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PubMed1996

[Fluid attenuated inversion recovery sequences: indications in neuroradiology].

The acronym FLAIR refers to fluid attenuation inversion recovery sequences, which are T2-weighted MR pulse sequences with liquor signal saturation by a long TI. They are characterized by long TR and TE and therefore the acquisition time is very long in the conventional mode, while fast imaging (the Turbo mode) reduces acquisition time to less than 2 minutes. Our study was aimed at codifying the use of this type of sequence in neuroradiologic studies. All the exams were performed with an MR unit with a 1-Tesla magnetic field. We carried out 150 neuroradiologic exams with this pulse sequence on patients with cerebral, medullary or orbital conditions. This technique is very useful to study periventricular or cortical lesions in multiple sclerosis and in other multifocal cerebral conditions (e.g., multiple metastases or lacunar infarcts), but we pointed out the following other advantages: better definition of the extent of infiltrative white matter lesions (i.e., gliomatosis cerebri and lymphomas), better differentiation of cystic from necrotic cavities and exact characterization of cortical damage in cerebral ischemic lesions (useful also for the differential diagnosis). Moreover, FLAIR pulse sequences could diagnose some globe conditions, such as amelanotic uveal melanomas and malformations with no need of contrast agent administration. In contrast, they were useless to study deep ischemic areas, solid neoplasms, hemorrhagic lesions, poroencephalic areas, intrinsic medullary lesions and intra-orbital and extra-ocular conditions. In conclusion, the FLAIR technique is a major diagnostic tool in neuroradiologic MR studies because they overcome such limitations of Turbo SE PD sequences as blurring artifacts; moreover, their acquisition time is always very short. In some cases, FLAIR images are decisive for the diagnosis.

Brain Diseases↗
PubMedDate not supplied

MR arthrography in chondromalacia patellae diagnosis on a low-field open magnet system.

OBJECTIVE: The purpose of this study was to compare the diagnostic efficacy conventional MRI and MR arthrography (MRA) in the diagnosis of chondromalacia patella (CP) on a low-field open magnet system (LFOMS), correlated with arthroscopy. SUBJECTS AND METHODS: Forty-two patients (50 knees) with pain in the anterior part of the knee were prospectively examined with LFOMS, including T1-weighted, proton density-weighted and T2-weighted sequences. All were also examined T1-weighted MRI after intraarticular injection of dilue gadopentetate dimeglumine. Two observers, who reached a consensus interpretation, evaluated each imaging technique independently. Thirty-six of the 50 facets examined had chondromalacia shown by arthroscopy, which was used as the standard of reference. The sensitivity, specificity and accuracy of each imaging technique in the diagnosis of each stage of CP were determined and compared by using the McNemar two-tailed analysis. RESULTS: Arthroscopy showed that 16 facets were normal. Four (30%) of 13 grade 1 lesions were detected with T1. Four lesions (30%) with T2 and three lesions (23%) with proton-weighted images were detected. Seven (53%) of 13 grade 1 lesions were detected with MRA. Grade 2 abnormalities were diagnosed in two (33%) of six facets with proton density-weighted pulse sequences, two (33%) of six facets with T1-weighted pulse sequences, in three (50%) of six facets with T2-weighted pulse sequences, in five (83%) of six facets with MRA sequences. Grade 3 abnormalities were diagnosed in three (71%) of seven facets with proton density- and T1-weighted images, five (71%) of seven facets with T2-weighted pulse sequences, six (85%) of seven facets with MRA sequences. Grade 4 CP was detected with equal sensitivity with T1-, proton density- and T2-weighted pulse sequences, all showing seven (87%) of the eight lesions. MRA again showed these findings in all eight patients. All imaging techniques were insensitive to grade 1 lesions and highly sensitive to grade 4 lesion, so that no significant difference among the techniques could be shown. CONCLUSION: All imaging technique studied had high specificity and accuracy in the detection and grading of CP; however, MRA was more sensitive than T1-weighted and proton density-weighted MR imaging on a LFOMS. Although the arthrographic techniques were not significantly better than T2-weighted imaging, the number of false-positive diagnosis was greatest with T2-weighted MRI.

Adult↗
PubMed1994

[Turbo-Spin-Echo sequences in focal liver diseases].

Turbo Spin-Echo (Turbo SE) pulse sequences represent a new type of fast sequences characterized by a dramatic reduction in acquisition time with similar intrinsic contrast to that of conventional Spin-Echo (SE) pulse sequences. For a preliminary clinical comparison of turbo SE and SE sequences, a consecutive series of 25 patients with focal liver lesions was investigated by means of MRI at 1.0 T. The patients were selected on the basis of US or CT positive liver scans for focal lesions. All patients underwent MRI following the same protocol: axial T1-weighted (T1W) gradient echo (GE) sequences (TR/TE = 110/6 ms) with the "breath-hold" technique, T2-weighted (T2W) SE sequences (TR/TE = 1730/90 ms; FL = 60, M = 128 x 256, 2 AC, TA = 7 min 26 sec) and turbo SE sequences (TR/TE = 5100/112 ms, M = 240 x 256, 2 AC, TA = 2 min 48 sec). Slice thickness was 10 mm with a 1-mm gap in all cases. Qualitative (number, size and definition of the lesions) and quantitative (contrast-to-noise ratio (CNR) and relative CNR) variables were considered. SE and turbo SE sequences demonstrated the same number of lesions; the smallest lesion (3 mm) was identified by turbo SE sequences. Turbo SE sequences better depicted lesion margins than SE sequences. In all cystic conditions, CNR was higher in turbo SE than in SE sequences, while it was lower in all the extant conditions. This preliminary experience proves turbo SE pulse sequences to be a valuable tool in liver MRI, which calls for further clinical trials.

Adult↗
PubMed2001

Contributions of an adiabatic initial inversion pulse and K-space re-ordered by inversion-time at each slice position (KRISP) to control of CSF artifacts and visualization of the brain in FLAIR magnetic resonance imaging.

AIM: The aim of this study was to compare the performance of three fluid attenuated inversion recovery (FLAIR) pulse sequences for control of cerebrospinal fluid (CSF) and blood flow artifacts in imaging of the brain. The first of these sequences had an initial sinc inversion pulse which was followed by conventional k-space mapping. The second had an initial sinc inversion pulse followed by k-space re-ordered by inversion time at each slice position (KRISP) and the third had an adiabatic initial inversion pulse followed by KRISP. MATERIALS AND METHODS: Ten patients with established disease were studied with all three pulse sequences. Seven were also studied with the adiabatic KRISP sequence after contrast enhancement. Their images were evaluated for patient motion artifact, CSF and blood flow artifact as well as conspicuity of the cortex, meninges, ventricular system, brainstem and cerebellum. The conspicuity of lesions and the degree of enhancement were also evaluated. RESULTS: Both the sinc and adiabatic KRISP FLAIR sequences showed better control of CSF and blood flow artifacts than the conventional FLAIR sequence. In addition the adiabatic KRISP FLAIR sequence showed better control of CSF artifact at the inferior aspect of the posterior fossa. The lesion conspicuity was similar for each of the FLAIR sequences as was the degree of contrast enhancement to that shown with a T(1)weighted spin echo sequence. CONCLUSION: The KRISP FLAIR sequence controls high signal artifacts from CSF flow and blood flow and the adiabatic pulse controls high signal artifacts due to inadequate inversion of the CSF magnetization at the periphery of the head transmitter coil. The KRISP FLAIR sequence also improves cortical and meningeal definition as a result of an edge enhancement effect. The effects are synergistic and can be usefully combined in a single pulse sequence. Curati, W. L.et al. (2001)Clinical Radiology56, 375-384

Adult↗
PubMed2006

Slice-selected LED and BPPLED: application of slice selection to DOSY.

High-resolution DOSY (Diffusion-ordered spectroscopy) is a series of 2-dimensional and 3-dimensional NMR techniques based on the differing diffusivity of constituent molecules in the solution state, with which the individual NMR spectrum of each component in a chemical mixture can be observed. All of the DOSY pulse sequences are derived from the spin-echo or stimulated-echo techniques under the effect of PFG (pulsed field gradient). One of the requirements for successful DOSY experiments and data fitting is that PFG must be uniform across the active sample volume. However, PFG, in general, is not uniform across the active sample volume in commercial high-resolution NMR probes and this nonuniformity of PFG is known to produce systematic errors in DOSY experiments. In fact, a strong and uniform gradient field can be realized only in the central region of the gradient coil and the slice-selection technique, widely used in Magnetic Resonance (MR) imaging, can be employed in resolving problems associated with the nonuniformity of PFG. We have developed a slice-selection pulse block, which can be generally applied to any DOSY pulse sequence with proper care of the phase cycling and experimental parameters. We applied the slice-selection technique to LED and BPPLED pulse sequences, which are among the most popular DOSY pulse sequences, and obtained good experimental results for a chemical mixture.

Journal Article↗
PubMed2003

Normal human left and right ventricular dimensions for MRI as assessed by turbo gradient echo and steady-state free precession imaging sequences.

PURPOSE: To establish normal ranges of left ventricular (LV) and right ventricular (RV) dimensions as determined by the current pulse sequences in cardiac magnetic resonance imaging (MRI). MATERIALS AND METHODS: Sixty normal subjects (30 male and 30 female; age range, 20-65) were examined; both turbo gradient echo (TGE) and steady-state free precession (SSFP) pulse sequences were used to obtain contiguous short-axis cine data sets from the ventricular apex to the base of the heart. The LV and RV volumes and LV mass were calculated by modified Simpson's rule. RESULTS: Normal ranges were established and indexed to both body surface area (BSA) and height. There were statistically significant differences in the measurements between the genders and between TGE and SSFP pulse sequences. For TGE the LV end-diastolic volume (EDV)/BSA (mL/m(2)) in males was 74.4 +/- 14.6 and in females was 70.9 +/- 11.7, while in SSFP in males it was 82.3 +/- 14.7 and in females it was 77.7 +/- 10.8. For the TGE the LV mass/BSA (g/m(2)) in males was 77.8 +/- 9.1 and in females it was 61.5 +/- 7.5, while in SSFP in males it was 64.7 +/- 9.3 and in females it was 52.0 +/- 7.4. For TGE the RV EDV/BSA (mL/m(2)) in males was 78.4 +/- 14.0 and in females it was 67.5 +/- 12.7, while in SSFP in males it was 86.2 +/- 14.1 and in females it was 75.2 +/- 13.8. CONCLUSION: We have provided normal ranges that are gender specific as well as data that can be used for age-specific normal ranges for both SSFP and TGE pulse sequences.

Adult↗
PubMed1991

1H NMR spectroscopic imaging of the monkey brain using binomial water suppression in a stimulated-echo sequence.

A new proton, two-dimensional pulse sequence for 1H NMR spectroscopic imaging (chemical shift imaging) was tested in phantoms and in the monkey brain. The pulse sequence consisted of one binomial chemically selective pulse and two spatially selective pulses in the stimulated-echo sequence. The point-spread function (which is influenced by k-space filtering and the number of phase-encoded steps) of a 1 mm source phantom was measured using a 16 x 16 spatial matrix and was found to have a FWHM of 10 mm (100 mm field of view) with very little rippling outside the main lobe. The binomial excitation profile was measured in order to correct the NMR intensity for the variable flip angles. Spectroscopic images were measured in the monkey brain with a 15 mm slice thickness and a 16 x 16 spatial matrix. Proton spectra derived from the brain contained sharp resonances of choline, creatine and N-acetyl aspartate with minimal lipid contamination. Proton spectra derived from the subcutaneous fat and adipose tissue behind the eyes contained large lipid resonances.

Animals↗
PubMed2006

General algorithm for automated off-center MRI.

A general formula was derived that automatically modifies any MRI pulse sequence to realize arbitrary field-of-view (FOV) shifts. Unlike conventional techniques for implementing off-center MRI, the new method is completely automatic and can therefore be incorporated into the scanner hardware or software, thereby simplifying the development of MRI pulse sequences. The algorithm was incorporated into a visual pulse sequence programming environment, and several pulse sequences were programmed and tested at various off-center locations using the new technique. Unless there is significant background field inhomogeneity or gradient nonlinearity, research sequences employing the automatic technique need only be programmed and tested at the gradient isocenter, whereas with conventional methods, artifacts can sometimes depend on the position of the FOV.

Algorithms↗
PubMedDate not supplied

Pulsed magnetization transfer versus continuous wave irradiation for tissue contrast enhancement.

Pulsed magnetization transfer and continuous wave irradiation techniques are analyzed and compared for saturation efficiency and radio-frequency (RF) power requirements at 1.5 and 0.5 T. Binomial RF pulses transmitted on resonance are a more power-efficient method of exciting saturation transfer and are easily implemented with any pulse sequence. Binomial pulses selectively excite all short T2 species and behave as 0 degrees pulses for on-resonance, long T2 species.

Humans↗
PubMed2005

Simultaneous measurement of blood and myocardial velocity in the rat heart by phase contrast MRI using sparse q-space sampling.

PURPOSE: To measure cardiac blood flow patterns and ventricular wall velocities through the cardiac cycle in anesthetized Wistar Kyoto (WKY) rats. MATERIALS AND METHODS: A gradient-echo cine pulse sequence incorporating pulsed field gradients (PFGs) provided phase contrast (PC) motion encoding. We achieved a range of velocity sensitivity that was sufficient to measure simultaneously the large flow velocities within the cardiac chambers and aortic outflow tract (up to 70 cm s(-1) during systole), and the comparatively small velocities of the cardiac wall (0-3 cm s(-1)). A scheme of sparsely sampling q-space combined with a probability-based method of velocity calculation permitted such measurements along three orthogonal axes, and yielded velocity vector maps in all four chambers of the heart and the aorta, in both longitudinal and transverse sections, for up to 12 time-points in the cardiac cycle. RESULTS: Left ventricular systole was associated with a symmetrical laminar flow pattern along the cardiac axis, with no appearance of turbulence. In contrast, blood showed a swirling motion within the right ventricle (RV) in the region of the pulmonary outflow tract. During left ventricular diastole a plume of blood entered the left ventricle (LV) from the left atrium. The ventricular flow patterns could also be correlated with measurements of left ventricular wall motion. The greatest velocities of the ventricular walls occurred in the transverse cardiac plane and were maximal during diastolic refilling. The cardiac wall motion in the longitudinal axis demonstrated a caudal-apical movement that may also contribute to diastolic refilling. CONCLUSION: The successful measurements of blood and myocardial velocity during normal myocardial function may be extended to quantify pathological cardiac changes in animal models of human cardiac disease.

Algorithms↗
PubMed1997

A method for simulation of NOESY, ROESY, and off-resonance ROESY spectra

A formalism is proposed for simulation of NOESY, ROESY, and, more specifically, off-resonance ROESY nuclear magnetic resonance spectra. The off-resonance ROESY experiment has several advantages compared to standard NOESY and ROESY experiments. A simplified formalism which allows rapid computer simulation of the development of magnetization, including relaxation, in the presence of an RF field is of general use, in particular in the implementation and interpretation of off-resonance ROESY experiments. The relevant matrix equations can be derived either from the classical Bloch and Solomon equations or from the quantum mechanical homogeneous master equation in the basis of the Cartesian product operators. Examples of simulated spectra and behavior of magnetization during pulse sequences are shown. In addition, we present the full quantum mechanical theory for a two-spin system derived from the homogeneous master equation. The proposed formalism, here applied to off-resonance ROESY, has many potential applications, e.g., in the development and analysis of ROESY and TOCSY mixing sequences, selective pulses, and decoupling in which the complete spin dynamics, including relaxation, is taken into account. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1998

Multiple-pulse mixing sequences that selectively enhance chemical exchange or cross-relaxation peaks in high-resolution NMR spectra.

Rotating-frame NMR experiments which either emphasize or suppress cross relaxation, and which simultaneously suppress TOCSY, COSY, and zero-quantum peaks in NMR spectra, are presented and analyzed. The new experiments rely on mixing sequences which follow naturally from the transverse-ROESY (Tr-ROESY) sequence of Hwang and Shaka, and which are applicable to larger molecules in solution (spin diffusion limit). In the first variant a modified Tr-ROESY sequence, called multiple-pulse ROESY (MP-ROESY), is used to enhance cross-relaxation peak intensity compared to Tr-ROESY; in the second, called phase-modulated CLEAN chemical exchange (CLEANEX-PM), cross-relaxation peaks are greatly attenuated. The two methods are thus complementary: MP-ROESY is used to observe Overhauser peaks, and CLEANEX-PM is used to eliminate them, permitting clear observation of chemical exchange peaks alone. The new techniques are examined by theory and experiment. Practical guidelines that will result in high-quality spectra are given, including the judicious use of continuous weak static magnetic field gradients.

Magnetic Resonance Spectroscopy↗
PubMed1990

Brain water accumulation in pseudotumour cerebri demonstrated by MR-imaging of brain water self-diffusion.

We studied brain water self diffusion in pseudotumour cerebri by MR-imaging using single spin echo pulse sequences with pulsed magnetic field gradients of different magnitude. The methods is based on the fact that the movement of water molecules is restricted in brain tissue and that accumulation of water in the brain tissue will enhance the self diffusion of water. In order to evaluate the brain water content in pseudotumour cerebri we compared the water self diffusion coefficient in various regions of the brain in pseudotumour patients with that of healthy controls. Ten patients with pseudotumour cerebri were studied. All had increased ICP and increased resistance to CSF outflow. All patients had normal conventional MR spin echo images without focal lesions and a normal sized ventricular system. All patients had abnormal diffusion images showing increased water diffusion. Some patients had in particular increased diffusion in the periventricular regions, others in the whole brain. The diffusion coefficients in all brain regions of interest were significantly higher in patients than in controls. The findings suggest that patients with pseudotumour cerebri have a convective transependymal flow of water causing an interstitial brain oedema and in addition an intracellular brain water accumulation.

Adult↗
PubMed1999

The correlation between R2' and bone mineral measurements in human vertebrae: an in vitro study.

The aim of this study was to investigate whether MR imaging of trabecular bone structure using magnetic inhomogeneity measurements is related to the amount of bone mineral in human vertebrae. Weight, bone mineral content (BMCDXA), bone mineral per area (BMADXA) and bone mineral density (BMDCT) were determined in 12 defatted human lumbar vertebrae (L2-L4) by weighing, dual X-ray absorptiometry (DXA) and CT. Inhomogeneity caused by susceptibility differences between trabecular bone and surrounding water was studied with MR imaging at 1.5 T using the GESFIDE sequence. The pulse sequence determines the transverse relaxation rate R2(*) and its two components, the non-reversible transverse relaxation rate (R2) and the reversible transverse relaxation rate (R2'; i. e. relaxation rate due to magnetic susceptibility) in a single scan. Voxel size was 0.9 x 1.9 x 5.0 mm. Positive significant correlations between R2' and weight, BMCDXA, BMADXA and BMDCT were observed (r > 0.61 and p < 0.05 for all). Unexpectedly, R2 was also positively correlated with weight, BMCDXA and BMDCT (r > 0.66 and p < 0.05 for all), but not with BMADXA. Thus, R2' measurements are related to the amount of bone mineral, but they also provide information which is not obtainable from bone mineral measurements.

Absorptiometry, Photon↗
PubMed1995

The potential of magnetic resonance imaging (MRI) for quantifying articular cartilage thickness -- a methodological study.

The thickness of patellar articular cartilage was assessed in a cadaveric human knee joint by magnetic resonance imaging. Imaging was conducted at 1.0 T, using three-dimensional gradient-echo sequences. From each of the sequences the total cartilage volume, the size of the articular surface, the mean cartilage thickness and the regional distribution of cartilage thickness were determined by image analysis. These values were then compared with those obtained from anatomical sections. The fat-suppressed FLASH sequence was found to allow the most accurate evaluation of the total volume and the regional distribution of the articular cartilage. Slight underestimation of the cartilage thickness by about 5% may be due to the fact that the calcified layer is not made visible by magnetic resonance imaging. There is, however, a very high degree of similarity between the distribution patterns obtained from the MR images and the anatomical sections. The contrast-to-noise ratios and reproducibility were also highest with the fat-suppressedFLASH sequence. This pulse sequence can therefore be recommended for experimental and clinical use.

Journal Article↗
PubMed1992

In vivo magnetic resonance diffusion measurement in the brain of patients with multiple sclerosis.

Measurement of water self-diffusion in the brain in 25 patients with multiple sclerosis was performed by magnetic resonance imaging. Quantitative diffusion measurements were obtained using single spin-echo pulse sequences with pulsed magnetic field gradients of different magnitude. Twenty-two of these patients also underwent measurement of the transverse relaxation time (T2). Only one plaque was evaluated in each patient. Based on prior knowledge, 12 plaques were classified as being 3 mo or less in age, and 7 plaques were classified as being more than 3 mo old. In all 25 plaques, water self-diffusion was found to be higher than in apparently normal white matter. Furthermore, water self-diffusion was found to be higher in acute plaques compared with chronic plaques. Finally, a slight tendency toward a relationship between the diffusion capability and T2 was found. We believe that an increased diffusion capability signifies an increase of the extracellular water space, which probably is related to the degree of demyelination. Thus, measurement of water self-diffusion in multiple sclerosis plaques may contribute to the study of pathogenesis of demyelination.

Adult↗
PubMed1996

NMR study of the diffusion processes in gels.

Stimulated spin-echo and Carr-Purcell pulse sequences, with pulsed magnetic field gradients, have been used to study the self-diffusion processes of the water in swollen and in supercoiled polyacrylamide gels and in polysaccharides agarose gels, for different polymer concentrations. Restricted diffusion effects are evidenced in both types of gels through the time dependence of the diffusion constant. The results can be interpreted in the framework of diffusion model through permeable walls.

Acrylic Resins↗
PubMedDate not supplied

Permeability estimation from NMR diffusion measurements in reservoir rocks.

It is well known that in restricted geometries, such as in porous media, the apparent diffusion coefficient (D) of the fluid depends on the observation time. From the time dependence of D, interesting information can be derived to characterise geometrical features of the porous media that are relevant in oil industry applications. In particular, the permeability can be related to the surface-to-volume ratio (S/V), estimated from the short time behaviour of D(t), and to the connectivity of the pore space, which is probed by the long time behaviour of D(t). The stimulated spin-echo pulse sequence, with pulsed magnetic field gradients, has been used to measure the diffusion coefficients on various homogeneous and heterogeneous sandstone samples. It is shown that the petrophysical parameters obtained by our measurements are in good agreement with those yielded by conventional laboratory techniques (gas permeability and electrical conductivity). Although the diffusing time is limited by T1, eventually preventing an observation of the real asymptotic behaviour, and the surface-to-volume ratio measured by nuclear magnetic resonance is different from the value obtained by BET because of the different length scales probed, the measurement remains reliable and low-time consuming.

Diffusion↗