The use of Magnetic Resonance Imaging (MRI) scanners is growing rapidly in hospitals, clinics and healthcare centers around the world. Doctors increasingly rely on the machines for getting some of the highest-definition images possible of the brain, musculoskeletal structure, cardiovascular system and even visualization of functions such as heartbeat. The market for such scanners tripled from 1996 to 2007 and is expected to exceed $5 billion in 2010.
Demand is especially strong for MRI scanners in the range of three tesla (T), a measure of their magnetic field strength. With a definite edge over standard 1.5T commercial units with half this strength, 3T models can’t be beat when it comes to image quality and scan speed: important performance criteria for medical facilities in handling as many patients as possible – and in boosting profit margins.
On the downside, MRI scanners inherently produce very loud knocks and taps during operation as magnetic field gradients are rapidly turned on and off. Noises are so loud that hearing protection must be worn for some imaging sequences. Even so, many patients still find the muffled noise uncomfortably annoying, with some calling it quits before the scan is complete while others refuse to undergo the procedure altogether. Unfortunately, the stronger the magnetic field, the louder the noise levels. Indeed, one of the major challenges for manufacturers is lowering noise levels in the powerful 3T models.
Quieting the world’s fastest scanners
At the forefront of efforts to lower MRI scanner noise is GE Healthcare Systems, a $17 billion unit of the General Electric Company. The firm provides a wide range of medical products including anesthesia delivery system, respiratory therapy equipment and imaging systems such as CT scanners, x-ray equipment, PET-CT systems, ultrasound and MRI scanners.
GE Healthcare is one of the dominant players in the burgeoning MRI market. The Discovery MR750 is a 3T machine billed as one of the world’s fastest MRI scanners. The unit is reported to boost imaging performance five-fold over previous generations with 60 percent greater coverage and resolution. Routine liver exams that typically take 40 minutes can be done in only 15 minutes, and a full breast exam can be completed in only two scan sequences verses four or five sequences previously. Units are designed, tested and manufactured at the company’s engineering and manufacturing center in Florence, South Carolina in the US. Some of the most advanced noise reduction initiatives being pursued jointly at Florence and at the GE Global Research Center in NY are aimed at quieting the next generation of 3T MRI scanners..
A key element in these noise-reduction efforts is the recent implementation of an LMS Test.Lab data acquisition and analysis system from LMS International. Dr. Washington de Lima, Acoustic and Vibration Systems Engineer at the Florence facility, said he and his colleagues use the system extensively for modal analysis in determining the natural frequencies and mode shapes of individual components – primarily gradient coils and structural members they’re connected to that are the source of the loudest scanner noises.
“These three coils (one for each of the x, y and z planes) produce structural vibration by rapidly turning on and off as they work in conjunction with the powerful 3T main core magnet in mapping a detailed three-dimensional image of the body,” explained Dr. de Lima.
Streamlining acoustic testing
The new test system enables engineers at the Florence center to perform acoustic and vibration testing and analysis without shipping the existing LMS equipment from the Global Research Center more than 700 miles away in Schenectady, New York.
“We previously had a data acquisition system with very limited capability. While personnel from the GE Global Research Center in Schenectady, New York were gracious enough to allow us to borrow their LMS system, now we can perform modal analysis when we need to without any delays for travel or scheduling,” said Dr. de Lima. “Plus, LMS Test.Lab has the capability for us to expand the scope of our work beyond the services of the Global Research Center. Part of these plans include operational modal analysis done while the MRI scanner is running. Analysis data will include operational model shapes, frequency responses and modal parameters such as damping and stiffness values for the structure. In addition, we have replaced our outdated former system for vibration/acoustics measurements with the new system, which gives us many more data-output options including spectrograms and real-time octave bands.”
Noise abatement methods
The goal, according to Dr. de Lima, is to trace vibration transfer paths from the source – in this case, the gradient coils – through the interior structure of the machine to exterior panels that produce the airborne noise heard by patients. Modal and vibro-acoustic testing and analysis provide valuable insight into these transfer paths, enabling engineers to identify which parts of the structure could possibly be excited to vibrate at their natural frequencies and thereby magnify noise amplitudes.
Many noise abatement methods are being tested by GE Healthcare.. When the approach is refined, Dr. de Lima noted that plans are to extend use of the method to reducing noise levels in all of GE Healthcare’s lines of MRI scanners-
Patient well-being is top priority
“Addressing noise issues in an MRI scanner takes considerable knowledge of complex vibro-acoustic behavior of the entire system,” noted Dr. Tim Havens, Electromagnetic and Systems Engineering Manager in Florence. “The data acquisition and analysis capabilities of LMS Test.Lab give our engineers the insight needed to significantly lower emitted noise for an improved patient experience.”
According to Dr. Havens, after an exhaustive comparison, the solution was selected based on the global reputation of LMS in vibro-acoustic testing and analysis in automotive and a wide range of industries. “The LMS Test.Lab solution leverages the expertise of our engineers in bringing innovative, industry-leading scanners to the medical community for the well-being of patients, which continues to be our number one priority and at the foundation of our product development efforts.”
