Oral Health Group
Feature

Cone Beam Volumetric Tomography

June 1, 2007
by Martin Bourgeois, DDS, Dip. Oral Rad. M Ed; Paula Sikorski DDS, MSc, FRCD(C), Dip ABOMR; Sarah Taylo


Cone Beam Volumetric Tomography (CBVT) also known as Cone Beam CT was first commercially introduced to the dental profession in 2001. Cone beam CT is an imaging modality that produces volume imaging in an easier and quicker fashion than conventional medical CT. Cone Beam CT was specifically designed for the dental profession to provide accurate, multiplanar 3-dimensional imaging.

There are a number of Cone Beam CT imaging systems commercially available. We have seen a rapid implementation of these machines throughout the United States and Canada.

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The purpose of this article is to introduce the general dentist to this new technology and to describe the advantages of cone beam CT in clinical practice. We will discuss how an image is acquired; compare medical and cone beam CT as well as discuss average radiation dosages. The clinical applications of cone beam CT will be outlined followed by a discussion of available software programmes to enable manipulation of cone beam CT data and application to dentistry.

CONVENTIONAL MEDICAL CT VS. CONE BEAM CT

Conventional CT utilizes a fan shaped beam that images an object slice by slice then stacks the slices to obtain a 3-D representation of the object. Conventional CT’s are large and expensive and deliver high radiation doses due to the multiple slice acquisition and inefficient use of x-ray photons. They are designed for full body medical imaging and are not always practically available to the dental community.

Cone beam scanners utilize a cone-shaped beam to scan the entire region of interest in a single rotation and are ideal for dedicated imaging of the maxillofacial complex. The volumes are manipulated by computer software into multiple slices for viewing. This technology not only reduces the size and cost of the scanners, but reduces the inefficiency of x-ray photon use resulting in a considerable reduction of patient radiation exposure (Table 1).

The dose from cone beam CT is approximately 1/10th the dose delivered in medical CT imaging and falls well within the range of doses delivered during conventional dental radiography.

FEATURES OF CBCT

Acquiring the image

The patient is usually sitting but may be in a supine position depending on type of cone beam CT unit. A full volume scan takes anywhere from 10 seconds to 70 seconds. Patient acceptance is very high as it is a non-invasive procedure similar to that of a panoramic exposure.

X-ray beam collimation

Most cone beam scanners allow the operator to reduce the field of radiation by collimating the primary x-ray beam to the desired region of interest. For example, one can image only the maxilla while collimating the mandible out of the field of radiation. Of course both jaws can be imaged simultaneously during a single scan. Similarly, when imaging the TM joint, the field of view is limited to the planes of the joints and areas superior or inferior to the joints. When needed, the entire craniofacial complex may be imaged such as may be the case in orthognathic evaluations.

Viewing

After completion of a scan, the first images to appear on the computer screen are in mutiplanar format (MPR) ie. the images appear in three planes of section — the axial, coronal and sagittal. These are not static images like conventional films, but images of tissue volumes, so for each plane of section, the operator can scroll through the entire depth of tissue in multiple slices. Software manipulation allow anterior-posterior coronal reconstructions and slices can be as thin as 0.4mm eliminating the problem of superimposition that plagues conventional imaging. The MPR format is only a starting point for image viewing.

The proprietary software for cone beam scanners allows the operator to customize any plane of section for viewing. For example, by mapping through the plane of the arch, a panoramic-like image can be created and from this, the software automatically generates cross-sectional images through the arch. Similarly the volumes can be mapped to create lateral or PA ceph-like projection, sagittal and frontal views through the TM joints. Again, unlike conventional imaging, patients do not have to be re-exposed to view different planes of section, the volume can be manipulated to produce any plane of section. Proprietary software also includes such features as a measuring tool, a mechanism for 3-D reconstruction and the image enhancement tool, MIP.

Image accuracy

The images are dimensionally accurate; non-magnified and measurements are taken at a 1:1 ratio.

Image artifact

Image artifact from metals is inherent with all CT imaging systems. However, even with extensive prosthodontic restorations, the image scatter is not significant enough to contraindicate imaging with cone beam CT and a relatively clear diagnostic image is produced.

Image format

Cone Beam CT produces images in a DICOM format. These files can then be exported into third party software programmes allowing the clinician to manipulate and treatment plan accurately. Common examples of these treatment software programmes include Simplant from Materialise and Procera from Nobelbiocare. These are just two examples implant planning software.

For the orthodontist/oral surgeon, planning orthognathic surgery is made much more precise and predictable with the use of software such as SimPlant CMF from Materialise, Dolphin and 3d MD. Utilizing the combined technology of Cone Beam CT and these software programs, clinicians can create and take advantage of 3D planning for virtual cranio-maxillofacial osteotomy and distraction surgeries.

CLINICAL INDICATIONS

Pre-implant imaging

The clinician placing dental implants needs to determine the best possible sites with respect to restorative, esthetic, biomechanical and functional requirements. CT images will allow the clinician to assess the following: bone height and width dimensions, bone quality, long axis of alveolar bone, internal anatomic considerations, external jaw boundaries and presence of pathology. Any point in space in the CT scan can be reference from a known intra-oral anatomic landmark (eg: tooth, mental foramen, nasopalatine canal) to allow for the transfer of radiographic information to the clinical site. For clinical/radiographic referencing, some clinicians prefer the use of imaging stents with radiopaque markers. Because of image artifact from metals, gutta percha is the marker of choice with cone beam CT.

Localizations of impacted teeth

Traditionally, radiologists have been asked to aid in the localization of impacted teeth with respect to proximity and relationship to other teeth and anatomic structures. Prior to cone beam CT, radiologists and clinicians relied on plane films (panoramic, occlusal and periapical) and film geometry such as the buccal-object rule to localize impacted teeth. These techniques were limited in delineating exact anatomical relationships and often failed to demonstrate root resorption.

With cone beam CT we now can accurately view in three planes, without superimposition, the relationship of impacted teeth to other anatomical structures. Plain films are able to demonstrate apical resorption but are limited at demonstrating resorption on the buccal and lingual aspect of roots. With cone beam CT, the axial plane in particular has been instrumental in demonstrating this buccal or lingual resorption.

Localization of foreign body

As with localizations of impacted teeth, cone beam CT allows the radiologist to accurately localize foreign bodies in the jaws with respect to neighboring teeth and anatomical structures in a multi-planar model. This allows the surgeon to easily retrieve the foreign body with limited surgical exploration and less morbidity to the patient.

Pathology

Patients with pathological lesions of unknown origin are referred to the radiologist for diagnosis and accurate anatomic delineation of the lesion. Traditionally, the radiologist relied on
plain films to diagnose these lesions. Cone beam CT, because of its multiplanar format and software manipulations, gives the radiologist a more detailed image of the lesion facilitating the diagnostic process and demonstrates precise anatomic relationships facilitating surgery.

TMJ

Imaging of the temporo-mandibular joint continues to be an area of interest to the dentist and often imaging of the joints is requested to support clinical findings and/or rule out pathology. With cone beam CT, we can visualize the condyle and fossa in multiple planes of section from a single scan and assess anatomical changes not previously seen on plain films.

Sinus

Before CT technology, dentistry did not have an accurate means to view the maxillary sinus and relied on conventional medical imaging. It is only with the advent of cone beam CT that dentistry can now view the entire maxillary antra and appreciate its anatomical variations and accurately detect and localize abnormalities. The other paranasal sinuses can also be well imaged with Cone bean CT.

Third molar nerve relationship

One of the most challenging imaging requests for the radiologist is to provide accurate anatomical relationship between impacted third molars and the inferior alveolar nerve canals. Traditionally, surgeons have relied unsuccessfully on the panoramic film to help in this dilemma. Now with cone beam CT, the radiologist can depict this spatial relationship in all planes of section.

Orthognathic evaluation/ asymmetry

From a single full scan of the head, specific software manipulations allow us to recreate any traditional orthodontic plane of interest such as the corrected oblique lateral, PA ceph, lateral ceph and panoramic view. Cone Beam CT allows us to localize landmarks without the superimpositon inherent in traditional plain film studies. Measuring tools allow the radiologist to accurately measure length and width of bone such as mandibular lengths and magnitude of deviation from norm (eg. cant of occlusal plane, midline discrepancy….) and in this way CT is proving particularly helpful in asymmetry assessments. The merging of CT and orthodontics is an exciting field of study and promises to retrain us to look at the analysis of the craniofacial complex in a whole new light. Proprietary software allows not only to view in multiple planes but also 3D reconstructions of the head and a new format of image viewing called maximum intensity projections or MIP to create virtual orthodontic models.

Third party software programmes can utilize cone beam CT images to allow surgeons in concert with the orthodontist to plan and simulate orthognatic surgeries.

Image output

So how does do we get this information from radiologist to clinician? Well, there are several “flavours” to choose from.

An increasing number of clinicians have now incorporated Cone Beam CT into their requests when referring patients for pre-treatment imaging and a number are also introducing third party software into their practices such as SimPlant from Materialise. The site or arch of preference is supplied on CD and/or emailed. Unfortunately for some this can be cost prohibitive, as the financial output for software can range anywhere from $4,000.00 to $10,000 USD.

To solve this problem, a number of companies are now producing dynamic imaging viewing software that can be supplied at no cost that allows clinicians to manipulate images, create any desired plane of section, locate anatomical landmarks take required measurements and adjust window levels to name but a few options. E-Film and iCAT Vision are two examples of this software. E-Film is compatible with dicom files supplied from both medical and cone beam CT units and the viewing software along with the patient images is usually supplied by the imaging lab. iCAT Vision is a proprietory software and is only compatible with files acquired from an iCAT Cone Beam Scanner. The software is a free download from Imaging Sciences International. The patient scan must be acquired using the iCAT Cone Beam CT.

Of course there are always options for the clinicians who are not so computer savvy. Static copies can be reproduced and printed out, placed on CD or emailed as PDF or JPEG files. As these images are a 1:1 ratio, taking measurements is a simple as placing a ruler over an image.

It is anticipated that the dental community will take its’ cue from the medical community when it comes to securely sharing files and images by implementing a PACS system. This will, for instance, allow the radiologist in Toronto to simultaneously view and discuss a patients’ images with the Oral Surgeon in North Bay, all while the patient is still in the radiologists’ office.

DISCUSSION/CONCLUSION

This technology is here and is already revolutionizing the field of dentistry. It will continue to evolve as new software programs, new CT units with shorter scan times and lower radiation doses are being continually developed. The specialty field of Oral and Maxillofacial Radiology is currently developing a position paper at the National level in collaboration with the American Association of Oral and Maxillofacial Radiology on the use of Cone Beam CT in dentistry and will make recommendation on issues of quality control, dosimetry, operator training and skill and interpretation of these scans.

The mandate of this collaboration is to make evidence based recommendations to the profession on those clinical situations where Cone Beam CT imaging should be the standard of care and also to ensure that this technology will be used appropriately with respect to the ALARA principle. Unfortunately abuses of this technology are already evident. Strict selection criteria must be adhered to prior to patient exposure because large volume of tissue are exposed. It is also critically important that these full volume scans be interpreted by skilled diagnosticians trained in the field of Oral and Maxillofacial Radiology.

The general dentist should be aware of this new technology and the clinical indications for patient referral for a Cone Beam scan. This article was a brief introduction to this exciting advanced imaging technology and it is the hope of the authors that it will serve as a template for further education as the technology evolves.

Paula Sikorski is Secretary Treasurer of the Ontario Society of Oral Radiologists and the H.M. Worth Radiology and Oral Medicine Study Club. She is in private practice specializing in Oral and Maxillofacial Radiology with Martin Bourgeois and on staff in the departments of Dentistry at Mount Sinai Hospital, Sunnybrook Health Sciences Centre and the Hospital for Sick Children.

Martin Bourgeois is Radiology Rep. to the Ontario Association of Dental Specialists; President of the Ontario Society of Oral and Maxillofacial Radiologists and the H.M. Worth Radiology and Oral Medicine Study Club. He is in private practice specializing in Oral and Maxillofacial Radiology.

Images courtesy of 3D Diagnostics, Imaging Sciences International, Materialise.

Oral Health welcomes this original article.

REFERENCES

1.Danforth RA. Cone Beam Volume Tomography: A New Digital Imaging Option for Dentistry. CDA. Journal.Vol.31.No.11, November 2003.

2.Scarfe WC, Farman AG, Sukovic P. Clinical Applications of Cone-Beam Computed Tomography in Dental Practice. JCDA.Vol.72, No.1, February 2006.

3.Danforth RA, Peck J, Hall P. Cone Beam Volume Tomography: An Imaging Option for Diagnosis of Complex Mandibular Third Molar Anatomical Relationships. CDA Journal.Vol.31.No.11, November 2003.

4.Hatcher DC, Dial C, Mayorga C. Cone Beam CT for Pre-Surgical Assessment of Implant Sites. CDA.Journal.Vol.31.No.11, November 2003.

5.Winter AA, Pollack AS, Frommer HH, Koenig L. Cone Beam Volumtric Tomography vs. Medical CT Scanners. NYSDJ. June/July 2005.

6.Ludlow JB, Davies-Ludlow LE, Brooks SL, Howerton WB. Dosimetry of 3 CBCT Devices for Oral and Maxillofacial Radiology: CB
Mercuray, NewTom 3G and I-CAT. Dentomaxillofacial Radiology (2006) 35, 219-226.

7.Nakajima A, Sameshima GT, Arai Y, Homme Y, Shimizu N, Dougherty Sr H. Two and Three-dimensional Orthodontic Imaging Using Limited Cone Beam-Computed Tomography. Angle Orthodontist, Vol. 75, No. 6, 2005.

8.Moore WS. Cone Beam CT: A New Tool for Esthetic Implant Planning. Texas Dental Journal. April 2005. 334-340.

9.Tyndall AA, Brooks SL. Selection criteria for Dental Implants Site Imaging: A position paper of the American Academy of Oral and Maxillofacial Radiology. OOOOE 2000: 89(5):630-7.

10.Bjerklin K; Ericson S. How a Computerized Tomography Examination Changed the Treatment Plans of 80 Children with Retained and Ectopically Positioned Maxillary Canines. Angle Orthodontist, Vol 76, No. 75, 2006.

Table 1

Dosimetry:

CBCT

A) Maxilla / Mandible36

B) Maxilla20

C) Mandible35

2. Conventional CT314

3. Panoramic6

4. Lateral Ceph2.3

(PA Ceph ~ 15%)

5. Background Radiation

3 mSv/yr, ~8 Sv/day

Department of Diagnostic Sciences and General Dentistry, University of North Carolina School of Dentistry, Chapel Hill, North Carolina, USA; 2 Ann Arbor, Michigan, USA; 3 Department of Oral Medicine/ Pathology/Oncology, University of Michigan School of Dentistry, Ann Arbor, Michigan, USA


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