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At least 1,279 records · Page 71Linked to original sources
PubMed2006

Chemical shift anisotropic mapping of a coherent optical absorber using magnetic field induced quantum beats.

A new method for mapping the spatial structure of optical coherent materials which relies on imposing a set of linear orthogonal gradient magnetic fields for a controlled hyperfine splitting of energy levels to create characteristic quantum beats when illuminated with a laser pulse with sufficient bandwidth to excite these levels is proposed. In this approach, a spectroscopic fingerprint of the dopant sites due to concentration and field susceptibilities in the sample is achieved through a Fourier decomposition of the radiative relaxation decay in an approach analogous to nuclear magnetic resonance spectroscopy due to the imposition of a controlled spatial-spectral encoding scheme. A three pulse sequence necessary to interrogate a gradient resolved voxel is also discussed. This three pulse approach can be combined with the conventional confocal imaging technique to provide information about the underlying chemistry of dopant distribution along each imaging plane which is useful in guiding the design and manufacturing process of optical crystals. In combination with gradient induced quantum beats, the entire inhomogeneous bandwidth can be interrogated. The proposed approach would scan this entire bandwidth at much faster rate enabling characterization of a large number of crystals than is currently possible through mechanical scanning with a confocal microscopy based spectroscopic technique as well as providing functional dopant profiling which is not currently possible with conventional approaches.

Journal Article↗
PubMed2002

The acquisition of multidimensional NMR spectra within a single scan.

A scheme enabling the complete sampling of multidimensional NMR domains within a single continuous acquisition is introduced and exemplified. Provided that an analyte's signal is sufficiently strong, the acquisition time of multidimensional NMR experiments can thus be shortened by orders of magnitude. This could enable the characterization of transient events such as proteins folding, 2D NMR experiments on samples being chromatographed, bring the duration of higher dimensional experiments (e.g., 4D NMR) into the lifetime of most proteins under physiological conditions, and facilitate the incorporation of spectroscopic 2D sequences into in vivo imaging investigations. The protocol is compatible with existing multidimensional pulse sequences and can be implemented by using conventional hardware; its performance is exemplified here with a variety of homonuclear 2D NMR acquisitions.

Animals↗
PubMed1991

Lung parenchyma: projection reconstruction MR imaging.

Magnetic resonance (MR) imaging of lung parenchyma is limited by the low proton density and short T2 in the lung as well as the effects of susceptibility and motion. The MR imaging appearance of lung parenchyma was investigated with a pulse sequence that offers some solutions to these problems. This sequence employs projection reconstruction (PR) acquisition gradients and a section-selective excitation pulse designed to eliminate the need to refocus and to allow low-frequency k-space data to be collected with minimal delay. Echo times as short as 50 microseconds can be achieved, producing a proton-density-weighted image. An excised inflated lung specimen and specimens from human subjects with normal lungs (n = 3), pulmonary arteriovenous malformations (n = 1), bronchogenic carcinoma (n = 1), and bullous lung disease with lung metastases (n = 1) were examined. Signal intensity from lung parenchyma and visibility of pulmonary structures were superior on images obtained with the PR MR imaging technique compared with spin-echo images.

Humans↗
PubMed1996

Detection of hyperacute primary intraparenchymal hemorrhage by magnetic resonance imaging.

BACKGROUND: MRI has become increasingly used in the acute setting to manage patients with stroke. There has been concern that MRI may not be sensitive in the detection of acute intracranial hemorrhage. We assessed whether strongly susceptibility-weighted MRI would be sensitive to intraparenchymal hemorrhage in the first few hours. CASE DESCRIPTIONS: In the course of our ongoing studies of MRI of acute ischemic stroke in more than 200 patients, 35 patients had MR studies within 6 hours. Six of these patients who presented with acute focal symptoms with definite time of clinical onset (2.5 to 5 hours) were found to have evidence of intraparenchymal hemorrhage. Standard T1- and T2-weighted MR scans were performed. In 5 of the patients, echo-planar imaging and gradient-echo sequences were performed to increase the sensitivity of magnetic susceptibility effects of the pulse sequences. Four of the cases were of putaminal hemorrhage and 2 were lobar hemorrhages. The hemorrhage was most evident as foci of T2* hypointensity (signal loss) and unambiguous on the more susceptibility-weighted sequences, particularly echo-planar gradient-echo images. CONCLUSIONS: MRI can detect hemorrhage within 2.5 to 5 hours of onset of clinical symptoms as regions of marked signal loss due to susceptibility effects, whereas conventional MR scans of ischemic stroke may appear normal. These results demonstrate that MR susceptibility sequences may be sensitive to hyperacute hemorrhage and suggest that MR may be an adequate screen for primary intraparenchymal hemorrhage.

Acute Disease↗
PubMed1991

[The imaging diagnosis of hepatic focal nodular hyperplasia].

Focal nodular hyperplasia (FNH) is a rare benign hepatocellular tumor occurring in noncirrhotic patients, mostly females, 20-50 years of age. It is usually asymptomatic. The authors took the lead from 5 cases of FNH studied over last year to analyze the different patterns exhibited by the condition on the various imaging techniques currently available. At scintigraphy with 99mTc DISIDA or with TcSC, FNH can be hyper, normal, or hypocaptating. On US scans, the lesion is often homogeneous and isoechoic, but it can also be hyper/hypoechoic. With Doppler US, high-flow signals can be observed. On unenhanced CT scans the lesion is solid, well-demarcated, isodense or slightly hyperdense; sometimes it shows a central hypodense area corresponding to fibrovascular scar. On postcontrast scans it appears hyper/isodense. At dynamic CT the lesion density, which is high during the arterial phase, decreases quickly in the parenchymal and the venous phases and reaches equal/inferior values to surrounding liver parenchyma. On liver angio-CT it is sometimes possible to visualize the bile ducts in the central scar. At angiography, FNH is hypervascular and homogeneous. On MR scans, in T1-weighted SE sequences, the condition is isointense or slightly hypointense, whereas on T2-weighted pulse sequences it is slightly hyperintense; the central scar is hypointense on T1, and hyperintense on T2, weighted scans. As we have no pathognomonic patterns but only orientative ones, a reliable differential diagnosis with hepatocellular adenoma (HA) and fibrolamellar hepatocellular carcinoma (FL-HCC) must be based on biopsy or cytology or, even better, histology. The differential diagnosis is nevertheless necessary because, while FNH does not usually require a surgical approach but only a radiological follow-up, both HA (due to possible bleeding and degeneration) and FL-HCC require surgery.

Adult↗
PubMed1990

[Staging of spinal metastasis using special techniques of magnetic resonance imaging].

The diagnosis of spine metastasis is a problem of great interest which leaves many questions unanswered. In this field MR imaging plays a fundamental role, as the only technique able to directly demonstrate the changes in bone marrow tissue, bound to tumoral activity. The introduction of gradient-echo (GE) sequences has helped reduce examination time. Moreover, with the accurate choice of pulse-sequence parameters (TR, TE, flip angle) additional information is acquired which is not yielded by conventional spin-echo (SE) sequences. Our study was aimed at evaluating MR sensitivity in the different stages of bone metastatic evolution. The comparative adequacy was evaluated of combined bone scintigraphy and conventional radiology versus MR imaging in 62 patients with vertebral metastases. Time interval between bone scan and/or radiological study and MR exam ranged from 10 days to 8 months. SE and GE T1-weighted images, and SE and GE T2-weighted images on the sagittal plane were employed, and axial images; coronal images were rarely acquired. Metastases were demonstrated by MR imaging at 122 vertebral levels, versus 88 true positives of combined scintigraphy and conventional radiology. Scintigraphic false-positives were observed at 15 vertebral levels, versus 9 with radiography. GE sequences were superior to SE ones in detecting vertebral morphologic lesions and bone marrow involvement thanks to their improved resolution and sensitivity. Moreover, GE sequences demonstrated tumoral bone marrow spread and persistent tumoral activity in the follow-up of spine metastases. Our results point to GE sequences as those of choice because of their higher resolution and sensitivity, which also allow response to treatment to be evaluated.

Humans↗
PubMed1998

A new approach for on-resonance magnetization transfer parameter optimization.

On-resonance radio frequency pulse sequences for magnetization transfer are optimized using a frequency domain approach. The method presented here was developed using binomial pulses and it is demonstrated that a simple analysis leads to accurate sequence parameters that can be used directly for magnetic resonance imaging. With thus optimized parameters it is possible to produce an efficient saturation of protons having short transverse relaxation time while protons with long transverse relaxation time are kept nearly unaffected by the radio frequency. The method is particularly well suited to the design of new magnetization transfer sequences and to the estimation of the limits of the accuracy of a T2 selection. Additionally in vitro tests have been performed on beef tendon oriented with a zero fiber to field angle.

Computer Simulation↗
PubMed2002

Pulsed Z-spectroscopic imaging of cross-relaxation parameters in tissues for human MRI: theory and clinical applications.

A new method of pulsed Z-spectroscopic imaging is proposed for in vivo visualization and quantification of the parameters describing cross-relaxation between protons with liquid-like and solid-like relaxation properties in tissues. The method is based on analysis of the magnetization transfer (MT) effect as a function of the offset frequency and amplitude of a pulsed off- resonance saturation incorporated in a spoiled gradient-echo MRI pulse sequence. The theoretical concept of the method relies on an approximated analytical model of pulsed MT that provides a simple three-parameter equation for a pulsed steady-state Z-spectrum taken far from resonance. Using this model, the parametric images of cross-relaxation rate constant, content, and T(2) of the semisolid proton fraction can be reconstructed from a series of MT-weighted images and a coregistered T(1) map. The method was implemented on a 0.5 T clinical MRI scanner, and it provided high-quality 3D parametric maps within an acceptable scanning time. The estimates of cross-relaxation parameters in brain tissues were shown to be quantitatively consistent with the literature data. Clinical examples of the parametric images of human brain pathologies (multiple sclerosis and glioma) demonstrated high tissue contrast and clear visualization of the lesions.

Brain↗
PubMed2000

Protein dynamics measurements by TROSY-based NMR experiments.

The described TROSY-based experiments for investigating backbone dynamics of proteins make it possible to elucidate internal motions in large proteins via measurements of T(1), T(2), and NOE of backbone (15)N nuclei. In our proposed sequences, the INEPT sequence is eliminated and the PEP sequence is replaced by the ST2-PT sequence from the HSQC-based experiments. This has the benefit of shortening the pulse sequences by 5.4 ms (=1/2J) and results in an increase in the intrinsic sensitivity of the proposed TROSY-based experiments. The TROSY-based experiments are on average of 13% more sensitive than the corresponding HSQC-based experiments on a uniformly (15)N-labeled Xenopus laevis calcium-bound calmodulin sample on a 750-MHz spectrometer at 5 degrees C. The amide proton linewidths of the TROSY-based experiments are 2-13 Hz narrower than those of the HSQC experiments. More sensitivity gain and higher resolution are expected if the protein sample is deuterated.

Amides↗
PubMed2005

Diffusion tensor imaging in children and adolescents with tuberous sclerosis.

BACKGROUND: Tuberous sclerosis (TS) is characterised by benign hamartomatous lesions in many organs. Diffusion tensor imaging (DTI) can detect microstructural changes in pathological processes. OBJECTIVE: To determine apparent diffusion coefficient (ADC) and fractional anisotropy (FA) maps in children with TS and to investigate the diffusion properties in cortical tubers, white-matter lesions, perilesional white matter, and contralateral normal-appearing white matter, and to compare the results with ADC and FA maps of normal age- and sex-matched volunteers. MATERIALS AND METHODS: Seven children and adolescents (age range 2-20 years) suffering from TS were included. MRI was performed on a 1.5-T scanner using a transmit/receive coil with T1-W and T2-W spin-echo and FLAIR sequences. DT images were acquired by using a single-shot echo-planar pulse sequence. Diffusion gradients were applied in six different directions with a b value of 1,000 s/mm(2). RESULTS: ADC was higher in cortical tubers than in the corresponding cortical location of controls. ADC values were higher and FA values were lower in white-matter lesions and perilesional white matter than in both the contralateral normal-appearing white matter of patients and in controls. There were no significant differences for both ADC and FA values in the normal-appearing white matter of patients with TS compared to controls. CONCLUSIONS: DTI provides important information about cortical tubers, white-matter abnormalities, and perilesional white matter in patients with TS.

Adolescent↗
PubMed2000

Magnetic resonance imaging of intramedullary spinal cord tumors.

The advent of MRI has significantly changed the diagnosis of spinal cord tumors. Standard imaging provides excellent localization and characterization of the tumor in a noninvasive fashion. Exact histologic diagnosis of the two most common tumors, ependymoma and astrocytoma, remains elusive but there are some suggestive imaging characteristics. It is hoped that some of the newer MR imaging sequences will improve characterization of the tumor and thereby influence therapy. Several of these pulse sequences are already used routinely in brain imaging. Evaluation of new imaging sequences in the spine has lagged behind brain MR imaging, mainly due to technical factors. Work on animal spine models and extrapolation from brain imaging studies suggest that it is only a matter of time before some of these techniques become clinically relevant.

Astrocytoma↗
PubMed2004

Measurement of left ventricular velocities: phase contrast MRI velocity mapping versus tissue-doppler-ultrasound in healthy volunteers.

The aim of this study was the comparison of phase contrast magnetic resonance imaging (PCMRI) measurements of left ventricular velocities in a physiological in vivo setting with tissue-Doppler-ultrasound (tissue Doppler imaging: TDI) data in healthy volunteers. Images were acquired in short axis view using a flow compensated black blood k-space segmented gradient echo sequence. Velocity encoding was performed by adding a bipolar gradient after each rf-pulse to the otherwise identical pulse sequences. Full in-plane velocity information of the moving heart was obtained in 16 heartbeats within one breath-hold measurement. Twenty-nine healthy volunteers (mean age=25 years) were examined with both imaging modalities. Both PCMRI and TDI demonstrate a biphasic profile of radial velocities over the cardiac cycle. Intraindividual comparison of left ventricular velocity data acquired using PCMRI and TDI show a very good correspondence with r-values of 0.97. The in vivo study in 29 healthy volunteers demonstrates a high validity of time-resolved phase contrast measurements for the analysis of left ventricular myocardial velocities.

Adult↗
PubMed1993

Rapid dynamic contrast-enhanced magnetic resonance imaging of the liver and portal vein.

The purpose of this study was firstly to show the diagnostic value of a rapid acquisition multislice sequence (TurboFLASH) during bolus injection of gadolinium-DTPA by comparing it with the pulse sequences currently used for abdominal studies and secondly to develop improved scanning protocols for the liver. Patients were referred for upper abdominal studies including portal vein assessment. 40 patients were imaged in the coronal plane using a multislice TurboFLASH (TF) sequence (TR = 100; TE = 4) acquired during a breath-hold period of 19 s. The short echo time allows up to 11 slices of 5 mm thickness to be acquired simultaneously. Images were obtained before, during and after bolus administration of Gd-DTPA. The slices from each acquisition were combined using a maximum intensity projection algorithm to include all the vessels on a single image. Initially each patient was scanned using a conventional axial T2 weighted spin-echo sequence (T2W-SE) (TR2000; TE = 45/90) and a coronal T1 weighted spin-echo sequence (TR450; TE = 15). The clarity of the demonstration of vascular anatomy was compared and scored for all sequences by two radiologists. Vessel patency, the conspicuity of mass lesions and the spread of tumour to adjacent structures were also scored. The anatomy of the main portal vein was significantly better shown by coronal TF images after bolus injection than on T2W-SE images or TF before Gd-DTPA. The right and left portal veins were equally well shown by coronal TF with Gd-DTPA and T2W-SE images. There was no significant difference between contrast enhanced TF and T2W-SE imaging in visualization of the hepatic veins. More lesions were demonstrated by post-contrast TF than by T2W-SE imaging. Portal venous occlusion was better appreciated by post-contrast TF. Our results demonstrate that Gd-DTPA TF imaging improves visualization of the main portal vein compared with SE sequences and provides a more accurate assessment of vessel patency. The segmental anatomy of the liver is better appreciated and the demonstration of focal liver lesions compared with T2W-SE images is improved during the non-equilibrium phase of enhancement. TF acquisitions before and after Gd-DTPA are obtained in approximately 5 min; thus a marked reduction in examination time can be achieved.

Gadolinium DTPA↗
PubMed2000

[MRI of drug-resistant epilepsies: contribution of FLAIR sequence in a series of 150 patients].

We studied the usefulness of the Fluid-Attenuated Inversion-Recovery (FLAIR) pulse sequence for patients with intractable seizures by reviewing the MR images of 150 consecutive patients including a standard imaging protocol and a FLAIR sequence. FLAIR images revealed a cortical lesion in 8 of the 81 patients with no lesion detected on the standard MR imaging protocol. In addition, FLAIR images provided additional informations for 13 patients of the 69 patients for whom a lesion was already detected on the standard MR imaging protocol. Therefore, our results indicated that FLAIR sequences were a useful imaging tool for patients with intractable seizures since it improved the MR diagnosis in 21 of 150 patients (14%).

Adolescent↗
PubMed1994

[Magnetic resonance of the cartilages of the large joints].

MRI of the articular cartilage requires a careful technical approach since this structure is very thin, with a peculiar internal architecture between the supporting matrix and the cell component. On MR images the normal articular cartilage has a zonal appearance. To optimize the variables for best visualization of the internal architecture of the hyaline articular cartilage, an ex vivo and in vivo study was carried out. Accurate T1 and T2 relaxation times of the articular cartilage were obtained with a particular mixed sequence and then used to create isocontrast intensity graphs. The latter allowed, in all pulse sequences (SE and GRE), the best combination of TR, TE and FA to optimize signal differences between cartilage areas. A trilaminar pattern was demonstrated, with a superficial and a deep hypointense areas, in all sequences, together with an intermediate area which was moderately hyperintense on SE images and markedly hyperintense on GRE images. In the current literature, MRI appears to have been widely used to investigate hyaline cartilage conditions. In many series, the technique proved its efficacy in assessing both acute (traumatic cartilage fractures, osteochondritis dissecans, arthritis) and chronic (arthrosis, chondromalacia patellae, Hoffa's disease, synovial plica syndrome) conditions of the articular cartilage. T2-weighted sequences (both SE and GRE) are widely known as the most accurate in assessing cartilage conditions, which depends mainly on the arthrographic effect of synovial fluid on T2-weighted images. Of late, also MR arthrography, especially MR arthrography after the i.v. administration of Gadolinium, has emerged as an outstanding technique to investigate articular cartilage conditions. On the basis of MR arthrography findings, some authors suggested a classification of osteochondritis dissecans, arthrosis and chondromalacia patellae on MR images. MR stages seem to be closely correlated with the histologic classification suggested for these conditions.

Cartilage Diseases↗
PubMed2006

Temporal diffusion spectroscopy: theory and implementation in restricted systems using oscillating gradients.

The theory of temporal diffusion spectra is reviewed. In contrast to q-space spectroscopy, which measures the displacement spectrum of spins in a spatial domain, the spectral density of the velocity correlation function (VCF) in the temporal domain is considered. It is demonstrated that casting diffusion in this domain may facilitate measurements of microscopic geometry and the decomposition of the diffusion signal into components due to disperse flow and restricted diffusion. An oscillating gradient (OG) method of diffusion spectroscopy was developed and implemented. Microscopic pore sizes, surface-to-volume ratios (S/Vs), and diffusion path tortuosities were extracted from model systems using this method. Cases are discussed in which this type of experiment may allow the characterization of pore geometry when spatial domain experiments fail. OGs may be combined with imaging sequences to map complex patterns of diffusion and flow. Moreover, scalar apparent diffusion coefficient (ADC) measurements in complex biological systems may be subtly dependent on specific pulse sequence parameters. Thus, scalar ADC measurements using gradient pulses with different frequency spectra may give different results. Conversely, the frequency dependence of motion-sensitizing gradient pulses may be exploited to deduce the origin of ADC changes.

Diffusion Magnetic Resonance Imaging↗
PubMed1998

Excitation of complicated shapes in three dimensions.

We experimentally verified a recently proposed technique for the excitation of a complicated three-dimensional profile (CARVE, completely arbitrary regional volume excitation). CARVE is based on a generalized DANTE RF pulse sequence and a synchronous string of gradient steps. Provided there is no limitation in the number of pulses, CARVE can generate an excitation profile of any shape with any resolution. However, hardware limitations and sample properties restrict the number of RF pulses and gradient steps and, thus, limit attainable resolution of the excitation profile. We theoretically and experimentally showed that spatial resolution can be increased by distributing a long sequence among several CARVE experiments and summing up their signals. This is particularly important for three-dimensional excitation profiles where an n-fold increase in resolution requires an n3-fold increase of the number of events in the sequence. The potential use of three-dimensional CARVE might be in spectroscopic imaging where the excitation profile can be tailored to match the shape of a selected organ or body part.

Magnetic Resonance Spectroscopy↗
PubMed1999

The relationship between synovitis and bone changes in early untreated rheumatoid arthritis: a controlled magnetic resonance imaging study.

OBJECTIVE: The interrelationship between synovitis and bone damage in rheumatoid arthritis (RA) is a subject of controversy. Using magnetic resonance imaging (MRI), this study followed the bone changes in early RA and determined their relationship to synovitis. METHODS: Thirty-one patients with early RA who had swelling of the metacarpophalangeal (MCP) joints and 31 healthy control subjects with no clinical evidence of arthritis underwent MRI of the second through fifth MCP joints of the dominant hand by use of a 1.5T scanner. Coronal T1-weighted and T2-fat suppressed (FS) sequences were performed to evaluate bone edema, and gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) pulse sequences were obtained to evaluate synovitis. Bony abnormalities were described as bone edema (low signal on T1-weighted sequences and intermediate/high signal on T2 FS sequences adjacent to the bone cortex) or as bone cysts (circular juxtacortical abnormalities with low signal on T1-weighted images and with very high signal on T2 FS sequences). Contrast and noncontrast MRI films were scored in a blinded manner, and Fisher's exact probability test was used to determine differences between groups. RESULTS: Twenty-one of the 31 RA patients (68%) had bone edema, which was seen in 43 of 124 joints (35% of joints) and 3 of the 31 control subjects had bone edema seen in 3 of 124 joints (2% of joints) (P < 0.0001). Thirty RA patients (97%) had Gd-DTPA-confirmed MCP joint synovitis, and bone edema was seen in 40 of the 75 joints with Gd-DTPA-proven synovitis (53%), but in only 3 of 49 without (6%) (P < 0.0001). CONCLUSION: MCP joint bone edema is present in the majority of patients with RA at presentation, but is seen only occasionally in normal control subjects. The fact that bone edema occurred rarely in the absence of synovitis in patients with RA suggests that bony changes in RA are secondary to synovitis.

Adult↗