SENSE: Sensitivity encoding for fast MRI

Pruessmann, Klaas P.; Weiger, Markus; Scheidegger, Markus B.; Boesiger, Peter

doi:10.1002/(sici)1522-2594(199911)42:5<952::aid-mrm16>3.3.co;2-j

Cited by 2,447 publications

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“…In the sensitivity encoding (SENSE) method, aliased images are unfolded in the image domain, based on the low resolution sensitivity maps introduced for phased array reconstruction ( Figure 12E). 29 In the generalized auto-calibrating partially parallel acquisition (GRAPPA) method, missing data points are estimated in k-space for each coil based on the so-called 'auto-calibration signal' (ACS) data instead of sensitivity maps ( Figure 12F). 30…”

Section: Parallel Imaging In Csimentioning

confidence: 99%

CSI and SENSE CSI

Schär

Strasser

Dydak

2016

eMagRes

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Chemical shift imaging (CSI) is a magnetic resonance spectroscopy (MRS) method of localizing spectra from multiple locations at the same time. For localization, the principle of phase encoding as used in MRI is applied. Because the applied phase-encoding gradients are strong and short, CSI localization does not suffer from image warping and chemical shift displacement artifacts. The main disadvantage of CSI is the long acquisition time. For example, acquiring the minimum of a single encoding step per phase encode leads to an acquisition time of more than 34 min for a twodimensional acquisition with a 32 × 32 matrix and a typical 2 s repetition time. The actual spatial resolution can be described with a point spread function, which exhibits strong side lobes due to the limited extent of the encoding matrix and tissue heterogeneity, leading to contamination from neighboring voxels. Filtering reduces the side lobes and their associated contamination at the cost of resolution and signal-to-noise ratio (SNR) per unit volume. Acquisition weighting can be used to simulate a filter during acquisition and thus optimize SNR. Inhomogeneities in the static magnetic field cause shifts in the frequency domain of spectra from voxels at different locations. Those shifts can be corrected using a field map or the water signal from an additional CSI acquisition without water suppression. Water and lipid suppression are performed with the same approaches as in single volume acquisitions. As MRS is challenged by the low metabolite concentrations, phased-array coils are often used to improve the SNR compared to volume coils. Because of the nonuniform sensitivity profiles of each coil, the signal amplitudes and phases from different coils vary among voxels from different locations. Combining data from different channels can be done using the initial points of the FIDs from each voxel. An SNR-optimized coil combination can be achieved using sensitivity maps, and the signals in the different voxels scaled to display a homogeneous sensitivity distribution. Acquisitions with phased arrays can also be accelerated with parallel imaging methods, typically shortening the above-mentioned 34 min acquisition to 9 min or less.

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Section: Parallel Imaging In Csimentioning

confidence: 99%

CSI and SENSE CSI

Schär

Strasser

Dydak

2016

eMagRes

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“…B 1 + shimming calibration data is then acquired. 44 Individual receiver data is used to calculate receiver-sensitivity maps and geometry-factor maps, 43 The magnitude of the B 1 + is mapped with two fully relaxed gradient echo images collected with excitations nominally set to 60 • and 120 • (double angle method). 45 Additionally, the 60 • excitation gradient echo image is carefully post-processed so that the result is in SNR units.…”

Section: Test and Evaluationmentioning

confidence: 99%

TEM Body Coils

Vaughan

2012

Encyclopedia of Magnetic Resonance

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A radiofrequency body coil is included with virtually all clinical MRI systems and many research systems, and is most often built into the bore of the MRI magnet immediately behind the patient bore tube. The body coil is commonly used to excite a uniform RF field required for stimulating the nuclear magnetic resonance signal response from a region of interest within the coil's volume. The most ubiquitous body coil design is the birdcage, a monolithic resonator composed of a ladder network of rung elements subtending a cylindrical form and connected by rails or endrings providing manifold feed and return paths for the rungs. The design is ideally suited for imparting a highly homogeneous, circularly polarized RF field centered within its volume, at lower field strengths where longer Larmor wavelengths are relatively uniform even within a body in the coil. As field strengths climb to 3T and above however, circuit lengths increase in wavelength proportion. Large circuits such as the birdcage become less efficient because of increased radiation resistance. Field-induced eddy current losses to the tissue conductor and displacement current losses to the tissue dielectric increase as well, resulting in increased heat and noise in the body load. Interference patterns in the excitation (and reception) fields result in highly non-uniform fields and inhomogeneous images. The TEM body coil has proved to be a good candidate for compensating these high field problems. It preserves some of the qualities of the birdcage, such as the rung structure for higher homogeneity. The TEM coil however replaces the cage coil's end rings with a slotted cavity for the rung elements' return path, rendering the TEM coil as an array of independent transmission line element resonators. These independent elements are electrically short and therefore more efficient at higher frequencies. And these TEM elements can be independently driven or controlled, over five degrees of freedom to manipulate the coil's field in magnitude, phase, space, time, and frequency. By means collectively referred to as parallel transmit and B1 shimming, the ''multichannel'' TEM coil can be used to mitigate high field problems of image inhomogeneity and SAR incurred by conventional imaging. Additionally, the TEM body coil makes entirely new approaches to imaging possible, such as RF localization of ROIs and optimization of RF-dependent parameters therein. Alternatively, the RF field can be dynamically swept through time, space, frequency, phase, and/or magnitude to ''scan'' a body that could otherwise not be uniformly excited. Military terms for these approaches are target acquisition, frequency hopping, and beam steering. As with military arrays, the TEM body coil lends itself to significant gains in transmission efficiency and cost reduction by incorporating distributed power amp modules into the TEM elements individually. A sixty-four-channel, 32 kW TEM body coil, for example, would have 64 TEM elements each mated with a 500 W power transistor, distributed in three dim...

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“…UTE imaging generates one line of k-space from each pair of excitations, which is a substantial potential penalty and so is justifiable only if the data that is recovered is significant (something that is helped by way in which the excess acquisitions improve the central parts of images). The form of the data recovery also does not lend itself to the receiver coil array techniques such as SENSE 11 and SMASH 10 and their derivatives, so that useful acceleration is very hard to achieve though not impossible. 13 In principle, this approach to imaging short T 2 components and the indirect methods using traditional MR approaches involve no specialist hardware.…”

Section: Practical Implementationmentioning

confidence: 99%

Overview of Short and UltrashortT₂/T₂* Echo Time (UTE) Imaging

Young

2012

Encyclopedia of Magnetic Resonance

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Imaging of tissues (or, indeed, anything else) with short and ultrashort T 2 /T 2 * components is still very much in its infancy, in spite of the significant time since the first clinical applications were published. A key aspect of this topic is the use of short and ultrashort TE pulse sequences, but a consequence of the early stage of development is that there is not yet an agreed definition of what precisely is meant by the term UTE Imaging. The purpose of this article is to deal with issues of this type and provide a framework into which other, more detailed, articles can be fitted.

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SENSE: Sensitivity encoding for fast MRI

Cited by 2,447 publications

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CSI and SENSE CSI

CSI and SENSE CSI

TEM Body Coils

Overview of Short and UltrashortT₂/T₂* Echo Time (UTE) Imaging

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SENSE: Sensitivity encoding for fast MRI

Cited by 2,447 publications

References 0 publications

CSI and SENSE CSI

CSI and SENSE CSI

TEM Body Coils

Overview of Short and UltrashortT2/T2* Echo Time (UTE) Imaging

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Overview of Short and UltrashortT₂/T₂* Echo Time (UTE) Imaging