• Patient Preparation
• Artifacts Caused by Metallic Restorations
• Using Radiopaque Markers To Make A CT Scanning Guide
• Choosing The CT Scanning Plane
• Interpreting CT images

Patient Preparation

This guide will help you to gain the maximum benefit from your IDT Dental Scan. Inadequate or incorrect patient preparation may significantly reduce the diagnostic value of the procedure.

1. Ensure that your patient is familiar with the procedures involved and stress repeatedly the importance of keeping completely still for the duration of the scan.
2. Give your patient a copy of the IDT sheet "Information for the Patient", or read our 'Frequently Asked Questions'.
3. Provide your patients with a suitable means of maintaining their jaws in a fixed relationship with the bite open 1-2mm vertical dimension. This is best achieved by using custom made diagnostic stents with "markers" to provide tooth position indices and cold-cure occlusal stops to keep the jaws slightly open.
   
   Please click here for more information about making scanning stents.
4. If the patient has a deep overbite and/or metal restorations, be sure to use some form of inter-arch separation to create a clear region between the opposing occlusal surfaces. This will prevent artifact from teeth in one jaw compromising the images in the region of interest of the opposing jaw.
5. Provide the radiographer with clear and concise instructions as to whether the patient is to remove or wear their existing dentures for the scan, or if they are to use wax bite rims or diagnostic stents provided by you.
6. Provide the radiographer with clear instructions as to whether scans are to be taken parallel to the occlusal plane. the hard palate, or the lower border of the mandible (see Guides & Publications).

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Artifacts Caused by Metallic Restorations

Metallo-ceramic restorations and large amalgam fillings may obscure detail at the level of the occlusal plane but rarely affect bony detail below the alveolar crest. The main exceptions are root canal fillings and gold or precious metal alloy posts that extend down into the bone.

Titanium and titanium alloy produce much less severe artifacts. Consequently, existing titanium implants are well visualised by CT. (see Guides & Publications).

Artifacts are always more pronounced on the axial slices than on the panoramics or cross sections. This is because computer generation of the panoramics and cross sections involves algorithmic processes that minimise the artifact in these reformatted images. Additionally, standard image processing techniques can sometimes be introduced to minimise the effects of artifact.

Judicious patient positioning often allows artifact to be directed away from the region of interest. (see Guides & Publications).

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Using Radiopaque Markers To Make A CT Scanning Guide

The positions of the desired implant sites can be indicated radiographically in one or more of the following ways:

• Small markers inserted into the buccal flanges of a custom-made diagnostic stent. These should be approximately 3 mm in diameter and made from model stone , glass ionomer, or gutta percha. Leave air spaces in the stent indicating the positions of the missing teeth. These will produce black outlines clearly visible on the CT images.
• Radiopaque teeth made from 15-20% barium sulphate powder stired into liquid acrylic.
  
  Barium sulphate powder can be obtained from:
  Dent-o-care Ltd., 7 Cygnus Business Centre, Dalmeyer Road, London, UK, NW1O 2XA
  - Tel. +44 (0)20 8459 7550.


Radiopaque teeth can be purchased from:
Ivoclar Vivadent Ltd., Warrens Business Park, Leicester, UK, LE19 4SD
- Tel. +44 (0)116 284 7880.

Use only a very small quantity of radiopaque material. CT is many times more sensitive than dental x-ray film. Too much contrast will result in unsightly artifacts.

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Choosing The CT Scanning Plane

The optimal scanning plane depends on the architecture of the patient's mouth, and is therefore left to the referring dentist / surgeon to specify. The desired CT scanning plane should be indicated in writing to the radiographer on the referral form.

The reformatted panoramic and cross-sectional images are always generated at right angles to the CT scanning plane. To ensure that these images have the same vertical orientation as the intended implant placements, choose the CT scanning plane to be at right angles to the direction in which you want the implants to go.

Implants are usually orientated so that the biting forces pass vertically along their axes, i.e. at right angles to the occlusal plane. This means that, in most cases, the best choice of CT scanning plane is parallel to the occlusal plane (See Figure below). The correct choice of CT scanning plane can help minimise the effects of artifact from metallic restorations. It is always best to scan parallel to any metallic restorations.

Figure: patient scanned parallel to the occlusal plane.

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Interpreting CT images

Under IR(ME)R 2000 regulations in the UK and SI 478 in Ireland, all x-rays must be "read" and the results recorded. The referring dentist is normally the person responsible for reading and interpreting the CT images. All referring dentists are advised to obtain the training necessary to read and interpret CT images. IDT does not interpret the images or provide a radiologist's report.

When taking distance measurements from cross-sectional images, care must be taken to ensure that the cross-sectionals are parallel to the direction in which the implants will be placed, otherwise misleading information may result. This is especially important when working from hard copy such as film. The cross-sectional views are always generated at right angles to the axial slices and on hard copy, this direction cannot be adjusted.

With interactive software such as SimPlant the implants can be adjusted on the 3D view, thus the correct three-dimensional distance can always be obtained.

[Click to enlarge image]

In the posterior mandible for example, one might argue that it is more intuitive to measure at right angles to the alveolar ridge, however this is not necessarily the direction of the cross-sectional images (see example below). In such cases it is important to take measurements from both the cross-sectional and panoramic views, and if necessary to make a mathematical correction for the oblique direction of the implant with respect to the reformatted planes.

CT images are extremely accurate, however, failure to take the direction of measurement into account (or to use suitable implant planning software) may lead to unexpected or erroneous results.

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