Receiver Loop Arrays

Wright, Steven M.

doi:10.1002/9780470034590.emrstm1129

Cited by 5 publications

(6 citation statements)

References 18 publications

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“…11 is that the improvement in SNR due to the upright coils relative to that from the surface coils alone does not change much with depth into the phantom (averaged across the three sagittal slices and ±10.5 cm in the direction of B 0 ). The SNR gain due to the upright coils is therefore appreciable throughout the phantom, in contrast to increasing array density by decreasing coil size in a surface coil array, which leads to SNR improvements that taper off with depth (7,14,22). The eight-element surface array achieves a higher SNR than that of the composite array only to a depth of about 7 cm below the surface, or approximately one diameter of the large surface coil.…”

Section: Snr Comparisonsmentioning

confidence: 99%

“…channel density) as the coil‐dominated loss regime is approached . Experimentally, only minor increases or overall decreases in SNR at depths greater than the individual element's diameter have been observed with high‐density arrays , and indeed, as coil losses become dominant, the layer in which SNR is expected to increase with channel density becomes thinner . Composite arrays can provide SNR and parallel imaging benefits at the surface without loss at greater depths because they do not require increasing the influence of coil losses to achieve higher channel density.…”

Section: Introductionmentioning

confidence: 99%

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Experimental verification of SNR and parallel imaging improvements using composite arrays

et al. 2014

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Composite MRI arrays consist of triplets where two orthogonal upright loops are placed over the same imaging area as a standard surface coil. The optimal height of the upright coils is approximately half the width for the 7 cm coils used in this work. Resistive and magnetic coupling is shown to be negligible within each coil triplet. Experimental evaluation of imaging performance was carried out on a Philips 3 T Achieva scanner using an eight-coil composite array consisting of three surface coils and five upright loops, as well as an array of eight surface coils for comparison. The composite array offers lower overall coupling than the traditional array. The sensitivities of upright coils are complementary to those of the surface coils and therefore provide SNR gains in regions where surface coil sensitivity is low, and additional spatial information for improved parallel imaging performance. Near the surface of the phantom the eight-channel surface coil array provides higher overall SNR than the composite array, but this advantage disappears beyond a depth of approximately one coil diameter, where it is typically more challenging to improve SNR. Furthermore, parallel imaging performance is better with the composite array compared with the surface coil array, especially at high accelerations and in locations deep in the phantom. Composite arrays offer an attractive means of improving imaging performance and channel density without reducing the size, and therefore the loading regime, of surface coil elements. Additional advantages of composite arrays include minimal SNR loss using root-sum-of-squares combination compared with optimal, and the ability to switch from high to low channel density by merely selecting only the surface elements, unlike surface coil arrays, which require additional hardware.

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Section: Snr Comparisonsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Experimental verification of SNR and parallel imaging improvements using composite arrays

et al. 2014

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“…If the signal is far stronger than the random variations in the intensity due to noise, then one can have confidence that apparent features in the image (a bright spot in a particular location) reflect the physical characteristics of the sample. When detected by the receiver, the noise may start as Gaussian white noise, but will generally take on a Rician shape and be further altered by the reconstruction process, particularly with Array Coils …”

Section: Part A: Basic Concepts and Overviewmentioning

confidence: 99%

RF coils: A practical guide for nonphysicists

Gruber

Froeling

Leiner

et al. 2018

Magnetic Resonance Imaging

171

136

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Radiofrequency (RF) coils are an essential MRI hardware component. They directly impact the spatial and temporal resolution, sensitivity, and uniformity in MRI. Advances in RF hardware have resulted in a variety of designs optimized for specific clinical applications. RF coils are the “antennas” of the MRI system and have two functions: first, to excite the magnetization by broadcasting the RF power (Tx‐Coil) and second to receive the signal from the excited spins (Rx‐Coil). Transmit RF Coils emit magnetic field pulses ( normalB1+) to rotate the net magnetization away from its alignment with the main magnetic field (B0), resulting in a transverse precessing magnetization. Due to the precession around the static main magnetic field, the magnetic flux in the receive RF Coil ( normalB1−) changes, which generates a current I. This signal is “picked‐up” by an antenna and preamplified, usually mixed down to a lower frequency, digitized, and processed by a computer to finally reconstruct an image or a spectrum. Transmit and receive functionality can be combined in one RF Coil (Tx/Rx Coils). This review looks at the fundamental principles of an MRI RF coil from the perspective of clinicians and MR technicians and summarizes the current advances and developments in technology. Level of Evidence: 1 Technical Efficacy: Stage 6 J. Magn. Reson. Imaging 2018;48:590–604.

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“…SNR was calculated using the Sum-of-Squares (SoS) method ideal for high input SNR [36] and the Maximum Available (MA) method [15].…”

Section: Materials Testsmentioning

confidence: 99%

“…The properties of the sample and the coil, contribute to the SNR through the resistance at the coil terminals (R eff ) and the sensitivity pattern of the coil. The noise signal U Noise in any MRI experiment is basically thermal noise generated by the receiver coil and the sample scaled to the bandwidth used in detecting the signal [15]. So, by improving the filling factor, the coil resistance R Coil , also called equivalent-series-resistance (ESR), is minimized by making the unloaded Q high, and the sample resistance R s (through the induced eddy current losses in the conductive sample) can be minimized by choosing the coil size to match the target Field of View (FoV).…”

Section: Introductionmentioning

confidence: 99%

Anatomically Adaptive Coils for MRI—A 6-Channel Array for Knee Imaging at 1.5 Tesla

et al. 2020

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Purpose: Many of today's MR coils are still somehow rigid and inflexible in their size and shape as they are intentionally designed to image a specific anatomical region and to fit a wide range of patients. Adaptive coils on the other hand, are intended to follow a one-size-fits-all approach, by fitting different shapes, and sizes. Such coils improve the SNR for a wide range of subjects by an optimal fit to the anatomical region of interest, and in addition allow an increased handling and patient comfort as one MRI receive-coil is maintained instead of multiple. Material and Methods: To overcome the SNR losses by non-fitting and thus poorly loaded RF coils, we propose a stretchable antenna design. Each loop has the ability to reversibly stretch up to 100% of its original size, to be anatomically adaptive to different shapes and sizes, and therefore make the coil usable for a wide patient population. Besides the mechanical challenge to find a robust but flexible conductive material, various other problems like frequency and matching shifts affect the SNR. Through bench measurements and MR Imaging at 1.5 T, we investigated different stretchable conductor materials, that fit the defined requirements. Finally, a rigid reference coil and an adaptive 6-channel array for knee imaging at 1.5 Tesla were developed to investigate the potential improvement in SNR. Results: The material tests identified two potentially useful materials: Highly ductile copper and a silver-plated stranded copper wire. Although, the adaptivity causes a frequency shift of the resonance frequency, which entails in variations of the impedance that each coil presents to its connected pre-amplifier, there are strategies to mitigate these effects. The adaptive array prototype made of partly-stretchable loops, showed an improved SNR of up to 100% in 20 mm depth from the phantom surface, and therefore demonstrates the effectiveness of adaptive coils.

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Receiver Loop Arrays

Cited by 5 publications

References 18 publications

Experimental verification of SNR and parallel imaging improvements using composite arrays

Experimental verification of SNR and parallel imaging improvements using composite arrays

RF coils: A practical guide for nonphysicists

Anatomically Adaptive Coils for MRI—A 6-Channel Array for Knee Imaging at 1.5 Tesla

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