Page 38 - VETcpd - Vol 2 - Issue 3

VETcpd - Radiology

Thoracic radiology

IntroductionRadiography of the thorax remains the mainstay of investigation for a variety of thoracic diseases. It is complemented by ultrasonography in some instances and more recently and increasingly with CT. (Figure1A, Figure1B Figure1C).

This article uses images of thoracic radiographs to show the normal anatomy and normal anatomical variation which will be encountered when interpreting thoracic images in every day practice. It gives recommendations for technique and interpretation in a practical and visual format.

Key words: normal thorax, normal variation, examples of pathology

Figure 1A

Figures 1: Thoracic imaging modalities. Radiography (A), computed tomography (B) and ultrasound (C). Three images of a Flat Coated Retriever with a primary lung tumour and associated lymphadenopathy. Note the different appearance that each modality has. Ultrasound is not appropriate in every case. CT provides excellent detail and shows nodules which are not seen on the radiographs (arrows). Radiography provides a very useful overview.

RadiographyThe general principles of thoracic radiography remain unchanged despite the increasing use of digital x-ray systems. The appearance of a lateral projection of the thorax as a standard film and a digital image are shown side by side (Figures 2 and 3). They have a different appearance and the digital appearance can take a little time to get used to. Digital images can be windowed so that lung, bone and soft tissue are displayed optimally whereas this is not possible with a standard film system.

TechniquePatients are positioned in lateral and either sternal or dorsal recumbency and there are a wide variety of aids to help with this.

Technique is vital for thoracic radiography and a variety of texts deal with this in great detail. While there is no need to reproduce it all here, some important points to refresh are:

Figure 1C

Recommended: orthogonal projections At least two orthogonal views of the chest are required – commonly right lateral and DV. The DV/VD projections must be taken before the lateral to avoid the effects of atelectasis (Figure 4). Occasionally two lateral projections are preferred.

Key Points: Always take dorsoventral / ventrodorsal view before the lateral.

Recommended: inflated Views Because identification of structures in the lung depends on the contrast between the air in the lung and the soft tissue or bone, it is important to try to take the radio-graph when the lung is inflated. However, be aware of the full expansion of the lung when inflated and do not over collimate the primary beam (Figure 5). Manual overinflation can also make small nodules less visible, make areas of pathological atelectasis and emphysema more difficult to assess and make the heart and pulmo-nary vessels appear small.

Paddy MannionBVMS DVR DipECVDI MRCVSRCVS & European Specialist in Veterinary Diagnostic Imaging

Paddy qualified from Glasgow in 1989. She has worked in large, mixed and small animal practice as well as in academic and private

referral centres in Britain and Switzerland. She did an 18 month mixed medicine and surgery internship at the RVC and did her postgraduate diagnostic imaging residency at the QVSH, Cambridge. She obtained the Certificate in Veterinary Radiology in 1994, the RCVS Diploma in 1996 and the European Diploma in Veterinary Diagnostic Imaging in 1997.

She established Cambridge Radiology Referrals in 1998 and has provided specialist outpatient imaging since then.

Heike Rudorf DrMedVet DVR DipECVDI MRCVS

Dr. Heike Rudorf qualified in 1988 from the “Freie Universität Berlin” (Germany) and was awarded the title Dr. med. vet from the same university

in 1994. She holds the Certificate and Diploma in Veterinary Radiology and is a Diplomat of the European College of Veterinary Diagnostic Imaging (DECVDI). She has worked as a veterinary radiologist in vet schools in Great Britain, Belgium, Sweden, Australia, Chile, and South Africa. Between 2006 and 2011 she worked as a research assistant in the Department of Cardiac Surgery at the University of Bonn and returned to QVSH as a clinical radiologist in 2012.

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VETcpd - Radiology

Figure 2: Standard film radiograph of the thorax. This is a thoracic radiograph of reasonable quality when assessing the skeleton, heart and great vessels. However, it is overexposed for lung.

Figure 4: Atelectasis Note the poor inflation and subsequent increased opacity of the left lung fields (arrows) and the shift of the apex of the heart (*) to the left. This is due to mediastinal shift and is typical of atelectasis. In this instance the patient was in lateral recumbency prior to the DV radiograph being taken. Atelectasis develops more quickly in sedated / anaesthetised patients and will not resolve during the examination making accurate interpretation difficult.

Figure 5: Inflated views Ideally, thoracic radiographs are taken when the lungs are inflated – either on full inspiration or with manual inflation. This radiographs shows just how far the inflated lungs extend over the diaphragm and illustrates the need for a wide enough collimation to allow the full extent to be included in the radiograph.

Figure 3: This digital radiograph shows good lung detail as well as showing the heart and great vessels well. It can be windowed to allow these to be even more clearly defined. This obviates the need for a repeat or second exposure.

When to choose 3 views? In general three views (right lateral, left lateral and a VD or DV) are advocated for checking for metastasis. The ability to distinguish structures on a thoracic radiograph is due to the different opac-ity of soft tissue, bone and air. When the patient is recumbent, the dependant lobe will collapse slightly, reducing the differentiation and allowing a lesion to be masked (Figures 6 A,B,C).

Figures 6 a,b,c: The Importance of Three Views

Figure 6A: The DV projection shows a busy lung pattern and as such it is easy to overlook the soft tissue nodule in the left cranial lobe (arrows). The mainstem bronchi are not obviously deviated.

Figure 6B: The left lateral projection shows the mass but its edges are not well defined. There is poorly defined increased opacity at the heart base (arrows) and this is suspicious for tracheobronchial lymphadenopathy. This is enough to suggest CT for this patient.

Figure 6C: The right lateral projection shows the mass most clearly as might be expected as it is within the uppermost and therefore aerated lung maximising its chance of detection. The increased opacity at the heart base is less clearly defined here.

Figure 6A

Figure 6B

Figure 6C