Iodine potassium iodide improves the contrast-to-noise ratio of micro-computed tomography images of the human middle ear

High-resolution imaging of middle-ear geometry is necessary for finite-element modeling. Although micro-computed tomography (microCT) is widely used because of its ability to image bony structures of the middle ear, it is difficult to visualize soft tissues - including the tympanic membrane and the suspensory ligaments/tendons - because of lack of contrast. The objective of this research is to quantitatively evaluate the efficacy of iodine potassium iodide (IKI) solution as a contrast agent. Six human temporal bones were used in this experiment, which were obtained in right-left pairs, from three cadaveric heads. All bones were fixed using formaldehyde. Three bones (one from each pair) were stained in IKI solution for 2 days, whereas the other three were not stained. Samples were scanned using a microCT system at a resolution of 20 m. Eight soft tissues in the middle ear were segmented: anterior mallear ligament, incudomallear joint, lateral mallear ligament, posterior incudal ligament, stapedial annular ligament, stapedius muscle, tympanic membrane and tensor tympani muscle. Contrast-to-noise ratios (CNRs) of each soft tissue were calculated for each temporal bone. Combined CNRs of the soft tissues in unstained samples were 6.1 3.0, whereas they were 8.1 +/- 2.7 in stained samples. Results from Welch's t-test indicate significant difference between the two groups at a 95% confidence interval. Results for paired t-tests for each of the individual soft tissues also indicated significant improvement of contrast in all tissues after staining. Relatively large soft tissues in the middle ear such as the tympanic membrane and the tensor tympani muscle were impacted by staining more than smaller tissues such as the stapedial annular ligament. The increase in contrast with IKI solution confirms its potential application in automatic segmentation of the middle-ear soft tissues. Lay description Micro-computed tomography (microCT) is a high resolution X-ray based imaging technique that can be used to visualize small structures in 3 dimensions. It is often used to image the small structures of the middle ear. Although microCT provides superb contrast for the bony structures of the middle ear, soft tissues such as the eardrum, muscles, tendons and ligaments are difficult to visualize because of poor contrast. The objective of this work was to investigate the efficacy of iodine potassium iodide (IKI) solution as a contrast-enhancement agent. Three cadaveric human middle ears were imaged after staining them with IKI, and another three were imaged without staining. Results indicate that IKI staining enhanced contrast of all middle-ear soft tissues, but it was most effective for larger tissues than smaller ones. In conclusion, IKI staining is useful for visualizing both the bony and soft-tissue structures of the middle ear.