Indian Journal of Radiology Indian Journal of Radiology  

   Login   | Users online: 1403

Home Bookmark this page Print this page Email this page Small font sizeDefault font size Increase font size     

 

HEAD AND NECK IMAGING Table of Contents   
Year : 2006  |  Volume : 16  |  Issue : 3  |  Page : 385-392
Imaging of cochlear implants


Dept. of Radiodiagnosis, Command Hospital, Bangalore, India

Click here for correspondence address and email
 

   Abstract 

Aim : To evaluate the role of various imaging modalities in pre and post operative evaluation of cochlear implant candidates. Materials and methods : 30 patients were evaluated by HRCT and 15 of these subsequently underwent MRI temporal bones prior to cochlear implant device insertion. All implanted patients were subjected to post operative radiography to assess position of implant. Results : Both modalities provided critical information on abnormalities of the otic capsule, pneumatisation of the mastoid, middle ear abnormalities, cochlear ducts patency and vascular abnormalities- thus helping to assess the suitability of the ear for implantation, determine the side to be implanted and to find any associated abnormality which could adversely influence the surgery or post op period. HRCT missed two cases of luminal ossification and MRI was more reliable in detecting early luminal obstruction. Conclusion : Based on our findings, we offer a realistic and practical protocol for imaging in cochlear implants in the Indian setting. HRCT is recommended in all patients for pre implant analysis of the temporal bone morphology due to its reliability and easy availability. MRI is recommended in all cases of post meningitic deafness and in others with doubtful CT findings.

Keywords: Cochlear implants, HRCT temporal bone, MRI cochlea

How to cite this article:
Chaturvedi A, Mohan C, Mahajan S B, Kakkar V. Imaging of cochlear implants. Indian J Radiol Imaging 2006;16:385-92

How to cite this URL:
Chaturvedi A, Mohan C, Mahajan S B, Kakkar V. Imaging of cochlear implants. Indian J Radiol Imaging [serial online] 2006 [cited 2020 Jul 3];16:385-92. Available from: http://www.ijri.org/text.asp?2006/16/3/385/29025

   Introduction Top


There is renewed interest in cochlear implants with an indigenous implant being developed in India by the Defence Research and Development Organisation (DRDO). Multi channel cochlear implants aim to provide complex sound analysis by stimulating auditory cortex over a wide range of frequencies. To achieve this goal, the implant must be placed well within the cochlear lumen. Therefore a detailed preoperative and post operative radiological assessment of the temporal bone has become vital for implantation of these multichannel cochlear implant devices. Imaging is required to determine the suitability of the ear to receive an implant, the choice of the ear to be operated upon and also detects additional findings that may have a bearing on the surgery or subsequent patient management [1].

In this report we review our experience in the pre and post operative imaging assessment of patients for cochlear transplants. Discrepancies in the radiological and surgical findings are discussed, CT and MR findings are compared to assess the relative utility of the modalities and a practical, viable imaging protocol is suggested for use in a cochlear implant program.


   Patients and methods Top


Patients

Thirty patients (20 male and 10 female) aged 02 - 66 years were evaluated radiologically for a multichannel cochlear implant insertion after a thorough otological assessment. The majority of patients (57%) were of pediatric age group [Table - 1].

An attempt was made to ascertain the etiology of deafness by means of a meticulous history, otological examination, audiometry, tympanometry and radiological findings. In almost a third of the patients, no specific cause could be determined. The remaining patients presented with diverse etiologies as detailed in [Table - 2].

14 patients received an implant after imaging. Three patients were rejected for cochlear implant on the basis of imaging. Remaining 13 patients are awaiting implants and six of these are presently on a hearing aid trial.


   Radiological techniques Top


HRCT Temporal Bones:

All 30 patients underwent HRCT of temporal bone which was performed on a spiral CT Scanner (GE Sigma, Wisconsin, Milwaukee, USA). Contiguous 1mm axial sections were obtained parallel to the orbitomeatal line. When patient tolerance allowed, direct coronal contiguous 1 mm sections were also obtained. The images were interpreted with a comprehensive checklist [Table - 3] so as to ensure consistency of reporting and not to miss any finding.

MRI Temporal bones:

15 patients subsequently underwent MRI in addition to HRCT. Patients with unequivocally normal CT findings did not undergo MRI. MRI was only done in cases of post meningitic deafness, cases where cochlear patency was in doubt and in some cases for academic interest. All MRI examinations were performed on a 1-Tesla system (Seimens Magnetom Harmony) using a standard head coil. T2 weighted contiguous slices of 2 mm thickness through the temporal bone in axial and coronal planes were the most informative. The images were focused especially for the cochlea and the internal auditory meatus.

Post Operative Imaging

i. Plain X-rays: Intra and post operative plain radiography was done in all patients to determine optimum positioning of the electrode array. An oblique anti Stenver's view was found most useful to show the electrode in the cochlea.

HRCT: In four cases with suspected malpositioned electrodes, axial HRCT was done to determine the exact position of the electrode and if possible, the cause for the failed insertion.


   Results Top


HRCT - Preoperative Findings

Data from all the preoperative HRCT examinations was reviewed and collated as described in [Table - 4].

1. Three patients were rejected on the basis of following imaging findings:

a) Congenital cochlear malformations in two patients. One case had bilateral cochlear hypoplasia [Figure - 1]a and b.The second patient had a common cavity on the right and cochlear hypoplasia on the left. These anomalies are not an absolute contraindication for an implant but since surgical experience was limited early in our series, these cases were provisionally excluded from surgery.

b) Severe bilateral cochlear ossification was seen in one patient with virtually no significant lumen available for implantation [Figure - 2].

2. In three patients, HRCT findings determined the side selected for implantation as follows:

a) A young man with multiple facial fractures and bilateral post traumatic sensorineural deafness had a fracture running through the right cochlea [Figure - 3]. The opposite cochlea was therefore chosen and successfully implanted.

b) Bilateral asymmetrical cochlear ossification in a case led to selection of the less affected side for the implant [Figure - 4]a&b.

c) Right sided jugular bulb ectasia in a case alerted the surgeon to approach the opposite side for a safe surgery [Figure - 5].

Two cases of cochlear ossification were missed on HRCT temporal bone. One of them was discovered per-operatively. Failure of detection in this patient was retrospectively attributed to relative inexperience of the radiologist during the early part of the learning curve. In the second patient, the ossification was subtle and early and therefore not picked up on CT. Since this patient was a child with post meningitic deafness with a high index of suspicion for cochlear ossification, an MRI examination done subsequently fortunately picked up bilateral asymmetrical luminal ossification.

MRI -Preoperative Findings

The salient MRI findings were as follows:

1. Normal study 08

2. Congenital malformations 02

(Also picked up on HRCT)

3. Cochlear ossification 03

(One case with a negative HRCT)

4. Mastoiditis 02

Since MRI picks up cochlear luminal ossification earlier and with greater reliability than HRCT, all patients with post meningitic deafness were investigated with a pre operative MRI. In the two patients with congenital cochlear malformations MRI was done for academic purposes with the aim to better visualize the cochlear anatomy [Figure - 4]b. MRI also picked up minor degrees of mastoiditis earlier than HRCT. This helped in timely treatment of the infection and ultimate selection of side for implant.

Post Operative Imaging

Post operative radiographs revealed the following:

a) Normally positioned implant - 10 patients [Figure - 6]

b) Malpositioned Implant - 02 patients [Figure - 7]

c) Equivocally positioned implant - 02 patients

The malpositioned and equivocal category of patients further underwent HRCT which revealed the following:

a) Normal implant tip position - 02 patients

b) Implant tip in hypotympanum - 01 patient [Figure - 8]

c) Implant tip in epitympanum -01 patient [Figure 9]


   Discussion Top


Aims of Preoperative Imaging

Both HRCT and MRI have their proponents for preoperative evaluation . Whichever the imaging modality, a step wise approach is employed to answers the following questions:

1. Are there cochleo - vestibular anomalies which preclude implantation?

2. Is there evidence of luminal obstruction?

3. Are there additional findings that can complicate surgery or post op recovery? [2]

Cochleo-vestibular anomalies

Approximately 20% of patient with congenital hearing loss have some degree of inner ear morphological anomaly which may or may not be detectable radiologically. Jackler et al proposed a classification scheme for the common inner ear deformities based on the different embryological stages of development as outlined in [Table - 5] [3]. This scheme is by no means all inclusive and a variety of disorders may defy classification. However, it serves as a useful basis for systematic interpretation of the anomalies. It is important to describe the specific cochlear anatomy, its relationship to the vestibule, patency of the labyrinth and nature of the aqueducts.

Theoretically, all these anomalies do not preclude implantation. The only absolute contraindication for an implant would be complete cochlear aplasia or absence of the cochlear nerve. The other anomalies possibly are only relative contraindications due to the high perilymph pressures in these systems observed by some workers which may cause sealing problems post operatively. There is also a possibility of random coiling of the implant within the deformed inner ear. Detection of inner ear anomalies is also useful in that the better ear can then be selected for surgery [4].

Luminal Obstruction

It is but logical that for the implant to be implanted successfully the cochlear lumen must be patent. The lumen can be obstructed by in growth of fibrous tissue or new bone formation due to a variety of causes as shown in [Table - 6] [5].

Typically cochlear ossification presents as replacement of the normal membranous labyrinth by high-density bone. In our study two cases of labyrinthine ossification were missed on HRCT. One was subsequently picked up on MRI. This was probably because early fibro-ossific changes do not manifest on CT and can be missed [6]. It must also be emphasized that radiologists in the early part of the learning curve may overlook these subtle early changes. It is our observation, also supported by reports in literature that increased radiologist experience significantly improves the detection rate of luminal obstruction [7].

MRI, specifically T2W sequences, is invaluable in demonstrating early labyrinthine ossification. The hyperintense fluid signal within the patent cochlear lumen is replaced by hypointense fibro-osseous tissue and can be picked up reliably. Since a large percentage of patients with post meningitic deafness have varying degrees of labyrinthine obstruction, most workers advocate a very high index of suspicion in post meningitic patients and strongly advocate a supplementary MRI even if the CT appears normal [8]. On the other hand, there are other studies which suggest that CT, using axial and semi-longitudinal planes, is equivalent to MRI in predicting cochlear patency [9]. Cochlear implant is contraindicated only if there is total or near total bilateral cochlear ossification. Fortunately, in most cases the ossification is partial and imaging can guide the surgeon to the less affected side [10].

Additional Complicating Findings

Some of the anatomical variants that can cause surgical problems are: [2]

1. Vascular anatomy:

a) Sigmoid Sinus - Anteriorly placed

b) Jugular Bulb - High riding / Dehiscent

c) Carotid canal - Dehiscent / aberrant course

2. Facial Nerve:

a) Aberrant course

b) Dehiscence of intratympanic portion

3. Mastoid and Tympanic cavity

a) Degree of mastoid pneumatisation or mastoiditis.

b) Chronic ear disease

Detection of these anatomical variants alerts the surgeon and helps him to select the side of surgery. Any chronic middle ear infection needs to be detected before implantation so that it can be tackled with appropriate drugs and by surgery if necessary. Subsequent implantation can be then performed in a stable, well protected and obliterated mastoid cavity.

Recent Advances

There is a growing opinion in otological literature that mere visual inspection on HRCT is insufficient to diagnose subtle inner ear malformations such as lateral semi circular canal hypoplasia and cochlear hypoplasia [11]. Some workers have attempted to establish normative cochlear and vestibular measurements in normal as well as abnormal inner ears on HRCT [12]. They opine that these measurements can increase the pick up rate of subtle inner ear abnormalities. However, we found that the acquisition and comparison of these measurements was time consuming and did not influence decision making for cochlear implantation. Therefore we did not include them in our protocol.

The MRI incompatibility of cochlear implant devices is a major disadvantage due to generation of excessive electromagnetic and magnetic interference [2]. Recent in vitro trials have now shown that the electromagnetic interference between the implant and a 1.5T scanner is within acceptable limits. But the torque generated on the internal magnet by the magnetic field still remains a major hazard in the in-vivo situation and MRI should be performed only if there is a strong medical indication with appropriate safety measures [13]. A magnet less(Clarion multi- strategy) implant has recently undergone clinical trials in 11 patients in Germany. It has been found to be safe and MRI compatible but is yet to be available commercially [14]. As on today no commercially available implant device has been cleared by the US FDA as MRI compatible.


   Conclusions Top


Review of literature and our experience has led us to the following conclusions:

1. HRCT temporal bone continues to be the mainstay for detection of inner ear abnormalities due to its reliability and easy availability. CT is superior to MRI for detection of a high riding jugular bulb. Another important advantage of CT over MRI is in tracing the course of the facial nerve. In cases of complex congenital malformations, the facial nerve may have an aberrant course and thus liable to injury during implantation [13].

2. MRI has gained popularity in recent times primarily due to its superiority over HRCT in detecting labyrinthine ossification. Early fibrotic obstruction may be missed on CT even by experienced radiologists and will require MRI for early detection. Another potential advantage of MRI is in identification of the cochlear nerve. An absent cochlear nerve is an absolute contraindication for an implant. MRI is superior to CT in detecting the large vestibular aqueduct syndrome [15].

3. A realistic and practical imaging protocol should comprise of a preoperative HRCT temporal bone in all patients. MRI is required in post meningitic patients who have a higher incidence of luminal obstruction. MRI is optional for other patients in whom the CT findings are equivocal. For post operative monitoring, plain radiography is sufficient and HRCT is required only in problematic cases.

4. Imaging of the complex ear is time consuming and involves subtle findings that can influence major and often very expensive decisions. Therefore, we emphasize strongly the requirement of trained and dedicated radiologists who are an inherent part of the cochlear implant program.

 
   References Top

1.Gleeson TG, Lacy PD, Bresnihan M, Gaffney R, Brennan P, Viani L. High resolution computed tomography and magnetic resonance imaging in the pre-operative assessment of cochlear implant patients. J Laryngol Otol. 2003 Sep; 117(9):692-5.   Back to cited text no. 1    
2.Fishman AJ, Holliday RA. Principles of Cochlear Implant Imaging. Cochlear Implants. Thieme Medical Publishers Inc: 2000.pgs 79-103  Back to cited text no. 2    
3.Jackler RK, Luxford WM, House WF. Congenital Malformations of the inner ear: a classification based on embryogenesis. Laryngoscope 1987; 97:2-14.  Back to cited text no. 3    
4.Harnsberger HR, Dart DJ, Parkin JL, Smoker WR, Osborne AG. Cochlear Implant candidates: assessment with CT and MR imaging. Radiology 1987 164: 53-57.  Back to cited text no. 4    
5.Schwartz JD, Mandell DM, Faerber EN et al. Labyrinthine ossification: etiologies and CT findings. Radiology 1985; 157:395-98.  Back to cited text no. 5    
6.Abdullah A, Mahmud MR, Maimunah A, Zulfiqar MA, Saim L, Mazlan R. Preoperative high resolution CT and MR imaging in cochlear implantation. Ann Acad Med Singapore. 2003 Jul; 32(4):442-5.  Back to cited text no. 6    
7.Jackler RK, Luxford WM, Schindler Ra, McKerrow WS. Cochlear patency problems in cochlear implantation. Laryngoscope1987;97:801-805  Back to cited text no. 7  [PUBMED]  
8.Parry DA, Booth T, Roland PS. Advantages of magnetic resonance imaging over computed tomography in preoperative evaluation of pediatric cochlear implant candidates. Otol Neurotol. 2005 Sep; 26(5):976-82.  Back to cited text no. 8    
9.Bettman R, Beek E, Van Olphen A, Zonneveld F, Huizing E. MRI versus CT in assessment of cochlear patency in cochlear implant candidates. Acta Otolaryngol. 2004 Jun; 124(5):577-81.  Back to cited text no. 9    
10.Frau GN, Luxford WM, William WM, Berliner K. High Resolution Computed Tomography in Evaluation of Cochlear Patency in Implant Candidates. J of Laryngology and Otology1994; 108:743-748.  Back to cited text no. 10    
11.Lemmerling M, Vanzeileghem B, Dhooge I, et al. CT and MRI of the semicircular canals in normal and diseased temporal bone. Eur Radiol 2001; 11:1210-9.  Back to cited text no. 11    
12.Purcell D, Johnson J, Fischbein N, Lalwani A. Establishment of normative cochlear and vestibular measurements to aid in the diagnosis of inner ear malformations. Otol Head Neck Surg 2003; 128:78-87.  Back to cited text no. 12    
13.Magnetic resonance imaging and cochlear implants: compatibility and safety aspects. Teissl C, Kremser C, Hochmair ES, Hochmair-Desoyer IJ. J Magn Reson Imaging. 1999 Jan; 9(1):26-38.  Back to cited text no. 13    
14.Clinical results of the CLARION magnet less cochlear implant. Weber BP, Neuburger J, Goldring JE, Santogrossi T, Koestler H, Battmer RD, Lenarz T. Ann Otol Rhinol Laryngol Suppl. 1999 Apr; 177:22-6.  Back to cited text no. 14    
15.Ellul S, Shelton c, Davidson, HC, Harnsberger HR. Preoperative Cochlear implant imaging: Is Magnetic Resonance Imaging Enough? American Journal of Otology 2000; 21:528-533.  Back to cited text no. 15    

Top
Correspondence Address:
A Chaturvedi
Dept of Radiodiagnosis, Command Hospital, Air Force Airport Road, Bangalore
India
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-3026.29025

Rights and Permissions


    Figures

[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8]

    Tables

[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6]

This article has been cited by
1 The role of imaging modalities in cochlear implant planing [Postavenie zobrazovacích metód pred kochleárnou implantáciou]
Sláviková, K. and Bilická, Z. and Kabátová, Z. and Profant, M.
Ceska Radiologie. 2011; 65(4): 298-306
[Pubmed]
2 Cochlear implantation in India: A public health perspective
Garg, S. and Singh, R. and Chadha, S. and Agarwal, A.
Indian Journal of Medical Sciences. 2011; 65(3): 116-120
[Pubmed]
3 The cochlear-carotid interval: Preoperative assessment for cochlear Implant
Hassanein, E. and Geneidi, E. and Taha, M.
Mediterranean Journal of Otology. 2008; 4(1): 19-23
[Pubmed]



 

Top
 
  Search
 
    Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
    Email Alert *
    Add to My List *
* Registration required (free)  


    Abstract
    Introduction
    Patients and methods
    Radiological tec...
    Results
    Discussion
    Conclusions
    References
    Article Figures
    Article Tables

 Article Access Statistics
    Viewed10094    
    Printed178    
    Emailed3    
    PDF Downloaded1202    
    Comments [Add]    
    Cited by others 3    

Recommend this journal