Petersilge C A, Lewin J S, Duerk J L, Yoo J U, Ghaneyem A J
Department of Radiology, University Hospitals of Cleveland, Case Western Reserve University, OH 44106, USA.
AJR Am J Roentgenol. 1996 May;166(5):1213-8. doi: 10.2214/ajr.166.5.8615272.
This study examined the contribution and interdependence of multiple imaging parameters in clinical imaging sequences to aid practicing radiologists in minimizing artifacts during MR imaging of the spine after implantation of titanium pedicle screws.
A lumbar spine specimen with titanium pedicle screws implanted in the pedicle was imaged with a 1.5-T scanner. Sequence type, voxel volume, TE, and bandwidth varied. Different voxel volumes were achieved by altering section thickness, field of view (FOV), and matrix size. Artifact size was measured on sagittal and axial images at the midpedicle level. Artifact size was expressed as a percentage of actual screw size, and mean artifact size was calculated for each sequence. Analysis of variance without replication was done.
Mean artifact size ranged from 231% to 364% of actual screw size. Artifact size was independent of voxel volume for voxels greater than 3 MM3 (p<.001). Artifact size decreased significantly (p<.001) when voxel volume was less than 1 mm3. When we increased slice thickness and maintained a constant voxel volume, artifact size decreased. Decreases in artifact size correlated with a reduction in the ratio of the FOV to the number of pixels in the frequency-encoding direction (Nx). Artifact sizes were smallest when fast spin-echo sequences were used. Conventional spin-echo sequences produced artifacts that were smaller than the artifacts produced by the gradient-echo sequences. Decreasing the TE did not diminish artifact size for conventional spin-echo images in larger voxel volume.
Although voxel volume has been recognized as a factor that affects artifact size, the role of other contributing factors--slice thickness, number of phase-encoding steps, and FOV/Nx--has not been evaluated before. Artifact reduction proved to be dependent only on FOV/Nx. Artifact size was reduced by the use of fast spin-echo sequences. With conventional spin-echo sequences, TE should be minimized, although other technical factors may outweigh the gain in artifact reduction.
本研究探讨了临床成像序列中多个成像参数的作用及相互依赖性，以帮助执业放射科医生在钛椎弓根螺钉植入术后的脊柱磁共振成像中尽量减少伪影。
使用1.5-T扫描仪对植入椎弓根钛椎弓根螺钉的腰椎标本进行成像。序列类型、体素体积、回波时间（TE）和带宽各不相同。通过改变层厚、视野（FOV）和矩阵大小获得不同的体素体积。在椎弓根中部水平的矢状面和轴位图像上测量伪影大小。伪影大小以实际螺钉大小的百分比表示，并计算每个序列的平均伪影大小。进行无重复方差分析。
平均伪影大小为实际螺钉大小的231%至364%。对于大于3立方毫米的体素，伪影大小与体素体积无关（p<0.001）。当体素体积小于1立方毫米时，伪影大小显著降低（p<0.001）。当我们增加层厚并保持体素体积恒定时，伪影大小降低。伪影大小的降低与视野与频率编码方向上像素数量之比（Nx）的降低相关。使用快速自旋回波序列时伪影大小最小。传统自旋回波序列产生的伪影小于梯度回波序列产生的伪影。对于较大体素体积的传统自旋回波图像，降低TE并不能减小伪影大小。
尽管体素体积已被认为是影响伪影大小的一个因素，但其他影响因素——层厚、相位编码步数和FOV/Nx——的作用此前尚未得到评估。事实证明，减少伪影仅取决于FOV/Nx。使用快速自旋回波序列可减小伪影大小。对于传统自旋回波序列，应尽量减小TE，尽管其他技术因素可能比减少伪影带来的益处更重要。
