A practical and flexible implementation of 3D MRI in the Earth’s magnetic field

Halse, Meghan E.; Coy, Andrew; Dykstra, Robin; Eccles, C.D.; Hunter, Mark; Ward, Rob; Callaghan, Paul T.

doi:10.1016/j.jmr.2006.06.011

Cited by 59 publications

(34 citation statements)

References 16 publications

(22 reference statements)

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“…The total received signal is obtained by integrating over the entire excited plane and includes the phase and frequency encoding terms. The received signal becomes, sðt x ; t y Þ ¼ ZZ x ; y in plane mðx; yÞ expðÀi gG y yt y Þ expðÀi g G x xt x Þ expðÀi o tÞdx dy: [12] The signal s is identified with the parameters t x and t y . For a given experiment, t y is held fixed, while the acquisition time t x is running.…”

Section: The Signal Equationmentioning

confidence: 99%

“…The low field regime has several additional advantages, like open coil geometries, increased relaxation times which improve the image contrast (9), narrow NMR line widths because of low susceptibility differences (10), and the ability to perform MRI in the presence of metals (11). Low field methods even enable imaging in the Earth's magnetic field (12). All of these features motivate the development of MRI at low fields.…”

Section: Undesired Gradientsmentioning

confidence: 99%

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Undesired gradients in low‐field magnetic resonance imaging

Ullah

Anwar

2009

Concepts Magnetic Resonance

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Magnetic resonance imaging (MRI) is a beautiful application of the phenomenon of nuclear magnetic resonance (NMR). Classically, the field strength is regarded as one measure of its quality because high field strength gives higher signal to noise ratios, better resolution and reduced scan times. So high field MRI has historically drawn a lot of attention. However, high-field MRI has some disadvantages like reduced relaxation times and high susceptibility gradients. Furthermore, high-field MRI systems are bulky, immovable, and very expensive. These reasons have motivated interest in the subject of low field MRI. The downside is that in the low field regime, we encounter the problem of undesired gradients appearing along with the desired ones. The presence of these additional gradients, generally known as concomitant gradients, directly follows from the fundamental Maxwell equations. The undesired gradients cause strong image distortions. In this article we discuss undesired gradients and work out their quantitative contribution towards the resulting image distortion. The gradient field is expressed in terms of a consistent tensorial description, including terms to arbitrary order. It is explored that the exact form of the undesired gradients can have an impact even in certain high-field MRI cases. Finally, we also appreciate the effects of these gradients in other physical systems such as the SternGerlach experiment and the steering of magnetic species in solution.

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Section: The Signal Equationmentioning

confidence: 99%

Section: Undesired Gradientsmentioning

confidence: 99%

Undesired gradients in low‐field magnetic resonance imaging

Ullah

Anwar

2009

Concepts Magnetic Resonance

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“…routine examination in underdeveloped countries. A practical implementation utilizing a Faraday coil as the detector for capsicum imaging in the earth's magnetic field was presented in 2006 [16]. Au unshielded 7-channel SQUID system was implemented by Espy et al in 2015 [17].…”

Section: Introductionmentioning

confidence: 99%

“…These two techniques made MRI possible in an urban environment, leading us to construct a 4-channel unshielded system for such applications in 2015 [21]. However, power-line harmonic interference and fixedfrequency noise peaks constitute a problem both in unshielded environment and in a shielded room, faced both by SQUID and inductive detection [16,22]. For instance, in vivo human brain imaging using SQUIDs [23] and coils [24] as detectors, and lung imaging based on hyperpolarized gases in a conductively shielded room [25] may all endure the power-line harmonic interference.…”

Section: Introductionmentioning

confidence: 99%

Adaptive suppression of power line interference in ultra-low field magnetic resonance imaging in an unshielded environment

Huang

Dong

Qiu

et al. 2018

Journal of Magnetic Resonance

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Power-line harmonic interference and fixed-frequency noise peaks may cause stripeartifacts in ultra-low field (ULF) magnetic resonance imaging (MRI) in an unshielded environment and in a conductively shielded room. In this paper we describe an adaptive suppression method to eliminate these artifacts in MRI images. This technique utilizes spatial correlation of the interference from different positions, and is realized by subtracting the outputs of the reference channel(s) from those of the signal channel(s) using wavelet analysis and the least squares method. The adaptive suppression method is first implemented to remove the image artifacts in simulation. We then experimentally demonstrate the feasibility of this technique by adding three orthogonal superconducting quantum interference device (SQUID) magnetometers as reference channels to compensate the output of one 2 nd -order gradiometer. The experimental results show great improvement in the imaging quality in both 1D and 2D MRI images at two common imaging frequencies, 1.3 kHz and 4.8 kHz. At both frequencies, the effective compensation bandwidth is as high as 2 kHz. Furthermore, we examine the longitudinal relaxation times of the same sample before and after compensation, andshow that the MRI properties of the sample did not change after applying adaptive suppression. This technique can effectively increase the imaging bandwidth and be applied to ULF MRI in both an unshielded environment and a shielded room made from aluminum sheets.

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“…Alternative detection techniques must be implemented for ultra-low-field MRI because conventional Faraday detection has poor sensitivity in low magnetic fields (16). Superconducting quantum interference devices (SQUIDs) provide a sensitivity that is independent of the field strength (17), and this technique has successfully imaged objects within a metallic enclosure and has produced undistorted images in the presence of a titanium bar (14).…”

mentioning

confidence: 99%

Flow in porous metallic materials: A magnetic resonance imaging study

Harel

Michalak

et al. 2008

Magnetic Resonance Imaging

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Purpose:To visualize flow dynamics of analytes inside porous metallic materials with laser-detected magnetic resonance imaging (MRI). Materials and Methods:We examine the flow of nuclearpolarized water in a porous stainless steel cylinder. Laserdetected MRI utilizes a sensitive optical atomic magnetometer as the detector. Imaging was performed in a remotedetection mode: the encoding was conducted in the Earth's magnetic field, and detection is conducted downstream of the encoding location. Conventional MRI (7T) was also performed for comparison.Results: Laser-detected MRI clearly showed MR images of water flowing through the sample, whereas conventional MRI provided no image. Conclusion:We demonstrated the viability of laser-detected MRI at low-field for studying porous metallic materials, extending MRI techniques to a new group of systems that is normally not accessible to conventional MRI. MAGNETIC RESONANCE IMAGING (MRI), conventionally performed in a strong homogenous magnetic field, is a versatile imaging modality for materials research (1). Its advantage of noninvasiveness allows imaging of opaque porous materials not possible by optical methods. Sederman et al (2) used MRI to study the structureflow correlation in packed beds. Seymour et al (3) studied biofouling of a homogeneous model porous media. Callaghan and Khrapitchev (4), and Blü mich et al (5) developed various pulse sequences to investigate timedependent flow velocities in porous media. Swider et al (6) employed high-resolution MRI to study both localized and global flow in porous biomaterials. Recently, Granwehr et al (7) developed a time-of-flight MRI scheme that utilized remote detection to study the flow behavior of xenon gas in porous rocks. In addition, different fluids can be selectively encoded based on their respective chemical shifts (8). The samples involved in these studies with conventional MRI are limited to electrically nonconductive materials.Flow dynamics of liquids in porous metallic materials is of extensive practical interest, as these materials are widely used in filtration, catalysis, and biomedicine (9 -12). Conventional MRI techniques, however, are not applicable to these materials for two fundamental reasons. First, the penetration depth of radiofrequency radiation, which is inversely proportional to the square root of the frequency, is only several micrometers in metals for the strong magnetic fields (Ͼ1 T) used in conventional MRI scanners (13). The nuclear spins within an electrically conductive sample that reside deeper than the penetration depth will not be sufficiently excited, and thus, will not contribute to the MR images. Even if some nuclear spins could be excited within a region sufficiently below the penetration depth of the metallic sample, these spins would not be detected at the pickup coil, because their oscillating field would be similarly screened out. Second, the large magnetic-susceptibility gradients intrinsically associated with porous metallic materials significantly distort the field homogeneity a...

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A practical and flexible implementation of 3D MRI in the Earth’s magnetic field

Cited by 59 publications

References 16 publications

Undesired gradients in low‐field magnetic resonance imaging

Undesired gradients in low‐field magnetic resonance imaging

Adaptive suppression of power line interference in ultra-low field magnetic resonance imaging in an unshielded environment

Flow in porous metallic materials: A magnetic resonance imaging study

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