rel="nofollow" href="#ulink_864449ad-1140-55f7-a5ff-e512443f2277">93]. Semi-automated volume ratio measurement of endolymph from images obtained 4 h after single dose intravenous GdC using short (8 min) and long (18 min) acquisition times [57] has been recommended. The correlation of the volume ratio between the long and short acquisition time images was high, ranging from 0.77 (endolymphatic hydrops in the cochlea) to 0.99 (endolymphatic hydrops in the vestibule); the Pearson’s correlation coefficients were all statistically significant (p < 0.001). Later they demonstrated that 3-Inversion-recovery turbo spin echo with real reconstruction (3D-real IR) showed higher contrast between the non-enhanced endolymph and the surrounding bone [94] (Fig. 6). Regular contrast 3D-FLAIR cannot readily visualize cochlear hydrops after single dose IV-Gd, especially in apical turn. Recently, Naganawa et al. [85, 95] developed the positive endolymph image method, which visualizes endolymph as both a bright signal and subtraction image (HYDROPS images, HYbriD of reversed image of positive endolymph signal and native image of positive perilymph signal images) and allowed more easily interpretable images. In our experience, using heavily T2-weighted 3D-FLAIR positive perilymph image and positive endolymph image and subtracted images (HYDROPS technique) are useful to compensate for the lower concentration of Gd by IV. A further developed technique for generating improved HYDROPS (i-HYDROPS) images allows for a higher contrast to noise ratio per unit time compared to conventional HYDROPS imaging; this is accomplished by elongating the repetition time and increasing the refocusing flip angle [96]. In the study, the size of the endolymphatic space was comparable in both i-HYDROPS and 3D-real IR images. The 3D-real IR does not require post-processing for subtraction and might be more robust towards slight compositional alterations in endolymph than i-HYDROPS imaging based on magnitude reconstruction, and the scan time for 3D real IR images was 10 min.
Fig. 5. A 42-year-old man with a clinical diagnosis of definite Ménière’s disease of the right ear. Images were obtained 24 h after IT-Gd in the right ear and 4 h after IV-SD-GBCM. The right ear shows the combined IT + IV effect while the left ear shows only the IV-Gd effect. Note that only on the IT + IV side is the conventional 3D-FLAIR and 3D-real IR sufficient to show enhancement of the perilymph in order to distinguish the endolymphatic space; however, heavily T2-weighted 3D-FLAIR and HYDROPS2 allows the differentiation between the perilymphatic and endolymphatic space in both the IV side and IT + IV side. Significant endolymphatic hydrops (arrows) is seen in both the cochlea and vestibule on the right side, but no endolymphatic hydrops is observed in the left cochlea. Absence of endolymphatic hydrops in the left vestibule is confirmed in lower-level slices (not shown). With permission of Jpn J Radiol [58].
Fig. 6. 3D Real reconstruction inversion recovery MRI of the right ear illustrates high signal-to-noise ratio and severe cochleovestibular endolmyphatic hydrops in a patient with Ménière’s disease 24 hours after intratympanic GdC application (Magnevist 1:8 diluted). Section thickness 0.3 mm. Siemens Verio scanner, 32-channel head coil. Endolymph appears black, perilymph appears white, temporal bone appears grey. The sections are positioned from left to right and from top to bottom so that they move through the inner ear in a caudal-to-cranial direction. The cochlea displays endolymphatic hydrops in all three turns. The vestibulum displays severe endolymphatic hydrops, with only a weak perilymph signal at its outer borders. The horizontal semicircular canal is completely visualized by its perilymph signal. (With kind permission of Prof. B. Ertl-Wagner, Institute of Clinical Radiology, University of Munich).
Figure 4 (after intravenous injection of GdC) and Fig. 5 (after intratympanic injection plus intravenous injection of GdC) demonstrates the inner ear fluid compartments, anatomical structures and endolymphatic hydrops. Nakashima et al. [59], Pyykko et al. [71], and Fiorino et al. [97] demonstrated, with MRI, that endolymphatic hydrops was present in all living patients with definite MD, which is different from the reports by Shi et al. [98] in which endolymphatic hydrops was absent in some definite MD. Recently, it has been demonstrated that endolymphatic hydrops can affect the cochlear and vestibular compartments differently and cause different complaints [71]. However, the association between clinical symptoms and endolymphatic hydrops in individual patients is not yet clarified, as hearing can be relatively well preserved despite prominent endolymphatic hydrops [67, 99] and the extent of endolymphatic hydrops seems to vary along the course of the disease: it may increase, decrease or remain stable [100–102]. With new imaging techniques, endolymphatic hydrops can be demonstrated in vivo and can confirm the diagnosis. Furthermore, it has become possible to evaluate MD using new functional tests, such as VEMP frequency tuning measurements, in patient populations with clinically and morphologically (by MRI detection of endolymphatic hydrops) confirmed diagnosis of MD [101, 103].
The current challenges in inner ear imaging are to improve the delivery of the contrast agent so that the concentration of GdC in the inner ear exceeds the detection limit. The transtympanic and intravenous administrations have different indications [66]. If the aim is to demonstrate endolymphatic hydrops, then transtympanic injection of GdC is preferred. Usually the transtympanic administration provides stronger uptake and is easier to assess than intravenous injection. In principle, the sensitivity of the intravenous and the transtympanic method to demonstrate endolymphatic hydrops in the inner ear should be similar based on sufficient uptake of GdC in the inner ear, as both methods measure the same phenomenon [87]. A technique in which the images of inverted grey-scale positive endolymph are subtracted from images with native positive perilymph images is useful when inner ear loading of GdC is low. This subtraction significantly improves the contrast noise ratio and assists in the separation of endolymph, perilymph, and bone [104] or when combining intravenous injection with transtympanic injection [68].
The development of dynamic imaging techniques of the inner ear has provided several important new insights into MD; (1) the cochlear and vestibular compartments can be differently affected and (2) in about 24–75% of the cases the disease is bilateral [71, 105]. (3) The extent of endolymphatic hydrops can vary with time in individual patients [102].