When it comes to imaging modalities, we have plenty of options. That can be an advantage when working up a challenging case, but it can also be overwhelming! In some cases, it may be difficult to determine which imaging modality is going to give you the most benefit for a particular patient.
Each imaging modality produces a different image, even when you’re imaging the exact same body part or object. The trick lies in determining what type of imaging will give you the most useful information for a particular case.
Although there are a wide variety of imaging modalities available, ranging from contrast studies to scintigraphy, let’s focus on the big four: radiographs, ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI).
An x-ray machine is available in nearly every veterinary clinic, without a need for referral. All veterinarians are trained in how to take and interpret radiographs. Therefore, radiography is the first imaging modality to be used for the majority of patients.
Radiographs are created by passing x-rays through an object, onto a screen. They can be helpful in assessing both soft tissue and orthopedic conditions. However, radiographs are limited by the fact that they cannot distinguish between fluid and soft tissue, or between tissues of different densities. They’re a great initial screening test, but not always adequate for a diagnosis.
Ultrasound was introduced as an imaging modality in the 1990s. This modality utilizes high-frequency sound waves, which are created by hitting crystals with electrical impulses to induce vibration. When the ultrasound transducer is placed in contact with the skin, these sound waves travel into the body. Images can be collected from these sound waves in several ways, depending on the ultrasound mode that is used, but all are based upon how sound waves travel through tissues of different densities.
In recent years, ultrasound has become more accessible to veterinarians in general practice. While ultrasound was a referral procedure just 10-20 years ago, a growing number of general veterinary practices now have their own ultrasound unit. Even in practices without an ultrasound unit, an increased availability of mobile ultrasonographers has made ultrasound accessible to a large number of clients, without the need to travel to a referral hospital.
Ultrasound is typically regarded as the best test for workup of cardiac conditions (echocardiography). Additionally, ultrasound is often utilized to evaluate the urinary tract and reproductive organs. Focused Assessment with Sonography for Trauma (FAST scans) is used by many emergency clinics in the management of trauma cases. Finally, ultrasound is often used when screening for abdominal abnormalities, such as neoplasia.
A CT scan incorporates a series of x-ray images, taken at a variety of different angles. The x-ray beam travels through a 360-degree arc, creating cross-sectional images on a series of detectors. These images are combined together, using a computer program, to create individual “slices” through the body.
CT requires referral to a specialty hospital or a veterinary teaching hospital. In many cases, CT can be performed with heavy sedation, avoiding the need for general anesthesia. CT imaging is also a relatively rapid process, which means that imaging and management can be combined in a single anesthetic/sedation episode. CT imaging is also typically less expensive than MRI.
Because CT imaging uses x-rays, it is best for discriminating between different tissue densities. Like a radiograph, tissues of similar densities cannot be distinguished on CT. Additionally, metal hardware in the area of the scan (in a patient with a prior history of surgery) will interfere with imaging.
Common applications of CT imaging include evaluation of trauma and the workup of thoracic or abdominal disease. CT is also often used for whole-body tumor grading.
An MRI uses a powerful magnetic field, combined with radio frequencies, to create an image of internal structures. MRI relies upon differences in the magnetic properties of tissues, not differences in density. Both T1- and T2-weighted images are collected, with T1-weighted images emphasizing anatomical characteristics and T2-weighted images emphasizing differences in fluid content.
MRI requires referral to a specialist or a veterinary teaching hospital. MRI imaging is time-consuming, potentially taking up to two hours, and requires general anesthesia. Because MRI imaging takes a long time, imaging cannot typically be combined with other anesthetic procedures (such as surgery). MRI is also often more expensive than CT imaging.
MRI is typically considered the “gold standard” for evaluating the central nervous system. Both brain and spinal cord imaging are typically performed using MRI, when possible. Additionally, MRI is superior to CT in diagnosing subtle orthopedic abnormalities, because MRI often provides more information about bone structure. MRI may also be useful in the evaluation of ligaments, tendons, and menisci.
While all of these considerations are valuable, there’s one thing to keep in mind: the best imaging modality is the one that the client will allow you to perform (as long as it’s expected to provide helpful information). The reality is that advanced imaging modalities such as CT and MRI may be out of the financial reach of some veterinary clients. Some clients may be able to pursue a CT but not an MRI, which means that you will need to determine whether a CT will provide enough information to be a worthwhile pursuit. As a veterinarian, your role is to recommend what’s best and then work with the client to provide the best possible care for their pet.