Introduction

A synthetic MRI (Syn.MRI), that generates several kinds of contrast-weighted images by synthesizing quantitative parameter maps of proton density (PD), T1-longitudinal relaxation time (T1), and transverse relaxation time (T2) [1] obtained using acquired fast-spin echo (FSE) data, is beginning to be used clinically because of the advantages of saving total examination time and the freedom to select MRI acquisition parameters of repetition time (TR), inversion time (TI), and echo time (TE). However, a synthetic T2-weighted (T2W) Fluid-attenuation inversion recovery (FLAIR) introduces hyper intense artifacts at the border zone of tissue and ventricle or the surface of the brain, and it is considered that the cause is partial volume effects (PVE) of brain tissue and cerebral spinal fluid (CSF) [2,3]. To solve those artifacts, several methods were proposed [4,5]. However, those methods were not essential or required complicated procedure.
In contrast, a synthetic MRI combined with T2-based water suppression technique (Wsup-Syn.MRI) was proposed [6]. However, in exchange for reducing CSF-PVE artifacts, the tissue SNR was reduced by ~ 1/√2 for the standard technique by subtracting long-TE image from standard images; and also it was difficulty in optimal suppression.
The purpose of this study was to propose and assess a modified Wsup-Syn.MRI without loss of tissue SNR.

Methods

Theory
When a unit voxel consists of two components of water and tissue, MR signal in the voxel is based on a two-compartment model [6]. Our proposed water suppression was based on the technique of subtracting additionally acquired long TE data of water signal dominant from the shorter TE data [7]. A processing flow for our proposed technique of water-suppressed synthetic MRI using minimum number of acquired images is shown in Fig. 1 for whole, and the detail of “Water suppression for acquired images” is as follows (Fig, 2).
a) TE-dependent Water Scaling: To obtain optimal signal intensity (S.I.) of water considering T2 decay, TE-dependent scaling factor (α(TE_n)) was obtained after measuring a pure water S.I., S_w(TE_n) in n’th TE, TE_n, followed by divided by a long-TE S.I., S_w(TE_long) as: α(TE_n)=S_w(TE_n)/S_w(TE_long).
b) Spatially-dependent Water Masking: Since the S.I. of long-TE image is assumed to be proportional to water volume, V_w , the S.I. after normalizing by the pure water S.I. ( e.g. maximum) in long-TE image is regarded to be a V_w map. A spatially-dependent water mask (Mask) is generated from long-TE image by setting a threshold of V_w (ThV_w) followed by applying spatial filter to smooth the boundary, where the ThV_w must be decided considering the S.I. of water and tissue noise level.
c) Finally, water suppressed acquired images are obtained by subtraction from the standard image with weighting to S_w(TE_long) using α(TE_n) and Mask.

MRI Experiments
In MR experiments, the first four data in Fig. 1 were acquired for our proposed Wsup-Syn.MRI. A healthy volunteer study was performed on a MRI (‘Galan 3T ZGO’, Canon Medical Systems corp., Otawara, Japan) with a 32-channel head coil after obtaining informed consent. A fast spin echo sequence was used and the acquisition parameters were: parallel imaging (SPEEDER) of speed-up factor 2, acquisition matrix of 256 x 256, display matrix of 512 x 512 after sinc interpolation, FOV=23cm, slice thickness=5mm, the number of slices was selected at maximum, NAQ=1, TR1=10000ms, TE1=20ms, TE2=100ms, TElong=500ms, TI1=1000ms, an adiabatic inversion pulse was used for IR to reduce B1 inhomogeneity. Quantitative maps and synthetic contrast images of SE and FLAIR were compared between masking subtraction and uniform subtraction.

Results

For the effects of our proposed Wsup-images with masking compared with the Wsup-images without masking, CSF signals were comparably suppressed and the tissue SNRs were increased by 50% by selecting optimal ThV_w value (Fig. 3 and Table 1). For the Wsup-synthetic T2W images of SE and FLAIR, signal intensities in CSF portions were reduced almost optimally and the tissue SNRs were kept comparably as the standard synthetic images (Fig. 4). Furthermore, for synthetic-images, the gray-white matter contrast ratio for our proposed masking subtraction was ~5% better than those for spatially uniform subtraction and equivalent to those for the standard (non-subtraction) method. (Table 1).

Discussion

We proposed a modified water suppressed synthetic MRI (Wsup-Syn.MRI) technique to optimally reduce the CSF-PVE artifacts in conventional synthetic MRI without loss of tissue SNR by applying a spatial mask to separate water and tissue portions when subtraction, while the CSF-PVE artifacts were almost perfectly reduced and the tissue SNR and GM-WM contrast in quantitative maps and synthetic images of SE and FLAIR were kept. Our proposed Wsup-Syn.MRI is very simple and easily combined to the conventional Syn.MRI technique and has a backward compatibility. Our proposed Wsup-Syn.FLAIR will be easily applied to clinical use. Although T2-base Wsup-technique [7] has still not generally applied clinically despite the advantage of tissue SNR. Since our proposed Wsup-Syn.MRI can compare both easily just by calculation, it is a possibility to widely applied in future.

Conclusion

Although further optimization of pulse-sequence and processing techniques, and clinical assessments especially for long T2 lesion, our proposed synthetic MRI technique with T2-based water suppression without loss of tissue SNR will solve the problem of CSF-PVE artifacts in current synthetic FLAIR and is expected to become clinically further useful.

Acknowledgements

We express our sincere thanks to Yuki Takai and Ryo Shiroishi in Canon Medical Systems corporation for supporting data acquisition and analysis in this study.