The rationale for using reformatted axial cuts. This is written mainly for physicians who are ordering CT scans to diagnose SCD.
Timothy C. Hain, MD Page last modified: July 21, 2020
In the "superior canal dehiscence" syndrome, or SCD, the roof of the superior semicircular canal is missing. The missing bone is identified using high-resolution CT scan. A good idea whether bone is missing or not can also be obtained from a 3T coronal T2 MRI (Browaeys et al, 2013). See the second half of this page for MRI to suggest or rule out SCD..
The CT-temporal bone is the "gold standard" for SCD, but because there is considerable radiation, it is best to save it fo situations where there is both lab evidence (i.e. a VEMP test), and where surgery is a possibility, as otherwise there is some risk (radiation) without great benefit.
A CT scan of the temporal bone or SCD is generally obtained in persons with sound or pressure sensitivity. CT of the temporal bone is supposedly very accurate in identifying canal fistulae (Fuse et al, 1996). As SCD is a type of canal fistula and it is moderately common (about 2% of population), the main reason for this procedure is to check for SCD.
Note that most temporal bone CT scans are "high radiation" procedures because enough Xray energy must be used to "see" into a very hard bone (temporal bone). All Xrays increase cancer risk. Accordingly, CT scans should not be done to "screen" for SCD. Safe tests such as the VEMP should be used first. CT scans also should not be done in a "successive approximation" mode -- i.e. you don't start with a poor scan, and then get a good one. If you are going to do it - - do it right the first time. Be extra cautious in scanning children, as radiation exposure increases cancer risk, and children have a long life expectancy (Tunkel et al, 2012).
An exception to the general rule that temporal bone CT scans are "high radiation" are "cone beam CT scans". These scanners have far lower radiation. They are commonly found in dentist offices, but so far, have not been favored in hospital settings. We hope that eventually all CT scans for SCD are done with this new, lower radiation technology.
The CT should be done of the temporal bone with at 0.6 mm resolution or better (lower is better). It may be impossible to get a CT scan with a resolution < 0.6 mm. This is often sufficient, but don't accept lower resolution (i.e. more than 1 mm is not good enough).
Conventional CT scans of the brain are nearly always useless to diagnose SCD as their cut resolution is 8-10mm -- this is almost as big as the entire inner ear ! There is also a trade-off between radiation and resolution.
The bottom line is that it is generally best to do high-resolution axials, with coronal and oblique reformatting. If the facility or radiology is unfamiliar with the radiology entailed with diagnosis of SCD, then it is best to do true-coronal high-resolution. Even better is to go elsewhere for the CT scan.
Cone-Beam CT scans may be superior to ordinary CT scans as they provide better resolution with far less radiation. Cone beam CT scans are standard equipment for dentists, but have not yet become very common outside of dentistry.
Below are a few examples of SCD diagnosed from Coronals. It works, but the axial technique is better.
|Coronal CT scan of the temporal bone clearly showing missing bone at the top of the left anterior (superior) semicircular bone. Also the tegmen (roof of middle ear) is dehiscent. This is common.|
|Another Coronal thin cut CT scan showing superior canal dehiscence (SCD). This patient was reported in detail in (Ostrowski, Hain and Wiet, 1997)|
It is generally best to perform direct axial and to perform reconstructed coronal and oblique parasagittal views. This is to reduce radiation exposure and the reformatting makes visualization easier. The oblique images are essential for complete evaluation of SSCC. The reconstruction that is in the plane of the superior canal is referred to as the Poschel view. The Poschel is usually obtained with the Stenver's view, which is 90 degrees rotated. These two are somewhat complementary as one can get a better idea about a slice of the SCC from the Stenvers, but a better idea about the length of the dehiscence from the Poschel.
|Temporal bone CT scan with reformatted images taken in plane of superior canal (i.e. Poschel view). There is a wide area of dehiscence seen at the top.|
The superior semicircular canal is a loop that is angled at roughly 45 degrees to the sagittal plane of the head . It has a back (i.e posterior portion), middle and front. If one has the Poschel view, one can use the measurement software in the radiology system, basically to create a chord and measure its length. This is a little crude and slow as well. A faster way is to divide up the superior canal into thirds. The back and front are lower than the middle. Castellucci et al (2020) used anatomic landmarks to name the three parts of the canal going from back to front, namely the superior petrosal sinus, the arcuate eminence, and the ampullary arm of the canal.
The back part of the canal is adjacent to the superior petrosal sinus. The middle-top part of the canal is covered by the arcuate eminence. The front part of the canal contains the ampulla (the motion sensitive part). So it is called the ampullary arm. A good discussion of the bony anatomy can be found here (Petrous part of the temporal bone). We think a naming system such as "A, B, C", and then calling a dehiscence an "B", or an "AB", or "ABC", would be preferable, but this is the way it is right now.
So in other words, one might have just SCD involving the top (arcuate eminence), or some combination of the top and the front, back or both. Larger defects are more difficult to close, and defects that are closer to the center (i.e. petrosal) are harder to access.
Errors in interpreting temporal bone scans for SCD are far more common than other types of inner ear imaging. Radiologists almost never miss acoustic neuromas, given that contrast is given, but they miss SCD, roughly 10% of the time. This is probably because of two things: 1. Radiologists are sometimes unfamiliar with the temporal bone. 2. There are laboratory tests available that can provide a clue that something was missed (i.e. oVEMP).
Because of the relatively high error rate, If your patient gets a CT scan of the temporal bone -- you HAVE TO LOOK AT IT. OR if you can't read these scans, send your patient to someone who can.
1. The usual type of error in missing SCD is not looking for SCD. That is why someone else should review their results -- if you are the ordering doctor, this should be you. If you are not competent to check them yourself, and there was a good reason to do the test in the first place, send them to someone who is.
2. The second most common error in missing SCD is having suboptimal CT images. Everything is easier when the images are "optimal", which translates into having someone set up the right images BEFORE the patient is in the scanner. This means both the resolution of the scan and the angles. This requires not only that there be a competant person in your radiology department who knows how to do these scans, but also that there is someone who can read the incoming radiology requisitions and transfer them into whatever software your radiology department is using. Thus perhaps two people.
If there are no coronal or Poschel/Stenvers reconstructions, SCD many not be visible from axials. If the resolution is 1 mm or more, SCD is also easily missed. Generally speaking, if the radiology department didn't do Poschel reconstructions (these are the perpendicular ones), they could certainly miss SCD. These are generally errors of ignorance on the part of the radiology department, who may simply not be used to doing these types of Xrays, but sometimes are errors in transfering information into the radiology department from outside physicians.
3. Sometimes the suboptimal images from the 2nd type of error results in a third type of error -- diagnosing SCD when there is no SCD, or on the wrong side.
If you have a patient in whom the temporal bone CT was not optimal, rather than doing it over, we suggest starting with a screening tests like an oVEMP and/or a threshold cVEMP. This is to avoid radiation to the head and eyes.
In our opinion, this is what your order for a CT scan of the temporal bone should generally say:
CT scan of the temporal bone, with high resolution (1 mm or less). Direct axial and reformatted coronal and reformatted oblique views parallel and perpendicular to the plane of the Superior Semicircular Canals. No contrast.
A properly done MRI can rule out SCD, as well as point strongly towards SCD (Browaeys et al, 2013). The method is to use a newer 3T scanner, and obtain T2-Coronal views. On T2, the fluid filled semicircular canals stand out from the bone, and one can usually see that the canal goes "right to the dura" -- in other words, there is no dark bone between the loop of the canal and the brain. The image above shows a man with SCD identified from his very large VEMP on the right side. His CT-temporal bone showed SCD.
An example of a coronal with no SCD is here (T2, 4 mm cut size): Note that there is space between the loop of the canal and the brain. Thus no SCD.
The huge advantage of using MRI is that there is no radiation. Temporal bone CT scans need a lot of X-ray radiation to see through the hard bone of the temporal area. This problem may eventually be handled through use of the newer cone-beam CTs.
On the other hand, MRI is more expensive and slower. We think MRI is underutilized, as most patients get MRI's for other purposes. We suggest that the proper MRI should be a 3T (high field) MRI, with T2 direct coronal cuts, with resolution ideally of 3 mm. Browaeys et al (2013) used higher resolution Fiesta imaging in a lesser field strength scanner (1.5T). We are not sure which is better, but we think the 3T T2 images are very reasonable.
An MRI is also useful to exclude potentially confounding entities such as cholesteatoma or tumor. MRI is not as good a test for dehiscence as a temporal bone CT because it doesn't show the bone and the resolution is not as good as a temporal bone CT scan. However, MRI is the best way of showing other possibly confounding problems such as acoustic tumors, cholesteatoma, or multiple sclerosis plaques. Note that "basic" CT scans such as are done in emergency rooms are always useless for diagnosis of SCD.
Sone et al (2015) reported that of 5 patients with SCD, 4 of them showed "severe hydrops" on their MRI. We find this finding difficult to understand, but bears repeating. We think the 3T MRI method is "bleeding edge" with respect to vestibular diagnosis. In other words, don't make any serious decisions based on 3T MRI findings of hydrops.