R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

26 Medical modelling
Unlike CT and MR, these techniques capture only the exterior topography
of the patient. This allows models to be made of the skin surfaces of
patients. These techniques are not yet considered regular medical imaging
modalities, and it is therefore possible that a non-medical scanning facility
will have to be used to capture the data. Many product development facilities
have access to this kind of equipment, although the nature of the equipment
can vary signifi cantly.
Non-contact surface scanning can be time consuming and potentially
expensive but is completely safe. The non-contact nature of the scanning
means there is less discomfort for the patient and no distortion of soft
tissues caused by the pressure applied when taking casts or impressions.
This advantage, in combination with the ability to manipulate data, makes
the approach particularly well suited to applications in prosthetic reconstruction
and rehabilitation. It is diffi cult, for example, to take a satisfactory
impression of a breast, therefore non-contact scanning may be used in the
creation of symmetrical prostheses for mastectomy patients. However, care
should be taken when scanning the body surface to ensure it is in the position
that relates to the intended use. For example, body parts that are
weight bearing will distort according to the position and posture of the
patient.
Data manipulation and high accuracy can be a signifi cant aid in prosthesis
manufacture, especially for large or complex cases. These techniques may
be a valuable aid to shaping and positioning the prosthesis, but the skill and
knowledge of the clinicians will always be required to determine the best
method of creating, colour matching and attaching the prosthesis to the
patient. Case studies illustrating some applications of non-contact scanning
are described in Rehabilitation applications in Chapter 6.
2.4.2 Anatomical coverage
Basic mistakes can be made through poor communication between the clinician
and technician. Detail can be lost when the scans do not cover the
whole region of interest or do not include suffi cient margins around the
anatomy of interest. Most scanners have a limited fi eld of view and several
scans may be required.
In addition, these techniques rely on ‘line of sight’. This means that areas
that are obscured or at too great an angle to the line of sight will not appear
in the scan data. Therefore, several scans may have to be taken from different
viewpoints to ensure all of the required details are captured. This
can be achieved by moving either the object or the scanner and repeating
the process. Depending on the shape of the object, many scans may be
necessary. When scanning faces, for example, a single scan will not acquire
data where the nose casts a shadow (see Fig. 2.14). However, this is