R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

34 Medical modelling
be lossless or preferably avoided completely. Any loss of information may
reduce the accuracy of models made from the data set. In addition, some
compressed formats are unreadable by third party software, rendering the
data useless. Compression for medical modelling is not often necessary. In
normal circumstances, although the image data may be several hundred
megabytes, it can usually be accommodated on a single CD-ROM.
3.1.4 Manual import
When the data is not in a DICOM compatible format or is from a manufacturer
that is not directly supported by the software application being
used, manual import must be used. Error free manual import of medical
scan data will thus require access to all of the parameters of the data. Table
3.1 lists the parameters that must be known to import data successfully.
3.2 Point cloud data
The data captured by a non-contact surface scanner is merely a ‘point
cloud’. This is a collection of thousands of point co-ordinates in threedimensional
space. It is typical to convert this into a more useful format
before applying the data to analysis or visualisation. There are a number
of formats used for point cloud data and the format will depend on the type
of scanner being used.
The simplest forms of export format are polygon meshes, such as the
commonly used STL fi le format (the STL fi le format is fully described in
Section 4.6.2). However, as the export data format used will depend very
much on the anticipated use of the data, this area is explored in greater
depth in Chapter 4. Some techniques can be applied directly to the point
cloud data. These are typically carried out to remove erroneous points or
simply reduce the number of them and to decrease noise. This has the effect
of cleaning up the data set and reducing the fi le size.
Table 3.1 Information required for manual data
import
Number of images Integer
File header size In bytes
Inter image fi le header size In bytes
Image size In pixels e.g. 512 × 512
Slice distance In millimetres
Field of view or pixel size In millimetres
Gantry tilt In degrees
Scan orientation Left–right or right–left