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At least 775 records · Page 43Linked to original sources
PubMed1987

Differentiation of adrenal masses by magnetic resonance imaging.

Eighty-one adrenal masses in 68 patients were examined with magnetic resonance imaging (MRI). Masses included nonfunctioning adenomas (17), metastases (25), adrenocortical carcinomas (10), and pheochromocytomas (23). T1-weighted pulse sequences depicted the anatomy with a resolution comparable to that of computed tomography (CT). T2-weighted pulse sequences provided some histologic specificity separating nonfunctioning adenomas with low signal-intensity from metastases with intermediate signal-intensity and pheochromocytomas with high signal-intensity. Pheochromocytomas could always be distinguished from other adrenal masses. In 20% of the cases, metastases with low signal-intensity could not be distinguished from nonfunctioning adenomas.

Adenoma↗
PubMed1985

Strategies and tactics in NMR imaging relaxation time measurements. I. Minimizing relaxation time errors due to image noise--the ideal case.

The effect of NMR image noise on errors in calculated values of relaxation times is quantitatively assessed by use of relaxation time noise figures, which are derived on the basis of statistical principles as functions of pulse delay, repetition, and recovery intervals for several types of pulse sequences. Two strategies for determining relaxation times are considered: two point (ratio of intensities for two experiments) and multipoint (least-squares fit of intensities to pulse-sequence functions for n experiments). For given total measurement times, values of pulse interval times are found which give minimum relaxation time noise figures. A comparison of ratio methods shows that the best is a combination saturation-recovery, inversion-recovery (SR/IR) technique. For short measurement times (less than about 10T1) this optimized SR/IR ratio determination is also superior to the best multipoint method, a series of inversion-recovery experiments with equally incremented inversion-recovery times. An examination of the effect of signal averaging on the relaxation time noise shows that up to a measurement time characteristic of the particular method used (e.g., for times up to about 5T1 for the SR/IR ratio determination, 100T1 for the multipoint inversion-recovery method), increased measurement time is more effectively allotted to longer pulse intervals than to signal averaging. Numerical examples are tabulated which can help one to set optimum values for pulse intervals, given a rough estimate of the relaxation time to be determined.

Magnetic Resonance Spectroscopy↗
PubMed1996

[Comparative assessment of conventional spin echo sequences and turbo spin echo in the study with magnetic resonance of the lesions of the spinal cord].

In the last few years, new magnetic resonance (MR) pulse sequences called Fast or Turbo Spin Echo (TSE) sequences have become available. This kind of T2-weighted images is particularly useful for the study of spondylosis and degenerative spinal conditions, because it both reduces involuntary motion artifacts and its acquisition time is shorter than that of conventional SE T2-weighted images. Our study was aimed at assessing the diagnostic gain of this new type of pulse sequences in intrinsic spinal cord conditions. Therefore, we acquired TSE and conventional SE sequences, consequently, in 36 patients with intrinsic spinal cord conditions, which were apparent on T2-weighted images, and then compared the sensitivity, contrast resolution, and depiction of lesion margins and extent in both acquisition techniques. The results of our study follow: even though all lesions were identified with both techniques, contrast resolution was higher with TSE than with conventional SE images. Lesion margins and extent were substantially equally depicted with both techniques. Finally, TSE sequences had the same, and sometimes even higher, diagnostic yield than conventional SE sequences: therefore, TSE can be considered the sequence of choice in the study of intrinsic spinal cord conditions.

Adult↗
PubMed1992

Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation.

Neuronal activity causes local changes in cerebral blood flow, blood volume, and blood oxygenation. Magnetic resonance imaging (MRI) techniques sensitive to changes in cerebral blood flow and blood oxygenation were developed by high-speed echo planar imaging. These techniques were used to obtain completely noninvasive tomographic maps of human brain activity, by using visual and motor stimulus paradigms. Changes in blood oxygenation were detected by using a gradient echo (GE) imaging sequence sensitive to the paramagnetic state of deoxygenated hemoglobin. Blood flow changes were evaluated by a spin-echo inversion recovery (IR), tissue relaxation parameter T1-sensitive pulse sequence. A series of images were acquired continuously with the same imaging pulse sequence (either GE or IR) during task activation. Cine display of subtraction images (activated minus baseline) directly demonstrates activity-induced changes in brain MR signal observed at a temporal resolution of seconds. During 8-Hz patterned-flash photic stimulation, a significant increase in signal intensity (paired t test; P less than 0.001) of 1.8% +/- 0.8% (GE) and 1.8% +/- 0.9% (IR) was observed in the primary visual cortex (V1) of seven normal volunteers. The mean rise-time constant of the signal change was 4.4 +/- 2.2 s for the GE images and 8.9 +/- 2.8 s for the IR images. The stimulation frequency dependence of visual activation agrees with previous positron emission tomography observations, with the largest MR signal response occurring at 8 Hz. Similar signal changes were observed within the human primary motor cortex (M1) during a hand squeezing task and in animal models of increased blood flow by hypercapnia. By using intrinsic blood-tissue contrast, functional MRI opens a spatial-temporal window onto individual brain physiology.

Animals↗
PubMed2000

Broadband heteronuclear Hartmann-Hahn sequences with short cycle times.

For some applications, broadband heteronuclear Hartmann-Hahn sequences with very short cycle times are required. A quality factor is presented that makes it possible to assess the relative sizes of cycle time, bandwidth, and maximum RF amplitude for any given multiple-pulse sequence. This quality factor is determined for multiple-pulse sequences that are commonly used in HEHAHA experiments and for some favorable sequences that were so far only discussed in the context of heteronuclear decoupling.

Magnetic Resonance Spectroscopy↗
PubMed1985

The short TI inversion recovery sequence--an approach to MR imaging of the abdomen.

Initial clinical experience with magnetic resonance imaging (MRI) of the abdomen using short TI inversion-recovery (STIR) pulse sequences is described and compared with X-ray CT in a variety of abdominal disease. The extent of abnormality shown with MRI was greater than that with CT in 21 of 30 cases and equal in 9 cases. Lesion contrast was greater with MRI in 15 cases, equal in 14 and less in 1. The level of artefact was equal in 27 cases and greater with MRI in 3 cases. The STIR pulse sequence has significant advantages in producing high soft-tissue contrast, controlling respiratory artefact, avoiding confusion with intra-abdominal fat and identifying bowel loops.

Abdomen↗
PubMed2003

Dynamic T1 estimation of brain tumors using double-echo dynamic MR imaging.

PURPOSE: To assess the clinical utility of a new method for real-time estimation of T1 during the first pass of contrast agent by using this method to examine brain tumors. MATERIALS AND METHODS: The multi-phase spoiled gradient-echo pulse sequence using the double-echo magnetic resonance (MR) technique was modified. In the first half of the pulse sequence, the flip angle was varied systematically. Then, static T1 values were calculated using differences in MR signal intensities between different flip angles. In the latter half of this sequence, changes in absolute T1 were calculated using differences in signal intensities before and after injection of contrast agent. The double-echo MR data were used to minimize the T2* effect. Five cases of neurinoma and seven cases of meningioma were examined. Changes in T1 during the first pass of contrast agent were compared between neurinoma and meningioma. RESULTS: Changes in absolute T1 were clearly demonstrated on the parametric map. Although the changes in absolute T1 during the first pass of contrast agent did not allow differentiation between the two types of tumors, the mean gradient after the first pass was statistically higher for neurinoma than for meningioma (P < 0.05; meningioma, 0.011 +/- 0.012 second(-1)/second; neurinoma, 0.034 +/- 0.020 second(-1)/second). CONCLUSION: The present method appears to be useful for estimation of dynamic T1 changes in brain tumors in clinical settings.

Adult↗
PubMed1990

Improved quantification of in vivo 1H NMR spectra by optimization of signal acquisition and processing and by incorporation of prior knowledge into the spectral fitting.

Quantification of localized in vivo brain 1H spectra is in general very difficult due to excessive spectral overlap. In addition, intensity distortions may result from the effects of the NMR pulse sequence on the spins. This paper describes an approach to solving these problems. It comprises optimization of the pulse sequence; correction of the experimental lineshape; determination of intensity distortions, of relative line positions, and of linewidths using model solutions; and incorporation of the thus obtained prior knowledge into a nonlinear least-squares spectral fitting procedure. This approach resulted in greatly improved accuracy, precision, and reliability of the quantitation of our in vivo spectra of rat brain, and enabled us to estimate absolute metabolite concentrations.

Animals↗
PubMed1986

Evaluation of the prostate by magnetic resonance imaging.

Forty-seven male patients with suspected prostatic disease underwent magnetic resonance imaging (MRI) of the pelvis on a Picker resistive magnet operating at 0.15 T; 33 had histologically proved adenocarcinoma, 12 benign prostatic hypertrophy, 1 a transitional cell carcinoma, and 1 a seminoma. Eleven normal subjects also were included in the study. The study attempted to (1) define the MRI characteristics of the normal prostate, benign prostatic hypertrophy, and prostatic adenocarcinoma, (2) evaluate various pulse sequences in imaging the prostate, and (3) compare MRI findings with clinical, pathologic, and computed tomography results. Various pulse sequences, including inversion recovery and spin-echo with short and long TE and TR, were used. MRI was sensitive in detecting intracapsular and extracapsular prostatic disease. The finding of inhomogeneous signal texture throughout the gland was a sensitive but nonspecific finding for adenocarcinoma. A focal nodule with prolonged T1 and T2 relaxation times was the most specific MRI finding for adenocarcinoma. Extracapsular spread of neoplasm was often demonstrated, and because of its superior soft-tissue contrast ability, MRI was more accurate than computed tomography in delineating extracapsular extension.

Adenocarcinoma↗
PubMed2003

Transient oscillations under strongly mismatched Hartmann-Hahn conditions.

Based on the standard 2D Polarization inversion experiment, a new pulse sequence (PI-TAPF) is proposed. It represents a combination of Polarization Inversion and TAPF (time averaged precession frequency) sequences. The depolarization period consists of phase-alternating intervals of different duration in the I channel. The pulse sequence yields transient oscillations under the strongly mismatched HH conditions where the required power for the dilute spins is reduced by a factor of 5. Experimental results recorded for a sample of ferrocene powder are well reproduced by numerical simulations.

Algorithms↗
PubMed1985

Early detection of canine myocardial infarction by magnetic resonance imaging in vivo.

This study was undertaken to assess the ability of proton magnetic resonance imaging (MRI) to detect myocardial ischemia shortly after coronary artery occlusion. Fifteen dogs were studied before and serially for up to 6 hr after anterior descending or circumflex coronary artery ligation in vivo by gated MRI with a 0.15 tesla resistive magnet (resonant frequency of hydrogen 6.25 MHz). Image acquisition was by single-spin echo, with echo times (TE) of 30 msec and TE 60 msec, and modified inversion recovery pulse sequences. Excellent anatomic definition was observed. By 4 hr after coronary artery occlusion the signal in the infarct zone increased to 36 +/- 20% greater than that in the adjacent normal myocardium for the TE 30 msec sequence (p less than .01) and to 116 +/- 100% for the TE 60 msec sequence (p less than .05). The most intense increase in signal was noted with the TE 60 msec pulse sequence and because normal myocardium is not well visualized by this technique, acutely ischemic myocardium was clearly delineated. Inversion recovery imaging did not show areas of ischemia. Changes seen on MR images correlated well with the location of ischemic changes noted on microscopic examination of the excised hearts. MRI thus provides a noninvasive means for detection of ischemia early in the course of myocardial infarction.

Animals↗
PubMed1994

MR imaging of bone marrow lesions: relative conspicuousness on T1-weighted, fat-suppressed T2-weighted, and STIR images.

OBJECTIVE: Fat-saturation pulse sequences offer important potential advantages for depiction of bone marrow lesions on MR images. The objectives of this study were to evaluate the relative conspicuousness of bone marrow lesions on images obtained by using two of the most widely available fat-suppression techniques, short-TI inversion recovery (STIR) and fat-saturation T2-weighted imaging, and to analyze the effect of these methods on image quality. In addition, we sought to determine if either or both of these sequences provide significant advantages relative to conventional T1-weighted spin-echo images for the evaluation of bone marrow lesions. SUBJECTS AND METHODS: T1-weighted (600/15 [TR/TE]), STIR (2500/20/160 [TR/TE/TI]), and fat-saturation T2-weighted (2500/20-70) MR images were obtained with a 1.5-T system in 34 consecutive patients with suspected bone marrow lesions. The conspicuousness of 36 lesions was evaluated subjectively by three radiologists, who also evaluated the MR images for how well they showed margination and extent of the lesion, image uniformity, motion artifacts, and overall image quality. In addition, lesion contrast on these sequences was compared quantitatively by using percentage contrast measurements. RESULTS: Lesions were qualitatively equally conspicuous with all four pulse sequences. Quantitative measurements indicated that lesions were more conspicuous on fat-saturation T2-weighted and STIR images than on T1-weighted images (p < .001). Differences between the first two sequences were not significant. Factors related to image quality, including reduction in motion artifacts and image uniformity, were generally superior on T1-weighted images. CONCLUSION: T1-weighted, fat-saturation T2-weighted, and STIR sequences all provide a high degree of sensitivity for depiction of most types of bone marrow abnormalities. Although the conspicuousness of lesions is similar on fat-saturation T2-weighted and STIR images, the former sequence has several practical advantages, including acquisition of more slices per unit time and improved tissue specificity. The combination of T1-weighted and either fat-saturation T2-weighted or STIR images is highly effective for the evaluation of bone marrow lesions.

Adolescent↗
PubMed1988

[Magnetic resonance imaging (MRI) of intracranial chordomas].

MR images of 5 patients with intracranial chordoma were evaluated and compared with those of other clival lesions (1 clival osteomyelitis, 1 metastatic clival tumor, 3 clival meningiomas). The MR examination was performed using a 0.5 T superconductive magnet, with approximately 10 mm section thickness, one average and a 256 x 256 matrix. T1 weighted images were obtained by inversion recovery (IR) with TR 2100-2500 msec, TI 600 msec and TE 40 msec. T2 weighted images were obtained by spin echo pulse sequence with TR 1800-2500 msec and TE 120 msec (long SE). In several cases, the spin echo pulse sequences with TR 1000 msec and TE 40 msec (short SE) were also done. Multiplaned images were obtained. Four of 5 intracranial chordomas were low in intensity compared to cerebral gray matter on T1 weighted images, and all of 5 chordomas were as high in intensity as cerebrospinal fluid or higher than that of cerebrospinal fluid on T2 weighted images. Clival fatty marrow is high intensity on T1 weighted images. Clival involvement by a tumor was a clearly demonstrated as disappearance of this high intensity in all cases. In two cases, the tumor extended to the retropharyngeal space and this was detected clearly on short SE image. Although clival fatty marrow was disappeared, osteomyelitis and metastatic tumor in clivus were iso-intense to cerebral gray matter on both T1 and T2 weighted images.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed2003

Simultaneous acquisition of multiple orders of intermolecular multiple-quantum coherence images.

Recent studies have demonstrated the ability to detect images based on intermolecular multiple-quantum coherences (iMQCs) that correspond to flipping of two or more separated spins simultaneously, as opposed to conventional magnetic resonance where only one spin is flipped at a time. Until now, iMQC imaging has only acquired one coherence signal per pulse sequence. Here we report a new sequence that successfully detects five orders of coherence (2, 1, 0, -1, and -2-quantum coherence images) in one pulse sequence, with each signal having its full intensity. The simultaneous acquisition highlights substantial contrast differences between conventional and iMQC images, and between the different types of iMQC images.

Humans↗
PubMedDate not supplied

A quantitative index of ventricular and extraventricular intracranial CSF volumes using MR imaging.

A new technique is described that utilises a novel magnetic resonance pulse sequence to produce a quantitative index both for ventricular and, for the first time, extraventricular intracranial CSF volumes. The pulse sequence is a combination of a null-point inversion recovery sequence with an extended spin-echo read (echo time = 400 ms), which produces a contrast of CSF to white or grey matter of approximately 120:1. A series of experiments are performed on phantoms representing CSF filled ventricles and sulci over a wide range of volume values, and it is found that the standard deviation of differences between true and estimated values is 3.9% for ventricles, 4.6% for total cranial CSF, and 7.9% for CSF within the sulci. Normal volunteer reproducibility studies revealed corresponding standard deviations of less than 5.5%. Using the technique to produce absolute estimates of CSF volumes in normal subjects and patients produced results in good agreement with previously published necropsy studies. The technique has wide neurological and neurosurgical applicability particularly in terms of differential diagnosis and as an objective monitor of therapy or progression in conditions such as hydrocephalus, atrophy, and benign intracranial hypertension.

Adolescent↗
PubMed1986

Measurement of solute proton spin-lattice relaxation times in water using the 1,3,3,1 sequence.

1H NMR spin-lattice relaxation times (T1) of the N-CH3 proton resonances of phosphocreatine (PCr) and creatine (Cr) in water solutions were obtained using the 1,3,3,1 pulse sequence. These T1 values were equivalent to those obtained in D2O and water using either the conventional inversion-recovery experiment or the 1,3,3,1 pulse sequence. Thus, the 1,3,3,1 sequence of proton NMR can provide an independent means along with phosphorous NMR for assess PCr and for the study of the creatine kinase reaction (PCr + ADP in equilibrium ATP + Cr) in aqueous solutions and perhaps in biological preparations.

Adenosine Diphosphate↗
PubMed1988

Magnetic resonance imaging of the parotid glands using inversion-recovery sequences at 0.08 T.

One hundred and eight examinations on 103 patients with suspected disease of the salivary glands were studied using a 0.08 T resistive magnet and inversion-recovery pulse sequences. Sixty-eight patients who had a mass lesion within a salivary gland later had surgery, and specimens were obtained for histological diagnosis. The remaining 35 patients were diagnosed on clinical grounds. In all cases the clinical findings were compared with the appearances on magnetic resonance imaging (MRI). The use of inversion-recovery pulse sequences allowed accurate localization of all tumour masses and, because of the clarity with which blood vessels were displayed, the precise relationship of any parotid mass to the retromandibular vessels and hence the facial nerve was possible. Whilst MRI did not display any pathognomonic features to allow the differentiation of malignant cell types or the differentiation of invasive malignant tumours from chronic inflammatory disease, it was possible to differentiate parotitis from Mikulicz's disease and to diagnose cysts and pleomorphic adenomas by their appearances on MRI.

Cysts↗