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3D breath-hold contrast-enhanced MRA: a preliminary experience in aorta and iliac vascular disease.

PURPOSE: Our goal was to describe a 3D breath-hold (3D BH) contrast-enhanced MRA technique and apply the technique to patients with known or suspected aortic and iliac artery disease. METHOD: A fat-suppressed 3D GRE pulse sequence was designed with a total of 16 partition encodings. This took < 24 s for data acquisition in the abdomen and pelvis and was easily achieved during a single breath-hold. The technique was applied to 26 patients who presented with either known or suspected abdominal aortic or iliac vascular diseases. For comparison, in 19 patients a 2D TOF MRA pulse sequence with a traveling saturation band was used. Angiographic correlation was made in 18 studies. RESULTS: The 3D BH MRA was easily applicable in the evaluation of vascular anatomy and pathology. In three cases, it was superior to 2D TOF and conventional angiography for visualizing clot within the wall of an aneurysm in the abdominal aorta. In 20 cases, both MRA techniques overestimated the degree of stenosis in the lower peripheral vessels; however, this was more pronounced on 2D TOF. In five cases, the aneurysm wall was clearly defined by 3D BH MRA, whereas there was considerable signal loss in 2D TOF due to complex flow. With 3D BH MRA, the entire vessel territory both in abdominal aorta and in iliac vessels was visualized in all cases without signal falloff in the FOV. Breath-holding provided static images of the vessels that were free of blurring due to respiratory motion. CONCLUSION: Preliminary experience suggests that 3D BH with its distinct advantage of speed may serve as a useful screening tool for patients who cannot have conventional angiography or tolerate a lengthy MR examination of the abdominal aorta and iliac arteries.

Adult↗

MRI of the neonatal brain: optimization of spin-echo parameters.

OBJECTIVE: Our aim was to measure the relaxation times of the neonatal brain and to use these to derive pulse sequence parameters that enhance the signal-to-noise ratio (SNR) and contrast of MRI scans of the neonatal brain. SUBJECTS AND METHODS: The transverse (T2) and longitudinal (T1) relaxation times were measured for 10 healthy neonates, and the average relaxation times were calculated for both gray and white matter. Simulations using these values were then performed to estimate the optimal pulse sequence parameters. Images were obtained in three neonates using both the optimized and conventional sequence parameters. RESULTS: The measured (mean +/- SD) relaxation times of the neonatal brain at 1.5 T were T1 equals 1712 +/- 235 msec and T2 equals 394 +/- 52 msec in white matter and T1 equals 1144 +/- 245 msec and T2 equals 206 +/- 26 msec in gray matter. The optimized T1-weighted imaging used a turbo spin-echo sequence with an echo-train length of 3 and TR/TE of 850/11 msec and showed increases in both the contrast and the SNR. The optimized T2-weighted sequence used a TE of 270 msec and markedly increased the contrast but at the expense of a reduction in the SNR. CONCLUSION: Parameters of MRI turbo spin-echo sequences for scanning neonates are different from those required for adult studies, and appropriate protocols should be used.

Artifacts↗

Simulation of MRI cluster plots and application to neurological segmentation.

The advent of magnetic resonance imaging has provided new opportunities for volume measurement of tissues, with applications increasing dramatically in recent years. Cluster classification techniques have proved the most popular for volume measurement, yet little attention has been paid to how the choice of images for analysis affects the quality and ease of segmentation. To address this issue, we have developed a system to simulate MRI cluster plots using multicompartmental anthropomorphic software models of anatomy, and components for image contrast, signal-to-noise ratio, image nonuniformity, tissue heterogeneity, imager field strength, the partial volume effect, correlation between proton density, T1 and T2, and a variety of data preprocessing techniques. The effect of these components on tissue cluster size, shape, orientation, and separation is demonstrated. The simulation allows an informed choice of pulse sequence, acquisition parameters, and data preprocessing for cluster classification to be made as well as providing an aid to interpretation of acquired data cluster plots and a valuable educational tool. The system has been used to choose suitable images for neurological segmentation of grey matter, white matter, CSF, and multiple sclerosis lesions using spin-echo, inversion recovery, and gradient-echo pulse sequences. Constraints on image selection are discussed.

Brain↗

Constant time steady gradient NMR diffusometry using the secondary stimulated echo.

A pulse sequence producing a second stimulated echo is suggested for the compensation of relaxation and residual dipolar interaction effects in steady gradient spin echo diffusometry. Steady field gradients of considerable strength exist in the fringe field of NMR magnets, for instance. While the absolute echo time of the second stimulated echo is kept constant throughout the experiment, the interval between the first two radiofrequency pulses is augmented leading to a modulation of the amplitude of that second stimulated echo by self-diffusion only. The unique feature of this technique is that it is of a single-scan/single-echo-signal nature. That is, no reference signals neither of the same pulse sequence nor of separate experiments are needed. The new method was tested with poly(ethylene oxide) melts and proved to provide reliable data for (time dependent) self-diffusion coefficients down to the physical limit (D approximately 10(-15)m(2)/s) when flip-flop spin diffusion starts to become effective.

Journal Article↗

A systematic approach to magnetic resonance imaging interpretation of sports medicine injuries of the shoulder.

Magnetic resonance imaging has developed as a useful imaging modality in the evaluation of the athlete with shoulder pain. The multiplanar capabilities of magnetic resonance imaging make it ideal for detecting the anatomical variations of the osseous outlet that may contribute to the clinical syndrome of impingement. Its superb soft tissue contrast and spatial resolution allow for accurate differentiation between tendinopathy, partial-thickness tear, and full-thickness tear of the rotator cuff and also allow for detection of the subtle lesions of the capsule and labrum that are associated with shoulder instability. However, to accurately interpret the numerous images and pulse sequences obtained in a standard magnetic resonance examination of the shoulder, it is helpful to have a systematic approach to ensure that each of the pertinent anatomical structures are evaluated. This article will provide a systematic approach to the interpretation of a magnetic resonance examination of the shoulder. The normal imaging appearance of each anatomical structure will be described, and the most useful pulse sequences and imaging planes for the evaluation of each structure will be discussed. Finally, the signs of injury will be described and illustrated.

Acromion↗

Simultaneous assessment of flow and BOLD signals in resting-state functional connectivity maps.

We have recently demonstrated using functional magnetic resonance imaging the presence of synchronous low-frequency fluctuations of signal intensities from the resting human brain that have a high degree of temporal correlation (p < 0.0001) both within and across the sensorimotor cortex. A statistically significant overlap between the resting-state functional connectivity map and the task-activation map due to bilateral finger tapping was obtained. Similar results have been obtained in the auditory and visual cortex. Because the pulse sequence used for collecting data was sensitive to blood flow and blood oxygenation, these low-frequency fluctuations of signal intensity may have arisen from variations of both. The objective of this study was simultaneously to determine the contribution of the blood oxygenation level signal and the flow signal to physiological fluctuations in the resting brain using the flow-sensitive alternating inversion recovery pulse sequence. In all subjects, the functional connectivity maps obtained from BOLD had a greater coincidence with task-activation maps than the corresponding functional connectivity maps obtained from blood-flow signals at the same level of statistical significance. Results of this study suggest that while variations in blood flow might contribute to functional connectivity maps, BOLD signals play a dominant role in the mechanism that gives rise to functional connectivity in the resting human brain.

Adolescent↗

MR measurement and numerical simulation of steady flow in an end-to-side anastomosis model.

Magnetic resonance phase contrast velocity imaging ('MR PC velocimetry') is a powerful tool for measuring blood velocity. While it has been demonstrated that MR PC velocimetry can accurately measure velocity profiles in simple, unidirectional flows, errors can arise when measuring the more complex, multidirectional flows of clinical importance. In this work, we have compared MR PC velocity measurements with numerical simulations of steady flow in a physiologically relevant model of an end-to-side anastomosis, which produces many of the complex flow features expected in vivo. MR PC velocity profiles, measured using both 2D and 3D pulse sequences, showed very good agreement with numerically computed profiles, although at 2D PC data showed slightly larger errors than the 3D PC data in regions of high accelerations and oblique flow, likely due to displacement artifacts. We conclude that, under ideal conditions, MR phase contrast velocimetry can measure complex flow patterns to within 15% accuracy, but that care should be taken when using 2D pulse sequences to measure such complex flows.

Anastomosis, Surgical↗

Ferrite particles: a superparamagnetic MR contrast agent for enhanced detection of liver carcinoma.

The potential of superparamagnetic ferrite particles to enhance detection of liver carcinoma at magnetic resonance (MR) imaging was studied with in vitro MR spectroscopy and in vivo MR imaging in animal models. After intravenous administration of ferrite, MR spectroscopy showed selective shortening of T2 relaxation times in normal liver but not in tumor. MR imaging showed that ferrite produced profound signal loss only from normal liver and not tumor; thus, differences in signal intensity between liver and tumor were greatly enhanced, especially on pulse sequences with T2-dependent contrast. Motion artifacts were reduced as well because of less signal from liver. Microscopic analysis showed sequestration of ferrite particles in hepatic reticuloendothelial cells but not in hepatocytes or tumor tissue, and there was no evidence of cellular injury. Ferrite particles efficiently and predictably enhance signal differences between normal liver and tumor and permit considerable latitude in selection of pulse sequence and timing parameters. Thus, they have considerable promise as a tissue-specific MR contrast agent for improved detection of liver carcinoma.

Adenocarcinoma↗

Ultrashort echo time (UTE) MRI of the spine in thalassaemia.

Back pain is common in adult patients with homozygous thalassaemia, and degenerative disc disease is increasingly recognised as a cause. Ultrashort echo time (UTE) pulse sequences, which are sensitive to the presence of short T(2) relaxation components in tissue produced by iron deposition and other processes, were used to examine the lower thoracic and lumbar spine in symptomatic patients with beta-thalassaemia major or intermedia. Three patients were studied with fat suppressed as well as both fat suppressed and long T(2) suppressed UTE (TE=0.08 ms) pulse sequences. Conventional 2D Fourier transformation T(1) and T(2) weighted scans were also performed for comparison. Normal controls showed narrow high signal areas in the region of the end-plate and annulus fibrosus. Patients showed hyperintense bands adjacent to the vertebral end plate in lower thoracic and lumbar spine discs using a UTE sequence with both long T(2) component and fat suppression. The extent of the changes was most marked in the patient with the most severe degenerative change. In the patient with minimal disease, findings of this type were present in discs which did not show evidence of degeneration with conventional MR imaging. High signal changes of a type previously not described were observed in each patient. The effect may be due to organic iron entering the disc and decreasing its T(1) and T(2), but susceptibility effects from iron in the vertebral bodies, fibrosis and other causes also need to be considered.

Adult↗

A systematic approach to magnetic resonance imaging interpretation of sports medicine injuries of the knee.

Magnetic resonance imaging is performed more commonly on the knee than on any other joint, and it is an excellent diagnostic tool that can aid in the evaluation of a host of sports-related injuries involving the ligaments, tendons, menisci, osseous structures, and articular surfaces. A thorough evaluation of the images, however, can be a daunting task, as the study often contains dozens of images obtained with multiple pulse sequences and in several imaging planes. A systematic approach will facilitate an accurate and timely evaluation of this complex examination and will ensure that all of the clinically relevant structures are adequately assessed. This article will provide a systematic approach to the interpretation of a magnetic resonance examination of the knee. The normal imaging appearance of each anatomical structure will be described, and the optimal pulse sequence and imaging plane for the evaluation of each structure will be discussed. Finally, the signs of injury will be described and illustrated.

Anterior Cruciate Ligament Injuries↗

MRA of the thoracic vessels.

Magnetic resonance imaging (MRI) is well suited for the noninvasive evaluation of the thoracic vasculature, and with improvements in scanner technology, the ability of MR to illustrate the thoracic vessels has significantly improved. Dedicated vascular software and pulse sequences have become commercially available, and fast imaging, in particular, has facilitated the time-efficient and comprehensive MR evaluation of most thoracic vascular lesions. Over the years, a host of black and bright blood MRI methods have evolved into practical tools for illustration of the thoracic vessels. As with other MR applications, successful vascular depiction relies significantly on the proper selection and prescription of imaging pulse sequences. In this article, these methods with their specific technical and practical pitfalls for thoracic magnetic resonance angiography (MRA) will be discussed. Current clinical indications for thoracic MRA will also be illustrated.

Aortic Diseases↗

Labral injuries: accuracy of detection with unenhanced MR imaging of the shoulder.

PURPOSE: To prospectively assess the accuracy of unenhanced magnetic resonance (MR) imaging in the detection and localization of labral injuries. MATERIALS AND METHODS: One hundred three patients with clinically suspected shoulder injuries were prospectively examined with unenhanced MR imaging. A combination of gradient-echo and high-resolution fast-spin-echo axial pulse sequences were used. Surgical correlation was obtained in all patients. RESULTS: At surgery, 37 torn anterior, 36 torn superior, and 19 torn posterior labral were identified. The sensitivity for detection of these tears with MR imaging was 100%, 86%, and 74%, respectively; the specificity was 95%, 100%, and 95%, respectively. Overall, unenhanced MR imaging was 95% accurate in the detection of labral injuries. CONCLUSION: With appropriate pulse sequences, unenhanced MR imaging of the shoulder is an accurate technique for the detection and localization of labral injuries.

Adult↗

Use of an enhanced gradient system for diffusion MR imaging with motion-artifact reduction.

PURPOSE: A spin-echo diffusion-sensitized pulse sequence using high gradients (23 mT/m) is introduced. MATERIAL AND METHODS: In order to minimize motion artefacts, velocity-compensating gradients, ECG-triggering and post-processing with phase correction and raw data averaging using navigator echoes was performed. The in vitro ratio of diffusion coefficients for water and acetone was determined and the water self-diffusion coefficient at different temperatures was evaluated. The pulse sequence was tested in 7 healthy volunteers and in 2 tumour patients with astrocytomas of grades I-II and III-IV. Both single-slice and multi-slice techniques were used. RESULTS: The incorporation of phase correction clearly improved the quality of both diffusion-encoded images and the calculated diffusion maps. Mean values of the diffusion coefficients in vivo were for CSF 2.66 x 10(-9) m2/s and for white and grey matter 0.69 x 10(-9) m2/s and 0.87 x 10(-9) m2/s, respectively. CONCLUSION: Velocity-compensating gradients in combination with a high gradient strength were shown to be useful for in vivo diffusion MR imaging.

Adult↗

Relationship between magnetic field strength and magnetic-resonance-related acoustic noise levels.

The need for better signal-to-noise ratios and resolution has pushed magnetic resonance imaging (MRI) towards high-field MR-scanners for which only little data on MR-related acoustic noise production have been published. The purpose of this study was to validate the theoretical relationship of sound pressure level (SPL) and static magnetic field strength. This is relevant for allowing adequate comparisons of acoustic data of MR systems at various magnetic field strengths. Acoustic data were acquired during various pulse sequences at field strengths of 0.5, 1.0, 1.5 and 2.0 Tesla using the same MRI unit by means of a Helicon rampable magnet. Continuous-equivalent, i.e. time-averaged, linear SPLs and 1/3-octave band frequencies were recorded. Ramping from 0.5 to 1.0 Tesla and from 1.0 to 2.0 Tesla resulted in an SPL increase of 5.7 and 5.2 dB(L), respectively, when averaged over the various pulse sequences. Most of the acoustic energy was in the 1-kHz frequency band, irrespective of magnetic field strength. The relation between field strength and SPL was slightly non-linear, i.e. a slightly less increase at higher field strengths, presumably caused by the elastic properties of the gradient coil encasings.

Acoustics↗

Activation mapping as a percentage of local excitation: fMRI stability within scans, between scans and across field strengths.

Functional magnetic resonance imaging (fMRI) does not typically yield highly reproducible maps of brain activation. Maps can vary significantly even with constant scanning parameters and consistent task performance conditions (Liu et al., Magn. Reson. Med., 2004, 52:751-760). Reproducibility is even more of a problem when comparing fMRI signal magnitude and spatial extent of activation across scans involving different task performance levels, scan durations, pulse sequences or magnetic field strengths. In this report, the consistency of fMRI was reexamined by considering the relative spatial and temporal distribution of fMRI blood oxygen level dependent (BOLD) activation signals separately from the absolute magnitude of the activation signal in each brain area. Subjects repeatedly performed the same simple motor task but under a variety of imaging conditions, using both spiral and standard echo-planar pulse sequences and at 1.5- and 4.0-T magnetic field strengths. The results demonstrate that the absolute amplitude of BOLD statistical activation signals varied significantly across time and scanning conditions, but the relative spatial pattern of BOLD activation was highly reproducible across all conditions. Analysis of realistic simulated fMRI data sets indicates that stability of relative activation patterns could provide a useful tool for assessing the accuracy of fMRI maps.

Adult↗

Continuous measurement of cell volume changes in perfused rat salivary glands by proton NMR.

Changes in intracellular and extracellular water content have been measured in perfused rat salivary glands by repetitive application of an inversion recovery (IR) pulse sequence. The relaxation reagent Gd-DTPA (10 mM) was included in the perfusate so that the intracellular and extracellular water proton signals could be distinguished by their different longitudinal relaxation times. Changes in water content in response to altered perfusion pressure and perfusate osmolarity were determined at 30-s intervals and indicated a clear separation of the intracellular and extracellular components. Using a modification of the IR pulse sequence, changes in intracellular water content were also measured at 6-s intervals. With this time resolution, differences in the rates of cell shrinkage in response to hyperosmotic perfusates and the secretomotor agonist acetylcholine were observed. The results suggest that this approach offers a relatively noninvasive method for studying cell volume regulation in intact, perfused tissues and organs.

Acetylcholine↗

Simultaneous detection of multiple components of motion with MRI.

OBJECTIVE: Simultaneous detection of two or more components of motion using new magnetic resonance pulse sequences was investigated. MATERIALS AND METHODS: The technique employs Fourier phase encoding to encode the first component, and phase contrast detection to encode the second. Although the technique can be generalized to any number of spatial dimensions and motional orders, applications in which one or two spatial dimensions are obtained with a single Fourier velocity or acceleration dimension are most likely to be useful. For example, Fourier-encoded velocity and phase-contrasted acceleration information can be combined into the same image. RESULTS: Several variations of the pulse sequence were investigated in phantoms and human volunteers. The first variation acquired images having an appearance similar to that of Fourier velocity-encoded images in which signal displacement is proportional to velocity, but with pixel intensity determined by acceleration. In another variation two spatial dimensions were acquired with a third dimension that uses Fourier velocity encoding to measure axial velocity within a curved tube. Radial velocity components were determined simultaneously with a second velocity-encoding gradient pulse. CONCLUSION: The phantom and in vivo results presented here suggest that simultaneous detection of two or more components of motion is feasible.

Adult↗

Formulation of radiographically detectable gastrointestinal contrast agents for magnetic resonance imaging: effects of a barium sulfate additive on MR contrast agent effectiveness.

Complete and homogeneous distribution of gastrointestinal (GI) contrast media are important factors for their effective use in computed tomography as well as in magnetic resonance (MR) imaging. A radiographic method (using fluoroscopy or spot films) could be effective for monitoring intestinal filling with GI contrast agents for MR imaging (GICMR), but it would require the addition of a radiopaque agent to most GICMR. This study was conducted to determine the minimum amount of barium additive necessary to be radiographically visible and to evaluate whether this additive influences the signal characteristics of the GICMR. A variety of barium sulfate preparations (3-12% wt/vol) were tested in dogs to determine the minimum quantity needed to make the administered agent visible during fluoroscopy and on abdominal radiographs. Solutions of 10 different potential GI contrast agents (Gd-DTPA, ferric ammonium citrate, Mn-DPDP, chromium-EDTA, gadolinium-oxalate, ferrite particles, water, mineral oil, lipid emulsion, and methylcellulose) were prepared without ("nondoped") and with ("doped") the barium sulfate additive. MR images of the solutions in tubes were obtained at 0.38 T using 10 different spin-echo pulse sequences. Region of interest (ROI) measurements of contrast agent signal intensity (SI) were made. In addition, for the paramagnetic contrast media, the longitudinal and transverse relaxivity (R1 and R2) were measured. A 6% wt/vol suspension of barium was the smallest concentration yielding adequate radiopacity in the GI tract. Except for gadolinium-oxalate, there was no statistically significant difference in SI for doped and non-doped solutions with most pulse sequences used. In addition, the doped and nondoped solutions yielded R1 and R2 values which were comparable. We conclude that barium sulfate 6% wt/vol added to MR contrast agents produces a suspension with sufficient radiodensity to be viewed radiographically, and it does not cause significant alteration in the MR signal appearance of most GICMR. These formulations can be useful for achieving optimal filling of the gastrointestinal tract prior to MRI.

Animals↗