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Singapore Med J 1999; Vol 40(01):

Non-Contrast High Resolution Fast Spin Echo Magnetic Resonance Imaging of Acoustic Schwannoma
T Y Tan

ABSTRACT

Aim of Study: The current gold-standard of examination for the exclusion of acoustic schwannomas is contrast-enhanced magnetic resonance (MR) imaging. Many patients however, still cannot afford to pay for the cost of this examination. As a result, many clinicians still resort to contrast-enhanced computed tomography (CT) scan; an examination which could miss small intracanalicular acoustic schwannomas. The aim of this study was to report on our experience on the usage of the more affordable high-resolution fast spin-echo (FSE) MR imaging for the diagnosis of acoustic schwannoma.

Methodology: A study involving 123 patients with symptoms of sensorineural hearing loss, vertigo and tinnitus was carried out between August 1996 and March 1997. All cases were scanned with a 1.5 T MR unit using a quadrate head coil. The section thickness was 1.5 mm, with TR/TE of 4000/96. Any mass arising from the vestibulocochlear nerve was considered to be an acoustic schwannoma, and most of these positive cases underwent further contrast-enhanced MR imaging to confirm the diagnosis.

Results: A total of 7 acoustic schwannomas were detected on high resolution FSE MR imaging and for 5 of these cases, the diagnosis was confirmed with contrast-enhanced MR imaging. Three of the 7 positive patients had contrast-enhanced CT scan done just prior to the MR study and the tumours were not detected on CT scan. The smallest acoustic schwannoma detected on high-resolution FSE MR imaging had a dimension of 0.5 x 0.4 x 0.5 cm.

Conclusion: High resolution FSE MR imaging is more sensitive than contrast-enhanced CT scan in the diagnosis of acoustic schwannoma. Although the sensitivity is less than that of contrast-enhanced MR imaging, high resolution FSE MR is more affordable and therefore can play a role in the screening for acoustic schwannomas in selected groups of patients.

Keywords: intra-canalicular tumour, sensorineural hearing loss, cost, internal auditory canal, vestibulocochlear nerve

INTRODUCTION

Contrast-enhanced spin-echo magnetic resonance (MR) imaging has high sensitivity and specificity for the diagnosis of acoustic schwannoma and is now the investigative modality of choice for this condition(1). However, despite the ready availability of MR imaging in Singapore, a fair number of patients in most restructured hospitals are still subjected to contrast-enhanced computed tomography (CT) of the internal auditory canals (IAC). This method is known to miss small intra-canalicular acoustic schwannomas(2), although combination with air cisternography could increase the sensitivity(3). The procedure of injecting air into the cerebrospinal fluid (CSF) space has its complications(3) and should not be done presently with the availability of MR scanner. The main reason why contrast-enhanced CT scan of the IAC is still being carried out today is that most of these patients cannot afford the cost of contrast-enhanced MR imaging; even at a subsidised rate. The purpose of this paper is to report on our experience on the usage of high-resolution fast spin-echo (FSE) MR for the diagnosis of acoustic schwannoma. This technique does not require the use of intravenous gadolinium and the total scanning time is about half that of contrast-enhanced MR. All these translates to cheaper cost of examination, thereby making it more affordable. In our series, high-resolution FSE MR could detect acoustic schwannomas with a diameter of 4 mm or more. Three of these patients with acoustic schwannoma had contrast-enhanced CT scans of the IAC done prior to the MR imaging and the lesions were missed on CT. Therefore, high-resolution FSE MR imaging should replace contrast-enhanced CT scan in those patients who cannot afford the cost of contrast-enhanced MR. It can also be used as a screening test for those whose presenting symptoms suggest that their chances of having acoustic schwannomas are low.

MATERIALS AND METHODS

Between August 1996 and March 1997, 123 patients underwent high-resolution FSE MR of the IAC in Tan Tock Seng Hospital. Their presenting symptoms included sensorineural hearing loss, vertigo and tinnitus. The purpose of the examinations was for the exclusion of acoustic schwannomas. All cases were scanned in a 1.5-T MR unit (Signa, GE Medical Systems). A quadrature head coil was used. The imaging parameters were: FSE sequence without fat

RESULTS

The most common mass lesion of the cerebellopontine angle found to be responsible for unilateral sensorineural hearing loss is acoustic schwannoma(1). The current gold-standard of examination for the assessment of this condition is contrast-enhanced conventional spin-echo MR imaging(4), which provides high sensitivity and specificity for the diagnosis of acoustic schwannoma(1). One major drawback of this examination is the high cost. In Tan Tock Seng Hospital (TTSH), it costs SGD 950 for a full-paying patient and half that amount for a subsidised patient. Even at a cost of SGD 475, many patients still cannot afford the treatment. It is for this reason that contrast-enhanced CT scans of the IAC are still being requested frequently in many of the restructured hospitals. It is customary to have at least one such request per day at TTSH. It costs only SGD 88 for a subsidised patient, and SGD 430 for a full-paying patient for this examination.

Contrast-enhanced CT scan, however, is not as sensitive as contrast-enhanced MR in the detection of acoustic schwannoma(2). This is especially so for small (less than 1 cm in diameter) intra-canalicular tumours. The 3 cases of acoustic schwannomas in this series that were missed initially on contrast-enhanced CT scans could bear witness to this fact. The consequence of missing small acoustic schwannomas can be grave especially for younger patients as they can present years later with large cerebellopontine angle tumours. Besides this, missing small tumours would mean that the opportunity to remove them with functional preservation (eg, facial expression and hearing) may be lost forever(5). Contrast-enhanced CT scan of the IAC is therefore not an ideal alternative examination for patients who cannot afford to pay for contrast-enhance MR. Before the era of MR, CT air cisternography was used to improve the sensitivity of diagnosing acoustic schwannoma(3). This examination, however, was not fool-proof as false positive results were not that infrequent and it also had its fair share of complications eg. persistent headache that required blood patch(3). The availability of MR imaging has confined CT air cisternography to the history book.

The yield of contrast-enhanced MR for acoustic schwannomas varies with different series. This was partly dependent on the selection criterias of patients subjected to the examination. In Armington’s series(6), where only patients with unilateral or asymmetrical sensorineural hearing loss were subjected to the examination, 17 acoustic schwannomas were detected out of a total of 176 cases (9.7%). A recent review was done on 314 cases of contrast-enhanced MR performed in TTSH(7). A total of 13 acoustic schwannomas (4%) were detected. We were less stringent on our selection criterias and this could explain the lower yield. Besides those with unilateral or asymmetrical progressive sensorineural hearing loss, we scanned a number of patients who were at a lower risk of having acoustic schwannomas on clinical grounds, eg. those with sudden onset of sensorineural hearing loss, those with bilateral symmetrical sensorineural hearing loss, and those with vertigo and/or tinnitus without any hearing loss. It was noted however that all the 13 acoustic schwannoma patients had sensorineural hearing loss(7). The yield therefore of scanning a patient presenting with vertigo and/or tinnitus without sensorineural hearing loss with contrast-enhanced MR was exceptionally low. Hence it would not be cost-effective to subject such patients to a contrast-enhanced MR. This is not to say that these patients should not be investigated as there were uncommon instances where acoustic schwannomas presented with vestibular symptoms and tinnitus without any hearing loss(8). Therefore, there is a need for a more cost-effective but fairly sensitive screening test for this group of patients and obviously, contrast-enhanced CT scan is not the answer.

Some authors have recently published papers on the use of non-enhanced FSE MR sequence for the detection of acoustic schwannomas and other cerebellopontine angle mass lesion(1,9). In the FSE sequence, the CSF in the IAC and in the cisterns around the pons appears hyperintense with the facial and vestibulocochlear nerves appearing as linear structures transversing through the cerebellopontine angle and the IAC(1). In the usual T2-weighted images with TR of 2000 and TE of between 20 - 90, acoustic schwannomas appear as hyperintense mass lesions which would make them less conspicuous against the high signal CSF. However, if the TR is increased to greater than 3000 and the TE greater than 90, the schwannomas would appear as hypointense masses whereas the CSF would remain hyperintense(9). Using these parameters would make acoustic schwannomas more conspicuous.

We started to use high resolution FSE MR technique since August 1996 for patients who otherwise would have undergone contrast-enhanced CT scan of the IAC because they could not afford the cost of contrast-enhanced MR. Up to March 1997, a total of 123 patients were examined. A total of 7 acoustic schwannomas were detected by this technique. Five of these cases had contrast-enhanced MR performed to confirm their diagnosis whereas for the last 2 cases, we were confident enough to make the diagnosis based on the high resolution FSE images alone. One case of congenital abnormality of the membranous labyrinth was also detected. The smallest tumour had a dimension of 5 x 5 x 4 mm. Using dual 3-inch phased-array receiver multicoils, Allen(1) was able to detect tumours as small as 1 x 1 x 2 mm. We used a standard head receiver coil which was not able to give as good a resolution as that given by a dual 3-inch phased-array receiver multicoils but with our setup, we were able to pick up tumours of at least 3 mm and above.

CSF flow artefacts can be a problem when using this technique. Firstly, they can obscure the medial ends of the nerves as in 8 of our patients, and secondly they can mimic as a mass lesion. In the latter case, a post-contrast T1-weighted spin echo scan can be performed and a true mass lesion excluded. By preventing the visualisation of the medial ends of the nerves, however, may result in the failure to detect small tumours adjacent to the cerebellopontine angle. Nevertheless, the chances of an acoustic schwannoma originating from the medial end of the vestibulocochlear nerve is small as most of the time, the tumour originates near the Scarpa’s ganglion(9) which is usually located within the IAC near the fundus where there are hardly any CSF flow artefacts.

One limitation in our series was that we did not know the true false-negative rate as contrast-enhanced MR scans were not carried out for most of the patients undergoing the examination. Allen(1) had a false-negative rate of 4% whereas Fukui(9) missed 3 out of 50 tumours (6%) when they used similar technique in their series. All the tumours missed were 4 mm or less in diameter. Therefore, it would be reasonable to assume that if we were to scan all patients suspected of having acoustic schwannomas using the high-resolution FSE technique, we could be missing some of the smaller acoustic schwannomas, perhaps of about 6% to 8%(5) of them. Despite this limitation, high resolution FSE MR of the IAC, was still more sensitive in detecting acoustic schwannomas than contrast-enhanced CT scan of the IAC, something which we had also demonstrated in this series.

1) Those who cannot afford the cost of contrast-enhanced MR of the IAC and who would otherwise have opted for contrast-enhanced CT scan instead.

2) Those whose presenting symptoms suggest that they are at low risk of having acoustic schwannomas, eg. those with vertigo and/or tinnitus without sensorineural hearing loss.

3) Elderly patients and those medically infirmed patients with short life expectency where no real harm would be done to the patients even if tiny tumours were missed.

4) Those with a long history of sensorineural hearing loss; so that if it was due to an acoustic schwannoma, the tumour would be of the size to be easily detected by high-resolution FSE MR.

5) Those with congenital sensorineural hearing loss where the abnormality is suspected to be in the membranous labyrinth or the endolymphatic system.

Conclusion

T Y Tan, MBBS, FRCR (UK),
FAMS
Consultant