CT metal artefact reduction of total knee prostheses using angled gantrymultiplanar reformation
Mark Lewis a,⁎, Andoni P. Toms a, Karen Reid b, William Bugg a
a Radiology Academy, Cotman Centre, Colney Lane, Norwich, Norfolk, NR4 7UB, United Kingdomb Department of Radiology, Norfolk & Norwich University Hospital, Colney Lane, Norwich, Norfolk, NR4 7UY, United Kingdom
Article history:Received 27 November 2009Received in revised form 18 February 2010Accepted 19 February 2010
Keywords:CTKneeMetalArtefactReduction
This study was designed to determine whether or not acquiring CT images of total knee prostheses by usingan angled gantry and multiplanar reformation can reduce beam hardening artefact. A CT phantom wascreated with a total knee prosthesis suspended in gelatine with a known attenuation. CT data was acquiredwith a gantry angled at 0°, 5°, 10° and 15° in both craniocaudal oblique planes. Axial images where thenreformatted from these datasets. Two independent observers selected regions of interest to measure themean and standard deviation (SD) of attenuation in the gelatine for all reformatted axial images. Artefactwas measured as SD of the background attenuation and areas under the curve of SD for each gantry angleacquisition were compared. Inter-observer reliability was excellent (ICC=0.89, CI 0.875–0.908). The mostaccurate mean attenuation values for tissues around a TKR were obtained with a CT gantry using 10° to 15°anteroinferior to posterosuperior angulation. The smallest area under the curve for SD of attenuation for thewhole prosthesis, and the femoral component in isolation, was obtained with a 5° gantry angle in the samedirection. The smallest area under the curve for the tibial component in isolation occurred with a gantryangle of 15°. We conclude that acquiring CT data with a gantry angle can reduce metal artefact around a TKR.Optimal overall metal artefact reduction can be achieved with a small angle from anteroinferior toposterosuperior. Further selective artefact reduction around the tibial component can be achieved withlarger angles.
Metallic artefact can be a significant problem in the assessment ofjoint replacements using computed tomography (CT). The very highattenuation of most metallic implants can lead to severe streakingartefacts. These are caused by two principal mechanisms. The firstproblem is beam hardening which is caused by preferential absorp-tion, by the implant, of lower energy photons in the beamwhich skewsthe attenuationprofile for all pixels along the line of the beam resultingin streaks and dark bands projected in the reconstructed image [1].This is compounded by the second problem, which is that the meanattenuation of themetal is outside the range expected by the computerresulting in incomplete attenuation profiles [1]. A number oftechniques have been described for reducing both metallic artefactand beam hardening artefact. Improved software reconstructionalgorithms can include expected attenuation profiles for metallicprostheses [2,3]. Increasing mAs and Kv will increase the number ofphotons, reducing noise, and narrow the profile of photon energies[4,5]. Increased slice thickness will improve the signal to noise ratio,
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but can be associated with increased partial volume artefacts [4].Increasing the CT scale will improve the appearance of streak artefacteven if it does not improve the severity of the artefact [6]. CT imaging oftotal hip replacement has improved considerably using these adapta-tions. However in total knee replacement CT is still difficult because ofthe complex morphology and large volume of metal in the prosthesis.This aim of this study was to determine whether or not metal artefactreduction could be achieved in TKRs usingmultiplanar reformatted CTacquired with an angled gantry.
2. Materials and methods
A phantom was created using a chromium steel alloy TKR (DePuyInternational Ltd, Leeds UK), including the polyethylene tray. The TKRwas suspended in gelatine containing iodinated contrast medium(Omnipaque™ GE Healthcare, Bucks, UK, diluted to 0.4% of its originalconcentration) with an approximate attenuation of 48 HU (Fig. 1). CTstudies were performed using a GE Lightspeed plus (GE Healthcare,Bucks, UK) with the following imaging parameters: slice thickness1.25 mm, 512×512 matrix, and 440 mA and 120 Kv. The wholeprosthesis was covered in a single acquisition with the gantry verticaland repeatedwith the gantry tilted at 5°, 10° and 15° anterosuperior toposteroinferior and 5°, 10° and 15° posterosuperior to anteroinferior
(Fig. 2). Axial datasets where then reformatted using the preset bonealgorithm for each acquisition. Reformatted datasets where thenreviewed on a DICOM viewer (Osirix v3.1–32-bit) where twoindependent observers drew 10 cm2 regions of interest (ROI) at thesame position on the background gelatine and recorded the meanattenuation values and standarddeviation (Fig. 3). The reliability of theobservers' measurements was calculated using intra-class correlation(SPSS 16.0 for Windows). The mean values, of the two observers, forstandard deviation were used as a measure of artefact [8]. The relativeamount of artefact, for each gantry angle, was calculated bymeasuringthe area under the curve (trapezoid method) generated by graphs ofstandard deviation of attenuation from the ROIs (Figs. 4 and 5).
3. Results
The intra-class correlation for the two observers was excellentr=0.89 (95% CI, 0.875–0.908) (Table 1). The mean backgroundattenuation was closest to the true value of 48 HU in those datasetsacquired at −10 and −15° of gantry angle (Table 1). With a gantryangle in the anterosuperior to posteroinferior plane the meanattenuation rose to 59.2 HU at +15° and 107.37 HU at +15° for thewhole phantom and for the femoral component respectively. For thetibial component the opposite occurred. Although the gantry angle of−10° gave the attenuation closest to background (37.6 HU) in theopposite direction themean attenuation fell to 1.9 HU at+15° (Fig. 4).
When the phantom was considered as a whole the minimumstandard deviation for attenuation values (least artefact) in thebackground fat was achieved at a gantry tilt angle of −5° (SD 64.3HU) and the maximum (worst artefact) at +5° (SD 81.7 HU). Whenthe femoral component alone was considered optimal artefactreduction was similarly at −5° (SD 87.65 HU) but much worse withincreasing anteroinferior to posterosuperior gantry angle (SD 122.1HU at −15°). Artefact around the tibial component appears toimprove with any gantry angle and any in any direction whencompared to directly acquired axial images (SD 50.27 HU) but artefactreduction was at its best with a gantry angle of −15° (SD 31.21 HU).
Fig. 4. Graph demonstrating the areas under the curve for measurements of standarddeviation in background attenuation adjacent to the tibial component. The smaller theamplitude of the curve the less artefact there is.
Table 1Demonstrating changes in mean attenuation and the standard deviation for variousparts of the phantom with varying gantry angles.
This study suggests that CT of total knee replacements may beimproved by adding a gantry angle particularly in the anteroinferior toposterosuperior direction and then reconstructing images in conven-tional orthogonal planes. Measures of artefact in the adjacent tissuescan in general be reduced by a 5 to 10° tilt or artefact reduction can bemore specifically targeted to the tibial componentwith an angle of 15°.In reality this is artefact displacement rather than true reduction. Theangled gantry spreads the artefact overmore of the reconstructed axialslices than if acquired as direct axial images but has the effect ofreducing the amount of artefact in the areas of interest. The optimalgantry angle depends on the particular oblique axial plane that crossesthe least amount of metal.
Angling the gantry tilt to reduce artefact in the region of interest tothe radiologist is by no means a new technique [7] and is used inpractice, for instance,when imaging the brain to avoid thedenseboneatthe base of the skull. However,wewere unable tofind an instance in theliterature describing this technique in relation to joint replacements.
There are some limitations to this study. Demonstrating an im-provement in measures of artefact in a phantom does not necessarilytranslate to subjectively improved clinical images or an improvementin the conspicuity of lesions such as areas of periprosthetic osteolysis.Despite the improved measures the mean standard deviations remainhigh. It is also recognised that the gantry tilt angles described arerelative to the CT table not to the long axis of the prosthesis. In thephantom the TKR was set in slight extension (2–3°) although axial
Fig. 5. Graph demonstrating the area under the curve for measurements of standard deviation in background attenuation adjacent to the femoral component.
282 M. Lewis et al. / The Knee 17 (2010) 279–282
reconstructions were performed parallel to a plane through the tibialtray. Therefore the data from this study is useful only to describe trendsin artefact reduction rather than absolute measures. In fact the trendsare more important than absolute measures when trying to translatethesefindings into clinical practice. In clinical practice it is unlikely thata TKR would be in perfect alignment with the axis of the CT machineand there are, of course, numerous variations on the basic design of aTKR which are likely to influence any absolute measures. We have notfully assessed the reproducibility in multiple patients. This has notbeen addressed in this project and is a potential aspect for furtherresearch.
CT reconstruction and post processing algorithms have madesignificant contributions to artefact reduction in total hip replace-ments but the complex morphology and the volume of metal in a TKRhave so far resisted similar advances. The tilted gantry angle may offerone approach to addressing this. It is possible to demonstrate theeffect that changing the gantry angle has on image sharpness at keyareas (Fig. 6).
5. Conclusion
The most accurate attenuation values for tissues around a TKR canbe obtained with a CT gantry using 10 to 15° anteroinferior toposterosuperior angulation. Optimal overall metal artefact reduction
for the TKR, and for the femoral component in isolation, can beobtained with a small (approximately 5°) angulation in the samedirection. Specific CT imaging of the tibial component can be furtherimproved by using even larger gantry angles (approximately 15°).
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