Hybrid ultra‐low‐field MRI and magnetoencephalography system based on a commercial whole‐head neuromagnetometer

Ultra‐low‐field MRI uses microtesla fields for signal encoding and sensitive superconducting quantum interference devices for signal detection. Similarly, modern magnetoencephalography (MEG) systems use arrays comprising hundreds of superconducting quantum interference device channels to measure the magnetic field generated by neuronal activity. In this article, hybrid MEG‐MRI instrumentation based on a commercial whole‐head MEG device is described. The combination of ultra‐low‐field MRI and MEG in a single device is expected to significantly reduce coregistration errors between the two modalities, to simplify MEG analysis, and to improve MEG localization accuracy. The sensor solutions, MRI coils (including a superconducting polarizing coil), an optimized pulse sequence, and a reconstruction method suitable for hybrid MEG‐MRI measurements are described. The performance of the device is demonstrated by presenting ultra‐low‐field‐MR images and MEG recordings that are compared with data obtained with a 3T scanner and a commercial MEG device. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Assessing liver function using dynamic Gd‐EOB‐DTPA‐enhanced MRI with a standard 5‐phase imaging protocol

Purpose:

To evaluate liver function obtained by tracer‐kinetic modeling of dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI) data acquired with a routine gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd‐EOB‐DTPA)‐enhanced protocol.

Materials and Methods:

Data were acquired from 25 cases of nonchronic liver disease and 94 cases of cirrhosis. DCE‐MRI was performed with a dose of 0.025 mmol/kg Gd‐EOB‐DTPA injected at 2 mL/sec. A 3D breath‐hold sequence acquired 5 volumes of 72 slices each: precontrast, double arterial phase, portal phase, and 4‐minute postcontrast. Regions of interest (ROIs) were selected semiautomatically in the aorta, portal vein, and whole liver on a middle slice. A constrained dual‐inlet two‐compartment uptake model was fitted to the ROI curves, producing three parameters: intracellular uptake rate (UR), extracellular volume (Ve), and arterial flow fraction (AFF).

Results:

Median UR dropped from 4.46 10^?2 min^?1 in the noncirrhosis to 3.20 in Child–Pugh A (P = 0.001), and again to 1.92 in Child–Pugh B (P < 0.0001). Median Ve dropped from 6.64 mL 100 mL^?1 in the noncirrhosis to 5.80 in Child–Pugh A (P = 0.01). Other combinations of Ve and AFF changes were not significant for any group.

Conclusion:

UR obtained from tracer kinetic analysis of a routine DCE‐MRI has the potential to become a novel index of liver function. J. Magn. Reson. Imaging 2013;37:1109–1114. © 2012 Wiley Periodicals, Inc.

     

Multiple‐bolus dynamic contrast‐enhanced MRI in the pancreas during a glucose challenge

Purpose:

To assess the feasibility of multiple‐bolus dynamic contrast‐enhanced (DCE) magnetic resonance imaging (MRI) in the pancreas; to optimize the analysis; and to investigate application of the method to a glucose challenge in type 2 diabetes.

Materials and Methods:

A 4‐bolus DCE‐MRI protocol was performed on five patients with type 2 diabetes and 11 healthy volunteers during free‐breathing. Motion during the dynamic time series was corrected for using a model‐driven nonlinear registration. A glucose challenge was administered intravenously between the first and second DCE‐MRI acquisition in all patients and in seven of the healthy controls.

Results:

Image registration improved the reproducibility of the DCE‐MRI model parameters across the repeated bolus‐acquisitions in the healthy controls with no glucose challenge (eg, coefficient of variation for K^trans improved from 38% to 28%). Native tissue T₁ was significantly lower in patients (374 ± 68 msec) compared with volunteers (519 ± 41 msec) but there was no significant difference in any of the baseline DCE‐MRI parameters. No effect of glucose challenge was observed in either the patients or healthy volunteers.

Conclusion:

Multiple bolus DCE‐MRI is feasible in the pancreas and is improved by nonlinear image registration but is not sensitive to the effects of an intravenous glucose challenge. J. Magn. Reson. Imaging 2010;32:622–628. © 2010 Wiley‐Liss, Inc.     

Noise amplification in parallel whole‐head ultra‐low‐field magnetic resonance imaging using 306 detectors

In ultra‐low‐field magnetic resonance imaging, arrays of up to hundreds of highly sensitive superconducting quantum interference devices (SQUIDs) can be used to detect the weak magnetic fields emitted by the precessing magnetization. Here, we investigate the noise amplification in sensitivity‐encoded ultra‐low‐field MRI at various acceleration rates using a SQUID array consisting of 102 magnetometers, 102 gradiometers, or 306 magnetometers and gradiometers, to cover the whole head. Our results suggest that SQUID arrays consisting of 102 magnetometers and 102 gradiometers are similar in g‐factor distribution. A SQUID array of 306 sensors (102 magnetometers and 204 gradiometers) only marginally improves the g‐factor. Corroborating with previous studies, the g‐factor in 2D sensitivity‐encoded ultra‐low‐field MRI with 9 to 16‐fold 2D accelerations using the SQUID array studied here may be acceptable. Magn Reson Med 70:595–600, 2013. © 2012 Wiley Periodicals, Inc.     

RT‐GROG: parallelized self‐calibrating GROG for real‐time MRI

A real‐time implementation of self‐calibrating Generalized Autocalibrating Partially Parallel Acquisitions (GRAPPA) operator gridding for radial acquisitions is presented. Self‐calibrating GRAPPA operator gridding is a parallel‐imaging‐based, parameter‐free gridding algorithm, where coil sensitivity profiles are used to calculate gridding weights. Self‐calibrating GRAPPA operator gridding's weight‐set calculation and image reconstruction steps are decoupled into two distinct processes, implemented in C++ and parallelized. This decoupling allows the weights to be updated adaptively in the background while image reconstruction threads use the most recent gridding weights to grid and reconstruct images. All possible combinations of two‐dimensional gridding weights GG are evaluated for m,n = {?0.5, ?0.4, …, 0, 0.1, …, 0.5} and stored in a look‐up table. Consequently, the per‐sample two‐dimensional weights calculation during gridding is eliminated from the reconstruction process and replaced by a simple look‐up table access. In practice, up to 34× faster reconstruction than conventional (parallelized) self‐calibrating GRAPPA operator gridding is achieved. On a 32‐coil dataset of size 128 × 64, reconstruction performance is 14.5 frames per second (fps), while the data acquisition is 6.6 fps. Magn Reson Med 64:306–312, 2010. © 2010 Wiley‐Liss, Inc.     

Feasibility of shutter‐speed DCE‐MRI for improved prostate cancer detection

The feasibility of shutter‐speed model dynamic‐contrast‐enhanced MRI pharmacokinetic analyses for prostate cancer detection was investigated in a prebiopsy patient cohort. Differences of results from the fast‐exchange‐regime‐allowed (FXR‐a) shutter‐speed model version and the fast‐exchange‐limit‐constrained (FXL‐c) standard model are demonstrated. Although the spatial information is more limited, postdynamic‐contrast‐enhanced MRI biopsy specimens were also examined. The MRI results were correlated with the biopsy pathology findings. Of all the model parameters, region‐of‐interest‐averaged K^trans difference [ΔK^trans ≡ K^trans(FXR‐a) ? K^trans(FXL‐c)] or two‐dimensional K^trans(FXR‐a) vs. k_ep(FXR‐a) values were found to provide the most useful biomarkers for malignant/benign prostate tissue discrimination (at 100% sensitivity for a population of 13, the specificity is 88%) and disease burden determination. (The best specificity for the fast‐exchange‐limit‐constrained analysis is 63%, with the two‐dimensional plot.) K^trans and k_ep are each measures of passive transcapillary contrast reagent transfer rate constants. Parameter value increases with shutter‐speed model (relative to standard model) analysis are larger in malignant foci than in normal‐appearing glandular tissue. Pathology analyses verify the shutter‐speed model (FXR‐a) promise for prostate cancer detection. Parametric mapping may further improve pharmacokinetic biomarker performance. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.