WO2009150576A1

WO2009150576A1 - Cryocooling system for mri providing reduced artifacts caused by vibrations

Info

Publication number: WO2009150576A1
Application number: PCT/IB2009/052349
Authority: WO
Inventors: Paul R. Harvey; Chandra T. R. Reis; Adrianus M. Machielsen; Johannes A. Overweg; Glen G. Pfleiderer
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-06-10
Filing date: 2009-06-03
Publication date: 2009-12-17

Abstract

The invention relates to a magnetic resonance imaging cooling system (148) fo cooling a superconducting magnet (122) of a magnetic resonance imaging system the cooling system (148) comprising a cold head (154), wherein the cold head (154) is drivable at a variable stepping frequency, the cooling system (148) being adapted for varying the stepping frequency in a frequency range of continuous cold head operation, such as to reduce image artifacts caused by vibrations of the MRI system generated by the operation of the cold head.

Description

Cryocooling system

TECHNICAL FIELD

The invention relates to a magnetic resonance imaging cooling system, a controller, a method of operating a magnetic resonance imaging cooling system and a computer program product.

BACKGROUND AND RELATED ART

Magnetic resonance imaging (MRI) is a state of the art imaging technology which allows cross sectional viewing of objects like the human body with unprecedented tissue contrast. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The basis of both NMR and MRI is the fact, that atomic nuclei with non-zero spin have a magnetic moment. In medical imaging, usually nuclei of hydrogen atoms (i.e. protons ¹H) are studied since they are present in the body in high concentrations like for example as water. Radio frequency waves are directed into nuclei in strong external magnetic fields, which lead to an excitation of protons and a relaxation of the protons. Due to the relaxation of the protons, radio signals are emitted which can be detected and computer processed to form an image.

A fundamental part in modern MRI systems is a superconducting magnet which is required in order to provide the strong external magnetic field, Bo. In order to operate a superconducting magnet, the superconducting material needs to be cooled to critical low temperatures, for example below 4.2K, which typically requires the usage of liquid helium as coolant for the superconducting materials. In order to reach these critical temperatures and to maintain the temperatures, a cooling system, typically a cryocooler, is used which is a fundamental part of a modern superconducting MRI system. The cooling system consists of a cold head and a compressor unit. The cold head is integrated with the vessel of the magnet and consists of an electric motor connected to a displacer. The displacer is located in a tube which makes direct or indirect thermal contact with the magnet. Typically, the stepper motor has 48 steps and is driven at a stepping frequency of 50 Hz by a synchronous power supply. In this example, it takes 0.96 seconds for one complete rotation. The displacer commonly contains material that has a strong reaction to magnetic field, such as Holmium Copper. Alternatively the 'displacer' can be a pulse of compressed gas as in a pulse tube. The following concepts apply to all cold heads that have an inherent frequency that can translate as vibration to the magnet. As with all physical systems, the magnetic resonance imaging system has at least one inherent mechanical resonance frequency. Optimal cooling is obtained when the cold head is in close physical contact with the cryostat. Because of this contact, the step motion generated by the cold head stepper motor gives rise to vibrations of the imaging system structure close to the stepping frequency. Typically these vibrations are further modulated by the rotational frequency of the stepper motor. Vibrations of the imaging system structure are translated to oscillations of the main magnetic field around the frequency of the vibration. These vibrations of the magnet field, typically referred to as Bo modulations, then cause image artifacts in acquired MR images. Additional artifacts can develop due to the motion of the displacer material in the magnetic field, even if the vibration transfer is prevented.

Many MR scans use very short repetition times (TRs). For these scans it is possible that the scan is complete within one cycle of the cold head motor and the scan is so short that it is immune to any kinds of Bo disturbances. In other cases however, the scan may be long enough to make the scan sensitive to magnetic field disturbances for example resulting from vibrations induced due to the moving parts of the stepper motor.

This is a known problem in the state of the art. Special shielding has been developed to absorb the induced magnetic changes due to the movement of the displace materials. Complicated methods have been developed to reduce the vibration transfer from the cooling system to the magnet vessel. These include hanging heavy weights from the cold head to act as a mechanical damper, locating the coldhead at some distance from the magnet and pumping the cold liquid or gas, or even turning off the required cooling during imaging. This last leads to a need for the MRI system to sit idle for a significant portion of every day to recover the lost cooling time.

The present invention aims on providing an improved magnetic resonance imaging cooling system, controller, method, and computer program product.

SUMMARY OF THE INVENTION

The present invention provides a magnetic resonance imaging cooling system for cooling a superconducting magnet of a magnetic resonance imaging system, the cooling system comprising a cold head, the cold head comprising a cold head stepper motor, wherein the stepper motor is drivable at a variable stepping frequency; and a displacer. Alternatively the cold head can comprise a pulse tube or any means of cooling a cryogenic system that transmits vibration to the magnet or cryostat. The term 'stepping frequency' is used to describe the driving frequency of the cold head. The cooling system also comprises a compressor in fluid communication with the cold head. The compressor operates at the compressor frequency. Typically the compressor is in electrical communication with the cold head. The cooling system further comprises or is in communication with a controller or controllers to vary the compressor frequency or to vary the stepping frequency in a frequency range of continuous cold head operation.

'Continuous cold head operation' is defined for the purpose of this patent as an operation mode of the cold head in which the drive assembly of the stepper motor is continuously stepping or the pressure wave generated by the cycling cold head is continuously cycling, i.e. the situation in which the cold head is turned off or in which the drive assembly stops stepping is excluded.

The preferred embodiment of the invention has the stepping frequency of the cold head selected for resonance image artifact suppression, the artifacts being induced by operation of the cooling system. The stepping frequency is communicated to the cold head by a controller. Additionally, it is an advantage to adapting the stepping frequency of the cold head to control in an improved manner the cooling characteristics of the cooling system which even permits saving energy.

Further, by controlling with an optional inline unit between the compressor and the cold head, the method of varying the stepping frequency applied at magnetic resonance imaging duty times and idle times is retrofϊttable to existing units, with the possibility of noticeably increasing the imaging time available on said units.

In one embodiment of the invention, a stepping frequency is chosen that differs from any known mechanical resonance frequency of the magnet system. By selecting the stepping frequency to differ from the mechanical resonance frequency of the MR system an excitation of these mechanical resonances can be avoided and thus vibration transfer to the magnetic resonance imaging system can be limited. This avoids artifacts from cold head operation such that the influence of cold head induced Bo modulation has a negligible effect on the quality of acquired MR images. Additionally, varying the stepping frequency changes the speed of the displacer which changes the magnitude of the induced disturbances from the motion of the displacer. Reducing the disturbances allows the shields to absorb nearly all of the change in magnetic field, thereby substantially minimizing or even preventing disturbances of B₀.

However, it has to be noted here that the magnetic resonance imaging system may comprise more than one single vibrational resonance frequency. In this case the stepping frequency is chosen to lie outside said vibrational resonance frequencies.

Depending on the length of the scan, in accordance with an embodiment of the invention the effect of Bo disturbances can be further limited by means of one of several stroboscopic synchronizations. 'Stroboscopic synchronization' is defined for the purpose of this patent as coordinating the motion of the displacer with the details of the imaging, such details including but not limited to the scan repetition time, scan duration, or k-space lines. In one embodiment of stroboscopic synchronization, the cold head stepping frequency can be chosen with a half period that is greater than or equal to the scan duration. This results in a scan acquisition that is performed during a linear change of Bo modulation induced by the moving of the cold head moving parts. Such nearly linear change of a cold head induced Bo modulation can be easily corrected in the resulting acquired MR image. For example, a scan lasting 25 ms would use a cold head stepping frequency of 20Hz. With a stepper motor with, 48 steps per revolution, one complete motor cycle will take 2.4 seconds. In an alternative embodiment, additionally, the start of the data acquisition is synchronized with the cold head stepper motor motion. Typically, one complete cycle of displacer motion driven by the stepper motor requires a fixed number of steps. In an alternative embodiment of stroboscopic synchronization the stepping frequency is set such that the stepping frequency equals that fixed number of steps divided by the scan repetition time. In this embodiment, each repeated start of a scan sequence will be at the same relative displacer position, corresponding to the same cold head induced Bo modulation for all repeated MR scans. In other words, by such a method the position of the displacer is guaranteed to be at the same place for each repetition so that the Bo disturbance contribution is the same for each k-space line. Since the Bo disturbance is constant for each k-space line, no artifacts arise in the acquired MR image.

In another alternative embodiment of stroboscopic synchronization the stepper motor frequency and start time are coordinated such that the displacer is moving at minimum speed when the scan is passing through the centre of k-space data acquisition. In this embodiment, the cold head operation is synchronized with the scan in such a way, that the displacer is at or close to the top or bottom of its stroke when the scan is passing through the centre of k-space. This ensures that the residual effect of motion during a single profile is minimized for the most critical profile.

This embodiment has the advantage that data acquisition at the centre of k- space data acquisition is synchronized with the repetition time, and, due to the almost non- moving displacer, the variation of Bo modulation over time is minimized. In contrast, when the displacer is located in between the top and bottom of the stroke coincident with the scan being at the center of k-space, the variation in Bo modulation is maximal.

Performing the application of the scan sequence in such a way that the displacer motion in minimized when the scan is passing through the centre of k-space data acquisition is an important aspect for the application of long scan sequences in which a scan cannot be completed in less than one stroke.

In another alternative embodiment of stroboscopic synchronization a non- sinusoidal or interrupted sinusoidal motion is imposed on the displacer, keeping it for an extended time at a constant position, for example in a critical part of a scan, and then letting it quickly traverse through the rest of the cooling cycle in order to maintain adequate refrigeration power. Alternatively, it is also possible to slow down the movement of the displacer such that the displacer is kept at a quasi-constant position for a given period of time.

All embodiments of stroboscopic synchronization may require that the cooling system or an external monitoring device communicates the displacer position or motion information to a device coordinating the scans, or that information about the scans is communicated to the cooling system or controller.

In one embodiment of the invention, the stepping motor stepping frequency can be varied independently of the compressor frequency. Typically, the compressor and cold head operate at the same, fixed frequency. The capacity of this system is rated to correspond to the maximum daily expected heat load. In cases of low heat load, typically corresponding to time with no imaging (also referred to as idle mode), additional heat has to be added to the magnet system to compensate for the cooling. Varying the cold head frequency to match the cooling requirements minimizes or removes the need for additional heat added. This can result in an energy savings. Alternatively, the compressor power is determined by the voltage and frequency of the electric power to which it is connected. It is possible to vary the electric frequency of the incoming power to reduce or increase the compressor power draw in concert with the heat load. In an alternative embodiment of the invention, the superconducting magnet further comprises an optional sensor or sensors that can measure cryogenic fluid pressure or a cryogenic component temperature or other desired parameters. The stepping frequency is then varied depending on the data from the sensor. In one example of this embodiment, the stepping frequency can be varied to assist in actively controlling the pressure of the cryogenic fluid. The stepping frequency can be increased to compensate for pressures higher than a determined threshold or decreased in order to compensate for pressures lower than the determined threshold. The same holds with respect to varying the stepping frequency for actively controlling the temperature of the at least one cryogenic component. Further, it has to be noted here that using the data from the sensors it is possible to tune the cold head to optimal performance on many different magnets: every magnet has a slightly different characteristics, such as thermal balance between its thermal shields and the bath temperature. By selecting an optimal frequency of the cold head when the system is not imaging, the same cold head universally used on different kinds of superconducting magnets can be tuned to optimal performance. This simplifies field service requirements for various product lines with a large degree of interchangeable parts. Further, an optimal performance with respect to energy consumption can be obtained.

In another aspect, the invention relates to a controller for controlling the stepping frequency of a cold head, the cold head being adapted for being used in a magnetic resonance imaging cryocooling system for cooling the superconducting magnet of a magnetic resonance imaging system, wherein the controller is adapted for varying the stepping frequency in a frequency range of continuous cold head operation.

In another aspect, the invention relates to a method of operating a magnetic resonance imaging cryocooling system for cooling a superconducting magnet of a magnetic resonance imaging system.

In one embodiment of the invention, the stepping frequency is reduced during when the MRI system is not scanning (idle mode) relative to the stepping frequency applied at magnetic resonance imaging duty times. During idle mode, magnetic resonance imaging is not performed which means that no additional heat due to switching of magnetic field gradients is induced in the cryogenic system. Therefore, less cooling power is required to maintain the desired state of the magnet system. By reducing the stepping frequency during idle mode the cold head lifetime can be extended. A low frequency operation reduces the wear on the cold head and increases the time between servicing, thus decreasing both the operation cost and the downtime of an MRI system. One common type of MRI magnet is a zero boiloff magnet system. Additionally, for this specific type of MRI magnet system, typical state of the art pressure control is performed by adding heat into the cryogenic bath to compensate for the cooling power of the cold head. For this specific embodiment, this is an additional power consumption that can be minimized by simple reduction of the cooling power.

In an alternative embodiment, the operation frequency of the compressor could be varied. However, this has the major disadvantage that the compressor functionality is extremely sensitive to frequency variations, allowing only a small range of operation frequency changes. By tuning the stepping frequency of the cold head, cooling power control can be performed more easily and accurate.

Further, the characteristics of a cooling system may change over time, due to diurnal imaging load, wear, contamination, oxidation, or changes in the ambient environment, or other internal or external disturbances. The heat leak into the magnet may also change over time, due to degradation of insulation, change in helium level, parasitic heat inputs, etc. Thus, based on detected parameters, an optimum combination of stepping frequency and compressor frequency can be selected so that the setting of cold head and compressor follows the changing characteristics of the MRI hardware. Preferably, the controller of the cooling system controlling the compressor frequency and the stepping frequency should be configured so that the settings of cold head and compressor follow the changing characteristics of the system. The settings can also be monitored and be used to plan preventive or corrective maintenance.

Further, the differential pressure of the compressor can be reduced temporarily when a new patient enters the scan room since this reduces drastically the acoustic noise of the system and reduces a patient's anxiety. In this case, the compressor frequency may be reduced but the stepping frequency may be increased to maintain the same cryogenic cooling power. It has to be noted here, that typically the compressor is not located within the patient room. Nevertheless, the pressurized flow of Helium vapor pumped from the compressor to the cold head can be rather noisy such that reducing the differential pressure of the compressor temporarily will thus also reduce the acoustic noise originating from the pressurized Helium flow.

In another aspect, the invention relates to a computer program product comprising computer executable instructions to perform the method according to the invention. The method according to the invention can also be implemented my means of control electronic, wherein these control electronics can be in communication with any combination of the frequency controller, the gradient system, the RF system, the magnet system or any other components of the MRI system. These control electronics can consist of a computer adapted to execute said computer executable instructions or PLC or any other controller known in the state of the art.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention are described in greater detail by way of example only making reference to the drawings in which:

Fig. 1 is a schematic illustrating an MRI system according to the invention, Fig. 2 illustrates a stepwise cold head displacer motion,

Fig. 3 illustrates frequencies excited by a step sinusoid displacer motion, Fig. 4 illustrates Bloch equation simulation ghostings, Fig. 5 illustrates the preferred range of stepping frequencies of a cold head motor, Fig. 6 is a flowchart illustrating a method of setting the stepping frequency of a cold head stepping motor.

DETAILED DESCRIPTION

Fig. 1 is a schematic illustrating an MRI system according to the invention. Only major components of a preferred MRI system which incorporate the present invention are shown in fig. 1. The magnetic resonance imaging apparatus comprises control electronics which can be for example implemented by means of a data processing system 100, wherein the data processing system 100 may comprise a computer screen 102 and an input device 104. Such an input device could be for example a keyboard or a mouse. The MRI system in fig. 1 may further comprise a memory 106 and an interface 108. The interface 108 is adapted for communication and data exchange with typical hardware MRI components.

In general, the control electronics can be a computer or a PLC or any means known in the state of the art to automatically monitor signals and direct changes in that system or others based on the signals. In this embodiment, one set of control electronics 100 is connected to all systems. Control electronics 100 could be multiple controllers that can communicate with each other as needed using any communication method known to the state of the art, including but not limited to USB, serial, GPIB, or Ethernet.

Typical MRI components are for example a main field control unit 130 adapted for controlling the main field of the magnet 122. Preferably, the magnet system, comprising a vessel 152 and a magnet 122, preferably a superconducting magnet. The MRI system further comprises gradient system 124 and an RF system 128. The combination of gradient and RF systems may be used in image acquisition. They may be separate or combined into one unit. The gradient system 124 and the RF system 128 may be replaced by any alternative method of image acquisition in an MRI system and are only shown here as one example configuration.

Further shown in fig. 1 is a cooling system 148. This cooling system is adapted for cooling the superconducting magnet 122 by means of a cryogenic fluid. The cooling system 148 includes among other components a compressor 158 and a cold head 154. Essential components of the cold head 154 in the configuration shown in fig. 1 are a cylinder 164 and a displacer 162 which is mechanically coupled to a cold head stepper motor 160. This invention applies to any cooling system as known in the state of the art that comprises at least one compressor and at least one cold head. The cooling system also contains a controller 150 that can vary the frequency of the cold head 154 or the compressor 158 or both. Controller 150 can be one controller or multiple controllers. Multiple controllers do not require direct communication between them. Controller 150 may be built into control electronics 100 or built into the compressor 158 or built into the cold head 154 or included in the cabling between or attached to compressor 158 by separate cabling or attached to cold head 154 by separate cabling or any combination thereof. Further included in the magnet system is an optional sensor 166. Sensor 166 may be located in the vessel 152 or in the cooling system 148 or in any fashion in communication therewith. Sensor 166 might be a single sensor or multiple sensors. Multiple sensors might measure different properties or the same property at different physical location or over different ranges of conditions. The sensor 166 includes but is not limited to temperature sensors or pressure sensors. The sensor 166 may be in communication with control electronics 100 or controller 150 or any combination thereof. For example, the sensor 166 might sense the pressure build up within the vessel 152 upon vaporization of the cryogenic fluid greater than pressure threshold, leading to controller 150 increasing the operation frequency of the cold head 154 to compensate. Alternatively or additionally, a communication between the cooling system

148 and the control electronics 100 can be performed by means of a communication link. The communication link can be any means known to the state of the art, including but not limited to analogue or a digital communication. In one embodiment, the control electronics 100 signals the controller 150 to the start of a certain scan sequence to be carried out with a certain scan repetition time. Thereupon, the controller 150 adapts the stepping frequency of the stepping motor 160 accordingly. In this embodiment, the stepping frequency may be selected via stroboscopic synchronization such that the stepping frequency equals the set of steps required by the stepper motor for one complete cycle of displacer motion, divided by the scan repetition time. In another embodiment, the control electronics 100 signals the controller 150 that the system will go to idle mode. In this embodiment, controller 150 may change the stepping frequency of the cold head 154, or the frequency at which the compressor 158 is operating, or any combination thereof, in such a manner that energy can be conserved. Controller 150 may change the frequency once or several times to match cooling power requirements to system status.

It is a known problem in the state of the art that there is an interaction between cold head operation and image quality. Several patents already exist, including US5363077, US5701744, DE10221640, JP2001218751 documenting complicated and non-retrofittable solutions developed by a variety of companies. The data processing system 100 further comprises a processor 110 which is adapted for execution of computer executable instructions of the computer program product 112. In the present embodiment the data processing system 100 comprises a computer program product 112 by means of a data acquisition module 114 which is adapted to control the above described hardware units. Data acquisition is performed and the acquired data is analyzed via the analysis module 116.

The computer program product 112 further comprises various modules 120, these modules can be for example adapted for determining an optimal cold head frequency and an operation frequency of the compressor 158. However, such modules can be also comprised either in hardware or software in a controller module 150 or as a control module included in the cooling system 148.

In one embodiment of a typical cold head, the ideal motion of a displacer can be represented by a sinusoid in which one period takes about 0.96 seconds for a 50 Hz stepping frequency. However, the displacer movement is not smooth due to the discreet step nature imposed by the stepper motor operation. This actual displacement comprising the steps induced by the stepper motor operation is shown in fig. 2 for a case where the stepper motor takes 48 steps to complete one cycle of displacer motion. Optimal cooling is obtained when the cold head is in close physical contact with the cryostat. Because of this contact, the step motion of the displacer gives rise to vibrations of the cryostat structure close to the stepping frequency. As already mentioned above, the cryostat itself has natural mechanical resonances which are excited by these vibrations. Vibration of the structure leads to vibration of components in the magnet that are translated to perturbations of the main magnetic field around the frequency of the cold head stepper motor. Due to the inherent sensitivity of the MRI process, these small perturbations can be seen as undesirable artifacts in many images. Fig. 3 shows one example of frequencies excited by a step sinusoid as shown in fig. 2. In this embodiment, the step sinusoid leads to peaks 300 and 302 at 49 Hz and 51 Hz, respectively. In this embodiment, the magnet system has a mechanical resonance frequency near 50Hz. By selecting a stepping frequency which differs from said mechanical resonance frequencies, Bo disturbances occurring due to excitation of mechanical resonances can be substantially reduced. In this embodiment, not shown, reducing the stepping frequency to 40 Hz will reduce the peaks 300 and 302 to less than half.

Further, by means of 'stroboscopic synchronization' the effect of Bo disturbances can be further limited. 'Stroboscopic synchronization' is defined as coordinating the motion of the displacer with the details of the imaging, such details including but not limited to the scan repetition time, scan duration, or k-space lines. In one embodiment of stroboscopic synchronization, the stepping frequency of the cold head motor is selected as Fcoidhead = Nsteps / TR. Here, N_steps is the number of steps in the stepper motor (i.e. 48) and TR is the scan repetition time. By this stroboscopic synchronization method, the position of the displacer is guaranteed to be at a same place for each TR of a scan sequence, which means for each repetition at fixed intervals TR of scan sequences. So, the Bo disturbance contribution is the same for each acquired k-space line. Since the Bo disturbance is constant for each k-space line no artifacts arise in the image. This method is quite effective as shown in fig. 4. Figs. 4a and 4b show Bloch equation simulation ghostings that can occur in a multi- echo spin echo scan with TR = 1500 ms. For clarity the images are displayed with logarithmic scaling. Fig. 4a shows the case when the cold head stepping frequency is set to 50 Hz (normal operation). According to the above mentioned equation, the required stepping frequency for a TR of 1500 ms is 32 Hz. The result is shown in fig. 4b. When the cold head stepping frequency is adjusted in synchrony with the scan TR the ghost levels and artifacts are significantly minimized or disappear. Fig. 5 shows the range of stepping frequencies that might be expected for the range of TRs used in clinical scans. Applying the above mentioned formula F_coidhead = N_steps / TR, curve 500 as the solid line is obtained in fig. 5. It has to be mentioned here, that while shown for multi-echo spin echo, this method will also work in any scan method know to the state of the art, including turbo spin echo (TSE) scans provided the cold head stepping frequency is correctly set before the TSE pre-scan calibration. In this way the TSE calibration will correctly compensate the now static Bo errors.

It further should be noted that in the preferred embodiment of applying the formula F_coidhead = N_steps / TR, it is not necessary that the position of the cold head displacer is known or that the cold head operation is synchronized precisely with respect to the start of the scan. The choice of the correct frequency as described ensures that a cold head motion is essentially frozen with respect to the scan acquisition.

It has been observed that, due to the mechanical transfer function of the magnet cryostat, the level of Bo disturbance reduces significantly below a lower bound stepping frequency or above an upper bound stepping frequency. Thus for TRs that would demand a stepping frequency outside these ranges based on the above formula, it is adequate to limit the frequency to the closest bounding limit. This alternative embodiment is shown in fig. 5 as the curve 502 which corresponds to a constant upper bound stepping frequency for all scan repetition times up to a lower limit repetition time, then using the above mentioned formula until a higher limit repetition time is reached, whereupon the stepping frequency is maintained at a lower bound stepping frequency for all greater repetition times. In the specific example shown in curve 502; the upper bound frequency is 60 Hz, the lower limit repetition time is 800ms, the higher limit repetition time is 1200ms, and the lower bound stepping frequency is 40 Hz. Thus, in this embodiment, all scans with TR greater than 1200 ms use a stepping frequency of 40 Hz and all scans with a TR lower than 800 ms use a stepping frequency of 60 Hz. These values are given for illustration only, and would be individually adjusted to the mechanical transfer functions of each magnet.

Fig. 6 is a flowchart illustrating a method of selecting the stepping frequency of a cold head stepper motor. The procedure starts in step 700 which assumes that the cryogenic system is stepping at an idle mode stepping frequency. In a practical example the cooling system may operate at an idle mode stepping frequency of 45 Hz. In step 702 the system is compared to the desired characteristics. If the system has deviated from the desired characteristics the stepping frequency can be modified in step 704. Deviations might be due to diurnal usage pattern, wear, contamination, oxidation, changes in the ambient environment, any combination thereof, or any other disturbance, internal or external, that causes the system to deviate. In a practical example, the cryogenic bath pressure might be high due to a heavy imaging load. In this example, the system idle mode stepping frequency would be adjusted up until the pressure starts to drop. When the pressure is at or near the desired level, the idle mode stepping frequency will be reduced. Preferably, the steps 702 and steps 704 are performed in a self calibrating process of the cooling system. Note that the system can repeat steps 702 and 704 an unlimited number of times before the system returns to duty mode. If, in step 702, it turns out that the system characteristics have not changed, step 706 is carried out. In step 706, a determination is made if the system is in duty mode, i.e. it checks if a data acquisition needs to be performed. This determination can be made by communication from the MR system or from the cooling system, or from an outside source, or any other method of determining imaging. In turn, if in step 706 it is determined that the system is in duty mode, in step 708 accordingly a duty mode stepping frequency is calculated by any of the above described methods and set for the cold head stepping motor.

After step 708, again step 706 is performed. When in step 706 it is determined that the system has finished the MR acquisition, the system goes back to idle mode (step 700) with the operation of the stepper motor at idle mode stepping frequency.

REFERENCE NUMERALS:

100 Data processing system

102 screen

104 input device

106 memory

108 interface

110 processor

112 computer program product

114 data acquisition

116 data analysis

120 modules

122 superconductive magnet

124 gradient system

128 RF system

130 control unit

132 control unit

134 control unit

148 cooling system

150 control unit

152 vessel

154 cold head

158 compressor

160 stepper motor

162 displacer

164 cylinder

Claims

CLAIMS:

1. A magnetic resonance imaging cooling system (148) for cooling a superconducting magnet (122) of a magnetic resonance imaging system, the cooling system (148) comprising a cold head (154), wherein the cold head (154) is drivable at a variable stepping frequency, the cooling system (148) being adapted for varying the stepping frequency in a frequency range of continuous cold head operation.

2. The system of claim 1, wherein the magnetic resonance imaging system comprises at least one mechanical resonance frequency, wherein the cooling system (148) is further adapted for selecting the stepping frequency to lie outside said at least one mechanical resonance frequency.

3. The system of claim 1, wherein the cooling system (148) is further adapted for selecting the stepping frequency by stroboscopic synchronization.

4. The system of claim 3, wherein the magnetic resonance imaging system is adapted to perform a magnetic resonance image data acquisition by applying an imaging scan sequence with a scan repetition time, wherein the stroboscopic synchronization is such that one half of the stepping period is greater than or equal to the repetition time.

5. The system of claim 3, wherein the cold head is connected to a mechanical displacer (162) for cyclically driving the displacer (162) with a cycling period, wherein a complete cyclic displacer (162) motion driven by the cold head requires a set of cold head motor steps, wherein the stroboscopic synchronization is performed such that the stepping frequency equals the set of steps divided by the scan repetition time.

6. The system of claim 3, wherein the cold head is connected to a mechanical displacer (162) for cyclically driving the displacer (162) with a cycling period, wherein the cooling system (148) is further adapted for performing an application of the scan sequence in such a way that the displacer (162) is moving at minimum displacement motion speed when the scan is passing through the centre of k- space data acquisition.

7. The system of claim 1 , wherein the cooling system further comprises a compressor (158) cyclically operable with a compressor (158) frequency, wherein the cooling system (148) is adapted for varying the cold head stepping frequency independently of the compressor (158) frequency.

8. The system of claim 1, wherein the superconducting magnet (122) further comprises at least one of a cryogenic fluid pressure sensor (166) or a cryogenic component temperature sensor (166), wherein the cooling system (148) is adapted for varying the cold head stepping frequency depending on the sensor value.

9. The system of claim 8, wherein the cold head (154) operates with at least one system determined frequency throughout a scan sequence such that idle time for the purpose of regaining desired operating conditions is minimized.

10. A controller (100; 150) for controlling the stepping frequency of a cold head (154), the cold head (154) being adapted for being used in a magnetic resonance imaging cooling system (148) for cooling a superconducting magnet (122) of a magnetic resonance imaging system, wherein the controller is adapted for varying the stepping frequency in a frequency range of continuous cold head operation.

11. A method of operating a magnetic resonance imaging cooling system (148) for cooling a superconducting magnet (122) of a magnetic resonance imaging system, the cooling system (148) comprising a cold head (154), the method comprising selecting a stepping frequency for driving the cold head (154), wherein the stepping frequency is selected from a frequency range of continuous cold head operation.

12. The method of claim 11, wherein the magnetic resonance imaging system is adapted to perform a magnetic resonance image data acquisition, wherein the stepping frequency is selected for a magnetic resonance image artifact suppression, the artifacts being induced by operation of the cold head (154).

13. The method of claim 12, wherein the stepping frequency is selected using stroboscopic synchronization.

14. The method of claim 11, wherein the stepping frequency is reduced during idle or low load magnetic resonance imaging scan times relative to the stepping frequency applied at magnetic resonance imaging duty times.

15. The method of claim 11, wherein the cooling system further comprises a compressor (158) cyclically operable with a compressor frequency, the method further comprising selecting the compressor frequency based on the stepping frequency.

16. A computer program product (120) comprising computer executable instructions to perform any of the method steps as claimed in any of the previous claims 11 to 15.