Essential considerations for spectral localization using indirect gradient encoding of spatial information

Mareci, Thomas H.; Brooker, H. R.

doi:10.1016/0022-2364(91)90267-w

Cited by 71 publications

(70 citation statements)

References 8 publications

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“…In our application, the Hanning window has been used as a weighting function in order to calculate the DW CSI sampling matrix. This window function is a good trade-off between SNR efficiency and sidelobe suppression (9). However, other functions (e.g., Gaussian, Cos-Bell, Hamming, or sinc) can be chosen as weighting functions for DW.…”

Section: Resultsmentioning

confidence: 99%

“…A Hanning-window (Fig. 1b) has been shown to represent a good compromise between sensitivity and spatial resolution (9). Without restricting the use of acquisition-weighting with other weighting functions, a Hanning window will be used in the following.…”

Section: Acquisition-weighted Phase-encodingmentioning

confidence: 99%

“…Pohmann et al (9) reported that due to the reduced FOVefficiency in AW, the choice of smaller voxel sizes at constant experiment durations leads to a small FOV that can result in strong aliasing, predominantly of the chest muscle signal in the PCr maps. This problem is overcome by density-weighting, as is demonstrated in the following experiments.…”

Section: In Vivo Experiments: 31 P Csi Of the Human Heartmentioning

confidence: 99%

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Efficient k‐space sampling by density‐weighted phase‐encoding

Greiser

Kienlin

2003

Magnetic Resonance in Med

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Acquisition-weighting improves the localization of MRI experiments. An approach to acquisition-weighting in a purely phaseencoded experiment is presented that is based on a variation of the sampling density in k-space. In contrast to conventional imaging or to accumulation-weighting, where k-space is sampled with uniform increments, density-weighting varies the distance between neighboring sampling points ⌬k to approximate a given radial weighting function. A fast, noniterative algorithm has been developed to calculate the sampling matrix in one, two, and three dimensions from a radial weighting function w(k), the desired number of scans NA tot and the nominal spatial resolution ⌬x nom . Density-weighted phase-encoding combines the improved shape of the spatial response function and the high SNR of acquisition-weighting with an extended field of view. The artifact energy that results from aliasing due to a small field of view is substantially reduced. The properties of density-weighting are compared to uniform and to accumulation-weighted phase-encoding in simulations and experiments. Density-weighted 31 In metabolic MRI in vivo, low concentrations commonly lead to low signal-to-noise ratios (SNRs). Although fast imaging methods with readout encoding offer comparably high sensitivity (1-4), pure phase-encoding (PE) in all spatially resolved dimensions is still the gold standard in sensitivity-limited MR experiments (5). The low achievable spatial resolution results in severe signal contamination from neighboring regions due to the sidelobes of the spatial response function (SRF). The SRF characterizes the spatial origin of the signals contributing to a distinct voxel (6). If averaging is necessary, acquisition-weighting by variable accumulation numbers for different k-space positions can strongly suppress the sidelobes of the SRF, resulting in a significant reduction of the contamination (7,8). This kind of acquisition-weighting, which is based on variable accumulation of the different PE steps, is in the following referred to as "accumulation-weighting" or "AW." The term "conventional" is used in this context for a uniformly sampled (UW) experiment with equal accumulation numbers. Compared to conventional uniform (UW) weighting, AW improves the SRF without any loss of SNR at identical resolution and experiment duration (6 -8). However, due to the higher accumulation numbers in central k-space and the need for an increased number of different PE steps to preserve spatial resolution, ⌬x nom , AW results in a smaller achievable field of view (FOV) compared to a UW experiment with identical total number of accumulations and spatial resolution. An improved form of acquisition-weighting is presented, "densityweighting" (DW).DW is based on a variation of the density of sampling points in k-space. Compared to AW, DW preserves the favorable shape of the SRF, but extends the accessible FOV. In DW the overall number of different sampling positions is maximized to the total number of scans. Furthermore, the sampling poin...

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Section: Resultsmentioning

confidence: 99%

Section: Acquisition-weighted Phase-encodingmentioning

confidence: 99%

Section: In Vivo Experiments: 31 P Csi Of the Human Heartmentioning

confidence: 99%

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Efficient k‐space sampling by density‐weighted phase‐encoding

Greiser

Kienlin

2003

Magnetic Resonance in Med

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“…This makes such a technique particularly vulnerable to Fourier artifacts such as Gibb's ringing, and an improvement of the PSF will lead to a significant enhancement of the image quality (12). In contrast to data weighting as a postprocessing filter, acquisition-weighting reduces the side-lobes of the PSF without any tradeoff in spatial resolution or SNR (11). In particular, as shown by the numerical simulation, the suppression of the side-lobes of the PSF significantly reduces the propagation of signal from ventricular blood into the myocardium.…”

Section: Discussionmentioning

confidence: 99%

“…It is based on a variation of the density of the samples in k-space. DW leads to an improved shape of the PSF, in substantially reduced spatial signal contamination between neighboring compartments (11,12), and preserves optimal signal-to-noise efficiency. In the same time, it achieves a larger FOV compared to conventional acquisition-weighted schemes, because central k-space is sampled with a smaller increment ⌬k.…”

Section: Methodsmentioning

confidence: 99%

Improved cardiac sodium MR imaging by density‐weighted phase‐encoding

Greiser

Haase

Kienlin

2004

Magnetic Resonance Imaging

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Purpose: To show that density-weighted (DW) k-space sampling improves the quality of human cardiac sodium imaging, a novel method was implemented that combines the high signal-to-noise efficiency of three-dimensional phase-encoding with the advantageous localization performance of nonuniform sampling. A simulation demonstrates substantially reduced blood contamination in the myocardium. Materials and Methods:At 2.0 T, DW cardiac "fast" sodium images with a voxel size of 844 L in seven minutes and "high-resolution" scans in 30 minutes with a voxel size of 570 L were acquired. For comparison, conventional gradient-echo imaging was also performed. Results:In the DW images, a myocardial signal-to-noise ratio (SNR) of 16.0 in the left ventricle and 8.5 in the septum (N ϭ 4) was measured. With longer experimental duration (about 30 minutes; N ϭ 3), the image quality and the SNR could be further improved (voxel size: 570 L; SNR: blood 16.1, septum 10.6). Compared to the gradient-echo images, the image quality was substantially improved. Conclusion:This new method for human cardiac sodium imaging provides high image quality combined with optimal sensitivity and thus may improve the clinical applicability of 23 Na cardiac MRI.

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Spectroscopic Imaging and Multivolume Localization

Graaf

Robin²

2007

In Vivo NMR Spectroscopy

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Essential considerations for spectral localization using indirect gradient encoding of spatial information

Cited by 71 publications

References 8 publications

Efficient k‐space sampling by density‐weighted phase‐encoding

Efficient k‐space sampling by density‐weighted phase‐encoding

Improved cardiac sodium MR imaging by density‐weighted phase‐encoding

Spectroscopic Imaging and Multivolume Localization

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