2.1 Patients
In this study, a total of 42 patients (23 males, 19 females; age range, 7-65 years; mean, 40.1 years) with surgically confirmed cholesteatoma were enrolled, and patients with congenital cholesteatoma have been excluded according to the clinical diagnosis from October 2018 to April 2019. Of the 42 patients, 37 had cholesteatoma in the middle and 5 had cholesteatoma in the external auditory canal. Clinicopathological results were the gold standard for all patients.
Ethics and Consent to Participate
The study was approved by the Review Committee of Eye & ENT Hospital of Fudan University, and written informed consent was obtained from all patients.
2.2 Imaging technique
All patients underwent DWI (both a prototype TGSE BLADE DWI sequence and a commercially available RESOLVE DWI sequence) using a 3 T scanner (MAGNETOM Prisma, Siemens Healthcare, Erlangen, Germany) with a 64-channel brain coil. However, especially for small attic lesions, a golden standard has not been reached at present and axial plane is considered acceptable.
The parameters for TGSE BLADE DWI were as follows: TR/TE = 4000/62 ms; slice thickness/gap = 2/0.2 mm; slices = 21; bandwidth = 520 Hz/Px; field of view (FOV) = 280 × 280 mm2; matrix = 192 × 192; voxel size = 1.5 × 1.5 × 2.0 mm3; number of excitations (NEX) = 1; diffusion mode = 4 scan trace; b = 0, 1000 s/mm2; turbo factor = 13; EPI factor = 3; and data acquisition time =3 min 46 s. For RESOLVE DWI, the imaging parameters were as follows: TR/TE = 5020/53 ms; slice thickness/gap = 2/0.2 mm; slices = 21; bandwidth = 766 Hz/Px; FOV = 230 × 230 mm; matrix = 192 × 192; voxel size = 1.2 × 1.2 × 2.0 mm; diffusion mode = 4 scan trace; b = 0, 1000 s/mm2; and data acquisition time = 3 min 46 s.
2.3 Image assessment
2.3.1 Qualitative analysis of image quality
All images obtained in the 42 included patients by TGSE or RESOLVE were evaluated by two radiologists with 10 years of experience in ear MRI evaluation.
Each observer randomly and blindly evaluated the images without knowing the type of DWI sequence and compared the two different DWI sequences using the side-by-side display method. A final decision was made based on mutual consultation when there was a divergence in the assessment results.
Qualitative evaluation of images obtained by TGSE and RESOLVE was performed according to four criteria: image sharpness, geometric distortion, ghosting artifacts, and overall image quality. Image sharpness was assessed on a scale from 1 to 3. Both geometric distortion and ghosting artifacts were evaluated on a scale from 1 to 4, and the evaluation of geometric distortion included two parts: the whole image and the lesion in the ear. Overall image quality was also graded on a scale of 1 to 5. The detailed qualitative evaluation criteria are shown in Table 1. In Figure 1, images C and E show geometric deformations with a score of 4 (no distortion) and 2 (moderate distortion), respectively.
2.3.2 Quantitative analysis of image quality
The SNR, contrast and contrast-to-noise ratio (CNR) were the main evaluation criteria for the quantitative analysis of images obtained using the two sequence. The apparent diffusion coefficients (ADCs) of the two sequences were compared simultaneously. On the b1000 TGSE and RESOLVE images, the SNR of the lesions in the region of interest (ROI) was defined as the ratio of the mean signal intensity of the lesion (SROI) to the standard deviation of the background noise (σBG) (SNR = SROI/σBG)9. The SNR of the brainstem was calculated by the same method as follows: SNR=SB/σBG. Contrast was defined as the ratio of the signal intensity of the lesion (SROI) to the signal intensity of the brainstem (SB) on the b1000 map (contrast=SROI/SB).
The CNR was defined as the difference between the SROI and SB divided by the standard deviation in the lesion ROI (σROI) and the brainstem ROI (σB)6-8,10, as follows: (see Equation 1 in the Supplemental Files)
The ROI of the lesion on the b1000 and ADC maps was manually drawn as 1 mm2 at the level of the maximum diameter of the lesion, and the corresponding signal intensity and standard deviation were automatically generated on the MRI workstation. The ROI of the brainstem was defined by selecting 10 mm2 of the brainstem, and the signal intensity and standard deviation of each ROI were automatically generated. A circular ROI of 10 mm2 was set in the background on the b1000 map for all patients, and the standard deviation of the ROI was automatically generated. In selecting the ROI, areas affected by susceptible artifacts or volume effects were avoided.
The parameters were measured independently and randomly by the two raters at an interval of 2 weeks. The mean value of the two measurements was selected as the final data for further analysis. For brain tissue evaluated on DWI sequences, the diagnostic criterion for cholesteatoma was a very high signal intensity that corresponded to limited diffusion on the ADC maps(3,20). The sizes of all lesions were determined on T2-weighted imaging (T2WI) based on the size and location of the lesions observed on the TGSE and RESOLVE sequence images and the premise of avoiding artifacts at air-bone interfaces as much as possible.
2.4 Statistical analysis
All statistical analyzes and plots were performed and created using the SPSS 24.0 software package (Chicago, IL, USA), and P < 0.05 was considered statistically significant. The normality of all measurements obtained using the TGSE and RESOLVE sequences was tested using the Shapiro-Wilk test. Significant differences in qualitative parameters between TGSE and RESOLVE DWI were determined using the Wilcoxon rank-sum test, and significant differences in quantitative parameters were determined using the paired t-test. The interreader correlation of the ADC as a quantitative index was evaluated using the intraclass correlation coefficient (ICC). The range of the ICC coefficient was set from 0 to 1.00, and the ICC was defined as follows: < 0.40, poor; 0.41-0.60, moderate; 0.61- 0.80, good; > 0.81, excellent16,17. The mean ADCs of the lesions and brainstem measured by the two observers were further calculated, and the differences between them were assessed by paired t-test.