Page 410 - Adams and Stashak's Lameness in Horses, 7th Edition

376 Chapter 3

COMPUTED TOMOGRAPHY

VetBooks.ir Mathieu Spriet

INTRODUCTION patient remains stationary during image acquisition as
the gantry itself travels over the area to be imaged. The
Computed tomography (CT) is a cross‐sectional horse can be positioned on a standard equine table, sim­
imaging technique, relying similarly to radiography, on plifying the logistics.
differential X‐ray attenuation by the tissues being In addition to the table design, the gantry diameter is
imaged. Images are acquired as slices of the anatomy of a limitation to the range of body areas that can be
the imaged subject, providing three‐dimensional (3D) imaged. Most classic human gantries have a diameter of
information. These slices of anatomy are obtained by 55–75 cm, limiting applications in horses to the distal
rotating an X‐ray generator around the imaged body limbs and head to cranial neck (typically to the level of
area and recording attenuation of the X‐ray beam at the third or fourth cervical vertebrae). Larger gantries
multiple different angles. The attenuation data are then have been designed for bariatric human patients with a
digitally processed to reconstruct an image. diameter up to 90 cm. These systems provide the ability
The imaging principles, relying on X‐ray attenuation, to image larger body areas such as the equine caudal
are similar to radiography. CT is excellent at imaging neck and stifles. Most recently scanners designed for
bones, with dense bones appearing white (hyperattenu­ intraoperative use in human neurosurgery even offer
ating), whereas gas will show up black due to a lack of larger gantries, with a diameter over 100 cm.
X‐ray attenuation. Soft tissue structures will display an The recent development of cone‐beam CT technology
intermediate (gray) opacity. Based on calibration of the brought versatility in the design of scanners. Cone‐beam
attenuation data, CT is better than radiography at iden­ scanners use a flat panel detector, similar to a digital
tifying small differences in tissue density. For this rea­ radiography plate. This allows modification of the con­
son, tendon, synovial fluid, and connective tissue can be figuration of the classic gantry with asymmetric shape
differentiated on CT, while they typically have the same and larger aperture. A robotic arm CT system has also
opacity on radiographs. CT also presents the advantage been developed: the classic gantry has been replaced by
over radiographs to eliminate superimposition of struc­ two robotic arms, one carrying the X‐ray generator and
tures. The combination of the increased contrast resolu­ one the detector plate. This potentially provides more
tion and abolition of superimposition explain why CT versatility for scanning standing horses and imaging
detects lesion not recognized on radiographs. larger areas. Cone‐beam CT provides an excellent spa­
CT has been used in the horse since the late 1990s, tial resolution, but limitations exist regarding contrast
mostly for head and distal limb imaging. Due to techno­ resolution and sensitivity to motion.
logical progress, CT cost and availability have greatly As an X‐ray‐based imaging technique, the generator
improved in the recent years. Also modification of the current intensity (mA) and voltage (kV) are important
size and configuration of the scanners have led to the technical parameters for CT similar to radiography.
development of new clinical applications, such as imag­ Additional parameters, including slice thickness, recon­
ing of the neck and more proximal areas of the limbs. struction algorithm, field of view, and matrix size, are
In this chapter, the basic principles of CT will be also important to understand the optimization of CT
described along with the different systems that are avail­ image quality. Depending on the type of tissue imaged,
able. The clinical applications of CT will be limited to the technique will be different.
musculoskeletal imaging due to the focus of this publi­ In order to optimize images for bone, it is important
cation. Imaging of the equine head, although a major to have a high spatial resolution and sharp edges. This is
indication for CT imaging, will not be covered. typically achieved with using thin slices, an edge
enhancement algorithm, a small field of view, and a
EQUIPMENT AND PRINCIPLES OF CT large matrix (Figure 3.194). The slice thickness in stand­
ard multislice CT is typically less than 1 mm, classically
Classic CT scanners consist of a gantry, containing a 0.65 mm. The in‐plane spatial resolution is governed by
rotating X‐ray generator and an array of detectors, and the matrix size and the reconstruction field of view.
a table translating the patient through the gantry for Most CT images will use a 512 × 512 matrix. The pixel
image acquisition. Obviously, the standard human table size is obtained by dividing the field of view by the
needs to be adapted to the size and weight of the equine matrix size. This is important to keep in mind as most
patient. This is typically accomplished by adding a large classic CT scanners can image a 50‐cm field of view;
table top over the human table but presents a techno­ however in the configuration the pixel size will be ~1 mm
logical challenge as the patient needs to be moved pre­ (500/512). Reducing the field of view to 25 cm, which is
cisely through the gantry during image acquisition. usually enough to cover the limb anatomy to image, per­
A few commercial solutions are available, but many sys­ mits bringing the pixel size to 0.5 mm (250/512),
tems rely on custom‐made tables. An interesting varia­ doubling the in‐plane spatial resolution. The edge
tion in the scanner design is the configuration of a few enhancement algorithm is a filter that is applied to the
systems having a translating gantry. In these systems, the acquired data to help make the image edges more