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Significant reduction of metallic artifacts with spectral detector CT

Philips CT Clinical Science Philips Healthcare • USA

Purpose of the study
This study was performed to evaluate the benefits of virtual Monoenergetic (MonoE) imaging to improve the image quality in areas around unilateral and bilateral hip replacements. The researchers from the Radiology Department of Isala in Zwolle, Netherlands, used a total hip arthroplasty phantom scanned on the Philips IQon Spectral CT for the evaluation.

The following is a summary of the study published by RHH Wellenberg, et al. in the European Journal of Radiology 88 (2017).

Metallic prosthetic implants in the hip can generate beam hardening artifacts, deteriorating the image quality and significantly affecting the diagnostic performance of CT. This could affect the detection of pathologies such as pseudotumors, capsular reactions and other soft tissue and bone pathologies.

On the IQon Spectral CT, conventional CT images can be created along with an image series at a range of different energy levels (keV). These are called virtual MonoE images and are created from perfectly matched low and high-energy projection data from the CT scanner. These keV’s range from 40-200 keV. When you apply high virtual MonoE’s, they will help to reduce image artifacts caused from the inability of the X-ray beam to penetrate the area of anatomy—what is referred to as beam hardening. When applying low virtual MonoE’s, you can improve contrast enhancement of structures that have been given IV contrast injections.

The study used several unilateral and bilateral hip prostheses consisting of different metal alloys inserted and combined in a water-filled total hip arthroplasty phantom. The phantom was then scanned on the IQon Spectral CT at 120 kVp, and then again at 140 kVp using a standard CT radiation dose of 20 mGy CTDI. The images were reconstructed and analyzed at virtual MonoE levels ranging from 40 to 200 keV. The high virtual MonoE images were compared to the conventional images that were obtained. There is a noticeable reduction in the beam hardening artifacts using the higher virtual MonoE.

The researchers rated metal artifacts seen on the scans into three categories:

The three measurements evaluated:
  • CT number accuracy
  • SNR, which compares the level of a desired signal to the level of background noise
  • CNR, which measures image quality
Figure 1 Improvements in CNR measurements at 130 keV and 200 keV virtual MonoE image as compared to the standard reconstruction at 140 kVp.
Figure 1
Improvements in CNR measurements at 130 keV and 200 keV virtual MonoE image as compared to the standard reconstruction at 140 kVp.

The virtual MonoE levels that improved CNR and reduced metal artifacts varied from 74 to 150 keV for different hip prostheses materials. For example, in lighter materials, such as titanium alloy, the virtual MonoE images resulted in less severe artifacts compared to the heavy metals such as cobalt alloy, where the reduction of artifact was not as significant.

The use of virtual MonoE images resulted in a significant reduction of mild and moderate metal artifacts by improving CNR and decreasing noise in a total hip phantom. The optimal virtual MonoE energy for reducing mild and moderate artifacts was 130 keV with values ranging from 74 keV to 150 keV for different prostheses made of different materials.

Clinical Relevance
Virtual MonoE imaging conducted on the IQon Spectral CT demonstrates a significant reduction of metal artifacts. This results in a clinical opportunity to significantly increase the performance of CT in the detection of pathologies near certain prostheses.

Wellenberg, R.H.H., et al. (2017) Quantifying metal artifact reduction using virtual monochromatic dual-layer spectral CT imaging in unilateral and bilateral total hip protheses. European Journal of Radiology, 88, 61-70.

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Jun 29, 2017

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IntelliSpace Portal, IQon Spectral CT
artifacts, Body, hip, image quality, implant, MonoE, Musculoskeletal, phantom, prothesis

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