History
A 75-year-old female patient was presented to the hospital due to an episode of instability. A routine examination revealed a progressive conductive hearing loss on the right side. An ultra-high resolution (UHR) scan was performed using a dual source photon-counting detector (PCD) CT, NAEOTOM Alpha®, to investigate the etiology of her hearing loss.
Diagnosis
UHR images showed a heterogeneous mass, measuring 7 mm × 6 mm × 4 mm in size, located anteriorly in the epitympanum with some extension into the mesotympanum. It was medially in contact with the tympanic segment of the facial nerve canal and laterally with the malleus. The mass was partially calcified with a semolina-like appearance which was highly suggestive of tophaceous gout. Subsequently, a surgical intervention, involving tympanoplasty with resection of the mass and reestablishment of the ossicular chain continuity through incus remodeling, was performed. The histopathological analysis of the specimen confirmed the CT suspicion of a gout tophus.
A follow-up, six months post-surgery, revealed only a marginal hearing improvement on audiometry alongside with favorable healing on otomicroscopy. A subsequent CT examination, performed with an energy-integrating detector (EID) CT in an external facility, showed suboptimal contact between the remodeled incus and the stapes. A millimetric calcified mass was also noted anteriorly in the middle ear. A revision tympanoplasty was performed, revealing a residual or recurrent gout tophus, which was resected. Additionally, a total ossicular replacement prosthesis (TORP) was implanted for a fracture of the anterior crus of the stapes to improve hearing.
Three months later, a follow-up CT, performed again with a dual source PCD-CT, showed a dislocation of the medial part of the TORP now located in the retrotympanon instead of centered on the oval window. A revision surgery was performed replacing the first TORP by a second one (Medel mXACT Total Prothesis 5 mm). A further follow-up CT, performed 4 months later, confirmed the correct position of the second TORP and the patient’s outcome had improved.

Fig. 1: An axial UHR image acquired with PCD-CT (Fig. 1a, 0.2 mm, 11.5 mGy) and a coronal MPR image (Fig. 1b) show the mass of tophaceous gout (arrows) in contact with the malleus (dotted arrows) and the tympanic segment of the facial nerve (dashed arrow). The incus (arrowhead) is not affected.

Fig. 2: An axial UHR image acquired with EID-CT (Fig. 2a, 0.4 mm, 47.8 mGy) shows a millimetric calcified mass (arrow) anteriorly in the middle ear, suggesting a residual or recurrent gout tophus. In a paracoronal image (Fig. 2b), a gap between the stapes head (dotted arrow) and the incus interponate (arrowhead) is seen, suggesting a suboptimal contact between the remodelled incus and the stapes.

Fig. 3: Oblique MPR images (Fig. 3a & 3b) and cVRT images (Fig. 3c & 3d) show a dislocated medial part of the TORP in the retrotympanon (arrows) and a revised TORP centered on the oval window (dotted arrows). UHR images are acquired with PCD-CT.
Comments
Tophaceous gout of the middle ear is a rare occurrence causing conductive hearing loss. It may not manifest any clinical or biochemical signs, such as this case – the patient had no typical gout symptoms in other joints and her serum uric acid levels were within the normal range. It is often mistaken for other more common entities, such as cholesteatoma or osteoma. The treatment of choice is surgical resection with ossicular chain reconstruction. The characteristic imaging presentation of this rare disorder is a semolina-like mass in the middle ear. Visualization of the small anatomical details requires high spatial resolution in the CT scan of the temporal bones. Techniques, such as adding metal grids reducing the detector aperture to enable ultra-thin slice acquisition and the use of sharp reconstruction kernels, have been applied previously with EID-CT to improve spatial resolution. The drawbacks, however, are the compromised radiation dose efficiency and increased image noise. PCD-CT provides energy-resolved CT data with increased spatial resolution without electronic noise. The UHR mode applies a fine collimation of 120 x 0.2 mm acquiring data from small detector sub-pixels. These pixels are defined by a strong electric field and are read out separately to increase the spatial resolution. Additional mechanical separation is not necessary, and the dose efficiency is therefore not compromised. As shown in this case, the CTDIvol with PCD-CT is reduced to 11.5 mGy (versus 47.8 mGy with EID-CT) while retaining an optimal image quality. Despite the ultra-thin slices of 0.2 mm reconstructed with a very sharp kernel of Hr80, the low noise level of the UHR images allows for a high-quality rendering of life-like, three-dimensional images using cinematic volume rendered technique (cVRT), which intuitively demonstrates the position of the prosthesis and helps to accurately plan the revision surgery.
Examination Protocol