Abstract:
A system for isolating vibration and reducing acoustic noise in the RF coil of an MR imaging apparatus is presented. The system positions an RF conductor in its operative position proximate to an RF support form. The RF conductor is sandwiched between a vibration decoupling layer and a mass loading layer. The vibration decoupling layer is affixed to the RF support form so that the vibration decoupling layer is positioned between the RF conduit and the RF support form while the mass loading layer is located exterior of the RF conductor. By this arrangement, the acoustic energy is decoupled from the RF support form by the vibration decoupling layer while the vibration is reduced by the mass loading layer located exterior of the RF conductor.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to an RF coil used in an MR imaging system, and, more particularly, to an RF coil having enhanced acoustic deadening properties. 
   With MR scanners, the apparatus basically includes an RF coil that surrounds the subject and which directs the RF energy toward the subject or which receives RF energy from the subject, in carrying out the scanning process. 
   One of the difficulties of such MR scanners, however, is that the noise level can become uncomfortably loud, both for the patient, or subject, and for the operators. The source of such acoustic noise can be many and varied, however, the RF coil has been shown to be a major contributor. 
   The noise from the RF coil is due to Lorentz forces set up in the RF conductors and, while other acoustic noise sources in the MR scanner can be addressed by standard vibration isolation techniques, the acoustic noise from the RF coil is more difficult to control due to its close proximity to the patient, or subject, bore. 
   There have been attempts at reducing the acoustic noise from the RF coil. Such attempts have included breaking up the RF conductor, where possible, to reduce eddy currents and constrained layer damping to reduce the RF support form vibration. These attempts, however, have not been able to eliminate all of the acoustic noise from the RF coil. 
   It would therefore be desirable to have a RF coil having a reduced acoustic output by providing vibration isolation between the RF conductors and the RF support form as well as providing damping to reduce the vibration from the RF conductor to the RF support form. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention provides a system and method of providing reduced acoustic output by providing vibration isolation between an RF conductor and an RF support form of an MRI system. 
   In accordance with one aspect of the invention, an MR scanning apparatus includes an isolating decoupling layer that is located between an RF conductor and an RF support form, and a mass loading layer is attached to the RF conductor. 
   In accordance with another aspect of the invention, a method of constructing an RF coil that includes affixing a vibration decoupling layer between an RF conductor and an RF support form, and affixing a mass loading layer to the RF conductor. 
   In accordance with another aspect of the invention, a magnetic resonance imaging system that includes an RF transceiver system and a gradient coil assembly, wherein the RF transceiver system includes an RF conductor, an RF support form, a vibration decoupling layer attached between the RF conductor and the RF support form, and a mass loading layer attached to the RF conductor. 
   Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a schematic block diagram of an MR imaging system for use with the present invention. 
       FIG. 2  is a cross-sectional view of a present RF conductor affixed directly to an RF support form; and 
       FIG. 3  is a cross-sectional view of a RF conductor and mass loading layer affixed to an RF support form in accordance with the preferred embodiment of the present invention. 
       FIG. 4  is a cross-sectional view of a RF conductor and mass loading layer affixed to an RF support form in accordance with an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A system is shown to isolate vibration, reduce the vibration natural frequency, and reduce acoustic noise in the RF coil of an MR imaging apparatus. 
   Referring to  FIG. 1 , the major components of a preferred magnetic resonance imaging (MRI) system  10  incorporating the present invention are shown. The operation of the system is controlled from an operator console  12  which includes a keyboard or other input device  13 , a control panel  14 , and a display screen  16 . The console  12  communicates through a link  18  with a separate computer system  20  that enables an operator to control the production and display of images on the display screen  16 . The computer system  20  includes a number of modules which communicate with each other through a backplane  20   a . These include an image processor module  22 , a CPU module  24  and a memory module  26 , known in the art as a frame buffer for storing image data arrays. The computer system  20  is linked to disk storage  28  and removable storage  30  for storage of image data and programs, and communicates with a separate system control  32  through a high speed serial link  34 . The input device  13  can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription. 
   The system control  32  includes a set of modules connected together by a backplane  32   a . These include a CPU module  36  and a pulse generator module  38  which connects to the operator console  12  through a serial link  40 . It is through link  40  that the system control  32  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  38  operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module  38  connects to a set of gradient amplifiers  42 , to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module  38  can also receive patient data from a physiological acquisition controller  44  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module  38  connects to a scan room interface circuit  46  which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  46  that a patient positioning system  48  receives commands to move the patient to the desired position for the scan. 
   The gradient waveforms produced by the pulse generator module  38  are applied to the gradient amplifier system  42  having Gx, Gy, and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated  50  to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  50  forms part of a magnet assembly  52  which includes a polarizing magnet  54  and a whole-body RF coil  56 . A transceiver module  58  in the system control  32  produces pulses which are amplified by an RF amplifier  60  and coupled to the RF coil  56  by a transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil  56  and coupled through the transmit/receive switch  62  to a preamplifier  64 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  58 . The transmit/receive switch  62  is controlled by a signal from the pulse generator module  38  to electrically connect the RF amplifier  60  to the coil  56  during the transmit mode and to connect the preamplifier  64  to the coil  56  during the receive mode. The transmit/receive switch  62  can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode. 
   The MR signals picked up by the RF coil  56  are digitized by the transceiver module  58  and transferred to a memory module  66  in the system control  32 . A scan is complete when an array of raw k-space data has been acquired in the memory module  66 . This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor  68  which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  34  to the computer system  20  where it is stored in memory, such as disk storage  28 . In response to commands received from the operator console  12 , this image data may be archived in long term storage, such as on the tape drive  30 , or it may be further processed by the image processor  22  and conveyed to the operator console  12  and presented on the display  16 . 
   Turning now to  FIG. 2 , there is shown a cross-sectional view of a conventional RF coil  56  and which comprises an RF support form  70  that is cylindrical. The RF support form  70  is normally of a fiberglass material and surrounds the patient. An RF conductor  72  is present to generate or receive the RF energy used in the scanning process. The RF support form  70  has an interior surface  74  that faces towards the patient and an exterior surface  76  that faces outwardly. As such, the interior surface  78  of the RF conductor  72  contacts the exterior surface  76  of the RF support form  70  and is adhesively bonded thereto. 
   As can be seen in  FIG. 2 , the conventional system has the RF conductor  72  directly affixed to the RF support form  70  and, therefore, vibrations can travel directly from the RF conductor  72  to the RF support form  70 , such that the RF support form  70  acts as a large sounding board to transmit vibration to the surrounding environment resulting in acoustic noise. 
   Turning now to  FIG. 3 , there is shown, a cross-sectional view of an RF coil  80  constructed in accordance with the preferred embodiment of the present invention. The RF conductor  72  is affixed exterior, or radially outward, to the RF support form  70 , as in the  FIG. 2  embodiment; however, as can be seen, there is decoupling layer  82  of a vibration, or mechanical, decoupling material that is located between the RF conductor  72  and the RF support form  70 . The decoupling layer  82  is in the form of a layer of a decoupling material and is a vibration isolating material, such as a foam, and which decouples and isolates the acoustic energy of the RF conductor  72  from the RF support form  70 . Basically, a soft foam material can be used for the intermediate decoupling layer  82 ; however, other material could be used that provides the isolation of acoustic energy. 
   Thus, the intermediate decoupling layer  82  is affixed to the interior surface  78  of the RF conductor  72  and the exterior surface  76  of the RF support form  70 . That affixation can be accomplished by a suitable adhesive. 
   There is also present a mass loading layer  84  that is located on the exterior surface  86  of the RF conductor  72 . The mass loading layer  84  replaces the loading effect of the RF support form  70  and reduces the natural frequency of the overall RF conductor  72 /decoupling layer  82  combination. As such, the mass loading layer  84  reduces the overall transferred vibration energy to the RF support form  70  and, with the mass loading layer  84 , the vibration of the RF conductor  72  decreases and reduces the overall acoustic noise. The mass loading layer  84  can be a heavy material such as a vinyl material having, for example, barium salt contained therein to increase the mass of the material. 
   As can now be seen, the RF conductor  72  is basically fixed in an operative position proximate to the RF support form  70  and is sandwiched between the vibration decoupling layer  82  and the mass loading layer  84 . The RF conductor  72  is therefore affixed to the RF support form  70  providing a combination of decoupling and mass loading that effectively reduces the vibration energy generated by the RF conductor  72  from reaching and affecting the RF support form  70 , thus reducing the acoustic noise of the system. 
   Referring to  FIG. 4 , there is shown, a cross-sectional view of an RF coil  100  constructed in accordance with an alternate embodiment of the present invention. The RF conductor  72  is affixed interior, or radially inward, to the RF support form  70 . However, as can be seen, there is decoupling layer  82  of a vibration, or mechanical, decoupling material that is located between the RF conductor  72  and the RF support form  70 . The decoupling layer  82  is in the form of a layer of a decoupling material and is a vibration isolating material, such as a foam, and which decouples and isolates the acoustic energy of the RF conductor  72  from the RF support form  70 . Basically, a soft foam material can be used for the intermediate decoupling layer  82 ; however, other material could be used that provides the isolation of acoustic energy. 
   Thus, the intermediate decoupling layer  82  is affixed to the exterior surface  98  of the RF conductor  72  and the interior surface  96  of the RF support form  70 . That affixation can be accomplished by a suitable adhesive. 
   There is also present a mass loading layer  84  that is located on the interior surface  106  of the RF conductor  72 . The mass loading layer  84  replaces the loading effect of the RF support form  70  and reduces the natural frequency of the overall RF conductor  72 /decoupling layer  82  combination. As such, the mass loading layer  84  reduces the overall transferred vibration energy to the RF support form  70  and, with the mass loading layer  84 , the vibration of the RF conductor  72  decreases and reduces the overall acoustic noise. The mass loading layer  84  can be a heavy material such as a vinyl material having, for example, barium salt contained therein to increase the mass of the material. 
   The RF conductor  72  is basically fixed in an operative position proximate to the RF support form  70  and is sandwiched between the vibration decoupling layer  82  and the mass loading layer  84 . The RF conductor  72  is therefore affixed to the RF support form  70  providing a combination of decoupling and mass loading that effectively reduces the vibration energy generated by the RF conductor  72  from reaching and affecting the RF support form  70 , thus reducing the acoustic noise of the system. 
   While the description of the preferred embodiments has been focused on a cylindrical RF coil which surrounds the subject, the invention also may have application for other type of RF coils. These include but are not limited to transmit/receive surface coils and receive only surface coils. 
   The present invention has been described in terms of the preferred embodiment and an alternate embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.