Technology
- Conventional CT scanners make use of a fan-beam.
- Transmitted radiation takes the form of a helix or spiral.
- The data are then interpolated or re-binned by the scanner into a set of slices making up a volume.
- Large anatomical regions of the body can be imaged during a single breath hold, reducing the possibility of artifacts caused by patient movement.
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- Cone Beam Computed Tomography (CBCT) or Dental Volume Tomography (DVT) scanners (such as the i-CAT) utilise a cone beam, which radiates from the x-ray source in a cone shape, encompassing a large volume with a single rotation about the patient.
- Images are then reconstructed using algorithms to produce 3-dimensional images at high resolution.
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Design of machine
- Conventional CT makes use of a lie-down machine with a large gantry.
- When patients lie down, the soft tissues tend to collapse. This is of particular importance to orthodontists when predicting the tissue changes likely to result from specific tooth movements.
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- Because the i-CAT is a sitting-up machine, it offers more accurate information for dental practitioners.
- The radiation dose from a conventional CT also does not justify taking a CT scan of, for example, a child in order to make soft tissue measurements.
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Size of machine
- The size of a conventional CT scanning machine precludes its installation and usage in a dental surgery.
- A conventional CT scanner has to be large (and utilizes heavy duty engineering) because the gantry rotates at a very high speed.
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- The i-CAT is approximately the same size as a DPT/OPG machine, which makes it compact and easy to install (Height = 183cm, Width = 127cm, Depth = 149cm taking into account the space required for rotation of the gantry).
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Radiation Exposure
- The radiation exposure to a patient from a conventional CT is approximately 100-300 microsieverts (µSv) for the maxilla and 200-500 µSv for the mandible.[1]
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- The radiation exposure (for both mandible and maxilla) from the i-CAT is between 34-102 microsieverts (µSv) depending on the time and resolution of the scan.[2]
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Type of exposure |
Radiation Dose |
Conventional CT |
Maxilla – 250µSv
Mandible – 480µSv |
| Cone Beam CT |
Double jaw – 30-100µSv |
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Patient positioning
- Conventional CT require the patient's head to be manually tilted to create images suitable for the dentist's needs (e.g. parallel to the occlusal plane, the hard palate, or the lower border of the mandible). When positioning to the lower border of the mandible, the patient's jaw is tilted quite far upward with strain to the neck, which patients find uncomfortable.
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- Patient positioning is the same for all patients in the i-CAT. The patient's lower jaw is positioned in the chin cup and the forehead stabilised using Velcro straps if necessary. The scan is taken and the images can be re-positioned if necessary using the software.
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Artifacts
- Artifacts arising from metal restorations are more severe using conventional CT.
- More imperative to scan the patient parallel to the occlusal plane to eliminate artifacts in all the slices.
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- Artifacts that arise from metallic restorations are less severe with the i-CAT.
- It is less imperative to scan parallel to the occlusal plane to eliminate artifacts when using the i-CAT.
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Protocol selection
- Protocol selection (e.g., slice thickness) is often problematic with conventional CT. Occasionally, the technical scanner settings are not correct and not enough information can be gathered from the scan.
- The patient may need to be exposed a second time using conventional CT.
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- The options on the i-CAT allow for easy selection of either the mandible, maxilla or both with no need to select the slice thickness or how many slices are necessary thus decreasing the likelihood of re-exposing the patient.
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Training
- Conventional CT scanners require extensive training and scans need to be taken by radiographers.
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- The i-CAT is simple to use and training is provided by a qualified i-CAT engineer. The training required by an i-CAT operator is very similar to that required to operate a DPT or OPG.
- Since adequate training by way of image capturing and data interpretation is provided to any and all members of staff, there is no reason why a dentist and his staff will not be able to operate the i-CAT safely and effectively.
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References:
1. Dula K, Mini R et al. Hypothetical mortality risk associated with spiral computed tomography of the maxilla and mandible. Eur J Oral Sci 1996; 104: 503-10
2. Brooks SL. Effective dose of two cone-beam CT scanners: i-CAT and NewTom 3G. Quarterly Publication of the American Association of Dental Maxillofacial Radiographic Technicians, Winter 2005
