Least Squares Magnetic-Field Optimization for Portable Nuclear Magnetic Resonance Magnet Design

Paulsen, Jeffrey L.; Franck, John M.; Demas, Vasiliki; Bouchard, Louis

doi:10.1109/tmag.2008.2001697

Cited by 22 publications

(13 citation statements)

References 34 publications

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“…To illustrate adjustability and imaging functionalities of the sensor, we picked 2 nonoverlapping sensitive volumes centered at a distance 1.0 and 1.8 cm from the magnet surface (or equivalently, 3.0 and 3.8 cm from the center of the magnet rods). We note that the magnet array can also produce sensitive volumes over a continuous range of distances (17). Each of these volumes was characterized by acquiring a magnetic field map as shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…1B). As the sensitive volume is pushed away from the sensor's surface, it shrinks along the direction of the magnet rod axes because homogeneity along this axis relies on the extent to which these rods approximate rods of infinite length; this approximation breaks down at distances on the order of a quarter of the rod length (17).…”

Section: Resultsmentioning

confidence: 99%

“…The sensor was designed by an inverse technique we developed based on a least-squares fit of optimal dipole magnitudes to produce a desired target field (17). For arrays of parallel permanent magnet rods, it yields optimal rod radii and orientations of the magnetization given a desired target field and rod locations.…”

Section: Methodsmentioning

confidence: 99%

“…The rod orientations for volume 1 corresponds to the original optimization in ref. 17, whereas volume 2 corresponds to a sensitive volume located an additional 1 cm away from the surface. All final rod orientations may need to be adjusted from their calculated values because the rods' magnetizations will vary on the order of several percent from their ideal values.…”

Section: Methodsmentioning

confidence: 99%

“…Our magnetic-field orientation and RF field configuration are optimized for low power requirements and high sensitivity ‡ . The design follows a recently-developed inverse magnet design method (17) that is adapted to a cylindrical geometry that leads naturally to the feature of adjustability. In terms of the above discussion on optimal geometry, we have opted for an intermediate field strength (Ϸ600-700 G) so that The authors declare no conflict of interest.…”

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confidence: 99%

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Volume-selective magnetic resonance imaging using an adjustable, single-sided, portable sensor

Paulsen

Bouchard²,

Graziani³

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

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Portable, single-sided NMR sensors can operate under conditions inaccessible to conventional NMR while featuring lower cost, portability, and the ability to analyze arbitrary-sized objects. Such sensors can nondestructively probe the interior of samples by collecting images and measuring relaxation and diffusion constants, and, given careful shimming schemes, even perform chemical analysis. The inherently strong magnetic-field gradients of single-sided sensors developed so far has prevented imaging of materials with high water content, such as biological tissues, over large volumes whereas designs with more homogeneous fields suffer from low field strength and typically cannot probe volumes larger than Ϸ10 cm 3 . We present a design with a continuously adjustable sensitive volume, enabling the effective volume to be enlarged several fold. This capability allows for imaging in reasonable times of much bigger objects and opens the door to the possibility of clinical imaging with portable sensors. We demonstrate MRI in axial and sagittal planes, at different depths of the sensitive volume and T 1-weighted contrast in a tissue sample. adjustable magnet ͉ ex situ NMR ͉ mobile NMR ͉ single-sided NMR ͉ permanent magnet N uclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) can provide noninvasive chemical analysis and imaging by using the endogenous contrast of materials, regardless of optical opacity. NMR can ascertain physical parameters such as diffusion, flow, and structure of porous materials and biological tissue, and can probe length scales from nanometers to meters. Clinical MRI is usually performed at high magnetic fields for an improved signal-to-noise ratio (SNR) and associated image resolution, but it requires bulky, expensive, immobile, and often hazardous magnets. Low-field NMR, however, has emerged as an attractive option for imaging in the field and in the presence of metal (1), and it provides more versatile contrast for relaxation weighted images (2). Because these smaller sensors typically operate in geometric environments where the static field is weak and inhomogeneous, the development of single-sided sensors has necessitated the development of novel methodologies and pulse sequences (3-13) designed to minimize image distortions and improve our ability to perform traditional chemical-shift spectroscopy (14). Therefore, objects can be nondestructively probed and one may obtain relaxation, diffusion, or image data over relatively small regions and obtain chemical-shift spectra with careful magnet shimming schemes (15). In spite of these advances, only relatively small volumes can be imaged (10 cm 3 or less) or analyzed spectroscopically (20 mm 3 or less).Mobile NMR sensors can vary in 2 aspects that have a fundamental impact on their performance: (i) the orientation of the field relative to the surface of the sensor and (ii) the type of sensitive region that the magnet produces (homogeneous field over a volume vs. thin slices in a strong gradient). The first case dictates the type of ...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

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