Method and apparatus for reducing acoustic noise in MRI scanners

#The present invention relates generally to the field of magnetic resonance imaging systems and, more particularly, to a technique for reducing acoustic noise in such systems.

#Magnetic resonance imaging (MRI) systems have become ubiquitous in the field of medical diagnostics. Over the past decades, improved techniques for MRI examinations have been developed that now permit very high quality images to be produced in a relatively short time. As a result, diagnostic images with varying degrees of resolution are available to the radiologist that can be adapted to particular diagnostic applications.

#In general, MRI examinations are based on the interactions among a static or primary magnetic field, a radio frequency (RF) magnetic field and time varying magnetic field gradients, with nuclear spins within the subject of interest. Specific nuclear components, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of such nuclear components can be influenced by manipulation of the fields to obtain RF signals that can be detected, processed, and used to reconstruct a useful image.

#FIG. 1 is a diagrammatical representation of an MRI system for use in medical diagnostic imaging and implementing certain aspects of the present shielding technique;

#FIG. 2 is a block diagram of functional components of an exemplary pulse sequence description module in a controller for a system of the type illustrated in FIG. 1;

#FIG. 3 is a graphical representation of an exemplary pulse sequence description for an MRI examination which may be implemented in the system of FIG. 1;

#Turning now to the drawings, and referring first to FIG. 1, a magnetic resonance imaging (MRI) system 10 is illustrated diagramatically as including a scanner 12, scanner control circuitry 14, and system control circuitry 16. While MRI system 10 may include any suitable MRI scanner or detector, in the illustrated embodiment the system includes a full body scanner comprising a patient bore 18 into which a table 20 may be positioned to place a patient 22 in a desired position for scanning. Scanner 12 may be of any suitable type of rating, including scanners varying from 0.5 Tesla ratings to 1.5 Tesla ratings and beyond.

#Scanner 12 includes a series of associated coils for producing controlled magnetic fields, for generating radio frequency excitation pulses, and for detecting emissions from gyromagnetic material within the patient in response to such pulses. In the diagrammatical view of FIG. 1, a primary magnet coil 24 is provided for generating a primary magnetic field generally aligned with patient bore 18. A series of gradient coils 26, 28 and 30 are grouped in a coil assembly for generating controlled magnetic gradient fields during examination sequences as described more fully below. A radio frequency coil 32 is provided for generating radio frequency pulses for exciting the gyromagnetic material. In the embodiment illustrated in FIG. 1, coil 32 also serves as a receiving coil. Thus, RF coil 32 may be coupled with driving and receiving circuitry in passive and active modes for receiving emissions from the gyromagnetic material and for outputting radio frequency excitation pulses, respectively. Alternatively, various configurations of receiving coils may be provided separate from RF coil 32. Such coils may include structures specifically adapted for target anatomies, such as head coil assemblies, and so forth. Moreover, receiving coils may be provided in any suitable physical configuration, including phased array coils, and so forth. As discussed in greater detail below, the scanner 12 includes energy-dampening laminate structures disposed at various locations about the primary magnet coil 24, the gradient coils 26, 28 and 30, and the radio frequency coil 32 for reducing emitted acoustic noise from the scanner during use.

#The coils of scanner 12 are controlled by external circuitry to generate desired fields and pulses, and to read emissions from the gyromagnetic material in a controlled manner. As will be appreciated by those skilled in the art, when the material, typically bound in tissues of the patient, is subjected to the primary field, individual magnetic moments of the paramagnetic nuclei in the tissue attempt to aligned with the field but precess in a random order at their characteristic or Larmor frequency. While a net magnetic moment is produced in the direction of the polarizing field, the randomly oriented components of the moment in a perpendicular plane generally cancel one another. During an examination sequence, an RF frequency pulse is generated at or near the Larmor frequency of the material of interest, resulting in rotation of the net aligned moment to produce a net transverse magnetic moment. Radio signals are emitted following the termination of the excitation signals. This magnetic resonance signal is detected in the scanner and processed for reconstruction of the desired image.