Dental imaging


expensive implant work fail and to avoid the “welling up” of red stuff in the floor of mouth due to pranging a neurovascular canal or misaligning the implant completely, perhaps a timely bit of imaging is required. So we dust off that radiolo- gy book and apply the knowledge. Conventional views have served for a num- ber of years with well-placed per-apicals (paralleling tech- nique) and the trusty panoramic, but the illusive third dimension is missing. There are those at confer-


ences who will sell the line that they can eyeball an implant into the mandible from a thousand yards, using skills honed over a thousand cases. The realities may be quite different to the novice operator on the implant scene.


nerves are is all-important in the quest to give a predictable result and avoid operator hyperten- sion. A bit larger than an OPG machine, obtaining its images by circling the head in anything between 10 and 40 seconds, the CBCT produces cross-section- al images that can be viewed from three plains: axial, coronal and sagittal. Variable thickness- es in section can be shown, down to 0.25mm.


In addition, using a software


package, the data can be made so that the acquired volume (field of view) can be viewed like the trusty old friend “the dental panoramic”. 3D recon- structed images of the jaw are possible and can be manipulat- ed at the chairside, sending the patient into raptures of delight as they see their inner-self revealed.


Having spent a serious amount of cash acquiring the impressive looking “Daddy of all dental imagers”, the question arises as to who is going to take the scan?


Alas, 3D imaging systems can


offer some peace of mind. Visualisation of the bucco-lin- gual dimension is now possible – seen in the coronal plane, remember. Sagittal, coronal and axial, the orthogonal planes used by anatomists and radiol- ogists to describe sections. The fog of the undergraduate memory clears.


The cone beam CT scanner Since units have become avail- able on the high street, dentistry has been quick to appreciate the benefits of an office-based (how American), low-dose (compared to med- ical CT), 3D imaging device, the cone beam CT (CBCT) scanner can address many of the problem solving aspects of diagnosis. With regard to implants, knowing where those pesky


So, CBCT imaging – can’t be that difficult, can it? Perhaps I should get one for the practice?1CBCT comes in vari- ous shapes and sizes and are classified as large field of view (FOV and small FOV. The small- er field size can offer dose reduction and limited viewing of teeth and immediate sur- rounding structures. Super-crisp images in the


brochures tempt one with res- olution “to die for”. Was the picture produced on a patient though, I hear you say? What is not immediately


apparent is that there are nuances to the CBCT systems. For example, the 0.4mm voxel setting gives better pictures than the 0.2 mm voxel setting on iCat. This comes down to the maths involved in the imaging algorithm. The system utilises a limited tube current in order to


keep dose down. No more, you are losing me, you might say… Put simply, this is essentially good news for the patient as it cuts down the rays. Having spent a serious amount of cash acquiring the impressive looking “Daddy of all dental imagers”, the question arises as to who is going to take the scan? In the opinion of this author, it should be a medical/ dental professional with appro- priate training. This may include a radiographer, or a den- tist. One should appreciate that the use of dental nurses or hygienists even with a dental radiography qualification is contentious in the acquisition of CBCT images at this time.


IRMER issues In order to comply with the radiation regulations, a great deal of effort has to go into informing the Health and Safety Executive that you have the CBCT scanner and into docu- menting your IRMER 2000 and IRR1999 protocols. CBCT room design will have


to allow for radiation protection features to cope with a 120KV beam energy. This 120KV beam has increased penetrating power, compared with dental sets at approximately 65KV to 70KV. A radiation protection advisor (a medical physicist) will have to verify this has been carried out correctly in compliance with IRR1999. All the IRMER documents must be in place, which entitle the various staff members to carry out their roles under IRMER with regard to the CBCT machine. For example, who is acting as employer, refer- rer, operator and practitioner. Protocols must be in place


itemising all possible CBCT imaging situations, detailing who can refer for them and under what kind of situations – that is, what “justification” is required. All must be docu- mented and be robust enough to withstand an IRMER inspec- tion. This gets a little complicated, but is distillable with some effort. Also, in terms


Continued » Ireland’s Dental magazine 17


Fig 1: Mass in the left piriform fossa (see axial section)


Fig 2. Lateral lingual canal situated in canine region of mandible (sagittal section)


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