PEEP—A rapid chemical‐shift imaging method

It is shown that the noise level in EPI (echo planar imaging) can increase very significantly if the final image is deconvolved to remove the ghost artifacts. This noise amplification is caused by divisions of small numbers, which scale the noise variance up. The scaling factor is a function of the frequency variable in the uniform sampling direction, and is small for low-frequency components and large for high-frequency components. A window function to provide an inverse scaling of data so that the noise scaling factor is canceled exactly is proposed. Since the signal energy is concentrated in the low-frequency range, it is not reduced significantly when windowing is applied. The window function is compared to the commonly used Hamming window and is shown to have good frequency response. The algorithm has been tested with computer simulations and has been verified to be able to raise the signal-to-noise ratio by 50% in the final image.

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“…(n) is a point spread function (PSF) and is given by p,(n) = -j sin (nAv)6(n + N / 2 ) + 6(n) + j sin (nAv)6(n -N / 2 ) ( 5 ) and…”

Section: Generalized Fourier Transform For Echomentioning

confidence: 99%

“…Since the phase accumulation due to chemical shift is a linear continuous function of time, it is impossible to create step phase changes in the chemical shift direction. Therefore, when using EPI for chemical shift imaging, one has always to deal with a zig-zag trajectory [4], [ 5 ] .…”

Section: Introductionmentioning

confidence: 99%

Noise reduction in MR echo planar image reconstruction

Yan

Mao

1991

IEEE Trans. Med. Imaging

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“…This combination of phase encoding and echo planar imaging is known as PEEP. 6 The improvement in total scan time is not as dramatic as in the EPSM approach but the spatial resolution and SNR are much improved. Recently, this approach has been successfully used for measurement of time-dependent regional distribution of brain metabolites.…”

Section: Introductionmentioning

confidence: 96%

REST‐PEEP: reduced scan time phase‐encoded echo planar imaging

Guilfoyle

Hrabě

2003

NMR in Biomedicine

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Echo planar methodology can be used for rapid acquisition in three dimensions of k-space. There are two groups of sequences which have thus far been implemented. The first group is characterized by three-dimensional acquisition from a single RF excitation. Echo planar shift mapping and echo volume imaging are single-shot chemical shift imaging and three-dimensional spatial imaging techniques, respectively. Even though these methods are extremely fast, their spatial and spectral resolutions are poor. In the second group of sequences, echo planar imaging acquisition is repeated for every phase-encode gradient step to improve upon the spatial resolution and signal-to-noise ratio at the expense of acquisition time. Here we report a novel combination of these two groups of sequences aimed at providing additional flexibility in trade-offs between the spectral bandwidth, signal-to-noise ratio, spatial resolution and imaging time. Preliminary results for both chemical shift imaging and three-dimensional spatial imaging are presented. The chemical shift imaging was optimized for excitation of the N-acetyl aspartate (NAA). An interesting consequence of our approach is that the Nyquist ghost is transferred from the spectrum to the metabolite image.

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“…Also, a large number of fast techniques which acquire N x 1 N s data points per excitation fections. Following the 180Њ pulse, the magnetization is refocused using a periodically oscillating field gradient G x prohave been reported (11)(12)(13). This sliced-k-space approach means that it is necessary to perform N y independently phase-ducing M echo trains, all of which contain N echoes (spaced T from each other) corresponding to N excitation RF pulses; encoded excitations.…”

mentioning

confidence: 99%

“…The presence of different chemical species provides continuous phase encoding of the chemical shift: the delay between the different gradient echo trains and the time origin (center of the Hahn spin echo) is increased. This chemical-shift encoding procedure is equivalent to that described as PEEP in (11).…”

mentioning

confidence: 99%