[PDR03] RECOMMENDED CT-SCAN PROTOCOLS

Transcription

1 SURGICAL & PROSTHETIC DESIGN [PDR03] RECOMMENDED CT-SCAN PROTOCOLS WORK-INSTRUCTIONS DOCUMENT (CUSTOMER) RECOMMENDED CT-SCAN PROTOCOLS [PDR03_V1]: LIVE 1

2 PRESCRIBING SURGEONS Patient-specific implants, guides, surgical plans, and medical model designs rely on digitall models as a foundation - produced from 3D Computed Tomography (CT) data. If you have not already requested the scan then you should read the following Information for Radiographers and Radiologists and base your instructions on them. You should inspect the results of the 3D scans to ensure they cover the area of interest plus additional margin. RECOMMENDED CT-SCAN PROTOCOLS [PDR03_V1]: LIVE 1

3 RADIOGRAPHERS & RADIOLOGISTS For the best results, the distance between the slices should be as small as possible. A slice distance of <1 mm to 1.5 mm will give excellent results. The quality of the virtual models and custom device accuracy will suffer slightly if the slice distance exceeds 2 mm. If possible, the slice thickness should be equal to the slice distance. For models involving the orbits or ethmoid bones, a colimation of 1 mm with 0.5mm overlap is preferred. Both stepped and spiral scans are suitable. Preferably, gantry tilt should be 0 degrees. If you have image options Smooth images usually perform better. Most scanner formats can be translated but there are some problems associated with compressed images. Whenever possible uncompressed DICOM images should be used. RECOMMENDED CT-SCAN PROTOCOLS [PDR03_V1]: LIVE 2

4 ARTEFACTS Occasionally when conducting CT scans artefacts can occur. This happens when dense objects scatter the X-rays resulting in a streaked appearance in the scan image. Objects that cause artefacts are usually metal items such as fillings, braces, screws, plates and implants. Artefacts in CT image. With regard to producing a digital model for device design, these effects can be edited out to produce a normal looking model. However, this does mean that the accuracy of the model in areas affected by the artefacts cannot be guaranteed. In most cases, this does not usually affect the usefulness of the whole model. 3D reconstruction. As cases showing artefacts usually occur in and around the teeth, a dental cast can always be used in conjunction with any physical medical models. Edited 3D reconstruction. RECOMMENDED CT-SCAN PROTOCOLS [PDR03_V1]: LIVE 3

5 NOISE Occasionally when conducting CT scans noise can occur. This happens when dense anatomy requires high doses that result in a fuzzy scan image. Although these images may be visually acceptable, they are impractical for producing digital 3d models as a basis for device design. Good modelling depends on identifying a smooth boundary between bone and soft tissue. Noise destroys this boundary which results in poor 3D reconstructions and consequently poor digital (and physical) models. Noise in CT image. If the effect only occurs in a few images, it may be possible to edit them out to produce a normal looking digital model - smoothing algorithms can be applied by PDR engineers. If the data is judged to be too poor for accurate model production, you will be notified. Close up of noise. Please note that this affect commonly affects areas through the shoulders and hence vertebrae in the lower neck and upper back Poor 3D reconstruction. RECOMMENDED CT-SCAN PROTOCOLS [PDR03_V1]: LIVE 4

6 SMOOTH VERSUS SHARP (Trial results from CT data kindly supplied by the Neuro-CT Department of Frenchay Hospital): Sharp Axial data after initial thresholding for bone. Smooth Axial data after initial thresholding for bone. RECOMMENDED CT-SCAN PROTOCOLS [PDR03_V1]: LIVE 5

7 Although the smooth image contrast appears poor on screen (Windows can only show 256 greyscales), taking a density reading shows that the actual contrast is good and that the smooth data allows a much lower threshold to be used. Thresholding Sharp Axial data. Thresholding Smooth Axial data. In general, if a medical model is required the smooth kernel should always be used for 3D CT scans. Even with these simple examples, it can be seen that the smooth data gives better results. RECOMMENDED CT-SCAN PROTOCOLS [PDR03_V1]: LIVE 6

8 A low quality 3D reconstruction of sharp data. A low quality 3D reconstruction of smooth data. RECOMMENDED CT-SCAN PROTOCOLS [PDR03_V1]: LIVE 7