Youliv4 Imaging Referrals, London, UK; The Ralph Veterinary Referral Centre, London, UK
Introduction
Radiography remains a fundamental technique for thoracic imaging. However, in some situations, thoracic ultrasound proves to be a better alternative or complementary to radiology. Ultrasound is very useful in feline thoracic medicine; it does not require sedation and is less stressful than radiology or computed tomography. Furthermore the feline conformation is amenable to producing good quality thoracic ultrasound images.
Ultrasound Technique
The thorax can be examined by using a subcostal, intercostal or a thoracic inlet window. When using a subcostal window the thorax is imaged through the liver.1 To optimize this approach it is preferable to examine the patient when the stomach is empty (12 hours fasting). Fasting is also indicated if sedation or general anaesthesia is needed (e.g., uncooperative patients or when biopsies are indicated).
Most cats are relatively cooperative when examined standing, furthermore the standing position proves more comfortable for most dyspnoeic cats. When using an intercostal window with the patient in sternal or standing position, the thorax should be flexed away from the ultrasonographer in order to increase the space between the ribs. Flexion of the trunk in the direction of the ultrasonographer should be avoided as by doing so the intercostal space is reduced. A sternal or standing position is recommended to visualise and sample a small volume of pleural fluid as the fluid collects ventrally.
Sector probes are often used for the thorax as they have a small footprint and they can easily fit into the intercostal spaces. Mechanical probes (no longer produced) are inconvenient because the close ultrasound field is not displayed. Linear probes provide excellent detail but penetrate less and have the disadvantage of a large footprint so the ribs get often in the way causing acoustic shadow.
Use of Diagnostic Imaging in Clinical Cases
Pleural effusion is a common cause of dyspnoea in cats. The list of common causes of a moderate to large pleural effusion in cats is relatively short; it has been reported that feline infectious peritonitis (FIP), congestive heart failure (CHF), pyothorax and neoplasia together accounted for 88–100% of cases with a confirmed etiology.2 Thoracic ultrasound offers a fast and definitive diagnosis in patients with suspected pleural effusion. Therefore ultrasound should be considered as the first imaging modality to use in dyspnoeic cats. Furthermore, ultrasound is more sensitive than radiography to detect small volume pleural effusion. The echogenicity of the pleural fluid may give an idea of the nature of the fluid; transudate, modified transudate and chylothorax are anechoic or hypoechoic whereas pyothorax and haemothorax are generally hyperechoic. In case of localized pyothorax, thoracic radiographs are helpful to guide the ultrasonographic examination.
Thoracic ultrasound is an excellent method to check for presence of mediastinal, parietal, pleural or pulmonary masses, particularly in patients with pleural effusion, whereas on radiographs, the presence of pleural fluid often obscures these lesions. Mediastinal masses are frequently neoplastic, with some exceptions (e.g., mediastinal cysts). Mediastinal cysts are easily recognized on ultrasound; they are round or oval, have thin, well-defined walls, and, usually, anechoic contents.
In presence of thoracic masses biopsies are required to reach a final diagnosis. Ultrasound guided biopsies (fine needle aspirate or tissue core biopsies) should be performed with the patient under general anaesthesia or heavy sedation to allow a level of control over the patient and the lung movements. Possible complications are: pneumothorax, haemorrhage and pyothorax.
The ultrasonographic appearance of normal lung is mostly composed of artefacts. A series of horizontal lines, parallel to the pleural line are seen in the deeper region of the field of view; these are called “A-lines” and represent reverberation artefacts. The distance between A-lines is equal to, or a multiple of, the distance between the skin and the pleural line. In the normal lung the pleural line moves synchronously with respiration: this dynamic horizontal movement is called lung sliding.
It is not possible to visualize structures positioned behind air (e.g., an air-filled pulmonary bulla cannot be identified with ultrasound). Atelectasis, pulmonary consolidation and pulmonary masses can be visualised when there is little or no air in between a lesion and the probe. Therefore the lesions listed above are seen either when they are in contact with the thoracic wall or when they are surrounded by pleural fluid. Pulmonary lesions can also be visualised from a trans-hepatic window when they are located against the diaphragm.
Lung ultrasound is used in human patients to diagnose cardiogenic pulmonary oedema (CPO). In case of CPO the normal ultrasonographic appearance of a pleural line with A-lines is lost because of artefacts called “B-Lines” (also called comet-tail, ring-down, or lung rockets artefacts). These artefacts are perpendicular to the pleural line and cause disruption of the A-lines.3 However, it is very important to keep in mind that B-lines are not a pathognomonic sign of CPO. In dogs and cats lung ultrasonography cannot be used to differentiate CPO from other causes of diffuse interstitial or alveolar disease.4
Feline pleural effusion can result from left and/or right CHF causing a transudate, a modified transudate or a chylous effusion. Feline CHF may cause pulmonary oedema, pleural effusion or both. Cardiogenic pulmonary oedema in cats has a variable radiographic appearance and is associated with a range of pattern and distribution.5 Pulmonary congestion precedes pulmonary oedema but is not always identified radiographically. Therefore radiographically feline cardiogenic oedema is often confused with respiratory disease. The non-characteristic appearance of feline cardiogenic pulmonary oedema, plus the fact that feline cardiac disease does not always result in radiographic cardiomegaly, complicates the radiographic diagnosis of cardiac disease. Echocardiography is necessary in these cases to demonstrate the underlying cardiac disease.
In cases of suspected pulmonary disease and thoracic trauma, the suggested imaging technique to start with is radiography or, when available, CT. Radiographs offer an overview of the thorax, including the musculoskeletal system, and are used to determine whether there are any lesions that could be visualized by ultrasound in order to guide biopsies. When a mass is not in contact with the thoracic wall, CT guided biopsies should be considered as an alternative to ultrasound guided biopsies.
Radiology, ultrasound or a combination of these two imaging modalities is generally sufficient to diagnose diaphragmatic ruptures. When using a trans-hepatic approach, the mirror image artefact should not be misinterpreted as a diaphragmatic rupture.
In case of pneumothorax the normal sliding movement of the pleural line is lost however ultrasound is not a reliable technique to diagnose pneumothorax in dogs and cats following trauma.6 CT is the most accurate technique to detect pneumothorax and the preferred imaging modality in cases of trauma because of its ability to demonstrate also minimally displaced fractures and subtle pulmonary changes. CT pulmonary angiography (CTPA) is used to identify pulmonary thromboembolism (PT); however more work is needed to determine the accuracy of CTPA for PT in dogs and cats.7
References
1. Neelis DA, Mattoon JS, Nyland TG. Chapter 7: thorax. In: Small Animal Diagnostic Ultrasound. 3rd edition. Saunders; 2015.
2. Beatty J, Barrs V. Pleural effusion in the cat: a practical approach to determining aetiology. J Feline Med Surg. 2010;12:693.
3. Gargani L, Volpicelli G. How I do it: lung ultrasound. Cardiovasc Ultrasound. 2014;4:12.
4. Ward JL, Lisciandro GR, Keene BW, et al. Accuracy of point-of-care lung ultrasonography for the diagnosis of cardiogenic pulmonary edema in dogs and cats with acute dyspnea. J Am Vet Med Assoc. 2017;15;250(6):666–675.
5. Benigni L, Morgan N, Lamb CR. Radiographic appearance of cardiogenic pulmonary oedema in 23 cats. J Small Anim Pract. 2009;50(1):9–14.
6. Walters AM, O’Brien MA, Selmic LE, et al. Evaluation of the agreement between focused assessment with sonography for trauma (AFAST/TFAST) and computed tomography in dogs and cats with recent trauma. J Vet Emerg Crit Care. 2018;28(5):429–435.
7. Goggs R, Chan DL, Benigni L, et al. Comparison of computed tomography pulmonary angiography and point-of-care test for pulmonary thromboembolism diagnosis in dogs. J Small Anim Pract. 2014;55(4):190–197.