Use of multicoil arrays for separation of signal from multiple slices simultaneously excited

368

436

The CAIPIRINHA (Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration) concept in parallel imaging has recently been introduced, which modifies the appearance of aliasing artifacts during data acquisition in order to improve the subsequent parallel imaging reconstruction procedure. This concept has been successfully applied to simultaneous multislice imaging (MS CAIPIRINHA). In this work, we demonstrate that the concept of CAIPIRINHA can also be transferred to 3D imaging, where data reduction can be performed in two spatial dimensions simultaneously. In MS CAIPIRINHA, aliasing is controlled by providing individual slices with different phase cycles by means of alternating multi-band radio frequency (RF) pulses. In contrast to MS CAIPIRINHA, 2D CAIPIRINHA does not require special RF pulses. Instead, aliasing in 2D parallel imaging can be controlled by modifying the phase encoding sampling strategy. This is done by shifting sampling positions from their normal positions in the undersampled 2D phase encoding scheme. Using this modified sampling strategy, coil sensitivity variations can be exploited more efficiently in multiple dimensions, resulting in a more robust parallel imaging reconstruction. Magn Reson Med 55:549 -556, 2006.

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confidence: 99%

Controlled aliasing in volumetric parallel imaging (2D CAIPIRINHA)

Breuer

Blaimer

Mueller

et al. 2006

368

436

“…This reduces acquisition times by decreasing the number of phase encoded lines of k-space that must be acquired. There are three implementations of parallel imaging reported in the literature, known as SENSE (1)(2)(3)(4), SMASH (5-7), and SPACE-RIP (8). All these methods require information about the coil sensitivity profiles (reference data), which is used to regenerate a full image dataset from a subsampled k-space acquisition (target data).…”

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confidence: 99%

“…The reference data is usually obtained as a separate acquisition, often with the subject in situ, although some implementations integrate the reference data acquisition with the target data acquisition (7,9). SENSE (1)(2)(3)(4) operates in the image domain for both the target image data and the coil reference data. We use the name SENSE to refer to the Cartesian sampled form.…”

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confidence: 99%

Generalized SMASH imaging

Bydder

Larkman

Hajnal

2001

Self Cite

107

123

A generalized parallel imaging method has been developed that uses coil profiles to generate missing k-space lines. The proposed method is an extension of SMASH, which uses linear combinations of coil sensitivity profiles to synthesize spatial harmonics. In the generalized SMASH approach described here, coil sensitivity profiles are represented directly in the Fourier domain to provide a general description of the spatial properties of the coils. This removes restrictions imposed by conventional SMASH, so that the choice and position of the receiver coils can be made on the basis of sensitivity to the volume of interest rather than suitability for constructing spatial harmonics. Generalized SMASH also intrinsically allows the freedom to accommodate acquisitions with uniform or nonuniform k-space sampling. Parallel imaging makes use of spatial sensitivity differences between individual coils in an array to partially replace gradient encoding during image acquisition. This reduces acquisition times by decreasing the number of phase encoded lines of k-space that must be acquired. There are three implementations of parallel imaging reported in the literature, known as SENSE (1-4), SMASH (5-7), and SPACE-RIP (8). All these methods require information about the coil sensitivity profiles (reference data), which is used to regenerate a full image dataset from a subsampled k-space acquisition (target data). The reference data is usually obtained as a separate acquisition, often with the subject in situ, although some implementations integrate the reference data acquisition with the target data acquisition (7,9). SENSE (1-4) operates in the image domain for both the target image data and the coil reference data. We use the name SENSE to refer to the Cartesian sampled form. The method used with other k-space trajectories will be referred to explicitly as non-Cartesian SENSE. In SENSE, the target data is acquired with a reduced field of view (FOV), which results in aliasing, and is then unfolded to the full FOV on a pixel-by-pixel basis using the reference data. Reduced FOV imaging imposes a requirement of uniformly spaced samples in the phase encode direction in k-space. Since all processing is done in the image domain, individual pixels in the reduced FOV data get unfolded by integer numbers of final pixels (i.e., 131, 132, 133, etc). This requires the solution of a set of linear simultaneous equations in which pixel intensities are weighted by the coil sensitivity at the final pixel locations. The numerical condition of these equations determines the local noise properties of the unfolded image (3,10), so that the signal-to-noise ratio (SNR) varies from pixel to pixel (3). The resulting patterns of noise variation generally reflect the coil geometry and can have a strong perceptual effect. The SENSE method is very flexible and can be used with any coil geometry, provided satisfactory numerical conditioning is achieved at the pixel locations of interest.SMASH (5,6) operates in k-space for the target image data but uses a...

“…The intervening slices could be filled with the conventional method of multislice interleaving. The technique is fully compatible with conventional multi-channel receiver parallel imaging methods (10,11,21), potentially enabling high acceleration factors in scan time. The simultaneous nature of the slice excitation opens up the possibility of ultra-fast flow measurement along the slice select direction, which could allow rapid measurement and depiction of gas-flow patterns in the airways (7,22).…”

Section: Discussionmentioning

confidence: 99%

Parallel imaging of hyperpolarized helium‐3 with simultaneous slice excitation

Teh¹,

Lee²,

Paley³

et al. 2006

Hyperpolarized (HP) gas imaging of the lungs is an ideal potential application for parallel imaging. This is due to the fact that there is limited scan time (breath hold of 20 s) and limited non-renewable polarization. Reduced phase encode parallel imaging is demanding on hardware in that it requires multiple receivers. In this work, simultaneous parallel acquisition of hyperpolarized (HP) 3 He images from multiple slices was demonstrated in phantoms and in vivo using a simultaneous slice excitation method, at a field strength of 1.5 T. The pulse sequence allows simultaneous acquisition of n slices per RF excitation, thus reducing the number of RF pulses needed to fully cover a given volume with multi-slicing. Unlike conventional parallel imaging, this method does not require prior reference scan information, which would consume some of the finite longitudinal polarization in lung ventilation studies with HP gas. Recent advances in laser optical pumping techniques allow mass production of hyperpolarized (HP) noble gases (He-3, Xe-129). Noble gas atoms have a 1 ⁄2 nuclear spin property and low gyro-magnetic ratio (␥ 3He ϭ 0.75 ␥ 1H and ␥ 129Xe ϭ 0.27 ␥ 1H ) (1). These gases are also stable and non-radioactive and thus enable for the first time, MR imaging of ventilation in the airways and airspaces with high SNR, spatial and temporal resolution. Ever since successful preliminary in vivo studies of hyperpolarized gas lung imaging on live animals (2,3) were demonstrated, there has been a large interest in human in vivo clinical studies. Despite growing clinical interest, there are multiple technical constraints and challenges remaining in HP gas MRI of the lungs. One of the main constraints is the choice of an optimum imaging strategy due to the nonrecoverable polarization of the spin population. We can approximate the longitudinal magnetization decay of the HP 3 He transverse signal within a single breath-hold as, M y (4):In Eq.[1] M 0 is the magnetization before first RF pulse, n is the n th RF pulse with flip angle ␣ and repetition time T R,