In this study, we developed a new dual echo sequence that enables us to acquire both 3D T1- and T2*-weighted images in the optimal conditions simultaneously. We optimized flip angles for 3D T1 and T2*-weighted imaging by conventional dual-echo in vivo experiments and computer simulations, and then implemented a dual-echo sequence with an echo-specific K-space reordering scheme to satisfy the optimal flip angles for both T1 and T2* contrast. The echo-specific k-space reordering scheme with high flip-angle changes between k-space regions caused ringing artifacts. We also proposed two strategies to suppress the ringing artifacts in the dual echo sequence with the echo specific k-space reordering scheme: (i) implementing smooth transition regions between the k-space regions of the abrupt flip angle jumps and (ii) discarding the transition regions as dummy PE steps. The proposed dual-echo scheme with ringing artifact suppression achieved optimal contrast for both T1 and T2* simultaneously with no artifacts, and thus may potentially be applicable to routine clinical applications for simultaneous high resolution T1 and T2*-weighted imaging.
Furthermore, dynamic acquisition using the proposed technique combined with compressed sensing was performed to test the feasibility which is potentially applicable to simultaneous acquisition of dynamic contrast enhanced (DCE) and dynamic susceptibility contrast (DSC) images with single shot injection of contrast agent. The temporal resolution was reduced to ~4 sec, because both DCE and DSC need high temporal resolution. Short scan time per measurement in the dynamic scan not only produces artifacts in images, but also changes the image contrast, since k-space center of each echo was not acquired in the steady state. To suppress artifacts in down-sampled image and achieve image contrast similar to that of the full sampled data, we acquired the k-space center at a later phase encoding steps that are closer to the steady state. The proposed scheme efficiently suppressed the ringing artifact in the image and produced the image contrast similar to the full sampled images. We could achieve simultaneous acquisition of T1 and T2* weighted images dynamically with temporal resolution of 4 sec and reasonable spatial coverage by increasing acceleration factor up to 8, which can potentially be applicable to simultaneous acquisition of DCE and DSC,. However, improving the temporal resolution below 4 s sacrificed the image contrast because the steady state could not be reached during the whole scan. Further study is necessary to improve the temporal resolution further without distorting image quality and thus to exploit accurate parameters of DCE and DSC.
현재 임상에서 뇌 질환을 진단하기 위해 주로 CT와 MR같은 비침습적 영상장비를 이용한다. CT는 MR에 비해 비교적 촬영시간이 짧아서 많이 이용되고 있지만, 방사선을 이용해 촬영하므로 환자를 위험에 노출시킨다. 반면, MR은 촬영시간은 길지만 자기장을 이용해서 촬영하기 때문에 환자의 안전이 보장된다. 따라서 MR의 촬영시간을 단축시킨다면 많은 이점을 가져온다. 보통 MR로 T1영상이나 T2*영상을 얻어서 뇌 질환을 진단한다. 이 두 영상은 반대되는 대조를 지니고 각각 다른 정보를 제공하므로 동시촬영이 가능해지면 더욱 정확한 진단이 가능해진다. Dual echo 기법을 이용하면 두 영상의 동시촬영이 가능하지만 T1영상과 T2*영상을 얻기 위해서는 상반된 parameter들을 사용해야 하기 때문에 동시촬영은 불가능하다고 여겨져 왔다.
본 연구에서는 echo specific k-space reordering scheme과 dual echo기법을 접목시켜 T1과 T2*영상의 동시촬영을 가능하게 하고 k-space reordering scheme응 이용함으로써 발생하는 artifact 제거 방법 두 가지를 제안한다. 또한 이 기법을 compressed sensing과 접목시켜 DCE와 DSC 동시촬영을 가능하게 함으로써 촬영시간 단축뿐만 아니라 환자에게 위험성이 있는 조영제 투여 양을 줄일 수 있게 한다.
