Patent Application
20080048658
One embodiment of the invention will now be described in conjunction with the accompanying drawing in which:
The concept is demonstrated schematically in
Similarly, where the desired signal from the imaging volume is detected by the environmental noise sense coils, that signal is scaled down and eliminated from the noise signal. Otherwise the signal of interest would be considered noise and would be subtracted from the image, reducing the signal-to-noise ratio (SNR) of the image. An auto-correlation process which detects the relative size of the signals in the noise sense coils and the imaging coil can be used to automatically detect that the imaging signal is stronger in the imaging coil and thus should not be subtracted from the signal in the imaging coil.
The four sensors in
The sense coils and their associated electronics can be designed in different ways, depending on the processing algorithms that are used to subtract the noise signal. If the sense coil receive electronics are designed to match the spectral sensitivity of the imaging coil receive electronics a simple scale function can be used to subtract the noise.
In some cases this is not sufficient because the attenuation of the environment may not be uniform across all frequencies, so a processing step is required to apply a frequency-dependent scale factor. A broadband receiver system with uniform sensitivity across as wide a bandwidth as possible is advantageous in this scenario.
The signal can be processed in the time domain or in the frequency domain. If signals are processed in the time domain, frequency dependent delays must be eliminated to maintain coherency in the signals. Equivalently, if signals are processed in the frequency domain, the phase of the signals must also be considered because MRI is a phase sensitive technique.
Thus during the analysis of the RF signals, the method acts to effect subtracting a correlated noise signal in the pickup coil from the signals obtained by the imaging coil so as to reconstruct a reduced noise image.
A scale factor is applied on the noise signal so that its magnitude is correlated to the required level to extract the noise without interfering with the actual signal to be sensed. The scale factor on the noise signal can be frequency dependent, that is the scale factor is different at different frequencies in the signals.
In another case a frequency dependent phase shift is applied on the noise signal. That is the noise signal is shifted in phase before being subtracted from the detected signals and the phase shift is varied at different frequencies in the signals.
There may be provided a plurality of noise pickup coils and an identical processing is applied for the multiple noise pickup coils with independent scale factor, frequency dependent scale factor and frequency dependent phase shift factors which can be applied to the signals for each noise pickup coil before the signals are subtracted from the detected signals.
In a calibration step prior to the processing of the signals, a series of signal acquisitions are used to determine all scale and/or shift factors, frequency dependent or not. Thus a series of signal acquisitions are made that can be used to determine all scale and/or shift factors, frequency dependent or not.
In one mode of processing the subtraction is done in the time domain. However in an alternative mode of processing, the subtraction is done in the frequency domain after FFTs have been performed. This is shown in
As a further step in the initial or calibration process, there is provided a series of pre-scan steps in which one step in the pre-scan process steps looks at any image signal received by the noise pickup coils and senses that it is larger in the imaging coil than in the noise pickup coil and subsequently excludes it from the noise signal.
The pickup receiver system for the noise signals is selected such that it has properties which are arranged to match the properties of the image signal receiver system.
In the alternative the pickup receiver system has at least one property that does not match the properties of the image signal receiver system and, in this case, a signal processing unit is built into it to match the properties of any coil/receiver combination used for imaging.
All processing is done automatically as part of a processing system which controls the transmission signals and generates the images from the received signals.
The magnet can be mounted in a room which does not use a passive RF shield (Faraday cage) for environmental electronic noise suppression.
Alternatively, it provides electronic noise suppression if mounted within a passively RF shielded room (Faraday cage).
Since various modifications can be made in this invention as herein above described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without department from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.
This application claims the benefit under 35U.S.C.119 from Provisional Application 60/839714 filed Aug. 24th 2006. This invention relates to a method for effecting magnetic resonance imaging experiments which allows reduction in the deleterious effects of noise from other RF sources.
| Number | Date | Country | |
|---|---|---|---|
| 60839714 | Aug 2006 | US |
