Receive Coil Array Research Articles

Capacitive approach to restore decoupling between channels for four‐element MR coil array

Multichannel coil arrays are increasingly being used to improve signal-to-noise ratio in magnetic resonance imaging (MRI). The decoupling between coils is an important parameter in array design. Indeed coupling between elements affects the resonance frequency of each single coil and decreases its sensitivity. Many solutions have been developed to achieve decoupling between elements of the multichannel coil array. Presented is a capacitive solution to restore channel decoupling of a specific four-channel receiver coil array using common conductors. The principle of an effective decoupling was first demonstrated by circuit simulations of |S|-parameters. A receive-only four-channel coil array was design for rat head MRI. Experimental |S|-parameter measurements validated the proposed capacitive approach by restoring decoupling between elements and particularly between external loops.

A 22‐channel receive array with Helmholtz transmit coil for anesthetized macaque MRI at 3 T

The macaque monkey is an important model for cognitive and sensory neuroscience that has been used extensively in behavioral, electrophysiological, molecular and, more recently, neuroimaging studies. However, macaque MRI has unique technical differences relative to human MRI, such as the geometry of highly parallel receive arrays, which must be addressed to optimize imaging performance. A 22-channel receive coil array was constructed specifically for rapid high-resolution anesthetized macaque monkey MRI at 3 T. A local Helmholtz transmit coil was used for excitation. Signal-to-noise ratios (SNRs) and noise amplification for parallel imaging were compared with those of single- and four-channel receive coils routinely used for macaque MRI. The 22-channel coil yielded significant improvements in SNR throughout the brain. Using this coil, the SNR in peripheral brain was 2.4 and 1.7 times greater than that obtained with single- or four-channel coils, respectively. In the central brain, the SNR gain was 1.5 times that of both the single- and four-channel coils. Finally, the performance of the array for functional, anatomical and diffusion-weighted imaging was evaluated. For all three modalities, the use of the 22-channel array allowed for high-resolution and accelerated image acquisition.

RF Surface Receive Array Coils: The Art of an LC Circuit

The radiofrequency (RF) receive array coil is a complicated device with many inductors and capacitors and serves as one of the most critical magnetic resonance imaging (MRI) electronic devices. It directly determines the achievable level of signal-to-noise ratio (SNR). Simply put, however, the RF coil is nothing but an LC circuit. The receive array coil was first proposed more than 20 years ago, evolving from a simple arrangement with a few electronic channels to a complicated system of 128 channels, enabling highly sophisticated parallel imaging, at different field strengths. This article summarizes the basic concepts pertaining to RF receive coil arrays and their associated SNR and reviews the theories behind the major components of such arrays. This includes discussions of the intrinsic SNR of a receive coil, the matching circuits, low-noise preamplifiers, coupling/decoupling amongst coils, the coupling between receive and transmit coils, decoupling via preamplifiers, and baluns. An 8-channel receive array coil on a cylindrical former serves as a useful example for demonstrating various points in the review.

Radiofrequency field enhancement with high dielectric constant (HDC) pads in a receive array coil at 3.0T.

To investigate the use of a new high-dielectric constant (HDC) material for improving SNR and transmission efficiency for clinical MRI applications at 3 Tesla (T) with cervical spine imaging. Human subjects were imaged using a commercial cervical spine receive array coil on a clinical system with and without pads containing Barium Titanate beads in deuterium water placed around the neck. Numerical electromagnetic field simulations of the same configuration were also performed. Experimental and simulated maps of transmit and receive fields showed greater efficiency for imaging the cervical spine when the pads were present. Experimental measurements showed a significant improvement in SNR with the pads present and an average input power reduction of 46%. Use of HDC material can enhance SNR and transmission efficiency for clinical imaging of the cervical spine at 3.0T.

Comparison of three multichannel TX/RX coils for anatomic and functional CMR at 7.0T

Background Ultrahigh field cardiac MR (CMR) is an area of vigorous ongoing research and is regarded as one of the most challenging MRI applications since image quality is not always exclusively defined by signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). The progress in the field has been driven by pioneering explorations into hardware developments which focused on novel multichannel transmit and receive coil arrays technology to tackle the challenges of B1-field inhomogeneities. Consequently, transmit-receive (TX/RX) structures are not a nicety but a necessity for ultrahigh field CMR including a trend towards a large number of transmit and receive channels. For all these reasons, this study was designed to compare the quality of anatomical and functional CMR at 7.0 T under clinical aspects using cardiac optimized transceive coils that use loop structures with the number of TX/RX elements ranging from 4-16. Methods

Ultra high spatial and temporal resolution breast imaging at 7T

There is a need to obtain higher specificity in the detection of breast lesions using MRI. To address this need, Dynamic Contrast-Enhanced (DCE) MRI has been combined with other structural and functional MRI techniques. Unfortunately, owing to time constraints structural images at ultra-high spatial resolution can generally not be obtained during contrast uptake, whereas the relatively low spatial resolution of functional imaging (e.g. diffusion and perfusion) limits the detection of small lesions. To be able to increase spatial as well as temporal resolution simultaneously, the sensitivity of MR detection needs to increase as well as the ability to effectively accelerate the acquisition. The required gain in signal-to-noise ratio (SNR) can be obtained at 7T, whereas acceleration can be obtained with high-density receiver coil arrays. In this case, morphological imaging can be merged with DCE-MRI, and other functional techniques can be obtained at higher spatial resolution, and with less distortion [e.g. Diffusion Weighted Imaging (DWI)]. To test the feasibility of this concept, we developed a unilateral breast coil for 7T. It comprises a volume optimized dual-channel transmit coil combined with a 30-channel receive array coil. The high density of small coil elements enabled efficient acceleration in any direction to acquire ultra high spatial resolution MRI of close to 0.6 mm isotropic detail within a temporal resolution of 69 s, high spatial resolution MRI of 1.5 mm isotropic within an ultra high temporal resolution of 6.7 s and low distortion DWI at 7T, all validated in phantoms, healthy volunteers and a patient with a lesion in the right breast classified as Breast Imaging Reporting and Data System (BI-RADS) IV.

Parallel MR imaging

Parallel imaging is a robust method for accelerating the acquisition of magnetic resonance imaging (MRI) data, and has made possible many new applications of MR imaging. Parallel imaging works by acquiring a reduced amount of k-space data with an array of receiver coils. These undersampled data can be acquired more quickly, but the undersampling leads to aliased images. One of several parallel imaging algorithms can then be used to reconstruct artifact-free images from either the aliased images (SENSE-type reconstruction) or from the undersampled data (GRAPPA-type reconstruction). The advantages of parallel imaging in a clinical setting include faster image acquisition, which can be used, for instance, to shorten breath-hold times resulting in fewer motion-corrupted examinations. In this article the basic concepts behind parallel imaging are introduced. The relationship between undersampling and aliasing is discussed and two commonly used parallel imaging methods, SENSE and GRAPPA, are explained in detail. Examples of artifacts arising from parallel imaging are shown and ways to detect and mitigate these artifacts are described. Finally, several current applications of parallel imaging are presented and recent advancements and promising research in parallel imaging are briefly reviewed.

MO-D-213CD-01: Cartesian Methods for Rapid Time-Resolved MR Angiography.

1. Understand recently developed physics techniques which have allowed a 20x improvement in the speed of data acquisition for MR angiography 2. Understand how Cartesian sampling of k-space facilitates the practical and effective implementation of these techniques 3. Show how contemporary implementation of these physics techniques has provided a significant improvement in MRA image quality over the last decade.

The SENSE-Isomorphism Theoretical Image Voxel Estimation (SENSE-ITIVE) model for reconstruction and observing statistical properties of reconstruction operators

The acquisition of sub-sampled data from an array of receiver coils has become a common means of reducing data acquisition time in MRI. Of the various techniques used in parallel MRI, SENSitivity Encoding (SENSE) is one of the most common, making use of a complex-valued weighted least squares estimation to unfold the aliased images. It was recently shown in Bruce et al. [Magn. Reson. Imag. 29(2011):1267-1287] that when the SENSE model is represented in terms of a real-valued isomorphism,it assumes a skew-symmetric covariance between receiver coils, as well as an identity covariance structure between voxels. In this manuscript, we show that not only is the skew-symmetric coil covariance unlike that of real data, but the estimated covariance structure between voxels over a time series of experimental data is not an identity matrix. As such, a new model, entitled SENSE-ITIVE, is described with both revised coil and voxel covariance structures. Both the SENSE and SENSE-ITIVE models are represented in terms of real-valued isomorphisms, allowing for a statistical analysis of reconstructed voxel means, variances, and correlations resulting from the use of different coil and voxel covariance structures used in the reconstruction processes to be conducted. It is shown through both theoretical and experimental illustrations that the miss-specification of the coil and voxel covariance structures in the SENSE model results in a lower standard deviation in each voxel of the reconstructed images, and thus an artificial increase in SNR, compared to the standard deviation and SNR of the SENSE-ITIVE model where both the coil and voxel covariances are appropriately accounted for. It is also shown that there are differences in the correlations induced by the reconstruction operations of both models, and consequently there are differences in the correlations estimated throughout the course of reconstructed time series. These differences in correlations could result in meaningful differences in interpretation of results.

An implanted 8-channel array coil for high-resolution macaque MRI at 3T.

An 8-channel receive coil array was constructed and implanted adjacent to the skull in a male rhesus monkey in order to improve the sensitivity of (functional) brain imaging. The permanent implant was part of an acrylic headpost assembly and only the coil element loop wires were implanted. The tuning, matching, and preamplifier circuitry was connected via a removable external assembly. Signal-to-noise ratio (SNR) and noise amplification for parallel imaging were compared to single-, 4-, and 8-channel external receive-only coils routinely used for macaque fMRI. In vivo measurements showed significantly improved SNR within the brain for the implanted versus the external coils. Within a region-of-interest covering the cerebral cortex, we observed a 5.4-, 3.6-fold, and 3.4-fold increase in SNR compared to the external single-, 4-, and 8-channel coils, respectively. In the center of the brain, the implanted array maintained a 2.4×, 2.5×, and 2.1× higher SNR, respectively compared to the external coils. The array performance was evaluated for anatomical, diffusion tensor and functional brain imaging. This study suggests that a stable implanted phased-array coil can be used in macaque MRI to substantially increase the spatial resolution for anatomical, diffusion tensor, and functional imaging.

Chang G, Wiggins GC, Xia D, et al. Comparison of a 28-channel receive array coil and quadrature volume coil for morphologic imaging and T2 mapping of knee cartilage at 7 T. J Magn Reson Imaging 2012;35:441-448

Chang G, Wiggins GC, Xia D, et al. Comparison of a 28-channel receive array coil and quadrature volume coil for morphologic imaging and T2 mapping of knee cartilage at 7 T. J Magn Reson Imaging 2012;35:441-448

Rat brain MRI at 16.4T using a capacitively tunable patch antenna in combination with a receive array

For MRI at 16.4T, with a proton Larmor frequency of 698 MHz, one of the principal RF engineering challenges is to generate a spatially homogeneous transmit field over a larger volume of interest for spin excitation. Constructing volume coils large enough to house a receive array along with the subject and to maintain the quadrature symmetry for different loading conditions is difficult at this frequency. This calls for new approaches to RF coil design for ultra-high field MR systems. A remotely placed capacitively tunable patch antenna, which can easily be adjusted to different loading conditions, was used to generate a relatively homogeneous excitation field covering a large imaging volume with a transversal profile similar to that of a birdcage coil. Since it was placed in front of the animal, this created valuable free space in the narrow magnet bore around the subject for additional hardware. To enhance the reception sensitivity, the patch antenna was combined with an actively detunable 3-channel receive coil array. In addition to increased SNR compared to a quadrature transceive surface coil, we were able to get high quality gradient echo and spin-echo images covering the whole rat brain.

Metabolic MR imaging of regional triglyceride and creatine content in the human heart

An optimized echo-planar spectroscopic imaging sequence is proposed to facilitate spatial mapping of triglyceride and total creatine content in the human heart. The sequence integrates local-look field of view reduction, cardiac and respiratory gating, and dedicated reconstruction steps to account for gradient channel delays, field inhomogeneity, and phase incoherence due to residual motion. The technique is demonstrated in 12 volunteers in comparison to single voxel point-resolved spectroscopy in the septal wall at 1.5 T. Triglyceride-to-water and total creatine-to-water ratios derived from echo-planar spectroscopic imaging (0.48 ± 0.18% and 0.06 ± 0.03%) and point-resolved spectroscopy (0.52 ± 0.17% and 0.07 ± 0.02%) were found to agree well. In the septal region, intraclass correlation coefficients ranging from 0.67 to 0.72 were estimated. A relatively weak agreement (intraclass correlation coefficients: 0.34 and 0.52) was found for sectors in the lateral wall due to field gradients induced by the posterior vein and limited sensitivity of the receive coil array in this area. On the basis of the findings, it is concluded that fast spectroscopic imaging of both cardiac triglyceride and total creatine content is feasible. Shimming and sensitivity challenges in the lateral region remain, however, to be addressed.

アレイコイルを使用した臨床画像の SNR測定におけるROI設定の影響

In many clinical imaging procedures using arrays of multiple receiver coils, a uniform sensitivity process is performed using the sensitivity distribution from the body coil. This causes the noise to be uneven, and background noise cannot be used when measuring the signal-to-noise ratio (SNR). The SNR of clinical images with sensitivity correction using arrays of multiple receiver coils sets the region of interest (ROI) in the region where the signal is uniform, and is limited to the identical ROI method where measurements are taken with noise from the identical region. When SNR is measured with the identical ROI method, uneven noise caused by sensitivity correction as well as the signal strength distribution within the ROI of the object is reflected in the noise. Therefore, evaluation must be performed in as localized a position as possible. Measurement error becomes small on images with higher resolution, and if ROI larger than 10×10 pixels can be set in a region of even signal, SNR measurement of clinical images with less underestimation may be possible.

Stretchable coil arrays: Application to knee imaging under varying flexion angles

To fit high-density receiver arrays for MRI closely around individual target anatomies, there is a need to provide a high degree of geometric adjustability with ease of handling and patient comfort. In this work, this is accomplished by the construction of a coil array that is stretchable such that it automatically conforms to a given anatomy's shape and size. Stretchability is implemented by creating the coil conductors from braided wire mounted on an elastic textile substrate. The signal-to-noise ratio yield of such coils is measured by MRI experiments at 3 T, and the signal-to-noise ratio effect of coil stretching is investigated with and without adjustment of the matching between each coil and the respective preamplifier. Four-channel and eight-channel arrays of stretchable receiver coils are evaluated in phantoms as well as for in vivo imaging of the human knee. Exploiting stretchability, it is demonstrated that the knee can be imaged under varying flexion angles up to 60° while maintaining closely coupled array detection, high signal-to-noise ratio, and uniform coverage of the entire joint.

Null space imaging: nonlinear magnetic encoding fields designed complementary to receiver coil sensitivities for improved acceleration in parallel imaging.

To increase image acquisition efficiency, we develop alternative gradient encoding strategies designed to provide spatial encoding complementary to the spatial encoding provided by the multiple receiver coil elements in parallel image acquisitions. Intuitively, complementary encoding is achieved when the magnetic field encoding gradients are designed to encode spatial information where receiver spatial encoding is ambiguous, for example, along sensitivity isocontours. Specifically, the method generates a basis set for the null space of the coil sensitivities with the singular value decomposition and calculates encoding fields from the null space vectors. A set of nonlinear gradients is used as projection imaging readout magnetic fields, replacing the conventional linear readout field and phase encoding. Multiple encoding fields are used as projections to capture the null space information, hence the term null space imaging. The method is compared to conventional Cartesian SENSitivity Encoding as evaluated by mean squared error and robustness to noise. Strategies for developments in the area of nonlinear encoding schemes are discussed. The null space imaging approach yields a parallel imaging method that provides high acceleration factors with a limited number of receiver coil array elements through increased time efficiency in spatial encoding.

Comparison of a 28‐channel receive array coil and quadrature volume coil for morphologic imaging and T2 mapping of knee cartilage at 7T

To compare a new birdcage-transmit, 28-channel receive array (28-Ch) coil and a quadrature volume coil for 7T morphologic MRI and T2 mapping of knee cartilage. The right knees of 10 healthy subjects were imaged on a 7T whole body magnetic resonance (MR) scanner using both coils. 3D fast low-angle shot (3D-FLASH) and multiecho spin-echo (MESE) sequences were implemented. Cartilage signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), thickness, and T2 values were assessed. SNR/CNR was 17%-400% greater for the 28-Ch compared to the quadrature coil (P ≤ 0.005). Bland-Altman plots show mean differences between measurements of tibial/femoral cartilage thickness and T2 values obtained with each coil to be small (-0.002 ± 0.009 cm / 0.003 ± 0.011 cm) and large (-6.8 ± 6.7 msec/-8.2 ± 9.7 msec), respectively. For the 28-Ch coil, when parallel imaging with acceleration factors (AF) 2, 3, and 4 was performed SNR retained was: 62%-69%, 51%-55%, and 39%-45%. A 28-Ch knee coil provides increased SNR/CNR for 7T cartilage morphologic imaging and T2 mapping. Coils should be switched with caution during clinical studies because T2 values may differ. The greater SNR of the 28-Ch coil could be used to perform parallel imaging with AF2 and obtain similar SNR as the quadrature coil.

A statistical examination of SENSE image reconstruction via an isomorphism representation

In magnetic resonance imaging, the parallel acquisition of subsampled spatial frequencies from an array of multiple receiver coils has become a common means of reducing data acquisition time. SENSitivity Encoding (SENSE) is a popular parallel image reconstruction model that uses a complex-valued least squares estimation process to unfold aliased images. In this article, the linear mathematical framework derived in Rowe et al. [ J Neurosci Meth 159 (2007) 361–369] is built upon to perform image reconstruction with subsampled data acquired from multiple receiver coils, where the SENSE model is represented as a real-valued isomorphism. A statistical analysis is performed of the various image reconstruction operators utilized in the SENSE model, with an emphasis placed on the effects of each operator on voxel means, variances and correlations. It is shown that, despite the attractiveness of models that unfold the aliased images from subsampled data, there is an artificial correlation induced between reconstructed voxels from the different folds of aliased images. As such, the mathematical framework outlined in this manuscript could be further developed to provide a means of accounting for this unavoidable correlation induced by image reconstruction operators.

High temporal and spatial resolution 3D time-resolved contrast-enhanced magnetic resonance angiography of the hands and feet

Methods are described for generating 3D time-resolved contrast-enhanced magnetic resonance (MR) angiograms of the hands and feet. Given targeted spatial resolution and frame times, it is shown that acceleration of about one order of magnitude or more is necessary. This is obtained by a combination of 2D sensitivity encoding (SENSE) and homodyne (HD) acceleration methods. Image update times from 3.4-6.8 seconds are provided in conjunction with view sharing. Modular receiver coil arrays are described which can be designed to the targeted vascular region. Images representative of the technique are generated in the vasculature of the hands and feet in volunteers and in patient studies.

A fast integral equation method for simulating high-field radio frequency coil arrays in magnetic resonance imaging

A fast full-wave numerical approach was developed for simulating high-field multi-channel radio-frequency (RF) receive coil arrays in magnetic resonance imaging. To improve the efficiency, the impedance matrix was compressed by a multilevel adaptive cross approximation method. Furthermore, careful organization of multiple coil simulations was applied so that the impedance matrix associated with biological subjects is constructed and pre-conditioned only once. Numerical examples demonstrate the efficacy of the proposed approach for RF coil simulations.