Original contribution
Resolution recovery in Turbo Spin Echo using segmented Half Fourier acquisition

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Abstract

Turbo Spin Echo (TSE) is a sequence of choice for obtaining T2-weighted images. TSE reduces acquisition time by acquiring several echoes within each TR, at the cost of introducing an exponential weighting in the k-space that leads to a certain image blurring. This is particularly important for short-T2 structures, which can even disappear if their size in the phase encoding direction is comparable to the degree of blurring. This article suggests the use of a combination of Half Fourier (HF) and segmented (multishot) TSE (sHF-TSE) to recover the original resolution of the SE images. The improved symmetry of the dataset achieved by HF reconstruction is used to increase the resolution of the TSE images. The proposed combination, available in most clinical scanners, reduces the blurring artifact inherent to the TSE sequence without increasing the scan time or the number of acquisitions, but at the cost of a slight reduction of the signal-to-noise ratios (SNR). Qualitative and quantitative results are presented using both numerical simulation and imaging. Significant edge enhancement has been achieved for structures with short T2, (narrowing of the full width at half maximum [FWHM] up to 45%). The proposed sequence is more sensitive to movement artifacts but has proven to be superior to the conventional TSE for imaging static structures.

Introduction

The most widely used clinical magnetic resonance imaging techniques for the diagnosis of parenchymal disease employ heavily T2-weighted sequences to detect long T2 components in tissue. But tissues also contain short T2 components that are not detected or only poorly detected with conventional sequences. These components are the majority species in tendons, ligaments, menisci and other related tissues, and the minority in many other tissues that have predominantly long T2 components [1]. In this work we propose to use the properties of the segmented Half Fourier Turbo Spin Echo (sHF-TSE) sequence as a strategy to increase the spatial resolution of T2 weighted images and, in particular, the conspicuity of short T2 structures.

Turbo or Fast Spin Echo (TSE, FSE) [2], formerly RARE (Rapid Acquisition with Relaxation Enhancement), is a faster alternative to the classical spin echo (SE) sequence [3] proposed by Hennig et al. [4] for the acquisition of T2-weighted images. The TSE sequence shortens the acquisition time by filling in several lines of the k-space in each shot or repetition time (TR). The number of echoes (k-space lines) acquired within each TR is called echo train length (ETL) and typically ranges from 3 to 30 [5]. The k-space thus becomes divided into segments, each one containing the echoes acquired at the same TE [6], [7]. The number of segments corresponds to the number of echoes in the ETL.

The particular phase encoding strategy followed in TSE can significantly influence the quality of the images, since the echo amplitude is modulated as a function of its position in the k-space: each segment becomes weighted according to the TE of their constituent echoes. The effects of this modulation in TSE imaging are ghosting and blurring artifacts that lead to a resolution loss in the phase encoding direction. These problems are particularly severe for short-T2 small objects that span only a few pixels in the phase encoding direction imaged using long echo train lengths; these structures may even disappear if the image is acquired using a long enough ETL [8].

Several phase encoding strategies for TSE imaging have been proposed to minimize these undesired effects [9], [10], obtaining either edge-enhanced images, when high spatial frequencies are collected at the early echoes, or blurred images with reduced truncation artifact, when the high spatial frequencies are collected at late echoes.

An alternative approach suggested to reduce the effects of the T2 modulation of the k-space is the use of postprocessing techniques [11]. Most of these procedures, however, require additional data acquisitions to average the signal decay for the echo train, and the resulting images show lower signal-to-noise ratios (SNR) than the original ones. Another method proposed to reduce the modulation of the echo amplitudes is the use of a small flip angle refocusing pulse [12]. This technique generates a series of echoes that represent a complex combination of spin echoes and stimulated echoes. The resulting images show mixed T1–T2 contrast and also lower SNR than the images acquired by the standard TSE technique.

This work proposes to exploit the properties of the sHF-TSE sequence to reduce the effect of the k-space modulation in TSE images. The sHF-TSE sequence is a Turbo Spin Echo sequence in which the final echoes of each echo train are calculated from the initial ones instead of acquired. This sequence increases the symmetry of the echo train, compensating for the T2 decay at the second half of the echo train, and giving rise to images with higher resolution. Segmented Half Fourier imaging can be implemented with routine software in most clinical scanners, which makes this strategy easy to be adopted.

This work includes a systematic comparative study between the sHF-TSE and its equivalent T2-weighted TSE sequence, based both on numerical simulation and MRI datasets obtained from both phantoms and patients.

Section snippets

Segmented Half Fourier in Turbo Spin Echo: theoretical basis

Half Fourier (HF) or partial Fourier imaging is a reconstruction method that allows the reduction of imaging time by up to almost 50%. Using HF technique, the entire MR image is reconstructed from as few as one-half of the number of phase-encoding steps. The lacking k-space data are calculated as the symmetric complex conjugate of each element with respect to the origin (center) of the matrix (Hermitian symmetry). In practice, image data sets always contain some phase errors that prevent a

Evaluation

The sHF-TSE sequence has been characterized by measuring the signal level and blurring on three types of images: software phantoms, true images of physical phantoms, and patient images.

Software phantom images allow the study of the effects derived purely from the phase encoding strategy, excluding any other influence of the imaging system. In particular, a one-pixel-line phantom (delta phantom) provides the point spread function (PSF)1

Results

The sHF-TSE sequence proposed has been analyzed in terms of blurring, signal level and ringing, according to the percentage of acquired data and the echo train length used to generate or acquire the images, as explained in the previous section.

Discussion

The study of the combination of TSE sequences with Half Fourier techniques has been classically centered on the particular case known as HASTE (Half Fourier Acquisition Single Shot Turbo Spin Echo) [24], [25]. The HASTE sequence fills half of the k-space in one single shot, thus being a very fast sequence generally used to acquire relatively low resolution images, either for breath-hold abdominal imaging or for fast scanning of children or uncooperative patients.

This article proposes the use of

Conclusion

The use of TSE secuences for the acquisition of heavily T2-weighted images produces a severe blurring of short-T2 small objects that became hardly conspicuous or may even disappear from the image. The segmented Half Fourier TSE (sHF-TSE) sequence proposed in this paper improves image quality, producing sharper T2 images. Significant edge enhancement (up to 45% decrease in FWHM) with respect to a full acquisition has been achieved for pA = 60% and T2 = 30 ms (equivalent to muscle). Different

Acknowledgements

This work was partially funded by grants FIS-00/36, Red Temática IM3 (FIS) and III-PRICIT (Comunidad de Madrid).

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