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Principles of Computed Tomography and Magnetic Resonance Imaging
1
Robert Cole and Adrien-Maxence Hespel 2
1 Department of Clinical Sciences, College of Veterinary Medicine, Auburn, AL, USA
2 College of Veterinary Medicine, University of Tennessee, Knoxville, TN, USA

2.1 Introduction of CT and MR, the elimination of superimposition and the
improved contrast resolution make up for this deficit [1, 2].
Computed tomography (CT) and magnetic resonance Computed tomography and MR share some similarities
imaging (MR) are advanced imaging modalities that have of image formation. Each image is composed of numerous
proven invaluable in small animal imaging. Although these pixels defining a three‐plane matrix; the three planes are
techniques are commonly reserved for referral practices height, width, and depth. A volume constituted by one unit
and universities, they are becoming more mainstream. It is among each of these planes is referred to as a voxel
important to have a basic understanding of these modali- (Figure 2.1) [1–3]. Each voxel is spatially localized and pro-
ties to better assess their clinical utilities, indications, and cessed by the computer. The voxel will contain information
limitations even if not currently available in your practice. about the attenuation values (CT) or signal intensity (MR).
The primary advantage of CT and MR over conventional This will result in some variation in the shade of gray dis-
imaging lies in the superior contrast resolution and tomo- played for each tissue. More attenuation on CT or more
graphic acquisition these modalities afford. These cross‐ signal on MR will result in a white voxel, whereas less
sectional technologies permit the evaluation of the tissue attenuation or no signal will result in a black voxel. This
of interest without superimposition. This chapter will voxel value is an average of all attenuation or signal in the
introduce the reader to the basics of CT and MR imaging voxel. If the tissue is heterogeneous, this variable attenua-
with respect to system designs and image acquisition. tion or signal will be averaged when the voxel is displayed
as a two‐dimensional pixel on the computer screen. As a
larger voxel will be more likely to contain a variety of tissue
2.2 Cross-sectional Imaging and because only the mean value of the voxel will be dis-
General Concepts played, a larger voxel is usually associated with a decrease
in contrast resolution [1–3]. The typical limiting factor for
Contrast resolution is the ability of the system to represent both CT and MR is slice thickness as it defines the depth of
differences in tissue characteristics [1, 2]. This is related to the voxel (largest factor in voxel size). Much of the work
X‐ray attenuation (CT) and signal intensity (MR). The bet- with cross‐sectional imaging is focused on achieving thin-
ter the contrast resolution, the more likely the users are to ner slices and smaller voxels in an attempt to achieve iso-
define the difference between tissues. The difference in tropic resolution (same dimension on all three planes) [2].
contrast is displayed on the computer screen as changes
in pixel brightness or shades of gray. MR and CT both
far exceed radiographs in terms of contrast resolution. 2.3 Computed Tomography
Surprisingly, the spatial resolution of CT and MR is more
limited when compared to radiography. Spatial resolution is Computed tomography was established as a diagnostic tool
the ability to resolve the separation between adjacent high‐ in the late 1970s that generates cross‐sectional imaging in a
contrast objects. Although spatial resolution is inherently plane parallel to the gantry. The CT unit consists of a gan-
better with radiography, because of the tomographic nature try, patient table, and computer‐based operator console

Feline Diagnostic Imaging, First Edition. Edited by Merrilee Holland and Judith Hudson.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.