고자장 MRI에서의 영상 영역에 대한 B₁⁺ 균질성

김홍준(Hongjoon Kim),손혁우(HyeokWoo Son),조영기(Youngki Cho),유형석(Hyoungsuk Yoo) 한국전자파학회 2012 한국전자파학회논문지 Vol.23 No.1

정 자장(B?)의 세기가 7 T(Tesla) 또는 9.4 T 고자기장 MRI(Magnetic Resonance Imaging) 시스템은 정 자장의 세기가 1.5 T 또는 3 T MRI(Magnetic Resonance Imaging) 시스템에 비하여 인가된 RF(Radio Frequency) 필드의 높은 불균질성을 보여준다. 다채널(multi-channel) RF 코일에서는 인가된 RF 자장(B₁?)의 불균질성을 개선시키기 위해서 각각의 코일 소자(element)에 인가되는 전류의 크기와 위상을 독립적으로 조절할 수 있다. 선택된 관심 영역에서의 RF 자장이 균일하도록 RF 코일의 각 요소로 들어가는 최적화된 전류의 크기와 위상 값을 얻기 위해서 iterative 방법과 함께 convex 최적화 방법이 사용된다. 이러한 방법을 입증하기 위하여 9.4 T MRI 시스템에 RF 코일의 공진 주파수가 400 ㎒을 가지는 다채널 전송 선로 코일이 인체 두상 모형과 함께 모델링되었으며, 이 코일에 의하여 자장이 얻어진다. 9.4 T MRI 시스템을 위한 시뮬레이션 결과가 자세히 논의된다. In high static field magnetic resonance imaging(MRI) systems, B? fields of 7 T and 9.4 T, the impressed RF field shows larger inhomogeneity than in clinical MRI systems with B0 fields of 1.5 T and 3.0 T. In multi-channel RF coils, the magnitude and phase of the input to each coil element can be controlled independently to reduce the non-uniformity of the impressed RF B₁? field. The convex optimization technique has been used to obtain the optimum excitation parameters with iterative solutions for homogeneity in a selected ROI(Region of Interest). To demonstrate the technique, the multichannel transmission line coil was modeled together with a human head phantom at 400 ㎒ for the 9.4 T MRI system and B₁? fields are obtained. In this paper, all the optimized B₁? in each isolated ROIs are combined to achieve significantly improved homogeneity over the entire field of view. The simulation results for 9.4 T MRI systems are discussed in detail.

Frequency-splitting dynamic MRI reconstruction using multi-scale 3D convolutional sparse coding and automatic parameter selection

Nguyen-Duc, Thanh,Quan, Tran Minh,Jeong, Won-Ki Elsevier 2019 Medical image analysis Vol.53 No.-

<P><B>Abstract</B></P> <P>In this paper, we propose a novel image reconstruction algorithm using multi-scale 3D convolutional sparse coding and a spectral decomposition technique for highly undersampled dynamic Magnetic Resonance Imaging (MRI) data. The proposed method recovers high-frequency information using a shared 3D convolution-based dictionary built progressively during the reconstruction process in an unsupervised manner, while low-frequency information is recovered using a total variation-based energy minimization method that leverages temporal coherence in dynamic MRI. Additionally, the proposed 3D dictionary is built across three different scales to more efficiently adapt to various feature sizes, and elastic net regularization is employed to promote a better approximation to the sparse input data. We also propose an automatic parameter selection technique based on a genetic algorithm to find optimal parameters for our numerical solver which is a variant of the alternating direction method of multipliers (ADMM). We demonstrate the performance of our method by comparing it with state-of-the-art methods on 15 single-coil cardiac, 7 single-coil DCE, and a multi-coil brain MRI datasets at different sampling rates (12.5%, 25% and 50%). The results show that our method significantly outperforms the other state-of-the-art methods in reconstruction quality with a comparable running time and is resilient to noise.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Convolutional dictionary reconstructs high-frequency component of MRI images well. </LI> <LI> Temporal total variation reconstructs low-frequency component of MRI images well. </LI> <LI> Multi-scale dictionary improves MRI reconstruction quality. </LI> <LI> Elastic net regularization works better than L1 or L2 regularization only. </LI> <LI> Genetic algorithm automatically finds optimal parameters for MRI reconstruction. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

3.0 T MRI를 위한 Parallel-Transmission RF 코일 구조의 비교와 최적화

오창현(Chang-Hyun Oh),이흥규(Heung K. Lee),류연철(Yeun-Chul Ryu),현정호(Jung-Ho Hyun),최혁진(Hyuk-Jin Choi) 대한전기학회 2007 대한전기학회 학술대회 논문집 Vol.2007 No.4

In high field (> 3 T) MR imaging, the magnetic field inhomogeneity in the target object increases due to the nonuniform electro-magnetic characteristics and relatively high Larmor frequency. Especially in the body imaging, the effect causes more serious problems resulting in locally high SAR(Specific Absorption Ratio). In this paper, we propose an optimized parallel-transmission RF coil element structure and show the utility of the coil by FDTD simulations to overcome the unwanted effects. Three types of TX coil elements are tested to maximize the efficiency and their driving patterns(amplitude and phase) optimized to have adequate field homogeneity, proper SAR level, and sufficient field strength. For the proposed coil element of 25 ㎝ × 8 ㎝ loop structure with 12 channels for a 3.0 T body coil, the 73% field non-uniformity without optimization was reduced to about 26% after optimization of driving patterns. The experimental as well as simulation results show the utility of the proposed parallel driving scheme is clinically useful for (ultra) high field MRI.

Two Paralleled Four Quadrant DC Chopper for Gradient Coil Magnetic Fields in MRI System

Park, Hyung-Beom,Mun, Sang-Pil,Park, Han-Seok,Woo, Kyung-Il The Korean Institute of IIIuminating and Electrica 2009 조명·전기설비학회논문지 Vol.23 No.11

This paper presents a two-paralleled four quadrant DC chopper type PWM power conversion circuit in order to generate a gradient magnetic field in the Magnetic Resonance Imaging (MRI) system. This circuit has 8-IGBTs at their inputs/outputs to realize further high-power density, high speed current tracking control, and to get a low switching ripple amplitude in a controlled current in the Gradient Coils (GCs). Moreover, the power conversion circuit has to realize quick rise/fall response characteristics in proportion to various target currents in GCs. It is proposed in this paper that a unique control scheme can achieve the above objective DSP-based control system realize a high control facility and accuracy. It is proved that the new control system will greatly enlarge the diagnostic target and improve the image quality of MRI.

Two Paralleled Four Quadrant DC Chopper for Gradient Coil Magnetic Fields in MRI System

Hyung-Beom Park,Sang-Pil Mun,Han-Seok Park,Kyung-IL Woo 한국조명·전기설비학회 2009 조명·전기설비학회논문지 Vol.23 No.11

This paper presents a two-paralleled four quadrant DC chopper type PWM power conversion circuit in order to generate a gradient magnetic field in the Magnetic Resonance Imaging (MRI) system. This circuit has 8-IGBTs at their inputs/outputs to realize further high-power density, high speed current tracking control, and to get a low switching ripple amplitude in a controlled current in the Gradient Coils (GCs). Moreover, the power conversion circuit has to realize quick rise/fall response characteristics in proportion to various target currents in GCs. It is proposed in this paper that a unique control scheme can achieve the above objective DSP-based control system realize a high control facility and accuracy. It is proved that the new control system will greatly enlarge the diagnostic target and improve the image quality of MRI.

7 T MRI에서의 다양한 RF 코일에 대한 B₁⁺ 비교

김홍준(Hongjoon Kim),유형석(Hyoungsuk Yoo) 한국전자파학회 2012 한국전자파학회논문지 Vol.23 No.8

정 자장(B?)의 세기가 7T(Tesla) 고자기장 MRI(Magnetic Resonance Imaging) 시스템은 정 자장의 세기가 1.5 T 또는 3 T MRI 시스템에 비하여 인가된 RF(Radio Frequency) 필드의 높은 불균질성을 보여준다. 이러한 문제점를 극복하기 위한 방법으로써, 관심 영역에서의 RF 자장이 균일하도록 RF 코일의 각 요소로 들어가는 최적화된 전류의 크기와 위상 값을 얻기 위해서 convex 최적화 방법이 사용된다. 이러한 방법을 7 T MRI 시스템에 다양한 RF 코일에 적용하여 각각의 B₁? 필드 값을 비교하여 그 성능을 파악하였다. This paper demonstrates the use of the convex optimization to localize the transverse magnetic B₁?field in regions of interest for recently proposed multi-sectioned alternating impedance coils and the traditional transmission line coil. An approach based on different axial slices to identical RF coils except upper stripline structure is investigated. Electromagnetic simulation results are compared for RF coils and discussed in detail at 7.0 T.

High Acceleration Three-Dimensional T1-Weighted Dual Echo Dixon Hepatobiliary Phase Imaging Using Compressed Sensing-Sensitivity Encoding: Comparison of Image Quality and Solid Lesion Detectability with the Standard T1-Weighted Sequence

남주강,이정민,이상민,강효진,이은선,허보윤,윤정희,EunJu Kim,Mariya Doneva 대한영상의학회 2019 Korean Journal of Radiology Vol.20 No.3

Objective: To compare a high acceleration three-dimensional (3D) T1-weighted gradient-recalled-echo (GRE) sequence using the combined compressed sensing (CS)-sensitivity encoding (SENSE) method with a conventional 3D GRE sequence using SENSE, with respect to image quality and detectability of solid focal liver lesions (FLLs) in the hepatobiliary phase (HBP) of gadoxetic acid-enhanced liver MRI. Materials and Methods: A total of 217 patients with gadoxetic acid-enhanced liver MRI at 3T (54 in the preliminary study and 163 in the main study) were retrospectively included. In the main study, HBP imaging was done twice using the standard mDixon-3D-GRE technique with SENSE (acceleration factor [AF]: 2.8, standard mDixon-GRE) and the high acceleration mDixon-3D GRE technique using the combined CS-SENSE technique (CS-SENSE mDixon-GRE). Two abdominal radiologists assessed the two MRI data sets for image quality in consensus. Three other abdominal radiologists independently assessed the diagnostic performance of each data set and its ability to detect solid FLLs in 117 patients with 193 solid nodules and compared them using jackknife alternative free-response receiver operating characteristics (JAFROC). Results: There was no significant difference in the overall image quality. CS-SENSE mDixon-GRE showed higher image noise, but lesser motion artifact levels compared with the standard mDixon-GRE (all p < 0.05). In terms of lesion detection, readeraveraged figures-of-merit estimated with JAFROC was 0.918 for standard mDixon-GRE, and 0.953 for CS-SENSE mDixon-GRE (p = 0.142). The non-inferiority of CS-SENSE mDixon-GRE over standard mDixon-GRE was confirmed (difference: 0.064 [-0.012, 0.081]). Conclusion: The CS-SENSE mDixon-GRE HBP sequence provided comparable overall image quality and non-inferior solid FFL detectability compared with the standard mDixon-GRE sequence, with reduced acquisition time.

Dual-source Parallel Radiofrequency Excitation ACR Phantom Magnetic Resonance Imaging at 3 T: Assessment of the Effect of Image Quality on High-contrast Spatial Resolution, Percent Signal Ghosting, and Low-contrast Object Detectability in Comparison with

이경배,박용성,최보영 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.63 No.8

The purpose of the present study was to assess dual-source parallel radiofrequency (RF) excitationAmerican College of Radiology (ACR) phantom magnetic resonance (MR) imaging at 3Tcompared with conventional single-source RF transmission and compared with the standard ACRMRI phantom test. We used a 3T MR scanner equipped with dual-source parallel RF excitationand an 8-channel head phased array coil. We employed T1- and T2-weighted fast spin echo (FSE)pulse sequences for an assessment of the impact of image quality on high-contrast spatial resolution,percent signal ghosting and low-contrast object detectability following the ACR MRI quality control(QC) manual. With geometric accuracy and identical slice locations, dual RFs using dual-sourceparallel RF excitation MR showed an advantage over single RF using dual-source parallel RF excitationMR and conventional MR in terms of high-contrast spatial resolution (p < 0.010), percentsignal ghosting (p < 0.010), and low-contrast object detectability (p < 0.010). The quality of theimage from the dual-source parallel RF excitation MR equipment was superior to that of the imagefrom conventional MR equipment for the ACR phantom. We need to pursue dual-source parallelRF excitation MR studies involving various clinical cases.

DRF-GRAPPA: A Parallel MRI Method with a Direct Reconstruction Filter

김동찬,정성원,박현욱 한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.73 No.1

A novel multi-channel image reconstruction algorithm for parallel MRI is proposed to reconstruct a complex-valued image with both magnitude and phase information in a reduced computation time, so called the DRF-GRAPPA. The DRF-GRAPPA combines multichannel images by separately obtaining magnitude information and phase information. The method is applied to parallel MRI to reconstruct an image from under-sampled k-space data by using a direct reconstruction filter that can simultaneously estimate full k-space signals and combine multi-channel images. To obtain both magnitude and phase information, the DRF-GRAPPA provides two different direct reconstruction filters. The accuracy of the magnitude and phase information is validated through simulations. In addition, the experiment results of spherical water phantom imaging and in-vivo imaging demonstrate that the DRF-GRAPPA can successfully reconstruct images with both magnitude and phase information within a reduced reconstruction time.

High-resolution fMRI with higher-order generalized series imaging and parallel imaging techniques (HGS-parallel)

Yun, SungDae,Oh, Sung Suk,Han, Yeji,Park, HyunWook Wiley Subscription Services, Inc., A Wiley Company 2009 JOURNAL OF MAGNETIC RESONANCE IMAGING Vol.29 No.4

<B>Purpose</B><P>To develop a novel approach for high-resolution functional MRI (fMRI) using the conventional gradient-echo sequence.</P><B>Materials and Methods</B><P>Echo-planar imaging (EPI) techniques have generally been used for fMRI studies due to their fast imaging time. However, it is difficult for studying brain function at the submillimeter level using this sequence. In addition, EPI techniques have some drawbacks, such as Nyquist ghosts and geometric distortions in the reconstructed images, and subsequently require additional postprocessing to reduce these artifacts. One way of solving these problems is to acquire fMRI data by means of a conventional gradient-echo imaging sequence instead of EPI. To provide a fast imaging time, the proposed method combines higher-order generalized series (HGS) imaging with a parallel imaging technique which is called the HGS-parallel technique.</P><B>Results</B><P>The proposed HGS-parallel technique achieves a 12.8-fold acceleration in imaging time without the cost of spatial resolution. The proposed method was verified through the application of fMRI studies on normal subjects.</P><B>Conclusion</B><P>This study suggests that the proposed method can be used for high-resolution fMRI studies without the geometric distortion and the Nyquist ghost artifacts compared to EPI. J. Magn. Reson. Imaging 2009;29:924–936. © 2009 Wiley-Liss, Inc.</P>