Textbook of Radiology for Cochlear Implants E-Book

• Herausgeber: Thieme Medical Publishers
• Kategorie: Fachliteratur
• Sprache: Englisch

Veröffentlichungsjahr: 2024

Beschreibung

Cochlear implantation, a hybrid intervention from medical engineering, comprises one of the most advanced, life-changing surgeries of ENT. The candidacy for cochlear implantation and the road map to this surgery is hugely dependent on radiology. The Textbook of Radiology for Cochlear Implants deals with various aspects of radiology in this field. With hundreds of photographs of radiology of temporal bone, the book takes the reader through a journey to understand the temporal bone and its pathologies radiologically, with beautifully colored scans for better understanding. Various sections of CT and MRI have been color-marked by the authors to make them more vivid. This book will aid in laying a strong foundation for surgeons interested in pursuing their career in otology, specially cochlear implants.

Highlights:

• Color-coded sections of CT and MRI
  
• All aspects of preoperative, intraoperative, and postoperative radiology covered
  
• Detailed explanation with pictographic representations of various inner ear anomalies
  
• Step-by-step methodical reading of CT and MRI of temporal bone important for cochlear implant surgery
  

This book includes complimentary access to a digital copy on https://medone.thieme.com.

Publisher's Note: Products purchased from Third Party sellers are not guaranteed by the publisher for quality, authenticity, or access to any online entitlements included with the product.

Leseprobe

Textbook of Radiologyfor Cochlear Implants

Editor-in-Chief

Mohnish Grover, MS (ENT), FACS, PGDHHM

Professor

Department of Otorhinolaryngology

Additional Superintendent

SMS Medical College and Hospital

Jaipur, Rajasthan, India

Associate Editors

Gaurav Gupta

Professor

Department of Otorhinolaryngology

SP Medical College

Bikaner, Rajasthan, India

C Preetam

Professor

Department of Otorhinolaryngology

All India Institute of Medical Sciences

Bhubaneswar, Orissa, India

ThiemeDelhi • Stuttgart • New York • Rio de Janeiro

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Contents

Foreword

Preface

1.Basics of CT and MRI

2.Radiology of External and Middle Ear Pertaining to Cochlear Implant

3.Radiology of Normal Inner Ear and Internal Acoustic Canal

4.Inner Ear Malformations

5.Inner Ear and Central Pathologies Pertaining to Cochlear Implants

6.Intraoperative and Postoperative Radiology Including Complications

7.Recent Advances

8.Preoperative Checklist Prior to Cochlear Implantation

Index

Foreword

In this age of the Internet, we are continuously and hopelessly drowning in a sea of information, forever filtering the useful from the useless, but not always knowing the difference. We increasingly rely on a Google search to answer every query—a search of over 40 zettabytes of information, with each zettabyte being a trillion gigabytes—that not surprisingly results in an equally large data dump, requiring time and energy to sift through to find the most meaningful facts and figures. With the publication of this textbook on cochlear implant radiology by Professor Mohnish Grover and his team, we now have a comprehensive resource to answer any conceivable question regarding imaging for cochlear implantation. Using his broad medical and surgical experience in cochlear implantation, as well as a deep understanding and encyclopedic knowledge of inner ear development, Professor Grover reviews the radiology of external, middle, and inner ear, central pathologies, as well as normal and abnormal anatomy, and their relevance for cochlear implantation. In a world filled with information noise, this book is a gift to all of us—Professor Grover has done the work for us and we no longer have to do mindless searches. This book is destined to be a classic!

Anil K. Lalwani, MD

Professor and Vice Chair for Research

Co-Director, Columbia Cochlear Implant Center

Department of Otolaryngology—Head & Neck Surgery

Associate Dean for Student Research

Columbia Vagelos College of Physicians and Surgeons

New York City, New York, USA

Preface

Over the past 17 years, as I treaded through the world of cochlear implants, I realized three things: First, the unknown inner ear anatomy has several intricacies than the well-explored complex middle ear. Second, the field of cochlear implants is one of the most rapidly changing areas of medical science. Last but probably the most important, reading and analyzing radiology is essential, especially because of the constant technological advances in the field and clinical significance of even the minutest details. During this period, I realized the need of a book which dealt with radiology related to cochlear implants in a detailed yet simplistic manner. This motivated me to write one. Authoring this book has been a wonderful learning experience for me too.

This book carries the essence of everything that I have learnt about application of radiology in the field of cochlear implants. I believe it will be an informative read for surgeons, radiologists, audiologists, and speech language pathologists who want to work in the field of cochlear implants. The images in the text with color-coded labeling will make the read interesting and easy to understand. Starting from basics, the chapters progress from normal radiology of temporal bones to more advanced topics such as malformations, ossification, and other inner ear and central pathologies. At the end there is a checklist which I recommend to be used during reporting of radiology for cochlear implants.

I thanks the coeditors Dr. C. Preetam and Dr. Gaurav Gupta for their help, and others who have contributed to this book in various ways. I thank my teachers at AIIMS, New Delhi, and PGIMER, Chandigarh, who have made me what I am in this field. I also gratefully acknowledge my colleagues at SMS Medical College, Jaipur, India (especially the ENT and Radiology Department) and families of cochlear implant recipients. I thank the publishing and editorial staff of Thieme, who guided the whole project from the first draft to finished textbook. Last but not the least, I thank my parents, my wife Shruti, and my children Nehal and Nibhish for bearing with me while I was busy writing this book.

Mohnish Grover, MS (ENT), FACS, PGDHHM

Chapter 1

Basics of CT and MRI

Introduction

Computed Tomography

Magnetic Resonance Imaging

Software

1 Basics of CT and MRI

Introduction

Cochlear implant (CI) has been a medical–engineering hybrid boon to patients with hearing loss. The results of CI are majorly dependent on candidacy, and radiology forms an imperative part of this work-up. Radiology not only helps in deciding the fitness for surgery but also is important as a roadmap for surgery. This helps in analyzing the possible risks during surgery and thereby helps the surgeon to counsel the patient in a better and honest manner.

High-resolution computed tomography (HRCT) of the temporal bone and magnetic resonance imaging (MRI) of the brain and temporal bone form the pillars of radiological evaluation for cochlear implant.1 Few centers across the world have started shifting toward either one of them;2,3 however, at least for people who are starting with cochlear implantation, the authors would recommend that both these investigations should be done in the best interest of the patient.

With the advent of technology, CT and MRI have become more accessible and economic. Also, better techniques have made them more useful and safe. These advantages have added safety to various neuro-otological surgeries.

It goes without saying that there should be a close collaboration between the surgeon and the radiologist so that the technology can be put to the best use. Surgeons often complain that radiologists do not provide correct sections or sequences or the reporting is suboptimal. At the same time, majority of radiologists feel that the surgeons do not provide adequate clinical details and the things required in reporting. The authors have found in their practice that good communication should be able to resolve majority of these issues.

Computed Tomography

Planes

To define the axis of various planes used in radiology it is imperative to define a true horizontal or transverse plane. To avoid any ambiguity, Reid’s baseline was defined. Initially, it was defined as a line drawn from the inferior orbital margin to the center of the orifice of the external auditory canal.4 However, in 1962, World Federation of Radiology changed the second point to the upper margin of the external auditory meatus.4,5 This is used as zero plane in radiology. With the head upright, this plane corresponds to approximately 7 degrees nose up with respect to the horizontal plane which we usually perceive.

The most important cut for CI radiology is the axial plane (like for other temporal bone pathologies). Axial plane in the HRCT of the temporal bone is not true horizontal. It is at 30 degrees to Reid’s baseline. Therefore, the axial plane is in the plane of the lateral semicircular canal. Coronal plane is perpendicular to the axial plane and therefore at 30 degrees to true vertical. Fig. 1.1 shows these planes in pictographic form. This is a major difference from the radiology of other areas in the head and neck such as CT of paranasal sinuses where axial and coronal planes are in plane of true horizontal and vertical, respectively.

Fig. 1.1 Planes in high-resolution computed tomography temporal bone. Red line: Reid’s baseline; Blue lines: axial planes; Green lines: coronal planes.

Hounsfield Units

Hounsfield unit (HU) is also called the “CT number.” It is a relative quantitative measurement of radiodensity used in the interpretation of CT images.6 HU is named after Sir Godfrey Hounsfield, a recipient of Nobel Prize in Physiology or Medicine in 1979 for the invention of CT.7 A CT image is made up of a large number of pixels of varying gray scale. The level of gray scale is dependent on the density of the material or the linear absorption/attenuation coefficient of radiation within a tissue. The physical density of tissue is proportional to the absorption/attenuation of the X-ray beam.8 HU is calculated based on a linear transformation of the baseline linear attenuation coefficient of the X-ray beam, where water is arbitrarily defined to be zero HU and air defined as −1000 HU.6 Denser tissue has greater X-ray beam absorption and thus appears bright and has positive values, whereas less dense tissue has less X-ray beam absorption, thus appears dark and has negative values (Table 1.1).

Table 1.1 Typical Hounsfield units of various tissues

Tissue

Hounsfield units

Air

−1000

Fat

−50 to −100

Water

0

White matter

20–30

Gray matter

37–45

Bone

+1000

Windows

Windowing is the process in which the appearance of a CT image is changed to highlight particular structures. The grayscale component of an image is manipulated via the CT numbers. It is therefore also called gray-level mapping.

Various terminologies that are used in windowing are discussed in subsequent text.

Window Width

It is defined as the range of HU that an image contains. A wide window typically has a large number of HU, for example, 400 to 2,000 HU. This would be good for areas where we want to study tissues of various HU together, for example, lungs where we have air, soft tissue, and vessels. Conversely, a narrow window characteristically has a lesser range of HU and is therefore used when areas of interest are of similar attenuation, for example, soft tissues.

Window Level/Window Center

The midpoint of range of HU displayed is referred to as the window level or the window center.

There are mainly the following two types of windows in head and neck.

Soft-Tissue Window

The usual range in soft-tissue window is −125 to +225 HU with window level at +50 HU. It is therefore used for soft tissues such as solid organs.

Bone Window

It is used to study bony details and therefore is very important for studying temporal bone radiology. In the bone window, the window level is +300 HU with a range of −700 to +1300 HU.

Components of a CT Machine

Fig. 1.2 depicts the components of a CT machine.

Fig. 1.2 Various components of the CT scan machine.

Filters

Filters in CT machine remove low-energy X-rays, which contribute to image formation but increase the dose of exposure of radiation to the patient, and thus are essential in creating a monochromatic beam.

Collimator

Collimator defines the slice thickness in single-slice scanners and helps to lower radiation dose to the patient.

Detector Array

A single-slice detector has one row of detectors. Multislice detectors have 8 to 128 rows. There are commonly 1,000 to 2,000 detectors in each row.

Gantry

Gantry is a slip-ring which enables continuous rotation of the CT scanner. The rotation time of the gantry is usually between 0.25 and 3 seconds.

Multislice Computed Tomography/Multidetector Computed Tomography

The multislice CT (MSCT), or multidetector CT (MDCT) row, is a CT system with multiple rows of CT detectors to create images in several multiple sections. Advances in MDCT technology with improved software led to significant improvement in the overall quality of cross-sectional images, and two-dimensional (2D) and three-dimensional (3D) reconstructions. MDCT allows us to visualize the anatomic structures of the middle and inner ear in greater detail and accuracy thus aiding in diagnosis and planning prior to surgery. With the advent of MDCT (16 slice onward), it is now possible to obtain multiplanar reconstructions with nearly isotropic resolution.9 The advantages of MDCT include better dynamic imaging due to faster scanning times, thinner slices, simultaneous acquisition of multiple slices, and it helps in 3D imaging and reconstructions.10

High-Resolution Computed Tomography

HRCT uses thin sections of CT images 0.625 to 1 mm slice thickness often with a high spatial frequency reconstruction algorithm. The usual slice thickness in HRCT is 0.6 to 0.7 mm. The resolution of the image means the ability to resolve small objects that are close together on an image as a separate form. The resolution of the image is highly important, as the anatomy of the temporal bone involves minute, small structures in close proximity. HRCT is a scan performed using a high spatial frequency algorithm to accentuate the contrast between tissue of widely differing densities such as air and bone, air and vessels. Collimation is of optimal importance to achieve high resolution. In routine practice, a collimator of 0.6 mm is commonly used during CT of the temporal bone.9 Collimation wider than 1 mm is not usually used as the resolution is often insufficient. Thicker slices are prone to volume averaging and thus reduce the ability to resolve smaller structures. For 40 to 64 detector scanners, the gantry cycle time is set at 1 cycle or gantry rotation per second. The kilovolt peak (kVp) used in HRCT is usually 120.11

Cone-Beam Computed Tomography

Cone-beam CT (CBCT) is a relatively new imaging technique, which was initially developed for angiography and has been used most commonly for dental and maxillofacial evaluation.12,13 CBCT presents a 3D approach for data acquisition, image display, image reconstruction, and image interpretation. More recently, CBCT has been used for a variety of otologic purposes. The first reported use of CBCT in cochlear implantation was on cadaveric temporal bones by the Freiburg group who demonstrated the superiority of CBCT over the conventional CT in the identification of electrode scalar position.14

CBCT uses a rotating gantry on which an X-ray tube and detector are attached. A cone-shaped X-ray beam is directed through the middle of the temporal bone onto a 2D X-ray detector. In contrast to conventional CT, which uses a narrow fan-shaped beam requiring multiple rotations around the patient to create a volume of data, CBCT requires only a single rotation of a cone-shaped beam (Fig. 1.3). In CT scan, HU is proportional to the degree of X-ray attenuation by the tissue and is assigned to each pixel to show the image that represents the density of the tissue; however, in CBCT, the degree of X-ray attenuation is shown by grayscale (voxel value).8 The resolution of CBCT is usually between 75 and 300 km which determines the size of the voxels and produces reconstructed images in the three orthogonal planes.14 The diagnostic importance of CBCT has been proven in the assessment of the position of CIs and visualization of alloplastic middle ear gold, titanium, or platinum implants.

Fig. 1.3 Depiction of conventional fan beam in multidetector computed tomography and cone beam in cone-beam computed tomography preventing radiation exposure to lens.

The advantages of CBCT over MSCT/HRCT include higher spatial resolution, reduced metal artifact, shorter acquisition and rapid scanning time, significantly lower radiation dose to the patient, and a reasonable price.8,15 The most important disadvantage of CBCT is its high sensitivity for motion artifacts as the patient has to hold the head perfectly still during the acquisition time of approximately 40 seconds.16

Safety Considerations

The obvious problem with CT is exposure to radiation which is detrimental to various tissues of the body. The biological effects of radiation can be either deterministic or stochastic. The deterministic effects occur only after a threshold dose is exceeded. The stochastic effects may occur at any level of dose.

In the case of HRCT temporal bone, the most detrimental deterministic effect is on the lens as it lies in the same area. Initially, the minimum single dose exposure required to produce a progressive cataract was considered to be 2 Gy.17 But in the latest statement on tissue reactions issued by the International Commission on Radiological Protection this threshold was decreased to 0.5 Gy.18 Therefore in HRCT of the temporal bone, the lens should not be in direct X-ray beam. If the lens is in a direct X-ray beam, approximately 0.03 to 0.06 Gy of radiation dose is exposed to the lens when HRCT temporal bone is done. If the patient is positioned such that the lens is not in direct X-ray beam, the dose is approximately 0.003 Gy.19 It goes without saying that the latter should be tried as far as possible. The stochastic effects include carcinogenesis and mutations, therefore unnecessary radiation exposure should be reduced as much as possible.

CT therefore follows the principle of as low as reasonably achievable (ALARA) with good quality images as needed for diagnosis. Various strategies and protocols are therefore followed by radiologists to minimize the dose of radiation as per the age of the patient, body part being imaged, and the equipment being used.

Magnetic Resonance Imaging

MRI is based on the magnetic resonance property of hydrogen atom which is the most common atom in our body. Hydrogen protons are electrically charged atoms and are considered as magnets with polarity.20 Each proton spins 360 degrees around its axis with a certain speed and this frequency is called Larmor frequency.

The majority of electromagnets used in the MRI scanner create a magnetic field strength of 1.5 Tesla (T); however, recently manufactured newer machines generate magnetic field strength up to 3 T.21 For research purposes, magnetic field strength up to 7 T is being used.

When the patient enters an MRI scanner with a strong magnetic field, hydrogen protons align parallel to the axis of the magnetic field. The MRI scanner also produces radiofrequency pulses exciting the protons to align at an angle to the magnetic field. Milliseconds after removal of the radiofrequency pulse the excited protons in the body relax, and a radiofrequency signal is detected by the receiver in the scanner and is transformed into images.

Relaxation of the protons occurs in following two ways:

1.Realignment of protons with the magnetic field.

2.Dephasing of spinning protons.

Sequences

Based on the type of relaxation, various sequences of MRI are available. On CT images, white means high density in contrast to MRI images where white means bright or high-intensity signal. In MRI generally, the terms low, intermediate, and high signal intensity are frequently used. Depending on the scan protocol, the tissue imaged as dark gray/black is low signal intensity, white is high signal intensity, and gray is intermediate signal intensity (Fig. 1.4).

The two basic types of MRI sequences available are: (1) T1-weighted and (2) T2-weighted.

Fig. 1.4 Color depiction of signals on magnetic resonance imaging. Black: low-intensity signal, gray: intermediate-intensity signal, and white: high-intensity signal.

T1-Weighted Sequence

In T1-weighted sequence, the signal is related to the speed of realignment with the magnetic field; the more quickly the protons realign, the greater is the T1 signal. The T1 images predominantly highlight fat tissue within the body and hence fat appears bright on T1 images (Figs. 1.5 and 1.6).

Fig. 1.5 Color depiction of T1-weighted sequence, with black (air, bone, blood vessels) as low-intensity signal and white (fat) as high-intensity signal.

Fig. 1.6 Magnetic resonance imaging brain T1-weighted axial section.

T2-Weighted Sequence

In T2-weighted sequence, the signal is related to the speed of proton spin dephasing, that is, the slower the dephasing, the greater is the T2 signal. The timing of radiofrequency pulse sequences used to make T2 images highlights both fat and water tissue within the body. So water and fat both appear white and bright on T2 images (Figs. 1.7 and 1.8). Cerebrospinal fluid (CSF) appears bright and white in T2-weighted images as it is fluid and appears dark in T1 images as it contains no fat.

Fig. 1.7 Color depiction Of T2-weighted sequence, with black (air, bone, nerves) as low-intensity signal and white (fluid and cerebrospinal fluid) as high-intensity signal.

Fig. 1.8 Magnetic resonance imaging brain T2-weighted axial section.

The other basic sequences available are:

1.Spin echo sequence that aims to remove the effects of the static field but leave the tissue characteristic T2 effect.

2.Gradient echo that is done by using a gradient to rephrase the spins with short TR.

MRI plays a crucial role in the selection of candidates for CIs, and high-resolution sequences using 3D gradient-echo techniques are required to display the fine anatomic structures of the internal auditory canal (IAC), cranial nerves, and the inner ear. 3D techniques that are currently performed at 3.0 T are 3D constructive interference in the steady state (CISS; Siemens AG, Berlin/Munich, Germany), which is also referred to as fast imaging employing steady-state acquisition with phase cycling (FIESTA-C; General Electric Healthcare, Waukesha, WI).

CISS/FIESTA Sequence

3D CISS/FIESTA is a T2-weighted and an ultrafast pulse sequence that produces high-resolution images that provide higher spatial resolution, clearer and brighter depiction of small structures like cranial nerves, and allows excellent visibility of all the three turns of cochlea, vestibule, and semicircular canal. An image of CSF, endolymph, and perilymph with homogeneous signal intensity is obtained in this technique. This sequence gives an exceptional image contrast between the CSF, perilymph, endolymph fluid to that of nerves, nerve branches, and vessels22,23 (Fig. 1.9).

Fig. 1.9 Magnetic resonance imaging temporal bone, axial sections, CISS/FIESTA sequence showing the cochlear and inferior vestibular nerves (in black) contrasting against the cerebrospinal fluid in the internal auditory canal (IAC).

MRI Planes and Cuts

1.5-T MRI and lately 3-T MRI with 3D reconstruction with eight-channel head coil is commonly used in most centers for MRI. Regularly used sequences in CI imaging are T1 weighted, T2 weighted, and 3D FIESTA/CISS in axial, coronal, and oblique sagittal planes.

Axial and Coronal Plane

Axial T1- and T2-weighted images are obtained through the temporal bone from the arcuate eminence superiorly to the mastoid tip and are perpendicular to the posterior margin of the brainstem described on a midsagittal image.24 The axial and coronal cuts taken in MRI are of similar angulation as in HRCT temporal bone. Axial CISS or 3D FIESTA images are obtained through the IAC and pons. The usual slice thickness in T1-weighted and T2-weighted sequences is 1 to 3 mm, and that in 3D FIESTA/CISS axial sequence is 1 mm.

Oblique Sagittal Plane

Oblique sagittal images are obtained in the plane perpendicular to the nerves of the IAC, so that we see a cross section of the nerves traversing the IAC (Fig. 1.10). These images are typically obtained in CISS/FIESTA sequences, which have been described earlier. This enables us to compare the thickness of the nerves and it will be discussed in detail in the chapters that follow.

Fig. 1.10 Magnetic resonance imaging temporal bone, CISS/FIESTA sequence oblique sagittal section through internal auditory canal (IAC) showing four nerves.

Safety Considerations

Due to powerful magnetic and radio-frequency fields, medical device implants with metallic or ferromagnetic components, such as CIs, can create problems during MR scans. Several precautions need to be followed by a cochlear implantee, prior to undergoing an MRI. MRI after CI was initially contraindicated and has been authorized only since 1995 under strict conditions, initially for 0.2-T MRI and then progressively up to 3-T MRI.25

MRI examinations performed under different conditions may result in severe injury or device malfunction. The recipient must remove all external components of their CI system before entering a room where an MRI scanner is located. The risks include the potential for device repositioning, localized heating, unusual sounds or sensations, pain or injury, demagnetization, and distortion of the MR image. Another problem posed by MRI post implantation is the artifact generated by either the magnet or the electrode. Image distortion may spread up to 6 cm around the implant on a 1.5-T MRI and may extend up to 12 cm on a 3-T MRI, when the magnet is left in place.26 So if we need to image the area near the implanted ear, then it is advisable to remove the magnet so that the artifacts are much lesser. The angle between the MRI magnetic field and the implant’s internal magnet must remain less than 90 degree to eliminate the risk of implant magnet demagnetization.27

To ensure MRI safety, various CI companies advise removal of the magnet or placing a compression bandage over the implant before the scan. The magnet is easy to remove and replaced if needed. Bandage applied should be 10 cm wide and should pass around the head in at least two layers. Of late, various CI companies have approved for MR scans under specific conditions at 1.5 T and 3 T with the magnet in place developing the feature of MRI compatibility.28

Software

The authors in their practice follow the rule of having the soft copy of images that can be studied in detail on any computer available in the hospital. Various softwares are available for this. The authors have personal experience of using Osirix and Horos on iOS systems and RadiAnt on Windows. These softwares allow many controls over the images and are good to plan the surgeries.

References

1.Yigit O, Kalaycik Ertugay C, Yasak AG, Araz Server E. Which imaging modality in cochlear implant candidates? Eur Arch Otorhinolaryngol 2019;276(5):1307–1311

2.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;26(5):976–982

3.Trimble K, Blaser S, James AL, Papsin BC. Computed tomography and/or magnetic resonance imaging before pediatric cochlear implantation? Developing an investigative strategy. Otol Neurotol 2007;28(3):317–324

4.Otake S, Taoka T, Maeda M, Yuh WT. A guide to identification and selection of axial planes in magnetic resonance imaging of the brain. Neuroradiol J 2018;31(4):336–344

5.Honda H, Watanabe K, Kusumoto S, et al. Optimal positioning for CT examinations of the skull base. Experimental and clinical studies. Eur J Radiol 1987;7(4):225–228

6.DenOtter TD, Schubert J. Hounsfield unit. In: StatPearls. Treasure Island, FL: StatPearls Publishing; May 11, 2020

7.Raju TN. The Nobel chronicles. 1979: Allan MacLeod Cormack (b 1924); and Sir Godfrey Newbold Hounsfield (b 1919). Lancet 1999;354(9190):1653

8.Razi T, Niknami M, Alavi Ghazani F. Relationship between Hounsfield unit in CT scan and gray scale in CBCT. J Dent Res Dent Clin Dent Prospect 2014;8(2):107–110

9.Debnath J, George RA, Satija L, et al. High resolution multi detector computed tomography of temporal bone: our experience in a tertiary care service hospital. Indian J Otolaryngol Head Neck Surg 2013;65(Suppl 3):512–519

10.Goldman LW. Principles of CT: multislice CT. J Nucl Med Technol 2008;36(2):57–68, quiz 75–76

11.Swartz JD, Loevner LA, eds. Imaging of the Temporal Bone. 4th ed. Thieme; 2009. Pg 1

12.Demirel O, Kaya E, Üçok CÖ. Evaluation of mastoid pneumatization using cone-beam computed tomography. Oral Radiol 2014;30(1):92–97

13.Sepúlveda I, Schmidt T, Platín E. Use of cone beam computed tomography in the diagnosis of superior semicircular canal dehiscence. J Clin Imaging Sci 2014;4:49

14.Saeed SR, Selvadurai D, Beale T, et al. The use of cone-beam computed tomography to determine cochlear implant electrode position in human temporal bones. Otol Neurotol 2014;35(8): 1338–1344

15.Miracle AC, Mukherji SK. Cone-beam CT of the head and neck, part 1: physical principles. AJNR Am J Neuroradiol 2009;30(6):1088–1095

16.Lemmerling M, De Foer B, eds. Temporal Bone Imaging. Berlin Heidelberg: Springer; 2015

17.Hall EJ, Giaccia AJ. Radiobiology for the Radiologist. 7th ed. Wolters Kluwer Health/Lippincott Williams & Wilkins; 2012

18.Stewart FA, Akleyev AV, Hauer-Jensen M, et al; Authors on behalf of ICRP. ICRP publication 118: ICRP statement on tissue reactions and early and late effects of radiation in normal tissues and organs—threshold doses for tissue reactions in a radiation protection context. Ann ICRP 2012; 41(1-2):1–322

19.Torizuka T, Hayakawa K, Satoh Y, et al. High-resolution CT of the temporal bone: a modified baseline. Radiology 1992;184(1):109–111

20.Berger A. Magnetic resonance imaging. BMJ 2002;324(7328):35

21.Panych LP, Madore B. The physics of MRI safety. J Magn Reson Imaging 2018;47(1):28–43

22.Cavusoglu M, Cılız DS, Duran S, et al. Temporal bone MRI with 3D-FIESTA in the evaluation of facial and audiovestibular dysfunction. Diagn Interv Imaging 2016;97(9):863–869

23.Erdogan N, Altay C, Akay E, et al. MRI assessment of internal acoustic canal variations using 3D-FIESTA sequences. Eur Arch Otorhinolaryngol 2013;270(2):469–475

24.Kopřiva J, Žižka J. Temporal Bone CT and MRI Anatomy. Springer International Publishing; 2015. Pg 157

25.Deneuve S, Loundon N, Leboulanger N, Rouillon I, Garabedian EN. Cochlear implant magnet displacement during magnetic resonance imaging. Otol Neurotol 2008;29(6):789–190

26.Orús Dotú C, Venegas Pizarro MdelP, De Juan Beltrán J, De Juan Delago M. Reimplantación coclear en el mismo oído: hallazgos, peculiaridades de la técnica quirúrgica y complicaciones. [Cochlear reimplantation in the same ear: Findings, peculiarities of the surgical technique and complications] Acta Otorrinolaringol Esp 2010;61(2):106–117

27.Bawazeer N, Vuong H, Riehm S, Veillon F, Charpiot A. Magnetic resonance imaging after cochlear implants. J Otol 2019;14(1):22–25

28.Precautions to Take for Children with Cochlear Implants. ENT for Children. Published December 20, 2017. Accessed May 31, 2020. https://entforchildren.com/precautions-take-children-cochlear-implants/

Chapter 2

Radiology of External and Middle Ear Pertaining to Cochlear Implant

Introduction

Axial Sections

Coronal Sections

Oblique Sagittal Reconstruction

Special Variants to Be Considered

2Radiology of External and Middle Ear Pertaining to Cochlear Implant

Introduction

Though cochlear implantation is a surgery that involves the inner ear, the route for entry in the inner ear is through the middle ear. Malformations of the external ear also impact this path which leads to the cochlea. In fact, radiology is very important when we want to predict many things related to this surgery as we go through the various parts of the middle ear cleft.

In this chapter, author will be discussing in detail about various aspects of radiology of the external and middle ear which plays an important role in cochlear implant surgery. For this purpose, author will be discussing various sections of high-resolution computed tomography (HRCT) temporal bone in relation to the above. By convention, the axial sections have to be read from superior to inferior and coronal sections from anterior to posterior.

Axial Sections

When we start reading the axial sections from superior to inferior, the first part of the middle ear cleft which we see is the mastoid air cells (Figs. 2.1 and 2.2). This pneumatization may vary from sclerotic to diploic to well-pneumatized to hyperpneumatized (Fig. 2.22). Operation on a sclerotic mastoid with a small antrum may sometimes be more time-consuming. At the same time, a hyperpneumatized mastoid may have a dehiscent facial nerve opening in a cell (Fig. 2.22).

Fig. 2.1 HRCT temporal bone, axial section, bone window. Superior most cut showing only mastoid air cells.

Fig. 2.2(a, b) HRCT temporal bone, axial section, bone window, at the level of internal acoustic canal (IAC) shows mastoid air cells, with corresponding colored structures, ( dark green) internal acoustic meatus and ( yellow) facial nerve–meatal and labyrinthine segment.

The first section where we start seeing the ossicles is usually at the level where we see the “signet ring” appearance of the lateral semicircular canal (LSCC) (Fig. 2.3). The head of malleus is the anterior bony shadow and the body of incus is the posterior bony shadow. They both, as we know, occupy the attic (epitympanum).