in the head region are poor delineation of bony struc- tures as well as current technical limitations. For ex- ample, a phase distortion of up to several millimeters may occur in MRI data sets. Accuracy is also compro- mised by the use of 3- to 4-mm slice thicknesses (thinner slices are extremely difficult to acquire for technical reasons). Moreover, the relatively long ac- quisition time increases the risk of motion unsharp- ness (especially of the eye), making the data difficult to interpret.
The limiting factors in the head region with regard to radiation exposure are the ocular lens and the thy- roid gland. The acquisition of axial CT scans can de- crease radiation exposure by a factor of 30 compared with coronal CT scans (Hassfeld 2000). The slice thickness used for CT data acquisition is 1 mm. Slice thicknesses of 2 mm or more may introduce errors of up to 2 mm, particularly along the Z-axis. With a slice thickness of 1 mm, a table increment of 1 mm, and a pixel size of 0.5 mm, an effective accuracy of approx- imately 0.4 mm can be achieved.
A sectional imaging study is needed in order to pro- duce a virtual 3D model of the patient. The software programs process data sets that have been acquired by CT, digital volume tomography (DVT), MRI, or a combination of these modalities. The advantages of MRI are better visualization of soft tissue and the ability to acquire images without exposure to ioniz- ing radiation. But since the diagnostic workup is based largely on bony structures, the principal source of image data is spiral CT, which may be supplement- ed by MRI data sets in selected cases. The advantages of CT include the markedly better delineation of bony structures, better visualization of foreign bod- ies, relatively short acquisition times, and consider- ably lower costs. The use of DVT can significantly re- duce the radiation exposure associated with CT scan- ning. DVT should not be used in tumor patients be- cause of its relatively poor soft tissue discrimination, but it is preferred over standard CT scans in implan- tology, dysgnathic surgery, and especially in trauma- tology. The disadvantages of MRI-acquired data sets
Imaging Procedures
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