Comparison of Echo-Planar Diffusion-Weighted Imaging and Delayed Postcontrast T1-Weighted MR Imaging for the Detection of Residual Cholesteatoma

Description of Patients

Thirty-one patients could be included in this study (M/F ratio = 1:0.93). Mean age at the time of the first surgery was 37.6 ± 4.05 years (median, 43.5 years; minimum, 4 years; maximum, 80 years). The mean delay between MR imaging and second-look surgery was 3.24 ± 1.12 months (median, 2 months; minimum, 0 months; maximum, 37 months). Mean delay between the 2 surgeries was 20.07 ± 3.10 months (median, 13 months; minimum, 10 months; maximum, 72 months). During the second-look surgery, residual cholesteatoma was observed in 19 patients (61.3%), whereas scar tissue, granulation tissue, and inflammatory tissue were observed in all other patients. Assuming that all patients without opacity on CT were free from disease (18/49), we could estimate the total rate of residual cholesteatoma at 38.78% (19/49) in this series of CWU tympanoplasties.

Interobserver Agreement

Interobserver agreement was good between the 2 radiologists for diffusion imaging (A.B. and V.P., κ = 0.81). However agreement between the radiologists and the otologic surgeon was poorer (κ = 0.75 and 0.56). Interobserver agreement was lower in cases of DPI (κ = 0.51 between the 2 radiologists’ evaluations and κ = 0.46 and 0.39 between the radiologists’ and the otologic surgeon's evaluations).

EPI DWI

Of 31 patients (Table 1), we observed 12 true-positive cases (TP, cases with cholesteatoma found during surgery and previously diagnosed by MR imaging), 3 false-positive cases (FP, cases with no cholesteatoma found during surgery though previously diagnosed by MR imaging), 8 true-negative cases (TN, cases with no cholesteatoma found during surgery and previously diagnosed as having no cholesteatoma by MR imaging), and 8 false-negative cases (FN, cases with cholesteatoma found during surgery though not previously diagnosed by MR imaging). Sensitivity, specificity, PPV, and NPV were 60%, 72.73%, 80%, and 50%, respectively, in the whole series (Tables 2 and 3). If we considered only the residual cholesteatomas of >3 mm, the sensitivity, specificity, PPV, and NPV were 75%, 84.21%, 75%, and 84.21%, respectively (Tables 2 and 3). Finally, considering only the residual cholesteatomas of >5 mm, we found that the sensitivity, specificity, and NPV increased to 100%, 88%, and 100%, respectively, whereas the PPV decreased to 66.67% (Tables 2 and 3).

DWI sequences displayed a high sensitivity as soon as the size reached 5 mm, with a poor detection rate for smaller cholesteatomas (Figs 1 and 2). Indeed, all FNs corresponded to small-sized residual pearl-like cholesteatomas (from 2 to 5 mm), with no misdiagnosis of large cholesteatomas. The smallest lesion detected was 3 mm. Specificity was lower than sensitivity whatever the size of the lesion. The FPs detected with this sequence comprised 2 patients with silastic sheets and 1 patient with granulation tissue (Fig 3).

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Fig 1.

EPI DWI and DPI for residual cholesteatoma detection. DWI axial b = 0 (A), b = 500 (B), b = 1000 (C) and coronal b = 0 (D), b = 500 (E), b = 1000 (F) EPI sequences. Early (G and I) and delayed (H and J) postconstrast T1-weighted axial (G and H) and coronal (I and J) sequences. With DWI, an increasing signal intensity (arrow) between b = 0 (A and D), b = 500 (B and E), and b = 1000 (C and F) sequences was observed in the left hypotympanum. A hypointense space-occupying lesion (arrows) was also observed in the left hypotympanum with early enhanced T1-weighted sequences (G and I). This lesion was more conspicuous on delayed imaging (H and J, large arrows), whereas scar tissue was intensely enhanced (small arrows). Thus, surgery confirmed a residual cholesteatoma of the left hypotympanum.

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Fig 2.

Mismatch between DWI and DPI, with a small lesion. EPI DWI axial b = 0 (A), b = 500 (B), b = 1000 (C) and coronal b = 0 (D), b = 500 (E), b = 1000 (F) sequences. Early coronal (H) and delayed postcontrast T1-weighted axial (G) and coronal (I) sequences. Progressive signal-intensity decay was observed in the right mastoid with DWI (A–F, small arrows). DWI was considered negative for residual cholesteatoma. Conversely, a hypointense lesion was observed on early-enhanced T1-weighted MR images (H, large arrow), with a ring enhancement on DPI (G and I, large arrows). At surgery, a 3-mm large pearl-like residual cholesteatoma was found.

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Fig 3.

FP examinations with DWI and DPI in 2 different patients. Axial (A) and coronal (B) EPI DWI b = 1000 and coronal DPI (C) sequences. Hyperintense foci were observed in the right mastoid with axial and coronal DWI (A and B, arrows). Lack of enhancement on coronal delayed T1-weighted MR image (C, arrow) led us to suspect a residual cholesteatoma. At surgery, only a silastic sheet filling the right mastoid was found. Axial (D) and coronal (F) EPI DWI b = 1000 and axial (E) and coronal (G) DPIs in another patient. DWI was negative for residual cholesteatoma (D and F). Note nonenhancing nodular masses in the left mesotympanum and mastoid on axial and coronal delayed T1-weighted MR images (E and G, arrows). Small-sized residual cholesteatomas were suspected. At surgery, only scar tissue was found.

DPI

Of 31 patients (Table 1), we observed 18 TP, 5 FP, 6 TN, and 2 FN cases. Sensitivity, specificity, PPV, and NPV were 90%, 54.55%, 78.26%, and 75%, respectively, in the whole series (Tables 2 and 3). If we considered only the residual cholesteatomas of >3 mm, then sensitivity, specificity, PPV, and NPV were 100%, 75%, 68.75%, and 100%, respectively (Tables 2 and 3). Finally, if we considered only the residual cholesteatomas of >5 mm, then sensitivity and NPV were unchanged, whereas specificity increased to 80% and PPV decreased to 55% (Tables 2 and 3).

DPI proved a highly sensitive examination with a sensitivity of 100% for lesions of >3 mm (Figs 1 and 2). The smallest lesion detected was 2 mm. The specificity was decreased by numerous FP cases. Most interesting, these cases included those observed with DWI: 2 cases of silastic sheet and 1 case of granulation tissue. The last 2 additional cases were scar tissue with silastic sheet (Fig 3).

DWI Sequences in Association with DPI

We calculated the intraclass correlation between the results of these 2 techniques and found an average concordance (κ = 0.56) between DWI and DPI. On examining the associations DWI or DPI and DWI and DPI, we observed that the results of the former were identical to those of DWI alone and the results of the latter were identical to those of DPI alone for any size of residual cholesteatoma.

EPI DWI for Residual Cholesteatoma Detection

Cholesteatoma is characterized by an increasing signal intensity along with b-value on EPI DWI. One explanation could be the decreased mobility of the water molecules in the residual cholesteatoma, so-called diffusion restriction.^17,18 Another explanation for the increased intensity of the epidermoid tumors could be the T2 shinethrough effect.¹⁶

Previous studies on the detection of residual cholesteatomas, including those with 17–45 patients, have reported sensitivities varying from 12.5% to 100%, depending on the study.^11-14 Conversely, the specificity has been more comparable among these studies, ranging from 91% to 100%. In the present study, the sensitivity, specificity, PPV, and NPV were 60%, 72.73%, 80%, and 50%, respectively.

The discrepancy concerning the sensitivity and the specificity cannot be related to interobserver differences, with a good interobserver agreement in both the present (κ = 0.81) and previous studies (κ = 0.92).¹² In our opinion, this difference is mainly due to a large variation in the size of the residual cholesteatomas between studies. Indeed, the lowest size of residual cholesteatoma detected seems to range from 4 to 5 mm according to previous articles.^11-14 Indeed, in a study by Vercruysse et al,¹⁴ only 29% of residual cholesteatomas were >3 mm and the corresponding sensitivity was 12.5%. Conversely, Dubrulle et al¹² observed no lesion of <5 mm, hence perhaps relating to the sensitivity of 100% reported in our study. If we consider only the residual cholesteatomas of >5 mm in the studies by Aikele et al,¹¹ Vercruysse et al,¹⁴ and Stasolla et al,¹³ the sensitivity reached 100%, which is consistent with that in our results.

Another explanation for the differences observed between studies could be the delay between MR imaging and surgery. This hypothesis cannot be confirmed, however, because this delay was shorter in series with lower sensitivities (mean delay of 3.24 months in the present study and 15 days in others¹⁴) than in other series (maximal delays of 5 and 11 months in studies performed by Dubrulle et al¹² and Aikele et al,¹¹ respectively).

DPI for Residual Cholesteatoma Detection

Although MR imaging with gadolinium injection allows a good discrimination between granulation tissue, inflammatory tissue, scar tissue, and cholesteatoma in nonoperated ears,¹⁹ detection of residual cholesteatoma is more problematic after CWU surgery. Granulation tissue, inflammatory tissue, and cholesterol granulomas can be easily diagnosed on T1, T2, and early postcontrast T1-weighted sequences. With these sequences however, the difference between scar tissue and cholesteatoma remains unclear. Scar tissue is constantly enhanced by gadolinium injection, though the enhancement achieved may be delayed due to poor vascularization. On the other hand, cholesteatomas are never enhanced following gadolinium injection. Cholesteatomas appear bordered by a ring enhancement attached to the enhancing mucosa. This difference forms the basis of DPI for residual cholesteatoma detection.

In a series including 18 patients, Williams et al¹⁵ reported diagnostic values by using DPI of 85.2% sensitivity, 92.6% specificity, 92.6% PPV, and 100% NPV. In a later series of 41 patients (including the 18 patients from the earlier series by Williams et al¹⁵), Ayache et al⁸ reported values of 90% sensitivity, 100% specificity, 100% PPV, and 92% NPV. Interobserver agreement was good (κ = 0.78,¹⁵) but less than that with DWI.

In our series, we noticed a lower interobserver agreement with DPI (κ = 0.51) compared with diffusion imaging. This reflects the high variability seen in the results, even with experienced neuroradiologists. Sensitivity, specificity, PPV, and NPV were 90%, 54.55%, 78.26%, and 75%, respectively, in the whole series. Our results are in accordance with those reported previously except for the specificity. Indeed, we observed 5 FP cases in our study. The sensitivity and the specificity rose to 100% and 80%, respectively, for cholesteatomas >5 mm. In previous studies,^8,15 the smallest residual cholesteatomas detected were 3 mm and the sensitivity for lesions >5 mm was 100%. Unfortunately, we have no data concerning the number of cholesteatomas >5 mm from either study, making it difficult to compare these results with ours.

EPI DWI versus DPI: Complementary or Competitor Techniques?

Owing to its good sensitivity, EPI DWI is efficient for the screening of large residual cholesteatomas. However, the specificity was low for the entire series in the present study. Only one FP case has been described by Dubrulle et al¹² (bone pate). We have described 3 additional cases in the present study involving a silastic sheet. The magnetic characteristics and the circular shape of this sheet in the mastoid could lead to misdiagnosed residual cholesteatomas.

DPI is more sensitive but less specific than DWI. Most interesting, the FP cases observed with diffusion imaging were also incorrectly identified with DPI. The problems occurred with the silastic sheet, in which the signal intensity characteristics and rounded shape probably misled the radiologist. The last 2 FP cases were scar tissue with a silastic sheet. In these cases, late enhancement with gadolinium was not observed, though it would be reasonable to assume that the delay between the injection and the last imaging was insufficient to reveal the scar tissue.

Silicone sheets like silastic may interfere with cholesteatoma detection. This could lead to a significant problem because these materials are not easily detected on CT (intermediate signal intensity similar to that of soft tissue). Radiologists should, therefore, be made aware of the presence of such foreign material to avoid an incorrect diagnosis of residual cholesteatoma.

The choice between these 2 techniques depends on the goal of the examination. Because the aim of imaging for residual cholesteatoma detection is to avoid undue surgery, we advocate the use of the more specific examination (ie, DWI). This technique is convenient, fast, and very robust, with good interobserver agreements. The main drawback would be the risk of a residual cholesteatoma lost to the follow-up. However, we agree with the view of other authors^8,15 that small residual cholesteatomas may be left if a close follow-up is performed. This imaging should be repeated with enough time left in between for a residual lesion to reach 5 mm. In our experience, DPI is more difficult to set up due to the delay between injection and imaging. DPI is a time-consuming examination, sometimes requiring general anesthesia, especially in children. Its interpretation requires more experience with a greater dependence on the examiner, as shown by the average interobserver agreements. Associating both techniques is interesting, mainly in cases of residual cholesteatoma of >5 mm detected with DPI. While keeping the same sensitivity, the use of a DWI and DPI paradigm could increase the specificity from 80% to 88%. In other words, any suggested large lesion observed with DPI but not with DWI would eliminate a residual cholesteatoma diagnosis. In our opinion, both techniques have equal strengths and weaknesses, and as such, we prefer to perform both techniques in each patient.