Speed of sound ultrasound: comparison with proton density fat fraction assessed with Dixon MRI for fat content quantification of the lower extremity.

OBJECTIVES: To compare speed of sound (SoS) ultrasound (US) of the calves with Dixon magnetic resonance imaging (MRI) for fat content quantification. MATERIALS AND METHODS: The study was approved by the local ethics committee. Fifty calf muscles of 35 women (age range 22-81 years) prospectively underwent an US and subsequent MRI (Dixon sequence) examination as well as body weight and impedance fat measurements. SoS (in m/s) was calculated positioning a reflector on the opposite side of a conventional US machine probe with the calf in between. Fiducial nitroglycerin markers were placed on the calf at the reflector and US probe end positions for later registration of the US sonification volumetric section. An automatic segmentation algorithm separated MRI adipose tissue, muscle and bone regions. MRI fat fraction of the entire leg slice (total) and intramuscular and adipose tissue fat fraction were calculated and correlation analysis and correlation coefficient comparison were performed. RESULTS: Median SoS demonstrated a very strong (r = - 0.83 (95% CI - 0.90; - 0.72); p < 0.001) correlation with MRI total fat fraction, a strong (r = - 0.61 (95% CI - 0.76; - 0.40); p < 0.001) correlation with MRI adipose tissue fat fraction and a moderate (r = - 0.54 (95% CI - 0.71; - 0.31); p < 0.001) correlation with MRI intramuscular fat fraction. Impedance body fat percentage correlated strongly with SoS (r = - 0.72 (95% CI - 0.85; - 0.51); p < 0.001) and MRI total fat fraction (r = 0.61 (95% CI 0.34; 0.78); p < 0.001). For electrical impedance, significantly lower correlations (p = 0.033) were found for MRI total fat fraction compared with SoS. CONCLUSIONS: Correlations of SoS with Dixon MRI fat fraction measurements were very strong to moderate. KEY POINTS: • Correlations of speed of sound with Dixon MRI fat fraction measurements of the same body location were very strong to moderate. • Speed of sound measurements showed a high repeatability. • Speed of sound provides a sufficient discrimination range for fat fraction estimates.

Evaluating Wideband Tympanometry Absorbance Changes in Cochlear Implant Recipients: Mechanical Insights and Influencing Parameters.

Background: Cochlear implant (CI) electrode insertion can change the mechanical state of the ear whereby wideband tympanometry absorbance (WBTA) may serve as a sensitive tool to monitor these mechanical changes of the peripheral auditory pathway after CI surgery. In WBTA, the amount of acoustic energy reflected by the tympanic membrane is assessed over a wide frequency range from 226 Hz to 8000 Hz. The objective of this study was to monitor changes in WBTA in CI recipients before and after surgery. Methods: Following otoscopy, WBTA measurements were conducted twice in both ears of 38 standard CI recipients before and in the range of 4 to 15 weeks after CI implantation. Changes from pre- to postoperative absorbance patterns were compared for the implanted as well as the contralateral control ear for six different frequencies (500 Hz, 750 Hz, 1000 Hz, 2000 Hz, 3000 Hz, 4000 Hz). Furthermore, the influence of the time point of the measurement, surgical access, electrode type, sex and side of the implantation were assessed for the implanted and the control ear in a linear mixed model. Results: A significant decrease in WBTA could be observed in the implanted ear when compared with the contralateral control ear for 750 Hz (p < 0.01) and 1000 Hz (p < 0.05). The typical two-peak pattern of WBTA measurements was seen in both ears preoperatively but changed to a one-peak pattern in the newly implanted ear. The linear mixed model showed that not only the cochlear implantation in general but also the insertion through the round window compared to the cochleostomy leads to a decreased absorbance at 750 and 1000 Hz. Conclusions: With WBTA, we were able to detect mechanical changes of the acoustical pathway after CI surgery. The implantation of a CI led to decreased absorbance in the lower frequencies and the two-peak pattern was shifted to a one-peak pattern. The result of the linear mixed model indicates that WBTA can detect mechanical changes due to cochlear implantation not only in the middle ear but also in the inner ear.