R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

24 Medical modelling
be uncomfortable and perhaps distressing for the patient, and the added
costs and delays incurred by the radiography department should be
considered.
2.3.5 Orientation
As with all medical imaging, anterior-posterior and inferior-superior orientation
is usually visually obvious but lateral orientation is ambiguous.
Usually this is not a problem with automatically imported data but, when
manually importing data, it is important that the correct lateral orientation
can be ascertained.
2.3.6 Image quality and protocol
MR image data is typically taken to investigate areas of specifi c illness or
locate pathology, such as a tumour. Usually, only as many images as are
required to identify the problem are conducted; consequently it is not
common practice to undertake three-dimensional MR scans. Conducting a
three-dimensional MR scan may take signifi cantly longer than a normal
session, and the compromise between scan time and the necessity of the
three-dimensional data has to be considered. As conducting MR scans is
expensive and a critical resource in most hospitals, increasing the scanning
time may cause problems and it increases the inconvenience for the patient.
Close collaboration with the radiographer is recommended to ensure that
the data are of suffi cient quality without creating problems. The confi guration
of the MR machine may be enhanced by the addition of more coils,
which has the effect of increasing the signal strength. Such confi gurations
may be used by specifi c specialities such as neurosurgery.
Unlike CT images, altering the protocol of an MR scan can dramatically
alter the nature of the image. By varying the sequence and timing of excitation
and emission of the radio waves, different effects can be achieved.
These may serve to improve the image quality for specifi c tissues or improve
contrast between similar adjacent tissues.
2.3.7 Artefacts
This is the general term for corrupted or poor data in MR images. These
may result from patient movement or magnetic effects.
Movement
A good quality MR scan depends on the patient remaining perfectly still
throughout the acquisition. Movement during the acquisition will lead to