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At least 361 records · Page 20Linked to original sources
PubMedDate not supplied

MR cholangiopancreatography: comparison between half-Fourier acquisition single-shot turbo spin-echo and two-dimensional turbo spin-echo pulse sequences.

BACKGROUND: To compare half-Fourier acquisition single-shot turbo spin-echo spin-echo (HASTE) magnetic resonance cholangiopancreatography (MRCP) with two-dimensional turbo spin-echo (2D TSE) MRCP for imaging pancreatobiliary diseases. METHODS: Twenty-seven patients with biliary or pancreatic disease underwent MRCP on a 1.0-T scanner with a body phased-array coil. A T2-weighted HASTE sequence (18 s) and a T2-weighted 2D TSE sequence (45 s) were used during a breath-hold by the patient. The source images and maximum intensity projection images of both sequences were reviewed independently by two radiologists. RESULTS: Motion artifacts were more severely pronounced with 2D TSE sequences than with HASTE sequences (p < 0.001). All obstructions and their sites were accurately identified with both sequences. Filling defects (calculi) in bile ducts were identified in all 22 segments (100%) with HASTE-MRCP, whereas calculi in 19 of 22 segments (86%) were identified with 2D TSE-MRCP (p = 0.25). Three missed sites on 2D TSE-MRCP were intrahepatic bile ducts. CONCLUSIONS: HASTE-MRCP is superior to 2D TSE-MRCP in terms of detecting motion artifacts and visualization of the pancreatic ducts. HASTE-MRCP is comparable to 2D TSE-MRCP for visualization of the biliary ducts and their obstruction and is superior to 2D TSE-MRCP for identification of calculi in intrahepatic bile ducts.

Adult↗
PubMedDate not supplied

T2-weighted MR imaging for hepatic hemangiomas: comparison of breath-hold and non-breath-hold turbo spin-echo pulse sequences with phased-array multicoil.

BACKGROUND: We compared T2-weighted and heavily T2-weighted breath-hold turbo spin-echo (TSE) sequences with T2-weighted non-breath-hold TSE sequence to evaluate hepatic hemangiomas on magnetic resonance (MR) with a phased-array multicoil. METHODS: Twenty-two patients with 27 hemangiomas were studied at 1.0-T scanner by using T2-weighted and heavily T2-weighted breath-hold TSE sequences (18 s each) and non-breath-hold T2-weighted TSE sequences with use of a phased-array multicoil. Images were quantitatively analyzed for tumor-to-liver signal-difference-to-noise ratios (SD/Ns) and tumor-to-liver signal intensity ratios (T/Ls) and qualitatively analyzed for tumor conspicuity and motion-induced image artifacts. RESULTS: Quantitatively, T2-weighted breath-hold TSE images showed the highest SD/Ns among the three sequences, although the differences from the heavily T2-weighted breath-hold TSE sequence and the T2-weighted non-breath-hold TSE sequence were not statistically significant (p = 0.61 and 0.06, respectively). Heavily T2-weighted breath-hold TSE images showed the highest T/Ls among the three sequences. The differences from the T2-weighted breath-hold TSE sequence and the T2-weighted non-breath-hold TSE sequence were statistically significant (p < 0.001). Qualitatively, breath-hold TSE images were superior to non-breath-hold TSE images in terms of tumor conspicuity (p < 0.01) and motion artifacts (p < 0.01). CONCLUSION: T2-weighted breath-hold TSE sequence is superior to T2-weighted non-breath-hold TSE sequence in the evaluation of hepatic hemangiomas on MR with a phased-array multicoil.

Adult↗
PubMedDate not supplied

MR imaging of advanced gastric cancer: comparison of various MR pulse sequences using water and gadopentetate dimeglumine as oral contrast agents.

BACKGROUND: To evaluate clinical usefulness of oral contrast agents (gadopentetate dimeglumine and water) and to assess proper magnetic resonance (MR) imaging in evaluating advanced gastric cancer (AGC) by comparing different MR imaging techniques. METHODS: Fifteen patients with AGC were imaged with a 1.0-T MR imager and body-array coil. All patients underwent surgery or laparascopic biopsy. Fast low-angle shot (FLASH), half-Fourier single-shot turbo spin-echo (HASTE), and true fast imaging with steady-state precession time (FISP) images were obtained after ingestion of 900 mL tap water in each patient, followed by postcontrast FLASH images after additional ingestion of gadopentetate dimeglumine (Gd-DTPA). Qualitative analysis including T-staging of AGC and scoring of imaging quality and quantitative analysis were performed prospectively. RESULTS: In image quality and diagnostic accuracy of T-staging, FLASH imaging showed results slightly superior to those of other imaging modalities, and there was no great difference between using water and Gd-DTPA as an oral contrast agent. As for cancer-to-gastric lumen contrast-to-noise ratio (CNR), HASTE and true FISP imaging were superior to FLASH imaging with Gd-DTPA (p < 0.0001). In cancer-to-pancreas CNR, FLASH imaging without Gd-DTPA showed the best result. CONCLUSIONS: The use of Gd-DTPA as a positive contrast agent may not be imperative, and T1-weighted FLASH imaging in combination with true FISP imaging with ingestion of tap water can be very useful in evaluating AGC with MR imaging.

Administration, Oral↗
PubMed2004

A comparative evaluation of a RARE-based single-shot pulse sequence for diffusion-weighted MRI of musculoskeletal soft-tissue tumors.

The purpose of this study was to evaluate the feasibility of a centric-reordered modified rapid acquisition with relaxation enhancement (mRARE) sequence for single-shot diffusion-weighted magnetic resonance imaging (DWI) of soft-tissue tumors in the musculoskeletal system. In the evaluation of this sequence, DWI was performed in a liquid phantom, in excised human tumor samples embedded in bovine muscle, and in nine patients suffering from different types of soft-tissue tumors. The measurements were compared to DWI using a spin-echo sequence and a single-shot echo planar imaging (EPI) sequence. The phantom measurements in water and dimethyl sulfoxide showed a difference of less than 5% when comparing the apparent diffusion coefficients (ADCs) determined by the mRARE sequence and the two other techniques. Comparing mRARE and EPI, the differences in the ADCs were about 10% in the excised tumor tissue and, typically, about 15% in vivo. ADCs between 0.8 x 10(-3) mm2/s and 1.4 x 10(-3) mm2/s, depending on the tumor type, were found in solid tumor tissue; in cystic tumor areas, ADCs greater than 2.0 x 10(-3) mm2/s were determined with the mRARE and the EPI sequences. Diffusion-weighted images of the mRARE sequence were less distorted than those acquired with the single-shot EPI sequence, and provided more anatomic information, since the muscle and fat signals were considerably higher.

Aged↗
PubMed2006

Detection of small pulmonary nodules in high-field MR at 3 T: evaluation of different pulse sequences using porcine lung explants.

To evaluate two MR imaging sequences for the detection of artificial pulmonary nodules inside porcine lung explants. 67 agarose nodules ranging 3-20 mm were injected into ten porcine lungs within a dedicated chest phantom. The signal on T1-weighted images and radiopacity were adjusted by adding 0.125 mmol/l Gd-DTPA and 1.5 g/l of iodine. A T1-weighted three-dimensional gradient-echo (T1-3D-GRE; TR/TE:3.3/1.1 ms, slice:8 mm, flip-angle:10 degrees ) and a T2-weighted half-Fourier fast-spin echo sequence (T2-HF-FSE; TR/TE:2000/66 ms, slice:7 mm, flip-angle:90 degrees ) were applied in axial orientation using a 3-T system (Intera, Philips Medical Systems, Best, The Netherlands), followed by CT (16x0.5 mm) as reference. Nodule sizes and locations were assessed by three blinded observers. In nodules of >10 mm, sensitivity was 100% using 3D-GRE-MRI and 94% using the HF-FSE sequence. For nodules 6-10 mm, the sensitivity of MRI was lower than with CT (3D-GRE:92%; T2-HF-FSE:83%). In lesions smaller than 5 mm, the sensitivity declined to 80% (3D-GRE) and 53% (HF-FSE). Small lesion diameters were overestimated with both sequences, particularly with HF-FSE. This study confirms the feasibility of 3 T-MRI for lung nodule detection. In lesions greater than 5 mm, the sensitivity of the 3D-GRE sequence approximated CT (>90%), while sensitivity and PPV with the HF-FSE sequence were slightly inferior.

Animals↗
PubMed1999

Focal nodular hyperplasia of the liver on ferumoxides-enhanced MR imaging: features on conventional spin-echo, fast spin-echo and gradient-echo pulse sequences.

The aim of this study was to assess the efficacy of a superparamagnetic iron oxide, ferumoxides, in the detection and characterization of focal nodular hyperplasia (FNH) on MR conventional spin-echo (SE), fast spin-echo (FSE) and gradient-echo (GRE) images. Fourteen adults with 27 FNHs were evaluated at 1.5 T before and after injection of ferumoxides. T1-weighted and T2-weighted SE, T2-weighted FSE and T2(*)-weighted GRE sequences were used and analysed qualitatively and quantitatively. One hundred percent of FNHs showed a significant postcontrast decrease in signal intensity on T2- and T2*-weighted images. Heavily T2-weighted SE images showed the maximum decrease in FNH signal-to-noise ratio (S/N). Postcontrast GRE T2(*)-weighted images improved the detection of the central scar and the delineation of FNHs and demonstrated the best lesion-to-liver contrast-to-noise ratio (C/N). Postcontrast T1-weighted SE images showed the least lesion-to-liver C/N. Ferumoxides-enhanced MR imaging can help detect and characterize FNH. Conventional pre- and postcontrast T2-weighted SE images and postcontrast GRE T2*-weighted images should be used preferentially.

Adult↗
PubMed1986

Errors in the assessment of the efficacy of MRI pulse sequences.

The effects of the echo delay TE on contrast in saturation recovery MRI have been analyzed both theoretically and with experimental brain scans. It is demonstrated that the influence of the TE values offered as minimal values by all major current equipment manufacturers is not insignificant, but produces a degree of T2 weighting that considerably degrades the useful contrast that could be obtained from T1 weighted studies. Consequently, conclusions drawn about the relative performances of T1 and T2 weighted sequences may be incorrect if based on saturation recovery images obtained with inappropriate echo times.

Brain↗
PubMedDate not supplied

A method for MR quantification of flow velocities in blood and CSF using interleaved gradient-echo pulse sequences.

The aim of this study was to establish a rapid method for in vivo quantification of a large range of flow velocities using phase information. A basic gradient-echo sequence was constructed, in which flow was encoded along the slice selection direction by variation of the amplitude of a bipolar gradient without changes in sequence timings. The influence of field inhomogeneities and eddy currents was studied in a 1.5 T interleaved sequences for calibration and in vivo flow determination were constructed, and flow information was obtained by pairwise subtraction of velocity-encoded from velocity non-encoded phase images. Calibration was performed in a nongated mode using flow phantoms, and the results were compared with theoretically calculated encoding efficiencies. In vivo flow was studied in healthy volunteers in three different areas using cardiac gating; central blood flow in the great thoracic vessels, peripheral blood flow in the popliteal vessels, and flow of cerebrospinal fluid (CSF) in the cerebral aqueduct. The results show good agreement with results obtained with other techniques. The proposed method for flow determination was shown to be rapid and flexible, and we thus conclude that it seems well suited for routine clinical MR examinations.

Aorta↗
PubMedDate not supplied

Abdominal applications of fast MR imaging: a comparison of fast field echo (FFE) and spin echo (SE) pulse sequences.

Fast gradient echo (GE) MR imaging during breathhold was performed in 73 abdominal MR examinations in order to determine the value of GE technique in the reduction of movement artefacts and improvement of image quality. The results were compared with conventional spin-echo (SE) technique. T1-weighted SE images consistently demonstrated normal anatomy and pathology. T2*-weighted and intermediate GE technique proved to be superior to T2-weighted SE technique in a considerable number of cases providing not only better delineation of normal structures but also better lesion detection. At this stage GE technique should be considered as an adjunct to T1-weighted SE technique. If the artefacts presently inherent to GE imaging could be reduced or compensated for, this technique of rapid MR imaging during suspended respiration could become an important tool being one of the fastest techniques for abdominal MR imaging.

Abdomen↗
PubMed1992

MR knee imaging: axial 3DFT GRASS pulse sequence versus spin-echo imaging for detecting meniscal tears.

The knees of 17 patients (34 menisci) referred for magnetic resonance (MR) imaging to evaluate knee pain were examined using thin axial three-dimensional Fourier transform (3DFT) gradient-refocused acquisition in a steady state (GRASS) images through the menisci, to determine if this method is sensitive and specific for detecting meniscal tears. Results were compared with spin-echo images with long TR and double-echo TE in both coronal and sagittal planes. Arthroscopy results, available in each case, were used as the "gold standard." Twelve meniscal tears were identified at arthroscopy. Axial 3DFT GRASS technique detected 10 of the 12 meniscal tears compared to 9 or 12 using spin-echo technique. With axial 3DFT GRASS technique one false-positive meniscal tear was reported, compared with two false-positive tears on spin-echo images. Axial 3DFT GRASS images were very useful in detecting peripheral tears, showing displaced meniscal fragments, and evaluating complex tears. In this small study, thin axial 3DFT GRASS images were comparable to spin-echo images for detecting meniscal tears, and were helpful in complicated cases in which they provided complementary information to that obtained from spin-echo images.

Adolescent↗
PubMed1994

Pulse sequence design for MR velocity mapping of complex flow: notes on the necessity of low echo times.

Lowering of the echo time (TE) has been proposed as a way to reduce effects of phase dispersion in MR velocity mapping, because a low TE reduces sensitivity to higher-order motion terms while first-order velocity sensitivity is maintained. Methods of lowering TE involves the use of extreme gradient ramp times and gradient strengths as well as reduction of the duration of transmit/receive windows, the latter method causing decrements in image resolution. When reducing higher-order sensitivity, however, it is not the overall TE that is the critical parameter, but rather the time pattern of the gradients used in the experiment. Hence, changes in TE without subsequent variations in gradient pattern would, according to theory, not affect quantitative measurements of complex flow and vice versa. In this study, we experimentally demonstrate this relation and utilize the experience to create a sequence robust towards complex flow without sacrifices in image resolution. Our experimental observations show that variations in TE alone while maintaining the time course of the velocity-encoding gradient does not significantly affect measurements of through-plane average complex flow in the studied velocity range. A parameter that cannot be measured as accurately if TE is increased is the peak flow. A phase mapping sequence with prolonged TE from 3 ms to 5 ms but with short duration of the velocity-encoding (section-selective) gradient and improved in-plane resolution was demonstrated in vivo.

Aorta↗
PubMed1996

Disappearance of multiple sclerosis lesions with severely prolonged T1 on images obtained by a FLAIR pulse sequence.

Although the superiority of FLAIR over conventional T2-weighted spin-echo (SE) is still unclear, one potential advantage would be its use in quantitative lesion load measurements. We have observed a potential pitfall regarding the use of FLAIR in image quantification. We report a case where a part of the hyperintense lesions on T2 SE, which was hypointense on T1 SE, subsequently was found to be isointense to brain on FLAIR images. Therefore, quantification of lesion load by means of FLAIR might underestimate the "true" lesion load, as seen on T2 SE.

Brain↗
PubMed1994

Application of the one-dimensional TOCSY pulse sequence in 750 MHz 1H-NMR spectroscopy for assignment of endogenous metabolite resonances in biofluids.

The complex 1H-NMR spectrum arising from an intact biofluid has been simplified using a one-dimensional homonuclear polarization transfer experiment (known as TOCSY or HOHAHA). This approach establishes connectivity between sequentially coupled multiplets, and the method is illustrated by the confirmation of the chemical shifts and hence resonance assignment of a number of endogenous metabolites in the 750 MHz 1H-NMR spectrum of seminal fluid. This has allowed the detection and assignment of pyroglutamate and uracil in this fluid for the first time.

Computer Simulation↗
PubMed1996

High resolution 1H spectra of powdered solids observed by Hahn echo pulse sequence with magic-angle spinning.

The 1H magic-angle spinning (MAS) spectrum for a typical powdered solid is composed of a high resolution component and a broadline component. The high resolution component can be well isolated from the broadline component by the Hahn echo sequence with a long echo time. Compared with CRAMPS experiment, which measures the proton system as a whole, the high resolution echo-MAS method measures only a fraction of the solid, which is usually small at room temperature and quite different from the majority of the solid in both molecular motion and chemical environment. It is shown that for a sample of fumaric acid monoethyl ester, the chemical shifts of the high resolution component are apparently distinguishable from the isotropic chemical shifts of the broadline component in the CRAMPS spectrum as the temperature approaches the melting point. In addition, for a sample of malonic acid, the echo-MAS spectrum is sensitive to moisture and temperature, while its corresponding CRAMPS spectrum is not. It is suggested that the molecules which produce the high resolution component are related to the lattice defects in a solid, including the surface disorder of the polycrystallites, while the molecules that generate the broadline component are located on the rigid lattice of the solid.

Chemical Phenomena↗
PubMed2005

Comparison of low-mechanical index pulse sequence schemes for detecting myocardial perfusion abnormalities during vasodilator stress echocardiography.

Myocardial contrast echocardiography has the potential to accurately detect functionally significant coronary artery disease during pharmacologic stress testing. Different low-mechanical index modalities, including triggering replenishment imaging (TRI) and real-time imaging (RTI), are currently used to identify myocardial perfusion defects. We compared the ability of TRI with that of RTI for detecting and localizing perfusion abnormalities. Thirty-six patients (62 +/- 14 years old, 15 men) underwent single-photon emission computed tomography (SPECT) with technetium-99m sestamibi and myocardial contrast echocardiography at baseline and after infusion of 0.56 mg/kg of dipyridamole. Sixteen of these patients also underwent quantitative angiography. Contrast-enhanced images were obtained in 4-, 3-, and 2-chamber views after intravenous bolus injections of lipid-encapsulated microbubbles (0.1 ml of Definity). A myocardial perfusion defect was defined by myocardial contrast echocardiography as a delay of >2 seconds in contrast replenishment after high-mechanical index flash impulse. The myocardial segments were divided into 3 major coronary territories. There was agreement in detecting perfusion defects between SPECT and TRI in 26 patients (72%, kappa = 0.46) and between SPECT and RTI in 27 patients (75%, kappa = 0.50). Agreements between myocardial contrast echocardiography and SPECT for localizing coronary territories with perfusion defects were 81% for TRI (kappa = 0.43) and 85% for RTI (kappa = 0.61). Accuracy of RTI for detecting >50% diameter stenoses by quantitative angiography was 79%, that of TRI was 71%, and that of SPECT was 65%. These data indicate that the different low-mechanical index imaging schemes are equivalent to radionuclide SPECT in accurately detecting diseased coronary artery territories during vasodilator stress.

Coronary Angiography↗
PubMed2006

Spatially encoded pulse sequences for the acquisition of high resolution NMR spectra in inhomogeneous fields.

We have recently proposed a protocol for retrieving nuclear magnetic resonance (NMR) spectra based on a spatially-dependent encoding of the MR interactions. It has also been shown that the spatial selectivity with which spins are manipulated during such encoding opens up new avenues towards the removal of magnetic field inhomogeneities; not by demanding extreme Bo field uniformities, but rather by compensating for the dephasing effects introduced by the field distribution at a radiofrequency excitation and/or refocusing level. The present study discusses in further detail a number of strategies deriving from this principle, geared at acquiring both uni- as well as multi-dimensional spectroscopic data at high resolution conditions. Different variants are presented, tailored according to the relative sensitivity and chemical nature of the spin system being explored. In particular a simple multi-scan experiment is discussed capable of affording substantial improvements in the spectral resolution, at nearly no sensitivity or scaling penalties. This new compensation scheme is therefore well-suited for the collection of high-resolution data in low-field systems possessing limited signal-to-noise ratios, where magnetic field heterogeneities might present a serious obstacle. Potential areas of applications of these techniques include high-field in vivo NMR studies in regions near tissue/air interfaces, clinical low field MR spectroscopy on relatively large off-center volumes difficult to shim, and ex situ NMR. The principles of the different compensation methods are reviewed and experimentally demonstrated for one-dimensional inhomogeneities; further improvements and extensions are briefly discussed.

Butanes↗
PubMed2005

Phantom materials for single point imaging pulse sequences.

The popularity of pure phase encode MRI techniques, including single point imaging (SPI), is steadily increasing, particularly in instances where the samples of interest are solid-like, or for other reasons possess short effective transverse relaxation times, T2*. As the interest in these techniques grows, so too does the need for a phantom material which is representative of this class of samples. The characteristics of such a phantom should include chemical and physical stability, straightforward preparation, high signal to noise ratio and relaxation times which are both easily manipulated and representative. To this end, we have developed a gelatin/sucrose-based gel which addresses the above criteria and behaves as a very flexible short T2* phantom. An order of magnitude variation in T1 and T2 can be achieved over a reasonable range of sucrose concentration. Even larger changes can be achieved with the addition of further doping agents.

Journal Article↗