Since the introduction of the Open MRI system in the late 1980’s, comparisons between the high field conventional systems have continued to be controversial. Many vendors of MRI equipment have begun to place more of their R&D (research and development) monies into improving the quality of the Open MRI systems. Today, these systems are continuing to improve and becoming more accepted in the medical community. Furthermore, patient education has made many patients ask their physicians to use the Open MRI. These requests are mainly contributed to the patient’s psychological needs such as claustrophobia or anxiety. Large patients also are in need of the Open systems.
Prior to the introduction of the Open MRI, large patients were simply not scanned when the machine limitations would not make it possible. This option has led to awareness that the healthcare system is being designed around the standard size patient. Many of these patients are very conscious of their special needs and require a sensitive caring staff that will show them that the Open center is here for them. It has been estimated that the Open systems now make up about one third of the systems used in clinical practice. It has also been estimated that it can be used for 90% of the imaging that is needed, and can produce, not only acceptable, but high quality diagnostic images. However, there has been a strong prejudice in favor of the high field systems. This discussion attempts to clarify some of the issues confronted in comparing and contrasting the high field closed systems with the lower field open systems.
Radiologists are becoming increasingly satisfied with the Open system and vendors are rising to the challenge of changing the high field bias. Sequences are being developed that can be performed on the Open systems that were originally designed for the high field systems. These special techniques such as fat suppression and fast spin echo, aid the radiologist in developing a differential diagnosis. Open systems are even finding their way into interventional/intraoperative, cardiac, and neurosurgical departments. The main debate slowing this process was the quality of the Open images. This notion can easily be extinguished with an understanding of the physical principles involved with MRI scanning. High field MRI techniques cannot be applied to the low field Open systems. This practice has lead to the myth that high field strength is essential in producing a high quality MRI image. This is simply not true. Many factors affect the quality of an MRI image. Proper parameter selection requires a specialized knowledge of the differences in the many factors that affect image quality. The main trade off between high field systems and low field Open systems is time. It takes roughly one and a half to two times longer to produce a comparable equivalent image.
One of the main factors influencing the image quality is signal-to noise ratio (SNR). In the early 1980’s, prior to the Open systems, the main component used to increase the SNR was the magnet. It was found that an increased magnetic field strength achieved a much greater SNR. However, the focus has shifted to increasing the SNR with other components such as the RF (radio-frequency) system, gradient system, and computer system. Another way to increase the SNR is with the use of surface coils. The Open systems have an advantage in this area. Due to a vertical vs. horizontal magnetic field, the use of a specialized coil (circular solenoid coils) allows for a further increase in the SNR values. Open systems utilizing this technology can achieve SNR values comparable to the high field systems. In fact, the images produced on high field systems five years ago are comparable to the routine images obtained on the Open systems today. Moreover, many of the high field magnets in clinical practice today are five or more years old and boast superior image quality based on their field strength.
Experts, Radiologists, and educators agree, image quality is not guaranteed with a high field system. Dr, Joseph C. George, MD, a member of the American College of Radiology, Director of Medical Imaging at The Heart Center of Indiana and Staff Radiologist at New Century Medical Imaging, states, "As a radiologist performing MRI examinations on both open and closed MRI platforms, I see the advantages and limitations of both technologies. The quality of open MRI images continues to improve, and with current technology, greater than 90% of orthopaedic and neurological studies are diagnostic. Open MRI remains the study of choice for large and claustrophobic patients, as well as those who are anxious and may benefit by having a family member or friend present during the exam” Testimonials such as this reinforce the advancements in Open MRI which are helping to dispel the myth that High filed is better. The diagnostic capabilities of the exam depend on the protocol selection of the Radiologist, and the ability of the Technologist to choose the proper parameters.
In the wrong hands, image quality can be greatly decreased with improper parameter choices. The American College of Radiology has addressed this issue with the advent of their MRI accreditation program. This program originated when Aetna US Healthcare announced they would require MRI providers to be accredited in order to receive reimbursement. The program design ensures a standard evaluation of personnel, equipment performance, quality control, and image quality. There is some controversy that these standards were based on the high field systems, but systems of all field strengths have passed the ACR accreditation process proving that there is no reason today’s low field magnets should have a high field bias placed against them. The bias should be with the training and accreditation.
Another common misconception came with the introduction of the high field short bore magnet. This magnet design, as the name implies, uses the conventional closed system design. However, the bore of the magnet was shortened from nearly nine feet to approximately six feet including a flared patient opening to further enhance the “open” concept. Manufacturers introduced this product in 1997 to compete with the increased patient comforts found in the Open systems. Taller patients may find that some exams will feel less claustrophobic in a short bore, but the truly claustrophobic patients will still be unable to complete exams without some form of sedative. Furthermore, the increase of four centimeters in diameter, which is found in most of the short bore magnets, is not quite an adequate increase for most large patients.
As you can see there are many issues involved in the choice of Open vs. Closed systems. However, referring physicians must be educated on the advancements in technology that have allowed the Open systems to be competitive in the high field market. High field no longer means better images; especially while Open technology continues to improve. Quality images are the product of proper parameter and protocol selection to ensure diagnostic capability. ACR accreditation can also be a standard indicator that an imaging facility is committed to quality, patient care, and physician satisfaction.
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Keith W. Smith is from Indianapolis, Indiana, and acquired his associate of Science in Radiography and Bachelor of Science in Medical Imaging Technology from Indiana University School of Medicine in Indianapolis. He has over 13yrs experience in MRI and CT and actively holds MRI certification through the American Registry of Radiologic Technologists. Keith has held numerous elected positions in both State and National societies for Radiologic Technologists. He has been published in the National Journal “Radiologic Technology” and invited to lecture on many occasions. Since 1994, Keith has been an adjunct faculty member for the IU Medical Imaging program teaching MRI and CT physics. Keith joined New Century Medical Imaging in February of 2001 and is currently the Director of Technical Operations for their three Indianapolis area clinics.