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Why Does My Dog Need a CT Scan, Not a Simple X-Ray?

We’re very lucky in veterinary medicine to have a wide-range of high tech equipment used to help diagnose and treat disease. One of the most cutting edge is a CT scanner. You might know that CT scans are related to x-rays, but why might your dog need one rather than a simple x-ray? Read on to find out.

What is a CT Scan? How Does It Work?

Computed Tomography, (also known as CT or CAT scanning), is an imaging mode used to look inside a patient at their bones, fluid and soft tissues. Other imaging modalities include x-rays, ultrasound and MRIs. The simplest explanation is that CT scans are three-dimensional versions of two-dimensional x-rays. 

CT scans work in practically the same way as normal x-rays 

This is particularly noticeable when compared to modern digital radiography. An x-ray machine powered by electricity is used – this has two parts, the cathode and anode. At the cathode, tiny particles called electrons are produced. These are attracted at high speeds to the anode, where they react and release their energy in the form of electromagnetic x-ray waves. These are then directed towards the patient.

X-rays are very high energy meaning they can pass through the different tissues of a body. However, every time they do this some energy is lost. The denser the tissue, the more energy is lost and the more the x-ray waves are reduced, or “attenuated”. The x-rays pass out the other side of the body and are detected by a sensor or plate. This reads the energy of each x-ray wave and converts it to a visible image on a computer screen. The more x-rays that hit one area of the sensor, the darker the image, and the less must have been blocked by the body. This is why air is black and bones are white on normal x-ray images.

Where CT scans differ significantly is that the x-ray machine rotates around a patient, taking multiple images, or slices, of the patient

Each view is a single 2D x-ray, but when combined makes a whole 3D image. A full CT scan can therefore produce a 3D model of an entire body part or even a whole patient in minute detail. Software can also be used to manipulate the 3D image further. Because of the duration a patient must be inside a CT scanner and the importance they do not move during the scan, most veterinary patients must be anaesthetised for a CT scan.

How Does It Compare to an X-Ray?

CT scans hold several advantages over traditional 2D x-rays. 

Firstly, by being three-dimensional, there is no risk of superimposition

This is where two or more tissues overlie each other on a 2D x-ray; which can result in problems being missed or something being overinterpreted. A common place this happens is in the skull. CT effectively eliminates this.

Furthermore, CT scans have a lot higher resolution than x-rays

This means that finer details can be seen, meaning subtle changes are less likely to be missed. CT scans can also better distinguish between two very similar density structures, such as different soft tissue densities of abdominal organs. Traditional x-rays are not often useful when looking at the abdomen for this reason. 

CT has also allowed some more invasive procedures to become superseded, such as some endoscopies, angiograms, barium swallows and more. This means there is less risk to the patient and greater information can be gained. 

However, CTs also have a few disadvantages compared to normal x-rays. 

Because CT scans take multiple x-rays, the exposure to ionising radiation to a patient is generally higher than a single x-ray

X-rays are ionising radiation – this means that exposure damages a living thing’s cells. Over time, this can lead to a greater risk of cancer. However, even with CT the risk is still comparatively low and the benefits of the scan are almost always worth it. But the risk does mean veterinary staff must exercise extra caution around CT scanners. And patients may have limits on the number of CT scans per year. Furthermore, multiple x-rays are often needed to view a complete body part, meaning in some cases a full CT scan may result in less radiation exposure compared to multiple x-rays being taken. 

CTs also have the disadvantage of cost

A good CT scanner could cost a veterinary practice many hundreds of thousands of pounds. These costs will need to be offset to the client somewhat. This means that a CT scan will cost more than an x-ray. The cost is justified in the information a CT scan will reveal, but it may not be affordable for some patients. An x-ray may cost £100 or less, but a full CT scan can reach £1000 or more.

Finally, as we discussed above, most pets will need to be sedated or anaesthetised for a full CT scan

While certain x-rays can be taken conscious, for very old, very young or unstable patients, this may mean a CT scan is not always practical or safe.

X-rays still bring immense benefits to vets, and they won’t likely be disappearing from your local practice anytime soon. But let’s next discuss some scenarios where a CT scan may be more appropriate than an x-ray.

When is a CT Scan Better Than An X-Ray?

Damage to Bones

Broken bones are one of the more common accidents seen in dogs and cats, especially after falls or being hit by a car. Most of the time diagnosis needs imaging, and here CT could be advantageous, especially for more complex fractures. Where a single-slice x-ray may only show one or two sites of damage, a whole CT scan will reveal much more information about the state of the bone, including subtler damage that may otherwise be missed. As well as for diagnosis, CT scans can be useful for fracture repair; so that orthopaedics can be tailor-made to fit a dog’s bones perfectly. CT scans of bones are also useful in looking at the progression of dysplasia, osteoarthritis and other similar diseases. 

However, it is important to remember that radiography has worked for decades in diagnosing and helping to treat bone diseases. In many cases a simple x-ray or two is sufficient. One significant advantage for radiography is its speed. For example, if a vet needs to check whether orthopaedic implants are in place properly, they will be unlikely to use a CT scanner unless they need pinpoint precision. 

Viewing Tumours

Unfortunately, a large percentage of dogs will get cancer in their life. Veterinary science has advanced to the point where many tumours are now treatable with advanced chemotherapy, radiotherapy or surgery. But to reach this stage, we have to know what kind of tumour and where the tumour is. 

CT can be of great help here, particularly for soft tissue masses that cannot be distinguished easily by x-ray. A CT scan will give precise detail of where the tumour is, how far it has spread and how invasive it is. These all directly affect the outcome of the disease. For example, a tumour on the cheek may look simple, but a CT scan could show it has invaded the bones of the face, altering how a surgery is performed. Many tumours like to spread away from the primary mass (metastases), and these secondary tumours can be difficult to spot, especially if they are inside an animal’s body. A CT scanner is precise enough to spot tiny changes that could indicate metastases. During treatment, repeated CT scans may allow vets to view changes that hopefully indicate the cancer is in remission, and alter therapy if not. 

Spine and Skull Imaging

Though x-rays are still of great use when imaging the legs of an animal, there have always been difficulties with imaging the head and spine. This is because there are many bones in these areas, resulting in a lot of superimposition when taking a traditional x-ray. As above, this creates risks of overinterpreting or missing critical information. CT eliminates this issue by viewing the whole skull or spine in 3D, providing excellent detail and stopping superimposition completely. For complex spinal or skull surgery, this makes CT scans essential. 

One more common example is the diagnosis of damage to the spinal cord, such as after disc disease. The spinal cord is soft tissue and will not show up on an x-ray (and it would be masked by superimposition regardless). A CT scan can show the exact dimensions of the spinal cord canal where the cord sits. And could show if there is calcification as a result of disc disease (although MRI is much better for spinal imaging even than CT). Previously, diagnosing disc disease on an x-ray was sometimes not even possible in all cases. A CT scan can also show the tiny cartilage structures of the nose in exquisite detail, something not possible with x-ray, allowing nasal diseases to be diagnosed.

Viewing the Lungs

Being large thin-walled sacs filled with air, there isn’t a lot in lungs that shows up on x-ray. In fact, we normally have to look at the blank black space to work out where the lungs are. Nevertheless, chest x-rays are useful when looking for damage to the tissue, fluid, or growths. But chest x-rays have similar issues to other sites in that there is a lot of superimposition, and not everything will show up on an x-ray. CT scans have revolutionised imaging the chest. 

Where before tiny nodules or inflammation were invisible on x-ray, they can now be seen and their exact location determined. This has made diagnosis of certain diseases, such as lung cancers, bronchitis, Westie lung and pneumonia much easier. For cancer in particular, a CT scan can identify spread to the lungs earlier than traditional x-ray; meaning therapy can be started promptly, giving a better outcome.

Abdominal Imaging

If you’ve ever seen an x-ray of someone’s abdomen, you probably won’t be able to identify much. Though there are subtle differences, and diseases can make some organs more prominent, most of an abdomen just looks uniformly grey on an x-ray. Not so on a CT scan, which can distinguish the different textures of the organs much more readily. This means a CT scan can look for inflammation, damage, cancer and more. Ultrasound is also a good imaging modality for abdominal imaging, but it lacks the instant 3D capabilities of a CT scan (a good ultrasonographer can view an organ in multiple dimensions but this takes practice!).

Final Thoughts

Though not every vet practice has a CT scanner, they are becoming more and more common. Many patients can access one via referral if their general practice does not have one. CT scanners have revolutionised how we diagnose and treat certain diseases, and even means previously non-diagnosable or treatable diseases are now possible to treat. And the science is only getting better and better, while costs may start to decline and CT scanners become more common. The next time your dog needs some investigation, ask if a CT scan is something that might be useful. 

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