Nishimaru Eiji, Ichikawa Katsuhiro, Hara Takanori, Terakawa Shoichi, Yokomachi Kazushi, Fujioka Chikako, Kiguchi Masao, Ishifuro Minoru
Department of Radiology, Hiroshima University Hospital.
Nihon Hoshasen Gijutsu Gakkai Zasshi. 2012;68(12):1637-43. doi: 10.6009/jjrt.2012_jsrt_68.12.1637.
Adaptive iterative reconstruction techniques (IRs) can decrease image noise in computed tomography (CT) and are expected to contribute to reduction of the radiation dose. To evaluate the performance of IRs, the conventional two-dimensional (2D) noise power spectrum (NPS) is widely used. However, when an IR provides an NPS value drop at all spatial frequency (which is similar to NPS changes by dose increase), the conventional method cannot evaluate the correct noise property because the conventional method does not correspond to the volume data natures of CT images. The purpose of our study was to develop a new method for NPS measurements that can be adapted to IRs. Our method utilized thick multi-planar reconstruction (MPR) images. The thick images are generally made by averaging CT volume data in a direction perpendicular to a MPR plane (e.g. z-direction for axial MPR plane). By using this averaging technique as a cutter for 3D-NPS, we can obtain adequate 2D-extracted NPS (eNPS) from 3D NPS. We applied this method to IR images generated with adaptive iterative dose reduction 3D (AIDR-3D, Toshiba) to investigate the validity of our method. A water phantom with 24 cm-diameters was scanned at 120 kV and 200 mAs with a 320-row CT (Acquilion One, Toshiba). From the results of study, the adequate thickness of MPR images for eNPS was more than 25.0 mm. Our new NPS measurement method utilizing thick MPR images was accurate and effective for evaluating noise reduction effects of IRs.
自适应迭代重建技术（IRs）可降低计算机断层扫描（CT）中的图像噪声，并有望有助于降低辐射剂量。为评估IRs的性能，传统的二维（2D）噪声功率谱（NPS）被广泛使用。然而，当一种IR在所有空间频率上都使NPS值下降时（这类似于剂量增加导致的NPS变化），传统方法无法评估正确的噪声特性，因为传统方法与CT图像的体数据性质不对应。我们研究的目的是开发一种可适用于IRs的NPS测量新方法。我们的方法利用了厚多平面重建（MPR）图像。厚图像通常是通过在垂直于MPR平面的方向（例如，对于轴向MPR平面为z方向）对CT体数据进行平均而生成的。通过将这种平均技术用作三维NPS的切割器，我们可以从三维NPS中获得足够的二维提取NPS（eNPS）。我们将此方法应用于使用自适应迭代剂量降低3D（AIDR - 3D，东芝）生成的IR图像，以研究我们方法的有效性。使用320排CT（Aquilion One，东芝）在120 kV和200 mAs条件下扫描直径为24 cm的水模体。从研究结果来看，用于eNPS的MPR图像的合适厚度超过25.0 mm。我们利用厚MPR图像的新NPS测量方法在评估IRs的降噪效果方面准确且有效。
