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Tissue Border Enhancement by inversion recovery MRI at 7.0 Tesla

Abstract

Introduction

This contribution presents a magnetic resonance imaging (MRI) acquisition technique named Tissue Border Enhancement (TBE), whose purpose is to produce images with enhanced visualization of borders between two tissues of interest without any post-processing.

Methods

The technique is based on an inversion recovery sequence that employs an appropriate inversion time to produce images where the interface between two tissues of interest is hypo-intense; therefore, tissue borders are clearly represented by dark lines. This effect is achieved by setting imaging parameters such that two neighboring tissues of interest have magnetization with equal magnitude but opposite sign; therefore, the voxels containing a mixture of each tissue (that is, the tissue interface) possess minimal net signal. The technique was implemented on a 7.0 T MRI system.

Results

This approach can assist the definition of tissue borders, such as that between cortical gray matter and white matter; therefore, it could facilitate segmentation procedures, which are often challenging on ultra-high-field systems due to inhomogeneous radiofrequency distribution. TBE allows delineating the contours of structural abnormalities, and its capabilities were demonstrated with patients with focal cortical dysplasia, gray matter heterotopia, and polymicrogyria.

Conclusion

This technique provides a new type of image contrast and has several possible applications in basic neuroscience, neurogenetic research, and clinical practice, as it could improve the detection power of MRI in the characterization of cortical malformations, enhance the contour of small anatomical structures of interest, and facilitate cortical segmentation.

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Acknowledgments

The authors would like to thank the nurse of the 7 T MR facility, Ms. Arianna Sugamosto, and all subjects who participated in this study. This work was supported by grant “Prog. 133/11” (to R.G.) approved by the Italian Ministry of Health and funded by Fondazione Pisa. The research leading to these results has received funding from the European Union Seventh Framework Programme FP7/2007-2013, project DESIRE (grant agreement no. 602531).

Ethical standards and patient consent

We declare that all human and animal studies have been approved by the ethics committee of Pisa University General Hospital (Comitato Etico dell'Azienda Ospedaliero-Universitaria Pisana) and have therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. We declare that all patients gave informed consent prior to inclusion in this study.

Conflict of interest

We declare that we have no conflict of interest.

Author information

Correspondence to Mauro Costagli.

Appendix

Appendix

The optimal TI for TBE (TITBE) can be determined for any pair of neighboring tissues of interest by imposing the following relationship:

$$ -{S}^{(1)}\left({\mathrm{TI}}_{\mathrm{TBE}}\right)={S}^{(2)}\left({\mathrm{TI}}_{\mathrm{TBE}}\right) $$
(1)

where S (1) and S (2) are the polarity-restored signal intensities of the two tissues of interest. In the simplest scenario where a standard SE-IR sequence is employed (as in the example in Fig. 1), the signal intensities can be approximated by the following:

$$ {S}^{(1)}\propto {\mathrm{PD}}^{(1)}\cdot \left(1-2{e}^{-\mathrm{TI}/T{1}^{(1)}}\right)\left(1-{e}^{-\mathrm{TR}/T{1}^{(1)}}\right)\cdot {e}^{-\mathrm{TE}/T{2}^{(1)}} $$
(2)

where PD(1), T1(1), and T2(1) indicate proton density, longitudinal relaxation time, and transverse relaxation time, respectively, of one of the two tissues. By substituting Eq. (2) into Eq. (1), it follows:

$$ -P{D}^{(1)}\cdot \left(1-2{e}^{-{\mathrm{TI}}_{\mathrm{TBE}}/T{1}^{(1)}}\right)\left(1-{e}^{-\mathrm{TR}/T{1}^{(1)}}\right)\cdot {e}^{-\mathrm{TE}/T{2}^{(1)}}={\mathrm{PD}}^{(2)}\cdot \left(1-2{e}^{-{\mathrm{TI}}_{\mathrm{TBE}}/T{1}^{(2)}}\right)\left(1-{e}^{-\mathrm{TR}/T{1}^{(2)}}\right)\cdot {e}^{-\mathrm{TE}/T{2}^{(2)}} $$
(3)

where (1) and (2) refer to the two tissues of interest.

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Costagli, M., Kelley, D.A.C., Symms, M.R. et al. Tissue Border Enhancement by inversion recovery MRI at 7.0 Tesla. Neuroradiology 56, 517–523 (2014). https://doi.org/10.1007/s00234-014-1365-8

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Keywords

  • Anatomical imaging
  • Inversion recovery
  • Tissue interface
  • Cortical malformations
  • Ultra-high-field MRI