The rationale for using reformatted axial cuts.
Timothy C. Hain, MD Page last modified: May 26, 2016
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 main SCD page.
This page is an attempt to outline the optimal technique for doing high-resolution temporal bone CT, when one is looking for SCD.
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. One might argue that the tiny lesions that can be discovered with 0.1 mm cuts are not worth the radiation load. This issue is presently unclear.
There are two choices for positioning in the scanner -- on the back (axial), or on the face (direct coronal). There are some pro's and cons of each. Ideally, for diagnosis, there should be both positions -- axial, and "direct" coronal. Unfortunately, this means doing the scan twice and doubling the amount of radiation. Lets look at each method in more detail. 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.
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.
True or direct coronal sections are intrinsically better than reconstructed or reformatted coronal images. The direct coronal view is taken in supine or prone with the neck hyperextended and with a tilted gantry (one cannot get true coronal because of limitation of neck and gantry angle). In young patient one can get good images but there are problems with older patients, those with neck problem or children.
Axials can be also "reconstructed" -- reformatted -- to provide a "pseudo" coronal view. Our general experience has been that when coronals are produced in this way, they are sometimes fuzzy and can be uninterpretable. However, it is possible to do a pretty good job with reconstructions (see below).
Although true coronals are more accurate, it is generally best to perform direct axial and to perform reconstructed coronal and oblique parasagittal views. This is to reduce radiation exposure. The oblique images are essential for complete evaluation of SSCC. This is the view at times referred to as the Poschel view. To do the oblique view properly, the CT technician has to identify the top axial section for SSCC, and connect the dots of the limbs of the SSCC. This is the reference section. Then one makes oblique images using this line. The radiology technician will use this line as reference and produce reformatted images parallel and perpendicular to the line). This results in an image that shows the entire SSCC (one of the images that are coplanar), as well as a series of images that show the canal and surrounding bone. The reformatted oblique perpendicular to the reference line is the best to see the PSSC in its plane. This is also critical for the anatomy of facial canal as well as the round widow in case there is a perilymphatic fistula.
When one does the direct coronal, a fixed reference line cannot be drawn as is the case for axial. The reason is that coronal sections depend on the neck extension and gantry angle and therefore is slightly different in each case. It is therefore very difficult to generate a reproducible oblique views from direct coronal.
It is not easy to get the Poschel view from direct coronals. By reformatting from axials ( which is easy for techs) we know that we get always the right images. For SSCC one depends on the oblique more than direct coronals, as the slope of tegmen and arcuate eminence/ fossa may give a false impression of a defect. All the SCCs can be evaluated by having axial and reformatted coronal and oblique images. Axial images are best for evaluation of VAQ and arachnoid granulation along the posterior petromastoid plate.
For resolution alone, the ideal study of temporal bone is the direct axial and direct coronal. However, since the multidirector spiral technique provides isotropic volumetric data, one should not lose any resolution on reformatted images. If there is no motion on axial images it is possible to get excellent reformatted images in true coronal and any other planes. There are some cases that reformatted coronal may not be as good as direct coronal. The issue of resolution is complicated and it is not only the matter of pixel size but rather the field of view, detector format, manufacturer of the software (Mafee, 2005)
3D reconstructions will show the full cortical bone
|Figure 2a. Coronal thin cut CT scan showing superior canal dehiscence (SCD). This patient was reported in detail in (Ostrowski, Hain and Wiet, 1997)||Figure 2b. Temporal bone CT scan with images taken in plane of superior canal. This is an oblique reformatting, sometimes called the Poschel view. There is a wide area of dehiscence seen at the top.|
In our opinion, this is what your test prescription 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.