Hanspeter W. Steinmetz, Mariano Makara
The Use of Computed Tomography and Magnetic Resonance Imaging in Zoo Animals
The uses of cross-sectional modalities such as computed tomography (CT) and magnetic resonance imaging (MRI) have become more widespread, representing a considerable advancement in the veterinary standard care. CT and MRI have superior diagnostic capabilities than conventional radiography or ultrasonography because of their increased contrast resolution and their tomographic nature. To understand the role of these modalities in the imaging workup, it is essential to first appreciate how these features help overcome the several limitations of conventional radiography or ultrasonography because of its two-dimensional nature and by poor contrast resolution.
Computed Tomography
Equipment and Procedure
CT scanners consist of a scanning gantry, x-ray generator, computer system, and operator’s console. The gantry houses the x-ray tube, which produces x-ray photons, and a set of detectors, which collects the information produced after the x-rays interact with the structure under examination. The x-ray tube and the detectors rotate around the patient, generating a series of x-ray projections. A number of these projections are progressively taken from slightly different angles as the tube rotates around the patient. The information collected from these projections is used to generate CT images. The images created are cross-sectional, representing slices of the anatomy at a particular level. The possibility of acquiring cross-sectional images represents a major advantage over conventional radiology. Conventional radiology produces a two-dimensional image from a three-dimensional object, which results in superimposition, which may create confusing opacities that do not represent a structure within the patient. As opposed to radiography, CT examines the tissue in thin sections, or slices, thereby eliminating superimposition.37,45
Physics
Anatomic structures in a CT image are displayed by varying shades of gray. The shades of gray depend on the interaction between the x-rays and the patient. The x-rays may either pass through the patient, be redirected (scatter), or be attenuated. The degree of attenuation depends on the x-ray energy and the characteristics of the structure under examination, including its electron density, physical density, thickness, and effective atomic number.8 After interacting with the patient, x-ray photons strike the detectors. A computer processes the information recorded by these detectors to generate a CT image. By convention, if an object absorbs only a small fraction of the x-ray beam, it will be represented as a black area on the image. If an object absorbs the entire x-ray beam, it will be represented as a white area on the image. Between these extremes, objects with intermediate absorption will be represented by various shades of gray.45 The amount of the x-ray beam that is absorbed or scattered as it passes through the object is expressed by the linear attenuation coefficient. Because of a higher atomic number and density, the linear attenuation coefficient will be higher for bone than it would be for muscle. In other words, bone allows fewer photons to reach the detectors, resulting in an image with a lighter shade of gray, than that representing muscle. These differences in linear attenuation coefficients between tissues determine the image contrast in CT and conventional radiography. Although the basic principles of CT are similar to those of radiography in that x-rays are used to create an attenuation map of the patient, CT has superior contrast resolution. This may be explained by two mechanisms: (1) optimization of the differences in x-ray attenuation between tissues using computer processing and (2) virtual elimination of scatter radiation.8,45 This improvement in contrast resolution allows CT to better discriminate between normal or pathologic processes, which is otherwise not possible by conventional radiography.
Image Acquisition and Image Formation
CT image acquisition is dependent on the information acquired by the detectors as they rotate around the patient, creating an attenuation profile of the object from many different angles. For these data to be presented as a diagnostically useful image, the data have to be reconstructed. In loose terms, this process consists of the projection of the attenuation profiles onto a matrix. This process of converting the data from the attenuation profile to a matrix is known as back projection.37
A CT image is composed of a matrix of thousands of small two-dimensional squares called picture elements or pixels. Each of these pixels displays information from a small volume element from the patient, also referred to as a voxel. The sum of the linear attenuation coefficients along the path of an x-ray beam through multiple voxels is used to calculate the individual contributions from each voxel.37 The attenuation value of each pixel is then standardized using a scale by which these values may be expressed relative to the attenuation of water. Attenuation values are expressed in Hounsfield units (HU). This scale arbitrarily assigned the number 0 to water, the number 1000 to bone, and the number −1000 to air.8 Pixels are assigned relative shades of gray based on the HU of the tissues within their voxels. Even though 2000 different shades of gray may be assigned to each of the HU values, the monitor use to show CT images may only display 256 shades of gray. An even more limiting factor is that the human eye can only recognize fewer than 40 shades of gray. To solve this problem, only a certain number of HU values are assigned to a level of gray. The number of these units represented on a specific image is determined by the window width. In other words, the window width will determine the quantity of HU to be displayed on an image. By increasing the window width, more HU values will be assigned a shade of gray. The window level determines the center HU value of the window width. For example, if a window level of 50 is used and the window width is 200, the range of HU values that would be displayed would include −50 to 150 HU. Any structure with a HU of −50 or below would appear totally black, and any structure with a HU of 150 or above would appear totally white. A wide window width is used when studying structures with a wide spectrum of HU, for example, the lung. A narrow window width is used when studying structures with a narrower spectrum of HU, for example, the white and gray matter of the brain or the ventricular system.8,37,45
Reconstruction
Other important parameters are the reconstruction filters, which determine the level of edge reinforcement applied while processing raw data. These parameters must be set carefully according to the region of interest. For example, a low-pass filter, commonly known as “standard or soft tissue filter” is recommended when soft tissue contrast must be emphasized, as in brain imaging, which, however, has the disadvantage of creating blurry images. “Bone filters,” conversely, maximize spatial resolution but introduce more noise, thus reducing contrast resolution. Such images are sharper but grainier.37
Magnetic Resonance Imaging
Since the beginning of the new millennium, MRI has become an important diagnostic tool in veterinary medicine. Its excellent contrast resolution allows detailed characterization of soft tissues. MRI is currently the imaging modality of choice for the evaluation of disease processes involving the nervous system and the musculoskeletal system.21
Equipment and Procedure
MRI systems may vary in size and shape. Scanners use electromagnets and radio signals to produce cross-sectional images and are composed of a magnet, radiofrequency (RF) generators, gradient coils, and a transceiver. The main coil surrounds the bore (which holds the patient) with a uniform strong magnetic field. Different gradient coils create varying magnetic fields, for example, from top to bottom across the scanning tube (y-coil), from the entrance to the exit of the scanning tube (z-coil), and from left to right across the scanning tube (x-coil). The transceiver sends RF signals to protons and receives signals from them. All of these components interact with a computer, which precisely synchronizes the transfer of energy to the hydrogen protons and processes the response from these protons. This information is then processed and converted into a diagnostically useful image.21
During the scanning procedure, which takes approximately 30 to 90 minutes, the patient is placed into the tube in the center of the scanner, which, as noted above, is known as the bore, and is about 1 meter (m) in diameter and varies between 1.5 m and 2.5 m in length. Any movement of the patient would alter image quality, so most animals are anesthetized for the procedure.
Physics
The clinical applications of MRI rely on the electromagnetic properties of the hydrogen nucleus. Hydrogen protons spin about their own axis and are oriented in a random fashion within the body. Because of their positive charge, these moving protons possess their own magnetic fields and therefore may be thought of as small bar magnets. When the patient is placed within the bore of an MRI scanner, the magnetic field of the hydrogen protons are forced to align with the uniform magnetic field of the scanner magnet. Some protons align parallel to the main field and others align opposite to it, canceling each other out. A few more protons align with the field rather than in the opposing direction. This number is proportional to the external magnetic field strength. In addition to spinning about their own axis, protons wobble like toy tops. The rate of wobbling is termed precession. By applying an RF pulse at the precession frequency, energy may be transferred to the spinning protons. This is the “resonance” part of the MRI.21 The protons excited by this pulse jump into a higher energy state. When the RF is turned off, the spins begin to return to their lower energy state by two distinct, but simultaneous, processes. At this time, the three gradient coils are turned on and off rapidly, altering the main magnetic field on a local level; The rising electrical current in the wires of the gradient coils opposing the main magnetic field makes a continual rapid hammering noise; the louder this noise is, the stronger is the main field. When the hydrogen protons slowly return to their natural alignment within the magnetic field, they release the energy absorbed from the RF pulses, and this signal is picked up by the transceiver and sent to the computer system and is converted to a picture.8,21
Image Acquisition and Image Formation
The release of energy from spins to their molecular environment is called T1 relaxation, and the release of energy by the interaction of spins with each other is called T2 relaxation.21 The rate at which T1 and T2 relaxations occur varies, depending on the tissues under examination. The energy released by these processes is captured by the receiver coil and represents the signal with which computers create a diagnostically useful image. The response of different tissues to this energy stimulus may be manipulated to accentuate the signal they emit by using instrument controls. The MRI system uses injectable contrast agents to alter the local magnetic field in the tissue being examined. Normal and abnormal tissues respond differently, thus giving different signals. Contrast in MRI therefore relies on multiple parameters, including differences in proton density, the response of protons to magnetic and RF fields, and the adjustment of technical parameters. An MRI may display more than 250 shades of gray to depict various tissues.21
Image Manipulation
The standard clinical imaging sequences commonly include T2-weighted, short tau inversion recovery (STIR), fluid attenuated inversion recovery (FLAIR), T1-weighted, and postcontrast T1-weighted images.21 In T2-weighted images, both fat and fluid are displayed as high signal intensity. Bright fluid is a desirable feature, as most disease processes have an increased fluid content. Unfortunately, this increased fluid content is not specific for any particular disease process. Lesions such as tumors, granulomas, abscesses, or edema all display a high signal in T2-weighted MRI scans. In T1-weighted images, fat is hyperintense, and fluid is hypointense. T1-weighted sequences are generally used after the administration of contrast agents. A precontrast T1-weighted sequence is mandatory if a postcontrast T1-weighted sequence needs to be performed. In some cases, breaks in tissue structure such as the blood–brain barrier allow the contrast agent to leak into the tissue and change the relaxation of the tissue, leading to increased signal intensity. STIR is a fluid sensitive sequence similar to the T2-weighted sequence with suppression of the fat signal. Because of this suppression, STIR sequences display fluid associated with lesions as bright images on a dark background. FLAIR sequences are similar to STIR sequences except that they suppress the signal from the cerebral spinal fluid (CSF). Therefore, hyperintense lesions located around the ventricular system are easily detected as they contrast with the black CSF.21
Safety
According to its frequency (measured in hertz [Hz]) and its ability to ionize an atom or molecule in biologic tissues, electromagnetic (EM) radiation is classified in two categories: ionizing and nonionizing.25 The term ionizing radiation refers to x-rays and γ-rays, which have the ability to produce ionization of the biologic tissues and cell damage. Nonionizing radiation in the electromagnetic spectrum refers to static electromagnetic fields, RF, and optical waves.25
Although it is well established that ionizing radiation (x-ray, CT) poses risks to animal and human health, mainly because of its ability to ionize tissues, available data on the possible effects of nonionizing radiation on patient and worker safety have not been elucidated.25 Accidents and noxious effects at the present time are mostly the result of failure to follow safety guidelines.
CT technology uses computer-processed x-rays, similar to conventional radiography. Both techniques have the disadvantage that the x-rays are energetic enough to ionize the deoxyribonucleic acid (DNA) or the surrounding molecules and thus may damage the DNA directly or indirectly. Direct damage occurs when x-rays create ions that physically break one or both of the sugar phosphate or break the base pairs of the DNA.40 Indirect damage is caused by the creation of radicals in surrounding tissue, which interact with nearby DNA and cause strand breaks or base damage.7 Most radiation-induced damage is rapidly repaired by various systems within the cell, but DNA double-strand breaks are less easily repaired, and occasional inappropriate repair may lead to induction of point mutations, chromosomal translocations, and gene fusions, all of which are linked to the induction of cancer.3 The risk increases with radiation dose, longevity of life expectancy, growing stage, and the exposed tissue.
The average yearly medical x-ray dose has been increased considerably in human medicine since CT technology has become more available, and a similar increase is already observed in veterinary medicine or may occur in the near future. Organ doses from CT scanning are larger than those for corresponding conventional radiography. For example, a conventional anterior-posterior abdominal radiographic examination results in an approximately 50 times smaller x-ray dose to the stomach than the corresponding CT scan.7 For better risk estimation, various quantitative measures have been developed to describe the radiation dose delivered by CT scanning. The most relevant being absorbed dose, organ dose, effective dose, and CT dose index (CTDI).7 The absorbed dose is the energy absorbed per unit of mass (Gray [Gy]). The organ dose is the distribution of the dose in the organ and largely determines the level of risk to that organ from the radiation.7 For risk estimation, the organ dose is the preferred quantity. The effective dose (Sievert [Sv]) is used for dose distributions that are not homogeneous; it is designed to be proportional to a generic estimate of the overall harm to the patient caused by radiation exposure. The effective dose allows for a rough comparison between different CT scenarios but provides only an approximate estimate of the true risk. The CTDI is the historical measurement (milli-Gray [mGy]) that was used to measure the radiation for a single slice in standard cylindrical acrylic phantoms.29 The CTDI was used for quality control but is not directly related to the organ dose or risk.7
A CT evaluation requires a clear indication and should be planned carefully. The expected information gain should outweigh the risks of potential side effects. The risk depends on various factors such as the growing stage of the patient, the affected organs, and the radiation dose. Compared with adults, growing animals are at greater risk from a given dose of radiation because they are inherently more radiosensitive and have more remaining years of life during which a radiation-induced cancer could develop. Most of the other factors are under the control of the radiologist, and ideally, they should be tailored to the type of study being performed and to the particular patient (species, age, size).31 Radiation doses to particular organs for any given CT study depend on a number of factors. The most important are the number of scans, the scanning time (milliamperes [mAs]), the size of the patient, the axial scan range, the scan pitch (the degree of overlap between adjacent CT slices), the tube voltage in the kilovolt peaks (kVp), and the specific design of the scanner being used.29 It is always the case that the quality (relative noise) of the CT images decreases as the radiation dose decreases, which means that a tradeoff always exists between the need for low-noise images and the desirability of using low doses of radiation.45 Reducing the energy dose is important in long-living animals, especially to protect sensible organs such as the thyroid gland, the retina, the lens, or the gonads from the side effects of the x-rays. Repeating a study because of poor quality images results in higher amounts of x-ray exposure compared with an adequate energy setup that produces high-quality images.
No largescale epidemiologic studies of the long-term risks associated with CT in veterinary medicine have been reported so far, so extrapolation from human medicine and conventional radiology has become necessary. Since evidence of an increased risk of cancer from exposure to x-ray has been reported, it is also necessary to use CT technology responsibly and consider all necessary measurements to reduce exposure of animals and employees to x-rays during CT (Box 66-1).
Box 66-1
Safety Considerations for Computed Tomography and Magnetic Resonance Imaging
| Computed Tomography (CT) | Magnetic Resonance Imaging (MRI) |
◆ Reduced exposure to personnel: • Personnel not present during study ◆ Reduced exposure to animals: • Protection of sensitive or growing tissue • Avoidance of repeated studies  Stay updated, free articles. Join our Telegram channel
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