Cephalograms are two dimensional representation of 3-D anatomy. Our ability to derive meaningful infor- mation from head films depends on the reliability with which the anatomic relations can be evaluated.
Fortunately, orthodontists around the world have agreed on a reasonably high level of standardization in the methods used to acquire a cephalogram. The head position and orientation, source object distance and radiographic enlargement have been standardized to a degree that permits a common descriptive language of dentofacial morphology and the develop- ment of consistent methods of anthropometric landmark identification.
The information inherent in the large data collec- tions that cephalometric studies entail is of funda- mental importance to orthodontic diagnosis and understanding craniofacial changes due to growth or orthodontic therapy.
The analysis and acquiring of cephalometric data can be streamlined and made more efficient by using a new approach or a new analytic tool. This techno- logy must be able to store, score, retrieve and ana lyze vast collections of information.
Th.is new approach has two important features. First, it applies powerful mathematical technologies to describe and analyze morphologic structures and secondly, this from can be analyzed by the computer. In the past two decades we have witnessed the development of number of systems for the computer aided encoding of data from lateral cephalograms for use in craniofacial research and clinical treatment planning,
follows:
DIGITIZATION
Digitization is the form by which analog information is converted to digital form. The methods involved can be either direct or indirect.
During digitization X-Y coordinates of cephalo- metric landmarks are recorded and stored in a data set. This data set is the starting point for the formulation of various computer generated VTO's and STO's.
Direct Computer Digitization
A digitizing tablet or digitizer is used for this purpose. Digitizers may be opaque, translucent or transparent.
Translucent or transparentdigitizers can be backlit, allowing direct digitization of cephalo-grams without any intermediate acetate tracing.
Resolution and Accuracy
Resolution is the smallest distance that can be resolved by the digitizer in the order of 1000 lines/inch.
Accuracy is the precision with which a digitizer can record reported movements over various regions on its surface. Tt should be in the order of±0.25 mm for cephalometric application.
The anatomical points are entered using on electronic pen or instrument. The digitizing tablet is made up of a fine electric grid that includes registration points as fine as 0.009 mm apart. This electronic instrument emits an electronic signal either on command or continuously. Various varieties of instruments are available for this purpose. The two most commonly used are:
• Electronic pen • Cross hair cursor.
Electronic pen An electronic pen is activated to emit a
signal when the tip of the pen is pressed against the film or a button on the pen is pressed. Electronic signals are emitted directly from the pen to the grid completing the circuit.
Cross hair Cl/rsor This potentiometer consists of two wires arranged in a cross hair pattern which are embedded into a glass window. The electronic
wires. The operator presses a button to activate the potentiometer.
The cross hair cursor is less popular now because: i. The digitizer is bulky and not very easy to use. ii. Glow from the glass in which the wires are
embedded prevents optimum mapping of the various landmarks.
Indirect Digitization
For indirect digitization, a video camera or mapper captures an image of the cephalometric radiograph and stores it in the computer. The video camera must be calibrated with the cephalometric film being plowed into the computer. The digital radiography is another method of data input into the computer.
The image is then displayed on a monitor and the landmarks are identified using a mouse. The only disadvantage of this method is that the digitizing resolution obtained is lesser than that obtained with a digitizer.
Mode of digitization • Point mode • Stream mode
Point mode (Fig. 12.3A) The user sequentially locates
landmarks in a pre-determined order recording one coordinate pair for each landmark. It basically involves the direct location of individual landmarks.
A visual representation of a cephalogram is generated by connecting discretely located points due to their proximity and sequence, making a visual representation of a cephalogram possible.
Stream mode (Fig. 12.3B)Here the operator 'traces' a
cephalogram using the digitizing device and the tracing thus obtained in the form of a stream of points controlled by a programmable option. The points are recorded as a specific number of coordinate points per second or after the cursor has moved a certain distance. The points when joined form audible contour and this analogy is easily accepted by the computer.
Fig. 12.3B:Streammode
Advantage and disadvantage
Point mode digitization is more time consuming
but more accurate.
Stream mode digitization is less accurate due to the manner in which the data is acquired. The location of hard and soft tissue landmarks must be provided to the predictive software. Point mode reliably provides location of the landmarks whereas the stream mode does not.
A coordinate pair may not be transmitted as the cursor passes over a landmark such as the sub- nasale. Thus, the accuracy of landmark location suffers here.
Also, to locate a point and determine a change in its position over time it is helpful to know the precise position of the landmarks relative to its origin. The accuracy of such information can be obtained only by way of point digitization. Direct Digitization
Computer softwares that allow direcl digitizalion of cephalometric radiographs are currently available. These software programs perform various orthodontic
and surgical movements based upon parameters entered by the user into the program.
'Dentofacial planner' program has a digitization regimen for the lateral-cephalogram consisting of 68 landmarks of which 43 are soft tissue landmarks and the remainder are hard tissue landmarks.
'Quick ceph', another commercially available program uses a 28 landmarks regimen withallpoints representing hard tissue landmarks.
Both programs can perform a variety of functions with the digital cephalometric radiograph including: • Cephalometric analysis (Fig. 12.4A)
• Superimposition (Fig. 12.4B) • Growth estimation
• Orthodontic treatment planning • Surgical prediction.
The hard tissue positions as predicted in the expected surgical procedure are fed into the program and the expected soft tissue changes are then eaIculated.
The soft tissue profile prediction is made possible by the application of ratios of soft tissue and hard tissue changes to the surgical movements that are within the program.
Fig. 12.46: Superimposition of profile photograph over lateral cephalogram and cephalometric plotting
These ratios are based upon retrospective studies of stability and soft tissue changes.
CEPHALOMETRIC ANAL VSIS