January 1, 2014
by David MacDonald, BDS, BSc(Hons.), LLB(Hons.), MSc, DDS(Edin.), DDRRCR, FDSRCPS, FRCD(C)
WHAT IS CONE-BEAM COMPUTED TOMOGRAPHY?
Cone-beam computed tomography (CBCT),1 unlike medical CT, uses a cone beam instead of a fan beam (Fig. 1). Since the cone irradiates a larger volume in a single rotation (nowadays with as little as 180 degree arc.) the radiation dose imparted is much lower than that by a fan beam, which needs many rotations to cover the same volume of patient.
FIGURE 1: The fan beam upon which medical (helical) computed tomography is based interrogates only a slice of tissue, whereas the cone beam of cone-beam computed tomography interogates a three dimenstional region with in a 360-degree rotation (now often less–as low as 180-degrees). Reprinted with permission from MacDonald-Jankowski DS, Orpe E. Computed tomography for oral and maxillofacial surgeons. Part 2: Cone-beam computed tomography. Asian Journal of Oral and Maxillofacial Surgery 2006;18: 85-92.
HOW ELSE IS CBCT SUPERIOR TO MEDICAL CT?
Because CBCT produces isotropic cuberilles (cubic pixels) directly from the dataset WITHOUT going through a VOXEL middleman (Fig. 2) – it produces excellent spatial resolution in ALL THREE planes. In addition CBCT can produce better spatial resolution than even the best medical CT.
Why has it become so important to the present-day dentist to at least know something about it, in particular who attends to children (and their parents)? In addition to the requirement of the dental professional to be informed about developments in clinical dentistry, due to press-activity the public (their potential patients) have already been informed about this technology and the risks it poses.2,3
Figure 2: The fan-beam computed tomography achieved 3-dimensional reconstruction by slicing the voxel into cuberilles, each with the same attentuation coefficient as the original voxel. Reprinted with permission from MacDonald-Jankowski DS, Li TK. Computed tomography for oral and maxillofacial surgeons. Part 1: Spiral computed tomography. Asian Journal of Oral and Maxillofacial Surgery 2006;18: 68-77.
Figure 3: Cone-beam computed tomography recontructs the three-dimensional images by generating cuberilles directly, each with its own attentuatuion coefficient. This allows three-dimensional reconstructions with better resolution in the z axis, in addition to the axial (XY) plane. Medical computed tomography, except the most modern multiple-detector computed tomographic (MDCT) units can only produce cuberilles secondarily from voxels (Fig. 2.) Reprinted with permission from MacDonald-Jankowski DS, Orpe E. Computed tomography for oral and maxillofacial surgeons. Part 2: Spiral computed tomography. Asian Journal of Oral and Maxillofacial Surgery 2006;18: 85-92.
Although the radiation dose of CBCT is generally lower than medical CT (this is not true for all makes),4 it is still higher than conventional radiographs. Conventional radiographs should used whenever radiography is indicated. For example, conventional cephalograms for orthodontic planning and parallax for unerupted teeth, such as maxillary canines.
The child patient is more radio-sensitive as he/she is developing, as well as growing. Furthermore, the child patient has a whole life-time ahead of him/her to develop tumours which were initiated in childhood. The methodologically-flawed paper which linked dental radiation exposure with brain tumours (meningiomas)5,6 may actually turn out, 20 to 40 years later, considering the current level of CBCT use on children today. Therefore, although that paper is flawed, it contains a kernel of truth that we ought to heed. The child-patient should be imaged gently!
The ‘Image Gently’ campaign was established in 2007 by the Alliance for Radiation Safety in Pediatric Imaging to reduce the radiation dose to the child-patient.7
WHAT IS THE EVIDENCE-BASIS FOR CBCT USE?
Today, there are 2345 published papers on PubMed under CBCT of which ‘CBCT and children’ account for 172; many pertaining to endodontics. The majority are case reports. More specific searches, ‘CBCT and Craniofacial Anomalies’ produced only eight (of which two were concerned with Turners Syndrome) and ‘CBCT and cleft lip and palate’ produced 14, all since 2010.
WHAT ARE ITS DOWNSIDES?
CBCT undoubtedly produces a higher radiation dose than conventional imaging. By reference to Benavides et al’s Table 2, a synthesis of dosimetry reports almost all published in the last five years, the CBCT radiation dose is at least 10 times more than a panoramic radiograph8 rather than just one or two panoramic radiographs as frequently suggested. Furthermore, the better the spatial resolution, the higher the dose. Although a higher spatial resolution is required in many cases referred to endodontic offices, the evidence of its indications in other areas has not yet been produced. Nevertheless, when a higher spatial resolution (SP) is deemed necessary, a small a field-of-view (FOV) should be used; usually this is five-by-five cm. Nevertheless, the reader should be aware that the radiation dose delivered by a certain CBCT unit of a small FOV with a high SP may exceed that of a large FOV used with a low SP.9
WHEN DOES CONE-BEAM COMPUTED TOMOGRAPHY PROPERLY COMPLEMENT THE WORK OF THE ORTHODONTIST AND THE PEDIATRIC DENTIST?
The need for CBCT for patients with craniofacial anomalies, including cleft-lip and palate, is clearly understood. Generally, although they deliver a much lower radiation dose than medical CT,4 which makes them more attractive for the more frequent exposures these patients require as a result of their staged treatments, they cannot completely displace medical CT. Those patients who require sedation and therefore need to be supine, will need to be investigated by medical CT, since almost all CBCT units require the patient to be standing or sitting vertically, except the NewTom VGi.
Other than craniofacial anomalies, CBCT assists the orthodontist in determining the precise position of unerupted teeth, particularly the maxillary canines and the extent of any root resorption. This can generally be achieved with a small FOV. Nevertheless, this should only be undertaken if conventional radiography, including parallax technique, have been equivocal. Figure 4 shows the CBCT of such a case.
FIGURE 4: Cone-beam computed tomographic (CBCT) recontructions of bilateral unerupted maxillary canines for whom the parallax technique did not suffic
e. The parallax technique is standard for all cases of unerupted canines(and other teeth) presenting to the orthodontists of the faculty of Dentistry of the University of British Columbia. The CBCT displays the intimate relationship between the crowns of unerupted canines and the roots of the erupted lateral incisors. Reprinted with permission from MacDonald-D, Bunting N, Ford N.CBCT: training and patient selection. The Bridge (The organ of the British Columbia Dental Association) Nov/Dec 2013. Pages 20-22.
Although, incidental findings are common on CBCTs taken for orthodontic purposes Drage et al, reported that they do not affect orthodontic treatment planning.10
HOW CAN CONE-BEAM COMPUTED TOMOGRAPHY TECHNOLOGY BE IMPROVED?
There are three technical improvements which will further transform the clinical applications of CBCT. They have better spatial resolution, improved (lower) radiation dose and improved software. Ultimately, these combined with a small FOV could mean improved imaging in caries diagnosis resulting in a 3D reconstruction of carious lesion. Needless to say, the radiation dose would need to be reduced to minimal levels, which are at least equal to a single bitewing investigation for the same area. Therefore we should not expect to see this facility on the market any time soon.
As with any technology, optimal use can only be acquired by proper training and practice. Hitherto, operators have relied solely on the dental trade to provide this training at the time of installation of the unit. The use of small FOVs mean that considerable technical expertise would be required to ensure that the patient is optimally positioned to avoid the need for a retake. Furthermore, the time spent on positioning the patient cannot be excessive as the child may tire and move, resulting in movement artifacts; see Aps’ Figure 2.11 Most CBCT units have a ‘scout’ view facility, which allows the operator to ensure that the patient is optimally positioned prior to making the definitive exposure. Although, this facility should only confer a minimal radiation dose in contrast to the full scan, this should be confirmed with the manufacturers, perferrably at the time of the initial purchase and be written into the mantenance contract. This ‘scout’ facility should be used routinely, particularly for the first scans performed by the novice operator and when a new or unfamiliar program is being used.
WHO ARE QUALIFIED TO PRESCRIBE AND REPORT CBCT DATASET?
Because of the great potential to overuse or misuse CBCT, dentists who prescribe and report these datasets do need to properly trained, examined and registered in order protect the public, particularly the more vulnerable children. In Canada, most provinces and territories have taken some step towards obligatory training, examination and registration. The most advanced in this regard is Ontario, with Alberta not far behind. The Royal College of Dental Surgeons of Ontario now recognises two types of practitioners permitted to prescribe and report CBCT datasets.12 Non-oral and maxillofacial radiology specialist dentists prescribing and reporting datasets made on CBCTs using FOVs of eight by eight or less are required to undertake and successfully complete, by examination, a two day (or more) theorectical and practical course affliated with an accrediated university. The investigation can only be confined to the dentoalveolar structures, excluding the temporomandibular joints, etc. For those who prescribe and report datasets of FOVs of eight by eight or more or image structures other than dentoalveolar structures, nothing less than successful completion of an accredited program in oral and maxillofacial radiology would suffice.12
The word ‘routine’ plays no role in medical or dental radiology. There must always be clear clinical indications for every exposure in order to keep the radiaiton dose as-low-as-reasonably-achieveable; ALARA! OH
Dr. David MacDonald is an Oral Maxillofacial Radiologist. He is a Fellow of the Royal College of Dentists of Canada and is Chair of the Division of Oral Maxillofacial Radiology at the Faculty of Dentistry, University of British Columbia.
Oral Health welcomes this original article.
1. MacDonald D. Oral and maxillofacial radiology; a diagnostic approach. Wiley-Blackwell. 2011. Pages 49-66. ISBN-13:978-0-8138-1414-8/2011.
2. Roslin A. Radiation safety of dental X-rays questioned. The Georgia Straight. 15-22 Aug, 2013. Pages 28-30. http://www.straight.com/life/409161/studies-link-dental-x-rays-brain-tumours-thyroid-cancer-and-low-birth-weight
3. Bogdanich W, McGinty JC. Radiation worries for children in dentists’ chairs. The new York Times. 22nd Novermber 2010. http://www.nytimes.com/2010/11/23/us/23scan.htmlLudlow JB, Ivanovic M. Comparative dosimetry of dental CBCT devices and 64-slice CT for oral and maxillofacial radiology. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2008 Jul;106(1):106-14.
4. Claus EB, Calvocoressi L, Bondy ML, Schildkraut JM, Wiemels JL, Wrensch M.Dental x-rays and risk of meningioma. Cancer. 2012 Sep 15;118(18):4530-7.
5. White SC, Hildebolt CF, Lurie AG. Dental x-rays and risk of meningioma. Cancer. 2013 Jan 15;119(2):464.
6. The Alliance for radiation safety in pediatric imaging. Image safely. 2007. http://www.pedrad.org/associations/5364/ig/
7. Benavides E, Rios HF, Ganz SD, An CH, Resnik R, Reardon GT, Feldman SJ, Mah JK, Hatcher D, Kim MJ, Sohn DS, Palti A, Perel ML, Judy KW, Misch CE, Wang HL. Use of cone beam computed tomography in implant dentistry: the International Congress of Oral Implantologists consensus report. Implant Dent. 2012 Apr;21(2):78-86.
8. Pauwels R, Beinsberger J, Collaert B, Theodorakou C, Rogers J, Walker A, Cockmartin L, Bosmans H, Jacobs R, Bogaerts R, Horner K; SEDENTEXCT Project Consortium. Effective dose range for dental cone beam computed tomography scanners. Eur J Radiol. 2012 Feb;81(2):267-71.
9. Drage N, Rogers S, Greenall C, Playle R. Incidental findings on cone beam computed tomography in orthodontic patients. J Orthod. 2013 Mar;40(1):29-37.
10. Aps JK.Cone beam computed tomography in paediatric dentistry: overview of recent literature.Eur Arch Paediatr Dent. 2013 Jun;14(3):131-40.
11. Royal College of Dental Surgeons of Ontario. Dental CT Scanners. http://rcdso.org/MemberResourceCentre/Dental%20CT%20Scanners