Introduction

As technology in medicine advances, and people become more aware of therapeutic options available for their pets, it is important as a profession that we veterinarians take an active role in understanding what tests we can offer our clients. Veterinarians are more aware than many professions that we have to balance cost with the importance of accumulating the information for our clients that can assist them in making important treatment decisions. Imaging of various types can be an invaluable tool for understanding cancer especially. Imaging plays a vital role in staging an animal, or helping to determine the extent of disease. What type of imaging we elect to offer depends on a lot of different factors.

When offering a specific type of imaging, it is important to understand the different advantages and disadvantages of each type of technology and determine if the information gained is worth the cost and degree of invasiveness of the procedure. The goal should always be to reach the most definitive diagnosis as possible regarding the extent and severity of the disease, with the least amount of invasiveness to the patient and at the least economical impact to the client. This is the cost:benefit ratio. In order to determine this, one must understand the sensitivity and specificity of a test, the unique information the test will provide and what its limitations and risks are.

The most common type of imaging, that is available at most practices, is radiographs. Many practices also have an ultrasound (US) machine. At specialty practices, 3 dimensional imaging such as computed tomography (CT) and magnetic resonance imaging (MRI), is becoming a more popular option. Only a few academic institutions have access to nuclear imaging and positron emission tomography (PET). The basics will be discussed in another lecture to help in understanding the benefits of referring for this type of testing.

Radiography

Radiographs are a 2D map of densities. We can distinguish 5 basic opacities on radiographs - air, fat, soft tissue/fluid, mineral and metal. Note that on radiographs, fluid and soft tissue look the same, so they are going to summate, meaning that you can't distinguish between the 2 of them. This is why if you have pleural effusion you won't be able to determine where the borders of the heart are. Or why if you have pericardial effusion, the heart just looks big and round, versus if you have an intrapericardial lipoma you can see the fat around the heart.

We take radiographs by directing a beam of low energy radiation at the body part we are interested in. Organs that are higher density (e.g., bones) versus organs that are lower density (e.g., lungs) stop more of the radiation. There is a detector (film or electronic) on the other side of the patient that catches all the radiation that makes it through. If almost no radiation makes it through the detector stays white, but if lots of radiation makes it through the detector turns black in that area. Everything else in between is variations of gray.

Advantages of radiographs:

 Best spatial resolution of any of the imaging modalities

 Can easily see how close one organ is to another organ, or measure how big something is quickly

 Generally inexpensive to obtain

 If you have a lot of contrast between the 2 organs you are looking at (e.g., ribs and lungs side by side), they are pretty easy to interpret

Disadvantages of radiographs:

 See shadows but not internal organ detail

 Not good for areas with low contrast

 May miss early or small lesions that do not change contour of organ

 Can't see complex anatomy due to superimposition

 30–50% bone loss must occur before it is radiographically visible

Ultrasound

Ultrasound creates images by sending sound waves into the body. These sound waves are reflected by various organs, and the returning sound waves are measured for time of return and strength to help create an image.

Ultrasound is particularly useful for imaging and staging organs in the abdomen. Remember, on radiographs all the organs look pretty much the same because they are the same opacity. On top of that fluid can summate with organs making evaluation difficult. Ultrasound can easily distinguish between different organs and pick up subtle changes in the parenchyma.

Ultrasound is not useful for imaging the lungs because air reflects sound waves (reverberation). It also can't get past bone and won't image deep to mineral (acoustic shadowing).

Advantages of ultrasound:

 Distinguish fluid from soft tissue

 Evaluate parenchymal changes

 Non-invasive

 Awake or sedated animals

 No radiation

 Ultrasound guided aspirates

 "Poor man's CT"

Disadvantages of ultrasound:

 Requires advanced skills

 Can't see deep to lung or bone

 Can't get the "big picture"

 Poor spatial resolution

 "You had to be there"

Computed Tomography

Computed tomography works similar to radiographs in that it produces a map of electron densities in the body. But instead of producing a flat 2D picture, it creates a 3D series of images, much like slices in a loaf of bread. These images are also obtained by lower energy radiation beams that are directed at a patient. But rather than having a single detector underneath an animal, there are a ring of electronic detectors around them.

CT is a superior way to evaluate the different tissues in all areas of the body in comparison to radiographs. Rather than seeing just 5 different opacities, we can see 1000's. We can measure densities quantitatively using something called Hounsfield units (HU's) to determine if something is fluid, soft tissue, mineral or metal. We can also give contrast and obtain a post contrast scan to help us determine how vascular different organs are.

CT advantages:

 CT identifies ∼2000 densities

 Radiography identifies only 5

 CT eliminates superimposition of structures

 Can view different windows

 Lung, soft tissue, bone

 Can 3-D reconstruct the image

CT disadvantages:

 General anesthesia

 Availability

 Cost ($1000–1500)

 Higher radiation doses to patient and personnel

 Spatial resolution << radiography (but this is far outweighed by other positives)

Magnetic Resonance Imaging (MRI)

MRI, in the simplest terms, measures water content in the different areas of the body. It accomplishes this by using a strong magnet. When the magnet is turned on, all of the water molecules in an organ line up together. When the magnet is turned off, those water molecules fall back into their natural disorganized form. The time it takes for an organ's water molecules to return to normal, as well as the number of water molecules in an organ and their orientation, create an image. MRI has superior soft tissue definition in comparison to all other imaging modalities. MRI is definitively the best way to image complex structures such as brain and spinal cord. On top of its superior soft tissue imaging capabilities, different, naturally occurring artifacts can be exploited to help us determine the composition of different tissues.

MRI advantages:

 Excels in soft tissue imaging

 Method of choice for brain and spinal problems

 Cancer - margins and staging

 Musculoskeletal/soft tissue injuries and disease

 No radiation exposure

 Metabolic imaging

MRI disadvantages:

 General anesthesia

 Availability

 Time

 Cost ($1000–1500)

 Spatial resolution << radiography

 No metal!

In conclusion, imaging is an invaluable part of treating cancer. It is used to diagnose, stage and monitor for response to treatment or disease progression. There is a wide variety of imaging modalities available, all with their own strengths and weaknesses. Our goal as primary clinicians is to determine what is the best way to achieve our goal with the least invasiveness to our patient and with the least economic impact on our clients.