Application of Magnetic Resonance Imaging (MRI) in Veterinary Practices
Magnetic resonance imaging, more commonly known as an MRI, is a highly useful tool used in veterinary medicine when more traditional diagnostic measures cannot provide an accurate diagnosis for diseases in the brain or the spinal cord. An MRI is a highly versatile tool for diagnosing diseases in the brain or the spinal cord, also known as magnetic resonance imaging. An MRI is particularly useful for looking at soft tissue structures, and it is more detailed than getting a computed tomography (CT) scan. An MRI can detect and help your vet diagnose many problems, including tumors, inflammation, stenosis (narrowing of a part of the body), herniated discs, tracking nerve fibers, typing brain tumors without needing a biopsy, assessing tumors for malignancy, and determining what type of treatment a tumor may need (surgery or radiation therapy).
Magnetic resonance imaging (MRI) is the most advanced imaging technique used in veterinary medicine. MRI provides detailed images of internal structures and reveal subtle abnormalities that cannot be identified with radiographs or CT scans. In some disease processes, MRI is the only imaging technique that can permit an accurate diagnosis. It is also the safest imaging modality for can evaluation of the spinal cord. MRI is considered the gold standard of diagnostic imaging for diseases of the brain and spinal cord.
Magnetic resonance imaging (MRI) is being used with increased frequency in veterinary medicine because of its ability to produce high-contrast, anatomically detailed tomographic images. Unlike radiography and computed tomography (CT), which use attenuation of an x-ray beam to create an image, techniques that use magnetic resonance (MR) do not reflect changes in tissue density. Instead, MR uses the electromagnetic signal emitted from protons in different tissues.
Magnetic resonance imaging (MRI) is an advanced imaging technique more and more available in the veterinary market. MRI system doesn’t use ionizing radiation but needs a magnetic field, radiofrequency pulses, and a computer in order to produce an image. The patient is placed in a magnetic field and protons in tissues are used to create a signal, which is detected by receiving coils and then encoded and processed to produce an image. Magnets producing the magnetic fields can be classified by their design (permanent, resistive and superconducting) or by their field strength (low field, high field). The clinical applications of this modality strongly depend on the type of machine and settings available. Low-field MRI units can provide adequate results in small animal practice but have some limitations such as limited field of view, long time of acquisition, and lower image quality, as compared to high-field units. On the other hand, a low-field MRI machine is certainly cheaper and has lower maintenance cost as compared to a high-field MRI system. A high-field MRI unit can provide exquisite contrast resolution and is capable of acquiring images with a large field of view in a limited amount of time.
What Is MRI?
- Magnetic Resonance Imaging (MRI) is a diagnostic imaging procedure that uses magnetic fields to look inside of humans and animals.
- Highly detailed images are generated from magnetic signals, which experienced veterinary radiologists then view and interpret via computer, allowing our clinicians to more accurately determine causes and locations of animal diseases and abnormalities.
Why Use MRI Technology?
Many diseases and conditions can be better diagnosed using MRI technology. Here are some examples:
- Neurologic diseases or cancer
- Brain, spinal cord and peripheral nerves
- Imaging of the spine for tumors, infection, trauma or intervertebraldisc disease or herniation
- Musculoskeletal abnormalities including, tendon, ligament or muscular tear, tumor or infection
- Hepatic neoplasm and metastatic disease
- Renal, adrenal, pancreatic neoplasm
- Portosystemic shunt or other congenital or acquired vascular abnormalities evaluation
- Vascular thrombus or tumor
- Surgical planing
How MRI Works
How MRI scanning works
Magnetic resonance imaging (MRI) is a diagnostic tool utilised to create highly detailed images of the structures within the body. In order to understand how the MRI scan is produced, it is important to be aware of atomic structure.
An atom is a cluster of protons and neutrons that form the nucleus, which is orbited by electrons. According to Squire and Novelline, ‘The MRI unit uses a powerful magnetic field to align the magnetization of atoms in the body. Radio frequency fields systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field which is detectable by the scanner’.1
Essentially, these protons align when placed in a magnetic field and when a radiofrequency is sent towards them they resonate. When the radio frequency signal is stopped, the protons return to their original position emitting energy. It is this energy that is picked up by the associated MRI computer to create an image.
This image can then be manipulated to provide sufficient detail of the area of interest. Since pathological processes change the properties of the affected tissues, these changes are then reflected on the cross-sectional 3-D image produced.
Until quite recently, MRI has had a limited application in the field of veterinary medicine, primarily owing to the expense of the imaging unit and associated requirements, such as the need for a specially constructed MRI suite to house the magnet and related equipment. Running an MRI suite requires specialist equipment to operate and produce the images, and experienced specialist staff to operate the equipment and interpret the images produced. The use of MRI scanning is becoming more common in the veterinary field as it provides good contrast between the soft tissues of the body, as well as detailed images of the skeletal structures .
Types of MRI scanner
MRI scanners can be either low strength (0.2 Tesla) open field, which do not fully enclose the patient; or high strength (1.5 Tesla) closed field magnets, which fully enclose the patient (Figure 2). To put this into context, the Earths magnetic field has a strength of 0.00006 Tesla.
The high strength, closed field magnets have several advantages over the smaller open field magnets in that the images produced are generally of greater quality, they provide superior contrast resolution and are better able to distinguish small details. Because of this, the required sequences can be completed in a shorter time frame which has advantages for the anesthetised patient through the reduction of the time the procedure takes.
It is worth noting that the actual magnet in the high strength superconducting units (1.5 Tesla) cannot be turned off, as it is supercharged and cooled by cryogens (liquid helium). They are only turned off (quenched) in emergency situations. ‘Quenching’ or shutting down the magnet presents many health and safety issues because it involves the venting of helium.
The strong static magnetic field can attract magnetic items with lethal force, and websites such as www.metrasens.com/ ferroguard graphically demonstrate this. There are, therefore, several important health and safety considerations with regard to the use of MRI:
- the entrance to the MRI room must be clearly marked and all equipment entering the room must be MRI compatible – non magnetic, non ferrous – this including all anaesthetic and monitoring equipment and tables used to transfer patients to the scanner.
- patients should be routinely checked to ensure that all metallic items are removed before transfer into the scan room in order to prevent serious injury. This includes patient collars, all items used for induction, and staff should also ensure ET tubes have plastic connectors only.
- staff must also remove all metallic items (scissors/pens/watches/hair clips/ mobiles etc) as all of these items will be pulled into the magnet with force .
Use of MRI in practice
Common applications for MRI use in practice include investigation and diagnosis of disease or injury of the spine and central nervous system, investigation of joint disease and the diagnosis and staging of neoplastic disease. Indeed, spinal MRI is considered to provide superior images and carry less associated risk for the patient than myelography.
MRI is especially useful in the treatment of neoplastic disease as it allows for the affected area to be mapped with great accuracy, which assists with surgical and treatment planning for radiotherapy. Patients undergoing MRI scans require general anaesthesia – or at the very least sedation for shorter procedures. The process is particularly noisy and patients must lie absolutely still or the resulting images will be poor and the sequence will have to be repeated.
The procedure varies in length according to the area under investigation and the sequences required. Some procedures can take over an hour – not including induction and recovery – particularly if contrast studies are required.
Patient preparation and recovery will largely depend on the individual, their associated condition and the number of sequences required. The clinician responsible for the patients care will select the most appropriate sedation or anaesthetic protocol; but there are several steps that nursing staff can take to facilitate the smooth running of the procedure and minimise the impact on the patient:
- gaining informed consent from the client as well as a complete history and any relevant radiographs
- ensuring patients are fasted
- completing a specific questionnaire relating to patient’s previous surgical history in order to assess if the patient has any surgical implants and the location of any microchip. (Surgical implants can produc
e heat during the scan process which presents a risk of thermal injury at the site of the implant; microchips will leave an artefact on the image produced) - ensuring the area to be scanned is clearly detailed on the patient records and these records stay with the patient at all times
- packing the patients ears with cotton wool will dampen the noise experienced within the scanner and can be particularly beneficial for patients that are sedated (Just remember to remove them when the patient is in recovery!).
- ensuring the bladder is empty before transfer, as patients may lose control of this organ while under anaesthesia or sedation and the presence of a liquid (urine) adjacent to the electrical coil could cause injury.
Repeated imaging can cause an increase in the temperature of the magnet and associated equipment, which is compensated for by the circulation of cold air. This then has a negative effect on the temperature of the patient. The use of additional items such as bubble wrap and blankets to insulate the patient while being scanned will help to reduce risk of hypothermia. Heat sources which have a metal component will not be suitable for use in this area.
Clinical Applications of MRI
In veterinary medicine the widest application of MRI is certainly linked to the nervous system and in minor extent to orthopaedic diseases. Research studies are available on cardiac and functional MRI but are still sporadic.
The exquisite contrast resolution and the absence of superimposition of structures allow studying neurological pathology with high diagnostic accuracy.
Congenital brain diseases (ex. hydrocephalus) can be initially evaluated with other imaging techniques (ultrasonography, computed tomography), but only with MRI is possible to clearly classify these pathologies. In cases of hydrocephalus for example, is necessary to understand if the disease process is obstructive (secondary to space-occupying lesions, stenosis) or compensatory (increased volume of the liquoral space secondary to brain atrophy).
Acquired inflammatory or neoplastic brain diseases need to be characterized with MRI; the number of lesions and their MRI features (signal intensity, margins, location, mass effect, etc.) are helpful in differentiating a neoplastic from a non-neoplastic pathology. Cerebrospinal fluid tap is often required to complete the MRI study and to provide answers to the questions of the clinician.
MRI studies of the spine, particularly with high-field units, provide a better contrast resolution as compared to CT and allow not only to characterize the site of potential spinal cord compression (intradural, extradural, intramedullary) but also its signal intensity. Intramedullary lesions, such as ischemic myelopathy, myelitis, and sometimes spinal cord neoplasia, can be undetectable with other imaging modalities (i.e., CT). MRI represents the gold standard imaging technique for these patients and is useful not only for pre-surgical planning, but also for providing important prognostic factors.
Advantages
Patients with a variety of clinical signs arising from a number of different organ systems can benefit from MRI. The technique is mostly helpful for reaching a diagnosis but can also be instrumental for planning therapy such as surgical excision or irradiation of a neoplasm. MRI is certainly not appropriate for every patient, and a diagnosis can often be derived with more traditional imaging modalities such as radiography or ultrasonography. However, some organ systems such as the central nervous system (CNS) are quite difficult to image with these traditional modalities. When the differential diagnosis includes diseases that are difficult or impossible to image with traditional methods, advanced imaging techniques such as CT, MRI, or magnetic resonance angiography (MRA) can help in formulating a shorter list of diagnostic differentials or in arriving at the final diagnosis. As with any other diagnostic test, MRI will never replace a thorough anamnesis and physical examination. In fact, the decision to conduct MRI, as with any other diagnostic test, should be dictated by the results of such initial evaluations.
Whether CT or MR techniques are better can be debated. A dying adage stipulated that CT is better for imaging bones, whereas MR techniques are better for imaging soft tissues. With technologic advances in both of these imaging modalities, this notion is becoming obsolete. Very few comparative studies exist in veterinary medicine, and their results are at best short-lived as advances in both technologies continue to improve imaging capabilities. Comparing CT with MRI may be similar to comparing a PC with an Apple computer: The results of the comparison are true only for as long as the technology for each computer does not change.
Disadvantages
Disadvantages of MRI examinations are related to cost, the necessity to use heavy sedation or general anesthesia, limited availability, and logistics of coordinating everything when MRI is not available on-site (i.e., scheduling the MRI examination, having staff available to help, transporting the patient, bringing the necessary material, inducing anesthesia outside the veterinary hospital, and being prepared for a possible anesthetic emergency). MRI is becoming more accessible, and a few veterinary colleges and private practices have the technology available on-site. Alternatively, some veterinarians have been able to make arrangements with human medicine imaging centers in their own areas. The typical cost of examining small animals via MRI ranges from $750 to $1,500, depending on the type of examination and the number of sequences required.
Benefits of magnetic resonance imaging in animals
One of the main benefits of this technique, over any other, is that it is not an invasive study. The resonance does not require the intervention or manipulation of the animal’s body to throw images about its internal behaviors.
On the other hand, it is an innocuous procedure. That is, it does not directly affect the patient’s health, since it does not involve the use of any substance, element or energy that could endanger the life of the animal.
In addition, resonance is one of the most accurate ways to obtain information in a very short period of time. It offers a wide variety of shots, including cuts other than the sections studied, without the need to move the patient and with the convenience of doing the study in a very fast time.
Magnetic resonance imaging is the best alternative to reach a timely diagnosis in the most critical cases, allowing veterinarians to quickly determine the origin or causes of certain symptoms and quickly design an action plan, including visualizing the relevance of any surgical intervention.
In this regard, resonance is the map of many surgical procedures, indicating to the veterinarian what are the areas to intervene and, even, the risks they may face before reaching the surgical field.
This study is the one that gives the highest diagnostic accuracy to veterinary doctors. It allows to make cuts as accurate as a millimeter thick, so it is easy to detect any anomaly or disease, however minimal it may be.
As if this were not enough, through resonance it is possible to visualize the blood vessels, without the need for contrast or intervention.
Risks of magnetic resonance imaging for the animal
Magnetic resonance imaging does not represent a greater risk to the health of animals. However, in the case of this type of patients it is also necessary that they be anesthetized since a key point of the resonance is that the patient remains completely immobile.
While the use of anesthesia in animals does not pose a very high risk, it does not cease to represent a certain level of risk, perhaps the only one that is linked to animal resonance studies. Complications with anesthesia in animals are always a probability, although it is usually remote and the animal is always under careful medical supervision.
The study lasts between 40 and 90 minutes and before beginning the race, weight, size and age of the animal is analyzed, since based on this the appropriate anesthesia process will be determined for each case.
On the other hand, it is important that, if the animal bears a localization tattoo, tracking chips or implants, it will be notified to the veterinarian before the exam. In some cases, such as when using a chip in small breeds, it may be necessary to remove it before the study.
In the other cases, the possible incidence in the veracity of the images must be taken into account, for any case of interference with the magnet.
Clinical situations in which magnetic resonance imaging is used
Veterinary professionals indicate the use of magnetic resonance imaging in the suspicion of certain conditions, including:
Lesions of the brain, cerebellum and brainstem, if the animal begins to experience seizures, vestibular syndrome, dizziness, sudden blindness, changes in behavior and personality, mental disorders, posture problems, eating disorders, problems with thermoregulation, sleep disturbances , tremors, tachycardias, among others.
Problems in the spinal cord, in animals with some perception deficit, spinal pain, spinal injuries, problems with reflex response, lack of control of the sphincters, postoperative relapses, and a wide variety of syndromes.
Abdominal masses, tumors, suspected fluid or hemorrhage and planning of surgical interventions of some organs such as kidneys, liver, spleen and bladder.
In addition, magnetic resonance imaging is used in cases of coagulation problems in any area of the entire circulatory system, ear diseases and optic nerve diseases, as well as throat and mouth diseases.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
Image-Courtesy-Google
Reference-On Request.
