Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-04-07
2002-10-08
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S418000, C600S421000, C381S071100, C381S072000
Reexamination Certificate
active
06463316
ABSTRACT:
RIGHTS OF THE GOVERNMENT
The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
BACKGROUND OF THE INVENTION
The field of the invention is noise minimization for magnetic resonance imaging and more specifically to delay based active noise cancellation for magnetic resonance imaging.
Magnetic resonance imagers (MRI) are extremely valuable tools for medical scanning. MRI systems are able to identify different types of tissue within the human body by placing the tissue in a strong magnetic field generated by a superconducting magnet and exciting the tissue (specifically the spin states of the molecules within the tissue) with a pulse of RF energy, and then measuring the electromagnetic emissions of the tissue as its spin states relax to the rest position. The MRI scanner is able to map the spatial locations of the different types of tissue by creating gradients in the magnetic field that change the response of the tissue as a function of spatial location within the body.
Researchers have long recognized the severity of the noise problem in MRI scanners and that they impair patient comfort and contribute to patient anxiety. Numerous studies have examined the acoustic characteristics of these noise fields. The MRI sound is created by “knocking” of the gradient coils. The gradient coils in the MRI are oriented orthogonally to the field of the static magnet. During the imaging, the current in these coils is turned on and off at 5-10 ms intervals. Each time the coil current is switched on in the magnetic field, a force is created that is orthogonal to both the direction of the current and the magnetic field (i.e. radially outward). This sudden force on the gradient coil causes it to “knock” against its mounting, creating acoustic noise. The characteristics of the noise are therefore directly related to the gradient pulse sequence used in the scan, and will vary substantially according to the scan performed. Studies have examined the acoustic characteristics of the noise fields by placing a condenser microphone inside the MRI. These studies, after verifying that the microphone was not effected by the electromagnetic fields in the scanner, have generally shown that the imager noise in a 1.5 Telsa MRI scanner is approximately 100-105 dB SPL during a relatively noisy MRI scan. More recent studies indicate that the peak noise level in a functional MRI scanner is approximately 118 decibel for a 1.5 Telsa MRI and 134 decibel for a 3.0 Telsa MRI scanner, suggesting that the more powerful MRI systems may be 15 decibel louder than the 1.5 Telsa scanners.
The noise generated by “gradient coil knocking” are disadvantageous for several reasons. First, the noise levels are sufficiently high that they could damage the patient's hearing. Occupational Safety and Health Agency guidelines limit occupational exposure to noise to 30 minutes per day at 105 decibels SPL and to less than 15 minutes per day at 115 decibel SPL. Since MRI scans can take 40 minutes or longer, patients who are scanned without hearing protection may exceed these limitations (particularly in the 3.0 Telsa scanners). The risk of exposure may be even worse in patients taking drugs that enhance the damaging effects of sound, such as aminoglycoside antibiotics. Second, the noise generated by the MRI scanner is annoying to patients and contributes to their anxiety about the MRI scan. It is now common practice to provide patients with pneumatic headphones that attenuate the sound field and mask the imaging noise with music. However, the imaging noise remains clearly audible over this music. Finally, the MRI noise inhibits the use of functional MRI scanners (FMRI) to examine the areas of the brain stimulated by acoustic stimuli. The FMRI compares the activity in the brain with no stimulus to the activity that occurs when the subject is exposed to a stimulus. The fMRI generates such a loud acoustic stimulus that it is impossible to conduct a controlled study of the processing of sound by the brain.
In the past, both passive and active attenuation have been used to mitigate MRI noise. The passive attenuation usually consists either of foam insert ear plugs or ear muffs. Under normal circumstances these systems can provide 20 -30 decibel of attenuation. Commonly these hearing protection devices are combined with a pneumatic-driven system carrying sound to the patient's ears via plastic tubes (similar to the type of headset once commonly used on commercial aircraft). The pneumatic sound system allows the operator to give verbal instructions to the patient or play music, but probably somewhat reduced the passive attenuation of the hearing protection devices. Recently, a noise-attenuating headset has been developed that uses shielded non-magnetic (piezoelectric) electro-acoustic transducers to generate an audio signal inside the MRI without the considerable delays inherent in pneumatic headphones (as sound propagates from the driver unit into the MRI magnet bore through a tube).
Known active MRI attenuation noise cancellation systems include a system that uses a standard pneumatic headset system in which polyethylene tubes carry an acoustic signal from a pneumatic driver unit outside the MRI system to a pair of earmuffs worn by a patient. The pneumatic headset is modified with two additional polyethylene tubes that carry the sound signal inside the earmuff away from the headset to a pair of electret microphones located inside the MRI room. The electret microphones are connected to an electro-optical transducer which carry the earmuff noise signal to the shielded control room via fiber optic cables. A “compensation amplifier” produces an anti-noise signal by filtering the noise signal measured by the microphones and playing this signal back through the pneumatic headphones. An average attenuation of 11.1 decibel was reported for this system.
However, there are a number of issues with this MRI noise-cancellation system which are unresolved. The first limitation involves the frequency spectrum where the noise is canceled. The system is only effective at low frequencies (from 40 Hertz to 500 Hertzl and this is inconsistent with measurements of imager noise which indicate that most of the MRI noise occurs around 1 kiloHertz. Indeed, in a T
1
pre-scan, 95% of the acoustic power in the noise signal was in the range from 840 Hertz-1920 Hertz. Thus, it does not appear that this prior art MRI system would be capable of attenuating the MRI scans by 10 dBA. The second issue or limitation with the system is the handling of the large time delays between the acoustic noise signal and its measurement by the electret microphones, and between the generation of the acoustic anti-noise signal by the compensation and its arrival at the listener's ear. Both the measurement and production of sound in the system are limited by the propagation speed of sound. The signal at the patient's ear must travel down the tube before reaching the electret microphones, a distance of at least 5 ft and a delay of at least 5 ms, and the antinoise signal must travel back down the tube before reaching the listener's ears, at distance of perhaps 20 ft and a delay of 20 ms. Thus there is a built in delay in the control loop of this system of at least 20 ms, or 10 periods of a 500 Hz signal. Such a delay would compromise most traditional noise canceling algorithms such as the LMS algorithm.
Another known prior art active MRI noise cancellation system uses non-magnetic microphones located at fixed locations inside the magnet bore to record the error signal, and non-magnetic piezoceramic loudspeakers located at fixed locations to generate the noise cancellation signal. Thus, the system cancels the MRI noise signal at fixed locations inside the MRI, rather than at the locations of the patient's ears. The system uses a two-stage adaptive processing algorithm to generate the noise cancellation signal. The system is capable of reducing acoustic nois
Holins Gerald B.
James Talaya
Kundert Thomas L.
Lateef Marvin M.
The United States of America as represented by the Secretary of
LandOfFree
Delay based active noise cancellation for magnetic resonance... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Delay based active noise cancellation for magnetic resonance..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Delay based active noise cancellation for magnetic resonance... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2984203