Department of Applied Mathematics, The Australian National University, Canberra, ACT 0200, Australia.
Med Phys. 2011 Oct;38(10):5459. doi: 10.1118/1.3633900.
In this paper we show that optimization-based autofocus may be used to overcome the instabilities that have, until now, made high-resolution theoretically-exact tomographic reconstruction impractical. To our knowledge, this represents the first successful use of theoretically-exact reconstruction in helical micro computed tomography (micro-CT) imaging. We show that autofocus-corrected, theoretically-exact helical CT is a viable option for high-resolution micro-CT imaging at high cone-angles (∼50°). The elevated cone-angle enables better utilization of the available X-ray flux and therefore shorter image acquisition time than conventional micro-CT systems.
By using the theoretically-exact Katsevich 1PI inversion formula, we are not restricted to a low-cone-angle regime; we can in theory obtain artefact-free reconstructions from projection data acquired at arbitrary high cone-angles. However, this reconstruction method is sensitive to misalignments in the tomographic data, which result in geometric distortion and streaking artefacts. We use a parametric model to quantify the deviation between the actual acquisition trajectory and an ideal helix, and use an autofocus method to estimate the relevant parameters. We define optimal units for each parameter, and use these to ensure consistent alignment accuracy across different cone-angles and different magnification factors. The tomographic image is obtained from a set of virtual projections in which software correction for hardware misalignment has been applied.
We make significant modifications to the autofocus method that allow this method to be used in helical micro-CT reconstruction, and show that these developments enable theoretically-exact reconstruction from experimental data using the Katsevich 1PI (K1PI) inversion formula. We further demonstrate how autofocus-corrected, theoretically-exact helical CT reduces the image acquisition time by an order of magnitude compared to conventional circular scan micro-CT.
Autofocus-corrected, theoretically-exact cone-beam reconstruction is a viable option for reducing acquisition time in high-resolution micro-CT imaging. It also opens up the possibility of efficiently imaging long objects.
本文展示了基于优化的自动对焦可用于克服迄今为止使高分辨率理论精确断层重建不切实际的不稳定性。据我们所知，这是理论精确重建在螺旋微计算机断层扫描（micro-CT）成像中首次成功应用。我们表明，自动对焦校正后的理论精确螺旋 CT 是在高锥角（约 50°）下进行高分辨率 micro-CT 成像的可行选择。高锥角可更好地利用可用的 X 射线通量，从而比传统的 micro-CT 系统缩短图像采集时间。
通过使用理论精确的 Katsevich 1PI 反演公式，我们不受低锥角范围的限制；理论上，我们可以从任意高锥角获取的投影数据中获得无伪影的重建。然而，这种重建方法对断层数据的错位很敏感，这会导致几何变形和条纹伪影。我们使用参数模型来量化实际采集轨迹与理想螺旋之间的偏差，并使用自动对焦方法来估计相关参数。我们为每个参数定义了最佳单位，并使用这些单位确保在不同的锥角和不同的放大倍数下具有一致的对准精度。断层图像是从一组已应用硬件未对准软件校正的虚拟投影中获得的。
我们对自动对焦方法进行了重大修改，使该方法可用于螺旋 micro-CT 重建，并表明这些改进使使用 Katsevich 1PI（K1PI）反演公式从实验数据进行理论精确重建成为可能。我们进一步证明了自动对焦校正后的理论精确螺旋 CT 如何将图像采集时间与传统的圆形扫描 micro-CT 相比缩短一个数量级。
自动对焦校正后的理论精确锥束重建是减少高分辨率 micro-CT 成像采集时间的可行选择。它还为高效成像长物体提供了可能性。
