Physical MR desktop data

T2 relaxometry with indirect echo compensation from highly undersampled data

PURPOSE

To develop an algorithm for fast and accurate T2 estimation from highly undersampled multi-echo spin-echo data.

METHODS

The algorithm combines a model-based reconstruction with a signal decay based on the slice-resolved extended phase graph (SEPG) model with the goal of reconstructing T2 maps from highly undersampled radial multi-echo spin-echo data with indirect echo compensation. To avoid problems associated with the nonlinearity of the SEPG model, principal component decomposition is used to linearize the signal model. The proposed CUrve Reconstruction via principal component-based Linearization with Indirect Echo compensation (CURLIE) algorithm is used to estimate T2 curves from highly undersampled data. T2 maps are obtained by fitting the curves to the SEPG model.

RESULTS

Results on phantoms showed T2 biases (1.9% to 18.4%) when indirect echoes are not taken into account. The T2 biases were reduced (< 3.2%) when the CURLIE reconstruction was performed along with SEPG fitting even for high degrees of undersampling (4% sampled). Experiments in vivo for brain, liver, and heart followed the same trend as the phantoms.

CONCLUSION

The CURLIE reconstruction combined with SEPG fitting enables accurate T2 estimation from highly undersampled multi-echo spin-echo radial data thus, yielding a fast T2 mapping method without errors caused by indirect echoes.

A nonlocal maximum likelihood estimation method for Rician noise reduction in MR images

Postacquisition denoising of magnetic resonance (MR) images is of importance for clinical diagnosis and computerized analysis, such as tissue classification and segmentation. It has been shown that the noise in MR magnitude images follows a Rician distribution, which is signal-dependent when signal-to-noise ratio (SNR) is low. It is particularly difficult to remove the random fluctuations and bias introduced by Rician noise. The objective of this paper is to estimate the noise free signal from MR magnitude images. We model images as random fields and assume that pixels which have similar neighborhoods come from the same distribution. We propose a nonlocal maximum likelihood (NLML) estimation method for Rician noise reduction. Our method yields an optimal estimation result that is more accurate in recovering the true signal from Rician noise than NL means algorithm in the sense of SNR, contrast, and method error. We demonstrate that NLML performs better than the conventional local maximum likelihood (LML) estimation method in preserving and defining sharp tissue boundaries in terms of a well-defined sharpness metric while also having superior performance in method error.

Complementary roles of sonography and magnetic resonance imaging in the assessment of fetal urinary tract anomalies

OBJECTIVE

The purpose of our study was to determine whether fetal magnetic resonance imaging (MRI) provides additional information that might affect the obstetric management of pregnancies complicated by sonographically diagnosed fetal urinary tract anomalies.

METHODS

Fetal MRI and sonography were used to study 39 women with suspected fetal urinary tract anomalies in the second and third trimesters of pregnancy.

RESULTS

In 24 of 39 cases (61%), fetal MRI confirmed the sonographic diagnosis. In 14 cases (36%), fetal MRI modified the initial sonographic diagnosis and counseling but did not change obstetric management. In 1 case (3%), the addition of fetal MRI resulted in a substantial change in the management of the pregnancy.

CONCLUSIONS

During the second and third trimesters of pregnancy, fetal MRI showed fetal urinary tract anomalies in excellent anatomic detail. Fetal MRI is a useful complementary tool in the assessment of sonographically diagnosed fetal urinary tract anomalies. In a small percentage of cases, it can have a substantial impact on obstetric management.

Reliable identification of the auditory thalamus using multi-modal structural analyses

The medial geniculate body (MGB) of the thalamus is a key component of the auditory system. It is involved in relaying and transforming auditory information to the cortex and in top-down modulation of processing in the midbrain, brainstem, and ear. Functional imaging investigations of this region in humans, however, have been limited by the difficulty of distinguishing MGB from other thalamic nuclei. Here, we introduce two methods for reliably delineating MGB anatomically in individuals based on conventional and diffusion MRI data. The first uses high-resolution proton density weighted scanning optimized for subcortical grey-white contrast. The second uses diffusion-weighted imaging and probabilistic tractography to automatically segment the medial and lateral geniculate nuclei from surrounding structures based on their distinctive patterns of connectivity to the rest of the brain. Both methods produce highly replicable results that are consistent with published atlases. Importantly, both methods rely on commonly available imaging sequences and standard hardware, a significant advantage over previously described approaches. In addition to providing useful approaches for identifying the MGB and LGN in vivo, our study offers further validation of diffusion tractography for the parcellation of grey matter regions on the basis of their connectivity patterns.

Liver and Spleen Volumetry with Quantitative MR Imaging and Dual-Space Clustering Segmentation

The purpose of this HIPAA-compliant, institutional review board-approved study was to assess the liver and spleen volumes calculated by using a semiautomated dual-space clustering segmentation technique, as compared with the volumes calculated by using the manual contour-tracing method. The quantitative magnetic resonance (MR) imaging data used as input were computed from images acquired by using a mixed fast spin-echo pulse sequence that was implemented with respiratory triggering. Linear regression analysis was used to assess agreement regarding the volumes calculated by using both segmentation techniques. There was strong agreement regarding the regression parameters for the liver (r = 0.98, P < .001) and the spleen (r = 0.99, P < .001) and the mean percentage volume differences for the liver (1.2%) and the spleen (0.9%). The mean segmentation time per patient was significantly shorter with use of the dual-space clustering method (P < .001).

Fast spectroscopic imaging strategies for potential applications in fMRI

Technical aspects of two general fast spectroscopic imaging (SI) strategies, one based on gradient echo trains and the other on spin echo trains, are reviewed within the context of potential applications in the field of functional magnetic resonance imaging (fMRI). Fast spectroscopic imaging of water may prove useful for identifying mechanisms underlying the blood oxygenation level dependence (BOLD) of the water signal during brain activation studies. Reasonably rapid mapping of changes in proton signals from brain metabolites, like lactate, creatine or even neurotransmitter associated metabolites like GABA, is substantially more challenging but technically feasible particularly as higher field strengths become available. Fast spectroscopic methods directed towards the 31P signals from phosphocreatine (PCr) and adenosine tri-phosphates (ATP) are also technically feasible and may prove useful for studying cerebral energetics within fMRI contexts.

T1 relaxation time at 0.2 Tesla for monitoring regional hyperthermia: feasibility study in muscle and adipose tissue

The purpose of this study was to characterize T(1), particularly in the hyperthermia temperature range (ca. 37-44 degrees C), in order to control regional hyperthermia with MR monitoring using 0.2 Tesla, and to improve T(1) mapping. A single-slice and a new multislice "T One by Multiple Read-Out Pulses" (TOMROP) pulse sequence were used for fast T(1) mapping in a clinical MRI hyperthermia hybrid system. Temporal stability, temperature sensitivity, and reversibility of T(1) were investigated in a polyamidacryl gel phantom and in samples of muscle and adipose tissues from turkey and pig, and verified in patients. In the gel phantom a high linear correlation between T(1) and temperature (R(2) = 0.97) was observed. In muscle and adipose tissue, T(1) and temperature had a linear relationship below a breakpoint of 43 degrees C. Above this breakpoint muscle tissue showed irreversible tissue changes; these effects were not visible in adipose tissue. The ex vivo results were confirmed in vivo under clinical conditions. T(1) mapping allows the characterization of hyperthermia-related tissue response in healthy tissue. T(1), in combination with fast mapping, is suitable for controlling regional hyperthermia at 0.2 T within the hybrid system.

Magnetic resonance imaging techniques

In this article we describe state-of-the art techniques for magnetic resonance imaging of the liver. T1-weighted, T2-weighted, and heavily T2-weighted pulse sequences are discussed. Gadolinium-enhanced hepatic parenchymal imaging and magnetic resonance angiography are also described. A comprehensive MR imaging examination of the liver affords evaluation of focal and diffuse hepatic parenchymal disease, biliary disease, and vascular pathology.

Vessel contrast at three Tesla in time-of-flight magnetic resonance angiography of the intracranial and carotid arteries

The effects of the increased field strength of 3T on blood vessel contrast in three-dimensional time-of-flight (TOF) MR angiography (MRA) of the intracranial and carotid arteries was evaluated. Bloch equation simulations based on measured longitudinal relaxation times suggested superior blood-to-background contrast might be expected at 3T over 1.5T when using typical 3D TOF MRA parameters. A 15-volunteer study found that 3T was preferable over 1.5T for visualising distal intracranial vessels and the carotid arteries, by providing superior background suppression and excellent fat suppression. The combination of improved background suppression and improved signal-to-noise at 3T, enabled high resolution intracranial 3D TOF MRA with voxel volumes as small as 0.14 mm(3) to be acquired.

Peripheral somatosensory fMRI in mouse at 11.7 T

The feasibility of performing extremely-high resolution somatosensory fMRI in anesthetized mice using BOLD contrast at 11.7 T was investigated. A somatosensory stimulus was applied to the hindlimb of an alpha-chlorolose anesthetized mouse resulting in robust (p < 4 x 10(-3)) BOLD changes in somatosensory cortex and large veins. Percentage modulation of the MR signal in cortex exceeded 7%. Experiments that artificially modulated the inspired oxygen tension were also conducted; the results revealed large, heterogeneous, BOLD contrast changes in the mouse brain. In addition, T(1), T(2), and T(2)* values in gray matter at 11.7 T were evaluated. Discussion of the sensitivity limitations of BOLD fMRI in the tiny mouse central nervous system is presented. These methods show promise for the assessment of neurological function in mouse models of CNS injury and disease.

Direct comparison of visual cortex activation in human and non-human primates using functional magnetic resonance imaging

We report a technique for functional magnetic resonance imaging (fMRI) in an awake, co-operative, rhesus macaque (Macaca mulatta) in a conventional 1.5T clinical MR scanner, thus accomplishing the first direct comparison of activation in visual cortex between humans and non-human primates with fMRI. Activation was seen in multiple areas of striate and extra-striate visual cortex and in areas for motion, object and face recognition in the monkey and in homologous visual areas in a human volunteer. This article describes T1, T2 and T2* values for macaque cortex, suitable MR imaging sequences, a training schedule, stimulus delivery apparatus and restraining hardware for monkey fMRI using a conventional 19 cm knee coil. Much of our understanding of the functional organization of the primate brain comes from physiological studies in monkeys. Direct comparison between species using fMRI such as those described here will help us to relate the wealth of existing knowledge on the functional organization of the non-human primate brain to human fMRI.

Signal formation in echo-shifted sequences

Echo shifted (ES) gradient-recalled echo sequences (whose TE > TR) have found important applications in fMRI and MR thermometry since their introduction. The technique increases T *(2) weighting in BOLD imaging and temperature change sensitivity in phase-based temperature imaging compared to FLASH sequences. Yet, inconsistent observations have previously been reported when variants of this technique were used in various MRI experiments. Previous understanding of the ES sequences (the "FLASH-like" postulation) was not sufficient to explain these observations. This work provides an in-depth study on the various ES sequences. It was found that there are two types of ES sequences: ES-FLASH that spoils coherent transverse magnetization and ES-GRE, which is based on SSFP signals. A signal expression was derived for the clinically popular TR-periodic ES-GRE sequence with one echo shift. This signal expression reduced to the "FLASH-like" postulation under certain conditions. The new knowledge about the echo formation mechanism in ES sequences helps explain the experimental observations previously reported. Moreover, the resonance offset angle based analysis demonstrates an elegant methodology to analyze short TR imaging sequences. Magn Reson Med 42:864-875, 1999.

MR cholangiopancreatography techniques

The tissue contrast principles and the technical aspects involved in the design of the imaging protocols currently used for clinical MR cholangiopancreatography are reviewed using a neutral terminology that is applicable to most of the high-field MRI equipment available from the major manufacturers. Furthermore, the technical discussions that follow are accompanied by a comprehensive set of tables listing the pulse sequence parameters used by the authors of the other articles in this issue. The tables are organized according to groups of parameters that determine the fundamental features of the protocols and of the generated images, specifically motion artifact reduction technique, scan geometry, image contrast, and recommended image post processing algorithm.

Multi-component apparent diffusion coefficients in human brain

The signal decay with increasing b-factor at fixed echo time from brain tissue in vivo has been measured using a line scan Stejskal-Tanner spin echo diffusion approach in eight healthy adult volunteers. The use of a 175 ms echo time and maximum gradient strengths of 10 mT/m allowed 64 b-factors to be sampled, ranging from 5 to 6000 s/ mm2, a maximum some three times larger than that typically used for diffusion imaging. The signal decay with b-factor over this extended range showed a decidedly non-exponential behavior well-suited to biexponential modeling. Statistical analyses of the fitted biexponential parameters from over 125 brain voxels (15 x 15 x 1 mm3 volume) per volunteer yielded a mean volume fraction of 0.74 which decayed with a typical apparent diffusion coefficient around 1.4 microm2/ms. The remaining fraction had an apparent diffusion coefficient of approximately 0.25 microm2/ms. Simple models which might explain the non-exponential behavior, such as intra- and extracellular water compartmentation with slow exchange, appear inadequate for a complete description. For typical diffusion imaging with b-factors below 2000 s/mm2, the standard model of monoexponential signal decay with b-factor, apparent diffusion coefficient values around 0.7 microm2/ms, and a sensitivity to diffusion gradient direction may appear appropriate. Over a more extended but readily accessible b-factor range, however, the complexity of brain signal decay with b-factor increases, offering a greater parametrization of the water diffusion process for tissue characterization.

A review of MRI pulse sequences and techniques in neuroimaging

BACKGROUND

The unmatched soft tissue contrast provided by magnetic resonance imaging (MRI) has made it the modality of choice for many neuroimaging examinations. The fact that signal intensity in MRI depends on many parameters, including spin-lattice and spin-spin relaxation times, proton density, and velocity, makes it possible to highlight various pathologies by appropriate choice of pulse sequences and pulse sequence parameters. It is somewhat overwhelming however, to filter through various pulse sequences and parameters in order to understand how their selection affects image contrast. This brief review is intended to highlight common pulse sequences and parameters as well as introduce new techniques currently being released for clinical use.

MATERIALS

Basic pulse sequences are described and the influence of the acquisition parameters on image contrast are illustrated. Such basic sequences include the ubiquitous spin echo, fast spin echo, and gradient echo sequences. Specialized techniques for fat suppression and magnetic resonance angiography are also presented. Currently approved contrast agents for use in MRI are briefly reviewed, and various advanced pulse sequences, such as those for diffusion and magnetization transfer contrast imaging, are briefly outlined.

RESULTS

The utility of basic and advanced pulse sequences are demonstrated by clinical examples and images of normal brain and spine. New sequences and techniques are briefly outlined with regard to their potential for improving neuroimaging examinations.

CONCLUSIONS

This brief review outlines how the choice of pulse sequence and acquisition parameters influences the resulting image contrast for a variety of basic and advanced imaging techniques.