MR bridge support

Details MR bridge support MR bridge support

2001-11-21

2002-08-13

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S318000, C324S320000

Reexamination Certificate

active

06433549

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to MR patient support configurations and more specifically to a patient support that mounts to a main MR magnet and supports a patient table bridge.
An MR imaging system or scanner commonly includes a cryostat, which contains a powerful superconductive main magnet positioned around a main magnet bore. The superconductive magnet is maintained at an extremely cold temperature and produces a strong static magnetic field Bo along a bore axis within the main magnet. Other essential components of the MR system include an RF coil, or RF antenna, and a gradient coil assembly, which comprises a hollow cylindrical structure. The RF coil may be operated in a transmit mode, to generate MR signals in an imaging subject, or may be operated in a receive mode to detect the MR signals.
The gradient coils are electrically excited to impose X-, Y-, and Z-gradient time varying magnetic fields on the primary magnetic fields that are required for imaging purposes. In a common arrangement, each gradient field is produced by a pair or set of gradient coils, wherein each coil is wrapped around one of two cylindrical coil forms. The two coil forms are placed in coaxial relationship, and the coil forms and respective X-, Y- and Z-gradient coils collectively comprise a gradient coil assembly. Arrangements of this type are described, for example, in U.S. Pat. No. 5,570,021, issued Oct. 29, 1996 and commonly assigned herewith to the General Electric Company. Such arrangements are also described in U.S. Pat. No. 5,760,584, issued Jun. 2, 1998 and likewise assigned to the General Electric Company. Typically, the gradient coil assembly is mechanically supported within the cylindrical bore of the main magnet. The gradient coil, RF coil and main magnet together form an imaging area about the main magnet bore axis.
To support a patient within the main magnet bore during data acquisition, a patient support structure is provided that typically includes an upright support, a bridge, a cradle and a bridge support. The upright support is a rigid member that rests on a floor adjacent an MR imaging system within an imaging room and includes an upper end for receiving a first end of the bridge. The bridge includes a stiff member that extends between first and second ends where the first end is mounted to the rigid upright support and the second end extends into and through the imaging area so that its length is generally parallel to the bore axis. In many designs the bridge includes tracks for receiving cradle wheels and guiding the cradle through the imaging area.
The cradle is typically a relatively flimsy member having upper and lower oppositely facing surfaces and a length that is generally sufficient to support a patient. The cradle includes a plurality of wheels mounted to its lower surface and arranged so as to be receivable within the bridge tracks for guidance there along. The cradle is capable of movement along the bridge into various positions with respect to the imaging area including a loading position outside the imaging area and at least one imaging position where at least a portion of a patient disposed on the cradle is positioned within the imaging area.
As configured above, when a patient (especially a relatively heavy patient) is supported on the cradle and the cradle is fully extended on the bridge, the bridge has a tendency to deflect slightly downward thereby causing a patient misalignment. To overcome this problem many support configurations include a bridge support. To this end, an exemplary bridge support includes a rigid member that is typically mounted to the inside of the gradient coil and extends upwardly to and is secured to the bridge relatively closer to the second end of the bridge than the first end. Thus, when a patient is positioned within the imaging area, the cradle and bridge are supported by the upright support on the first end and by the bridge support, gradient coil and main magnet on the second end.
Unfortunately, when a gradient coil is excited to generate magnetic gradients, the gradients interact with structure about the coils and the coils tend to be mechanically displaced (i.e., vibrates). The mechanical structure used to support the gradient coil assembly within the main magnet bore provides a path for transferring or coupling the vibrations of the gradient coils to the main magnet structure. Generally, the main magnet is supported on the floor of the building in which the MR system is operated. Accordingly, the gradient generated vibrations are often directly coupled from the magnet to the floor, and then travel through the floor to vibrate structures throughout the building. As a result, gradient coil vibrations can couple acoustically to rooms outside of the MR scan room, i.e., a room which is specially constructed to house the MR system.
In addition to the problems associated with transmitting gradient vibrations to other facility equipment and space, the rigid bridge support also causes the gradient vibrations to be transmitted to the patient support cradle and a patient thereon. While gradient related patient vibration in early MR systems was relatively minimal and therefore could essentially be ignored, characteristics of newer MR systems have resulted in greater adverse effects. For instance, gradient technology has evolved to the point where relatively high gradient fields are employed during data generation and acquisition so that the magnitude of gradient related vibrations is relatively greater in newer systems. In addition, the actual mass of the imaging components (i.e., the main magnet, coils, shields, etc.) has been reduced appreciably such that even small gradient fields sometimes cause appreciable vibration.
Cradle vibration has two adverse side effects. First, whenever a patient is exposed to a new or unfamiliar medical process, the patient typically and understandably experiences anxiety and nervousness about the experience. This is especially true of MR imaging procedures where noise and essentially uncontrolled gradient movement and vibrations are transmitted to the patient. Anxiety often causes patients to move or flinch during acquisition which can cause image artifacts in images generated with collected data. Second, even where a patient manages to remain essentially still relative to a supporting cradle, where the cradle and patient vibrate together relative to the RF data receiving coils, resulting images are typically polluted by image artifacts.
One configuration that essentially isolates the gradient coils from the main magnet and thereby mitigates transmission of gradient vibrations to the main magnet and surrounding facility equipment and space is described in U.S. Pat. No. 6,160,399 (hereinafter “the '399 patent”) which issued on Dec. 12, 2000 and is entitled “Apparatus For Supporting MR Gradient Coil Assembly”. According to the '399 patent, two mounting assemblies are mounted to the main magnet and extend axially to the gradient coils, a separate mounting assembly disposed at either end of the main magnet. The mounting assemblies transmit a static force from the main magnet to the gradient coil assembly to hold the coil assembly in place within the main magnet bore in coaxial relationship with the bore and in spaced-apart relationship with an internal bore wall. At the same time, the two mounting assemblies act to dampen or attenuate the gradient coil vibrations, and thus oppose passage transmission of the vibrations through the mounting assemblies to the main magnet. The '399 patent configuration provides no other path through which gradient coil vibrations can be transferred from the gradient coil assembly to the main magnet.
While addressing the problem of transmitting gradient vibrations from the MR configuration to facility equipment and space, unfortunately the mounting assemblies descr

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