RU2290863C2 - Method of inspection of condition of cryogenic cooling system of magnetic-resonance tomograph's screen - Google Patents

Abstract

FIELD: medical equipment.

SUBSTANCE: pressure of helium is monitored in cooling circuit. Audio signal of operation of cryogenic system is recorded inside cryostat area. Audio signal is dispersed to spectrum. Relative change of separate spectral components is estimated during some period of time due to comparison of their level with threshold values assigned for cryogenic system. Warning signals on necessity of checking of operation are formed on the base of the changes. Specific components of cryogenic system can be subject to replacement if necessary if levels of excited spectral components exceed specified thresholds.

EFFECT: improved reliability of magnetic-resonance tomography.

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Description

The invention relates to medical equipment and can be used in magnetic resonance imaging (MRI).

There is a method of monitoring the technical condition of the cryosystem for cooling MRI screens, which consists in taking into account the actual operating time of the system as a whole and its individual parts, monitoring the pressure of helium in the cooling circuit and the recommended replacement of system components after a theoretically and empirically adjusted time interval (RGD 5 \ 100- 2 Cold Head and RW 4000 \ 4200 Compressor. Service Manual 46-294439P4, revision 8, LEYBOLD VACUUM \).

However, this method entails high operating costs and does not provide a low probability of system failures due to the multi-parameter dependence of operational reliability on single and randomly coincident failures of the components of the cryosystem. Power outages and malfunctions of the cryocompressor, cryohead engine, dispenser, oil and gas vapors and drops falling into helium, and overheating of helium due to problems in the water cooling circuit can serve as examples of superposition and failure.

Closest to the technical nature of the claimed method is a method of monitoring the technical condition of the cryosystem for cooling MRI screens, consisting of empirical monitoring of changes in helium pressure in the cooling circuit, dew on the parts of the cryohead, increasing helium consumption in the cryostat and a deviation from the normal functioning of the main system components (dispenser, adsorber, cryocompressor) [Tomicon-Avance S50 Components, part 3. System manual, doc. No.1273, Bruker Medizintechnik. 1997].

The known method does not provide reliable operation of MRI due to unreduced failures in operating conditions, the presence of impurities in the gas and oil, uneven wear of the rubbing working parts of the cryohead, cryocompressor. As a result, despite the sometimes premature replacement of expensive system components (such as a dispenser, adsorber), emergency failures occur, leading to helium ejection from the cryostat, loss of the superconducting magnetic field, and stopping the magnetic resonance imager. This entails very high economic losses.

The purpose of the invention is to increase the reliability of the functioning of MRI due to the control of the spectral composition of the audio signal of the cryosystem cooling screens.

This goal is achieved by the fact that in the method of monitoring the state of the cryosystem cooling the screens of magnetic resonance tomographs tracking changes in helium pressure in the cooling circuit, a periodic recording of the audio signal of the cryosystem in the cryostat zone, its decomposition into the spectrum, and the assessment of the relative change in the levels of individual spectral components over time by comparing their level with the thresholds set for the cryosystem, the formation of warnings on the monitored dynamics of these changes editelnyh signals about the need to verify the operation, and then replace specific components of cryogenic system excitation levels of the spectral components of which exceeded the defined thresholds.

Figure 1 shows the spectrum of the audio signal of the cryosystem, taken immediately after the replacement of the main elements - dispenser and adsorber.

Figure 2 - spectrum of the audio signal of the cryosystem after 8000 hours of operation (recommended period of replacement of the dispenser recommended by the manufacturer). The spectral components 1, 2, 3 are changed slightly. There are no external manifestations of malfunctioning of the cryosystem. Displacer replacement is not required.

Figure 3 shows the control measurement of the spectral components 1, 2, 3 after 12000 hours of operation. Obviously a change in the controlled levels, but not below the established threshold values.

Figure 4 - spectrum of the audio signal of the cryosystem of example 1.

Figure 5 - the initial spectrum of the audio signal of the cryosystem of example 1.

6 is a block diagram of a method implementation.

The method works as follows. In accordance with the technical operating conditions, the gas pressure readings in the cooling circuit of the screens are read.

The audio signal in the MRI cabin is periodically shot during pauses of the superconducting magnetic resonance imager (between scans) using the radio signal of a standard intercom channel. A standard board of an analog-to-digital converter of a personal computer (sound adapter - SoundBlaster, for example, Creative AuduoPCI CT5880) sends a signal to a computer. With a standard recording program (for example, in Windows 98/2000 / NT, access via Start / Programs / Accessories / Entertainment / Sound recording), the signal is recorded in the file corresponding to the current date. Any of the many typical sound analyzer programs (for example, contained in the WinOnCD package) perform a visualization of the spectrum and either a visual or a machine assessment of the specific selected spectral components. Further, in the machine version of the processing, a programmatic assessment is made of the dynamics of a temporary change in the level of specific spectral components and these levels are compared with the set thresholds. As can be seen from the above spectrograms, the levels of specific spectral components indicate the operability of the corresponding components of the cryosystem. According to their state relative to pre-set thresholds, an enable signal is generated or, when the threshold is exceeded, an alarm signal (ALARM) is generated. When ascertaining the excess of the threshold level of a specific spectral component, a decision is made on the need to replace a specific component of the cryosystem (or several). In semi-automatic mode, the operator periodically evaluates the levels of spectral components and makes a decision on the necessary replacement of parts of the equipment.

As an example of the implementation of the method, an operator analysis of the state of the system when it is used on a Bruker Tomikon S50 superconducting magnetic resonance imager with the Magnits magnet itself. Coold Head RGD 5 / 100-2 helium compressors Coolpaak 6000 Coolpaak 6000 M manufactured by Leybold Vacuum are used to cool the screens of the cryostat of this magnet, providing the recommended maintenance of helium pressure in the cooling circuit of the screens in the range of 14-26 bar. Recommended by the manufacturer, the intervals for replacing adsorbers - 8000 hours, dispensers - 24000 hours of operation. Empirically, the optimal nominal pressure of helium in the cooling circuit of the screens is 20-22 bar, which contributes to increased reliability during the longest operation of these compressors.

In accordance with the technical operating conditions, maintenance personnel daily reads the helium pressure in the cooling circuit of the screens according to the pressure gauge installed on the Coolpaak 6000 helium compressor. In nominal value, it is set to 22 bar (20 bar for Coolpaak 6000 M). Using the standard radio-frequency channel between the doctor and the patient, weekly, in the intervals between scans, the audio signal from the microphone installed inside the cryostat tube (for the indicated tomograph) in the Faraday MRT cell is converted to the radio frequency signal of the audio communication channel. It is possible to remove the audio signal at any time (including during scanning). However, in this case, additional filtering of the magnetization reversal and gradient noise of a known spectrum, i.e. to additional operations. The signal level can be changed by the volume control from the operator console. It is set close to maximum in order to avoid self-excitation of amplifiers in case of overload by an audio signal and at the same time to provide a lower level of interference in the resulting spectrum. In the simplest case, an audio signal is recorded in any way. For example, removing the radio frequency signal from the output of the preamplifier of the audio communication channel by a pairing cable, connecting it to the standard audio input of a personal computer (PC). At the same time, using standard programs that are included, for example, in the Microsoft Office package (Sound Recorder), a typical audio file is created (for example, such as .wav). The recording time is determined based on the spectral analysis methods used below. Even for the simplest programs included in the typical audio recording packages of modern PCs (for example, the WinOnCD package), a few (1-5) seconds are enough to complete the analysis. This recording stage can also be simply implemented by connecting a standard microphone to a typical PC audio input PC by placing it at the output speaker of the operator’s console operator. In case of detection of a drop (fluctuation) in pressure in the cooling circuit of the screens (by 1-2 bar) during daily monitoring, the recording operation of the corresponding audio file is also carried out daily. The spectral composition of the received audio signal can be studied in various ways, both typical (included, for example, in packages for recording audio files), and specialized spectral analysis programs. Using, for example, the WinOnCD package routine, a spectral picture is obtained (Figs. 1, 5) by analyzing the resulting audio file. Initially, for a normally functioning cryosystem - with the new Cold Head (dispenser), compressor (adsorber) - the spectrogram shown in Fig. 5 is characteristic of the previously mentioned components of the cryosystem. It lacks both lower (less than 280 Hz) and upper (above 1.8 kHz) frequencies, and the spectral components of the range 280-344 Hz, which are typically excited by the DC 100 dispenser (Cold Head RGD 5 / 100-2), have an average (compared with the frequency components of 80-1000 Hz - approximately half) level. It should be noted that the location of the signal pickup (microphone) in the cryostat zone (MRI cabin), as well as the amplitude-frequency (with a known non-linearity) characteristic of the sound communication channel path, mainly determine the overall level in the spectral region, preserving the ratio of spectral components. Since the operations are carried out under the same conditions using the same elements, linear and nonlinear spectral distortions are easily normalized. Those. an "assessment of the relative change over time of the levels of individual spectral components" is made. Moreover, based on the initially obtained spectrum (Fig. 5), it is easy to set critical thresholds for specific spectral components. So, for the specific cryosystem specified above, for the spectral components of the range 280-344 Hz, an 80% critical threshold is advisable with respect to the levels of the spectral components of the range 800-1000 Hz. During operation (aging), the level of spectral components in the range of 280-344 Hz changes, gradually increasing. However, an abrupt (including temporary) nature of exceeding the specified 80% threshold is also possible. This is due to cryosystem malfunctions caused by various factors: from a violation of the tightness of the helium circuit to a power outage. If, for example, after eliminating failures (eliminating, for example, helium leakage in the cooling circuit of the screens due to a temporary violation of the sealing of the connecting pipelines), the level of the spectral component under consideration returned to a level close to the previous one, below the set threshold, further operation of the MRI without repair of the cryosystem is possible. In the case of a stable (for tens of hours) exceeding the threshold value, especially with further growth, the necessary replacement of at least a cold head dispenser. Both the threshold setting and the dynamics of the behavior given as an example, the level of the spectral component is individual (determined by the design features of the cryosystem components) and requires initial normalization (tuning). The operator, analyzing the current spectrogram, compares it with the original and previous, using the established threshold criteria. A set of features, including a change in the helium pressure in the cooling circuit of the screens, exceeding the established thresholds for specific spectral components, the nature of their exceeding (stability of the dynamics), allow the components of the cryosystem to be defective. In the case considered, a dispenser.

According to the second implementation example (Fig. 6) of the proposed method, the radio frequency signal of the audio communication channel (see the stage of the implementation example discussed above) is fed to a spectrum analyzer in which the installed spectral components are extracted. If we consider the same equipment components as in the previous example, then, for example, for a dispenser, the highlighted spectral component of 280-344 Hz will be of interest. The threshold shaper of alarm signals sets threshold values based on the 80% level of the selected spectral component of the range 800-1000 Hz for the component excited by the dispenser (280-344 Hz), 20% for the absorber (80-100 Hz), 50 % - for a helium compressor (40-50 Hz). Both the frequency of the component and the threshold level are specified during hardware and software configuration of the alarm driver. A specific implementation of the latter may contain integrating elements for dynamic averaging in the shaper of disturbance emission alarm signals. Alarms of the display device in combination with a change in pressure in the cooling circuit of the screens are the basis for replacing those elements of the cryosystem from which an alarm signal was generated accordingly.

In the third example of the implementation of the proposed method, the radio frequency signal of the audio communication channel is also fed to the standard audio input PC. An audio recording program is launched regularly on a computer (with a programmable period or by operator’s commands), the output file of which (for example, .wav format) is analyzed by a specialized spectral analysis program. The latter, after decomposing and filtering the signal in the spectral region, forms, for example, by analogy with the one considered in the electronic version, an analysis algorithm: an 80% level of the selected spectral component of the range 800-1000 Hz for the component excited by the dispenser (280-344 Hz), 20% - for an absorber (80-100 Hz), 50% - for a helium compressor (40-50 Hz), thresholds with which the selected components of interest are compared. The output of such a program may be alarm messages for components that have exceeded thresholds. Based on these messages, the operator, having previously checked, as mentioned earlier, the pressure in the cooling circuit of the screens, decides to replace the defective unit.

Claims (1)

A method for monitoring the state of the cryosystem for cooling screens of magnetic resonance tomographs by tracking changes in the pressure of helium in the cooling circuit, characterized in that it additionally introduces a periodic recording of the audio signal of the cryosystem in the cryostat zone, decomposing it into a spectrum, evaluating the relative change over time of the levels of individual spectral components by comparing them level with thresholds set for the cryosystem, the formation of warning changes by the monitored dynamics of these changes the need to verify the functioning, and then the replacement of specific components of the cryosystem, the levels of excited spectral components from which exceeded the established thresholds.

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