Victor Rendano Jr.
Dyspnea, coughing, wheezing, trauma, cardiac murmur, and neoplastic screening test are the most common reason for imaging the thoracic viscera. The conventional radiographic evaluation using an x-ray tube, grid, cassette/screens, and film allows for analysis of the intra-thoracic viscera. This technology is good to excellent for defining the size, shape, position, opacity, and contour of many of the intra-thoracic viscera. It allows us determine if there is gas, fat, soft tissue, and bone present. It is limited by artifacts, superimposition of structures, and resolution. It does not allow us to distinguish between the components of soft tissue unless contrast media is used. We can only detect a structure if it is adjacent to a structure of a different opacity. Some of these limitations can be overcome by using equipment and techniques that decrease artifacts, enhance resolution and contrast, and decrease superimposition of structures.
The common artifacts that may be removed or decreased are:
Improper patient positioning.
Under or overexposed images.
Dirt and hair in the cassette.
Improper film sensitivity for the screens being used.
Improper speed of the film/screen combination for the size of the animal or lesion.
Light leakage into the cassette or into the processing room, which exposes the film.
Improper safe light.
Processing problems that include exhausted chemistry, improper processing time and/or temperature, insufficient washing of the film.
To enhance resolution with conventional radiographic techniques use:
The small focal spot size of the tube.
Detail film-screen combination.
To decrease superimposition of structures use:
Multiple projections including oblique projections
When properly used, the conventional radiographic techniques will allow us to appreciate the size, opacity, and shape of the trachea; alterations in opacity and contour of the mediastinum; enlargement of the esophagus; the presence of metallic or calcific opacity foreign bodies; the presence of osseous lesions, masses in the lungs, fluid, or gas in the pleural space; the contour of the pericardial sac and diaphragm; the contour and size of the bronchi; the contour and size of the pulmonary vessels; and the status of the alveoli. Visualization of lesions, the distribution of the lesions, and the clinical and clinicopathologic findings will often help define the etiology for the patient’s clinical problems. However, even at its best, conventional radiography is still hampered by limited resolution and maximum superimposition.
Computed Tomography (CT/CAT) imaging has been developed to enhance resolution and remove superimposition. These two benefits allows for improved characterization of normal anatomy and disease processes. Similar to conventional radiography, CT imaging uses an x-ray tube and is dependent on differential absorption of the x-rays to help distinguish between gas, fat, soft tissue, and bone. The contrast media used for conventional radiography (barium, iodinated media, gas) are the same as those used for CT imaging. Some of the same artifacts that hamper conventional radiography, such as patient motion, also hamper CT image quality.
Unlike conventional radiography, CT imaging uses detectors rather than film to determine the amount of radiation that has penetrated the area of interest. The X-ray tube and/or the detector/detectors move around the patient during the exposure. The patient is imaged from multiple angles and a defined area of 1 to 10 mm is usually imaged per “slice.” Multiple slices are obtained by moving the patient through the center of the CT unit, the gantry. These multiple slices collectively define the tissues in the body cavity being imaged. The information from each detector is fed into the computer for processing and the image viewed on the computer screen. This electric image can be printed on film for viewing, similar to a conventional radiograph.
The conventional radiographic study can be equated to radiographing a whole loaf of bread, while the CT study is equated to cutting the loaf into multiple thin slices and analyzing each slice. Both methods use X-rays, both methods have similar and unique artifacts, both methods require knowledge of how the different ingredients come together to produce and image. The conventional radiograph gives you an appreciation of the “overall picture,” the CT image gives you the spatial resolution and a better appreciation of the ingredients of the disease process.
The enhanced image using CT technology allows for appreciation of lesions not defined with conventional radiographic techniques, for example, smaller metastatic lesions, smaller alveolar infiltrates, less extensive bronchial wall thickening, and less extensive pleural effusions.
Unless we do invasive procedures such as injecting contract medium, conventional and CT imaging technologies are limited in their ability to show the composition of soft tissues or to show dynamic activity, for example, cardiac muscle contractility and blood flow. Because of these limitations, other imaging methods, such as ultrasound, are required.
Ultrasound imaging uses high frequency non-ionizing electromagnetic waves. The frequencies used in veterinary imaging at this time vary from 2.5 to 10.0 megahertz. Crystals that are part of a transducer produce this sound. When the crystals are stimulated electronically, the high frequently sound is produced. This sound enters the patient, is reflected back from the tissues being imaged, and is detected by the crystals in the transducer. The crystals in the transducer thus act as both initiators and detectors of the ultrasound. The higher the frequency of the sound produced, the better the potential resolution but the poorer the penetration of the sound into the tissues. As such, a 10-megahertz transducer is excellent for imaging the eye since it gives excellent tissue resolution and does not require deep penetration into the patient. Such a transducer frequency would be inappropriate to image the heart of a large breed dog since there would be insufficient penetration. A 3.5 or 5.0 megahertz transducer would allow sufficient penetration to see the heart, sacrificing some resolution for the needed penetration.
The differential reflectivity of sound amongst tissues allows us to define their composition. Gas and bone are poorly imaged with ultrasound. Soft tissues and fat are most appropriate for imaging.
Gas and bone reflect a large amount of sound and thus appear very white in the image. Structures that are situation “deep” or “beyond” gas and bone are not visualized. Soft tissue and fat reflect much less sound per region of interest, thus there are finer gradations of tissue composition that can be obtained. Some soft tissues such as fluid can be easily defined with ultrasound because they reflect no, or very little, sound and appear black. The ultrasound composition of tissues is described by its echogenicity. The terms used to describe the echogenicity are anechoic, isoechoic, hypoechoic, and hyperechoic. Of the four terms, only anechoic is a non-relative term since it means without echoes; this is the character of fluid. The other three terms are relative terms since they define the echogenicity of a structure compared to anther structure with isoechoic being the same, hypoechoic being darker and hyperechoic being lighter/brighter. When imaging tissues in the thorax, only those tissues that are not behind bone or surrounded by gas can be seen. The composition of the body wall tissues, the pleura, mediastinum, and pericardial sac/heart are usually defined. Metastatic lesions in lung that are surrounded by aerated lung are not defined since the gas in the aerated lung obstructs visualization of the soft tissue metastatic lesion. A consolidated lung due to any disease process can be analyzed relative to its composition using ultrasound. A diseased lung behind rib or surrounded by aerated lung cannot be analyzed. The gas and/or bone obstruct the passage of the sound into the soft tissues and thus there is no opportunity or “window” for the sound to see through to interact in the soft tissues.
The ultrasound technology allows for real time dynamic studies of motion such as cardiac contractility and blood flow without the need for contrast medium in most situations. The cardiovascular dynamics may be assessed using B-mode, M-mode and Doppler methods. This imaging advantage has made ultrasound the primary method to assess cardiac and vascular dynamics.
In summary, the x-rays technology of conventional and CT imaging has, and continues to, be an excellent way to appreciate tissue composition in the broad categories of gas, fat, soft tissue, and bone. These technologies allow us to appreciate disease especially when there is alteration in the size, shape, position, contour, and opacity of a tissue/organ. The CT imaging improves our ability relative to lesion detection, location, and extent, because of its improved resolution and spatial orientation. Both imaging technologies are often not adequate for defining the dynamic status of the cardiovascular system and soft tissue composition. Ultrasound is excellent for meeting the needs of the void created by the x-ray technology. It is excellent for resolving soft tissue composition and assessing cardiovascular dynamics. With ultrasound, gas and bone are our foe and soft tissue and fat are our friends. With X-rays, gas, bone, and fat are our friends; soft tissue, especially fluid, is our foe.
Boon: Manual of Veterinary Echocardiography ISBN 0-683-00938-9
Nyland and Mattoon: Veterinary Diagnostic Ultrasound ISBN 0-7216-2745-5
Thrall: Textbook of Veterinary Diagnostic Radiology-Third Edition ISBN 0-7216-5092-9
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Veterinary Radiology and Ultrasound E-mail: firstname.lastname@example.org