Within the Temporal Bone · Chapter 04 · Part I · The laboratory
Reading Temporal Bone Imaging
From slices to the specimen in three stations
Bench mode: one step per screen, large type, screen kept awake. Tap, use the arrow keys or say “next”.
- Specimen
- The bone on the bench and its own CT, where available
- Bench time
- Before every dissection, and again when the field surprises you
- Instruments
- Thin-section bone-window CT in axial and coronal planes, a viewer, the specimen record
- View
- Axial and coronal series; reformats when needed
- Before this
- Chapter 3: surfaces and hidden compartments
By the end you can
- Confirm that a scan belongs to this specimen and read its side correctly
- Trace a structure through adjacent slices and into a second plane
- Translate findings into expectations for the exercise you plan
- Recognise when thin bone may only appear absent
- Correlate the dissected specimen with its images and record discrepancies
A written pre-dissection note that links scan findings to the planned exercise, and, after the dissection, every exposed structure matched to its image with discrepancies recorded.
Radiological diagnosis. A suspected dehiscence that would change the procedure calls for appropriate radiological review.
The landmark chain
Each landmark leads to the next. Tap any one to see every station where it appears.
04.1 Read the specimen in more than one plane
Imaging tells you, before the first cut, which landmark will be difficult in this particular bone. It does not replace the landmarks, and it does not authorise blind drilling. This chapter turns a CT into a pre-dissection note and closes the loop afterwards, when the exposed anatomy is compared with the slices that predicted it.
Where imaging of the actual specimen is available, verify its identity and side before using it. Examine thin-section bone-window images in the axial and coronal planes, with reformats when needed, and follow each structure through adjacent sections rather than treating one image as a complete description.
A teaching scan from another specimen may illustrate a relationship, but it cannot establish the dimensions or variants of the bone on the bench. The images in this chapter are real, labelled bone-window CT slices of one left ear from a published teaching case (Radiopaedia, courtesy of Craig Hacking), shown with their original numbering and legend.
Read a volume, not a slice.
04.2 · The 4 stations
Confirm the scan and the plane
- Match the specimen number and side on the scan to the label on the holder.
- Do not assume that the left of the display is the specimen’s left. In the standard radiological display the patient’s right is on the viewer’s left.
- Identify the plane, and relate it to the microscope view by turning the specimen in your hand or by drawing the orientation on the record.
Specimen, side and plane confirmed and written on the record.
The scan’s identity or side is uncertain. Do not use it for this bone.
Read the axial series
- Begin with the external canal and the middle ear to orient the study.
- Trace the ossicular region, then the cochlea, the vestibule and the semicircular canals; follow the internal auditory canal and the facial nerve.
- Inspect the mastoid air-cell system and its boundaries: the distribution of the cells and the position of the sigmoid sinus relative to the canal wall and the labyrinth.
- Scroll up and down through each structure.
Each structure traced through adjacent slices and named in a fixed order.
You are naming structures on a single slice. Scroll before you decide.
Read the coronal pair
- The coronal plane shows the roof, the lateral attic wall and the vertical relationships that the axial plane does not. Read one slice through the cochlea and one through the vestibule, and scroll either side of each.
- Through the cochlea: how low does the tegmen lie over the attic? Is the epitympanum aerated? Is the scutum sharp or blunted, and is Prussak’s space clear? Are the cochlear turns and capsule normal? Are both the labyrinthine and tympanic facial segments seen? Is the carotid canal wall seen throughout?
- Through the vestibule: is the bone over the lateral canal intact? Is the facial nerve seen below the lateral canal, above the oval window? Is the vestibule of normal size and shape? How high does the jugular bulb rise? Is there bone over the superior canal arc (thin bone may look absent)?
- Answer every question on the record. A low tegmen narrows the attic and the mastoid from above.
Every coronal question answered on the record for this bone.
You are calling thin bone absent. Correlate the planes, and seek review if it would change the plan.
Correlate during and after dissection
- Keep the study open during the exercise. When a finding differs from expectation, stop and compare the same region in more than one plane.
- Decide whether the difference reflects orientation, incomplete exposure, specimen damage or genuine variation.
- After the dissection, find each exposed structure on the pre-dissection images.
- Record important discrepancies and the explanation agreed with the instructor; keep an unresolved one as a documented question rather than silently revising the original reading.
Each exposed structure matched to its image, and every discrepancy recorded.
You are choosing a slice because it resembles the current view. Return to the series.
04.3 Read for the intended exposure
Describe the relevant anatomy before choosing a route. Each exercise asks its own questions of the scan.
| Exercise | What to read on the scan |
|---|---|
| Mastoid | Distribution of air cells (dense bone gives fewer transitions to guide orientation); contour of the tegmen; sigmoid sinus relative to the canal wall and the labyrinth |
| Facial recess and cochlea | Course of the facial nerve; configuration of the cochlea; round-window region and vascular anatomy |
| Inferior routes and petrous apex | Jugular bulb and carotid canal; available aerated tracts |
| Internal auditory canal | Its relation to the cochlea, the labyrinth and the petrous surfaces |
04.4 Absent, or only invisible?
A low roof or an anteriorly placed sinus can restrict the available field. These findings change the approach to exposure; they are not measurements that authorise blind drilling.
Do not infer normal anatomy from a structure that is simply difficult to see. Small bony defects can lie below the resolution of a particular acquisition, and thin bone may appear absent because of imaging limitations. If a suspected dehiscence would materially change the procedure, seek appropriate radiological review. A normal-looking scan does not replace direct anatomical caution.
Distances measured on CT must be read in the correct plane and within the individual specimen. Do not teach a single adult antrum depth as a universal endpoint: measurement refines the anatomical plan; it does not replace it.
The bone tells you the truth.
04.5 Fault finding
| What you see | Probable cause | Correction |
|---|---|---|
| Findings on the wrong side of the bone | Display left taken as specimen left | Check the side marker against the holder |
| A thin plate read as a defect | Resolution and reconstruction limits | Correlate planes; seek review if it would change the plan |
| A depth carried from another bone | Measurement used as a target | Measure in this specimen, in the right plane |
| Field and scan disagree | Orientation, exposure, damage or variation | Stop, compare planes, decide which |
04.6 Viva at the bench
The CT appears to show an absent thin bony covering. What remains unresolved, and what do you do?
Whether it is a true defect or thin bone below the scan’s resolution. Correlate in more than one plane, proceed with direct anatomical caution, and seek radiological review if it would change the procedure.
On a coronal slice through the vestibule, where does the tympanic segment of the facial nerve lie?
Below the lateral canal and above the oval window.
The field disagrees with the CT. Name four possible explanations.
Orientation, incomplete exposure, specimen damage, or genuine variation.
One best answer
04.7 My dissection log
Saved on this device only. Print it or send the summary to your fellowship coordinator for review.
Score the dissection · review order
| Item | 0 · not achieved | 1 · partly | 2 · achieved |
|---|---|---|---|
| Scan identity and side confirmed | |||
| Axial series read in a fixed order | |||
| Coronal questions answered | |||
| Pre-dissection note links a finding to the plan | |||
| Exposed structures matched to images | |||
| Discrepancies recorded, not revised away |
The review order is a teaching method, not a validated scoring instrument. Structured end-product scales such as the Welling scale have been studied for rater reliability; they assess the dissection, not operative outcome, but they make feedback specific.
Before your first CT-guided dissection
Sources and credits
- Hacking C. Normal CT temporal bone, axial and coronal series (labelled). Radiopaedia.org, rID 63096 and 63095. CC BY-NC-SA 3.0. radiopaedia.org/cases/63096
Real dissection photographs with numbered drawings: Bassett Collection, Stanford University (CC BY-SA 4.0). Classic anatomical and operative plates: Gray’s Anatomy of the Human Body (1918, drawings by H. V. Carter) and Burghard’s A System of Operative Surgery (1909), both public domain, credited beside each plate. Remaining diagrams and charts: Dr Bhushan Kathuria & Team, not to scale. Operating-theatre frames and clips: our theatre, patient consent taken, identity not shown. Atlas photographs: Open Access Atlas of Otolaryngology (CC BY-NC 3.0), with their authors credited beside each image. This edition uses no AI-generated images. For professional education alongside supervised instruction and local protocols.
Dr Bhushan Kathuria & Team · Reg. No. HN-017323, Haryana Medical Council · A GeNext MedEd Foundation surgical education initiative.
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