Postoperative imaging after cochlear implantation usually is performed by conventional cochlear view (X-ray) or by multislice computed tomography (MSCT). Conventional cochlear view is routinely used mainly in children due to short investigation time and low radiation dose. This technique only gives projective information on the fact that insertion into the cochlea has been successful, but analysis of exact electrode position with regard to the topography of the cochlea is impossible [
An additional imaging method is flat-panel computed tomography. The experimental use of flat-panel high-resolution CT in temporal bone specimens was evaluated with regard to position of electrode array and with special concern on documentation of the highest possible accuracy in cranial base navigation [
The aim of our study was to evaluate the clinical use of flat-panel CT as far as feasibility, artefacts, position within the cochlea and the temporal bone, and visualization of electrodes of different implant types are concerned.
Between December 2009 and September 2010 31 subsequently implanted adult patients (female = 11, male = 20; mean age 52 years) without inner ear malformations were included in this study one day after surgery. All received preoperative diagnostic imaging by CT scan and/or magnetic resonance imaging. 3 different cochlear implant devices were applied: Cochlear Nucleus CI 512 (
Flat-panel CT examinations were performed on a Philips Allura C-arc angiographic unit (Philips Medical Systems, Best, The Netherlands) connected to a 3DRA workstation (Philips Medical Systems).
With patients temporal bone in system isocenter, the scan was performed with a propeller movement covering 207° of the circular trajectory. 622 frames were exposed during the 20.7 s scan (30 frames/s), utilizing a detector format of 33 × 40 cm. Total examination time including bedding of the patient on the examination table demanded less than 2 minutes.
Source images were transferred to the workstation during and after the rotational acquisition, and a volume data set was created. The reconstruction appeared on the workstation monitor.
Multiplanar MIP reconstructions parallel to the cochlea were performed with a slice thickness of 1.5 mm and orthogonal to the cochlea with a slice thickness of 0.41 mm.
Images were analysed by two independent investigators (otorhinolaryngology 1, neuroradiology 1). Artefacts were characterised between very low (= 1) and very high (= 6). Identification of scala tympani, scala vestibule, and osseous spiral lamina as well as tip fold over, full insertion, identification of distance between electrode array and modiolus (next to (n) and far from (f)), and possibility of identification of single electrodes within the electrode array was of further interest.
An overview on all subjects is illustrated in Table
Patients and evaluation parameters of imaging.
Code | Age | Implant type | Artefact (1–6) | Full insertion | Tip fold over | Separation of electr. | Facial nerve | Differentiation of scalae | |
---|---|---|---|---|---|---|---|---|---|
Inv I/Inv II | |||||||||
PW49 | 60 | CN | 4 | 3 | + | − | − | ? | − |
KE36 | 73 | CN | 2 | 2 | + | − | − | + | − |
MG57 | 52 | CN | 2 | 3 | + | − | − | ? | − |
LE46 | 63 | CN | 3 | 3 | + | − | − | ? | − |
PB42 | 67 | CN | 2 | 2 | + | − | − | + | − |
WF52 | 57 | ME | 3 | 2 | + | − | + | + | − |
DM77 | 32 | ME | 4 | 3 | + | − | + | ? | − |
UA74 | 35 | CN | 2 | 3 | + | − | − | ? | − |
PU47 | 62 | CN | 3 | 3 | + | − | − | ? | − |
OG57 | 52 | AB | 3 | 3 | + | − | + | + | + |
DU62 | 47 | ME | 2 | 2 | + | − | + | − | − |
BN77 | 33 | CN | 2 | 2 | + | − | − | + | − |
OK53 | 57 | CN | 3 | 4 | + | − | − | + | − |
KD67 | 43 | CN | 2 | 2 | + | − | − | ? | − |
FM50 | 60 | AB | 3 | 3 | + | − | + | − | − |
WM65 | 45 | CN | 2 | 2 | + | − | − | + | − |
DR62 | 48 | CN | 2 | 2 | + | − | − | + | − |
LS71 | 39 | CN | 3 | 4 | + | − | − | + | − |
SP67 | 43 | CN | 2 | 3 | + | − | − | + | − |
BR63 | 47 | AB | 2 | 3 | + | − | − | + | − |
GH50 | 60 | CN | 3 | 3 | + | − | + | − | − |
ÖT67 | 43 | CN | 1 | 2 | + | − | − | + | + |
SM39 | 71 | CN | 2 | 3 | + | − | − | ? | − |
HA51 | 59 | CN | 3 | 4 | + | − | − | + | − |
CE39 | 71 | CN | 2 | 3 | + | − | − | + | − |
WJ76 | 34 | CN | 3 | 3 | + | − | − | ? | − |
GJ62 | 48 | CN | 4 | 4 | + | − | − | + | − |
ZT56 | 54 | CN | 1 | 2 | + | − | − | − | + |
TK75 | 35 | CN | 2 | 2 | + | − | + | + | − |
WJ40 | 70 | AB | 2 | 2 | + | − | + | ? | − |
PD88 | 22 | CN | 1 | 2 | + | − | − | ? | + |
Implant type: CN: Cochlear Nucleus CI512; ME: Medel Sonata Ti; AB: Advanced Bionics Hi Res 90.
Artefacts: 1: no artefacts and 6: evaluation impossible due to multiple artefacts.
Inv. I, II: investigator 1, investigator 2.
Full insertion +: all electrodes within cochlea; −: one or more electrodes outside cochlea.
Tip fold over +: tip of electrode is folded in the apical part; −: tip of electrode array is straight.
Separation of electrodes: +: single electrodes are clearly visible; −: no differentiation of single electrodes due to artefacts.
Position of facial nerve in projection to cochlea: +: clearly visible; −: not possible.
Differentiation of scalae tympani and vestibuli: +: clearly visible; /not possible.
Visualization of osseous lamina spiralis: +: clearly visible; −: not possible.
Radiological examinations after cochlea implantation are mostly performed by conventional X-ray (Figure
(a) Conventional X-ray cochlear view, (b) conventional multislice computed tomography.
(a)–(c) Differentiation of single electrodes in diverse implant types.
SonataTi, MedEl
Hi Res 90 Adv. Bionics
CI 512, Cochlear, Nucleus
(a)–(d) Identification of electrode array position with regard to modiolus and cochlear nerve fibers.
Visualization of scala tympani, scala vestibule, and osseous lamina spiralis.
Several attempts have been made to improve imaging. The technique of cone beam computed tomography as a low-dose imaging technique for postoperative assessment of cochlear implantation, which was tested in postoperative patients, seems to be promising with fewer artefacts and higher resolution than multislice CT [
Several publications deal with image quality of isolated temporal bone specimens [
Rotational computed tomography (RT) is based on three-dimensional digital subtraction angiography. Images are taken with a rotating C-arm in a single rotation [
Of course, tip fold over should be clearly visible. In our collective, no patient had one.
Electrode positioning within the scala tympani is clinically eligible. Otherwise, in case of positioning within the scala vestibuli, avoidable side effects like high impedances, reduced speech reception, and vertigo might occur due to damage of the intracochlear structures [
Exact positioning within cochlea was not assessable in our collective. As far as the literature is concerned, the assessment of positioning within the scala tympani most often could only be demonstrated in
Aschendorff et al. [
Some examinations have been performed to adjust radiation dose of FD CT in comparison to multislice CT. Radiation dose of FD CT is described to be lower than in MSCT.
The main weak point of our study is the lack of direct comparison of MSCT and flat-panel CT. Due to the high level of artefacts known from own experience, MSCT is not common in our house under this indication.
Our results indicate that flat-panel CT is a fast and accurate examination in the postoperative imaging of cochlear implants. It is of course superior to conventional X-ray, but it is also superior to MSCT mainly due to fewer artefacts. Additionally, radiation dose is lower than in MSCT.