X-ray & Ultrasound

What is the difference between X-ray and Ultrasound?

An X-ray uses charged electrons moving through specific parts of the body to cast an image onto an x-ray film.  This film is then processed to create a picture for analysis. Depending on the density of the part of the body we are looking at, different volumes of electrons are able to make it through the tissue onto the plate.  Bone is a very dense tissue and is able to stop all electrons passing through and so shows up white, whilst air has no density and allows all electrons through, showing up black. 

An Ultrasound Machine uses high frequency sound waves pulsing into parts of the body and records an image from the returning sound waves, the echo.  This echo is different depending on the density of the organ we are looking at.  The number of sound waves bouncing back depends on the softness of the organ, or if there are enough particles present to reflect the sound waves.  For example, water is black, muscles are grey, whilst air and bone are identified by being a solid white line.

Which type of imaging is better?

Neither imaging modality is “better”, which is why we use both.  Each technique has both strengths and limitations.   

Ultrasound

Broadly, ultrasound is great because it allows us to see organs and body parts in “real-time” and also allows us to look “inside” organs.  We can see the intestines moving, the heart beating and peer inside the bladder.   We can then take measurements of wall thicknesses, look at urine density and other techniques used to evaluate your pet’s health.  Doing this regularly we can track changes and monitor disease progression.  This last part is commonly used in heart-diseased pets, from which we can tailor medication to help keep your pet healthy. 

Whilst this is great, there are three notable limitations to ultrasound.

1. The first is that we can only look at very small parts at any one time.  This is like trying to look at a landscape when you can only look through a telescope.  Whilst possible, the wider impact, or bigger picture may be harder to put together. 

2. The second is that we can only look at a single “slice” at a time.  Imagine trying to determine the shape of a donut when all you are being shown is sliced sections in a straight line; it is not easy.  To accommodate, through practice and constantly moving the soundwaves, we can manage this limitation, but it does mean activities such as accurately counting puppies in a pregnancy, are all but impossible. 

3. The final limitation is that the ultrasound sound waves are unable to pass through bone or air.  This makes everything beneath the bone or air invisible to the ultrasound.  If we are concerned about lungs, or bones, then ultrasound is useless unless the disease is right on the surface and not containing air or bone!

X-ray

X-rays are a well understood method of getting images of inside the body.  An x-ray is great for bone and air, where the contrast between the two help to identify obvious and subtle changes. An X-ray can be used both as a means for a survey picture, such as screening lungs for potential disease spread, or as a focused picture, when needed to accurately measure bones to prepare for surgery.   A big benefit for x-rays is that we can change both how many electrons and how energetic they are when taking images.  This allows us to visualise many different densities of tissues that could be inaccessible to other imaging machines such as ultrasound. 

X-rays have two major limitations. 

1. The first limitation of x-ray is that it is not overly detailed when looking at internal organs of the abdomen, known as soft-tissues.  Many soft-tissues are full of water, and so from an x-ray perspective they can look a similar “colour” on the image. Because of this, we learn what normal silhouettes look like and are able to identify both normal and strange structures from these shapes.  

2. The second limitation is that we are looking at a 3-dimensional body on a 2-dimensional plate.   This results in body parts overlaying each other and can put strange lines, or cover joints, complicating the interpretation.

What can you do to overcome their limitations? 

Knowing what each machine is good for and what the limitations are is important to ensure the right technique is used for the right condition.  

A rule of thumb is X-ray is used for evaluating bone and lungs, whilst ultrasound is used for the abdomen. In many cases use of both machines is preferable because they complement each other beautifully.  For example, an x-ray can survey the scene whilst and ultrasound can focus on specific organs and their internal structures. 

What other types of imaging machines are there?

Other types of imaging machines more commonly used in veterinary diagnostics include magnetic-resonance imaging (MRI), computed tomography (CT), gamma-scintigraphy (bone scans), endoscopy and fluoroscopy.  

Such machines are very expensive and require highly trained teams to work them.  As a result, should your pet need the advanced imaging we would look to refer you to a specialist hospital.  Luckily this is not a common occurrence but knowing that the option is there is great for peace of mind.

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