Ecchordosis physaliphora (EP) is an uncommon benign lesion, usually asymptomatic, that arises from an ectopic notochordal remnant. It occurs intradurally in the prepontine cistern, with an attachment to the dorsal surface of the clivus.1 An intradural EP does not need any treatment unless it is symptomatic. A previous report described the magnetic resonance imaging (MRI) features of intradural EP using conventional 2-dimensional (2D) T1- and T2-weighted images.2 However, the spatial and contrast resolutions of this type of imaging are insufficient for the precise evaluation of small intradural lesions, and when using 2D magnetic resonance (MR) images from 3-T MRI, cerebrospinal fluid (CSF) inflow artifacts can hinder a clear depiction of the prepontine cistern lesions. To address the shortcomings of conventional MR sequences for visualizing small intradural EPs, several investigators have evaluated the efficacy of MR cisternographic 3-dimensional (3D) images, such as by using fast imaging employing steady-state acquisition (FIESTA).3–5 However, this technique is typically used for specific MR examinations of the inner ear or internal auditory canal and is not used for routine brain MRI protocols, so the incidental identification of intradural EPs is unlikely except in patients who undergo temporal region MR examinations.
Since the introduction of a 3-T MRI scanner at our hospital, we have used 3D fluid-attenuated inversion recovery (3D FLAIR) as a part of our routine brain MRI protocol. Owing to its excellent spatial resolution and the elimination of CSF inflow artifacts, this technique helps with detecting small intradural lesions in the posterior cranial fossa.6,7 Using this technique, we have noticed novel findings related to intradural EP; however, to the best of our knowledge, there has been no previous report on using 3D FLAIR to detect intradural EP. The aim of this study, therefore, was to use 3D FLAIR to investigate the prevalence and imaging features of intradural EP.
MATERIALS AND METHODS
This study retrospectively assessed the data of 3922 consecutive patients who underwent brain 3-T MR examinations at our hospital between March 2017 and August 2018. Of these, 13 patients with definite diagnoses of a posterior fossa brain tumor (n = 11), empyema (n = 1), and subarachnoid hemorrhage (n = 1) and 21 patients whose scans included a metal artifact (n = 15) or motion artifact (n = 6) were excluded because the prepontine cistern was not sufficiently visible with 3D FLAIR. Thus, the study cohort comprised 3888 patients.
This retrospective study was approved by our institutional review board, which waived the requirement for informed consent. Patient anonymity was ensured prior to the assessment of the data.
All MR examinations were performed with a 3-T MR scanner (Ingenia; Philips Healthcare, Best, the Netherlands) using a 32-channel phased-array head coil or a dStream HeadNeckSpine coil (Philips). Precontrast MR sequences included 2D spin-echo echo-planar diffusion-weighted imaging (DWI) in the axial plane, 2D turbo spin-echo (TSE) T2-weighted images in the axial plane, 3D fast field echo (FFE) T1-weighted images in the sagittal plane, and 3D FLAIR with fat suppression in the sagittal plane. Additional MR sequences included contrast-enhanced 3D T1-weighted images in the sagittal plane, including a TSE and/or FFE sequence, and 3D-driven equilibrium (DRIVE) as MR cisternography.
The imaging sequence parameters of 3D FLAIR were as follows: field of view (FOV), 250 mm; matrix, 256 × 184 (480 × 480 after reconstruction; in-plane resolution, 0.52 × 0.52 mm); section thickness, 1.14 mm, with 0.57-mm overlap; no parallel imaging; repetition time (TR)/echo time (TE), 6000 milliseconds/shortest (approximately 400 milliseconds); inversion time, 2000 milliseconds; variable flip angle (for brain view FLAIR); TSE factor, 203; T2-prep, 125 milliseconds (4 pulses); fat suppression with spectral presaturation inversion recovery; number of signals acquired, 2; and scan time, 4 minutes 54 seconds. Other MR sequence parameters were as follows: DWI: TR/TE, 4400/87 milliseconds; FOV, 220 mm; slice thickness, 3 mm; b value, 1000 s/mm2; and scan time, 52 seconds; 2D T2-weighted images: TR/TE, 8142/90 milliseconds; TSE factor, 17; FOV, 220 mm; slice thickness/gap = 3/0.5 mm; and scan time, 2 minutes 34 seconds; 3D TSE T1-weighted images: TR/TE, 300/17 milliseconds; TSE factor, 9; FOV, 250 mm; slice thickness, 0.85 mm; and scan time, 4 minutes 38 seconds; 3D FFE T1-weighted images: TR/TE, 8.3/4.6 milliseconds; TFE factor, 260; flip angle, 10°; FOV, 260 mm; slice thickness, 0.9 mm; and scan time, 4 minutes 42 seconds; and 3D DRIVE: TR/TE, 1500/300 milliseconds; FOV, 160 mm; slice thickness, 0.6 mm; number of signals acquired, 1; and scan time = 3 minutes 52 seconds. For the contrast-enhanced 3D T1-weighted images, the intravenous contrast agent used was gadoterate meglumine (Dotarem; Guerbet, Roissy, France), gadoteridol (ProHance; Bracco Diagnostics, Milan, Italy), or gadobutrol (Gadovist; Bayer Schering-Pharma, Berlin, Germany), administered at 0.1 mmol/kg body weight.
MRI Data Analysis
All the 3D FLAIR images with a 1-mm section thickness, including multiplanar reconstruction images, were evaluated by consensus by 2 neuroradiologists (R.K. and M.M.) with 5 and 27 years of neuroimaging experience, respectively.
Each intradural EP was classified as either “classical EP” or “possible EP.” A cyst-like lesion with or without an osseous stalk at the level of Dorello's canal was classified as a classical EP, whereas a hyperintense lesion with or without an osseous stalk at the level of Dorello's canal was classified as a possible EP. We analyzed the prevalence, size, and presence or absence of an intraosseous stalk on 3D FLAIR images for the classical and possible EPs. Some of the patients with EP also underwent 3D DRIVE imaging; in these cases, we compared the 2 types of images. Similarly, some of the patients underwent contrast-enhanced MRI; in these cases, the EP was assessed for the presence or absence of contrast enhancement. When follow-up MRI was performed before or during the period of this study, we assessed the EP for longitudinal changes in size.
Patient Symptoms Data Analysis
The EP rarely causes direct symptoms,8 although there have been some reports of dizziness or vertigo observed in patients radiologically diagnosed with intradural EP.4,5 We therefore investigated the presence or absence of dizziness or vertigo in the cases with intradural EP.
The statistical analyses were performed using SPSS version 25 (IBM Corp, Armonk, NY). The age of patients and the size of lesions were compared between the classical and possible EPs with paired t tests. P < 0.05 was considered statistically significant.
Of the 3888 cases examined, intradural EP was detected on 3D FLAIR in 50 (1.3%), including 36 classical EPs (0.9%) and 14 possible EPs (0.4%). There was no significant difference in mean age between the classical and possible EP groups (60.2 ± 17.3 and 62.6 ± 19.1 years, respectively; P = 0.67). The mean size for all the EPs was 7.0 ± 4.1 mm. The classical EPs were significantly larger than the possible EPs (8.2 ± 4.2 mm [range, 4.0–21.3 mm] vs 3.8 ± 1.4 mm [range, 2.0–6.8 mm]; P < 0.01).
Figures 1 and 2 show example images of classical EPs in 2 patients. On 3D FLAIR, the 36 classical EPs were shown as cyst-like lesions on the dorsal surface of the clivus at the level of Dorello's canal. Five of the EPs (14%) showed an osseous stalk. On DWI, 28 of the 36 classical EPs (77.8%) showed high signal intensity. Sixteen of the patients with classical EPs also underwent 3D DRIVE imaging; this detected the EP in all cases, and 11 of the 16 EPs (68.8%) showed an osseous stalk. Contrast-enhanced 3D T1-weighted images were obtained for 8 of the patients with classical EP. None of the lesions showed contrast enhancement. Follow-up MRI studies were performed for 22 patients over periods of 1 to 73 months (mean, 34.9 months); no change of size was observed in any of these lesions. Three of the 36 patients reported dizziness or vertigo.
Figure 3 shows example images of a possible EP. On 3D FLAIR, 14 possible EPs were detected as high-signal lesions on the dorsal surface of the clivus at the level of Dorello's canal, with 4 of these (29%) showing an osseous stalk. On T1-weighted images, all the possible EPs showed a low signal intensity, and 13 of the 14 possible EPs showed a high signal intensity on T2-weighted images. However, the possible EPs could not be detected with certainty with T1- and T2-weighted images alone. On DWI, 12 of the 14 possible EPs (85.7%) showed a high signal intensity. Three of the patients with possible EPs underwent 3D DRIVE imaging, which showed an osseous stalk in all cases but detected none of the possible EPs. Contrast-enhanced 3D T1-weighted images were acquired for 2 of the patients; the lesions did not show any contrast enhancement. Follow-up MRI studies were performed for 9 patients over a period of 1 to 60 months (mean, 21.2 months). None of the possible EPs changed in size. None of the patients with possible EPs reported dizziness or vertigo.
The prevalence of classical EP on MR images has been reported by various studies as 0.8% to 2.8%.2–5 Several investigators have used FIESTA sequences to evaluate findings for EPs.3–5 Fast imaging employing steady-state acquisition is a balanced steady-state free precession technique that provides a strong signal in tissues, such as CSF, with a high T2/T1 ratio, as well as high spatial resolution with the reconstruction of multiplanar images.9 As a result, it can clearly depict tiny intradural lesions that cannot be detected by conventional MR techniques. In the present study, the prevalence of classical EP on 3D FLAIR sequences was 0.9%, consistent with the range previously reported, even though the number of cases examined was 4 times larger than that of the previous studies.
At our hospital, 3D FLAIR is routinely used for 3-T MRI of the brain. It has several advantages for clinical neuroimaging, including thinner slices and multiplanar reformation capability, a black blood effect, high sensitivity to subtle T1 changes in fluids, high sensitivity to the T2 contrast of lesions, an absence of CSF inflow artifacts, and a reasonable scan time.10,11 A recent study using 3D FLAIR reported a novel entity, an intradural, small, high-signal lesion posterior to the vertebral artery at the foramen magnum.6,7,12 This shows that 3D FLAIR is suitable for the evaluation of small intradural lesions in the prepontine cistern.
The 3D DRIVE is a high-resolution 3D MR cisternography technique that provides excellent spatial and contrast resolution between the CSF and intradural normal structures or tiny lesions,13 as with FIESTA. In the present study, 3D DRIVE imaging had been acquired for 16 of the 36 cases of classical EP; in all cases, it detected the EP. Using multiplanar reformation of 3D DRIVE images, classical EP was sufficiently delineated as a cyst-like component on the dorsal surface of the clivus, as was reported using FIESTA.3–5 The findings of classical EP on 3D FLAIR were consistent with those on 3D DRIVE, indicating that the signal of the gelatinous content of an EP and the surrounding CSF is suppressed, whereas the signal of the outer margin is relatively high, as can be seen in Figures 1 and 2.
Our findings demonstrated that 3D FLAIR revealed the presence of possible EPs. This has never previously been reported with FIESTA, and 3D DRIVE in the present study was also unable to detect these EPs. The possible EPs were located at the midline of the surface of clivus and the level of Dorello's canal, and some showed an osseous stalk. We therefore infer that this finding may represent an atypical type of intradural EP. Using FIESTA, Chihara et al3 reported an osseous stalk without an intradural cyst-like lesion, considering this to be a possible EP that was a subtype of EP. This suggests that the possible EPs in the present study might represent parts of the possible EPs described by Chihara et al.3 The possible EPs in our study were detected by 3D FLAIR but not by 3D DRIVE, so FIESTA might also be unable to detect this lesion. It remains uncertain why this lesion was clearly delineated by 3D FLAIR but not by 3D DRIVE. We speculate that moderate amounts of physaliphorous cells and mucinous material coexist in the lesion, resulting in the lesion having relatively high intensity on both 3D DRIVE and 3D FLAIR. However, the surrounding CSF appears as a high signal on 3D DRIVE and a low signal on 3D FLAIR, which could explain why the possible EPs were detected only by 3D FLAIR. In addition, the mean size of the possible EPs was significantly smaller (less than half the size) compared with that of the classical EPs, which may also have contributed to the difference in detection.
Chihara et al3 reported that osseous stalks were delineated on FIESTA in 82.4% of the classical EPs. In the present study, osseous stalks were observed in 69% of the classical EPs on 3D DRIVE imaging but only in 14% on 3D FLAIR imaging. This suggests that 3D FLAIR is not suitable for the delineation of osseous stalks. This method suppresses the signals of both fat and CSF, resulting in poor contrast between the clivus with an osseous stalk and the adjacent CSF. The same is true for possible EPs, with only 29% showing an osseous stalk on 3D FLAIR.
The classical and possible EPs showed high signals on DWI, in 77.8% and 85.7%, respectively. Few previous reports have described DWI findings for intradural EPs, and there has been only a single case report of a large intradural EP, in which DWI showed hyperintensity of the lesion, mimicking an epidermoid cyst.14 Although DWI is unable to delineate the precise morphology of EP lesions because of the poor image quality, a tiny high signal in the prepontine cistern may suggest a diagnosis of intradural EP.
Golden and Small4 reported that 6 of 7 cases of classical EP presented with dizziness or vertigo, and Özgür et al5 reported that 24 of 31 cases of EP presented with these symptoms. However, these studies used FIESTA sequences specifically to evaluate patients suspected of having temporal or posterior fossa lesions, which may have biased the patients' symptoms in favor of dizziness or vertigo in those studies. In the present study, only 3 of the 50 cases with classical and possible EPs reported these symptoms. At our hospital, 3D FLAIR is used as part of the routine brain MR protocol, and so this study was able to evaluate data for 3888 consecutive patients in a clinical setting. Our findings suggest that these symptoms are infrequent and incidental among cases of intradural EP.
There were several limitations to this study. A major limitation was the absence of pathologic confirmation, which was not feasible because the patients presented with no significant symptom related to the EP. This has been reported in several previous studies that evaluated the MRI characteristics of EP in a large number of cases.2–5 Second, the MR findings do not exclude other entities than possible EPs as differential diagnoses, including intradural chordoma, epidermoid cyst, and neurenteric cyst. In the possible EP cases, the size of the lesion was very small, ranging from 2.0 to 6.8 mm, which is not consistent with intradural chordoma, which usually extends the prepontine cistern and compresses the brainstem.15,16 A small epidermoid cyst limited to the midline prepontine cistern without extending into the cerebellopontine angle would be atypical. Neurenteric cysts frequently show T1 hyperintensity of the lesion.17 Thus, the MR findings for possible EPs are inconsistent with those for the differential diagnoses. Finally, this study was limited by its retrospective nature. Many of the patients did not undergo 3D DRIVE and contrast-enhanced 3D T1-weighted imaging. Further investigation including a comparison of 3D DRIVE and contrast-enhanced 3D T1-weighted images is needed to clarify the 3D FLAIR findings of classical and possible EPs.
Our results showed that 3D FLAIR allowed the detection of classical EP as well as can be achieved with MR cisternography. In addition, the 3D FLAIR imaging suggested a previously unreported type of possible EP variant.
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