Nelson Geoff, Pigrish Vadim, Sarkar Vikren, Su Fan-Chi, Salter Bill
Department of Radiation Oncology, University of Utah, Salt Lake City, UT, USA.
J Appl Clin Med Phys. 2019 Mar;20(3):125-131. doi: 10.1002/acm2.12546. Epub 2019 Mar 9.
Two new tools available in Radiation Oncology clinics are Dual-energy CT (DECT) and Siemens' DirectDensity™ (DD) reconstruction algorithm, which allows scans of any kV setting to use the same calibration. This study demonstrates why DD scans should not be used in combination with DECT and quantifies the magnitude of potential errors in image quality and dose.
A CatPhan 504 phantom was scanned with a dual-pass DECT and reconstructed with many different kernels, including several DD kernels. The HU values of various inserts were measured. The RANDO man phantom was also scanned. Bone was contoured and then histograms of the bone HU values were analyzed for Filtered-Backprojection (FBP) and DD reconstructions of the 80 and 140 kV scans, as well as several virtual, monoenergetic reconstructions generated from FBP and DD reconstructions. "Standard" dose distributions were calculated on several reconstructions of both phantoms for comparison.
The DD kernel overcorrected the high-Z material inserts relative to bone, giving an excessively low relative electron density (RED). A unique artifact was observed in the high density inserts of the CatPhan in the monoenergetic scans when utilizing a DD kernel, due to the overcorrection in the DD scan of the material, especially at lower kV.
While DD and DECT perform as expected when used independently, errors from their combined use were demonstrated. Dose errors from misuse of the DD kernel with DECT post-processing were as large as 2.5%. The DECT post-processing was without value because the HU differences between low and high energy were removed by the DD kernel. When using DD and DECT, we recommend the use of a DD reconstruction of the high energy scan for the dose calculation, and use of a FBP filter for the low and high energy scans for the DECT post-processing.
放射肿瘤学临床中可用的两种新工具是双能CT（DECT）和西门子的DirectDensity™（DD）重建算法，该算法允许对任何千伏设置的扫描使用相同的校准。本研究证明了为何不应将DD扫描与DECT结合使用，并量化了图像质量和剂量方面潜在误差的大小。
使用双通DECT对CatPhan 504体模进行扫描，并用多种不同的内核进行重建，包括几个DD内核。测量各种插入物的HU值。还对RANDO人体模进行了扫描。勾勒出骨骼轮廓，然后分析80 kV和140 kV扫描的滤波反投影（FBP）和DD重建以及从FBP和DD重建生成的几个虚拟单能重建的骨骼HU值直方图。在两个体模的多个重建上计算“标准”剂量分布以进行比较。
相对于骨骼，DD内核过度校正了高Z材料插入物，导致相对电子密度（RED）过低。在使用DD内核的单能扫描中，由于材料在DD扫描中的过度校正，尤其是在较低kV时，在CatPhan的高密度插入物中观察到一种独特的伪影。
虽然DD和DECT单独使用时表现符合预期，但证明了它们联合使用时会产生误差。将DD内核与DECT后处理一起误用导致的剂量误差高达2.5%。DECT后处理没有价值，因为DD内核消除了低能和高能之间的HU差异。当使用DD和DECT时，我们建议使用高能扫描的DD重建进行剂量计算，并使用FBP滤波器对DECT后处理的低能和高能扫描进行处理。
