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By Keith W. Smith, BS, RT(R)(MR),
Director of Technical Operations, New Century Medical Imaging
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.
1.
Oldendorf, William Jr. Open MRI assumes permanent place
alongside high-field competition. Diagnostic Imaging Oct. 1998.
MR8-12.
2.
Davis, Mark. High- vs. low-field MR: What’s the difference.
Diagnostic Imaging Oct. 1988. MR28-30.
3.
Wagner, Steven K. Experts predict shift toward more power in
open systems. Diagnostic Imaging Oct. 1999. MR2-6.
4.
Wagner, Steven K. Vendors close differential between high and
low field. Diagnostic Imaging Oct 2000: 2-6.
5.
Wagner, Steven K. From Glendale to Bala Cynwyd, uses for open
MRI proliferate. Diagnostic Imaging Oct. 2000: 8-11
6.
Parizel, Paul M.., DeSchepper, A.M. Fathoming MRI physics at
lower field strength. Diagnostic Imaging Oct. 2000: 29-32.
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.
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