Kupffer cell function of hepatocellular adenoma: pulse sequence effects in superparamagnetic iron oxide-enhanced magnetic resonance imaging.
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Adenoma, Liver Cell↗
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Fast inversion recovery for myelin suppression (FIRMS). A new magnetic resonance pulse sequence.
Fast inversion recovery for myelin suppression is a new magnetic resonance sequence with the ability to increase gray-white matter contrast. This can improve the definition of normal anatomical structures.
Brain↗
An improved nuclear magnetic resonance diffusion coefficient imaging method using an optimized pulse sequence.
Two-dimensional diffusion coefficient maps (images) of a carefully controlled diffusion phantom have been measured by a new diffusion imaging sequence using a 0.6-T whole-body nuclear magnetic resonance (NMR) scanner having a gradient field strength of 2.5 mT/m. The free induction decay (FID) data for the diffusion coefficient images were collected by varying the duration of the readout gradient in the conventional two-dimensional Fourier imaging sequence. The experimental results obtained by the proposed NMR diffusion measurement technique indicate a close agreement with other previous measurements. The selection of optimum spin-echo time for maximum signal-to-noise ratio (SNR) in diffusion imaging is studied and also experimentally confirmed. Finally, a preclinical study with human volunteers has been performed and results are presented.
Humans↗
A numerical investigation of the dependence of NMR signal from pulsatile blood flow in CINE pulse sequences.
The Bloch equations have been solved using numerical techniques for a uniform fluid undergoing periodic pulsatile flow in an NMR imaging experiment. The magnetization and NMR signal have been calculated for experimental parameters appropriate for a CINE sequence (TR = 40 ms, (TE = 14 ms) applied to the study of pulsatile aortic or other arterial flows. The flow velocity profile is obtained by Fourier superposition of different harmonics and it is shown that the steady-state NMR signal has reduced high-frequency components. There is also a time delay between peak signal intensity and flow because the backflow effects that can be as much as 100 ms. The apparent pulsatility depends on the NMR sequence parameters. Some limitations of the phase contrast flow-imaging method are also discussed for nonuniform flow.
Blood Flow Velocity↗
Correcting for rf inhomogeneities in multiecho pulse sequence MRI dosimetry.
The growing interest in using MRI to measure distributions of dose in specialized materials has prompted the need to improve existing methods for measuring NMR relaxation rates throughout large volume phantoms. The change in transverse relaxation rate (R2) has proven to be a more sensitive measure of dose than longitudinal relaxation rate (R1) in some materials; however, the accuracy of R2 measurements is limited by the presence of artifacts resulting from imperfect MRI tip angles throughout large phantoms. A novel echo quotient technique allowing accurate measurement of changes in R2 (delta R2) in regions of imperfect tip angle is presented. Application of this technique to the measurement of a dynamically wedged photon beam dose distribution demonstrates the improved accuracy of the echo quotient technique compared with other methods. Agreement of the MRI data with film and ion chamber data is within 2.5% throughout the volume of interest, indicating the potential for delta R2 MRI dosimetry to become a clinically useful tool.
Gelatin↗
Spin echo technique selection: basic principles for choosing MRI pulse sequence timing intervals.
Several "rules of thumb" are presented here to guide the MRI practitioner in spin echo technique selection and in the interpretation of pathologic changes.
Brain Neoplasms↗
Suppression of cerebrospinal fluid and blood flow artifacts in FLAIR MR imaging with a single-slab three-dimensional pulse sequence: initial experience.
The authors compared high-signal-intensity flow-related artifacts present with a conventional two-dimensional (2D) fluid-attenuated inversion recovery (FLAIR) sequence with those seen with a single-slab, three-dimensional (3D) FLAIR sequence. Four readers graded the subarachnoid space and intraventricular artifacts, the pulsation artifacts, and the conspicuity of cranial nerves in the posterior fossa. For all comparisons, differences between 2D and 3D images were highly statistically significant, with 3D imaging being superior in all cases.
Adult↗
Myocardial viability: rapid assessment with delayed contrast-enhanced MR imaging with three-dimensional inversion-recovery prepared pulse sequence.
Contrast-enhanced magnetic resonance (MR) imaging allows detection of nonviable myocardium. The authors compared a one-breath-hold three-dimensional inversion-recovery gradient-echo MR sequence with a multiple-breath-hold two-dimensional inversion-recovery gradient-echo MR sequence for the detection of nonviable myocardium. On the basis of a quantitative and qualitative approach, total myocardial area and contrast material-enhanced area, as well as the presence and spatial extent of hyperenhancement, were analyzed separately for each MR image obtained with each sequence in 10 patients with chronic ischemic heart disease. Findings for total myocardial area and contrast-enhanced area agreed well between the two sequences. A high level of agreement was also found for the presence of hyperenhancement (kappa = 0.84), while agreement was poor for the transmural extent of hyperenhancement (kappa = 0.32), which was attributed to the blurred appearance of the three-dimensional MR images. Findings with the one-breath-hold three-dimensional MR sequence allow assessment of nonviable myocardium with good agreement with those with the multiple-breath-hold two-dimensional MR sequence.
Adult↗
Latest pulse sequence for displacement-encoded MR imaging incorporates essential technical improvements for multiphase measurement of intramyocardial strain.
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Algorithms↗
Magnetic resonance imaging of intervertebral disk disease. Clinical and pulse sequence considerations.
Sixty-five patients were examined with magnetic resonance imaging (MR) to determine what combination of operator-selectable controls would result in a thorough examination of the intervertebral disks. There were 20 normal subjects, 8 with degenerative lumbar disk disease, 27 with both degeneration and herniation, 5 with stenosis of the spinal canal, and 5 with disk space infection. T2 was significantly longer in the normal nucleus pulposus than in the degenerated disk. Based on plots of in vivo signal intensity vs. repetition time (TR) for various echo times (TE), a sagittal 30-msec. TE and a 0.25-sec. TR were used for anatomical delineation and rapid localization, while sagittal and/or axial 120-msec. TE/3-sec. TR images were used to evaluate the cerebrospinal fluid and disk. Comparison with radiographs, high-resolution CT scans, and myelograms showed that MR was the most sensitive for identification of degeneration and disk space infection, separating the normal nucleus pulposus from the annulus and degenerated disk. Herniation, stenosis of the canal, and scarring can be identified as accurately with MR as with CT or myelography.
Adult↗
Short TI inversion-recovery imaging of the liver: pulse-sequence optimization and comparison with spin-echo imaging.
Magnitude-reconstructed short inversion-time (TI) inversion-recovery (IR) sequences have the advantage of reducing the signal of fat while providing additive T1 and T2 contrast. A double-echo short TI IR sequence was implemented to offer different degrees of T1- and T2-dependent image contrast. In 50 consecutive patients with proved liver tumors (30 metastases, 13 hemangiomas, seven other primary liver tumors), images obtained with a double-echo IR sequence at a repetition time (TR) of 1,500 msec, echo time (TE) of 30 and 60 msec, and TI of 80 msec (TR/TE/TI = 1,500/30, 60/80) were compared with those obtained with spin-echo (SE) sequences at a TR of 275 msec and a TE of 14 msec (TR/TE = 275/14) and 2,350/60, 120, 180. Metastases-liver contrast-to-noise ratios were highest at SE 275/14, followed by IR 1,500/30/80 and SE 2,350/180. IR 1,500/30/80 and SE 275/14 sequences consistently showed higher sensitivity for the detection of metastases than T2-weighted SE sequences. Differential diagnosis of benign and malignant lesions was more reliable with T2-weighted SE sequences than T2-weighted short TI IR sequences.
Adult↗
Intracranial circulation: pulse-sequence considerations in three-dimensional (volume) MR angiography.
The technique and feasibility of magnetic resonance (MR) angiography of intracranial vessels were studied in 35 healthy volunteers. Variations in image orientation, repetition time (TR), and flip angle were evaluated to determine their effects on flow-related enhancement. Gradient modifications--including echo time (TE), motion compensation, bandwidth, and field of view--were also studied in an effort to reduce motion-induced phase shifts. Results indicated that a FISP (fast imaging with steady precession) sequence with a TR of 50 msec, TE of 15 msec, velocity compensation in the read and section-select directions, acceleration compensation in the read direction, anisotropic volume, and a 1.25-mm partition thickness produced three-dimensional angiographic MR images that were accurate and reproducible in the depiction of the major intracranial vessels. Difficulties with field of view, persistent signal void secondary to higher-order motion, and spatial resolution remain major problems requiring additional study.
Adult↗
MR imaging of the knee: comparison of three-dimensional FISP and two-dimensional spin-echo pulse sequences.
Two-dimensional (2D) spin-echo (SE) sequences and three-dimensional (3D) FISP (fast imaging with steady precession) sequences of the knee with the same section thickness and field of view were directly compared in 54 patients, 17 of whom underwent subsequent follow-up (15 arthroscopic and two arthrographic examinations). In those patients with follow-up, each sequence demonstrated 100% sensitivity, 80% specificity, and 94% accuracy for evaluation of the medial meniscus and 100% sensitivity, 100% specificity, and 100% accuracy for evaluation of the lateral meniscus. SE imaging demonstrated 100% sensitivity, 100% specificity, and 100% accuracy for evaluation of the anterior cruciate ligament, while 100% sensitivity, 82% specificity, and 88% accuracy were achieved with FISP imaging. In the 37 patients without follow-up, only two discrepancies were found between the 2D SE and the 3D FISP images for meniscal evaluation. Three discrepancies were found in the evaluation of the anterior cruciate ligament, two of which were likely false-positive 3D FISP results. We conclude that 2D SE and 3D FISP imaging provide comparable data for meniscal evaluation. FISP images are slightly less accurate than SE images of comparable resolution for the evaluation of the anterior cruciate ligament.
Adolescent↗
Intracerebral lesion contrast with spin-echo and fast spin-echo pulse sequences.
Fast spin-echo (FSE) magnetic resonance (MR) imaging was compared with conventional, peripherally gated T2-weighted spin-echo (SE) imaging in the detection of high- and low-signal-intensity lesions in the central nervous system. Lesion detectability was determined with percentage of contrast measurements and contrast-to-noise ratios with two different measurements for noise. All three measures of lesion detectability were similar. FSE and SE sequences were quantitatively equivalent in the detection of high-signal-intensity lesions. The SE sequence, however, was superior to the FSE sequence in the detection of small, low-signal-intensity lesions in the central nervous system caused by magnetic susceptibility effects.
Adolescent↗
Hypervascular malignant liver lesions: comparison of various MR imaging pulse sequences and dynamic CT.
PURPOSE: To compare the appearance of hypervascular liver lesions on gadolinium-enhanced fast low-angle shot (FLASH) imaging with T2-weighted fat-suppressed spin-echo imaging, dynamic nonequilibrium-phase FLASH imaging, and dynamic nonequilibrium-phase iodine-enhanced computed tomography (CT) and to characterize the appearance of lesions on serial postgadolinium FLASH images. MATERIALS AND METHODS: Twenty-nine patients with hypervascular malignant liver lesions were examined with dynamic contrast-enhanced CT and magnetic resonance (MR) imaging within a 1-month interval. MR sequences included T2-weighted fat-suppression, precontrast FLASH, and postgadolinium FLASH at 1 second (sinusoid phase), 45 seconds (nonequilibrium phase), and 10 minutes. RESULTS: More than five lesions were detected in 12 patients with CT, 15 patients with T2-weighted fat-suppression imaging, 16 with sinusoid-phase FLASH imaging, and 11 with nonequilibrium-phase FLASH imaging. In six patients, a statistically significant (P = .03) increase in the number of lesions detected, by category, was observed on sinusoid-phase FLASH images compared with CT images. CONCLUSION: Sinusoid-phase FLASH imaging is superior to nonequilibrium-phase imaging with MR or CT for the demonstration of hypervascular malignant lesions.
Adolescent↗
Focal hepatic lesions: the continuing search for the optimal MR imaging pulse sequence(s).
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Artifacts↗
Initial clinical experience in MR imaging of the brain with a fast fluid-attenuated inversion-recovery pulse sequence.
PURPOSE: To evaluate fast fluid-attenuated inversion-recovery (FLAIR) technique for imaging brain abnormalities. MATERIALS AND METHODS: A fast FLAIR sequence was developed that provided 36 5-mm contiguous sections in 5 minutes 8 seconds. Resulting images were compared with dual-echo T2-weighted spin-echo images of 41 consecutive patients with brain abnormalities. RESULTS: Contrast and contrast-to-noise ratios (C/Ns) (for contrast between the lesion and background and between the lesion and cerebrospinal fluid) for fast FLAIR exceeded the corresponding values for T2-weighted spin-echo images for all but the second-echo lesion-to-background C/N. Fast FLAIR provided equivalent or greater overall lesion conspicuity and enabled greater lesion detection in 98% and 100%, respectively, of the evaluations. Fast FLAIR images more often had image artifact, but this did not interfere with image interpretation in a significantly (P < or = .05) greater number of evaluations. CONCLUSION: Fast FLAIR provides images that are superior to proton-density- and T2-weighted images for many image quality criteria.
Artifacts↗
Comparison of breath-hold fast spin-echo and conventional spin-echo pulse sequences for T2-weighted MR imaging of liver lesions.
PURPOSE: To evaluate a breath-hold fast spin-echo (SE) technique for T2-weighted magnetic resonance (MR) imaging of liver lesions. MATERIALS AND METHODS: A fast SE technique was developed that enabled six sections to be imaged per 16-second breath hold with a single echo. Resulting images were compared with those obtained with the first echo of a conventional dual-echo T2-weighted SE sequence (16 minutes 55 seconds for 18 sections). Thirty-one patients with malignant focal hepatic lesions were studied prospectively. The images were compared quantitatively and qualitatively. RESULTS: Quantitatively, the contrast and contrast-to-noise ratios for the fast SE images were 20% +/- 5 and 19% +/- 8 greater, respectively, than those for the conventional T2-weighted SE images of the 54 representative lesions. Qualitatively, fast SE images had less image artifact, enabled comparable or better lesion sizing, and greatly improved depiction of extrahepatic structures compared with conventional T2-weighted SE images. CONCLUSION: The fast SE technique with breath holding provides diagnostically useful liver images in a greatly decreased acquisition time.
Adult↗