Limited to Whole Head and Everything in Between: A Guide to Field of View in CBCT

So you’ve purchased a Cone Beam CT (CBCT) machine. You’ve reviewed a lot of
technical details with the rep who sold it to you, but like any other imaging modality, the decision
when and how to use it comes down to your clinical judgement. There’s just one catch – unlike
other imaging used in dentistry, you now have a choice on how much of the patient you want to
capture in the image you acquire. Do you want to capture a few teeth or maybe just the
mandible? Or maybe you’re looking at using this ever affordable piece of technology to replace
a panoramic machine in your office. So the big question is, does the size of the volume you
acquire really matter? I’m here to say – size does matter!

While Cone Beam CT has been available in the United States since the early 2000s, it
has become increasingly more affordable and therefore more widespread in the practice of
dentistry. From implant treatment planning, visualization of pathology in three-dimensions, to
getting an image of that unknown entity in a panoramic, Cone Beam CT is a fantastic addition to
the dentist’s diagnostic toolbox. But a tool is only as good as it’s user, and most effective when
used in the appropriate situation. When utilizing Cone Beam CT, it is important to consider how
much of the patient we want or need to capture. This specific volume size, or as we like to call it
Field of View (FOV), of the patient, is simply the result of collimation of the x-rays to a specific
area. Like many aspects in radiology, there are trade offs or costs vs. benefits to what we do
(more on that later). First though, let’s talk about some common FOV sizes and reasons we
might use them.

The Limited Field of View (i.e. 4 x 5 CM or 5 x 5 CM)

The 5 x 5 (cm) or smaller, is often referred to as the limited FOV. The name is an
adequate description as we are often looking at an area the size of 3 or 4 teeth up to half of one
dental arch. This is an ideal view for implant treatment planning of single implants or endodontic
evaluation.

Endodontics: The American Academy of Endodontics (AAE) released a joint position
paper with the American Academy of Oral and Maxillofacial Radiology (AAOMR) in 2010 and
then revised it 2015 regarding the use of CBCT in endodontics. Taking into consideration the
level of detail or resolution required for diagnosis of the complex root systems, it is
recommended that the field of view “should only slightly exceed the dimensions of the anatomy
of interest” 1 The effects on resolution will be discussed further in the costs / benefits.

Implant treatment planning: While not a joint position paper, the AAOMR released a
statement regarding selection criteria, including FOV, for treatment planning dental implants.
While Panoramic and supplemental periapicals are recommended for initial diagnostic imaging
examination, CBCT is the modality of choice for cross-sectional imaging of potential implant
sites, especially when involving potential augmentation of the site. Tucked away in
recommendation 5 they conclude, “Although the FOV should be limited to the area of interest,
the FOV may extend beyond the implant site to include the maxillary sinus or opposing dental
arch.” 2

Dental Arches (i.e. 5 x 8 and 8 x 8 CM)

This FOV lends itself to evaluation of the entirety of the tooth bearing areas of the jaws.
If the patient is small enough, the TMJs can sometimes be captured in this view as well.
Ultimately, this is a comfortable place for most dentists because we are capturing images that
involve structures we are familiar with. It is an excellent choice to evaluate pathology as it will
allow us to gage it’s extent while simultaneously allowing us to compare to the contralateral side
of the jaws. It is also helpful for evaluating specific location of teeth to give us an appreciation of
it’s buccal/frontal or lingual/palatal position in relation to other teeth. Ultimately, this is a useful
FOV for the general dentist as it allows comparison of a region of interest to the contralateral
jaw or the opposing arch.

Both Arches including TMJs (i.e. 14 x 8 CM)

This is a common FOV I’ve seen in our practice and lends itself to evaluation of the
entirety of the tooth bearing areas of the jaws as well as the airway, TMJs, and sinuses. It is this
FOV that I most often see a reconstructed panoramic image created from. While some
machines have created more limited FOV for separate evaluation of the TMJs, this is the go-to
volume size to evaluate the articulating surfaces (the glenoid fossa and the mandibular condyle)
of the TMJ while also evaluating the positioning of the teeth to obtain a broader picture of
possible dysfunction. It is important to remember, however, that CBCT does not capture the soft
tissue and so the position of the disc cannot be definitively stated but rather inferred.
Evaluating airway as part of dental sleep medicine has also brought about updates in the
CBCT software to evaluate the volume of the oropharyngeal airway. This can be helpful in
determining appropriate referrals based on restriction of the airway to a physician trained in
sleep medicine for diagnosing sleep apnea.

Similar to the previous FOV, this size is useful for determining the extent of pathology. It
is also useful for surgical evaluation of fractures to the oral maxillofacial complex, obstruction of
the osteomeatal complex, and evaluation of sinus pathology.
The Whole Head (i.e. 17 x 23 CM)

Most dentists will not have a CBCT that has a FOV this large as it is not practical for
most day-to-day diagnosis of our patients. I will use this opportunity to discuss CBCT in
Orthodontics as it has sometimes been suggested as a replacement for traditional orthodontic
assessment.

The AAOMR has outlined clinical guidelines when it comes to utilizing CBCT in
orthodontics. And while not all encompassing, it does outline some general recommendations
that are useful. They are as follows: image appropriately according to clinical condition, assess
the radiation dose risk, minimize patient radiation exposure, and maintain professional
competency in performing and interpreting CBCT studies. You will notice that two of these
involve radiation dose to the patient which makes sense since pediatric patients have an
increased sensitivity to ionizing radiation. With this in mind the smallest FOV should be
considered. Where I have seen CBCT whole head being useful is if the dose acquired from this
one volume is less than if all other imaging was taken in the case of orthodontically complex
patients. This will not always be true depending on the CBCT machine. In the case of a pediatric
patient needing a whole head FOV, it is important that the patient is positioned in a way that
limits their ability to move (as children are not always cooperative for longer periods of time).
This is especially true in CBCT machines that use a technique to “stitch” two separate yet
sequential CBCT volumes together to acquire a larger FOV.

Since we are taking such a large FOV, often on pediatric patients, the staff taking the
CBCT needs to be appropriately trained to minimize the need to retake the volume and thus
impart more radiation to the patient. The dentist must also be mindful of the extent of the volume
acquired which is often outside most dentist’s comfort zone for interpretation. This large volume includes structures of the middle cranial fossa and cervical vertebrae which require additional
training outside of what is taught currently in most dental schools.

Costs / Benefits to Different Fields of View

There are two primary things to consider when selecting the field of view, the foremost
being patient exposure. As is true in any other acquisition in radiology, there is a dose imparted
to the patient. Reduction of the field of view can result in “patient dose reductions ranging from
25% to 66% depending on the machine, type of collimation (vertical or horizontal), amount of
mechanical collimation, and location (maxilla vs. mandible; anterior vs. posterior).” 4 And this
makes sense as the larger the volume that is acquired, the more dose is imparted to the patient.
This is particularly important when we are talking about pediatric patients and taking larger
volumes for orthodontics – the radiation dose should be considered when taking orthodontic
records as discussed previously.

The second may not be as obvious, and that is the image quality. I won’t get into the
physics here, but because of how CBCT is acquired there is the potential for a lot of excess
unusable photons or scatter that can result in degradation of the image or noise. How much of
an effect this has is very machine dependent but reducing the FOV does result in increased
contrast and decreased noise. 4

Unlike other imaging modalities used in dentistry, CBCT allows us to choose exactly how
much of the patient we capture at each imaging examination. As a diagnostic tool it is important
to not only know when to use it but how to use it effectively so that we can acquire the best
diagnostic image for the clinical question while considering the cost of radiation dose to the
patient. Hopefully you now understand that in selecting a FOV in CBCT, size does matter!

Ryan Holmes, DDS
Oral & Maxillofacial Radiologist / Co-Founder
True View Dental Radiology

References

1. American Association of Endodontists; American Academy of Oral and Maxillofacial
   Radiology. Use of cone-beam computed tomography in endodontics Joint Position
   Statement of the American Association of Endodontists and the American Academy of
   Oral and Maxillofacial Radiology. Oral Surg Oral Med Oral Pathol Oral Radiol.
   2015;120(4):508-12.
2. Tyndall D, Price JB, Tetradis S, Ganz S, Hildebolt C, Scarfe W. Position statement of the
   American Academy of Oral and Maxillofacial Radiology on selection criteria for the use
   of radiology in dental implantology with emphasis on cone beam computed tomography.
   Oral Surg Oral Med Oral Pathol Oral Radiol. 2012; 113(6):817-26.
3. American Academy of Oral and Maxillofacial Radiology. Clinical recommendations
   regarding use of cone beam computed tomography in orthodontics. Position statement
   by the American Academy of Oral and Maxillofacial Radiology. Oral Surg Oral Med Oral
   Pathol Oral Radiol. 2013; 116(2):238-57.
4. Mallya SM, Lam E. White and pharaoh’s oral radiology: principles and interpretation
   8th ed., St. Louis: Elsevier Mosby; 2018.