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At least 847 records · Page 47Linked to original sources
PubMed2005

Technology insight: MRI of the myocardium.

MRI is emerging as the method of choice for the evaluation of a wide variety of cardiovascular disorders. A major advantage of this technique over the other cardiac imaging modalities is the fact that it allows the operator--via special software programs called pulse sequences--to probe a vast array of biological properties while using the same machine. In this review, we provide the reader with a brief overview of the pulse sequence concept and how it enables MRI practitioners to pursue a multifaceted approach to evaluating the myocardium. We discuss how MRI technology makes this imaging method ideally suited to the assessment of cardiac morphology, contractile function, myocardial perfusion and infarction. In addition, we present clinical scenarios in which the performance of multifaceted imaging by MRI can alter clinical decision making.

Coronary Disease↗
PubMed1984

Mechanisms of contrast in NMR imaging.

Nuclear magnetic resonance pixel intensity and contrast-to-noise has been computed and presented in graphical form for various tissues in the normal central nervous system, on the assumption that the signal intensity is proportional to the macroscopic transverse spin magnetization at the time of detection. T1, T2, and spin density values were experimentally determined using chi-square minimization techniques. Additionally, spin density was derived from partial saturation scans obtained with a long repetition time compared with the spin-lattice relaxation time. Pulse sequences discussed comprise partial saturation, saturation recovery, spin echo, and Carr- Purcell - Meiboom -Gill ( CPMG ). The complicated dependence of signal and contrast-to-noise on the pulse timing parameters and the specific pulse sequence makes it appear desirable to display image intensity so that the dependence on the extrinsic (operator-selectable parameter) is eliminated. Whereas T2 images can be derived from CPMG scans without excessive time penalty, this is not the case for T1 and spin density.

Central Nervous System↗
PubMedDate not supplied

Spinal subarachnoid tumor seeding from intracranial metastasis: MR findings.

We describe the magnetic resonance findings in two cases of spinal subarachnoid seeding from an intracranial metastatic lesion. Magnetic resonance imaging techniques using T1 and intermediate T1-T2 pulse sequences improve anatomical definition of the spinal cord-thecal sac and allow for optimal contrast to define metastatic deposits in the subarachnoid space. Heavily T2 weighted pulse sequences do not appear to be useful in the diagnosis of subarachnoid metastases.

Brain Neoplasms↗
PubMed1984

Focal renal masses: magnetic resonance imaging.

Thirty patients with focal renal masses were evaluated on a .12-Tesla resistive magnetic resonance unit using partial saturation and spin echo pulse sequences. A short repetition time (TR = 143 ms) was employed for partial saturation images and a spin echo was present in each case (TE = 10 ms). Additional pulse sequences through regions of interest were also obtained. Fifteen patients had cystic lesions, nine patients had renal cell carcinoma, two had metastatic lesions, one had an angiomyolipoma, and three had focal bacterial infection. Cystic lesions were well circumscribed and demonstrated a range of signal intensities. Small intra-parenchymal cysts were difficult to identify. Renal cell carcinomas demonstrated areas of increased signal using a partial saturation sequence (TR = 143-415 ms, TE = 10 ms). Magnetic resonance imaging accurately detected perinephric extension and vascular invasion in all patients. Metastatic disease to the kidney was uniformly low in signal, in contrast to primary renal cell carcinoma; an angiomyolipoma demonstrated very high signal intensity. Two masses resulting from acute focal bacterial nephritis were uniformly low in signal. One additional case of a more indolent pyelonephritis demonstrated high signal in regions of replacement lipomatosis and low signal in sites of active infection. Magnetic resonance imaging appears to be an accurate way of detecting, identifying, and staging focal renal masses.

Adenocarcinoma↗
PubMed1992

Growth plate of the normal knee: evaluation with MR imaging.

The purpose of this study was to establish the characteristics at magnetic resonance (MR) imaging of the normal physis of the tibia and of the fibula during development and maturation. Sixty MR imaging studies of the knee were performed in male and female subjects aged 1-20 years who had no clinical symptoms or radiographic abnormality in the lower extremity. Images obtained with both T1-weighted and field-echo pulse sequences were reviewed to establish developmental characteristics of the distal femoral and proximal tibial growth plates. Four stages of physeal development were identified. A field-echo pulse sequence with repetition time of 700 msec, echo time of 20 msec, and a flip angle of 40 degrees best depicted growth-plate cartilage. Some coronal and sagittal images of the normal knee show discontinuity of the physeal cartilage (drop-out sign), which should not be mistaken for premature closure.

Adolescent↗
PubMed1991

Characterization of intrahepatic space-occupying lesions by magnetic resonance imaging.

Hepatic neoplasia can be accurately characterized with magnetic resonance imaging of architectural, signal and dynamic gradient-echo contrast-enhancing features. Architectural characteristics valuable in determining the nature of hepatic neoplasia include the outer and inner contours of the lesion; the presence of fibrous encapsulation; the presence, signal and enhancing characteristics of a central scar; the presence and distribution of hemorrhage or necrosis; the relation of the lesion to regional vasculature and bile ducts; and lesion multiplicity. The signal characteristics in spin-echo and gradient-echo pulse sequences of the adjacent hepatic parenchyma and the lesion itself provide further information. In certain instances hepatic neoplasia can be definitively characterized on dynamic gradient-echo pulse sequences obtained after administration of the contrast agent gadolinium.

Carcinoma, Hepatocellular↗
PubMed2003

The DIVAM sequence: selective excitation of signals from both rigid and mobile domains in a fluoropolymer.

Simulations have been carried out on z-magnetizations produced by (and hence of the resulting spectra from) the dipolar filter (DF) and the recently suggested "Discrimination Induced by Variable-Amplitude Minipulses" (DIVAM) pulse sequences [S. Ando et al., Polymer 42 (2001) 8137; Magn. Reson. Chem. 40 (2002) 97]. The strengths of dipolar interactions have been modelled by introducing different values for transverse relaxation times. The DF case has been extended by allowing the pulse angles to be smaller than 90 degrees. The pulse intervals have also been used as variables. For the DIVAM case, the variables are similarly the minipulse nutation angles and minipulse intervals. The computations show that DIVAM is superior to DF in terms of selectivity for spectra of heterogeneous materials such as semi-crystalline polymers. The effects of the pulse sequences on 19F spectra of poly(vinylidene fluoride) (PVDF) and of a copolymer of vinylidene fluoride and trifluoroethylene (p(VDF/TrFE)) are presented, together with fits of the experimental results by the simulations.

Journal Article↗
PubMed2002

NMR of multipolar spin states excitated in strongly inhomogeneous magnetic fields.

The possibility of exciting and filtering various multipolar spin states in proton NMR like dipolar encoded longitudinal magnetization (LM), double-quantum (DQ) coherences, and dipolar order (DO) in strongly inhomogeneous static and radio-frequency magnetic fields is investigated. For this purpose pulse sequences which label and manipulate the multipolar spin states in a specific way were implemented on the NMR-MOUSE (mobile universal surface explorer). The performance of the pulse sequences was also tested in homogeneous fields on a solid-state high-field NMR spectrometer. The theoretical justification of these procedures was shown for a rigid two-spin 1/2 system coupled by dipolar interactions. Dipolar encoded longitudinal magnetization decay curves, double-quantum and dipolar-order buildup curves, as well as double-quantum decay curves were recorded with the NMR-MOUSE for natural rubber samples with different crosslink density. The possibility of using these multipolar spin states for investigations of strained elastomers by NMR-MOUSE is also shown. These curves give access to quantitative values of the ratio of the total residual dipolar couplings of the protons in the series of samples which are in good agreement with those measured in homogeneous fields.

Magnetic Resonance Spectroscopy↗
PubMed1997

Reliability of interactive three-dimensional brain volumetry using MP-RAGE magnetic resonance imaging.

We assessed the reliability of an interactive 3-dimensional methodology for magnetic resonance imaging (MRI) brain tissue segmentation and volumetry using a 3-dimensional magnetization prepared rapid gradient echo pulse sequence (3D MP-RAGE). The methodology was intended to be practically useful to study structural brain changes in larger groups of patients investigated for suspected dementia. The pulse sequence combines volume acquisition, excellent tissue contrast and short patient scan-time. The volumetric method is fully interactive, requiring a minimum of image pre-processing. Ten healthy controls and 10 patients with dementia were included in the study. Six healthy controls were scanned twice. The method is based on thresholding combined with manual tracing in a 3D volume. The 3-dimensional measurement method reduces the measurement time considerably compared to that of slice by slice measurement and permits 3-dimensional display of measured volumes. For different brain regions the intra-study (0.5-1.3%), study-study (1.8-4.7%) and inter-operator (7.1%) variability of this method compared favourably with other manual or automated methods reported. The major advantages of the method are its simplicity and speed, which permits measurement and display of regional brain volumes and tissues in larger patient groups.

Aged↗
PubMed1987

Magnetic resonance imaging of retroperitoneal lymphadenopathy.

A prospective study was performed to determine the value of magnetic resonance imaging (MRI) in the detection of retroperitoneal lymphadenopathy. Fifty patients with known malignancy, who demonstrated enlarged lymph nodes on computed tomography (CT), were entered into the study. The best visualization of retroperitoneal lymphadenopathy was found with a multislice spin-echo pulse sequence having a repetition time of 2000 msec, and echo delay of 30 msec. Nodes had a signal intensity lower than fat and greater than muscle with this pulse sequence. More than 90% of the lymphadenopathies as shown by CT could be demonstrated by MRI. Due to the flow-void phenomenon, blood vessels did not have any signal and were easily visualized without the use of intravenous contrast medium. Sagittal and coronal images added to an appreciation of the size of nodes and the extent of retroperitoneal lymphadenopathy. Ten normal volunteers were also imaged by MRI and their retroperitoneal nodes were well demonstrated.

Humans↗
PubMed2005

Orientational dependence of intermolecular double quantum coherence (iDQC) signal from tendon tissue.

The proton signal changes as the long axis of tendon tissue is rotated with respect to the main magnetic field (B(0)). The orientational changes in the tendon signal obtained using the correlation spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED) sequence, which allows the effects of intermolecular dipolar interactions to be observed, were investigated and compared with the orientational changes of the signals produced using correlation spectroscopy (COSY), spin-echo (SE), and one-pulse sequences. The intermolecular double quantum coherence (iDQC) signal obtained using the CRAZED sequence showed a variation in the signal from tendon tissue, with sharper peaks and greater relative differences between minimum and maximum signal values compared to the variations in the signal obtained from the COSY, SE, and one-pulse sequences. This result is attributed to the orientational dependence of the transverse relaxation rate of single (SQC) and double (DQH) quantum coherences R(2) and R(2,2), respectively.

Animals↗
PubMed2000

Double-resonance J-edited (1)H-NMR detection of 6-(13)C-D-2-deoxyglucose uptake in glioma cells.

The C6 methylene protons were selectively detected in (1)H-NMR spectra of intact glioma cells incubated with 6-(13)C-D-2-deoxyglucose (6-(13)C-2dG), a (13)C-enriched glucose analog that is suitable for monitoring glucose utilization in brain tumors. Spectral editing via (1)H-(13)C scalar coupling was performed with twin spin-echo double resonance (T-SEDOR), a pulse sequence which combines chemical specificity and high sensitivity, requires no solvent pre-saturation, and can easily be adapted to imaging protocols. This work demonstrates the suitability of the pulse sequence for monitoring 6-(13)C-2dG uptake in living cells in vitro, in spite of line-broadening and the occurrence of other strong signals in the spectral region of interest (3.5-4.4 ppm).

Animals↗
PubMedDate not supplied

MR imaging and vascular access ports: ex vivo evaluation of ferromagnetism, heating, and artifacts at 1.5 T.

The purpose of our study was to assess ferromagnetism, heating, and artifacts associated with vascular access ports exposed to a 1.5-T MR system. Twenty-eight different vascular access ports were evaluated in this investigation. Ferromagnetism was determined by using two previously described techniques. Temperature changes were measured immediately before and after performing a pulse sequence on the vascular access ports for 60 min at a specific absorption rate of 3.1 W/kg. Artifacts were assessed in association with the use of a fast GRASS pulse sequence. None of the vascular access ports displayed ferromagnetism. Heating was 0.2 degrees C or less. The presence of artifacts varied, depending on the component materials. The lack of ferromagnetism and negligible heating indicates that MR imaging performed at 1.5 T or less may be conducted safely in patients with any of the vascular access ports tested. None of the associated artifacts produced by the vascular access ports is considered to pose a substantial problem for MR imaging.

Artifacts↗
PubMed1999

Characterization of millisecond time-scale dynamics in the molten globule state of alpha-lactalbumin by NMR.

The motional dynamics of the molten globule (MG) state of alpha-lactalbumin have been characterized using (15)N transverse relaxation rates (R2). A modified version of the Carr-Purcell-Meiboom-Gill (CPMG) R2 pulse sequence is proposed in order to overcome the loss of sensitivity that arises from extreme line broadening due to complex dynamics on the millisecond time-scale. Using this pulse sequence, chemical exchange rates were extracted by examining the (15)N transverse relaxation rates as a function of CPMG delay values. The results clearly illustrate that pervasive conformational exchange of 0.2-0.5 ms in the (15)N backbone resonances of the molten globule state of alpha-lactalbumin. The temperature dependence of the conformational exchange rates display standard Arrhenius kinetic behavior between 10 and 30 degrees C. Estimates of the activation energies range from 0.8 to 4. 4 kcal/mol, indicating a low energetic barrier to conformational fluctuations relative to native state proteins. The fluctuations and low energetic barriers may be critical for directing the search for contacts that will result in the transition from the MG state to the native state.

Lactalbumin↗
PubMed2006

Error propagation model for microscopic magnetic resonance elastography shear-wave images.

Microscopic magnetic resonance elastography is a high-resolution method for visualizing shear waves and assessing the biomechanical viscoelastic properties of small biological samples. In this work, we used error propagation to develop a simple analytical model that relates the signal-to-noise ratio of MR magnitude images to the variance in shear-wave maps collected using gradient-echo and spin-echo phase-contrast pulse sequences. Our model predicts results for shear-wave images in phantoms, which match the experimentally observed phase variance within 8%. This model can be used to optimize MR pulse sequences for elastography studies, as well as other phase-difference techniques in MRI.

Biomechanical Phenomena↗
PubMed2003

Reproducibility of brain ADC histograms.

The aim of this study was to assess the effect of differences in acquisition technique on whole-brain apparent diffusion coefficient (ADC) histogram parameters, as well as to assess scan-rescan reproducibility. Diffusion-weighted imaging (DWI) was performed in 7 healthy subjects with b-values 0-800, 0-1000, and 0-1500 s/mm(2) and fluid-attenuated inversion recovery (FLAIR) DWI with b-values 0-1000 s/mm(2). All sequences were repeated with and without repositioning. The peak location, peak height, and mean ADC of the ADC histograms and mean ADC of a region of interest (ROI) in the white matter were compared using paired-sample t tests. Scan-rescan reproducibility was assessed using paired-sample t tests, and repeatability coefficients were reported. With increasing maximum b-values, ADC histograms shifted to lower values, with an increase in peak height ( p<0.01). With FLAIR DWI, the ADC histogram shifted to lower values with a significantly higher, narrower peak ( p<0.01), although the ROI mean ADC showed no significant differences. For scan-rescan reproducibility, no significant differences were observed. Different DWI pulse sequences give rise to different ADC histograms. With a given pulse sequence, however, ADC histogram analysis is a robust and reproducible technique. Using FLAIR DWI, the partial-voluming effect of cerebrospinal fluid, and thus its confounding effect on histogram analyses, can be reduced.

Adult↗
PubMed1996

Acute subarachnoid haemorrhage: detection with magnetic resonance imaging.

The purpose of this study was the evaluation of fluid attenuated turbo inversion recovery (FLAT TIRE) MR pulse sequence for detecting acute subarachnoid hemorrhage (SAH). Seven patients with SAH were studied within 6 days of ictus. Six of them underwent both CT and MRI and one MRI only. Pulse sequences included T1 spin echo (SE), PD and T2 turbo spin echo (TSE) and FLAT TIRE (TR/TI/TE = 6500/1800/140-180). All studies were performed on a 0.5 T system (Gyroscan T5, Philips Medical Systems). Simulated acute SAH was also studied with MRI. The FLAT TIRE sequence was better than the SE and TSE in all seven cases and better than CT in two cases. In two cases MRI was equivalent to CT, and in another two MRI underestimated the extent of SAH. The simulated acute SAH could be detected easily with the FLAT TIRE sequence, with difficulty on the T1 weighted images and not at all on the PD/T2 weighted images. The specific FLAT TIRE sequence used seems promising for the detection of acute SAH.

Acute Disease↗