Synopsis

High resolution edema image is highly desired for reducing particle volume effect (PVE) on thin myocardium. In this work, we proposed a normalized segmented T₂STIR-bSSFP to improve base resolution from 192 to 320 in edema imaging as well as maintain high contrast of edema to myocardium. The results show that the proposed method can reduce PVE on thin myocardium and makes edema imaging for the right ventricle myocardium being possible.

INTRODUCTION

METHODS

Theory:

The single-shot T₂STIR-bSSFP was extended to segmented acquisition for achieving high resolution imaging. Since T₁ and T₂ values of edema are both elevated in T₂STIR-bSSFP, directly extending to the segmented version will reduce the contrast between edema and normal myocardium, due to insufficient recovery of longitudinal magnetization. Here, three strategies were proposed to maintain the contrast (see Figure 1), (1) 2 RR intervals were used for magnetization recovery between segments; (2) a saturation pulse was applied immediately after T₂STIR-bSSFP readout. This strategy accelerates the T₁ relaxation, since the longitudinal magnetization was still negative when readout was completed; (3) a low flip angle bSSFP readout was inserted in the next RR intervals to acquire a reference image. After these strategies, readout for each segment was adjusted to the same cardiac phase. Normalized image was obtained by dividing the T₁&T₂-weighted image by the reference image. The number of segments was limited to 4 for minimizing breath-holding time and the first segment was discarded. Finally, a total of 11 RR intervals were acquired.

Experiment:

Simulations: Bloch equation simulations were used to calculate the contrast in single-shot and segmented T₂STIR-bSSFP sequence with T₁/T₂ for normal myocardium (T₁/T₂=1149/44ms) and edema (T₁/T₂=1503/69ms) using MATLAB (MathWorks, USA). TE_prep of the T₂STIR module varied from 40ms to 70ms. RR interval=800ms. Other parameters are listed in Table 1(a).

Phantom experiments: Phantoms of myocardium and edema were prepared using two difference mixtures of agarose and NiSO₄·6H₂O to approximate the above T₁ and T₂ values. Phantoms were imaged with TE_prep from 40ms to 70ms with step 5ms. Imaging parameters are identical to the simulations.

In-vivo experiments: These sequences were performed on a 3T clinical MR system (TIM TRIO, Siemens, Germany). A 32-elements cardiac phased-array coil was used. TE_prep=60ms. Parallel imaging was used for 2× acceleration. Besides, T₁ map⁶ and T₂ map⁷ were performed to show the elevated T₁ and T₂ values in edematous myocardium. The experimental parameters are depicted in Table 1(b).

RESULTS

Figure 2 depicts the contrast ratio and intensity difference during all TE preparation times in simulation and phantom experiments. Contrasts in the normalized segmented method are better than that in the single-shot acquisition method. Figure 3 summarizes results from the phantom experiments when TE_prep=60ms.

In patient studies, we chose TE_prep=60ms to contain more T₂-weighted information for better characterizing pathological changes of myocardium. Figure 4 shows a patient case. Green arrow in normalized image indicates edema at anteroseptal and inferoseptal segments in agreement with T₁ and T₂ maps. Though subtle difference between edema and myocardium is demonstrated in T₂ map, intensity of edema is higher in normalized segmented image, compared to the remote myocardium.

DISCUSSION

The normalized method indeed improves image contrast compared to the single-shot method, as shown in Figure 2 and 3. In this study, contrast ratio instead of CNR was used to estimate the tissue contrast, since the scale of intensity has changed in normalized signals. Contrast ratio eliminates the influence of signal scale and the dependence on noise, therefore it can be used to estimate contrast and compare the two methods in the same TE preparation time.

Though no edema image using the single-shot method was shown here, as observed from the virtual boxes in T₁ map and T₂ map (see Figure 4), we expect blurring occurred near the edge of the heart in single-shot T₂STIR-bSSFP due to insufficient spatial resolution. Since base resolution has been improved to 320, edema imaging for the RV is possible with voxel size on the order of 1 mm.⁵

CONCLUSION

Acknowledgements

References

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