Textbook of Radiology for Cochlear Implants (eBook)

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.

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

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.

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...