Our study highlighted two major anatomic landmarks and sites in the middle ear (STR and ST); one should be familiar with these landmarks to be able to describe normal and pathologic middle ear findings and provide an accurate CT interpretation and preoperative surgical assessment. HRCT is a major tool for imaging evaluation of the middle ear. HRCT offers excellent delineation of soft tissue abnormalities against a background of air (middle ear cavity, EAC, mastoid air cells) and allows excellent preoperative assessment of bone changes and/or involvement.
The STR is a common site of recurrent cholesteatoma, in both congenital and acquired cholesteatomas; however, it was not clearly recognized why cholesteatoma formed so easily in the STR . The prevalence of residual cholesteatoma is high following mastoidectomy. Even though, otoendoscopy is helpful to reduce retrotympanic residual cholesteatoma, anterior attic (STR) residual still represents a problem particularly with the canal wall up technique . Thus, the surgeons directed their attention to the ST and STR due to their frequent involvement by cholesteatoma and their relations to important structures. But, the STR and ST have received scanty attention and in the radiological literature [12, 17,18,19] without detailed description. We tried in our current study to use HRCT to assess both STR and ST in details defining their measurements in all directions and provide a new classification of the STR.
HRCT of the temporal bone is therefore a useful guide to the surgeon because it provides information regarding the extent of disease as well as possible anatomic variations and potential complications that may be avoided during surgery . We used 64 HRCT scanner that is sufficient to improve the image quality and better delineation of middle ear structures [21, 22].
Effective doses in sinus CT are 0.5–0.9 mSv and in ear CT are 0.3–0.6 mSv . Because CT examinations of the paranasal sinus and ear presented the highest dose to eye lens of 35.3 mGy , lead lined eye shield was used in the current work to minimize exposure of the eye and lens to radiation.
The ST is a posterior outpouching of the retrotympanum delimited by the mastoid part of the facial nerve and pyramidal eminence laterally and vestibule medially [19, 24] (Fig. 4).
During cholesteatoma surgery, the cholesteatoma may be located and missed in the depth of the ST that represents an important surgical challenge due to the conformation and position of the ST . Marchioni et al.  stated that when cholesteatoma involves the ST, there are two clinically important risks: first is the potential risk of residual disease due to incomplete removal of the cholesteatoma, and second is the increased risk for ossicular discontinuity and hearing loss due to cholesteatoma within the ST. Therefore, the CT-derived morphology of the ST could help the surgeon in the preoperative assessment .
In our current retrospective study on 200 ears CT, we confirm the data published by Marchioni et al.  who reviewed HRCT scan of the temporal bones. We were able to confirm the presence of their described three ST variations and codify their incidence: 28% for type A, 71% for type B, and 1% for type C. So, based on our data, ST type B is the most common morphological ST conformation, and ST Type A is less common that is similar to previous findings [13, 19]. In addition, we defined the exact measurements of the ST.
On the other hand, STR is bounded anteriorly by the root of zygomatic arch, medially by the anterior part of the tympanic facial nerve and geniculate ganglion, posteriorly by the cog, laterally by the scutum, and superiorly by the tegmen tympani (separates it from the dura mater) .
The STR could be involved by different pathologic processes. In both congenital and acquired cholesteatomas, the STR has been shown to be the common site of recurrence [12, 18]. Familiarity with the CT appearance of this space facilitates recognition of its early pathologic changes or its involvement in extensive lesions.
Our study shows that the STR is an anatomic structure that is consistently identified on HRCT scans. Thus, preoperative CT assessment will give orientation for STR configuration.
We reported that STR is commonly formed of solitary cell (79.5%), while in 20.5% of the ears, it is formed of multiple cells. These results are in agreement with the results of the recent study of Hong et al. . STR depth in the current study was 4.17 ± 0.86 mm with a range of 2.36 to 7.85 that is near to mean detected by Marchioni et al.  (5.1 ± 1.46 mm).
We introduced a new grading system for classification of the depth (anterior-posterior depth) of the STR into three grades based on the HRCT findings: grade 1 (depth < 3 mm), grade 2 (depth 3–5 mm), and grade 3 (depth > 5 mm). We found that grade 2 STR was the most common (80%) followed by the deep STR (grade 3, 14%) while the shallow STR (grade 1) was the lease common (6%).
We found that there was no significant relationship between ST types and STR grading, so the depth of each of them is not associated with or expect depth of the other recess and the STR and ST should be assessed radiologically as a separate entity.
In deep STR (types 2 and 3), surgeons need to be ready to use of angled (45°) endoscope and drill the anterior portion of the scutum mostly.
These newly suggested preoperative CT measurements and new STR grades are applicable, measurable in children and adult, and reliable. Therefore, they can be used as good predictors for the STR visibility during cholestetoma surgery and predict the need for the incus and malleus head removal and or otoendoscopy use to access the recess with different measurement. In addition, these will help to increase attention of the radiologists and the otosurgeons to the CT evaluation of these important regions.
It seems intuitive that the deeper the ST and the larger STR, the higher the risk of residual disease. So, second look surgery could be expected more in type C ST and grade 3 STR. Adding of the ST types and STR grading to the preoperative CT check list is recommended. Further studies focused on that topic in cholesteatoma patients are required.
Our study has some limitations; the study was conducted on non-pathological temporal bones, and further studies discussing changes in ST and STR measurements in cases of cholesteatoma are required. Also, our study did not discuss the age-related changes in ST and STR parameters due to lack of a considerable number of subjects in childhood age.