1. Field of the Invention
The invention relates to an electromagnetic shielding device for nuclear magnetic resonance apparati.
2. Discussion of Related Art
U.S. Pat. No. 4,725,781 discloses such a shielding device. Particularly, the shielding device is provided as combined with a nuclear magnetic resonance image detecting apparatus, of the type adapted to contain, in its cavity, only one body part. The cavity, which is substantially defined in shape by the shape of the coil of the magnet for generating the static field is tubular and open at its ends. The stationary part of the electromagnetic shield is stably integrated into the magnetic structure of the apparatus, and is positioned radially inside the coils for generating the gradient field, and radially outside the coils for transmitting nuclear spin exciting pulses. At the open ends, the electromagnetic shield is also open, and may be entirely or partially closed by electrically conductive elements, which may be removably applied at the ends of the cavity and electrically connected to the stationary part of the shield.
The open end of the cavity, through which no body part is intended to pass, may be completely closed. The end of the cavity through which the body part is intended to enter into the detection area, is only partially closed by a removable shielding element, with an opening which is reduced, according to the body size, with respect to the span of the opening of the cavity. The shielding means are in contact with the body or the part thereof passing through them, to penetrate the cavity. Although the device disclosed by U.S. Pat. No. 4,725,781 allows to eliminate or anyway drastically reduce magnetic noise infiltrating inside the cavity, by entirely closing or minimizing the eventual opening/s through which the body to be examined may reach the detection area of the cavity, this effect is completely neglected in said document. The shielding is not aimed at preventing noise infiltrations, but at ensuring a higher concentration of the field lines of the RF transmitting and/or receiving coils. Particularly, no emphasis is placed on keeping the open areas of the detection cavity far from the image zone, that is from the receiving coil, on the contrary, in order to obtain the purposes stated in the document, the effectiveness of the shielding increases as the inner volume of the shield is decreased, i.e. as the latter approaches the image zone. Moreover, this type of shielding is absolutely ineffective against electrical noise. Particularly in nuclear magnetic resonance image detection machines for predetermined body parts, a considerable part of the body remains outside the detection cavity and acts as an antenna, through which electrical variations penetrate up to the detection area of the cavity, and particularly up to the body part under examination, and so up to the receiving coils.
Furthermore, since nuclear magnetic resonance apparati are used by operators which are unskilled in the knowledge of physics, the above mentioned patent does not deal with the problem of user-friendliness of shields, particularly with respect to the operations of assembling and disassembling the parts which are meant to be associated to the openings of the cavity or of the detection volume.
Appearance and comfort are also important for the patient who should not be shocked during his treatment, and especially should not be frightened but reassured by the appearance of the apparatus.
The article entitled "Shielded solenoidal probe for in vivo NMR studies of solid tumors" by Th. C. N G, J. D. Glickson, published in "Magnetic Resonance in Medicine", vol. 2, 1985 provides, for the study of cutaneous and subcutaneous tumors, the introduction of a mouse into a metal cylinder connected to ground, which cylinder has an opening with a variable span, from which a part of the tumor tissue to be examined is made to protrude. In this case, the combination of the animal and the shielding cylinder is inserted into the cavity and into the area being relevant for nuclear magnetic resonance image acquisition. The shielding is used to eliminate the echoes coming from the innermost parts of the body of the animal with respect to the outer areas, having the tumor cells to be examined. Electromagnetic noise which may eventually affect the quality of the acquired image and which come from outside are not a problem. The reason is that the tested element, that is the animal, may be completely introduced into the cavity for image detection in such a distant position from the openings that it is anyway not much exposed to electromagnetic noise.