Equivalent circuit for coil-patient interactions in magnetic resonance imaging

ABSTRACT:In high-field proton NMR, the signal-to-noise ratio SNR achieved with a ( ) close-fitting solenoidal microcoil is adversely affected by radio frequency RF losses in the coil, its leads, the capacitor used to tune it, and finally, the sample. In Part II, a rigorous description of these various losses is presented, and their severity is related to the details of coil design. Results not only provide a rational basis for defining a microcoil's optimal wire diameter and the number of turns, but also for evaluating how the SNR varies with coil size and NMR frequency in high-field proton NMR studies involving either conducting or non-conducting samples. ᮊ

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“…To illustrate this, it is helpful to consider the data Ž . originally presented by Van Heteren et al 19 .…”

Section: Dielectric Lossesmentioning

confidence: 96%

Solenoidal microcoil design—Part II: Optimizing winding parameters for maximum signal‐to‐noise performance

Minard

Wind

2001

Concepts Magn. Reson.

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“…[23] becomes larger than the term, bx 1=2 r . There have been efforts to experimentally quantify the relative amount of RF coil noise contributed by the sample and the coil for various coil designs and calculate the corresponding SNR (25,26). However, this is nontrivial since Q cannot provide a direct measurement of SNR, as both the signal and noise voltages scale linearly with Q (40).…”

Section: Rf Coil Quality Factormentioning

confidence: 99%

“…More recently, Redpath (24) has provided a review of SNR in MRI and discusses how to improve the SNR by using quadrature detection and phased-array RF coils. van Heteren et al (25) experimentally quantified the performance of several receiver coils, taking into account dielectric and inductive coupling between RF coils and their respective samples, and applied this to a theoretical SNR calculation (26).…”

Section: Receiver Coilsmentioning

confidence: 99%

RF coil loading measurements between 1 and 50 MHz to guide field‐cycled MRI system design

Gilbert

Scholl

Chronik

2008

Concepts Magn Reson

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Conventional magnetic resonance imaging (MRI) uses a single superconducting magnet both to polarize the nuclear magnetic moments in a sample and to provide the magnetic field environment under which an image is acquired. In field-cycled MRI (FCMRI), these two tasks are independently accomplished by two actively controlled resistive magnets. As a result, the expressions for signal-to-noise ratio for an FCMRI system, as with other systems making use of prepolarization techniques (such as hyperpolarized noble gas MRI), are fundamentally different than those of a conventional system. Once losses in the receiver coil are dominated by contributions from the sample, there is little benefit to increasing the receive frequency. The added freedom to independently vary the two field strengths can lead to substantial relative improvements in SNR over a range of Larmor frequencies, while precluding the difficulties associated with the operation of high-field systems. Radiofrequency coil loading measurements were recorded over a frequency range of 1-50 MHz. The optimal frequency range for an FCMRI scanner intended for small animal imaging was determined to be between 5 and 48 MHz, depending on the given receiver coil and sample conductivity. The implication of these results on the design of systems employing prepolarization techniques is provided, with an emphasis being placed on the design and operation of FCMRI systems.

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“…They may shift the coil resonance frequency (detuning) and increase its bandwidth, thus affecting the quality factor (coil loading effect) [19], [20], [33], [34]. The frequency shift may exceed the capabilities of the coil tuning circuit, in which case the coil can no longer be tuned to the proper resonance frequency for the MRI imager.…”

Section: B Conductivity-related Incompatibilitymentioning

confidence: 99%

“…This represents a significant challenge, owing to the stringent constraints of safety, intercompatibility and limited space imposed by the MRI environment [14], [15]. Improperly designed, such shields could severely affect the quality of the MRI acquisitions through susceptibility artefacts [16], [17], eddycurrent-induced artefacts and signal inhomogeneities [18], or coil loading [19], [20]. They might also represent a hazard for the patient or a risk of damage through eddy-current-related vibrations or heating [14], [21].…”

mentioning

confidence: 99%

Gamma shielding materials for MR-compatible PET

Strul

Cash

Keevil

et al. 2003

IEEE Trans. Nucl. Sci.

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Abstract-As for standard positron emission tomography (PET) scanners, MR-compatible PET scanners will require gamma shielding to suppress the influence of activity outside the PET field of view (FOV). Suitable materials must have very specific properties, including magnetic properties close to those of water, high density, high atomic number, and ideally a low conductivity. In order to identify potential suitable materials, we have selected several heavy-metal-based candidates based on the available data for magnetic and shielding properties. These materials include several nonferromagnetic metals and metal oxides, two scintillating crystals (bismuth germanate and lead tungstate) and two metal/epoxy compounds. The magnetic resonance imaging (MRI) compatibility of these materials was assessed under various conditions, both on a human and a small-animal MRI scanner. In parallel, we assessed the shielding efficiency at 661 keV of the most promising candidates. These experiments showed that there is a range of possibilities for the design of MR-compatible gamma shields. Lead has acceptable magnetic compatibility but can induce significant conductivity-related artefacts. Heavy-metal-based minerals are fully insulating and hot-pressed lead monoxide showed good MR compatibility combined with good shielding properties. Other possibilities include the use of lead based powders and heavy-metal oxide composites.Index Terms-Magnetic resonance imaging (MRI), positron emission tomography (PET), shielding.

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Equivalent circuit for coil-patient interactions in magnetic resonance imaging

Cited by 32 publications

References 11 publications

Solenoidal microcoil design—Part II: Optimizing winding parameters for maximum signal‐to‐noise performance

Solenoidal microcoil design—Part II: Optimizing winding parameters for maximum signal‐to‐noise performance

RF coil loading measurements between 1 and 50 MHz to guide field‐cycled MRI system design

Gamma shielding materials for MR-compatible PET

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