Objectives

 To understand the role of computed tomography (CT) in thoracic disease

 To recognize the advantages and disadvantages of CT in the diagnosis of thoracic disease

 To learn basic interpretation principles using selected diseases of the thorax

 Physics of CT image formation will not be covered

Similarly to radiographs, CT - also known as CAT scan (computed axial tomography) - is based in the attenuation of x-rays by the body. However, unlike radiography, it creates multiple slices of the area of interest. The term cross-sectional imaging is used to describe the ability of an imaging modality to depict the anatomy of a patient by generating slices of varying thickness of the area of interest. Cross-sectional imaging modalities include CT, MRI, and ultrasound. Rather than using analog film or detector panels, the CT unit uses multiple electronic detectors that receive the x-rays from a moving x-ray tube. The x-ray tube does not produce a fan-shaped x-ray beam but rather a pencil-like primary beam. Through analysis of the x-ray attenuation with the aid of a computer, CT depicts more shades of grey than the human eye can account for. It can also assign objective numeric values to the grey scale. These are known as Hounsfield units (HU) or CT numbers.

To assess the lung parenchyma, it is assumed that a high-resolution, multichannel helical unit is used. This ensures acquisition of high-resolution volume reconstruction in different planes. CT is used to better stage parenchymal disease found in radiographs. It provides a better understanding of the pathophysiology and etiology of the disease affecting the lung lobes.

CT versus Radiographs

Advantages: CT has a higher tissue contrast resolution than radiographs. As a result, on CT, the gallbladder can be distinguished from the surrounding liver parenchyma, and the lateral ventricles from the brain lobes. Tissues that contain scattered fibrotic tissue will likely appear denser than water. On the other hand, soft tissues with an inflammatory component will tend to show less density when compared to water. CT also has a higher spatial resolution than radiographs. This is one reason it has become the gold standard to assess the lung parenchyma by virtue of depicting, free of superimposition, high-generation bronchi and pulmonary vasculature. Advances in CT technology allow the patient's images to be reconstructed in different planes depicting images in three dimensions without changing the patient's position within the CT gantry. CT also excels in depicting bone. This is used in reconstruction of the skull, vertebrae, or ribs in cases of trauma. 3D images of the displaced bones can be generated to give a surgeon an accurate assessment of the area before surgery. CT is also an ideal technique for angiographic procedures that can be performed without the need for more invasive placement of catheters.

Disadvantages: CT requires general anesthesia and is more expensive than routine radiographic studies. Like radiography, it uses ionizing radiation, which can harm biological tissues. In fact, as in human medicine, CT is becoming the largest source of patient exposure to ionizing radiation in diagnostic imaging. The unit requires a higher degree of operational and servicing expertise.

CT versus Ultrasound

Advantages: CT provides a complete view of the entire thoracic cavity. When acquired correctly, CT exhibits fewer artifacts than ultrasound. It also has a higher soft-tissue and spatial resolution than ultrasound. CT is better suited to detect deeper lesions and with standard post-acquisition image processing software, provides volume reconstructions in infinite planes.

Disadvantages: CT images are more difficult to generate, and CT is more expensive than ultrasound. General practitioners are more familiar with interpretation of ultrasound than CT images. While CT can be used to guide a biopsy needle, this guidance is not in realtime with standard CT units. Ultrasound is better suited to assess superficial lesions and when there is a large amount of fluid in the pleural space.

Interpretation

(Correct terminology in bold type)

Three planes are commonly generated (veterinary terminology in bold type and the human medicine in parentheses): sagittal plane (sagittal), transverse plane (transaxial), and dorsal plane (coronal).

Structures that are white or bright on the screen are hyperdense or hyperattenuating. Structures that are dark or black on the screen are referred to as hypodense or hypoattenuating. Structures with the same level of grey are isodense.

Standard series can be acquired in bone or lung window where the spatial resolution is the highest or in a soft-tissue or abdominal window. An abdominal window increases conspicuity of the abdominal organs. Typical CT numbers for gas are -1000 HU; normal lung is -700 HU and diseased lung are less than -700 HU. Fatty tissue ranges between -90 and -100 HU, soft-tissue CT units are between 50 and 70 HU, and pleural effusion is between 0 and 30 HU. It is rare to find pleural effusion with a zero value. Hemorrhagic effusion exhibits a higher HU value due to the hemoglobin present in the fluid and mineral HU range from 100 to 1000 HU.

CT and Contrast Material

Low-osmolality, iodine-based compounds such as iohexol are used. Contrast-enhancing lesions are higher than 1000 HU. Post-contrast images taken soon after contrast intravenous injection will clearly show an increment in the attenuation of the vessels as contrast is seen in the lumen of the vasculature. Use of contrast material is no different than radiographs. Many of the routine radiographic contrast studies such as angiography, excretory urograms, and myelograms can also be performed with CT. However, CT excel at multiphase angiographic studies where the contrast can be imaged as it goes through the arterial, venous, and portal system.

Normal Anatomy

Lung parenchyma: The lung parenchyma is homogenously grey with the exception of areas of atelectasis where the lungs are hyperintense. The pleural space is only visible if fat, gas, or fluid are accumulating in the area. Fissure lines (borders between lung lobes) are often seen. The triads are also well seen. Highest-generation arterial branches can be noted toward the periphery of the lung parenchyma.

Airways: High level of anatomical detail is noted with high-resolution CT. The principal bronchi, lobar bronchi, segmental bronchi, as well as the bronchioles can be detected. Dogs have up to nine divisional bronchial branches before reaching the pulmonary acinus. The highest generation (terminal bronchiole) cannot be seen at the periphery of the lung on a CT image. However, the arteries and veins at this level can be seen. Size of the terminal bronchiole is less than 3 millimeters. Structures that are not detected in normal lungs include the respiratory bronchiole, alveolar duct, and alveolar sac.

Pulmonary interstitium: It is classified in three types: peripheral interstitium, which is along the subpleural space. The portion of the interstitium that follows the bronchovascular structures and surrounds the pulmonary acinus is the axial interstitium. The parenchymal interstitium is located where the gas exchange occurs.

Disease States

As a general rule, airway disease can result in increased visualization of airways of highest generation due to thickening of the airway wall. Bronchiolitis results in bronchiolar wall thickening and nodular-like structures which represent thicker alveolar ducts and terminal bronchioles. This is known as a "tree-in-bud" pattern. The HU values of the diseased lung parenchyma increase. Pulmonary fibrosis or interstitial pneumonia can result in CT numbers in the -500 HU to -600 HU range. The peripheral and axial interstitium becomes thicker and is noted as linear areas of increased attenuation.

Final Disclaimer Note

Attempts have been made to extrapolate the pattern of lung disease from human medicine. However, the anatomy of the human lung is different than that of a dog and cat. Hence, many of these patterns of lung disease might not accurately describe the pathophysiology in dogs of cats. Please note that even though advanced imaging techniques such as CT can give a glimpse to many pathological conditions of the lung parenchyma, the appearance or abnormal imaging findings are nonspecific. Therefore, histopathology is required (always) for a definitive diagnosis.