- Email: [email protected]
High-resolution flat-panel computed tomography imaging of cochlear implants
Operative Techniques in Otolaryngology (2014) 25, 321–326
High-resolution flat-panel computed tomography imaging of cochlear implants Monica S. Pearl, MD,a,b Alexis Roy, MSc,c Charles J. Limb, MDc,d From the aDivision of Interventional Neuroradiology, Johns Hopkins University School of Medicine, Baltimore, Maryland; bInterventional Neuroradiology, Children's National Medical Center, Washington, District of Columbia; cDepartment of Otolaryngology-Head and Neck Surgery, Johns Hopkins University School of Medicine, Baltimore, Maryland; and the dPeabody Conservatory of Music, Baltimore, Maryland KEYWORDS Flat-panel CT; cochlear implants
Metallic streak artifacts and the high density of the temporal bones can limit the radiological assessment of cochlear implants. Flat-panel computed tomography (CT) is a relatively new imaging modality that provides CT-like images; however, they are acquired with a C-arm–based x-ray system using flat-panel image detectors. Secondary reconstructions of the initial data set using a manually generated field of view, Hounsfield Units kernel type, and sharp image characteristic create even higher resolution images. Radiation dose is less than that of multislice CT imaging of the temporal bones and can be further reduced by collimating the volume of interest during image acquisition to only include the temporal bones. Flat-panel CT is emerging as a promising imaging tool for the evaluation of cochlear implant placement. r 2014 Elsevier Inc. All rights reserved.
Introduction Cochlear implantation is a proven intervention that has been shown to provide functional restoration of hearing in individuals with severe to profound hearing loss. Clinical outcomes after surgery, however, may vary considerably in the degree and quality of hearing recovery. Detailed postoperative imaging plays an important role for characterizing the cochlear implant (CI) electrode location,1,2 insertion depth,3 scalar localization,4 and relationship to the facial nerve canal.2,5 These data carry important Support for this work was provided in part by research grants from MedEl Corporation (Principal Investigators: Charles C. Limb and Alexis Roy) and Siemens Corporate Research (Principal Investigator: Monica S. Pearl) Flat-panel CT of cochlear implants. Address reprint requests and correspondence: Monica S. Pearl, MD, DABR, Division of Interventional Neuroradiology, The Johns Hopkins Hospital, 1800 E Orleans St, Bloomberg Building, 7218, Baltimore, MD 21287. E-mail address: [email protected] http://dx.doi.org/10.1016/j.otot.2014.09.003 1043-1810/r 2014 Elsevier Inc. All rights reserved.
prognostic information and have implications for surgical techniques and programming strategies for optimal auditory nerve stimulation. Although computed tomography (CT) is considered the current gold standard method for characterizing CI position,6 metallic artifacts significantly limit the postoperative imaging evaluation of the electrode array. Initial imaging of postoperative CI placement was performed with plain radiographs obtained in the Stenvers projection.7,8 Multislice CT (MSCT) replaced this method and has since become the gold standard imaging technique for assessing the electrode array location relative to intracochlear scalae, the electrode-modiolar interval, and the proximity of the electrode to the Fallopian canal.6 In addition, CT provides 3-dimensional positional information and excellent contrast for different tissue types9; however, dense-bone structures10 and metallic artifacts severely degrade image quality and obscure the electrode contacts and surrounding structures.8,11,12 Flat-panel CT (FPCT) is a relatively new imaging modality that provides CT-like images acquired with a
322
Operative Techniques in Otolaryngology, Vol 25, No 4, December 2014 using the following parameters: 109 kV, small focus, 2001 rotation angle, and 0.41 per frame angulation step.
Reconstruction parameters
Figure 1 FPCT of a right-sided cochlear implant (CI). Posteroanterior view of a collimated fluoroscopic acquisition for a 20-second FPCT in a patient with a right-sided CI. The skull above and below is excluded from the initial acquisition to reduce radiation exposure.
C-arm–based x-ray system using flat-panel image detectors. This technique provides superb visualization of highcontrast structures with superior spatial resolution when compared with MSCT.13 Secondary reconstructions of the initial FPCT data set using a smaller field of view produce even higher resolution images than can be obtained by standard FPCT reconstructions.14
Flat-panel CT imaging protocol FPCT (DynaCT, Siemens, Erlangen, Germany) evaluation can be performed using a flat-panel angiography system (Axiom Artis Zee, Siemens Healthcare AG, Germany) and commercially available software (Syngo DynaCT, Siemens Healthcare AG, Germany). The patient is placed supine on the angiography table, and the head is taped in place to limit patient motion. When preparing the DynaCT acquisition, attention is paid to collimate the volume of interest (VOI) to include only the temporal bones (craniocaudal collimation from just above the petrous ridges to just below the mastoid tip) (Figure 1). A 20-second FPCT of the head is performed
Postprocessing is performed on a commercially available workstation (Leonardo DynaCT, InSpace 3D software; Siemens Healthcare AG, Germany). To create higher resolution images, the operator must manually generate the VOI to include only the electrode array (Figure 2A). This decreases the voxel size to 0.07 mm-0.08 mm and creates higher resolution secondary reconstructions (Figure 2B and C). The secondary reconstruction parameters available for postprocessing include kernel type (Hounsfield units [HU] and edge enhancement) and image characteristics (very smooth, normal, auto, and sharp). The optimal combination of reconstruction parameters that produces the clearest depiction of the electrode array and surrounding labyrinthine structures is HU kernel type and sharp image characteristics (Figure 3).14 Coronal oblique images in the plane of the electrode array can be generated by aligning the multiplanar reconstruction axes on the axial and sagittal planes parallel to the basal turn and perpendicular to the modiolus (Figure 4). A slice thickness of 3 mm will enable visualization of all electrode contacts on 1 slice. The window width and contrast level should be adjusted until both the individual electrode contacts and surrounding labyrinthine structures can be visualized.
Visualization of insertion point and individual electrode contacts FPCT is able to depict the insertion point of the CI, defined as the interface between the air and osseous structures along
Figure 2 Secondary reconstruction of an initial FPCT data set and comparison of initial and secondary reconstructed images. (A) A secondary reconstruction is created by manually generating a smaller volume of interest to include only the electrode array (arrow). (B) The coronal oblique image created after the default reconstruction is blurry when compare with the higher resolution coronal oblique image generated after the secondary reconstruction (C).
Pearl et al.
Flat-panel CT of Cochlear Implants
323
Figure 3 Coronal oblique secondary reconstructions of the same CI patient. These images illustrate the variable image quality obtained with identical 3-mm slice thickness, window width (3681), contrast (1246), and kernel type (HU). Note the blurring and poor visualization of the electrode contacts with the very smooth (A) setting. A sharp (D) parameter produces optimal images with well-defined osseous structures and individual electrode contacts. (A) Very smooth, (B) normal, (C) auto, and (D) sharp. (Reproduced with permission from Pearl et al.14)
Figure 4 Multiplanar reconstruction images of a right-sided CI. Multiplanar reconstruction axes are aligned parallel to the basal turn on the axial (A) and sagittal (B) planes to generate a coronal oblique (C) image of the CI. This image is rotated slightly to visualize the vestibule and superior semicircular canal. The arrow in “C” denotes the insertion point. (Color version of figure is available online.)
Figure 5 Coronal oblique secondary reconstructions of CIs from Med-El and Cochlear America. FPCT is able to depict the individual electrode contacts in the Med-El medium 24-mm array (electrode contacts 1.9 mm apart) (A) and Med-El standard 31.5-mm array (electrode contacts spaced 2.4 mm apart) (B). More closely spaced electrode contacts, as in the Cochlear Nucleus implant by Cochlear America (C), which has 22 electrodes spaced 0.2-0.3 mm apart over 15 mm, are also discernible by FPCT.
the outer margin of the electrode. This radiographic technique also has the ability to depict the individual electrode contacts from various CI manufacturers including the Med-El standard 31.5-mm arrays and Med-El medium 24-mm arrays, with electrode contacts spaced 2.4 mm and 1.9 mm apart, respectively, as well as the more closely
spaced electrode contacts (between 0.2 mm and 0.3 mm) in the Cochlear Nucleus implant by Cochlear America (Figure 5). FPCT is also useful for visualizing the course of the electrode and can be particularly useful for identifying malposition due to kinking of the electrode array causing
324
Operative Techniques in Otolaryngology, Vol 25, No 4, December 2014 cannot always distinguish between the osseous spiral lamina and the CI to designate exact scalar location (Figure 7) throughout the entire course of the array. CT methods for scalar designation use oblique coronal images reconstructed perpendicular to the basal turn of the cochlea and parallel to the modiolus. The scalar position, however, is inferred in relation to the walls of the cochlea15 because the image resolution is insufficient to visualize the osseous spiral lamina and basilar membrane.16 Some reports document the ability to detect scalar position of the CI (ie, scala tympani vs scala vestibuli); however, this assessment was primarily demonstrated in ex vivo specimens.17
Measurement of angular insertion depth Figure 6 Malpositioned cochlear implants. FPCT depicts the course of the cochlear implant and is useful to determine kinking of the electrode array, which causes 2 electrode contacts to reside outside the cochlea (arrows).
electrode contacts to reside outside the cochlea or coiling of the array in the basal turn (Figure 6).
Identification of scalar localization Although the osseous spiral lamina is identifiable on FPCT in the absence of a CI, when an implant is present, FPCT
The 2-dimensional multiplanar reconstruction slice thickness can be reduced to 0.1 mm to determine the electrode array insertion point, seen in the axial images at the air to bone interface along the outer margin of the electrode and then confirmed in the sagittal and coronal planes. The insertion point is designated in the coronal images and used as the first reference point for measuring angular insertion depth. On the coronal oblique image (3-mm slice thickness), the 3 most apical electrodes are identified and used to create a circle depicting their course. A line drawn from the insertion point to the center of this circle serves as the reference (01) line. The angle θ between the most apical
Figure 7 FPCT for scalar localization. Multiple FPCT images in a patient with a left sided CI (A and C). The contralateral side is shown and flipped in orientation to demonstrate visualization of the osseous spiral lamina (arrow in B). The scalar position can be inferred, but when the implant is in place, the osseous spiral lamina cannot be visualized along the entirety of the CI.
Pearl et al.
Flat-panel CT of Cochlear Implants
325 multidetector CT showed a substantial dose reduction for the ocular lens in FCPT over MSCT (6.9 times lower dose to the lens in FCPT).18 These studies suggest that, in addition to improved spatial resolution, FPCT also offers lower radiation exposure to patients.
Conclusion FPCT imaging in patients with CIs is easy to perform, produces high-resolution images, and offers a radiation dose savings when compared with MSCT. This technique allows for visualization of individual electrode contacts as closely spaced as 0.2-0.3 mm, thus enabling measurement of the angular insertion depth of the most apical electrode contacts. The optimal imaging methods use a high-resolution secondary reconstruction algorithm with a manually generated VOI that includes only the electrode array, HU kernel type, and sharp image characteristics. As CI imaging continues to improve, so will our understanding of the relationship between cochlear anatomy, CI electrode placement, and auditory performance. Figure 8 Method for calculating the angular insertion depth of the most apical electrode. The insertion point, as determined by the axial and sagittal images (not shown), is designated in the coronal oblique image. A circle whose outer circumference represents the trajectory of the 3 most apical electrodes is drawn. A reference line is drawn between the insertion point and the center of this circle. The most apical electrode is then identified, and the angle θ is calculated between the electrode and the reference line. In this example, angular insertion depth is calculated by 360 þ θ. (Reproduced with permission from Pearl et al.14)
electrode and the reference line is calculated (Figure 8), and this angle is subtracted from or added to 360 or 720, depending on the relationship between the apical electrode and the number of turns across the reference line. For example, if the CI makes less than 1 complete turn, the calculated angular insertion depth is 360 θ, whereas if it makes more than 1 turn but less than 1.5 turns, then the angular insertion depth is 360 þ θ.
Radiation dose Previous authors have reported an important advantage of FPCT in terms of radiation dose reduction compared with standard temporal bone MSCT protocols.3 Struffert et al13 reported that the radiation dose of FPCT was half of the amount delivered using MSCT standard protocols. Ruivo et al15 calculated an effective dose of 80 μSv for a FPCT evaluation after CI, and compared the measured effective dose for MSCT and FPCT, showing much higher doses for 16-slice computed tomography (3600 μSv) and for 4-slice tomography (4800 μSv). A more recent study comparing the radiation dose between FPCT and a clinical 64-slice
References 1. Teymouri J, Hullar TE, Holden TA, et al: Verification of computed tomographic estimates of cochlear implant array position: A micro-CT and histologic analysis. Otol Neurotol 32:980-986, 2011 2. Bartling SH, Gupta R, Torkos A, et al: Flat-panel volume computed tomography for cochlear implant electrode array examination in isolated temporal bone specimens. Otol Neurotol 27:491-498, 2006 3. Trieger A, Schulze A, Schneider M, et al: In vivo measurements of the insertion depth of cochlear implant arrays using flat-panel volume computed tomography. Otol Neurotol 32:152-157, 2011 4. Lane JI, Driscoll CL, Witte RJ, et al: Scalar localization of the electrode array after cochlear implantation: A cadaveric validation study comparing 64-slice multidetector computed tomography with microcomputed tomography. Otol Neurotol 28:191-194, 2007 5. Vanpoucke F, Zarowski A, Casselman J, et al: The facial nerve canal: An important cochlear conduction path revealed by Clarion electrical field imaging. Otol Neurotol 25:282-289, 2004 6. Zeitler DM, Wang KH, Prasad RS, et al: Flat-panel computed tomography versus multislice computed tomography to evaluate cochlear implant positioning. Cochlear Implants Int 12:216-222, 2011 7. Cohen LT, Xu J, Xu SA, et al: Improved and simplified methods for specifying positions of the electrode bands of a cochlear implant array. Am J Otol 17:859-865, 1996 8. Xu J, Xu SA, Cohen LT, et al: Cochlear view: Postoperative radiography for cochlear implantation. Am J Otol 21:49-56, 2000 9. Skinner MW, Ketten DR, Vannier MW, et al: Determination of the position of nucleus cochlear implant electrodes in the inner ear. Am J Otol 15:644-651, 1994 10. Whiting BR, Holden TA, Brunsden BS, et al: Use of computed tomography scans for cochlear implants. J Digit Imaging 21:323-328, 2008 11. Whiting BR, Bae KT, Skinner MW: Cochlear implants: Threedimensional localization by means of coregistration of CT and conventional radiographs. Radiology 221:543-549, 2001 12. van Wermeskerken GK, van Olphen AF, Graamans K: Imaging of electrode position in relation to electrode functioning after cochlear implantation. Eur Arch Otorhinolaryngol 266:1527-1531, 2009
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Operative Techniques in Otolaryngology, Vol 25, No 4, December 2014
13. Struffert T, Hertel V, Kyriakou Y, et al: Imaging of cochlear implant electrode array with flat-detector CT and conventional multislice CT: Comparison of image quality and radiation dose. Acta Otolaryngol 130:443-452, 2010 14. Pearl MS, Roy A, Limb CJ: High-resolution secondary reconstructions with the use of flat panel CT in the clinical assessment of patients with cochlear implants. Am J Neuroradiol 35:1202-1208, 2014 15. Ruivo J, Mermuys K, Bacher K, et al: Cone beam computed tomography, a low-dose imaging technique in the postoperative assessment of cochlear implantation. Otol Neurotol 30:299-303, 2009
16. Verbist BM, Frijns JH, Geleijns J, et al: Multisection CT as a valuable tool in the postoperative assessment of cochlear implant patients. Am J Neuroradiol 26:424-429, 2005 17. Arweiler-Harbeck D, Monninghoff C, Greve J, et al: Imaging of electrode position after cochlear implantation with flat panel CT. ISRN Otolaryngol 2012. [Article ID: 728205], 2012 18. Kennedy T, Szczykutowicz T, Royalty K, et al: C-arm CT imaging of the temporal bone: Image quality and 3D radiation dose distribution [abstract]. 52nd Annual Meeting of the American Society of Neuroradiology, May 17-22, 2014, Montreal, Quebec, Canada, [Abstract no: O-110].
High-resolution flat-panel computed tomography imaging of cochlear implants Monica S. Pearl, MD,a,b Alexis Roy, MSc,c Charles J. Limb, MDc,d From the aDivision of Interventional Neuroradiology, Johns Hopkins University School of Medicine, Baltimore, Maryland; bInterventional Neuroradiology, Children's National Medical Center, Washington, District of Columbia; cDepartment of Otolaryngology-Head and Neck Surgery, Johns Hopkins University School of Medicine, Baltimore, Maryland; and the dPeabody Conservatory of Music, Baltimore, Maryland KEYWORDS Flat-panel CT; cochlear implants
Metallic streak artifacts and the high density of the temporal bones can limit the radiological assessment of cochlear implants. Flat-panel computed tomography (CT) is a relatively new imaging modality that provides CT-like images; however, they are acquired with a C-arm–based x-ray system using flat-panel image detectors. Secondary reconstructions of the initial data set using a manually generated field of view, Hounsfield Units kernel type, and sharp image characteristic create even higher resolution images. Radiation dose is less than that of multislice CT imaging of the temporal bones and can be further reduced by collimating the volume of interest during image acquisition to only include the temporal bones. Flat-panel CT is emerging as a promising imaging tool for the evaluation of cochlear implant placement. r 2014 Elsevier Inc. All rights reserved.
Introduction Cochlear implantation is a proven intervention that has been shown to provide functional restoration of hearing in individuals with severe to profound hearing loss. Clinical outcomes after surgery, however, may vary considerably in the degree and quality of hearing recovery. Detailed postoperative imaging plays an important role for characterizing the cochlear implant (CI) electrode location,1,2 insertion depth,3 scalar localization,4 and relationship to the facial nerve canal.2,5 These data carry important Support for this work was provided in part by research grants from MedEl Corporation (Principal Investigators: Charles C. Limb and Alexis Roy) and Siemens Corporate Research (Principal Investigator: Monica S. Pearl) Flat-panel CT of cochlear implants. Address reprint requests and correspondence: Monica S. Pearl, MD, DABR, Division of Interventional Neuroradiology, The Johns Hopkins Hospital, 1800 E Orleans St, Bloomberg Building, 7218, Baltimore, MD 21287. E-mail address: [email protected] http://dx.doi.org/10.1016/j.otot.2014.09.003 1043-1810/r 2014 Elsevier Inc. All rights reserved.
prognostic information and have implications for surgical techniques and programming strategies for optimal auditory nerve stimulation. Although computed tomography (CT) is considered the current gold standard method for characterizing CI position,6 metallic artifacts significantly limit the postoperative imaging evaluation of the electrode array. Initial imaging of postoperative CI placement was performed with plain radiographs obtained in the Stenvers projection.7,8 Multislice CT (MSCT) replaced this method and has since become the gold standard imaging technique for assessing the electrode array location relative to intracochlear scalae, the electrode-modiolar interval, and the proximity of the electrode to the Fallopian canal.6 In addition, CT provides 3-dimensional positional information and excellent contrast for different tissue types9; however, dense-bone structures10 and metallic artifacts severely degrade image quality and obscure the electrode contacts and surrounding structures.8,11,12 Flat-panel CT (FPCT) is a relatively new imaging modality that provides CT-like images acquired with a
322
Operative Techniques in Otolaryngology, Vol 25, No 4, December 2014 using the following parameters: 109 kV, small focus, 2001 rotation angle, and 0.41 per frame angulation step.
Reconstruction parameters
Figure 1 FPCT of a right-sided cochlear implant (CI). Posteroanterior view of a collimated fluoroscopic acquisition for a 20-second FPCT in a patient with a right-sided CI. The skull above and below is excluded from the initial acquisition to reduce radiation exposure.
C-arm–based x-ray system using flat-panel image detectors. This technique provides superb visualization of highcontrast structures with superior spatial resolution when compared with MSCT.13 Secondary reconstructions of the initial FPCT data set using a smaller field of view produce even higher resolution images than can be obtained by standard FPCT reconstructions.14
Flat-panel CT imaging protocol FPCT (DynaCT, Siemens, Erlangen, Germany) evaluation can be performed using a flat-panel angiography system (Axiom Artis Zee, Siemens Healthcare AG, Germany) and commercially available software (Syngo DynaCT, Siemens Healthcare AG, Germany). The patient is placed supine on the angiography table, and the head is taped in place to limit patient motion. When preparing the DynaCT acquisition, attention is paid to collimate the volume of interest (VOI) to include only the temporal bones (craniocaudal collimation from just above the petrous ridges to just below the mastoid tip) (Figure 1). A 20-second FPCT of the head is performed
Postprocessing is performed on a commercially available workstation (Leonardo DynaCT, InSpace 3D software; Siemens Healthcare AG, Germany). To create higher resolution images, the operator must manually generate the VOI to include only the electrode array (Figure 2A). This decreases the voxel size to 0.07 mm-0.08 mm and creates higher resolution secondary reconstructions (Figure 2B and C). The secondary reconstruction parameters available for postprocessing include kernel type (Hounsfield units [HU] and edge enhancement) and image characteristics (very smooth, normal, auto, and sharp). The optimal combination of reconstruction parameters that produces the clearest depiction of the electrode array and surrounding labyrinthine structures is HU kernel type and sharp image characteristics (Figure 3).14 Coronal oblique images in the plane of the electrode array can be generated by aligning the multiplanar reconstruction axes on the axial and sagittal planes parallel to the basal turn and perpendicular to the modiolus (Figure 4). A slice thickness of 3 mm will enable visualization of all electrode contacts on 1 slice. The window width and contrast level should be adjusted until both the individual electrode contacts and surrounding labyrinthine structures can be visualized.
Visualization of insertion point and individual electrode contacts FPCT is able to depict the insertion point of the CI, defined as the interface between the air and osseous structures along
Figure 2 Secondary reconstruction of an initial FPCT data set and comparison of initial and secondary reconstructed images. (A) A secondary reconstruction is created by manually generating a smaller volume of interest to include only the electrode array (arrow). (B) The coronal oblique image created after the default reconstruction is blurry when compare with the higher resolution coronal oblique image generated after the secondary reconstruction (C).
Pearl et al.
Flat-panel CT of Cochlear Implants
323
Figure 3 Coronal oblique secondary reconstructions of the same CI patient. These images illustrate the variable image quality obtained with identical 3-mm slice thickness, window width (3681), contrast (1246), and kernel type (HU). Note the blurring and poor visualization of the electrode contacts with the very smooth (A) setting. A sharp (D) parameter produces optimal images with well-defined osseous structures and individual electrode contacts. (A) Very smooth, (B) normal, (C) auto, and (D) sharp. (Reproduced with permission from Pearl et al.14)
Figure 4 Multiplanar reconstruction images of a right-sided CI. Multiplanar reconstruction axes are aligned parallel to the basal turn on the axial (A) and sagittal (B) planes to generate a coronal oblique (C) image of the CI. This image is rotated slightly to visualize the vestibule and superior semicircular canal. The arrow in “C” denotes the insertion point. (Color version of figure is available online.)
Figure 5 Coronal oblique secondary reconstructions of CIs from Med-El and Cochlear America. FPCT is able to depict the individual electrode contacts in the Med-El medium 24-mm array (electrode contacts 1.9 mm apart) (A) and Med-El standard 31.5-mm array (electrode contacts spaced 2.4 mm apart) (B). More closely spaced electrode contacts, as in the Cochlear Nucleus implant by Cochlear America (C), which has 22 electrodes spaced 0.2-0.3 mm apart over 15 mm, are also discernible by FPCT.
the outer margin of the electrode. This radiographic technique also has the ability to depict the individual electrode contacts from various CI manufacturers including the Med-El standard 31.5-mm arrays and Med-El medium 24-mm arrays, with electrode contacts spaced 2.4 mm and 1.9 mm apart, respectively, as well as the more closely
spaced electrode contacts (between 0.2 mm and 0.3 mm) in the Cochlear Nucleus implant by Cochlear America (Figure 5). FPCT is also useful for visualizing the course of the electrode and can be particularly useful for identifying malposition due to kinking of the electrode array causing
324
Operative Techniques in Otolaryngology, Vol 25, No 4, December 2014 cannot always distinguish between the osseous spiral lamina and the CI to designate exact scalar location (Figure 7) throughout the entire course of the array. CT methods for scalar designation use oblique coronal images reconstructed perpendicular to the basal turn of the cochlea and parallel to the modiolus. The scalar position, however, is inferred in relation to the walls of the cochlea15 because the image resolution is insufficient to visualize the osseous spiral lamina and basilar membrane.16 Some reports document the ability to detect scalar position of the CI (ie, scala tympani vs scala vestibuli); however, this assessment was primarily demonstrated in ex vivo specimens.17
Measurement of angular insertion depth Figure 6 Malpositioned cochlear implants. FPCT depicts the course of the cochlear implant and is useful to determine kinking of the electrode array, which causes 2 electrode contacts to reside outside the cochlea (arrows).
electrode contacts to reside outside the cochlea or coiling of the array in the basal turn (Figure 6).
Identification of scalar localization Although the osseous spiral lamina is identifiable on FPCT in the absence of a CI, when an implant is present, FPCT
The 2-dimensional multiplanar reconstruction slice thickness can be reduced to 0.1 mm to determine the electrode array insertion point, seen in the axial images at the air to bone interface along the outer margin of the electrode and then confirmed in the sagittal and coronal planes. The insertion point is designated in the coronal images and used as the first reference point for measuring angular insertion depth. On the coronal oblique image (3-mm slice thickness), the 3 most apical electrodes are identified and used to create a circle depicting their course. A line drawn from the insertion point to the center of this circle serves as the reference (01) line. The angle θ between the most apical
Figure 7 FPCT for scalar localization. Multiple FPCT images in a patient with a left sided CI (A and C). The contralateral side is shown and flipped in orientation to demonstrate visualization of the osseous spiral lamina (arrow in B). The scalar position can be inferred, but when the implant is in place, the osseous spiral lamina cannot be visualized along the entirety of the CI.
Pearl et al.
Flat-panel CT of Cochlear Implants
325 multidetector CT showed a substantial dose reduction for the ocular lens in FCPT over MSCT (6.9 times lower dose to the lens in FCPT).18 These studies suggest that, in addition to improved spatial resolution, FPCT also offers lower radiation exposure to patients.
Conclusion FPCT imaging in patients with CIs is easy to perform, produces high-resolution images, and offers a radiation dose savings when compared with MSCT. This technique allows for visualization of individual electrode contacts as closely spaced as 0.2-0.3 mm, thus enabling measurement of the angular insertion depth of the most apical electrode contacts. The optimal imaging methods use a high-resolution secondary reconstruction algorithm with a manually generated VOI that includes only the electrode array, HU kernel type, and sharp image characteristics. As CI imaging continues to improve, so will our understanding of the relationship between cochlear anatomy, CI electrode placement, and auditory performance. Figure 8 Method for calculating the angular insertion depth of the most apical electrode. The insertion point, as determined by the axial and sagittal images (not shown), is designated in the coronal oblique image. A circle whose outer circumference represents the trajectory of the 3 most apical electrodes is drawn. A reference line is drawn between the insertion point and the center of this circle. The most apical electrode is then identified, and the angle θ is calculated between the electrode and the reference line. In this example, angular insertion depth is calculated by 360 þ θ. (Reproduced with permission from Pearl et al.14)
electrode and the reference line is calculated (Figure 8), and this angle is subtracted from or added to 360 or 720, depending on the relationship between the apical electrode and the number of turns across the reference line. For example, if the CI makes less than 1 complete turn, the calculated angular insertion depth is 360 θ, whereas if it makes more than 1 turn but less than 1.5 turns, then the angular insertion depth is 360 þ θ.
Radiation dose Previous authors have reported an important advantage of FPCT in terms of radiation dose reduction compared with standard temporal bone MSCT protocols.3 Struffert et al13 reported that the radiation dose of FPCT was half of the amount delivered using MSCT standard protocols. Ruivo et al15 calculated an effective dose of 80 μSv for a FPCT evaluation after CI, and compared the measured effective dose for MSCT and FPCT, showing much higher doses for 16-slice computed tomography (3600 μSv) and for 4-slice tomography (4800 μSv). A more recent study comparing the radiation dose between FPCT and a clinical 64-slice
References 1. Teymouri J, Hullar TE, Holden TA, et al: Verification of computed tomographic estimates of cochlear implant array position: A micro-CT and histologic analysis. Otol Neurotol 32:980-986, 2011 2. Bartling SH, Gupta R, Torkos A, et al: Flat-panel volume computed tomography for cochlear implant electrode array examination in isolated temporal bone specimens. Otol Neurotol 27:491-498, 2006 3. Trieger A, Schulze A, Schneider M, et al: In vivo measurements of the insertion depth of cochlear implant arrays using flat-panel volume computed tomography. Otol Neurotol 32:152-157, 2011 4. Lane JI, Driscoll CL, Witte RJ, et al: Scalar localization of the electrode array after cochlear implantation: A cadaveric validation study comparing 64-slice multidetector computed tomography with microcomputed tomography. Otol Neurotol 28:191-194, 2007 5. Vanpoucke F, Zarowski A, Casselman J, et al: The facial nerve canal: An important cochlear conduction path revealed by Clarion electrical field imaging. Otol Neurotol 25:282-289, 2004 6. Zeitler DM, Wang KH, Prasad RS, et al: Flat-panel computed tomography versus multislice computed tomography to evaluate cochlear implant positioning. Cochlear Implants Int 12:216-222, 2011 7. Cohen LT, Xu J, Xu SA, et al: Improved and simplified methods for specifying positions of the electrode bands of a cochlear implant array. Am J Otol 17:859-865, 1996 8. Xu J, Xu SA, Cohen LT, et al: Cochlear view: Postoperative radiography for cochlear implantation. Am J Otol 21:49-56, 2000 9. Skinner MW, Ketten DR, Vannier MW, et al: Determination of the position of nucleus cochlear implant electrodes in the inner ear. Am J Otol 15:644-651, 1994 10. Whiting BR, Holden TA, Brunsden BS, et al: Use of computed tomography scans for cochlear implants. J Digit Imaging 21:323-328, 2008 11. Whiting BR, Bae KT, Skinner MW: Cochlear implants: Threedimensional localization by means of coregistration of CT and conventional radiographs. Radiology 221:543-549, 2001 12. van Wermeskerken GK, van Olphen AF, Graamans K: Imaging of electrode position in relation to electrode functioning after cochlear implantation. Eur Arch Otorhinolaryngol 266:1527-1531, 2009
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13. Struffert T, Hertel V, Kyriakou Y, et al: Imaging of cochlear implant electrode array with flat-detector CT and conventional multislice CT: Comparison of image quality and radiation dose. Acta Otolaryngol 130:443-452, 2010 14. Pearl MS, Roy A, Limb CJ: High-resolution secondary reconstructions with the use of flat panel CT in the clinical assessment of patients with cochlear implants. Am J Neuroradiol 35:1202-1208, 2014 15. Ruivo J, Mermuys K, Bacher K, et al: Cone beam computed tomography, a low-dose imaging technique in the postoperative assessment of cochlear implantation. Otol Neurotol 30:299-303, 2009
16. Verbist BM, Frijns JH, Geleijns J, et al: Multisection CT as a valuable tool in the postoperative assessment of cochlear implant patients. Am J Neuroradiol 26:424-429, 2005 17. Arweiler-Harbeck D, Monninghoff C, Greve J, et al: Imaging of electrode position after cochlear implantation with flat panel CT. ISRN Otolaryngol 2012. [Article ID: 728205], 2012 18. Kennedy T, Szczykutowicz T, Royalty K, et al: C-arm CT imaging of the temporal bone: Image quality and 3D radiation dose distribution [abstract]. 52nd Annual Meeting of the American Society of Neuroradiology, May 17-22, 2014, Montreal, Quebec, Canada, [Abstract no: O-110].