Conventional spiral and low-dose computed mandibular tomography for dental implant planning

Abstract

Absorbed doses to radiosensitive organs in the head and neck from pre-implant conventional hypocycloidal, conventional spiral and computed tomography (CT) were measured with thermoluminescent dosimeters in an anthropomorphic phantom head. From conventional tomography organ doses, except to the major salivary glands, were below 0.2 mGy. They were considerably higher with CT than conventional tomography. Tomographic images of posterior lower jaw regions were evaluated regarding measurement reliability in conventional spiral tomograms. Observers measured the distance marginal crest-mandibular canal, marked their measuring points and estimated the length of an intended Brånemark implant. The variability between observers, mainly due to the intra-observer variation, decreased with multiple readings and optimized image quality. The suggested implant length was the same as the one inserted in 70% of the cases. Tomographic images from mandibular body segments were used to test the influence of radiation exposure and scanning mode on image quality in CT and compare the quality of the CT images with that in conventional spiral tomograms. Mandibular canal and marginal bone crest visibility was unaffected by radiation dose in CT examinations. Differences among CT scanning modes and between CT and conventional spiral tomography were highly significant. Frontal CT scans or conventional spiral tomograms were preferred. In cross-sectional images of the posterior lower jaw image quality in conventional spiral and reformatted CT at 40 mAs and 80 mAs was compared. Observers graded the acceptability of images for implant planning and traced the contours of the mandibular body and canal. Conventional, spiral tomograms were subjectively preferred over reformatted CT images. Differences between CT images at different mAs settings were not statistically significant. The mandibular canal was more frequently untraceable in high-dose CT. The use of tomography for dental implant planning was studied by means of a survey among oral radiology clinics in Sweden and implantology clinics in other countries. It was used by 93.4% but varied markedly between and within anatomic regions. CT was used by 73%. Radiation doses varied considerably within and between different CT brands. The availability rather than the clinical need strongly influenced the choice of technique. From the point of view of radiation dose and information necessary for implant planning conventional spiral tomography is to be preferred over reformatted CT. However, when CT is the only technique at hand, it can be performed with a lower than standard mA-setting.