Tomoki Amemiya^{1}, Suguru Yokosawa^{1}, Yo Taniguchi^{1}, Ryota Sato^{1}, Hisaaki Ochi^{1}, and Toru Shirai^{1}

We propose a computational method for obtaining T2-weighted images from maps of proton density, T1, and T2* acquired by 3D RF-spoiled gradient echo. The proposed method uses a predetermined polynomial that approximates the relationship between the MR parameters and the intensity of T2WI on the basis of datasets of other subjects. Similarities between computed images and actually scanned images were improved compared with a computation method using T2* instead of T2 in the theoretical equation of the spin echo signal.

A three-dimensional RF-spoiled gradient echo sequence was performed on two healthy volunteers using a 3T MRI system (Hitachi, Ltd., Japan) and a 32-channel head coil. Seventeen images were obtained with different scan parameters (FA, TR, TE, and phase increment of RF (θ)), as shown in Table 1. The measurement resolution was 0.9×1.4×2.0 mm^{3}, and the total acquisition time was five minutes. Data from the volunteers were obtained according to the standards of the internal review board of the Research & Development Group, Hitachi, Ltd., following receipt of written informed consent.

Maps of proton density, B1, T1, and T2* were obtained from the scanned images using a previously developed method^{1, 2} that uses the method of least squares to fit a signal equation based on a Bloch simulation. A T2WI was calculated by the following steps, also shown in Figure 1. (a) Coefficients of a 7-order polynomial that calculates intensity from PD, T1, and T2* were determined by using a dataset of parameter maps and a T2WI actually scanned at the same position of a subject by using a fast spin echo (FSE) sequence. The least squares method was used to calculate the coefficient of the polynomial. (b) The T2WI of the other subject was calculated using the predetermined polynomial and parameter maps of the subject.

We compared the proposed method with other calculation method that calculates the theoretical intensity of spin echo by considering T2* as T2 in the following signal equation.

$$I_{T2W}=PD\cdot\{1-\exp(-TR/T1\ )\}\cdot\exp(-TE /T2)$$

Here, TR and TE were the same as in the actually
scanned (reference) T2WI (TE = 84 ms, TR = 4.3 s). The similarity between each
computational T2WI and reference T2WI by using an FSE sequence were evaluated
by analyzing the zero mean normalized cross correlation (ZNCC) of mean intensity
in 24 manually settled regions of interest (ROI) and by analyzing the ZNCC of
intensity in all voxels of a slice that include principal tissues in the brain
(ZNCC_{ROI} and ZNCC_{voxel}, respectively).

1. Taniguchi Y, Yokosawa S, Bito Y. Simultaneous T1, T2, and B1 Mapping Using Partially RF-Spoiled Gradient Echo. Proc. ISMRM, 2011; 4560.

2. Yokosawa S, Taniguchi Y, Amemiya T, et al. Fast fitting method for simultaneously quantifying multiple MR parameters using local optimization method with predetermined initial values. Proc. ISMRM, 2017; 1441.

3. Warntjes JBM, Leinhard OD, West J, Lundberg P. Rapid magnetic resonance quantification on the brain: optimization for clinical usage. Magn Reson Med, 2008; 60; 320–329.