이 연구의 목적은 영상의 화소 간 분석(voxel-based analysis)을 이용하여 자화율 가중 영상(SWI)에 나타난 위상 마스킹의 효과를 알아보는 것이었다. 20명의 정상 노인에서 SWI 영상의 정보를 획득하기 위하여 3차원 경사자장 에코 시퀀스를 이용하여 영상을 얻었다. SWI 영상에서의 위상 마스킹의 효과를 관찰하기 위해 원래의 경사자장 크기(magnitude) 영상에 위상 영상을 2번 곱한 SWI2 영상, 4번 곱한 SWI4 영상, 영상 내 정맥 혈관을 강조한 양의 위상 마스크 SWI 영상 (PSWI), 그리고 조직 부분을 강조한 음의 위상 마스크 SWI 영상(NSWI)을 만들었다. paired t-test를 이용한 PSWI와 NSWI간 신호강도의 차이, SWI2와 SWI4간의 신호강도의 차이, 그리고 경사자장 크기영상 영상과 위상 마스킹에서 얻은 SWI 영상의 신호강도의 차이를 voxel-based 분석으로 수행하였다. 신호 강도 차이는 magnitude과 SWI4 영상 간의 차이가 magnitude과 SWI2 영상 간의 차이보다 더 크게 나왔다. 또한, 신호강도 차이는 magnitude과 PSWI 영상 간의 차이가 magnitude과 NSWI보다 더 많았다. 그리고 NSWI2와 NSWI4간의 신호강도 차이가 PSWI2와 PSWI4간의 신호강도 차이 보다 더 크게 나타났으며, 그리고 NSWI4와 PSWI4간의 신호강도 차이가 NSWI2와 PSWI2간의 신호강도 차이보다 더 크게 나타났다. 위 실험은 화소 간 분석을 통한 SWI 영상 연구가 뇌 전체의 자화율 효과를 볼 때 매우 유용할 것이라는 사실뿐만 아니라, 각기 다른 위상 마스킹 방법을 응용함으로써 선택적으로 정맥 혈관 대비, 혹은 뇌 조직 대비를 강조할 수 있다는 사실을 입증하였다. 그러므로, 자화율 가중 영상의 화소 간 분석은 많은 임상 예에 적용될 수 있을 것이다.
이 연구의 목적은 영상의 화소 간 분석(voxel-based analysis)을 이용하여 자화율 가중 영상(SWI)에 나타난 위상 마스킹의 효과를 알아보는 것이었다. 20명의 정상 노인에서 SWI 영상의 정보를 획득하기 위하여 3차원 경사자장 에코 시퀀스를 이용하여 영상을 얻었다. SWI 영상에서의 위상 마스킹의 효과를 관찰하기 위해 원래의 경사자장 크기(magnitude) 영상에 위상 영상을 2번 곱한 SWI2 영상, 4번 곱한 SWI4 영상, 영상 내 정맥 혈관을 강조한 양의 위상 마스크 SWI 영상 (PSWI), 그리고 조직 부분을 강조한 음의 위상 마스크 SWI 영상(NSWI)을 만들었다. paired t-test를 이용한 PSWI와 NSWI간 신호강도의 차이, SWI2와 SWI4간의 신호강도의 차이, 그리고 경사자장 크기영상 영상과 위상 마스킹에서 얻은 SWI 영상의 신호강도의 차이를 voxel-based 분석으로 수행하였다. 신호 강도 차이는 magnitude과 SWI4 영상 간의 차이가 magnitude과 SWI2 영상 간의 차이보다 더 크게 나왔다. 또한, 신호강도 차이는 magnitude과 PSWI 영상 간의 차이가 magnitude과 NSWI보다 더 많았다. 그리고 NSWI2와 NSWI4간의 신호강도 차이가 PSWI2와 PSWI4간의 신호강도 차이 보다 더 크게 나타났으며, 그리고 NSWI4와 PSWI4간의 신호강도 차이가 NSWI2와 PSWI2간의 신호강도 차이보다 더 크게 나타났다. 위 실험은 화소 간 분석을 통한 SWI 영상 연구가 뇌 전체의 자화율 효과를 볼 때 매우 유용할 것이라는 사실뿐만 아니라, 각기 다른 위상 마스킹 방법을 응용함으로써 선택적으로 정맥 혈관 대비, 혹은 뇌 조직 대비를 강조할 수 있다는 사실을 입증하였다. 그러므로, 자화율 가중 영상의 화소 간 분석은 많은 임상 예에 적용될 수 있을 것이다.
To investigate effects of phase mask on susceptibility-weighted images (SWI) using voxel-based analyses in normal elderly subjects. A three-dimensional (3D) gradient echo sequence ran to obtain SWIs in 20 healthy elderly subjects. SWIs with two (SWI2) and four (SWI4) phase multiplications were achie...
To investigate effects of phase mask on susceptibility-weighted images (SWI) using voxel-based analyses in normal elderly subjects. A three-dimensional (3D) gradient echo sequence ran to obtain SWIs in 20 healthy elderly subjects. SWIs with two (SWI2) and four (SWI4) phase multiplications were achieved with positive (PSWI) and negative (NSWI) phase masks to investigate phase mask effects. The voxel-based comparisons were performed using paired t-tests between PSWI and NSWI and between SWI2 and SWI4. Differences of signal intensities between magnitude images and SWI4 were larger than those between magnitude images and SWI2s. Differences of signal intensities between magnitude images and PSWIs were larger than those between magnitude images and NSWIs. Moreover, the signal intensities from NSWI2s and NSWI4s were greater than those from PSWI2s and PSWI4s, respectively. More differences of signal intensities between NSWI4 and PSWI4s were found than those between NSWI2s and PSWI2s in the whole brain images. The voxel-based analyses of SWI could be beneficial to investigate susceptibility differences on the entire brain areas. The phase masking method could be chosen to enhance brain tissue contrast rather than to enhance venous blood vessels. Therefore, it is recommended to apply voxel-based analyses of SWI to investigate clinical applications.
To investigate effects of phase mask on susceptibility-weighted images (SWI) using voxel-based analyses in normal elderly subjects. A three-dimensional (3D) gradient echo sequence ran to obtain SWIs in 20 healthy elderly subjects. SWIs with two (SWI2) and four (SWI4) phase multiplications were achieved with positive (PSWI) and negative (NSWI) phase masks to investigate phase mask effects. The voxel-based comparisons were performed using paired t-tests between PSWI and NSWI and between SWI2 and SWI4. Differences of signal intensities between magnitude images and SWI4 were larger than those between magnitude images and SWI2s. Differences of signal intensities between magnitude images and PSWIs were larger than those between magnitude images and NSWIs. Moreover, the signal intensities from NSWI2s and NSWI4s were greater than those from PSWI2s and PSWI4s, respectively. More differences of signal intensities between NSWI4 and PSWI4s were found than those between NSWI2s and PSWI2s in the whole brain images. The voxel-based analyses of SWI could be beneficial to investigate susceptibility differences on the entire brain areas. The phase masking method could be chosen to enhance brain tissue contrast rather than to enhance venous blood vessels. Therefore, it is recommended to apply voxel-based analyses of SWI to investigate clinical applications.
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문제 정의
However, there are few studies on voxel-based changes of brain tissue on SWI. Therefore, the objective of this study was to investigate effects of the phase mask on SWIs using voxel-based analyses in normal elderly subjects. The phase mask effects were evaluated using two approaches.
The main objective of this study was to investigate phase mask effects on SWI in the voxel-based analysis. Because the post-processing step to utilize a phase image is crucial in creating final images, the image contrasts differ by which a method is chosen to be processed.
One limitation of this study was the uncertain relationship between phase signals and the susceptibility effects. Although phase images contain information on susceptibility differences, the phase contrast does not fully correspond to the susceptibility differences; therefore, one cannot easily conclude that the final SWIs produced by phase mask multiplications purely enhanced the susceptibility effects.
제안 방법
The phase mask effects were evaluated using two approaches. First, the SWIs processed by positive and negative phase masks were studied by comparing their signal differences. Second, the number of phase multiplication effects were investigated by performing voxel-wise comparisons among magnitude images and SWIs with two and four phase mask multiplications.
First, the SWIs processed by positive and negative phase masks were studied by comparing their signal differences. Second, the number of phase multiplication effects were investigated by performing voxel-wise comparisons among magnitude images and SWIs with two and four phase mask multiplications.
All voxel-based statistical analyses were also achieved using SPM5 software. In order to investigate multiplication effects of phase, the voxel-based comparisons of the whole brain images were performed using the paired t-tests between magnitude and PSWI2, between magnitude and PSWI4, between PSWI2 and PSWI4, between magnitude and NSWI2, between magnitude and NSWI4, and between NSWI2 and NSWI4. The family-wise error rate (FWE) was applied to account for the multiple comparisons, and a threshold for the significance of FWE p=0.
In order to investigate phase-mask effects, the voxel-based comparisons were also performed using a paired t-test between SWIs processed by the positive phase mask and that by negative one. Therefore, in these analyses, we compared between NSWI2 and PSWI2 and between NSWI4 and PSWI4 in the entire brains.
To investigate regional differences in specific brain areas rather than the whole brain, the ROI-based analysis was performed with different regions of interest (ROIs). The anatomical ROIs that are known for rich iron-contents were selected based on the previous studies on detecting rich iron-content regions of the brain (Haacke et al, 2005) using a WFU Pick-Atlas software (Wake Forest University School of Medicine, USA).
데이터처리
The following ROIs were selected; Amygdala, Globus Pallidus, Hippocampus, Pulvinar, Putamen, Red Nucleus, Thalamus and Precaneus. The average signal intensities from the selected ROIs were calculated using a Marsbar software(Matthew Brett, http://marsbar.sourceforge.net/), and a statistical analysis of the collected mean values for each ROI was performed by generating a one-way analysis of variance (ANOVA) test to compare differences among the image types (magnitude, SWI2 and SWI4, Degree of freedom (DOF)=2), between different phase mask effects (positive vs. negative phase mask, DOF=1), between brain tissue masking effects (with vs. without brain tissue mask, DOF=1), and among the selected ROIs (DOF=10). A post-hoc analysis was subsequently performed for those showing significant differences, where the Turkey method was used for multiple comparisons of image types and ROIs, while the LSD method was used for multiple comparisons of tissue masking effects.
성능/효과
In order to investigate multiplication effects of phase, the voxel-based comparisons of the whole brain images were performed using the paired t-tests between magnitude and PSWI2, between magnitude and PSWI4, between PSWI2 and PSWI4, between magnitude and NSWI2, between magnitude and NSWI4, and between NSWI2 and NSWI4. The family-wise error rate (FWE) was applied to account for the multiple comparisons, and a threshold for the significance of FWE p=0.0000000001 was chosen with a threshold looking for clusters with at least 10 contiguous voxels. The gender and age information of the subjects were included as covariates.
1. It is to be noted that the susceptibility image contrast increases from the magnitude image to PSWI2 and to PSWI4; in addition, PSWI2 and PSWI4 revealed better image contrasts than NSWI2 and NSWI4, respectively. Susceptibility image contrast is the greatest in PSWI4 and is the weakest in the magnitude image.
2. Magnitude images produced the highest signals compared to those from PSWI2 and PSWI4, and more differences were found between magnitude and PSWI4 than between magnitude and PSWI2. The main differences were found on the left and right culmen, on the right lingual gyrus, and on the left and right claustrum.
3. Magnitude images produced the highest signals compared to those from NSWI2 and NSWI4, and more differences were found between magnitude and NSWI4 than between magnitude and NSWI2. The main differences were found on the left precentral gyrus, left paracentral lobule, left medial frontal gyrus, left red nucleus and on the right lingual gyrus.
4. Signals from NSWI2 and NSWI4 were greater than those from PSWI2 and PSWI4, respectively, and more differences were found when comparing NSWI4 and PSWI4 than comparing NSWI2 and PSWI2 in the whole brain images. The main differences were found on the left and right caudate, the left and right lateral globus pallidus, the right red nucleus and the left and right anterior cingulate.
The one-way ANOVA test showed that the significant differences existed among the different image types (F=125.93, p=0.00000001), among the selected ROIs (F=101.88, p=0.00000001) and between brain tissue-masking effects (F=748.96, p= 0.00000001), but not for positive-negative phase mask (F= 2.8151, p=0.0934792). A post-hoc analysis revealed that the significant differences lied between magnitude images and SWI2, between magnitude and SWI4 and between SWI2 and SWI4.
4. Results of voxel-wise comparisons between susceptibility-weighted imaging (SWI) with the positive (P) phase mask and that with the negative (N) phase mask with the whole brain with the threshold of family-wise error rate (FWE) p=0.005. (a) Comparison between the whole brain negatively phase-masked SWI with 2 phase mask multiplications and positively phase-masked SWI with 2 phase mask multiplications (NSWI2 ＞ PSWI2).
The number of phase mask multiplication effects was also studied by comparing SWIs with two multiplications and those with four multiplications. The major findings of this study were the following; first, the general signal intensity values decreased as the number of phase mask multiplications increased, and second, positively phase-masked SWIs revealed lower signal intensities compared to those in negatively phase-masked SWIs, indicating that emphasizing venous blood signals attributed to more contrasts in SWI.
Our results also showed that PSWIs had lower signal intensities than NSWIs, which indicated that enhancing phase signals from venous blood veins using positive phase masks revealed better susceptibility effects in final SWIs. On the other hand, because negatively masked SWIs enhanced only negative phase signals present in soft tissues, the susceptibility effects were not as much boosted as those in the positively-masked SWIs.
The results of voxel-wise comparisons between the whole brain NSWI and PSWI showed that more differences lied between NSWI4s and PSWI4s than between NSWI2s and PSWI2s. This is because the signal intensities were reduced as the number of phase-mask multiplications increased.
This is because the signal intensities were reduced as the number of phase-mask multiplications increased. Because the positive phase masks are known to be more sensitive to susceptibility effects as previously mentioned in this study, the higher number of mask multiplications would have caused more signal losses in final PSWIs; conversely, because the NSWIs were less sensitive to susceptibility effects, the increasing number of mask multiplications would not have significantly influenced in signal losses due to the magnetic susceptibility. The differences between NSWI and PSWI would have thus augmented as the phase mask multiplications were performed four times.
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