Metal Artefact Reduction by Dual-energy Computed Tomography Using Monoenergetic Extrapolation: In-vitro Determination of Optimal Monoenergetic Level with Different Metallic Implants Using a Phantom Body
WCS Chan, JPK Tsang, WY Wong, PY Chu, VYK To, CY Lee, TW Yeung, OC Leung, NY Sin, OC Li, MK Yuen
Hong Kong J Radiol 2016;19:35-42
DOI: 10.12809/hkjr1615356
Objective: To identify the optimal monoenergetic level, balancing metal artefacts, and the amount of noise present for imaging of metal implants using dual-energy computed tomography (CT) and focusing on the assessment of peri-prosthetic soft tissue.
Methods: Four metallic implants commonly used in the hips were placed in a phantom body: unipolar hemiprosthesis, dynamic hip screw (DHS), intra-medullary (IM) nail, and titanium insert. The unipolar hemiprosthesis was imaged at two points: the head and stem. The head of the hemiprosthesis and DHS were imaged in two axes: one axial to and one resembling the angle at its expected position in the hip with respect to the scanner. The IM nail was assessed both at the level with and without a screw inserted. A region of interest to measure the noise level of the images was first performed with different monoenergetic levels (70-170 kV with increments of 10 kV). Four monoenergetic levels were then chosen (80, 90, 105, 120 kV) for each implant and were assessed and scored (presence of least to most artefacts: score 1-4) by nine radiologists who were blinded to the monoenergetic level. A total of eight sets of images were assessed. The scores for different monoenergetic levels were compared using analysis of variance.
Results: In the first part of the experiment, the images with the least amount of noise were in the range of 85-95 kV, thus we included 90 kV among the images for subsequent scoring. The mean score for different monoenergetic levels for all implants was as follows: 3.94 for 80 kV, 2.68 for 90 kV, 1.50 for 105 kV, and 1.88 for 120 kV (p<0.001), with 105 kV having the least metal artefacts. For subgroup analysis of individual implants, 105 kV was found to produce the best quality images with a statistically significant better score for hip stem, DHS, and IM nail. 120 kV trended towards being the best monoenergetic level when imaging the hip head and the IM nail with screw where relatively more artefacts were present. 90 kV trended towards being the best monoenergetic level when imaging the titanium insert where artefacts were nearly absent.
Conclusion: With regard to imaging the soft tissue around a metallic implant, the overall optimal monoenergetic level for reduction of metal artefacts using dual-energy CT is 105 kV. When more artefacts are inherently present, 120 kV trended towards being the best monoenergetic level. When artefacts are minimal, 90 kV trended towards being the optimal monoenergetic level with the least amount of noise present.
中文摘要
利用雙能量CT單能外推法減少金屬偽影：在假體內使用不同金屬植入物以確定最佳單能譜值
陳煥章、曾佩琪、王慧瑜、朱炳容、杜婉筠、李芷茵、楊子慧、梁安祥、單雅怡、李安慈、袁銘強
目的：在利用雙能量CT為金屬植入物作成像並觀察周圍骨與軟組織病變的過程中，找出最佳單能水平、讓金屬偽影最低並平衡噪聲分佈。
方法：把單極半髖假肢、動力髖螺釘（DHS）、髓內釘和鈦插件四種常見用於臀部的金屬植入物置入假體內。在單極半髖假肢頭和桿部兩點進行成像。半髖假肢頭和DHS在兩個平面進行成像：即相對於掃描儀的中軸，以及在其臀部的預期位置。髓內釘則分別在插入螺絲和沒有插入螺絲時進行評估。用興趣區量出不同單能量（70至170 kV，每個升幅為10 kV）影像的噪聲水平。然後在每個植體上選擇四個單能量（80、90、105和120 kV），讓9名放射科醫生為每個單能量水平圖像評分（他們並不知道圖像的單能量水平）；1分為最少金屬偽影，4分最多。共評估了8套成像。利用方差分析比較不同單能量圖像的分數。
結果：實驗第一部分發現最少噪聲的成像均在85-95 kV的範圍內，因此我們把90 kV納入以下評分。所有金屬植入物的各單能水平平均分數為：80 kV 3.94分、90 kV 2.68分、105 kV 1.50分、120 kV 1.88分（p<0.001）；105 kV出現最少金屬偽影。對於個別的金屬植入物，半髖假肢的桿部、DHS和髓內釘在105 kV的情況下會得到最佳的去除金屬偽影效果，統計顯著。半髖假肢的頭部和有螺釘的髓內釘則在120 kV的情況下產生最佳單能水平，但相對金屬偽影較多。鈦插件在90 kV的情況下會產生最佳單能水平，而且幾乎沒有金屬偽影。
結論：要為金屬植入物附近軟組織進行成像時，總體上使用雙能量CT 105 kV影像信號可達致最佳單能水平和最少金屬偽影。當金屬偽影無可避免時，使用120 kV影像信號可達致最佳單能水平。當偽影少時，使用90 kV影像信號可達致最佳單能水平並產生最少噪聲。