Patent Application: US-76633810-A

Abstract:
a magnetic resonance imaging system having an exterior cryogenic enclosure containing a device for creating an intense main magnetic field in a usable interior tunnel space an rf exciter , a set of solenoid gradient windings in a cylindrical space around the interior tunnel space and electronic control circuits . the cryogenic enclosure includes an interior cylindrical space vacuum at room temperature having the set of windings therein , at least one thermal screen in a temperature range of 20 ° k to 80 ° k , a cold box below 5 ° k , and a former supporting the exciter for creating an intense main magnetic field . to reduce acoustic noise and cryogenic losses , an additional envelope is between the set of windings and the vacuum enclosure , the additional envelope being of a conductive material having electrical resistivity at least 7 × 10 − 8 ω · m and having a characteristic frequency no more than the characteristic frequencies of each of the other components of the exterior cryogenic enclosure .

Description:
the invention applies to magnetic resonance imaging systems 7 such as those described above with reference to fig1 to 6 that use a cryostat 10 including a vacuum enclosure 102 at room temperature , for example made of stainless steel , containing a gradient generator system 2 , at least one thermal screen 103 , 104 made of a material that is a good conductor of heat , such as aluminum , and a cold box 105 , for example made of stainless steel , which is at the boiling point of helium , which is 4 . 2 k . the envelopes 102 to 105 are concentric . the cold box 105 incorporates a former 101 , for example made of aluminum alloy , supporting superconductor windings constituting the magnet 1 that creates a main magnetic field . the gradient generator system 2 , which interacts with the cryostat 10 , can be produced as shown diagrammatically in fig3 with gradient windings 21 in a direction x , gradient windings 22 in a direction y perpendicular to the direction x , and gradient windings 23 in a direction z perpendicular to the directions x and y and aligned with the axis of the imaging apparatus . this gradient generator system 2 is not described again . the various components 102 to 105 constituting the cryostat 10 are metal cylinders that are thin ( having a thickness of a few millimeters for a diameter of approximately one meter on the inside and a diameter of two meters on the outside ) and are connected by domed ends to constitute toroidal spaces of small substantially rectangular axial section ( see fig5 ). such envelopes constitute shells at different temperatures . for example , fig9 shows a vacuum enclosure 102 at room temperature , a first thermal screen 103 at a temperature of approximately 80 k , a second thermal screen 104 at a temperature of approximately 20 k , and a helium reservoir or cold box 105 at the boiling point of helium , which is 4 . 2 k . the former 101 and the superconductor windings constituting the magnet 1 are also at the boiling point of helium . according to the invention , there is inserted between the gradient generator system 2 and the interior cylinder 102 of the cryostat 10 at least one additional cylindrical envelope 107 that : has a characteristic frequency f 0 as different as possible from the characteristic frequencies of the other envelopes 102 to 105 present in the cryostat , which is not true of the usual good conductors such as copper as recommended by reference [ 9 ]; is a rather poor conductor , so as not to carry high eddy currents , which also goes against the teachings of reference [ 9 ]. a plurality of successive additional envelopes having the same properties can be inserted , for example two or three additional envelopes 107 , as a function of the space available . however , a single envelope already provides a considerable benefit and does not entail modifying the entire pre - existing structure of the cryostat . to address the first of the characteristics required for the additional envelope 107 , the above expression for f 0 shows that it is necessary to use materials with a low young &# 39 ; s modulus and a high density or vice - versa . the characteristic frequencies for 1 m diameter cylinders are as follows : the first two materials have the most “ abnormal ” frequencies but the toxicity of beryllium and its compounds in practice rules out their use . this material must therefore be excluded . moreover , its resistivity of 4 × 10 − 8 ω · m would be too low to satisfy the second criterion . in contrast , lead is entirely appropriate , with its relatively high resistivity of 21 × 10 − 8 ω · m and offering the possibility of “ adjusting ” both the characteristic frequency and the resistivity to their optimum values by producing alloys of lead with tin , bismuth , or cadmium in particular . examples of the properties of lead alloys are given in references [ 13 ] and [ 14 ], for example . alternatively , the additional envelope 107 can be made of tin , which has a characteristic frequency of 892 . 3 hz for a 1 m diameter cylinder and a resistivity of 10 × 10 − 8 ω · m . otherwise , depleted uranium is difficult to use and has a characteristic frequency that is too high ( 1080 . 8 hz for a 1 m diameter cylinder ). in the fig9 embodiment , a tubular additional intermediate envelope 107 is used that , in order to prevent direct transmission of vibration , is not mechanically connected to the gradient generator system 2 or to the vacuum enclosure 102 of the cryostat . for example , the additional intermediate envelope 107 can consist of a cylinder of ordinary lead with the following dimensions : mean radius 0 . 44 m , thickness 10 mm , length 3 . 6 m . this yields the fig1 energy dissipation spectra , which show a spectacular reduction in the harmful effects of the gradients and in particular the disappearance of the peaks in the vicinity of 1000 hz to 1500 hz relative to prior art apparatus with no additional envelope ( fig7 ) or with a cylindrical envelope made of a material that is a very good conductor , such as copper ( see fig8 and the curve a 106 corresponding to the spectrum of the power dissipated in a copper cylinder ). in fig1 , the curve a 107 is the spectrum of the power dissipated in the lead cylinder 107 . this power spectrum a 107 shows that the power dissipated in the lead cylinder 107 remains below approximately 200 w up to a frequency of the order of 1700 hz but produces a drastic power reduction for all the power spectra a 101 to a 105 of the other envelopes 101 to 105 . in particular , the power spectrum a 102 linked to the first stainless steel envelope 102 , which is usually the main cause of acoustic noise , has a peak of the order of only 100 w at around 500 hz , whereas in a prior art system with no additional envelope ( see fig7 ), this peak is in the range 1300 to 2000 w at around 1700 hz , and even with the insertion of a copper conductive envelope as taught by reference [ 9 ] ( see fig8 ), there is still a peak of the order of 750 w at around 1200 hz . in the same way , with a system of the present invention , by means of the presence of an additional cylindrical envelope made of lead , the dissipation peak of the power spectrum a 105 of the cold box 105 ( see fig1 ) is no more than approximately 50 w at a frequency of the order of 400 hz , whereas in a prior art system with no additional envelope ( see fig7 ) this peak is of the order of 600 w at around 1700 hz , and even with the insertion of a conductive envelope made of copper as taught by reference [ 9 ] ( see fig8 ), there is still a peak of the order of 250 w at around 1300 hz . the higher the electrical resistivity of the additional envelope 107 , the better the results , but obviously this must not be taken as far as choosing an insulative material , which would have no effect . this means that for a given machine ( gradient windings and cryostat ) there is an optimum choice for the combination ( f 0 , σ ) from among the materials available . the length and thickness of the cylinder 107 can equally be adjusted to obtain the best results for given a gradient generator system and a given cryostat . as a general rule , for standard mri systems , the additional envelope 107 can comprise a tubular structure having a mean radius in the range 0 . 4 m to 0 . 5 m , thickness in the range 8 mm to 12 mm , and a length in the range 2 m to 4 m . the fig9 embodiment uses an intermediate tube 107 that is not mechanically connected to the gradient generator system 2 or to the cryostat 10 . this is to prevent direct transmission of vibration . however , in another possible embodiment , a cylinder 108 of lead or equivalent material can be mechanically connected to the set 2 of gradient windings to modify and above all reduce their natural vibration ( fig1 ). a cylindrical layer 109 of lead or equivalent material can also be mechanically connected ( for example glued or welded ) to the interior tube 102 of the cryostat 10 to modify its vibrational properties ( fig1 ). for a given gradient generator system 2 and a given cryostat 10 , it is possible to determine the best configuration including one or more lead cylinders 107 , 108 , or 109 , whether or not they are mechanically connected to the gradient generator system 2 or to the interior tube 102 of the cryostat 10 , by modeling using the methods referred to above . a preferred embodiment remains one with a mechanically free intermediate tube 107 made of lead or lead alloy having a characteristic frequency f 0 that is as low as possible and electrical resistivity that is at least as high as that of lead . an embodiment with an intermediate tube made of tin or tin alloy constitutes an alternative solution offering slightly reduced performance but nevertheless achieving a significant improvement in terms of reducing acoustic noise and cryogenic losses compared to embodiments using tubes made of copper or other materials that are very good conductors of electricity . 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[ 11 ] a set of simple , accurate equations for circular cylindrical elastic shells , james g . simmonds , int . j . solids structures , 1966 , vol 2 pp 525 - 541 . [ 12 ] echo planar imaging at 4 tesla with minimum acoustic noise , d . g . tomasi & amp ; t . ernst , journal of magnetic resonance imaging , 18 , pp . 128 - 130 , 2003 . [ 14 ] structure , mechanical properties and electrical resistivity of rapidly solidified pb — sn — cd and pb — bi — sn — cd alloys , mustafa kamal , abu bakr el - blediwi , mohamed bashir karman , journal of material science , materials in electronics 9 , pp . 425 - 428 , 1998 .