Magnetic resonance imaging in dogs: main indications
Introduction
Diagnostic imaging techniques are essential in the veterinary clinician’s everyday practice. Technological advances mean that these techniques are now indicated for a lot more situations than 30 years ago. Most small clinics now have X-ray equipment and either directly or indirectly offer ultrasound services. In addition, an increasing number of medium-sized centres and large hospitals provide advanced diagnostic imaging techniques, such as computed tomography and magnetic resonance imaging.
Magnetic resonance imaging was first used in dogs during the last few decades of the 20th century, initially in a few research centres and later in large referral hospitals. However, since the beginning of the 21st century, their use has increased exponentially.1,2
Although the use of magnetic resonance imaging (MRI) in dogs has offered a breakthrough in the diagnosis of various diseases, we must remember that it is not a perfect diagnostic method, and it has its limitations. So, what are the correct indications for MRIs? The improper use of MRI can put the patient at risk, entail unnecessary costs for the client and detract from the technique’s usefulness.2
Basic principles of magnetic resonance imaging
MRI produces high-quality tomographic images of patient anatomy in any plane, without having to resort to the use of ionising radiation. The images are obtained by using a powerful magnetic field to study the interaction between protons in the hydrogen atoms of organic tissues/fluids in the area being examined and computer-generated radio frequency waves. The radio waves cause excitation then relaxation in the protons which temporarily alters their alignment relative to the magnetic field, finally they return to their original position when the radiofrequency pulse stops. The energy generated during the process is captured by the MRI system to produce the corresponding images. Different organs have a different appearance in MRI images depending on the proton density of each tissue and whether the images are T1-weighted (loss of longitudinal magnetisation) or T2-weighted (loss of transverse magnetisation).3,4
Consequently:
- In T1-weighted images, fat appears hyperintense (brighter) and liquids are hypointense (darker).
- In T2-weighted images, free water and fluids appear hyperintense, while fat is darker.
- Contrast agents, such as gadolinium, can also be used. These modify the relaxation properties of the tissues, thus allowing the differentiation of tissues that would otherwise produce a similar image.3
- Gadolinium is not registered for use in dogs. As such, it is important to obtain prior informed consent.
- Although reactions to this drug are rare, anaphylaxis, respiratory and cardiovascular signs, and seizures (more frequent in patients already suffering from seizures or those with intracranial lesions) have been reported.
- As it is excreted through the kidneys, caution is advised in patients with severe kidney disease.4
Indications for magnetic resonance imaging in dogs
In its early days, magnetic resonance imaging in dogs was used exclusively for neurological diagnosis. However, there are now many other indications. Nevertheless, given that MRIs on dogs generally involve having to travel to a specialist centre (although more and more centres are offering the service), general anaesthesia and fairly significant financial costs, other techniques should be to make an accurate diagnosis whenever possible.3
In any event, the indications for performing an MRI in dogs include:
- Assessment of patients with problems affecting the central nervous system, spinal cord and spine.
- Neoplasms: nasal, and when assessing patients with signs of cancer in the chest and abdomen.
- Orbital or optic nerve diseases.
- Assessment of the tympanic bulla.
- Musculoskeletal disorders where other techniques cannot be used to diagnose and assess different types of vascular abnormalities.
- Finally, magnetic resonance imaging has also occasionally been used in cardiology.4,5
Conclusions
Magnetic resonance imaging is a very powerful diagnostic tool, but it does have limitations. Patients must be selected carefully and the possibility of making an accurate diagnosis with another technique studied before referring them for an MRI. Interpretation is a key step in the magnetic resonance imaging process. Although a clinician may acquire experience in this field, whenever possible the results of MRI scans should be assessed by specialists in diagnostic imaging.
