To CT or to not CT - that is the question?

Dr Geoff Gibbons

The cost of a $1500 CT can sometimes have pet owners (and even their regular vet), questioning the need for expenditure.  In the below article, our Medicine Specialist Dr Geoff Gibbons, shares some case files and images, to show that sometimes this tool can even save unnecessary spending or treatment.

Radiographs are useful, but do you want a diagnosis?

CT

Having now worked with a CT unit in the Riverina, Adelaide and now in Newcastle at the Animal Referral and Emergency Centre, I am absolutely convinced of its value.
For instance, when investigating nasal discharge now, I never radiograph but go straight to CT before scope and biopsy.  Both procedures require a general anaesthetic, so the actual CT procedure fee is just over double that of the radiograph fee.  The difference is that frequently the CT provides a diagnosis and the radiographs do not.  The savings in unnecessary Vet and hospital time can be substantial.

Examine this diagnostic quality radiograph of a small dog with a unilateral nasal discharge:

Radiograph

Consider the changes you see and the possible aetiologies.

Radiograph 2
Radiograph 3

Now examine these two CT images:

The extensive, destructive, inoperable neoplasm is now obvious.  Radiation (not generally available) is the best treatment, but the prognosis is still poor to hopeless.

“Devil” an 11.5 year old, desexed male, domestic short hair, 6 kg cat was presented dyspnoeic with a three month history of weight loss.  Chest drains were inserted to drain the pleural fluid, the working diagnosis was pyothorax and Devil was referred to the Medical Specialist.  The cytological diagnosis was a minimally degenerate, inflammatory exudate which was not fully typical of pyothorax.  Radiographs were taken.

Radiograph 4

The specialist radiologist’s report included:

Conclusions:
1. Mild bilateral pleural effusion. 
2. Patchy non-consolidated alveolar pattern and diffuse unstructured interstitial lung pattern.
3. Documentation of pleural drainage tubes.
Additional comments:
The main differential diagnosis for the pulmonary changes is pneumonia with secondary pleural effusion.
Other differentials such as neoplasia or heart failure are unlikely given the history.
The reasons for this conclusion can be seen. However, as all the findings still did not quite fit, a CT was performed.

The same specialist radiologist’s report included:

Radiograph 5
Radiograph 6

Conclusions:
1. Moderate bilateral pleural effusion.
2. Diffuse pleural enhancement with space-occupying nodular lesions.
3. Sternal lymphadenopathy.
4. Atelectasis lung lobes.
5. Nodular lung pattern. 

Pleural neoplasia, such as mesothelioma, lymphoma, or carcinoma is very likely.
Pulmonary metastases are highly suspected.

Thus, prolonged and unsuccessful treatment for pyothorax and pneumonia was avoided.  The aerobic and anaerobic cultures subsequently came back negative and the cytology failed to identify any bacteria within neutrophils.  Lymphoma would shed cells into the pleural fluid but the other two probably not and they are very much less susceptible to chemotherapy.  Note also that the pulmonary metastases are 3 mm diameter and are visible with the CT but not in the radiographs.

CT is very useful in other cases too:

  • Ectopic ureters and other potentially surgical urinary tract problems
  • “Cancer hunts” particularly when metastases have to be ruled out.  Osteosarcoma could be an exception.
  • Brain tumours.  MRI, if available, is generally better for neural tissue but many brain tumours have been found on CT.  CT cannot identify the GME group of diseases.
  • Porto-caval shunts.  CT is essential to guide surgery.
  • Elbow dysplasias
  • Complicated fractures, particularly those involving joints
  • Vascular studies such as vascular ring anomaly
  • Contrast myelogram CT for spinal disc prolapse
  • CT-guided aspirates of eg disc lesions