Andrew Volp

Metal artifacts from titanium and steel screws in CT, 1.5T and 3T MR images of the tibial Pilon: a quantitative assessment in 3D.

Radiographs are commonly used to assess articular reduction of the distal tibia (pilon) fractures postoperatively, but may reveal malreductions inaccurately. While magnetic resonance imaging (MRI) and computed tomography (CT) are potential three-dimensional (3D) alternatives they generate metal-related artifacts. This study aims to quantify the artifact size from orthopaedic screws using CT, 1.5T and 3T MRI data. Three screws were inserted into one intact human cadaver ankle specimen proximal to and along the distal articular surface, then CT, 1.5T and 3T MRI scanned. Four types of screws were investigated: titanium alloy (TA), stainless steel (SS) (Ø =3.5 mm), cannulated TA (CTA) and cannulated SS (CSS) (Ø =4.0 mm, Ø empty core =2.6 mm). 3D artifact models were reconstructed using adaptive thresholding. The artifact size was measured by calculating the perpendicular distance from the central screw axis to the boundary of the artifact in four anatomical directions with respect to the distal tibia. The artifact sizes (in the order of TA, SS, CTA and CSS) from CT were 2.0, 2.6, 1.6 and 2.0 mm; from 1.5T MRI they were 3.7, 10.9, 2.9, and 9 mm; and 3T MRI they were 4.4, 15.3, 3.8, and 11.6 mm respectively. Therefore, CT can be used as long as the screws are at a safe distance of about 2 mm from the articular surface. MRI can be used if the screws are at least 3 mm away from the articular surface except for SS and CSS. Artifacts from steel screws were too large thus obstructed the pilon from being visualised in MRI. Significant differences (P<0.05) were found in the size of artifacts between all imaging modalities, screw types and material types, except 1.5T versus 3T MRI for the SS screws (P=0.063). CTA screws near the joint surface can improve postoperative assessment in CT and MRI. MRI presents a favourable non-ionising alternative when using titanium hardware. Since these factors may influence the quality of postoperative assessment, potential improvements in operative techniques should be considered.

USING MRI FOR THE IMAGING OF LONG BONES: FIRST EXPERIENCES

Many applications in medical research and development require virtual three dimensional (3D) models of bones. The current gold standard for the acquisition of such data is Computer Tomography (CT) scanning. Due to the amount of radiation involved, CT scanning is generally limited to the imaging of clinical cases and cadaver specimens [Messmer, 2007]. Magnetic Resonance Imaging (MRI) is not routinely used for the imaging of bones because of difficulties in precise segmentation between bone and certain soft tissue types as well as higher costs compared to CT. As MRI does not involve ionising radiation it is ideally suited for the imaging of volunteers, who can be recruited according to study specific requirements. This study aimed to develop a MRI scanning protocol suitable to image the legs of volunteers and to provide an initial validation of the geometrical accuracy of the reconstructed 3D models.

Anatomical MR imaging of long bones: Comparative performance of MRI at 1.5 T and 3 T

The current gold standard for the design of orthopaedic implants is 3D models of long bones obtained using computed tomography (CT). However, high-resolution CT imaging involves high radiation exposure, which limits its use in healthy human volunteers. Magnetic resonance imaging (MRI) is an attractive alternative for the scanning of healthy human volunteers for research purposes. Current limitations of MRI include difficulties of tissue segmentation within joints and long scanning times. In this work, we explore the possibility of overcoming these limitations through the use of MRI scanners operating at a higher field strength. We quantitatively compare the quality of anatomical MR images of long bones obtained at 1.5 T and 3 T and optimise the scanning protocol of 3 T MRI. FLASH images of the right leg of five human volunteers acquired at 1.5 T and 3 T were compared in terms of signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). The comparison showed a relatively high CNR and SNR at 3 T for most regions of the femur and tibia, with the exception of the distal diaphyseal region of the femur and the mid diaphyseal region of the tibia. This was accompanied by an ~65% increase in the longitudinal spin relaxation time (T1) of the muscle at 3 T compared to 1.5 T. The results suggest that MRI at 3 T may be able to enhance the segmentability and potentially improve the accuracy of 3D anatomical models of long bones, compared to 1.5 T. We discuss how the total imaging times at 3 T can be kept short while maximising the CNR and SNR of the images obtained.