In addition, such machines could lower the cost of MRI scans, because each patient would spend less time in a machine, Bourland says.
"In some cases, scans that previously required 15 minutes can now be done in one to one-and-a-half minutes," he says.
Though faster scanning techniques may bring improvements in image quality, Bourland says the combination of high-powered and fast-changing magnetic fields has potential to cause discomfort in patients. His group at Purdue is studying ways to combine power and speed without causing distress.
Initial findings from the study, funded by the National Institutes of Health, will be reported April 16 at the annual meeting of the International Society of Magnetic Resonance Medicine in Vancouver, British Columbia.
Bourland and Purdue electrical engineering Professor John Nyenhuis have studied the safety of MRI technology for more than eight years. Much of their early work focused on the magnetic field's effects on the heart and other organs. Their work played a role in the introduction of new "fast-scan" MRI systems last year, including the echo planar and hybrid imaging systems.
Though these new systems have improved image quality and speed, Bourland says there is room for more improvement.
"The MRI scanners used in hospitals are still fairly conservative in the amount of electromagnetic stimulation they use to attain images," he says. "Using additional power, however, must be approached slowly to avoid jeopardizing safety or causing discomfort to patients."
Magnetic resonance imaging allows physicians to "see" internal tissues by using magnetic fields to interact with the protons in hydrogen atoms in the patient's body.
When the patient enters the imager, the small internal magnetism of the protons partially lines up with the powerful static magnetic field that is always present inside the imager. A radio frequency magnetic field is then briefly applied to tilt the protons' magnetism into an orientation perpendicular to the static magnetic field, causing the protons to rotate about the static field to produce weak radio signals. Magnetic gradient fields then are applied to produce radio signals with a range of frequencies. Signal processing techniques convert the different frequencies into images of tissues.
The absence of ionizing radiation, such as that used in X-rays or CAT scans, means the MRI procedure carries few risks for patients, Bourland says.
"The magnetic component precludes some patients, such as those who have cardiac pacemakers or metal implants in their body," he says. "Otherwise, there are no known long-term effects caused by an MRI environment. After 13 years of medical experience, and millions of patients, MRI has proven to be one of the most effective and safe imaging tools available."
Increasing the speed and image quality of MRI systems, however, requires the use of high-powered, fast-changing magnetic fields, a combination that increases the potential for negative effects such as pain or discomfort.
"Studies indicate that nerves and muscles in some people may be stimulated by these pulsed magnetic fields, causing sensations such as tingling or a weak muscle contraction," Bourland says.
Nyenhuis says such effects may be caused by rapid changes in the intensity of the gradient magnetic fields, a process that is necessary to obtain images from MRI scanners.
"These magnetic fields are key to obtaining a three-dimensional view of the body," Nyenhuis says. "But pulses or fluctuations can induce electric currents in the patient, called eddy currents, which then may stimulate nerves and muscles."
Little human data are available on the stimulating effects of MRI gradient fields, Bourland says. "Our research is aimed at determining at what levels patients can feel the magnetic fields, and at what levels the symptoms are acceptable."
Bourland and Nyenhuis are using a unique system designed to determine the optimal ways to vary gradient fields without causing pain or discomfort to patients. The apparatus consists of a very powerful gradient amplifier assembly and magnetic coils to produce pulsed magnetic fields of greater intensity than can be applied in a commercial magnetic resonance imager.
This system, developed at Purdue over two years, will allow the group to manipulate and measure tiny changes in the timing and intensity in gradient fields.
The study, overseen by Purdue's Committee for the Use of Human Research Subjects, will provide detailed statistics on patients' responses to the strength and duration of various combinations of the fields. Human volunteers in the studies are subjected to a series of electromagnetic pulses, which last less than a quarter-second each. The subjects then rate each pulse zero through 10, with a rating of zero meaning it was not detectable and 10 meaning it was intolerable.
"We ask the volunteers to ask themselves while rating each pulse whether they would find that level of stimulation uncomfortable over a prolonged period such as several minutes," Bourland says.
The data can then be used by MRI system designers to devise optimal pulse sequences for image resolution and speed, Nyenhuis says.
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