The Sussex Cancer Centre is the first unit in the UK to acquire and use Philips new software package for the suppression of orthopaedic metal artefacts in CT imaging. Marilyn Seabrook, Senior Pre-treatment Lead Radiographer commenting on the image quality the Centre is obtaining with the new software says, “This easy-to-use software has allowed us to produce impressive images showing anatomy that is usually ‘hidden’ by the artefacts products by orthopaedic implants.”
Philips’ O-MAR (Metal Artefact Reduction through Orthopaedic Implants) is an efficient algorithm that offers a method of suppressing metal artefacts from orthopaedic implants. Artefacts from large metal objects, such as orthopaedic implants, can be problematic in treatment planning. These artefacts can make it difficult to contour anatomic structures accurately and may have to be manually compensated for in the treatment plan. Philips O-MAR algorithm isolates the effects of metal objects in the image data and uses this to reduce the associated artefacts. The system automatically produces conventional images and O-MAR images for the clinician to review. This can enhance the visualisation of critical structures and target volumes.
Marilyn continues, “The anatomy and Volumes of Interest that our clinicians need to outline for accurate radiotherapy planning can now be visualised more clearly. The difference in patients with bilateral hip replacements is remarkable. The radiotherapy planning software models the effect that tissue density has on a radiation beam for precise treatment planning. Before O-MAR, the artefacts caused by the implants gave false tissue densities that had to be managed; now the tissue density is far more representative of the anatomy imaged.”
At the core of O-MAR is an iterative loop where the output correction image is subtracted from the original input image. The resultant image can then become the new input image and the process can be repeated. The first step is to threshold the input image to create a metal-only image. The metal-only image consists of all the pixels set to zero except for those pixels categorised as metal. This image is then used to identify the projections within the sonogram data that have contributions from metal. If no large clusters of metal pixels are present in the image, then no further processing is performed. Therefore, O-MAR has no impact on non-metal images, and in addition, will not be applied to stents or similar small metal objects.