Shaihan J. Malik

Whole body fat: Content and distribution

Obesity and its co-morbidities, including type II diabetes, insulin resistance and cardiovascular diseases, have become one of the biggest health issues of present times. The impact of obesity goes well beyond the individual and is so far-reaching that, if it continues unabated, it will cause havoc with the economies of most countries. In order to be able to fully understand the relationship between increased adiposity (obesity) and its co-morbidity, it has been necessary to develop proper methodology to accurately and reproducibly determine both body fat content and distribution, including ectopic fat depots. Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) have recently emerged as the gold-standard for accomplishing this task. Here, we will review the use of different MRI techniques currently being used to determine body fat content and distribution. We also discuss the pros and cons of MRS to determine ectopic fat depots in liver, muscle, pancreas and heart and compare these to emerging MRI techniques currently being put forward to create ectopic fat maps. Finally, we will discuss how MRI/MRS techniques are helping in changing the perception of what is healthy and what is normal and desirable body-fat content and distribution.

Motion-Compensation Techniques in Neonatal and Fetal MR Imaging

SUMMARY: Fetal and neonatal MR imaging is increasingly used as a complementary diagnostic tool to sonography. MR imaging is an ideal technique for imaging fetuses and neonates because of the absence of ionizing radiation, the superior contrast of soft tissues compared with sonography, the availability of different contrast options, and the increased FOV. Motion in the normally mobile fetus and the unsettled, sleeping, or sedated neonate during a long acquisition will decrease image quality in the form of motion artifacts, hamper image interpretation, and often necessitate a repeat MR imaging to establish a diagnosis. This article reviews current techniques of motion compensation in fetal and neonatal MR imaging, including the following: 1) motion-prevention strategies (such as adequate patient preparation, patient coaching, and sedation, when required), 2) motion-artifacts minimization methods (such as fast imaging protocols, data undersampling, and motion-resistant sequences), and 3) motion-detection/correction schemes (such as navigators and self-navigated sequences, external motion-tracking devices, and postprocessing approaches) and their application in fetal and neonatal brain MR imaging. Additionally some background on the repertoire of motion of the fetal and neonatal patient and the resulting artifacts will be presented, as well as insights into future developments and emerging techniques of motion compensation.

Resting State fMRI in the moving fetus: A robust framework for motion, bias field and spin history correction

There is growing interest in exploring fetal functional brain development, particularly with Resting State fMRI. However, during a typical fMRI acquisition, the womb moves due to maternal respiration and the fetus may perform large-scale and unpredictable movements. Conventional fMRI processing pipelines, which assume that brain movements are infrequent or at least small, are not suitable. Previous published studies have tackled this problem by adopting conventional methods and discarding as much as 40% or more of the acquired data. In this work, we developed and tested a processing framework for fetal Resting State fMRI, capable of correcting gross motion. The method comprises bias field and spin history corrections in the scanner frame of reference, combined with slice to volume registration and scattered data interpolation to place all data into a consistent anatomical space. The aim is to recover an ordered set of samples suitable for further analysis using standard tools such as Group Independent Component Analysis (Group ICA). We have tested the approach using simulations and in vivo data acquired at 1.5 T. After full motion correction, Group ICA performed on a population of 8 fetuses extracted 20 networks, 6 of which were identified as matching those previously observed in preterm babies.

Tailored excitation in 3D with spiral nonselective (SPINS) RF pulses

Brain images acquired at 3T often display central brightening with spatially varying tissue contrast, caused by inhomogeneity in the transmit radiofrequency fields used for excitation. Tailored radiofrequency pulses can provide mitigation of radiofrequency field inhomogeneity, but previous designs have been unsuitable for 3D imaging in rapid pulse sequences. This article presents a nonselective pulse design based on a short (1 ms) 3D spiral k‐space trajectory that covers low spatial frequencies. The resulting excitations are optimized to produce a uniform excitation within a specified volume of interest covering the whole brain. B1 mapping and pulse calculation times were reduced by optimizing in only five slices within the brain. The method has been tested with both single and parallel transmission: in phantom experiments, normalized root‐mean‐square error in excitation was 0.022 for single and 0.020 for parallel transmission. The corresponding results in vivo were 0.066 and 0.055 respectively. A pilot brain imaging study using the proposed pulses for excitation within the Alzheimers disease neuroimaging initiative magnetization prepared rapid gradient echo (MP‐RAGE) protocol, yielded excellent image quality with improved signal to noise ratio in peripheral brain regions and enhanced uniformity of contrast compared with standard excitation. Greatest performance enhancement was achieved using parallel transmission, but single channel transmission offers significant improvement over standard excitation pulses. Magn Reson Med, 2012.

Parallel transmission for ultrahigh‐field imaging

The development of MRI systems operating at or above 7 T has provided researchers with a new window into the human body, yielding improved imaging speed, resolution and signal‐to‐noise ratio. In order to fully realise the potential of ultrahigh‐field MRI, a range of technical hurdles must be overcome. The non‐uniformity of the transmit field is one of such issues, as it leads to non‐uniform images with spatially varying contrast. Parallel transmission (i.e. the use of multiple independent transmission channels) provides previously unavailable degrees of freedom that allow full spatial and temporal control of the radiofrequency (RF) fields. This review discusses the many ways in which these degrees of freedom can be used, ranging from making more uniform transmit fields to the design of subject‐tailored RF pulses for both uniform excitation and spatial selection, and also the control of the specific absorption rate.

Impact of number of channels on RF shimming at 3T

ObjectAt high-field strengths (≥3T) inhomogeneity of the radio frequency (RF) field and RF power deposition become increasingly problematic. Parallel Transmission (PTx)—the use of segmented transmission arrays with independently driven elements—affords the ability to combat both of these issues. There are a variety of existing designs for PTx coils, ranging from systems with two channels to systems with eight or more. In this work, we have investigated the impact of the number of independent channels on the achievable results for both homogeneity improvement and power reduction in vivo.Materials and methodsA 3T Philips Achieva MRI system fitted with an 8-channel PTx body coil was driven so as to emulate configurations with 1, 2 4 and 8 independent channels. RF shimming was used in two different anatomies in order to assess improvements in RF homogeneity.ResultsSignificant homogeneity improvements were observed when increasing from 1 to 2, 2 to 4, and 4 to 8 channel configurations. Reductions in RF power requirements and local SAR were predicted for increasing numbers of channels.ConclusionIncreasing the number of RF transmit channels adds extra degrees of freedom which can be used to benefit homogeneity improvement or power reduction for body imaging at 3T.

Optimal linear combinations of array elements for B1 mapping

Accuracy of B1 mapping for array coils can be improved by mapping the fields produced by driving linear combinations of the array elements, chosen to produce a more uniform distribution of B1 amplitude. Quality of the resulting single element B1 maps is influenced by the transformation used both via the uniformity of the resulting linear combination fields and by the degree to which these linear combinations differ from one another. In this work we investigate the effect of using different transformations on the quality of B1 maps by simulating the B1 mapping process for two different techniques, using real data from a 3T 8‐channel body transmit system. Different transformations are generated using a single complex parameter. It is demonstrated that the optimal transformation within this framework is different for different imaging targets (pelvis and brain of healthy volunteers, and water and oil phantoms). For the same target (pelvis) the optimum condition, however, is similar for a number of subjects, suggesting that optimal configurations to be used for calibrating coils in specific anatomical contexts can be determined in advance. Potential gains may be translated into significant reductions in scan time for equivalent signal‐to‐noise ratio coil maps. Magn Reson Med, 2009.

x-f choice: Reconstruction of undersampled dynamic MRI by data-driven alias rejection applied to contrast-enhanced angiography

A technique for reconstructing dynamic undersampled MRI data, termed “x‐f choice,” was developed and applied to dynamic contrast‐enhanced MR angiography (DCE‐MRA). Regular undersampling in k‐t space (a hybrid of k‐space and time) creates aliasing in the conjugate x‐f space that must be resolved. When regions in the object containing fast dynamic change are sparse, as in DCE‐MRA, signal overlap caused by aliasing is often much less than the undersample factor would imply. x‐f Choice reconstruction identifies overlapping signals using a model of the full non‐aliased x‐f space that is automatically generated from the undersampled data, and applies parallel imaging (PI) to separate them. No extra reference scans are required to generate either the model or the coil sensitivity maps. At each location in the reconstructed images, g‐factor noise amplification is compared with predicted reconstruction errors to obtain an optimized solution. Acceleration factors greater than the number of receiver coils are possible, but are limited by the sparseness of the dynamic content and the signal‐to‐noise ratio (SNR) (in DCE‐MRA the latter is dominant). Temporal fidelity was validated for up to a factor 10 speed‐up using retrospectively undersampled data from a six‐coil array. The method was tested on volunteers using fivefold prospective undersampling. Magn Reson Med, 2006.

Spatially resolved extended phase graphs: Modeling and design of multipulse sequences with parallel transmission

A spatially resolved extended phase graph (SR‐EPG) framework is proposed for prediction of echo amplitudes in the presence of spatially variable radio frequency (RF) fields. The method may be used to examine any regularly repeating pulse sequence and provides a design framework for parallel transmission (PTx) systems; in this work signal homogeneity in static pseudo‐steady state (SPSS) turbo spin echo (TSE) imaging was investigated. Building on SR‐EPG calculations with PTx, a dynamic RF‐shimming approach is proposed in which, RF pulse amplitudes and phases are optimized on a per channel and per pulse basis to yield the desired signal response for all echoes. Results show significant improvements over “static” RF shimming (in which the relative amplitude/phase of the PTx channels are fixed for all pulses). SPSS‐TSE imaging using dynamic RF shimming resulted in excellent image quality, both in phantoms and in vivo, and confirmed SR‐EPG predictions. Magn Reson Med, 2012.

Slice profile correction for transmit sensitivity mapping using actual flip angle imaging

To enable clinical use of parallel transmission technology, it is necessary to rapidly produce transmit sensitivity (σ) maps. Actual flip angle imaging is an efficient mapping technique, which is accurate when used with 3D encoding and nonselective RF pulses. Mapping single slices is quicker, but 2D encoding leads to systematic errors due to slice profile effects. By simulating steady‐state slice profiles, we computed the relationship between σ and the signals received from the actual flip angle imaging sequence for arbitrarily chosen slice selective RF pulses. Pulse specific lookup tables were then used for reconstruction. The resulting σ‐maps are sensitive to T1 in a manner that depends strongly on the specific pulse, for example a precision of ±3% can be achieved by using a 3‐lobe sinc pulse. The method is applicable to any RF pulse; simulations must be performed once and thereafter fast reconstruction of σ‐maps is possible. Magn Reson Med, 2011.