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Measurement of the Diffusion-Selective T2 value in the Human Brain Using Diffusion-Weighted T2 map
Takuya Aoike1, Noriyuki Fujima2, Masami Yoneyama3, Suzuko Aoike1, Hiroyuki Sugimori4, Kinya Ishizaka1, and Kohsuke Kudo2

1Department of Radiological Technology, Hokkaido University Hospital, Sapporo, Hokkaido, Japan, 2Department of Diagnostic and Interventional Radiology, Hokkaido University Hospital, 3Philips Electronics Japan, 4Faculty of Health Sciences, Hokkaido University

Synopsis

The purpose of this study was to assess the variation of T2 values by T2 map with the pre-pulse of diffusion gradient (=DW-T2 map). T2 value of GM was gradually decreased as b-value increased. From this result, water diffusivity based selective removal of tissue component such as perfusion fraction or extracellular extravascular space can be obtained using DW-T2 map technique. This technique can reveal the more detain in the human brain tissue characteristics.

Introduction

The quantitative T2 value is important parameter which reflects the tissue specificity related to the tissue transverse relaxation. Measurement of T2 value enables to quantitative evaluation for the property of numerous substances. Clinical utility of quantitative T2 value was previously reported for the evaluation of diseases such as metastatic brain tumors, Sturge-Weber syndrome and multiple sclerosis.1 In general, T2 map can be obtained by multiple echo-time (TE) acquisitions with pixel by pixel basis. However, there are various components in each pixel such as the intracellular space, extracellular space and perfusion fraction. Measured T2 value in each pixel by conventional pixel by pixel basis T2 map was considered to be obtained as a mixed and averaged value between various T2 values in multiple components in the brain tissue. In contrast, diffusion gradient as a use for pre-pulse can result in signal loss according to the degree of the water diffusivity in various components, introduced as diffusion-weighted image (DWI). By using various degree of diffusion gradient (i.e. various b-values) as pre-pulse, roughly separation of diffusion based tissue component such as perfusion and non-perfusion related signal, or intracellular and extracellular space may be accomplished. Therefore, it is expected that T2 map with the pre-pulse of diffusion gradient can provide selectively T2 value based on multiple tissue component by using the various b-values. The purpose of this study was to assess the variation of T2 values by T2 map with the pre-pulse of diffusion gradient (=DW-T2 map).

Methods

1) Subjects

Three healthy volunteers with no history of the past brain disease were included in this study. Details of these volunteers were as follows; 3 male, age, 26 years in all subjects.

2) MR scanning

All subjects were scanned on a 3 Tesla MR scanner (Achieva TX; Philips Healthcare, Best, The Netherlands) with a 8-channel head coil. DW-T2 map used spin-echo echo-planar imaging (SE-EPI) with TR=4500ms, TE1=55ms, TE2=160ms, slice-thickness=5mm, matrix=128×128, FOV=240×240mm. The scanning range of DW-T2 map acquisitions were planned to cover the whole brain including the cerebellum at the lower end. Scans with b-values (s/mm2) = 0, 200, 500, 1000 and diffusion gradient was acquired after the 90° excitation pulse and 180° refocusing pulse (Fig. 1). Conventional T2 map was also acquired using the multiple echoes by SE readout.

3) Data analysis

In each T2 map, manual regions of interest (ROI) was placed bilaterally on gray matter (GM) and white matter (WM) in level of the evenly basal ganglia,2 after that, each T2 value of GM and WM in each ROI was determined as mean value in the ROI (Fig. 2). Twenty ROIs were placed in a subject. Finally, total sixty ROIs from three subjects were analyzed.

4) Statistical analysis

Analysis 1

The obtained T2 value in each ROI (GM and WM) was compared between DW-T2 map and conventional T2 map using Wilcoxon rank sum test.

Analysis 2

The obtained T2 value by DW-T2 map in each ROI (GM and WM) was compared between various b-values (0, 200, 500 and 1000) using multiple comparison (Steel-Dwass’ test with a bonferroni correction).

Statistical significance was set to p<0.05 for all tests.

Results

Analysis 1

In GM ROI, T2 value of conventional method (90±8) was significantly lower than T2 value obtained by DW-T2 map (96±12) (p<0.05). In contrast, T2 value in WM ROI of conventional method (76±8) was significantly higher than T2 value obtained by DW-T2 map (71±12) (p<0.05) (Table. 1).

Analysis 2

In GM ROI, as b-value increased, T2 value by DW-T2 map was linearly decreased (p<0.05). In WM ROI, T2 values by DW-T2 map were not differ significantly among all b-values (Fig. 3).

Discussion and Conclusion

In the comparison of T2 value between conventional and DW-T2 map, the difference of T2 value might be affected by the difference of readout. In the comparison of T2 value among various b-values in DW-T2 map, T2 value of GM was gradually decreased as b-value increased. From this result, water diffusivity based selective removal of tissue component such as perfusion fraction or extracellular extravascular space can be obtained using DW-T2 map technique. This technique can reveal the more detain in the human brain tissue characteristics.

Acknowledgements

No acknowledgements

References

1. Hagiwara A, Hori M, Yokohama K, et al. Utility of a Multiparametric Quantitative MRI Model That Assesses Myelin and Edema for Evaluating Plaques, Periplaque White Matter, and Normal-Appearing White Matter in Patients with Multiple Sclerosis: A Feasibility Study. AJNR Am J Neuroradiol originally published online on october 27, 2016, 10.3174/ajnr.A4977.

2. Wansapura JP, Holland SK, Dunn RS , et al. NMR relaxation times in the human brain at 3.0 tesla. Journal of magnetic resonance imaging, 1999, 9.4: 531-538.

Figures

Figure 1. Pulse sequence diagram of dual-echo DWI readout for DW-T2 map. Bipolar diffusion gradient was inserted as a pre-pulse of T2 map acquisition. To obtain the dual-echo of different TE, this technique uses two 180° refocusing pulses for one 90° excitation pulse.


Figure 2. Example of the manual placement of region of interest (ROI); the ROIs were placed bilaterally on gray matter (GM) and white matter (WM) on the level of the basal ganglia [circles (GM) and squares (WM), respectively].

Table 1. In both gray matter (GM) and white matter (WM), T2 value obtained by DW-T2 map among various b-values were presented.

Figure 3. The source images of DW-T2 map were acquired using first TE (55ms) (a), and second TE (160ms) (b). Finally, DW-T2 map was created by these two images (c).

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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