In this study, we quantified metal artifact reduction in a THA phantom using high energy VMI with and without O-MAR in comparison to conventional imaging with and without O-MAR. We found that 130 keV VMI with O-MAR resulted in the strongest metal artifact reduction of mild and severe metal artifacts.
A tube voltage of 140 kVp is often used to reduce metal artifacts and results in lower CT number inaccuracies, lower noise and higher SNR and CNR [11] in comparison to 120 kVp. However, increasing tube voltage may result in only a slight reduction of metal artifacts [12,13,14,15,16,17]. Moreover, a higher tube voltage leads to a higher required dose whereas extraction of VMI and application of O-MAR are not leading to a higher required dose.
In previous THA phantom studies, it was already reported that the use of O-MAR in conventional imaging reduces metal artifacts [1, 2, 18,19,20,21]. Our results confirmed the value of O-MAR in conventional imaging for metal artifact reduction. Although 130 keV VMI without O-MAR showed stronger metal artifact reduction for mildly affected pellets than conventional imaging with O-MAR, metal artifact reduction for severely affected pellets worsened in comparison to conventional imaging with O-MAR in our study. Besides, the metal artifact reduction in CT values of 92% when comparing 130 keV VMI without O-MAR and conventional imaging with O-MAR is probably heightened by the bright streak artifacts visible on the 130 keV VMI without O-MAR. Our finding that 130 keV VMI in only mildly affected pellets showed metal artifact reduction in comparison to conventional imaging may be explained by reduction of metal artifacts caused by beam hardening. VMI of 130 keV is less prone to beam hardening artifacts, which are dominant in mildly affected pellets, decreasing metal artifacts.
Severely affected pellets were only present in the images of the phantom with bilateral prostheses where photon starvation is dominant over beam hardening. These severe metal artifacts caused by bilateral implants were not reduced on 130 keV VMI without O-MAR, which is supported by previous studies [6, 11, 22]. On the other hand, conventional imaging with O-MAR did result in a reduction of metal artifacts in severely affected pellets. Similarly, 130 keV VMI with O-MAR resulted in a decrease of metal artifacts in severely affected pellets. In addition, 130 keV VMI with O-MAR resulted in a decrease of metal artifacts in mildly affected pellets compared to conventional imaging with O-MAR and therefore resulted in the strongest metal artifact reduction for both mildly as severely affected pellets.
Our quantitative analysis using a THA phantom shows that high energy VMI with O-MAR results in the strongest metal artifact reduction, and this is supported by studies incorporating subjective assessment by radiologists [5,6,7] and by studies incorporating quantitative assessment of metal artifact reduction [5, 7, 23]. However, these studies used General Electric (GE) or Siemens systems to extract VMI with metal artifact reduction software, while we used a Philips system to extract VMI with O-MAR. Although metal artifact reduction techniques of the different vendors show comparable results regarding metal artifact reduction using VMI and VMI with metal artifact reduction software, it remains difficult to compare the results. A phantom study comparing metal artifact reduction of the different vendors would provide more insight in the performance of the different DECT techniques of the vendors and their metal artifact reduction software.
Although 130 keV VMI with O-MAR resulted in the strongest relative metal artifact reduction, it should be noted that the CT values, CNR and SNR of the unaffected, mildly affected and severely affected pellets were lower in the 130 keV VMI with O-MAR in comparison to conventional images with O-MAR. This can be explained by the high virtual monochromatic energy of the 130 keV images, causing a reduction of the CT values in the images in comparison to conventional images. Although low SNR and CNR can affect the detectability of the pellets and therefore diligence is important, subjective analysis of patients shows that high energy VMI with O-MAR still can be assessed reliable [22]. Furthermore, the Rose criterion states that an object’s CNR must exceed 3–5 to be detectable [24, 25]. Both conventional imaging with O-MAR as well as 130 keV imaging with O-MAR exceeded this criterion for both mildly affected and severely affected pellets.
On our 130 keV images, bright artifacts were induced. Likewise, several studies described that VMI and metal artifact reduction software can induce secondary artifacts [7, 26, 27]. Hence, the radiologist should also review conventional images in addition to the images with metal artifact reduction. Furthermore, relatively high negative metal artifact reduction in CNR (− 69%) and SNR (− 124%) was observed in severely affected pellets while high p values (0.889 and 0.889, respectively) were found when comparing 130 keV VMI without O-MAR and conventional imaging without O-MAR, which seems contradictory. This may be explained by the large dispersion of CNR values and SNR values of the individual severely affected pellets in the 130 keV VMI without O-MAR, caused by the introduction of bright artifacts.
In this study, we focused on image quality in orthopedic prosthesis imaging. Hence, we used a phantom with pellets mimicking bone tissue and we did not assess soft tissue. However, assessment of structures such as the bladder, uterus and prostate is also hampered by metal artifacts, which is particularly important in oncology patients. A study to quantify metal artifact reduction using structures mimicking soft tissue will certainly be of interest.
Our study has limitations. First, we categorized the pellets in the images with the unilateral and bilateral prostheses in unaffected, mildly affected and severely affected which allowed comprehensible analysis of all pellets. However, mildly affected pellets were caused by unilateral and bilateral prostheses. Assignment of these pellets to one category may not be in accordance with clinical practice because a unilateral prosthesis causes different artifacts than bilateral prostheses Furthermore, pellet L9 in the bilateral image was categorized as unaffected despite bright streaking artifacts, and a more appropriate category may be mildly affected instead. Third, our results did not quantify metal artifact reduction in the close vicinity of the implant. Fourth, the pellets only represent one bone density, comparable to trabecular bone. Bone structures with higher densities such as cortical bone may alter the results. Finally, we chose to place the background ROI in a part of the image where no metal artifacts were present. Although this avoids metal artifacts in the background ROI, the calculated CNR does not quantify contrast between pellets and the background in the vicinity of the pellets.