X-Rays are a very commonly used diagnostic technique in veterinary clinics.

Feline patients are notoriously reticent when it comes to overt signs of pain and illness, and therefore diagnostic imaging plays a vital role in the assessment of cats in veterinary clinics.

Despite the inherent radiation risks, radiographs are a non-invasive tool with fewer complications or unwanted sequellae than more invasive diagnostic modalities, but a higher sensitivity and specificity for many conditions than most less invasive techniques. 

In many ways, we can consider radiographs to be the optimal middle ground in imaging technology for the diagnosis of a remarkable variety of conditions. 

Why use radiographs?

X-rays are a common imaging modality used in veterinary clinics due to their ability to penetrate tissue, and so reveal internal structures.

They are therefore used to assess solid structures and are particularly useful in areas with contrasting densities, such as in the thorax.

Radiographs can be an important part of a diagnostic pathway and should be used alongside a full clinical history, physical examination, and other diagnostic tools such as bloodwork and other imaging modalities. 

Why not use radiographs?

X-rays are usually considered non-invasive for patients in a veterinary setting, as they are unlikely to alter or exacerbate any underlying pathology.

However, it should be remembered that X-ray exposure can be harmful at certain levels, and positioning cats for X-rays may cause stress and discomfort. For this reason, chemical restraint is always preferred, when clinically possible.

Why are radiographs particularly useful in cats?

Cats are notorious for their ability to mask clinical indicators of pain, illness, and disease. Their tendency to withdraw at times of physiological, physical, and behavioral stress can lead to both owners and veterinarians missing clinical signs, and make a comprehensive physical assessment more difficult.

This means that objective measures of disease are important, with accurate diagnostic methods beyond physical assessment needed.  

Indications for X-rays in cats 

Radiographs have a role in the diagnosis of many conditions, but they are of particular value in disease processes affecting:

The thorax

One of the most commonly imaged areas in cats is the thorax. Due to the pleural potential space providing mechanical coupling between the chest wall and lung, invasive imaging techniques – such as thoracoscopy – are very high risk. In the acutely dyspnoeic patient, ultrasonography may be more rewarding and lower risk, but in most cases, radiography gives more useful information due to the radiolucent air-filled spaces preventing ultrasound penetration.

X-ray imaging of the thorax may be used for the determination of respiratory, cardiovascular, oncological, and traumatic conditions.

The standard radiographic projections should include left and right lateral and a VD or DV view; although the VD should be avoided in patients with severe pulmonary or cardiac disease, as the resulting V/Q mismatch may prove rapidly fatal in a brittle dyspnoeic cat. 

Fortunately, the range of densities from gas-filled lungs to solid bone provides high contrast, meaning that a resting “cat-o-gram" will often prove diagnostically useful, if the patient’s medical status precludes a more correctly positioned radiographic series.

However, findings can be non-specific and require further investigation, in particular, if there is profuse pulmonary, pleural, or pericardial fluid or solid lesions in the lungs. 

The abdomen

Abdominal radiographs have many diagnostic uses, including changes to the size, shape, or architecture of organs, foreign bodies, fluid or gas accumulations, and trauma.

Common views taken are the left and right lateral and a DV or VD. However, radiographic interpretation is not always straightforward, as the mass of overlapping soft tissue structures produces a complex palimpsest where three-dimensional relationships are obscured.

Moreover, while different fat: water ratios in different tissues do give subtly different radiographic density (and thus shade), these subtle variations are difficult to appreciate. In many cases, only the major organs are visible on a plain film radiograph. Remember too that obese patients can be more difficult to assess, due to the accumulation of fat around organs. 

However, even a simple survey radiograph has its advantages, as being relatively quick and easy, while an ultrasound scan can give different information, it is not possible to visualize the entire abdomen in a single view, unlike with a radiograph.

And while an exploratory laparotomy or even laparoscopy would indeed allow the surgeons to access and examine every structure in the cavity, these are invasive procedures with a relatively high complication rate.

The essence of obtaining – and interpreting – a diagnostic abdominal radiograph is in understanding the contrast. For example, intestinal gas – especially in the colon – shows a clear contrast to the soft tissue around it, as do radio-opaque foreign bodies.

For a more detailed examination of the intestinal or urinary tracts, contrast radiography is an under-utilized and powerful technique that bypasses the fundamental problem of poor tissue differentiation and should always be considered, especially if a structural or functional bowel obstruction is suspected.

Bones and joints

Fractures, deformities, and injuries of bony structures can all be assessed in cats using X-rays. Joint and soft tissue imaging can be more challenging, as the density of ligaments and tendons is extremely similar.

Orthopedic imaging is classically considered to revolve around radiography; however, if soft tissue lesions are suspected (or no bony lesions are detected), the parallel use of radiography and ultrasonography is an exceptionally powerful combination.

When performing orthopedic radiographs, orthogonal views are critical, as the X-ray produced is a two-dimensional image of a three-dimensional anatomic site, requiring the opposing view to fully visualize the area.3 In essence, with the exception of some of the more specialized glenohumeral and coxofemoral views, any limb radiograph can be considered to comprise a dorsopalmar/dorsoplantar view and a mediolateral view. 

It is usually important to image limbs from medial to lateral, as the limb being imaged should be as close to the imaging plate as possible. Imaging the upper, rather than lower, limb will result in excessive magnification due to an elongated object-film distance; in addition, due to the resultant angularity of the limb, variable magnification along its length may be seen. In both cases, it is impossible to accurately measure lesions or bones for the selection and fitting of plates.

In some cases, with subtle or highly focal pathology, additional oblique views may also be necessary to skyline the lesions.

These should be selected based on the location, size, and any suspicions based on simple DP and ML views.

However, in the majority of cats, the bone opacity is low enough that most lesions can be appraised from a simple paired view.

Dental X-rays

Dental disease is extremely common in cats, and management can be complex. Radiography allows for a thorough assessment of oral structures, including those below the gingival margin.

Using dental X-rays in cats allows for success in both treatment planning and evaluation.

Radiographs can provide a rapid, non-invasive, and clinically useful assessment of many internal structures in the cat.

However, X-rays should be used alongside other clinical options such as a physical exam, bloodwork, and other imaging. It is important to remember the limitations of a plain film radiograph (the reduction of three-dimensional structures into a flat palimpsest, and the limited differentiation between soft tissue structures).

As a result, a good knowledge of feline-specific anatomy and a good radiographic atlas are invaluable aids in interpreting the images. However, no other imaging modality is as versatile and as quick and easy to perform in the clinic, and with cats frequently presenting late into the course of a range of pathologies due to their masking behaviors, this makes radiography an excellent first choice for imaging most body systems. 

References

1. del Regato JA: Wilhelm Conrad Röntgen, in Radiological Physicists. American Institute of Physics, 1985. 

2. Larson, M. Feline Diagnostic Imaging. Published 2020 John Wiley. Ed. Holland & Hudson.  ISBN:9781118840948

3. Lavin L: Small animal soft tissue, in Lavin L (ed): Radiology in Veterinary Technology, ed 3. Philadelphia, WB Saunders, 2003

4. DuPont, G. & DeBowes, L. Atlast of Dental Radiography in dogs and cats. Saunders Elsevier Missouri, 2009

5. Ismael Hernandez-Avalos, Daniel Mota-Rojas, Patricia Mora-Medina, Julio Martínez-Burnes, Alejandro Casas Alvarado, Antonio Verduzco-Mendoza, Karina Lezama-García & Adriana Olmos-Hernandez (2019) Review of different methods used for clinical recognition and assessment of pain in dogs and cats, International Journal of Veterinary Science and Medicine, 7:1, 43-54

6. Borgeat, K. and Pack, M. (2021), Approach to the acutely dyspnoeic cat. In Practice, 43: 60-70. https://doi.org/10.1002/inpr.15