MRI And The Senior Pet
Patrick R. Gavin, DVM, PhD, DACVR (Radiology and Radiation Oncology)
The technology for magnetic resonance imaging is largely based on computer technology and, therefore, this is a rapidly evolving diagnostic method. However, there are some basic requirements that currently make such equipment rather expensive. The expense of the equipment is translated into the expense of the diagnostic procedure. The question is then asked, when is magnetic resonance imaging appropriate, given the relatively large cost? The answer lies in looking at the value of the procedure relative to the cost. This can be judged best based on the following question: Will visualization of the disease process be of benefit in the diagnosis and subsequent treatment options? If the answer to this question is yes, that visualization is important prior to deciding the diagnosis and treatment options, then magnetic resonance imaging may have the value required to proceed. Conversely, if visualization is of no benefit, then this diagnostic test would be inappropriate.
This program will provide you with examples of visualization of many disease processes that affect our senior pets. Virtually any organ system can be visualized; musculoskeletal, genital urinary, cardiovascular, and the nervous system. Magnetic resonance imaging is relative noisy, and therefore, the animals must be anesthetized to remain still during the diagnostic procedure. While there is always some risk associated with anesthesia, current anesthetic agents and anesthesia protocols have greatly diminished these risks, as the animals do not have to be in a deep surgical plane of anesthesia, but must merely be unconscious and unable to hear. At the IAMS Pet Imaging Center in Vienna, Virginia, most of the animals are extubated on the imaging table immediately after the end of the diagnostic procedure.
Magnetic resonance imaging (MRI) is another imaging modality like radiology, ultrasonography, computed tomography, and gamma scintimaging. Therefore, the ‘I’ for imaging is somewhat redundant, but the abbreviation MRI is deeply ingrained in our vocabulary and many of our clients have some appreciation for this modality. A typical MRI study will take 30-60 minutes and produce numerous series of images to aid in the identification of the disease process. An example would be a typical brain series, which utilizes a minimum of seven sequences, each containing 9-20 images. The question, “why so many sequences and why so many images?” will now be answered.
Let us use an example of a senior pet that has recently developed seizures. The pet has been examined and at the time of examination no abnormal findings are made on the physical examination and routine laboratory analyses, including a complete blood count and serum chemistry profile, are within normal limits. The differential diagnosis for such a condition could be epilepsy, metabolic disorders, cerebral infarcts, cerebral infections/inflammation, and brain tumors. Magnetic resonance imaging has the ability to diagnose virtually any macroscopic structural abnormality. The limit of detection is approximately 0.5 mm in diameter. Therefore, MRI could visualize any of the above conditions with the exception of epilepsy and metabolic disorders. Let us examine a typical MRI brain series.
We start the imaging process with a localization series within the magnet, as we need to have precise localization of the brain in all three planes. We will then proceed with numerous sequences in the transverse plane to visualize the brain and surrounding structures. We generally use sequences called a “dual echo” (proton density NT2-weighted images), a fluid attenuated series to make the cerebral spinal fluid dark (called a FLAIR), and a T1-weighted pre- and post-contrast series. If anything abnormal is visualized on the transverse series, we then include post-contrast images in the sagittal and coronal planes. An example of such a study is given in figures 1A through 1G.
It is easy to notice how the various components of the brain change in their appearance from white to dark and there appears to be no rational relationship to each other, yet the visualization of the anatomy and disease process is excellent. How can things appear so different? The answer lies in the fact that this is a new modality and we much use a new diagnostic approach. Magnetic resonance imaging relies on the appearance of the element hydrogen and the bonds to other elements. We must think of chemistry when we think of the magnetic resonance image. The different bonds, including C-H, N-H, S-H, O-H, and perhaps the physical nature of the material, cerebral spinal fluid, brain tissue, collagen, and cortical bone result in the intensity of the signal that will compose the image. The actual physics of MR imaging and the complexity of image acquisition is beyond the scope of this presentation. However, we must try to understand some of the basic principles in order to appreciate this wonderful, new diagnostic tool. I would like to utilize a cooking analogy to describe the MR principles. If one were to view the body and the various organs as ingredients in the refrigeration with which to make a meal, we would then take these ingredients, go to one of our favorite cookbooks and select a recipe. The recipe selected will have a large influence on the meal and its presentations. For example, one could select a sirloin steak, potato, and carrots and have a grilled steak with baked potato and cooked carrots. The same ingredients could be cooked another way, chopped up and mixed together and made into a stew. MR imaging utilizes the same principles. We use various recipes to look at the ingredients of the body and then we “present” the outcome with various techniques.
Often, there is a desire to produce numerous “pretty pictures” with magnetic resonance imaging due to its ability to depict anatomy and disease states so clearly. Actually, there can be a trade off between the resolution of structures and the sensitivity to detect disease processes. Therefore, a typical study will utilize both high resolution sequences and lower resolution high sensitivity sequences. Figure 2A and 2B demonstrate the difference between a high resolution sequence and a high sensitivity sequence. The high sensitivity sequences are often used for the rapid visualization of the abnormality, which coupled with the high resolution sequences allow precise anatomical localization. As we go through the various examples in the organ systems, try to remember that black can become white, white can become black, and anything in between. Try to remember that we are going to be visualizing chemistry and it is easy for us to see the difference in the kitchen between butter and water, steak, potato, and carrots, and it will be just as easy as we view these studies. I would be remiss if I did not mention that images are generally made from sequences that utilize the relaxation properties of these hydrogen bonds. There are two main relaxation properties called T1 and T2 relaxation, which coupled with the density of the hydrogen atoms yield the image elements. Therefore, while we might mention a T1 or T2-weighted image sequence, we must remember that these relaxations are always taking place with these protons (hydrogen nuclei) and they are just different recipes to visualize the structures.
Figure 2: Different image sequences for the visualization of a tumor involving the left scapula. A is a T1-weighted pre-contrast. Notice the excellent visualization of the muscles and adipose tissue within the fascial planes. While this is a pretty picture, it does not depict the lesion as well as 2B, which is a STIR sequence. This fat suppressed heavily T2-weighted image readily visualizes the image. No contrast has been used.
There can be some problems with metallic foreign material creating an artifact in the image of MRI. The metal must be ferrous in order to have a significant effect. Therefore, things such as BB’s and steel birdshot can create a large problem. Other things such as lead birdshot or other lead material creates virtually no significant artifact. There is no harm to the pet if it is placed in the magnet with a BB, as it will not become dislodged, but it will create a large imaging artifact.
We will now look at several examples of magnetic resonance imaging that would be appropriate for diagnosis of a disease process in our senior pets. We will operate under the assumption that visualization of the disease process is important for the diagnosis and subsequent treatment of the patient. I like to think of this as not replacing histopathology, but certainly something akin to an antemorten gross examination. This examination of the body will be better than any exploratory surgery because we will not only look at the center of the organs, but we will be able to slice them up and look at the inside of the organs and then replace everything back in there with no harm. I hope to convince you that we can visualize any organ system, including bone. MRI is extremely well suited to the visualization of abdominal diseases in animals, as the use of anesthesia removes most of the motion artifacts that can occur when human abdominal studies are performed. We can perform high quality studies on the thorax and heart, and if needed, we can even gait these with the respiratory or ECG triggers. Finally, we can even perform angiograms or myelograms without the need of any injections or contrast agents. We can perform our studies to any plane of the body’s axis and if I forget to give you enough information concerning the plane of imaging, please interrupt me and make sure that we all have an understanding and appreciation of the following examples.
Another advantage to magnetic resonance imaging will be the fact that these are digital images and they can be manipulated just as any digital image, changing the contrast, magnification, rotation, etc. Magnetic resonance imaging has improved the conspicuity of lesions. This is the buzz word of the millennium for the radiologists, meaning “objects become more conspicuous.” It is a combination of contrast and resolution, and magnetic resonance imaging clearly out performs the other modalities in this combination.
Central nervous system, including the brain and spinal cord is one of the main systems imaged with magnetic resonance. It provided excellent visualization of all structures. Remember, we can see anything greater than 0.5 mm in diameter. We also will visualize the chemistry of the brain, so we are not so dependent upon the damage to the blood-brain barrier and subsequent enhancement following the administration of intravenous contrast agents. Therefore, lesions that do not enhance are still readily visible on magnetic resonance imaging that would escape other modalities including computed tomography. If we do have to use a contrast agent, the contrast agent used in magnetic resonance imaging is extremely safe and there are virtually no hypersensitivity reactions, again, in contrast to the reactions that can occur with the iodinated contrast agents used in computed tomography. Magnetic resonance imaging for the central nervous system will yield significant information that, coupled with the physical examination, laboratory tests, and often cerebral spinal fluid analysis may be accurate for etiologic diagnosis. Obviously, the proper treatment depends on an accurate diagnosis and where possible, when images can provide and etiologic diagnosis, it is obviously a huge benefit in this system as typical biopsies and histopathology are more problematic with this organ system. In addition, studies that do not depict an abnormality, in conditions such as epilepsy, allow us to have a high degree of confidence that we are dealing with a nonstructural condition and allow the owner to have confidence in the diagnosis, and we have found better compliance with medication or rehabilitation in our epileptic and stroke patients. I will show you examples where we contrast the magnetic resonance imaging to computed tomography in the same patient. I am sure you will agree with me that magnetic resonance imaging clearly depicts the abnormalities that computed tomography failed to visualize.
We will illustrate various disease processes, including common tumors of the brain (meningioma, choroid plexus tumors, and pituitary tumors) and other disease states including cerebral infarction and infectious lesions. The time available precludes a lengthy discussion of any of these processes. However, all of these diseases and more occur in our senior pets. The examples are to illustrate the strength of the diagnostic modality.
Head and Neck
While the brain is often the organ of interest when the head is examined with MRI, any other structure of the head is also well visualized. We will show examples of the use for MRI for nasal disease and oral masses including lymph node detection. MRI has proven to be the best modality for visualization of neoplastic diseases. Identification of the gross extent of the tumor is necessary for proper treatment planning, whether the treatment will involve surgery, radiation therapy, chemotherapy, or any combination. Your speaker is also a radiation oncologist and will not treat a neoplastic condition other than skin tumors without the benefit of an MRI examination of the disease process. Conditions can be upgraded or downgraded significantly based on their MRI appearance. While computed tomography is often used for treatment planning, computed tomography generally visualizes the effect of the disease process on the skeletal system and is far inferior to the visualization of the nonosseous components of the disease process. Contrasting examples will be provided.
MRI is the only modality that allows direct visualization of the spinal cord and surrounding tissues. Conditions that we used to diagnose, based on the inability to find anything else, including fiber cartilaginous emboli and other conditions, are now readily visualized. We will use examples of spinal MRI to show common disk disease, tumors affecting the spinal cord and spinal nerves, tumor affecting the spinal column, and conditions that we used to be unable to diagnose including syrinx and hydromyelia.
The examples of MRI for the detection of disk herniation allow easy visualization of the disease process. Many would argue that myelography could indicate this disease process. We have found that magnetic resonance imaging far surpasses myelography’s ability to detect even simple disk disease. In addition, magnetic resonance imaging provides us with precise localization of the disk material and allows the surgeon to perform the hemilaminectomy on the side where the majority of the disk material is, greatly facilitating removal of disk material, which we believe is important to the patient outcome. We never perform myelograms in the dog or cat at Washington State University unless there is a metallic foreign body creating an artifact precluding MRI.
I used to consider spinal tumors (of the spinal column and of the spinal cord) as rare conditions. Magnetic resonance imaging has greatly improved visualization of these conditions and they are much more frequent than previously recognized. Early diagnosis of some of these lesions has led to early treatment, including surgery and post-surgical radiation therapy and has greatly improved the outcome in some cases. We will also show examples of a magnetic resonance myelogram, which again, is without the injection of contrast agent within the spinal cord; it is merely a depiction of the cerebral spinal fluid on the outside of the spinal cord. I believe you will agree with me that this myelogram is equal to and generally superior to any contrast myelographic procedure.
Musculoskeletal magnetic resonance imaging for our senior pets has significant benefit. If you or I were to injure our knee today, most of us would have an MRI examination prior to any surgery, including arthroscopy. The reason is simple; complete knowledge of the disease process prior to surgery is of benefit to the surgical approach and subsequent outcome. Examples of musculoskeletal MRI for the senior pet including diagnosis of conditions such as supraspinatous tenosynovitis, bicipital tenosynovitis, cranial cruciate ligament rupture, meniscal rupture, and tendon injures. Tumors of the musculoskeletal system that would evade radiographic detection are also readily visualized. We must remember that early diagnosis with these conditions generally will lead to an improved prognosis and outcome. Many or our diagnoses in the past have been based on secondary arthritic changes that occur in the musculoskeletal system and not on visualization of the primary process. Visualization of the primary disease process may allow us to have earlier treatment and improve our outcome. Use of MRI for foreign body detection is also extremely advantageous and examples will be shown.
Abdominal and Thoracic Studies
Use of MRI in the abdomen and thorax is in its relative infancy in the human field. It is actually developing more rapidly in the veterinary profession due to the anesthesia and lack of motion artifacts. We will show how magnetic resonance imaging can visualize all the organs in the abdomen in their entirely. It is an excellent modality for the detection of metastasis from neoplastic processes. It is the best method for visualization of adrenal masses and their potential impact on the caudal vena cava and for other conditions of the abdomen including the genital urinary system. In the thorax, we will show examples of anterior mediastinal masses that can evade detection with radiography, computed tomography, and even ultrasonography.
Cardiovascular MRI is rapidly developing the human field and will probably replace coronary angiography in the future. While coronary arterial disease is not a major problem in our age pets, other conditions such as heart based tumors and aortic body tumors are, unfortunately, not rare in our senior pets. MRI has the ability to detect these lesions and their extent, which again aids in the selection of proper treatments.
I will close with some examples of our smallest patients studied, including nontraditional pets such as the iguana, and some of our larger patients that we study including the horse that can weigh up to one ton.
I hope this lecture has provided you with the insight into the vast
potential of magnetic resonance imaging for numerous body organ systems
and the detection of various diseases of our senior pets. My telephone
number and e-mail address can be found at the end of the notes.
If you have any questions or desire additional information, please do
not hesitate to contact me.