**11.1 Rapid scan time**

Because CBCT acquires all projection images in a single rotation, scan time is comparable to panoramic radiography. This is desirable because artifact due to subject movement is reduced. Computer time for dataset reconstruction however is substantially longer and varies depending on FOV, the number of basis images acquired, resolution and reconstruction algorithm and may range from ~1 to 20 minutes [41, 43].

## **11.2 Beam limitation**

Collimation of the CBCT primary X-ray beam enables limitation of the X-radiation to the area of interest. Therefore an optimum FOV can be selected for each patient based on suspected disease presentation and region of interest. While not available on all CBCT systems, this functionality is highly desirable as it provides dose savings by limiting the irradiated field to fit the FOV.

#### **11.3 Image accuracy**

CBCT imaging produces images with sub-millimeter isotropic voxel resolution ranging from 0.4 mm to as low as 0.09 mm. Because of this characteristic, subsequent secondary (axial, coronal and sagittal) and MPR images achieve a level of spatial resolution that is accurate enough for measurement in maxillofacial applications where precision in all dimensions is important such as implant site assessment and orthodontic analysis [42].

### **11.4 Reduced patient radiation dose compared to conventional CT**

Published reports indicate that the effective dose (E) varies for various full field of view CBCT devices from 29 to 477 μSv depending on the type and model of CBCT equipment and FOV selected patient positioning modifications (tilting the chin) and use of additional personal protection (thyroid collar) can substantially reduce dose by up to 40%. These doses can be compared more meaningfully to dose from a single digital panoramic exposure, equivalent CT dose, or the average natural background radiation exposure for Australia (1500 μSv) in terms of background equivalent radiation time (BERT). CBCT provides an equivalent patient radiation dose of 5–80 times that of a single film-based panoramic radiograph, 1.3–22.7% of a comparable conventional CT exposure or 7–116 days of background radiation.

#### **11.5 Limitations of CBCT imaging**

While there has been enormous interest, current CBCT technology has some limitations related to the "cone beam" projection geometry, detector sensitivity and resolution which is contrast. These parameters create an inherent image "noise" that reduces image clarity such that current systems are unable to record soft tissue contrast at the relatively low dosages applied for maxillofacial imaging.

Another factor that impairs CBCT image quality is image artifact three types of cone-beam-related artifacts:

**59**

*Role of Radiographic Evolution: An Aid to Diagnose Periodontal Disease*

pixels, which results in image distortion, streaking artifacts.

normal CT, as the exposure time is ~18 seconds.

Partial volume averaging: it occurs when the selected voxel resolution of the scan

Under sampling: under sampling can occur when too few basis projections are provided for the reconstruction. A reduced data sample leads to misregistration and sharp edges and noisier images because of aliasing, where fine striations appear in the image. Cone-beam effect: the cone-beam effect is the major source of error, especially in the parts which are outside of the scan volume. Because of the divergence of as it rotates around the patient in a horizontal plane. The amount of data corresponds to the total amount of attenuation along a specific beam projection angle as the scanner completes an arc. Because the outer row pixels record less attenuation, whereas more information is recorded for objects projected onto the more central detector

1.CBCT has a scanning time which rapid as in comparison with panoramic

2.It allows reconstruction with proper three dimensional and display from an angle.

3.Its beam collimation makes limitation of X radiation to the area of interest.

4.Images clarity produces images ranging from 0.4 mm to as low as 0.076 mm.

6.The units of CBCT reconstruct the projection data to provide inter relational

7.Reformation which is multiplanar is possible by sectioning volumetric datasets

8.Multiplanar image can be "thickened" by increasing the number of voxels.

9.Volume rendering which is 3D is processed by direct or indirect technique.

11.Reduced image artifacts: CBCT projection geometry, together with fast acquisition time, results in a low level of metal artifact in primary and secondary

The only disadvantage is its cost. But considering the enormous benefits, this

5.Radiation dosage of patient is reduced (29–477 μSv) in comparison with conventional CT (~2000 μSv). Patient radiation dose is six times lesser than

images in three orthogonal planes (axial, sagittal, and coronal).

10.The three positioning beams make patient positioning easy.

Indications of cone-beam computed tomography:

• Pathological lesions which are bone and soft tissue;

1.Assessment of the jaw which includes:

is greater than the spatial or contrast resolution of the object to be imaged.

*DOI: http://dx.doi.org/10.5772/intechopen.88035*

**11.6 Advantages of CBCT**

nonorthogonally.

reconstructions.

cost effect can be overlooked.

**11.7 Disadvantages**

radiography.

*Role of Radiographic Evolution: An Aid to Diagnose Periodontal Disease DOI: http://dx.doi.org/10.5772/intechopen.88035*

Partial volume averaging: it occurs when the selected voxel resolution of the scan is greater than the spatial or contrast resolution of the object to be imaged.

Under sampling: under sampling can occur when too few basis projections are provided for the reconstruction. A reduced data sample leads to misregistration and sharp edges and noisier images because of aliasing, where fine striations appear in the image.

Cone-beam effect: the cone-beam effect is the major source of error, especially in the parts which are outside of the scan volume. Because of the divergence of as it rotates around the patient in a horizontal plane. The amount of data corresponds to the total amount of attenuation along a specific beam projection angle as the scanner completes an arc. Because the outer row pixels record less attenuation, whereas more information is recorded for objects projected onto the more central detector pixels, which results in image distortion, streaking artifacts.
