96‐Channel receive‐only head coil for 3 Tesla: Design optimization and evaluation

The benefits and challenges of highly parallel array coils for head imaging were investigated through the development of a 3T receive‐only phased‐array head coil with 96 receive elements constructed on a close‐fitting helmet‐shaped former. We evaluated several designs for the coil elements and matching circuitry, with particular attention to sources of signal‐to‐noise ratio (SNR) loss, including various sources of coil loading and coupling between the array elements. The SNR and noise amplification (g‐factor) in accelerated imaging were quantitatively evaluated in phantom and human imaging and compared to a 32‐channel array built on an identical helmet‐shaped former and to a larger commercial 12‐channel head coil. The 96‐channel coil provided substantial SNR gains in the distal cortex compared to the 12‐ and 32‐channel coils. The central SNR for the 96‐channel coil was similar to the 32‐channel coil for optimum SNR combination and 20% lower for root‐sum‐of‐squares combination. There was a significant reduction in the maximum g‐factor for 96 channels compared to 32; for example, the 96‐channel maximum g‐factor was 65% of the 32‐channel value for acceleration rate 4. The performance of the array is demonstrated in highly accelerated brain images. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Evaluation of multicoil breast arrays for parallel imaging

Purpose:

To evaluate three multicoil breast arrays for both conventional and SENSE‐accelerated imaging.

Materials and Methods:

Two commercially available 8‐element coils and a prototype 16‐element coil were compared. One 8‐element array had adjustable coils located next to the breast tissue and the other had a fixed coil arrangement; both were designed to allow parallel imaging in the left–right direction. The 16‐element coil was designed to have coil sensitivity variation in both the left–right and superior–inferior directions, and also had adjustable coils. Their performance was assessed in terms of signal‐to‐noise ratio (SNR), g‐factor, and uniformity with a custom‐built phantom.

Results:

The 16‐element array with adjustable coils provided the highest SNR, while the 8‐element coil with a fixed coil arrangement had the best uniformity. All coils performed well for SENSE acceleration in the left–right direction. The 8‐element coils did not have the capability for acceleration in the superior–inferior direction across the whole volume. The 16‐element coil enabled acceleration in the superior–inferior direction in addition to the left–right direction.

Conclusion:

Smaller, adjustable coil elements located next to breast tissue can provide greater SNR than larger, fixed coil elements. A multicoil breast array with high intrinsic SNR and low g‐factors enables high‐quality parallel imaging. J. Magn. Reson. Imaging 2010; 31: 328–338. © 2010 Wiley‐Liss, Inc.     

A 20‐channel receive‐only mouse array coil for a 3 T clinical MRI system

A 20‐channel phased‐array coil for MRI of mice has been designed, constructed, and validated with bench measurements and high‐resolution accelerated imaging. The technical challenges of designing a small, high density array have been overcome using individual small‐diameter coil elements arranged on a cylinder in a hexagonal overlapping design with adjacent low impedance preamplifiers to further decouple the array elements. Signal‐to‐noise ratio (SNR) and noise amplification in accelerated imaging were simulated and quantitatively evaluated in phantoms and in vivo mouse images. Comparison between the 20‐channel mouse array and a length‐matched quadrature driven small animal birdcage coil showed an SNR increase at the periphery and in the center of the phantom of 3‐ and 1.3‐fold, respectively. Comparison with a shorter but SNR‐optimized birdcage coil (aspect ratio 1:1 and only half mouse coverage) showed an SNR gain of twofold at the edge of the phantom and similar SNR in the center. G‐factor measurements indicate that the coil is well suited to acquire highly accelerated images. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

128-channel body MRI with a flexible high-density receiver-coil array

Purpose

To determine whether the promise of high‐density many‐coil MRI receiver arrays for enabling highly accelerated parallel imaging can be realized in practice.

Materials and Methods

A 128‐channel body receiver‐coil array and custom MRI system were developed. The array comprises two clamshells containing 64 coils each, with the posterior array built to maximize signal‐to‐noise ratio (SNR) and the anterior array design incorporating considerations of weight and flexibility as well. Phantom imaging and human body imaging were performed using a variety of reduction factors and 2D and 3D pulse sequences.

Results

The ratio of SNR relative to a 32‐element array of similar footprint was 1.03 in the center of an elliptical loading phantom and 1.7 on average in the outer regions. Maximum g‐factors dropped from 5.5 (for 32 channels) to 2.0 (for 128 channels) for 4 × 4 acceleration and from 25 to 3.3 for 5 × 5 acceleration. Residual aliasing artifacts for a right/left (R/L) reduction factor of 8 in human body imaging were significantly reduced relative to the 32‐channel array.

Conclusion

MRI with a large number of receiver channels enables significantly higher acceleration factors for parallel imaging and improved SNR, provided losses from the coils and electronics are kept negligible. J. Magn. Reson. Imaging 2008;28:1219–1225. © 2008 Wiley‐Liss, Inc.     

Custom‐fitted 16‐channel bilateral breast coil for bidirectional parallel imaging

A 16‐channel receive‐only, closely fitted array coil is described and tested in vivo for bilateral breast imaging at 3 T. The primary purpose of this coil is to provide high signal‐to‐noise ratio and parallel imaging acceleration in two directions for breast MRI. Circular coil elements (7.5‐cm diameter) were placed on a closed “cup‐shaped” platform, and nearest neighbor coils were decoupled through geometric overlap. Comparisons were made between the 16‐channel custom coil and a commercially available 8‐channel coil. SENSitivity Encoding (SENSE) parallel imaging noise amplification (g‐factor) was evaluated in phantom scans. In healthy volunteers, we compared signal‐to‐noise ratio, parallel imaging in one and two directions, Autocalibrating Reconstruction for Cartesian sampling (ARC) g‐factor, and high spatial resolution imaging. When compared with a commercially available 8‐channel coil, the 16‐channel custom coil shows 3.6× higher mean signal‐to‐noise ratio in the breast and higher quality accelerated images. In patients, the 16‐channel custom coil has facilitated high‐quality, high‐resolution images with bidirectional acceleration of R = 6.3. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Half‐fourier‐acquisition single‐shot turbo spin‐echo (HASTE) MRI of the lung at 3 Tesla using parallel imaging with 32‐receiver channel technology

Purpose

To assess the feasibility of half‐Fourier‐acquisition single‐shot turbo spin‐echo (HASTE) of the lung at 3 Tesla (T) using parallel imaging with a prototype of a 32‐channel torso array coil, and to determine the optimum acceleration factor for the delineation of intrapulmonary anatomy.

Materials and Methods

Nine volunteers were examined on a 32‐channel 3T MRI system using a prototype 32‐channel‐torso‐array‐coil. HASTE‐MRI of the lung was acquired at both, end‐inspiratory and end‐expiratory breathhold with parallel imaging (Generalized autocalibrating partially parallel acquisitions = GRAPPA) using acceleration factors ranging between R = 1 (TE = 42 ms) and R = 6 (TE = 16 ms). The image quality of intrapulmonary anatomy and subjectively perceived noise level was analyzed by two radiologists in consensus. In addition quantitative measurements of the signal‐to‐noise ratio (SNR) of HASTE with different acceleration factors were assessed in phantom measurements.

Results

Using an acceleration factor of R = 4 image blurring was substantially reduced compared with lower acceleration factors resulting in sharp delineation of intrapulmonary structures in expiratory scans. For inspiratory scans an acceleration factor of 2 provided the best image quality. Expiratory scans had a higher subjectively perceived SNR than inspiratory scans.

Conclusion

Using optimized multi‐element coil geometry HASTE‐MRI of the lung is feasible at 3T with acceleration factors up to 4. Compared with nonaccelerated acquisitions, shorter echo times and reduced image blurring are achieved. Expiratory scanning may be favorable to compensate for susceptibility associated signal loss at 3T. J. Magn. Reson. Imaging 2009;30:541–546. © 2009 Wiley‐Liss, Inc.     

A 128-channel receive-only cardiac coil for highly accelerated cardiac MRI at 3 Tesla.

A 128-channel receive-only array coil is described and tested for cardiac imaging at 3T. The coil is closely contoured to the body with a "clam-shell" geometry with 68 posterior and 60 anterior elements, each 75 mm in diameter, and arranged in a continuous overlapped array of hexagonal symmetry to minimize nearest neighbor coupling. Signal-to-noise ratio (SNR) and noise amplification for parallel imaging (G-factor) were evaluated in phantom and volunteer experiments. These results were compared to those of commercially available 24-channel and 32-channel coils in routine use for cardiac imaging. The in vivo measurements with the 128-channel coil resulted in SNR gains compared to the 24-channel coil (up to 2.2-fold in the apex). The 128- and 32-channel coils showed similar SNR in the heart, likely dominated by the similar element diameters of these coils. The maximum G-factor values were up to seven times better for a seven-fold acceleration factor (R=7) compared to the 24-channel coil and up to two-fold improved compared to the 32-channel coil. The ability of the 128-channel coil to facilitate highly accelerated cardiac imaging was demonstrated in four volunteers using acceleration factors up to seven-fold (R=7) in a single spatial dimension.     

Increased vessel depiction of the carotid bifurcation with a specialized 16‐channel phased array coil at 3T

The purpose of this work was to design and construct a multichannel receive‐only radiofrequency coil for 3T magnetic resonance imaging of the human carotid artery and bifurcation with optimized signal‐to‐noise ratio (SNR) in the carotid vessels along the full extent of the neck. A neck phantom designed to match the anatomy of a subject with a neck representing the body habitus often seen in subjects with carotid arterial disease was constructed. Sixteen circular coil elements were arranged on a semirigid fiberglass former that closely fit the shape of the phantom, resulting in a 16‐channel bilateral phased array coil. Comparisons were made between this coil and a typical 4‐channel carotid coil in a study of 10 carotid vessels in five healthy volunteers. The 16‐channel carotid coil showed a 73% average improvement in SNR at the carotid bifurcation. This coil also maintained an SNR greater than the peak SNR of the 4‐channel coil over a vessel length of 10 cm. The resulting increase in SNR improved vessel depiction of the carotid arteries over an extended field of view, and demonstrated better image quality for higher parallel imaging reduction factors compared to the 4‐channel coil. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Comparison of volume, four- and eight-channel head coils using standard and parallel imaging

Array coils can potentially offer increased signal-to-noise ratio (SNR) over standard coils adjacent to the array elements, while preserving the SNR at the center of the volume. The SNR advantage should theoretically increase with the number of array elements. Parallel acquisition techniques (PAT), on the other hand, can benefit acquisition times or spatial resolution at a cost to SNR as well as image quality. This study examines the question of whether SNR and image quality are still acceptable with two different array coils (four and eight channels) in conjunction with PAT when compared to standard imaging with a volume coil. All imaging was on a 1.5 T MR scanner. T2-weighted, FLAIR, diffusion-weighted, and time of flight (TOF) angiography images were performed with and without PAT in a phantom and in ten healthy volunteers. The phantom measurements demonstrated superior SNR for the eight-channel coil versus the four-channel and standard head coils. Using the eight-channel head coil for in vivo imaging, image quality with PAT (acceleration factor=2) was scored similar to images without PAT using the volume coil. The four-channel head coil suffered from inhomogeneity, lower SNR and poorer image quality when using PAT compared to standard imaging with the volume head coil. Both the in vivo and the phantom results indicate that the eight-channel head coil should be used for the highest quality brain images; this coil can be combined with PAT sequences for shorter acquisition time without a significant decrease in image quality relative to a volume coil without PAT.     

Sodium imaging of human brain at 7 T with 15‐channel array coil

Signal‐to‐noise ratio (SNR) is a major challenge to sodium magnetic resonance imaging. Phased array coils have been shown significantly improving SNR in proton imaging over volume coils. This study investigates SNR advantage of a 15‐channel array head coil (birdcage volume coil for transmit/receive and 15‐channel array insert for receive‐only) in sodium imaging at 7 T. Phantoms and healthy human brains were scanned on a whole‐body 7 T magnetic resonance imaging scanner using a customer‐developed pulse sequence with the twisted projection imaging trajectory. Noise‐only images were acquired with blanked radiofrequency excitations for noise measurement on a pixel basis. SNR was calculated on the root of sum‐of‐squares images. When compared with the volume coil, the 15‐channel array produced SNR more than doubled at the periphery and slightly increased at the center of the phantoms and human brains. Decorrelation of noise across channels of the array coil extended the SNR‐doubled region into deep area of the brain. The spatial modulation of element sensitivities on the sum‐of‐squares combined image was removed by performing self‐calibrated sensitivity encoding parallel image reconstruction and uniform image intensity across entire field of view was attained. The 15‐channel array coil is an efficient tool to substantially improve SNR in sodium imaging on human brain. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Signal-to-noise ratio enhancement of intermolecular double-quantum coherence MR spectroscopy in inhomogeneous fields with phased array coils on a 3 Tesla whole-body scanner

Purpose

To improve signal‐to‐noise ratio (SNR) of intermolecular double‐quantum coherence (iDQC) MRS on a 3 Tesla (T) whole‐body scanner.

Materials and Methods

A 32‐channel phased array coil was used to acquire iDQC signal of a MRS phantom in the presence of large field inhomogeneity. The obtained individual spectra from the array elements were combined together in the time domain using a multichannel nonparametric singular value decomposition algorithm. The results were compared quantitatively with those acquired with a circularly polarized (CP) head coil.

Results

The achieved gain in SNR ranges from 1.63 to 2.10 relative to the CP coil, mainly depending on the relative position between the surface of the phased array coil and the voxel of acquisition.

Conclusion

SNR enhancement of iDQC MRS in inhomogeneous fields on a 3T whole‐body scanner is feasible with phased array coils. This can facilitate iDQC applications of high‐resolution in vivo spectroscopy in the presence of field inhomogeneity for potential disease diagnosis in humans. J. Magn. Reson. Imaging 2011;33:698–703. © 2011 Wiley‐Liss, Inc.     

Multiple‐profile homogeneous image combination: Application to phase‐cycled SSFP and multicoil imaging

Signal inhomogeneities in MRI often appear as multiplicative weightings due to various factors such as field‐inhomogeneity dependencies for steady‐state free precession (SSFP) imaging or receiver sensitivities for coil arrays. These signal inhomogeneities can be reduced by combining multiple data sets with different weights. A sum‐of‐squares combination is typically used due to its simplicity and near‐optimal signal‐to‐noise ratio (SNR). However, this combination may lead to residual signal inhomogeneity. Alternatively, an optimal linear combination of the data can be performed if the weightings for individual data sets are estimated accurately. We propose a nonlinear combination to improve image‐based estimates of the individual weightings. The signal homogeneity can be significantly increased without compromising SNR. The improved performance of the method is demonstrated for SSFP banding artifact reduction and multicoil (phased‐array and parallel) image reconstructions. Magn Reson Med 60:732–738, 2008. © 2008 Wiley‐Liss, Inc.     

32‐channel phased‐array receive with asymmetric birdcage transmit coil for hyperpolarized xenon‐129 lung imaging

Hyperpolarized xenon‐129 has the potential to become a noninvasive contrast agent for lung MRI. In addition to its utility for imaging of ventilated airspaces, the property of xenon to dissolve in lung tissue and blood upon inhalation provides the opportunity to study gas exchange. Implementations of imaging protocols for obtaining regional parameters that exploit the dissolved phase are limited by the available signal‐to‐noise ratio, excitation homogeneity, and length of acquisition times. To address these challenges, a 32‐channel receive‐array coil complemented by an asymmetric birdcage transmit coil tuned to the hyperpolarized xenon‐129 resonance at 3 T was developed. First results of spin‐density imaging in healthy subjects and subjects with obstructive lung disease demonstrated the improvements in image quality by high‐resolution ventilation images with high signal‐to‐noise ratio. Parallel imaging performance of the phased‐array coil was demonstrated by acceleration factors up to three in 2D acquisitions and up to six in 3D acquisitions. Transmit‐field maps showed a regional variation of only 8% across the whole lung. The newly developed phased‐array receive coil with the birdcage transmit coil will lead to an improvement in existing imaging protocols, but moreover enable the development of new, functional lung imaging protocols based on the improvements in excitation homogeneity, signal‐to‐noise ratio, and acquisition speed. Magn Reson Med 70:576–583, 2013. © 2012 Wiley Periodicals, Inc.     

32‐Channel RF coil optimized for brain and cervical spinal cord at 3 T

Diffusion and functional magnetic resonance imaging of the spinal cord remain challenging due to the small cross‐sectional size of the cord and susceptibility‐related distortions. Although partially addressable through parallel imaging, few highly parallel array coils have been implemented for the cervical cord. Here, we developed a 32‐channel coil that fully covers the brain and c‐spine and characterized its performance in comparison with a commercially available head/neck/spine array. Image and temporal signal‐to‐noise ratio were, respectively, increased by 2× and 1.8× in the cervical cord. Averaged g‐factors at 4× acceleration were lowered by 22% in the brain and by 39% in the spinal cord, enabling 1‐mm isotropic R = 4 multi‐echo magnetization prepared gradient echo of the full brain and c‐spine in 3:20 min. Diffusion imaging of the cord at 0.6 × 0.6 × 5 mm³ resolution and tractography of the full brain and c‐spine at 1.7‐mm isotropic resolution were feasible without noticeable distortion. Improvements of this nature potentially enhance numerous basic and clinical research studies focused on spinal and supraspinal regions. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Multiple‐mouse MRI with multiple arrays of receive coils

Compared to traditional single‐animal imaging methods, multiple‐mouse MRI has been shown to dramatically improve imaging throughput and reduce the potentially prohibitive cost for instrument access. To date, up to a single radiofrequency coil has been dedicated to each animal being simultaneously scanned, thus limiting the sensitivity, flexibility, and ultimate throughput. The purpose of this study was to investigate the feasibility of multiple‐mouse MRI with a phased‐array coil dedicated to each animal. A dual‐mouse imaging system, consisting of a pair of two‐element phased‐array coils, was developed and used to achieve acceleration factors greater than the number of animals scanned at once. By simultaneously scanning two mice with a retrospectively gated cardiac cine MRI sequence, a 3‐fold acceleration was achieved with signal‐to‐noise ratio in the heart that is equivalent to that achieved with an unaccelerated scan using a commercial mouse birdcage coil. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Comprehensive quantification of signal‐to‐noise ratio and g‐factor for image‐based and k‐space‐based parallel imaging reconstructions

Parallel imaging reconstructions result in spatially varying noise amplification characterized by the g‐factor, precluding conventional measurements of noise from the final image. A simple Monte Carlo based method is proposed for all linear image reconstruction algorithms, which allows measurement of signal‐to‐noise ratio and g‐factor and is demonstrated for SENSE and GRAPPA reconstructions for accelerated acquisitions that have not previously been amenable to such assessment. Only a simple “prescan” measurement of noise amplitude and correlation in the phased‐array receiver, and a single accelerated image acquisition are required, allowing robust assessment of signal‐to‐noise ratio and g‐factor. The “pseudo multiple replica” method has been rigorously validated in phantoms and in vivo, showing excellent agreement with true multiple replica and analytical methods. This method is universally applicable to the parallel imaging reconstruction techniques used in clinical applications and will allow pixel‐by‐pixel image noise measurements for all parallel imaging strategies, allowing quantitative comparison between arbitrary k‐space trajectories, image reconstruction, or noise conditioning techniques. Magn Reson Med 60:895–907, 2008. © 2008 Wiley‐Liss, Inc.     

32-channel 3 Tesla receive-only phased-array head coil with soccer-ball element geometry.

A 32-channel 3T receive-only phased-array head coil was developed for human brain imaging. The helmet-shaped array was designed to closely fit the head with individual overlapping circular elements arranged in patterns of hexagonal and pentagonal symmetry similar to that of a soccer ball. The signal-to-noise ratio (SNR) and noise amplification (g-factor) in accelerated imaging applications were quantitatively evaluated in phantom and human images and compared with commercially available head coils. The 32-channel coil showed SNR gains of up to 3.5-fold in the cortex and 1.4-fold in the corpus callosum compared to a (larger) commercial eight-channel head coil. The experimentally measured g-factor performance of the helmet array showed significant improvement compared to the eight-channel array (peak g-factor 59% and 26% of the eight-channel values for four- and fivefold acceleration). The performance of the arrays is demonstrated in high-resolution and highly accelerated brain images.     

Size‐optimized 32‐channel brain arrays for 3 T pediatric imaging

Size‐optimized 32‐channel receive array coils were developed for five age groups, neonates, 6 months old, 1 year old, 4 years old, and 7 years old, and evaluated for pediatric brain imaging. The array consisted of overlapping circular surface coils laid out on a close‐fitting coil‐former. The two‐section coil former design was obtained from surface contours of aligned three‐dimensional MRI scans of each age group. Signal‐to‐noise ratio and noise amplification for parallel imaging were evaluated and compared to two coils routinely used for pediatric brain imaging; a commercially available 32‐channel adult head coil and a pediatric‐sized birdcage coil. Phantom measurements using the neonate, 6‐month‐old, 1‐year‐old, 4‐year‐old, and 7‐year‐old coils showed signal‐to‐noise ratio increases at all locations within the brain over the comparison coils. Within the brain cortex the five dedicated pediatric arrays increased signal‐to‐noise ratio by up to 3.6‐, 3.0‐, 2.6‐, 2.3‐, and 1.7‐fold, respectively, compared to the 32‐channel adult coil, as well as improved G‐factor maps for accelerated imaging. This study suggests that a size‐tailored approach can provide significant sensitivity gains for accelerated and unaccelerated pediatric brain imaging. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Echo‐planar spectroscopic imaging (EPSI) of the water resonance structure in human breast using sensitivity encoding (SENSE)

High spectral and spatial resolution MRI, based on echo‐planar spectroscopic imaging, has been applied successfully in diagnostic breast imaging, but acquisition times are long. One way of increasing acquisition speed is to apply the sensitivity encoding algorithm for complex high spectral and spatial resolution data. We demonstrate application of a complex sensitivity encoding algorithm to high spectral and spatial resolution MRI data, in a phantom and human breast, with 7‐ and 16‐channel dedicated breast phased‐array coils. Very low g factors are obtained using the breast coils, and the signal‐to‐noise ratio (SNR) penalty for water resonance peak height and water resonance asymmetry images is small at acceleration factors of up to 6 and 4, respectively, as evidenced by high Pearson correlation factors between fully sampled and accelerated data. This is the first application of the sensitivity encoding algorithm to characterize the structure of the water resonance at high spatial resolution. Magn Reson Med 63:1557–1563, 2010. © 2010 Wiley‐Liss, Inc.     

Phased Array Coils for Upper Extremity MRA

A phased array coil was constructed for imaging the upper extremity vasculature for patients undergoing dialysis treatment. The phased array coil exhibits improved signal-to-noise ratio (SNR) over the body coil and allows imaging of the entire upper extremity. SNR as a function of depth was measured on a homogeneous phantom with the arm coil and compared with the body coil. Near the coil there is an improvement of a factor of 7 and at a depth of approximately 6–7 cm, there is an improvement of factor 2. In vivo SNR measurements resulted in similar improvements. Images of the upper extremity vasculature of healthy volunteers were generated using 2D-time-of-flight (2DTOF) angiography. Blood flow velocity was assessed in a CINE-phase contrast study.