The invention relates to a magnetic resonance imaging (MRI) system having an open superconducting magnet system, which comprises a number of horizontally oriented superconducting coils and a cryogenic container for containing a liquid cooling medium for cooling the superconducting coils which are located within the cryogenic container, the cryogenic container being provided, at its top, with a recondensor for continuously liquifying cooling medium evaporating from the container.

An MRI system as mentioned in the opening paragraph is known from US Patent 6,011, 456. This patent describes an open architecture recondensing superconducting magnet having a cryogenic container, being a helium vessel, for a superconducting MRI magnet in which the so-called"Zero-Boil-Off' (ZBO) technique is used for conservation of the helium coolant. The ZBO technique itself is a technique that aims to prevent loss of helium (or any other coolant) by means of a recondenser that re-liquefies the helium gas somewhere in the top of the helium vessel instead of letting it escape as a gas. In a magnet with a recondenser, the helium that evaporates cannot escape the helium vessel, because in the exit path (somewhere in the neck at the top of the magnet) it will encounter the cold surface of the recondenser, which causes the helium gas to liquefy. The recondensed helium then drips into the helium vessel again. So, there is continuous circulation of the helium.

Typically, in an MRI magnet according to the invention, the heat leak is of the order of 1 W, causing an evaporation of 1.4 litres/hour of liquid helium. As a result, about 14 litres/hour of 4.2 K helium gas try to escape the helium vessel, which is quite a large amount compared to conventional MRI magnets not having an open structure. So strictly speaking, the term"zero- boil-off'is not correct.

A further detail of the ZBO technique is that uncontrolled operation of the cryocooler may lead to underpressure in the helium vessel which is undesirable. The solution consists in controlling the pressure by means of a heater at the bottom of the helium vessel. In fact the heater spoils the excess cooling capacity of the recondensor. This ensures a constant circulation of helium, regardless of the quality of the cryostat.

As indicated, the recondensor should prevent the evaporated helium from leaving the helium vessel and does so to a large extent. However in practice some loss of helium is inevitable during service actions, failures of the cryogenic system or the existence of small undetectable helium leaks through which gas can escape the helium vessel. In other words, the helium loss averaged over a long period of time is very small but not really zero.

An important quality factor of the helium vessel in this respect is the effective volume, which is defined as the difference between the maximum fill ratio of the cooling medium and the minimum fill ratio of the cooling medium between which the magnet is allowed to operate. For example, if the maximum fill ratio is 95 % of the total volume of the vessel and the minimum fill ratio is 15 %, the magnet can be filled with helium to 95 % and will have to be re-filled before the fill ratio drops below 15 %. In this case the effective volume is 80 % of the total volume of the helium vessel. Typically, for an open magnetic resonance imaging (MRI) superconducting magnet system of medical imaging systems according to the state of the art, the maximum fill ratio and the minimum fill ratio would be relatively close to each other, for example respectively 95% and 85 %, in which case the effective volume is only 10 %, even if the magnet is equipped with a recondenser (ZBO technique). A large effective volume is therefore interesting because it will increase the helium re-fill interval.

It is an object of the invention to provide a medical imaging system the cryogenic container of which needs a re-fill of the fluid cooling medium at increased intervals or even not at all anymore during the economical lifetime of such a system.

In order to achieve said object, an MRI system according to the invention is characterized in that the MRI system comprises a circuit for guiding the liquid cooling medium from the recondensor along at least part of the superconducting coils. The cooling medium within the circuit should be capable of sufficiently cooling the superconducting coils. This does not necessarily mean that the superconducting coil is directly in contact with the cooling medium. Alternatively, sufficient cooling of the superconducting coils can also take place, for example, by using a thermal conductive intermediate material, for instance of the circuit itself, between the cooling medium and the superconducting coils. Since the circuit only necessitates the presence of the liquid cooling medium in the circuit, Vmin can be decreased drastically, thus correspondingly increasing the effective volume. For example in this manner, the invention enables the effective volume to be increased by a factor of 8,

leading to an 8 times greater re-fill interval. In practice this can be the difference between re- filling every 2 years, or every 16 years. In the latter case the magnet is not expected to be re- filled at all because 16 years exceeds the economical lifetime of an MRI medical imaging system.

A particular embodiment of an MRI system according to the invention is characterized in that the circuit comprises at least one local reservoir for the liquid cooling medium, which is located close to an associated superconducting coil for cooling said associated superconducting coil. In this manner, a very efficient increase of the effective volume is achieved.

A further embodiment of an MRI system according to the invention is characterized in that the local reservoir comprises an overflow edge for the liquid cooling medium, which is located at or above a lower side of the associated superconducting coil. In this manner it is ensured that at least the lower part of the associated superconducting coil is immersed in the fluid cooling medium. Due to the very good thermal conductive properties of superconducting coils, immersing only the lower part of said superconducting coil in the fluid cooling medium is sufficient for maintaining the associated superconducting coil as a whole at the required reduced temperature.

A yet further embodiment of an MRI system according to the invention is characterized in that the local reservoir is at least partly ring-shaped. In this manner, the shape of the local reservoir is adapted to the general shape of the associated superconducting coil. It is not strictly necessary within the spirit of the invention that this particular ring shape encloses a full circle. Due to the good thermal conductivity properties, it also can be sufficient to limit the ring shape to for instance 15 degrees of the associated superconducting coil, thereby further decreasing the minimum fill ratio at which the magnet is allowed to operate.

A particular embodiment of an MRI system according to the invention is characterized in that the local reservoir comprises a winding body for the associated superconducting coil. Said winding body is for instance part of a so called coil former. The number of additional structural components within the cryogenic reservoir is reduced, because the winding body not only serves as a body around which the coil is wound during the manufacture of the superconducting coil, but also as a wall or restriction for the reservoir.

A similar advantage applies in a further embodiment of an MRI system according to the invention, wherein the local reservoir comprises a positioning body for the associated superconducting coil. Such a positioning body, which is for instance also part of a

so called coil former, is normally present in the cryogenic reservoir anyway for avoiding unwanted deformation of the superconducting coils due to generated Lorentz forces.

A yet further embodiment of an MRI system according to the invention is characterized in that the circuit comprises downwardly sloping guides for guiding the liquid cooling medium from a first superconducting coil to a second superconducting coil which is positioned below the first superconducting coil. In this manner, an optimal routing of the fluid cooling medium is achieved. The fluid cooling medium is guided along the successive superconducting coils, with a minimal need for fluid cooling medium to be present between the superconducting coils. This is especially advantageous if the respective superconducting coils are not exactly positioned above each other.

A particular embodiment of an MRI system according to the invention is characterized in that the MRI system comprises sensor means for determining the level of the liquid cooling medium within the cryogenic container. In this manner, the fill ratio of the fluid cooling medium within the cryogenic container can be monitored. By extrapolating data derived from the sensor means, one can anticipate when, and if, a next re-fill would be necessary.

Hereinafter, the invention will be explained further by a description of a preferred embodiment of a magnetic resonance imaging (MRI) system according to the invention. For this description reference is made to the Figures, in which Figure 1 diagrammatically shows a cross-section of an open superconducting magnet system of the MRI system according to the invention, however without features specific to the invention, Figures 2a and 2b diagrammatically show a cross-section of the magnet system according to Figure 1, including features specific for the invention, and including a maximum level Vmax and a minimum level Vmin of helium between which the magnet system is allowed to operate.

Figure 1 shows a schematic cross-section of an open superconducting magnet system 1, which forms part of a magnetic resonance imaging (MRI) system according to the invention. Figure 1 does not show the specific features of the invention. The remaining parts of the MRI system, including a frame, a patient support unit and a control unit, are not shown

in Figure 1 and may be of a usual kind known to the skilled person. The magnetic field and the axis 2 of the magnet system 1 are oriented in the vertical direction. The magnetic field is generated by ring-shaped superconducting coils 3,4, 5, 6,7, 8,9, 10 each lying in a horizontal plane. The two coils 3,4 having the largest diameter constitute shield coils, which provide active shielding of the fringe field. Active shielding is the state-of-the-art method of reducing the fringe field of the magnet system 1 in order to minimize the volume occupied by the MRI system in the hospital.

In order to become superconducting, the superconducting coils 3-10 should be cooled. For such cooling the superconducting coils 3-10 are located within a cryogenic container 11. The shape of the cryogenic container 11 corresponds to the specific shape of the magnet system 1, which means that the cryogenic container 11 comprises a disc-shaped upper part 12 and a disc-shaped lower part 13, both parts 12 and 13 being interconnected at one side by a post 14. It is also possible within the scope of the invention to provide more posts, e. g. for mechanical stability, but only one post is shown in Figure 1. Following these specific contours, a radiation shield 15 and a helium vessel 16 are present in the cryogenic container 11. This results in the helium vessel 16 being common for both the upper part 12 and the lower part 13. This is the most economical way, because if two separate helium vessels were used for the upper part 12 and the lower part 13, many components would be needed twice, including the rather expensive cryocooler, as will become clear from the description given below.

In the particular design as shown in Figure 1, there are four superconducting coils located in the upper part 12 of the cryogenic container 11 and four superconducting coils located in the lower part 13, but this is not essential to the invention. A more homogeneous version of the magnet system may require more coils, whereas a non-shielded version of the magnet system may require fewer coils. In Figure 1 the coils of the upper and the lower part 12 and 13 are symmetric, however, this is not necessary within the context of the invention.

The areas 17,18 represent the positions of further components of the magnet system 1, such as gradient coils, RF coils, and a shim system, which have been recessed in recesses in the respective upper part 12 and lower part 13 of the cryogenic container 11. In this manner, maximum space is achieved for the patient within the patient space 19 between the upper part 12 and the lower part 13.

For cooling helium present within the helium vessel 16, a cryocooler 20 is provided, which penetrates into the helium vessel 16 through a neck 21 on top of the magnet.

The cryocooler 20 has two heat stations 22,23. The fist heat station 22 is connected to the radiation shield 15 of the magnet system 1. The second heat station 23 floats in the helium vessel 16 and acts as a recondenser. In this particular embodiment, an advanced two-stage cryocooler is used, of which the second heat station 23 or the second stage reaches a temperature below 4.2 K and is therefore capable of recondensing helium, while the first heat station 22 or the first stage cools the radiation shield 16. However, combining the functions of recondensing and cooling the radiation shield in a single cryocooler is not essential to the invention. Alternatively, one could use a helium liquefier and a separate means to cool the radiation shield, e. g. a separate cryocooler or a nitrogen coolant.

In Figures 2a and 2b the helium vessel 16 is shown without the surrounding cryogenic container 11 and the radiation shield 15. To improve the intelligibility of the Figures no lines are shown (unlike Figure 1) for showing the rotating movement of several elements, such as the superconducting coils, within the helium vessel 16. In addition to the elements shown in Figure 1, figures 2a and 2b show winding bodies 24,25, 26,27, 28,, 29, 30,31 and coil formers 32,33, 34,35, 36,37, 38,39 for each superconducting coil 3-10.

The superconducting coils are wound around the winding bodies during production of the magnet system 1. The coil formers are used to support the superconducting coils mechanically against Lorentz forces. The winding bodies and the coil formers would be present within the helium vessel even without applying the present invention since they are necessary anyway. Furthermore the helium present within the helium vessel 16 is shown using a grey scale.

In Figure 2a the helium is shown at the maximum level 40 at which the magnet system 1 is allowed to operate, whereas in Figure 2b the helium is shown at the minimum level 41 at which the magnet system 1 is allowed to operate. The difference between maximum and minimum filling is the so called effective volume that can easily reach values between 80% and 90% of the total volume of the helium vessel 16.

As described above, a dynamic balance exists within the helium vessel between on the one hand the evaporation of helium and on the other hand the liquefaction of helium by the condensor. Helium liquefied by the condensor is guided along all superconducting coils by means of a circuit. This circuit starts right below the condensor by a downwardly sloping chute 42, via which the helium can flow to a first ring-shaped local reservoir 63 having a U-shaped cross-section. The legs of the U-shape are formed by part of the helium vessel 16 itself and by a wall 43 located at the inner side of superconducting coil 3, whereas the body of the U-shape is formed by a bottom 44. On the left hand side of Figure

2a an overflow edge 45 of wall 43 is shown, from which a downwardly sloping pipe 48 extends to just above superconducting coil 5. Coil 5 is positioned within a second ring- shaped local reservoir 64 having a U-shaped cross-section. The legs of this U-shape are formed by a part of winding body 25 and wall 46, having an overflow edge 47. The body of the U-shape is formed by a part of coil former 33. Helium overflowing overflow edge 47 will arrive at a third ring-shaped local reservoir 65 formed by winding body 27 for superconducting coil 7 and by parts 49,50 of the helium vessel 16. The upper edge 51 of winding body 27 should be considered to be an overflow edge for the helium. After having overflown this edge 51, the helium reaches a fourth ring-shaped local reservoir 66 also having a U-shaped cross-section for superconducting coil 7. Wall 53, part 52 of helium vessel 16 and winding body 27 constitute this fourth local reservoir 66. The height of wall 53 is smaller than the height of winding body 27. For this reason, if the fourth reservoir 66 is fully filled with helium, helium will tend to overflow upper edge 54 of wall 53. Next, due to gravity, helium will fall through the part of helium vessel 16 extending through post 14, onto the upper part of a downwardly sloping chute 55, from which the helium flows down into a fifth ring-shaped local reservoir 67. This fifth local reservoir 67 serves to jointly cool superconducting coils 6,8 and 10. The fifth local reservoir 67 is formed by winding body 30 for superconducting coil 6, which winding body 30 is at its upper end sealingly connected to the helium vessel 16. Furthermore, the fifth local reservoir 67 is formed by bottom 56 which extends underneath the three superconducting coils 6, 8 and 10, and by wall 57. On the right hand side of Figures 2a and 2b it can be seen that the upper edge 58 of wall 57 is positioned slightly underneath the wall of the helium vessel 16, allowing passage of helium therebetween onto downwardly sloping chute 59, which guides the helium to a sixth ring- shaped local reservoir 68. This local reservoir 68 is formed by part 60 of the helium vessel 16, by coil former 39 and by winding body 31 which is extended by extension part 61. After having overflown upper edge 62 of the extension part 61, the helium arrives at the bottom 63 of the helium vessel 16. At the bottom a heater (not shown) is present for controllably evaporating the helium in order to create a dynamic equilibrium with the recondensation of the helium by the cryocooler 20.

In order to monitor the helium level in the helium vessel 16, a level sensor 69, known to the men skilled in the art, is provided on the inside of the helium vessel. This enables continuous monitoring of the helium level. Monitoring the helium level is advisable in order to detect problems at an early stage. A drop of the helium level indicates a failure (e. g. a gas leak) that will eventually lead to loss of helium from the magnet system 1.

It will be clear from the above description and from Figure 2b (in comparison to Figure 2a), that in order to cool the superconducting coils to the required temperature it is sufficient if all local reservoirs 63,64, 65,66, 67,68 are filled with helium. This is already achieved if only a relatively small part of the helium vessel 16 is filled with helium, since the local reservoirs have volumes which are substantially smaller than the total volume of the helium vessel 16. Liquid helium, which overflows one of the local reservoirs, arrives at a lower local reservoir, where applicable, via guiding elements such as chutes or pipes. In this manner, a continuous flow of liquid helium is achieved from the condensor to the highest local reservoir, and further to the lowest local reservoir via the intermediate local reservoirs, so that it is ensured that all reservoirs remain filled despite local helium evaporation in each local reservoir.