Friday, May 25, 2007
One of the most difficult projections for a lot of radiographers continues to be the open-mouth odontoid view. Here are some alternate ways to image the odontoid:
Most technologists can acquire an open-mouth and at least get some of the C2 vertebral body, but commonly clip the dens. In this case, you could do a Fuchs (not to be confused with Scottish profanity) by aligning the CR perpendicular to the MML and centering 1" below the mental point.
You could simply adjust the elevation of the patient's chin or you could angle your tube cephalic/caudal depending on the need. Review the anatomy on the preceding image, remembering that you need to align the tips of the incisors with the base of the skull (or as I like to use, the mastoid tips which are easily palpable from the patient's side). And simply for review (not to insult anyone's intelligence), the labeled image at the beginning of the post could be improved upon by lowering the chin because the incisors are above the occipital bone. You would have to raise the chin if the incisors were below. All of these are common practices, and should probably be attempted before proceding to the next reccommendations.
How would you obtain an odontoid view on someone with a mandibular fracture, or with their jaw wired shut? Well, there are three ways:
1) You could blast right through the mandible. This would be the easiest, increasing your kVp to about 85 should do the trick. You can lower the SID to about 30" and this will minimize the detail of the mandible/teeth superimposing the dens.
The following two options will work if there is a lot of dental work or wiring in the jaw.
2) You could oblique the head and do unilateral views of the lateral masses. For the right side, you should attempt to keep your normal alignment of the incisors and mastoids while rotating the head to the patient's left. Do it just enough so that the mandibular rami (side up) crosses over the patient's midline over the neck, freeing the jaw from superimposition over the dens. Take your film with a perpendicular beam, and you will see the right lateral mass and most of the dens. Be cautious not to over-rotate the patient's head, or you will close the joint space between the dens and lateral mass on the side you're trying to image. You can do the same thing for the opposite side rotating the head in the opposite direction.
*I had trouble finding radiographs for this one, but I'll post one if I have the opportunity to perform this method in the near future.
3) You could perform tomograms. Just like for your IVP, lay the patient supine. With calipers, measure the distance from the table to the EAM, then add one cm. This should create a focal point right through the dens. You may have to take additional slices through the vertebral bodies. I would suggest 1/2 cm incriments posterior to the initial dens image. For example, if your initial measurement from table to EAM was 10 cm, then you add one, your first slice would be at 11 cm. I might try 10 1/2 cm next, then 10 cm and show the radiologist to see if any additional films are required.
And last, and probably least, another method for obtaining the dens can be performed, but not reccommended unless nothing else works. In my experience with radiograpy on patients from some Asian countries (the skull takes a less oblong shape), you may align the incisors and the base of the skull perfectly, but still will not see the dens free of superimposition. You can align the patient like you normally would, but lower the SID as close to the patient's open mouth as possible. This allows for a more dramatized beam divergence, eliminating the incisors and mandible from being able to superimpose the dens. I would not reccommend this be practiced under most circumstances due to radiation protection purposes. The only reason I would use this is if the patient's insurance didn't cover a CT scan, and all other options had failed or were unable to be performed.
Hopefully, we can become better as we practice more of these and have the ability to assess which methods would probably work best on the initial exposure. I hope this adds to your bag-of-tricks!