analysis. They provide an elegant simple notation for many problems. We show how one can specify the component of a spinor and then, by exactly solving the Bloch equations, synthesize a hard pulse sequence or a soft pulse which will yield the desired spinor. This is done by generalizing our previous approach to inverting the Bloch equations. This can be applied to synthesizing refocusing pulses. Finally, some simple consequences about symmetric pulses are derived. It is shown that given a symmetric inversion pulse, one can always synthesize an asymmetric inverting pulse of the same duration, which, as an inversion, is better.

Humans↗
PubMed1995

Optimized ultra-fast imaging sequence (OUFIS).

The DUFIS sequence can make ultra-fast images (approximately 10 ms) without the use of rapidly switched gradients. The RF excitation sequence is spatially selective so that only a small fraction of the magnetization in each imaging pixel is used which produces a poor imaging signal to noise ratio (SNR). We have developed several alternative RF sequences that use RF pulses with multiple phases, and also with just 0 degrees and 180 degrees phases to excite almost all the magnetization in a pixel and greatly improve the SNR. The optimization of these pulse sequences (now called OUFIS) have been conducted both analytically and by numerical searches, with various linear and nonlinear models. Both theoretical and computational methods used in the optimizations are described in detail. Preliminary experimental results are briefly presented with several possible applications of the OUFIS excitation sequences suggested.

Algorithms↗
PubMed2005

Optimized 1H MRS and MRSI methods for the in vivo detection of boronophenylalanine.

Boronophenylalanine (BPA) is used as Boron-10 carrier in boron neutron capture therapy, an experimental cancer radiotherapy. Results of quantitative, noninvasive in vivo detection and imaging of BPA in laboratory animals using (1)H NMR are presented for the first time. The purpose of this study was to implement and validate optimized techniques for the efficient detection of BPA. The (1)H NMR signals through which BPA is most readily detected in vivo are those from the aromatic ring of the molecule, which are part of a scalar-coupled spin system. The preferred detection method should therefore be based on a pulse sequence in which the effective TE is as short as possible. Modified versions of LASER (tau(CP) = 4.6 ms, TE = 27.6 ms) and double-echo slice-selective 2D MRSI (TE = 12 ms) were implemented for single-voxel spectroscopy and spectroscopic imaging of BPA, respectively. Chemical shift selective excitation was used for both sequences, based on a pulse that enabled narrow-band excitation without concomitant delay in TE. SI data without water suppression was used for absolute quantitation and for correction of B(0) variations. Experiments were conducted at 4.7 T in phantoms and in mice where the infused BPA was detected in the kidney.

Animals↗
PubMed2000

1H NMR studies: dynamics of water in gelatin.

Proton magnetic resonance was used to characterize the dynamics of water in gelatin. Both sol and gel states were investigated. Transverse relaxation rates (R2) were dependent on the proton frequency measurement. (R2) measured with the Carr-Purcell-Meiboom-Gill pulse sequence was dependent on pulse spacing. These observations were interpreted in terms of chemical exchanges between water protons and those of the macromolecules in the sol state, whereas in the gel state the contribution of diffusion through microheterogeneities in the sample seems to provide an additional transverse relaxation mechanism.

Algorithms↗
PubMed1980

Electron spin echo studies on chloroplasts. Spectral characteristics of electron transport components and light-induced transients.

Electron-spin resonance echoes are used to study the complex overlapping ESR spectra of whole chloroplasts. By varying the repetition rate of the microwave pulse sequence, delay time, and pulse width, signals with different longitudinal and transverse relaxation times were extracted. We have identified the echo signals due to plastocyanin and ferredoxins. In addition, we have found a strong signal at g = 4.3, that possibly arises from distorted cytochrome, and weak signals in the region g = 6-9. The strong echo signal at g = 2.0047 (Signal II), is made up of at least three "dark" components that differ in their relaxation times. Upon illumination at 1.2 K several of the echo signals including Signal II show reversible light-induced components. The kinetics of these transients depend on the addition of 3(3,4-dichlorophenyl)-1,1-dimethyl urea. Part of the transients are believed to arise from cyclic electron flow around Photosystem I.

Chloroplasts↗
PubMed2001

A comparative study of MR imaging profile of titanium pedicle screws.

OBJECTIVE: We compared the MR imaging profile of three different types of titanium pedicle screw implants in common usage in a human cadaveric model. We additionally compared the change in temperature during imaging among three constructs. MATERIAL AND METHODS: Titanium-based lumbar pedicle screw/rod constructs from three manufacturers were implanted sequentially in a human cadaveric spine. MR imaging was then performed using both conventional spin-echo sequences and advanced imaging pulse sequences. Changes in tissue temperature were also measured during imaging to assess differences among the various implants. MR images were compared in a blinded fashion by two neuroradiologists. RESULTS: No significant differences in imaging profile were noted between the three types of titanium implants with regards to their MR artifact profile. Fast spin-echo sequences led to a decrease in perceptible MR artifacts. Moreover, there were no significant differences in temperature increase among the three manufacturers (mean increase 0.5 degrees C) during imaging. CONCLUSION: Slight differences in the percentage of titanium among the three pedicle screw systems does not appear to result in artifact differences during MR imaging. Therefore, with regard to imaging profile considerations, the three systems studied should be considered interchangeable.

Artifacts↗
PubMed1992

Primary structures of neuropeptides isolated from the corpora cardiaca of various cetonid beetle species determined by pulsed-liquid phase sequencing and tandem fast atom bombardment mass spectrometry.

A peptide with the same retention time on gradient reversed-phase high-performance liquid chromatography was present in the corpora cardiaca of 5 scarabaeid beetles, subfamily Cetoniinae: the three fruit beetle species Pachnoda marginata, P. sinuata and P. aemulae and the two protea beetle species Trichostetha fascularis and T. albopicta. Crude corpora cardiaca material from P. sinuata had a small hypertrehalosaemic effect in American cockroaches and a very weak hyperlipaemic activity in migratory locusts. Injections into P. sinuata caused hypertrehalosaemia when a dose of 1.0 corpora cardiaca equivalents was injected. An identical neuropeptide was isolated, by RP-HPLC, and sequenced by pulsed-liquid phase sequencing employing Edman chemistry after enzymically deblocking the N-terminal 5-oxopyrrolidine-2-carboxylic acid residue, as well as by collision-induced decomposition tandem fast atom bombardment mass spectrometry. The peptide is a blocked octapeptide: Glu-Leu-Asn-Tyr-Ser-Pro-Asp-TrpNH2, previously designated Mem-CC. The synthetic peptide is able to elicit haemolymph carbohydrates in P. sinuata upon injection of low doses. Activity studies using synthetic analogues of this peptide revealed that Tyr4 may be important for receptor recognition/binding. The peptide is synthesized in intrinsic cells of the corpus cardiacum as shown by in vitro incorporation of [3H]Trp and [14C]Tyr in Mem-CC.

Amino Acid Sequence↗
PubMed1985

Magnetic resonance imaging of mediastinal and hilar masses: comparison with CT.

Magnetic resonance imaging (MRI) was compared to computed tomography (CT) of the mediastinum and/or hila in 37 patients with bronchogenic carcinoma (35 unresectable for cure) and 11 patients with other masses. Spin-echo pulse sequences using a short pulse repetition rate (TR) and short echo delay (TE) were most helpful for detection of abnormal soft-tissue mediastinal and hilar masses. The accuracy of MRI and CT in staging bronchogenic carcinoma for curative resectability/nonresectability was comparable. CT staged 35 of 37 cases appropriately, while MRI correctly staged 36 of 37 cases. Several pitfalls in MRI evaluation of the mediastinum were identified. By MRI the esophagus may be misinterpreted as an enlarged retrotracheal lymph node unless serial scans are studied. Scattered calcifications in enlarged mediastinal and hilar lymph nodes due to old granulomatous disease are not detectable by MRI. Small adjacent lymph nodes shown individually by CT may appear as a single enlarged lymph node by MRI due to partial-volume averaging. Small lung nodules may be undetected by MRI due to respiratory motion and partial-volume averaging. Certain patients are unsuitable for MR scanning. Because of the requirement for patient selection and the identified pitfalls of MRI, CT remains the radiologic procedure of choice in the staging of patients with bronchogenic carcinoma and the evaluation of other mediastinal and hilar masses at present. However, because of the ability to show blood vessels without an intravascular contrast agent, MRI is useful in evaluating patients with potential contrast allergy and solving diagnostic problems not solved by CT.

Calcinosis↗
PubMed1998

Optimal imaging protocol after intraocular silicone oil tamponade.

PURPOSE: Our purpose was to define the optimal protocol for imaging of the orbits after vitreous humor replacement with silicone oil. METHODS: Eleven eyes in 10 patients with tractional and/or rhegmatogenous retinal detachment were studied. Five CT scans and 18 high-field (1.5 T) MR images were obtained. Standard T1-weighted, T1-weighted with fat and silicone saturation, fast spin density-weighted, and T2-weighted orbital MR sequences were performed. Unique pulse sequences included fast spin density-weighted and T2-weighted imaging with and without fat saturation or silicone saturation, gradient-echo imaging, and short-tau inversion recovery imaging. RESULTS: The T1-weighted MR and CT studies were comparable in displaying the silicone. However, the fat- or silicone-saturated fast T2-weighted sequences always showed the fibrous bands and subretinal fluid to best advantage. In one case, the eye also contained inadvertently retained perfluorocarbon liquid, which blended with silicone oil on both saturated images, requiring companion T1-weighted sequences without saturation to demonstrate its presence. CONCLUSION: Simple, commonly available fat-saturated fast T2-weighted MR images supplemented by standard T1-weighted images are all that are needed to evaluate the eye efficiently after vitrectomy and tamponade.

Adult↗
PubMed1994

Magnetization transfer contrast imaging of hepatic neoplasms.

A method of performing magnetization transfer contrast (MTC) using a pulse sequence based on a series of on-resonance binomial pulses preceding a conventional spin-echo sequence has been recently described. We investigated this technique in the evaluation of circumscribed hepatic neoplasms on a 0.5 T imager. Conventional spin-echo imaging was performed in 18 patients with hepatic neoplasms, 15 with primary or metastatic hepatic neoplasms, and 3 with cavernous hemangiomas. The T1-weighted sequence was repeated following MTC saturation in 7 patients and the proton density sequence was repeated following MTC saturation in 13 patients (both were used in two patients). The mean lesion-to-liver contrast for neoplasms worsened with T1-MTC (0.87 vs. 1.04), but improved slightly with proton density-MTC (1.10 vs. 1.22) when compared to the corresponding non-MTC spin-echo images. However, these differences were not statistically significant (p = .19 and p = .16, respectively). The limited data on cavernous hemangiomas does not suggest these have consistently different MTC imaging characteristics from other hepatic neoplasms. In conclusion, MTC imaging using a binomial saturation pulse did not offer significant improvement in contrast between hepatic neoplasms and normal liver. Refinements of the binomial pulse sequence will be necessary if a consistent improvement in lesion sensitivity is to be achieved. Evaluation of its role in increasing diagnostic specificity will require further investigation.

Adenoma↗
PubMed1999

Calculations of multipulse sequence in NQR of spins 32.

The general formalism of the interaction representation with respect to an operator which is its own inverse is developed and applied to pure NQR of spins I = 32. Under the assumption of no relaxation and no dipolar coupling, it is shown that the calculation of the response to pure NQR multipulse sequences can be performed with the same concepts used in high field NMR, such as coherence pathways. All the tools and mathematical expressions to predict the time evolution of the signal created by a pure NQR multipulse sequence are presented explicitly. It takes into account the off-resonance irradiation as well as the angular dependence of the excitation and detection for every value of the electric field gradient asymmetry parameter. Particular attention is devoted to the powder average, which is performed via a probability function derived analytically for the first time, leading to a drastic reduction of simulation times. The theory is illustrated by the study of the optimization and excitation bandwidths of one- to three-pulse sequences and compared to experimental results on Chloranil. We show that the three-pulse "stimulated echo" sequence gives a more uniform excitation profile than the traditional two-pulse echo sequence for powder samples. Thus, the "stimulated echo" sequence could be useful to cover a large spectrum when the experiment duration, or the signal to noise ratio, are not critical parameters. Analytical expressions for the nutation spectra obtained by one or two-pulse sequences are also derived for the first time.

Chloranil↗
PubMed1998

MR imaging of hyaline cartilage at 0.5 T: a quantitative and qualitative in vitro evaluation of three types of sequences.

OBJECTIVE: To identify an optimal pulse sequence for in vitro imaging of hyaline cartilage at 0.5 T. MATERIALS AND METHODS: Twelve holes of varying diameter and depth were drilled in the cartilage of two pig knees. These were submerged in saline and scanned with a 0.5-T MR system. Sixteen T1-weighted gradient echo (GE), two T2-weighted GE, and 16 fast spin echo sequences were used, by varying repetition time (TR), echo time (TE), flip angle (FA), echo train length, profile order, and by use of fat saturation. Contrast-to-noise ratios (CNR) of cartilage versus saline solution and cartilage versus subchondral bone were measured. Cartilaginous lesions were evaluated separately by three independent observers. Interobserver variability and correlation between the quantitative and qualitative analyses were calculated. RESULTS: The mean CNRs of two specimens of cartilage versus saline solution ranged from 6.3 (+/- 2.1) to 27.7 (+/- 2.5), and those of cartilage versus subchondral bone from 0.3 (+/- 0.2) to 22.5 (+/- 1.4). The highest CNR was obtained with a T1-weighted spoiled 3D-GE technique (TR 65 ms, TE 11.5 ms, FA 45 degrees). The number of lesions observed per sequence varied from 35 to 69. Observer agreement was fair to good. The T1-weighted spoiled GE sequences with a TR of 65 ms, TE of 11.5 ms and FA of 30 degrees and 45 degrees were significantly superior to the other 34 sequences in the qualitative analysis. CONCLUSION: T1-weighted spoiled 3D-GE sequences with a TR of 65 ms, a TE of 11.5 ms, and a FA of 30-45 degrees were found to be optimal for in vitro imaging of cartilage at 0.5T.

Animals↗
PubMedDate not supplied

A multi-plane scout sequence using flash imaging.

A new pulse sequence is presented for multi-plane imaging which simultaneously acquires images in the axial, coronal, and sagittal planes. Combined with FLASH imaging or other similar techniques, T1- and T2-weighted images in all three planes can be acquired in approximately 12 and 30 seconds, respectively. The sequence is easily implemented by modifying a multi-slice sequence to provide a rotation of gradient assignments between slice excitations. This sequence has been used extensively as a rapid "scout" scan. Several clinical examples are included.

Arachnoid↗