An MRI essentially sends a strong magnetic field throughout the different tissues and parts of your body. Each part of your body has a different reaction to the presence of this magnetic field. Using advanced technology and computers, the information gathered from the MRI scan can be translated into a 3D image of the area that was scanned, such as your arm, leg, brain, or entire body.
As MRIs are completely painless, non-invasive, and do not expose you to any radiation unlike CT scans , they have become a very popular tool for diagnosing internal health issues of the organs and tissues of the body.
Because Magnetic Resonance Imaging uses a strong magnetic field to gather images of your body, any metal that has a strong reaction to magnetism can cause interference with the scan. Some more rare metals like nickel and cobalt are also ferromagnetic. In CT, metal artifacts are typically seen as bold and starburst streaks resulting from beam hardening, partial volume effects and missing projection data Susceptibility artifacts in MRI are seen as bright and dark blotches in images due to signal mismapping and dephasing The volume of these artifacts also depend on the size, shape, composition and the position of the implants with respect to the X-ray beams and magnetic fields of CT and MRI scanners respectively In spite of these shortcomings, recent studies have reported that suitable CT and MRI protocols can significantly reduce the amount of such artifacts, therefore help to minimise the amount of image distortions 16 , 17 , 19 - Although CT is often implicated with high radiation dosages, there have been recent technical advancements in the medical manufacturing industry with the development of suitable algorithms and protocols specifically catering for dose reduction 24 while at the same time aiming to preserve image quality An example is the iDose protocol Philips Medical Systems utilised in this study.
From the same manufacturer, a post-processing algorithm is also available for the metal artifact reduction for orthopaedic implants O-MAR to effectively reduce the amount of metal artifacts generated from orthopaedic implants and subsequently minimize image degradation and distortions There are numerous studies that have assessed the extent of metal related artifacts for different imaging modalities, implant materials, implant types and anatomical regions 16 , 18 , 28 , Most of these studies have focussed on 2D qualitative assessments.
Although Moon et al. To the best of our knowledge, there are no published manuscripts that specifically quantify and compare optimally reduced metal artifacts of common orthopaedic screws for pilon fracture treatment across three clinical imaging modalities CT, 1.
Therefore, the first objective of this study was to develop a simple method for the quantitative assessment of metal screw artifacts in 3D and in relation to the articular surface of the tibial pilon. The second objective was to apply this method to quantify and investigate the effects of imaging modality, screw type and material on the extent of the resulting metal related image artifact. The age of the specimen was 90 years old, and amputated from a left leg. This specimen was defrosted 24 hours prior to the surgical insertion of metal screws.
An L-shaped incision of about 6 cm in the anterolateral approach was made with a surgical blade to expose the tibial plafond. Utilising the C-arm fluoroscope for imaging and with the aid of K wires, three holes were drilled with a diameter of 2. Three metal screws of the same type and material were inserted into the holes. After the insertion of the screws, the skin flaps were closed with nylon thread, and the specimen was sealed in two plastic bags.
The specimen was first scanned on all modalities with TA screws. Subsequently they were replaced with a set of SS screws and the specimen was rescanned. The same process was repeated for the cannulated screw sets.
The specimen was positioned on the scan table by aligning the long axis of the tibia with the long axis of the CT scanner Philips Brilliance slice CT.
The following CT protocols were used: Tube voltage of kVp, X-ray tube current of mA, slice thickness of 1 mm, slice spacing of 0. The iDose function low dose was used for all of the CT scans. However, O-MAR post-processing was only applied for the TA and CTA screws as pilot scans showed that screws made of steel introduced grey streaks, thus reducing instead of improving the image quality. The specimen was positioned on the bed with the spine array in place and covered with a body matrix receive coil.
This process was repeated with the 1. Based on a semi-automatic threshold method developed by Rathnayaka [], the segmentation of bony contours was applied to reconstruct the 3D bone models. This process was repeated again to segment the boundary of the metal artifact so as to generate representative 3D models.
To quantify metal artifacts produced from each type of screw, the alignment of 3D screw models relative to the metal artifact models must first be established so that the measurements for the extent of metal artifacts can be calculated from the central axis of the screw to the boundary of the artifact in four orthogonal directions with respect to the distal tibia for each dataset.
To correctly position the screws relative to the CT scans, TA screws were used as a reference by aligning the three screw models provided by Synthes GmbH in the centre of the artifact model with a trackball function.
The position of the screw models was subsequently validated against the CT images Figure 1. This position was replicated for all other CT data by using a fine registration function to align the screw models to the CT-generated bone models. Fine registration is based on the iterative closest point algorithm ICP 30 , 31 Figure 1. For the positioning of all the screws in MRI scans, the fine registration function was used to align the MRI-generated bone model to the CT-generated bone model, then its screws were positioned relative to the CT bone model.
Again, the correct positions of the screws were validated against the MRI data. The above-mentioned procedure was repeated with three cannulated screw models, and then saved as model files MDL for quantitative analysis to be carried out in Rapidform Next, we do our research. Once we know you have an implant, we find out everything we can about it. Our technologists are specially trained to research these devices.
In more complicated cases, we do further research that may involve bringing in our MRI safety officers to do the legwork. And, in some cases, they may even be on hand for your scan. Meeting the conditions of a device can sometimes be complicated, explains Jeff Jahn, vice president of engineering and research at RAYUS.
The manufacturer says you can get a scan on a 1. MRI Compatible Metals. Facts About Magnets. Can Brass Be Magnetized? Science Fair Magnet Ideas. How to Identify a Metal. What Are the Dangers of Electromagnets? How Do Neodymium Magnets Work? Uses for Permanent Magnets.
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