The definition of open MRI has changed recently with the debut of higher field strength magnets that allow a more open experience for patients and more powerful exam for care-givers. But at the same time, traditional open MRI systems have gained speed and clinical utility, offering a full menu of brain, neurologic, spinal, musculoskeletal and abdominal imaging and MR angiography - studies that a few years ago would not have been considered on an open system. And most important, image quality is earning clinical confidence.
Could it be that the definition of an open MRI scanner resides in the eye of the beholder?
There's a debate raging in the world of magnetic resonance imaging about what constitutes the definition of an open MRI system. Traditionalists believe that the only open scanners are those that do not resemble a cylindrical shape in the least, but feature a design that employs either a four-poster design such as the Toshiba America Medical Systems Ultra Open 0.35 Tesla scanner, or a wide open two-sided design like the Philips Medical Systems Panorama 0.1 Tesla, or the 0.7 Tesla SIGNA OpenSpeed system by GE Healthcare.
On the other side of the equation, clinicians who use the Siemens Medical Solutions MAGNETOM Espree 1.5 Tesla Open Bore that provides heads-out and feet first scans for most patients, would argue that this cylindrical machine qualifies as an open scanner by virtue of its 70 cm bore that is 125 cm in length from cover to cover. Some clinicians have found this feature has eliminated the sensation of being confined within an enclosed space for those patients with fears around those matters.
But that's not the only discussion occurring within the MR community these days. While open systems are immensely popular with patients because they provide maximum comfort for those who may be larger in size or who suffer from claustrophobia, clinicians are unsure that those issues outweigh the longer scan times and diminished quality of images produced by the lower field strength magnets as compared to conventional high-field scanners. Meanwhile, manufacturers of the lower field strength machines have worked diligently to overcome the problems created by the physics of the original designs to insure that those concerns become less pronounced. And image quality in recent years has improved greatly.
Open in a traditional sense
Original open MR systems used resistive or permanent magnets, which were by their nature lower field strength machines because, built of iron, they became heavier and bulkier the higher the field strength. An additional reality governed by the laws of physics involved increases in sensitivity to vibration as the field strength gained power, leading to site challenges that would maintain imaging capabilities regardless of the heavy truck that drove by the hospital, or the elevator shaft that was located adjacent to the MR scanner.
Those issues were coupled with additional drawbacks of lower field strength scanners including reduced image quality while scan times were longer. The typical 0.2 and 0.3 Tesla machines are not well suited to fat suppression imaging. This becomes important in evaluating tumors that are surrounded by adipose tissue. Without fat suppression techniques, the mass appears the same as surrounding fatty structures, and borders of the tumor are more difficult to discern.
On the positive side, many of the lower field magnets are quite sufficient for a number of studies, and they certainly come in at a lower price point and weigh significantly less than their more powerful cousins. This coupled with manufacturers' efforts to enhance performance of the lower field strength equipment has improved their utility and acceptance.
Rik Baier, CMPE, executive director of M&M Orthopedics in Downers Grove, Ill., describes their 18-physician practice as comprised of five clinics in the Western suburbs of Chicago. Following an extensive review of available MR systems, they employed a "hands on" approach for determining the best imaging quality for their purposes. Baier served as a test "patient" who was scanned on existing functioning systems they were evaluating, and he brought the films back to the practice for comparison.
After evaluation, the practice chose the Toshiba Ultra Open 0.35 Tesla system that features a four-post design to minimize vibration effects and a superconducting magnet to permit a more stable field as well as advanced gradient technology that enables shorter scan times with higher spatial resolution. Cooled in a cryogenless environment similar to a refrigerant unit, the system can accommodate up to a 500-pound patient within the 65 cm opening, and has been used for a variety of imaging studies across the current installed base of 50 units.
"From an image quality standpoint, we've not been disappointed at all, and [the images produced] rival the high field magnet studies," says Baier. They perform lumbar as well as cervical spine studies, a full range of orthopedic exams, and their physicians have been quite pleased with the results.
Hollis Potter, MD, chief of MRI division, director of research for the department of radiology at the Hospital for Special Surgery and professor of radiology of the Weill Medical College of Cornell University experienced a paradigm shift as she took a new look at open MRI.
Prior to reviewing the image quality produced by the GE 0.7 Tesla OpenSpeed, Potter used to tell orthopedic surgeons not to rely on open MRI scans because they could not visualize cartilage well. Rheumatologists and other clinicians require excellent cartilage imaging because it changes the way they manage their patients' care.
Although they have several high field strength systems, they elected to add the OpenSpeed so that they could serve the patients they otherwise would lose, those who were claustrophobic and larger patients, such as high performance professional athletes. They routinely scan several patients from the National Football League, and Potter describes patients with a 52-inch plus chest who could not fit into a conventional scanner.
Once the OpenSpeed was operational, Potter decided to perform an experiment to have their referring clinicians guess which unit was used to produce the scans.
"The head of the foot and ankle service here orders a lot of MRI, and he uses it for tendons and ligaments, but he is particularly interested in the integrity of the articulate cartilage which is thick in the toes and mid-foot," Potter explains. The test proved successful: that physician could not tell whether the images were obtained on a high field or open scanner.
Current research at the Hospital for Special Surgery involves producing a prototype hip coil, because although they routinely do standard hip studies to look for occult fractures or vascular necrosis, they have less success with highly detailed work such as the central cartilage over the hip joint to be able to label tears in that tissue. The hip coil should facilitate those studies.
Roger Y. Shifrin, MD, chief of MRI, CT and 3D imaging at Radiology Associates in Daytona Beach, Fla., employs a number of low to mid-field MR scanners among his 10 MR systems, including an OpenSpeed with EXCITE from GE.
Using this system, they perform brain, neurologic, spinal, musculoskeletal, abdominal, MR angiography, and many other studies that a few years ago they would not have considered doing on an open system. Shifrin explains that the engineering innovations that have been added make him comfortable in using it for those studies, and he believes the quality of the images supports his confidence in the scanner's capability.
"On the hardware side, being able to have a vertical field magnet lets you use a solenoidal coil that provides a boost in efficiency in signal-to-noise ratio," Shifrin explains. "Having better gradients enables you to perform faster and more intensive scans." That is especially important when an imaging study is being performed on a moving part, such as during breathing. He describes completing a high-quality imaging study of a knee in about 2 minutes on a 1.5 Tesla system, which might take six or seven minutes on the open system. Patient movement becomes important in that scenario.
Shifrin reports that their center has been able to reduce the number of claustrophobic patients in their practice from 10 percent to 2 percent through use of special equipment such as the mirrored glasses that enable patients to see out, a goggle-type device that permits them to view a movie or images of wide open spaces, allowing family members to accompany patients and hold their hand, and by teaching technologists to utilize reassurance activities.
Nevada Imaging Centers, a general radiology practice with five imaging centers throughout Las Vegas, is having similar success with open MR, having installed a Philips Panorama 1.0T open-magnet system in November 2004. The area of the machine where the patient is scanned features a panoramic viewing angle to enable the patient to see outside the magnet at all times during the exam. Additionally, this design feature means that the clinical staff have easy patient access and optimal patient visibility. A feature called active shielding means that the magnet contains two concentric superconducting coil sets that enable the inner set to produce the primary magnetic field while the outer set reduces the fringe field outside the magnet to a minimum.
After thorough evaluation of the system in the factory in the Netherlands, Nevada Imaging's Medical Director Garey L. McLellan, MD, says he was stunned by the outstanding image quality, considering the images to be comparable to those produced by their 1.5 Tesla machine.
The architecture is very open, and patient acceptance has been excellent, McLellan reports. While originally their orthopedic and neurosurgeons had requested images be done on their 1.5 T short-bore scanner, when he asked them to let him try the Panorama Open system, the referring physicians were extremely pleased with the results.
"Before, using an open system was a compromise with image quality and speed of imaging," says McLellan. "But if you're looking for a 1.5T image quality, with this machine it's no longer a compromise." And he has found it valuable for large and difficult to image patients.
Adding short-bore systems to the mix
Siemens Magnetom Espree 1.5 MRI system features a 70 cm bore that is 125 cm in length and permits 60 percent of patient scans to be performed with the patient's head and feet outside the scanner. Patterned after typical CT scanner design, the system was introduced in August 2004.
Stephen T. Sweriduk, MD, medical director of Shields Health Care Group (Quincy, Mass.) explains that although they have multiple open magnets, their initial experience with the Espree has exceeded their expectations in image quality. They have installed two of these machines in separate centers.
Initially they had concerns about possible limitations in field of view, but quickly dispelled those misgivings. "I went into the machine myself as a volunteer for a thoracic spine," says Sweriduk. "I'm six foot two inches and I'm fairly large. The images were great, and we were able to cover from the mid-cervical spine to the mid-lumbar spine with no difficulties." Plus he describes the geometry of the machine as phenomenal. During his scan, he could lift his head and look out to see his feet.
"Basically, we have a high-field MRI machine that doubles as an open magnet," he concludes. They do not limit clinical scan indications. They've done angiograms, dynamic scans of the liver, diffusion imaging in the brain as well as more routine studies.
Mark E. Geller, MD, CEO of Hudson Valley Radiology Associates, director of radiology at Nyack Hospital in New York, and chairman of the physician advisory board for Radiologix (Dallas, Texas) is in the process of installing a Siemens Espree, which was due to go live at the end of June.
Following extensive use of various open MR units for the past eight to 10 years, they decided to evaluate the Espree because some of their referring physicians were not happy with the image quality they obtained at the mid-field level.
"The high field Espree can do everything we require," says Geller. All of the applications, gradients and coils that they would use on a high field unit are available on this system as well. "You can get thin slices, rapid scan times and breath-holding images which you could not do on mid-field opens." Acquisition times are reduced due to the field and gradient strengths on the system.
Michael H. Rowan, BS, ARRT(R)(CV), manager of imaging services at Palomar Pomerado Health in San Diego describes their use of the GE 1.5 T short bore Signa Horizon Echo Speed Plus scanner, with a coil gradient of 30 Tesla.
With a tube length of five feet and flanged ends, he describes it as appearing shorter for patients. "The design of this short bore reduces anticipation and anxiety that the patient may have," he says.
They have used the system for cardiac function studies including ejection fraction and standard imaging as well as neurologic scans and musculoskeletal exams. In addition, they have used it for breast imaging in their two hospitals that cover an 850-square mile catchment area.
The field of open MRI has entered exciting possibilities with some of the recent advancements in systems. While the driving forces for using these machines remains as alternatives for either very large patients or those who are claustrophobic, clinicians demand excellence in image quality to manage their patients and improvements in throughput to facilitate their practice. New systems have been designed to address those issues.