Review of Available MRI Systems
By Christy M. West • Apr 26, 2008 • Article #20985
The VMTH’s specialty services are supported by several ancillary services such as its diagnostic imaging service, which provides MRI, computed tomography, radiography, and fluoroscopy for equine patients.
Photo: Courtesy University of California Davis
Magnetic resonance imaging (MRI) is a very powerful tool for imaging limbs and even heads of horses to detect injuries and other problems. However, not all MRI units are created equal; differences in field strength affect their price tags and their abilities to demonstrate lesions. During the 2007 American Association of Equine Practitioners Convention, held Dec. 1-5, 2007 in Orlando, Fla., Natasha Werpy, DVM, Dipl. ACVR, assistant professor of veterinary clinical sciences at Colorado State University, presented a comparison of low- and high-field MRI systems.
"High-field systems have faster acquisition times and superior resolution; however, the substantial investment needed for purchase and maintenance make them prohibitive for many practices," she said. "Many injuries can be accurately characterized and diagnosed using a low-field MR system."
Field strength--High-field systems have a field strength of at least 1.0T (tesla; one tesla is approximately 20,000 times the strength of the earth's magnetic field); systems of 1.5T are common in equine referral practices and 3.0T systems have recently been introduced into human practices. They carry a price tag from $495,000 to millions of dollars and usually have expensive annual service contracts.
Low-field systems have field strengths of up to 0.3T, and are substantially less expensive to purchase. Low-field magnets currently in use for horses have 0.20 and 0.29T magnets.
Examination time--Similar MRI sequences (focusing on a single area) take 2-5 minutes longer on a low-field system vs. a high-field one, Werpy reported. A typical series of six to eight sequences will take 12-40 minutes longer. She noted that patient compliance is the biggest factor affecting examination time with a low-field system, as the horse is often standing and sedated compared with high-field systems, which always utilize general anesthesia.
Veterinarians use MRI to examine a horse at Alamo Pintado Equine Medical Center.
Due to the general anesthesia requirement for a high-field magnet, these systems are often called "down" magnets as opposed to low-field "standing" magnets.
Image quality--"Field strength is the most important factor determining image quality," said Werpy. More specifically, three factors affect image quality: Signal to noise ratio (amount of true information compared to artifacts), contrast (difference in signal intensity or gray scale of different tissue types) to noise ratio, and artifacts (false information). Higher field strength offers better image quality due to higher signal to noise and contrast to noise ratios, and fewer artifacts (although some artifact types are more common in high-field systems).
Image quality and diagnostic accuracy--Does a veterinarian really need the higher-quality images of a high-field system to make an accurate diagnosis? Sometimes, yes. A high-field system can detect smaller, lower-contrast lesions than a low-field system, possibly catching problems earlier in the disease process when treatment can do more good. However, this degree of imaging detail means practitioners often find many small lesions, and not all of them are necessarily causing clinical problems, at least not yet. In addition, there is a great range of individual variation in pain response to a particular lesion type and size. This is an area of great debate in both human and animal medicine--just what lesions are really significant?
In addition, Werpy noted that low-field images undergo post-processing such as filtering and smoothing. This results in an apparently accurate image, but some fine details such as trabecular bone architecture (important for identifying bone lesions common to the foot and fetlock) can be lost. These details can be seen on high-field systems.
In regards to standing studies of the upper limb (carpus and tarsus, or knee and hock), she commented, "It never hurts to try a standing MRI study, but be prepared that the motion may significantly affect the image quality. They are often not diagnostic for soft tissue lesions; it depends on the size and the severity of the lesions. The studies can be diagnostic for large bone lesions, again depending on the type."
Clinical Implications--"If the imaging results don't match the clinical picture, that doesn't mean there isn't a lesion," she noted. "It does mean you have ruled out several lesions. Can you have pain without structural changes seen on MRI yet or at all? Sure, or a lesion might be present but we don't or can't recognize it."
"Both high- and low-field MRI have helped thousands of horses get diagnoses that were not possible before," she concluded. "Low-field systems can accurately characterize many lesions; however, certain ones will need high-field imaging. As we continue to image horses and correlate findings with the results of clinical examination and other imaging modalities, we may be better able to determine which patients should be imaged with a high-field vs. a low-field system."