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At least 325 records · Page 18Linked to original sources
PubMed1999

Fat-suppressed T2-weighted MR imaging of hepatocellular carcinoma and metastases: comparison of conventional spin-echo, fast spin-echo, and echoplanar pulse sequences.

The purpose of our study was to compare the diagnostic accuracy of fat-suppressed T2-weighted magnetic resonance (MR) images obtained with conventional spin-echo (SE), respiratory-triggered fast SE, and breath-hold multishot SE echoplanar (EP) sequences for the detection of hepatocellular carcinoma and metastases. Images obtained with the three sequences in 17 patients (15 with cirrhosis) with 31 hepatocellular carcinomas and 14 patients with 45 metastases were retrospectively analyzed. Image review was conducted on a segment-by-segment basis; in all, 248 liver segments were reviewed separately and independently for detection of solid, malignant lesions. Diagnostic accuracy was evaluated with receiver-operating-characteristic (ROC) curve analysis. Diagnostic accuracy for hepatocellular carcinoma as determined by ROC curve analysis was better by a statistically significant amount for conventional SE (Az = 0.95) images when compared with respiratory-triggered fast SE (Az = 0.83, P < 0.05) and breath-hold multishot SE EP (Az = 0.80, P < 0.05) images. Conventional SE MR sequences should not be replaced with respiratory-triggered fast SE or breath-hold multishot SE EP sequences for T2-weighted MR imaging of patients with hepatocellular carcinoma in cirrhosis, unless sufficient contrast-enhanced dynamic MR imaging is subsequently performed.

Adult↗
PubMed2001

Malignant hepatic tumor detection with ferumoxides-enhanced MR imaging with a 1.5-T system: comparison of four imaging pulse sequences.

The purpose of our study was to compare observer performance in the detection of malignant hepatic tumors with ferumoxides-enhanced magnetic resonance (MR) images obtained with proton density-weighted spin-echo (SE), T2-weighted fast SE, T2*-weighted gradient-recalled-echo (GRE), and proton density-weighted echo-planar (EP) sequences. Ferumoxides-enhanced MR images obtained with the four sequences in 50 patients with 92 solid malignant and 64 nonsolid benign lesions were retrospectively analyzed. Image review was conducted on a segment-by-segment basis; a total of 397 liver segments was reviewed separately for solid and nonsolid lesions by three independent readers. Observer performance was evaluated with receiver operating characteristic analysis. Lesion-to-liver contrast-to-noise ratio was higher with SE and EP than with GRE and fast SE images for solid lesions (P < 0.05), and higher with fast SE and SE than with GRE images for nonsolid lesions (P < 0.01). Proton density-weighted SE and T2-weighted fast-SE images were superior to T2*-weighted GRE and proton density-weighted EP images for detection of malignant hepatic tumors. T2-weighted fast SE images were the best for detection of nonsolid lesions. T2-weighted fast SE images that were comparable to proton density-weighted SE images for solid tumor detection, that were the best for nonsolid lesion detection, and that had an acquisition time of one third to half of that of SE imaging may be able to replace SE images for ferumoxides-enhanced liver imaging.

Aged↗
PubMedDate not supplied

Three-dimensional fast spin-echo imaging: pulse sequence and in vivo image evaluation.

Three-dimensional (3D) magnetic resonance imaging allows thin-section acquisition and therefore more accurate multiplanar reconstruction than conventional two-dimensional spin-echo imaging. Unfortunately, addition of a third acquisition plane extends imaging time greatly. With gradient-echo techniques, 3D acquisitions have become clinically useful. These techniques are limited, however, by susceptibility and other field inhomogeneity artifacts and decreased signal-to-noise ratios compared with spin-echo techniques. The authors describe implementation of a true spin-echo 3D technique that, by using fast spin-echo parameters, reduces acquisition time to a clinically useful level. Potential applications of the technique are demonstrated.

Brain↗
PubMedDate not supplied

MR cholangiopancreatography: comparison between two-dimensional fast spin-echo and three-dimensional gradient-echo pulse sequences.

The purpose of our study was to perform a prospective comparative analysis of three-dimensional (3D) steady-state free precession (SSFP) and two-dimensional (2D) fast spin-echo (FSE) imaging in the evaluation of 26 patients with suspected bile duct obstruction. SSFP and highly T2-weighted FSE sequences were obtained for each patient in multiple planes. Both sequences were reviewed independently and results were compared with findings from direct cholangiography (n = 17) or from a combination of sonography and CT (n = 9). The extrahepatic bile duct [EHBD] and intrahepatic bile duct [IHBD] were dilated in 32% and 54% of patients, respectively. The EHBDs were visualized in 44% of patients with SSFP, versus in 96% with FSE. One or more IHBD segments were seen in 42% of the SSFP sequences and in 100% of the FSE sequences. A portion of, or the entire, pancreatic duct was seen in 23% of the SSFP sequences and in 65% of the FSE sequences. Our findings lead us to conclude that T2-weighted FSE sequences are superior to SSFP sequences in visualizing the biliary tree and pancreatic duct and that they should replace gradient-echo sequences in MR cholangiopancreatography.

Artifacts↗
PubMedDate not supplied

T1-weighted MR imaging of the brain using a fast inversion recovery pulse sequence.

The purpose of this paper was to develop and evaluate a fast inversion recovery (FIR) technique for T1-weighted MR imaging of contrast-enhancing brain pathology. The FIR technique was developed, capable of imaging 24 sections in approximately 7 minutes using two echoes per repetition and an alternating echo phase encoding assignment. Resulting images were compared with conventional T1-weighted spin echo (T1SE) images in 18 consecutive patients. Compared with corresponding T1SE images, FIR images were quantitatively comparable or superior for lesion-to-background contrast and contrast-to-noise ratio (CNR). Gray-to-white matter and cerebrospinal fluid (CSF)-to-white matter contrast and CNR were statistically superior in FIR images. Qualitatively, the FIR technique provided comparable lesion detection, improved lesion conspicuity, and superior image contrast compared with T1SE images. Although FIR images had greater amounts of image artifacts, there was not a statistically increased amount of interpretation-interfering image artifact. FIR provides T1-weighted images that are superior to T1SE images for a number of image quality criteria.

Artifacts↗
PubMedDate not supplied

MR of focal liver lesions: comparison of breath-hold and non-breath-hold hybrid RARE and conventional spin-echo T2-weighted pulse sequences.

To compare liver lesion detection rates, tissue signal and noise data, and qualitative parameters for breath-hold (BH) and non-breath-hold (NBH) hybrid rapid acquisition with relaxation enhancement (RARE) and conventional spin-echo (CSE) T2-weighted (CSE-T2) MR sequences, 20 patients were imaged using all three sequences. Lesion detection rates were 73.5% for the CSE-T2 sequence and 81.1% and 88.6% for the BH-RARE and NBH-RARE sequences, respectively (P = .027). Mean lesion-to-liver signal-difference-to-noise ratio for the NBH-RARE sequence was 14.0 +/- 11.5, significantly greater than 9.8 +/- 7.8 obtained for the BH-RARE sequence (P = .050) and 9.0 +/- 6.2 obtained for the CSE-T2 sequence (P = .015). The NBH-RARE sequence demonstrated fewer artifacts and greater overall image quality compared to the CSE-T2 sequence. The NBH-RARE sequence is a useful alternative to the liver signal-difference-to-noise ratio and lesion detection rate and better overall image quality.

Adult↗
PubMed2004

Multislice first-pass myocardial perfusion imaging: Comparison of saturation recovery (SR)-TrueFISP-two-dimensional (2D) and SR-TurboFLASH-2D pulse sequences.

PURPOSE: To compare signal-to-noise ratio (SNR), contrast-to-noise (CNR) ratio, and diagnostic accuracy of a newly developed saturation recovery (SR)-TrueFISP-two-dimensional (2D) sequence with an SR-TurboFLASH-2D sequence. MATERIALS AND METHODS: In seven healthy subjects and nine patients with coronary artery disease (CAD), contrast-enhanced perfusion imaging (with Gd-DTPA) was performed with SR-TrueFISP and SR-TurboFLASH sequences. Hypoperfused areas were assessed qualitatively (scale = 0-4). Furthermore, SNR and CNR were calculated and semiquantitative perfusion parameters were determined from signal intensity (SI) time curves. Standard of reference for patient studies was single-photon emission computer tomography (SPECT) and angiography. RESULTS: The perception of perfusion deficits was superior in TrueFISP images (2.6 +/- 1.0) than in TurboFLASH (1.4 +/- 0.6) (P < 0.001). Phantom measurements yielded increased SNR (143 +/- 34%) and CNR (158 +/- 64%) values for TrueFISP. In patient/volunteer studies SNR was 61% to 100% higher and signal enhancement was 110% to 115% higher with TrueFISP than with TurboFLASH. Qualitative and semiquantitative assessment of perfusion defects yielded higher sensitivities for detection of perfusion defects with TrueFISP (68% to 78%) than with TurboFLASH (44% to 59%). CONCLUSION: SR-TrueFISP-2D perfusion imaging provides superior SNR and CNR than TurboFLASH imaging. Moreover, the dynamic range of SIs was found to be higher with TrueFISP, resulting in an increased sensitivity for detection of perfusion defects.

Adult↗
PubMed2005

Implications of pulse sequence in structural imaging of trabecular bone.

PURPOSE: To investigate the SNR and image properties of 3D steady-state free precession (SSFP), fast large-angle spin echo (FLASE), gradient-recalled acquisition in steady state (GRASS), and spoiled GRASS (SPGR) for structural imaging of trabecular bone (TB). MATERIALS AND METHODS: SNR was examined theoretically and experimentally on phantoms, bone specimens, and in vivo. The bone volume fraction, TB thickness, and echo time (TE) dependence of the thickness were compared. The trabecula was modeled as a cylinder in simulations to examine the intra-voxel spin-dephasing in SSFP and GRASS. Images were acquired on a 1.5 T Siemens Sonata system (40 mT/m maximum gradient, 200 T/m/s peak slew rate). RESULTS: Within the hardware and safety limit constraints, SNR of FLASE was superior, followed by SSFP, GRASS, and SPGR. The trabecular thickness derived from gradient-echo images was 10-45% greater than that obtained with FLASE. Conversely, SSFP images delineated partial volume trabeculae better than FLASE. Simulations indicated that the artifactual thickening was more severe in SSFP than in GRASS, which was attributed to off-resonance effects from the induced gradients at the bone/marrow interface. CONCLUSION: FLASE had the highest SNR and was insensitive to susceptibility dephasing. Although SSFP has superior SNR compared to GRASS, off-resonance effects and duty cycle limitations may compromise its practicality in this application. Inc.

Algorithms↗
PubMed2004

Improved pulse sequences for pure exchange solid-state NMR spectroscopy.

Spin-exchange experiments are useful for improving the resolution and establishment of sequential assignments in solid-state NMR spectra of uniformly (15)N-labeled proteins oriented macroscopically in phospholipid bilayers. To exploit this advantage fully, it is crucial that the diagonal peaks in the two-dimensional exchange spectra are suppressed. This may be accomplished using the recent pure-exchange (PUREX) experiments, which, however, suffer from up to a threefold reduction of the cross-peak intensity relative to experiments without diagonal-peak suppression. This loss in sensitivity may severely hamper the applicability for the study of membrane proteins. In this paper, we present a two-dimensional exchange experiment (iPUREX) which improves the PUREX sensitivity by 50%. The performance of iPUREX is demonstrated experimentally by proton-mediated (15)N-(15)N spin-exchange experiments for a (15)N-labeled N-acetyl-L-valyl-L-leucine dipeptide. The relevance of exchange experiments with diagonal-peak suppression for large, uniformly (15)N-labeled membrane proteins in oriented phospholipid bilayers is demonstrated numerically for the G-protein coupled receptor rhodopsin.

Kinetics↗
PubMed1988

A volume-localized, two-dimensional NMR method for the determination of lactate using zero-quantum coherence created in a stimulated echo pulse sequence.

A volume-localized, two-dimensional NMR method which unambiguously measures lactate protons in the presence of interfering lipid resonances is presented. This technique employs the stimulated echo method for spatial selection and was used to observe lactic acid levels in a subcutaneously implanted RIF-1 tumor at a concentration of 9.6 mM.

Animals↗
PubMed1990

Application of reverse-DEPT polarization transfer pulse sequence to study the metabolism of carbon-13-labeled substrates in perfused organs by 1H NMR spectroscopy.

Metabolism of 13C-enriched metabolites can be advantageously studied by reverse-polarization transfer methods. In this work an improved reverse-DEPT sequence has been applied for the first time on perfused organs in a 20-mm probe. The metabolic fate of 99% enriched [2-13C]acetate perfused in excised rat liver and heart has been documented.

Acetates↗
PubMed1992

Use of fluid-attenuated inversion-recovery pulse sequences for imaging the spinal cord.

Fourteen patients with disease of the spinal cord were imaged with fluid-attenuated inversion-recovery (FLAIR) sequences in which the inversion time was chosen to substantially reduce or null the signal from CSF. Lesions were seen with greater conspicuity than with conventional contrast-enhanced and -unenhanced T1- and T2-weighted sequences in 11 cases.

Adult↗
PubMed1996

Off-resonance image artifacts in interleaved-EPI and GRASE pulse sequences.

Echo-time shifting (ETS) is used in GRASE and interleaved-EPI sequences to improve the phase evolutions for off-resonance signal sources. However, even with ETS the phase evolutions still exhibit discontinuities. In this work, we extend previous studies of ETS by quantitatively evaluating the magnitude and form of the image artifacts that result from these phase discontinuities. The functional form of the phase evolution is used to derive the general conditions under which artifacts are expected. The artifacts for two sequence structures are then evaluated as a function of off-resonance frequency and data sampling period by calculating point spread functions and simulated images. It was found that even when ETS is used to improve the phase evolutions, periodic phase discontinuities may degrade image quality by producing ghosting artifacts of edges. These artifacts are similar to those that commonly occur with periodic motion. From our results recommendations are derived for limiting the ghosting artifacts.

Artifacts↗
PubMed1996

Multisection T1-weighted hybrid-RARE: a pulse sequence for MR imaging of the entire liver during suspended respiration.

It is shown that the maximum average-data-collection-speed (ADCS) of multisection 2D hybrid-RARE sequences is independent of TR and TEeff, and a monotonically increasing function of echo-train-length (ETL). This result was used in the design of an optimized T1-weighted hybrid-RARE sequence that produces 20 images of the abdomen in 31 s divided into four breath-hold periods. The resulting ADCS is 58 lines in k-space per second. Twenty-four subjects (2 healthy volunteers and 22 patients) were imaged with a protocol that also included: (a) breath-hold T1-weighted FLASH which acquires data at 34 lines in k-space per second (49 s scan time), and (b) T1-weighted conventional spin-echo (9:44 minutes scan time) with respiratory compensation. The experiments show that this T1-weighted-hybrid-RARE sequence has: (1) a level of T1 weighting that is comparable with the conventional sequences, (2) very low vulnerability to susceptibility artifacts, (3) high data acquisition efficiency, and (4) higher SNR than T1-weighted-FLASH. In conclusion, the T1-weighted-hybrid-RARE sequence described herein is an efficacious and reproducible technique for rapid imaging of the upper abdomen during suspended respiration.

Adult↗
PubMed1997

A multiple echo pulse sequence for diffusion tensor imaging and its application in excised rat spinal cords.

A new imaging sequence for rapid determination of the apparent self-diffusion tensor of water was developed and tested on fixed excised rat spinal cords. To reduce the time required to determine the tensor, the sequence utilized a new single-shot approach with multiple spin echoes. An assumption of cylindrical symmetry in the sample was made, thus requiring the measurement of only four of the six unique elements of the tensor. This assumption was found experimentally to be valid, and the results obtained using the new sequence were found to be quantitatively the same as results obtained using a standard spin-echo sequence.

Animals↗
PubMed2005

Self-navigated multishot echo-planar pulse sequence for high-resolution diffusion-weighted imaging.

Single-shot techniques have preferentially been adopted for diffusion-weighted imaging due to their reduced sensitivity to bulk motion. However, the limited spatial resolution achievable results in orientational signal averaging within voxels containing a distribution of fibers. This leads to impaired performance of tracking algorithms. To combat partial volume effects, high-resolution multishot techniques can be used but, being more sensitive to motion, require phase correction to obtain artifact-free images. While separately acquiring 2D navigator echoes is an effective approach, it is not very efficient as the navigators do not contribute signal to the final image. Here a self-navigated interleaved echo planar imaging (EPI) sequence based on EPI with keyhole (EPIK) is proposed. The refocusing reconstruction method is successfully adapted to EPIK and compared to the standard linear approach. The resultant improvement in resolution is shown to lead to a significant increase in anisotropy in fiber-branching areas and can potentially offer a superior ability to detect fine tract splits.

Algorithms↗