History
A 74-year-old female patient, with a history of bilateral stapedectomy and stapes prosthesis insertion 10 years ago, came to the hospital complaining of a progressive, worsening conductive hearing loss on the left side for over the past 2 years. A CT examination using a ultra-high resolution (UHR) scan mode was performed for evaluation.
Diagnosis
CT images showed a piston wire prosthesis (PWP) on each side. The right PWP was normally positioned, with one end hooked onto the long process of the incus and the other attached to the oval window. A second PWP was also seen inside the right middle ear below the auditory ossicles. The left PWP was dislocated, with its loop slipped out of the long process of the incus and detached, from the other end, from the oval window. Otosclerosis was visualized bilaterally.
Subsequently, the patient was scheduled for revision surgery on the left. The second PWP on the right was considered a failed insertion attempt (10 years ago) that was accidentally left there, and a revision tympanoplasty for retrieval was not planned as the patient had no complaint at all on that side.
Courtesy of Medical Imaging Centre, Semmelweis University, Budapest, Hungary
Fig. 1: Oblique MPR images show a 3 mm long PWP on each side (Figs. 1a & 1b). The right PWP (Fig. 1a, arrow) is in a normal position, and the left PWP (Fig. 1b, dotted arrow) is dislocated. Otosclerosis is visualized bilaterally with subtle lucency in the bones near the cochlea (Figs. 1c & 1d, arrows).
Courtesy of Medical Imaging Centre, Semmelweis University, Budapest, Hungary
Fig. 2: cVRT images show the PWP, the auditory ossicles and the cochlea on both sides. The right PWP (Figs. 2a, 2c, 2e & 2g, arrows) is in a normal position, with one end hooked onto the long process of the incus and the other attached to the oval window. A second right PWP (arrowheads) is also seen inside the middle ear below the auditory ossicles. The left PWP (Figs. 2b, 2d, 2f & 2h, dotted arrows) is dislocated, with its loop slipped out of the long process of the incus and detached, from the other end, from the oval window. Note that the axial images used for cVRT creation are slices reconstructed at 0.2 mm with a very sharp kernel of Hr84.
Comments
The stapes is the innermost bone of the auditory ossicles in the middle ear. It has a footplate which seals the oval window and conducts vibrations to the cochlea. Otosclerosis is an abnormal bone remodeling which can affect the movement of the stapes causing conductive hearing loss. This may be corrected by stapedectomy, a surgical replacement of the stapes with a prosthesis. High resolution CT scans of the temporal bones have been routinely performed for both pre-surgery diagnosis as well as for post-surgery follow-up. The prosthetic position in the middle ear can be determined with high resolution CT. However, the thin metallic wires are difficult to detect due to volume averaging. [1]
This case is performed on NAEOTOM Alpha, a newly developed Dual Source CT scanner with photon-counting detectors (QuantaMax™), providing energy-resolved CT data at improved spatial resolution, without electronic noise. [2] A UHR mode featuring a slice collimation of 120 x 0.2 mm is applied in which the sub-pixels of the detector, with a size of 0.15 x 0.18 mm2 (at the isocenter), are read out individually. There are no physical septa between the pixels, each group of 4 x 6 subpixels is confined by collimator grids. Therefore, the full radiation dose efficiency of the detector is preserved in UHR mode. Furthermore, electronic noise is eliminated by setting up a predefined digital threshold for counting X-ray photons far above the electronic noise floor. A refined iterative reconstruction technique (Quantum Iterative Reconstruction QIR) is used to efficiently reduce image noise at ultra-high resolution without negatively affecting image sharpness or image noise texture. This combination – image noise reduction and ultra-thin image slices at full dose efficiency – allows not only for improved anatomic conspicuity, due to increased spatial resolution, but also for a photo-realistic three-dimensional visualization of anatomical details using cinematic volume rendering technique (cVRT). As shown in this case, even images reconstructed at 0.2 mm with a very sharp kernel of Hr84, can be used to render incredible details of the only 3 mm long PWP. CT findings clearly show the prosthesis dislocation, explaining the progressive hearing loss, and assist the otologist in making an appropriate surgical revision plan for the patient.
Examination Protocol