Field of the Invention
The invention concerns a method for operating a medical imaging examination apparatus having multiple sub-systems, as well as such a medical imaging apparatus and an electronically readable data storage medium that implement such a method.
Description of the Prior Art and Related Subject Matter
Medical imaging examination apparatuses such as a magnetic resonance apparatus or computed tomography apparatus, for instance, are complex equipment having a multiplicity of technical sub-systems. These include, for example for a magnetic resonance apparatus, a basic magnetic field system, a gradient system, a shim system, an RF transmit system and an RF reception system.
In order to use a magnetic resonance apparatus to generate images or spectroscopic data from a subject under examination, the subject under examination is placed in a strong homogeneous basic magnetic field, also called the B0 field, which is generated by the basic magnetic field system and has a field strength of 0.2 Tesla to 7 Tesla and higher, with the result that the nuclear spins of the subject are oriented along the basic magnetic field. In order to induce nuclear spin resonances, the subject under examination is exposed to radio frequency excitation signals (RF pulses) by suitable antenna devices in the RF transmit system, causing the nuclear spins of certain atoms, which have been excited to resonance by this RF field, to be tilted by a particular flip angle with respect to the magnetic field lines of the basic magnetic field. The induced nuclear spin resonances, i.e. the RF signals (also known as the magnetic resonance signals) emitted during precession of the nuclear spins, are detected by the RF reception system, usually digitized, and stored as complex numbers, normally (when there is a spatial reference) in a memory in a data set called a k-space matrix, (k-space data). For single-voxel spectroscopy measurements (without a spatial reference), the digitized data are stored as complex time signals, also known as “FID data”. The k-space data or FID data can be used as the basis for reconstructing MR images or determining spectroscopy data. Magnetic gradient fields, rapidly switched by the gradient system, are superimposed on the basic magnetic field for spatial encoding of the measurement data. The shim system is used to make the magnetic fields homogeneous.
All these technical modules must be suitably addressed in a coordinated manner by a computerized control system. The control system must also make the settings and switching operations needed for a specific imaging process for each of the subsystems, each at the correct time. Usually data from the volume to be imaged are acquired in an imaging procedure in sub-volumes, for instance in a number of slices for 2D imaging or in a number of slabs for 3D imaging. The sub-volumes captured in this way are then combined to form a total volume. Sub-volumes may also be defined, for example, by “Regions of Interest” (ROI) or “Volumes of Interest” (VOI), which can be defined specifically by the operator. Moreover, defining local saturation regions or local preparation or labeling pulses in magnetic resonance systems results in additional sub-volumes.
As already mentioned, sequence control data, usually based on what is known as a measurement protocol, are passed to the controller for coordinated control. The sequence control data define various functional sub-sequences of a complete measurement sequence. For a magnetic resonance acquisition, a first sub-sequence may be, for example, a pulse sequence for achieving saturation locally in a specific region. Further sub-sequences may contain, for instance, specific preparation pulses, and yet further sub-sequences are used for successive excitation and for receiving the magnetic resonance signals in different slices or slabs.
Standard magnetic-resonance based techniques such as tomographic imaging (MRT, magnetic resonance tomography) or spectroscopy (MRS, magnetic resonance spectroscopy) need “benign” underlying physical conditions to ensure an optimum quality of the acquired data. For instance, these conditions concern the spatial homogeneity, temporal stability and the absolute accuracy of the relevant magnetic fields and RF fields, i.e. of the basic magnetic field (B0) and of the gradient fields (ΔB) and RF fields (B1).
At present, differences from ideal underlying conditions can be corrected at least in part by system-specific corrections known as “tune-ups”, in particular with regard to eddy-current induced dynamic field disturbances or gradient sensitivities, or by corrections specific to the subject under examination, known as adjustments, in particular with regard to susceptibility-related static field disturbances or spatial variations in the RF field. These corrections, however, which are established before starting a measurement, usually apply throughout the measurement (“static” adjustment).
For spatially variable underlying conditions, which cannot be fully corrected, this means a compromise for data quality.
De Graaf et al. describes in “Dynamic shim updating (DSU) for multislice signal acquisition”, Proc. Intl. Soc. Mag. Reson. Med. 10, p. 536, 2002, a rudimentary form of dynamic adjustment of the shim currents in the field coils for the B0-shim in functional multislice MR imaging. For this purpose, a dedicated field determination sequence is employed for determining first-order or higher-order spatial field variations. Such a sequence must be exactly adapted to the relevant parameters (e.g. slice positions and orientations) of the desired imaging sequence. The field determination sequence acquires the data needed for determining the field, and analyzes such data in order to calculate therefrom optimized shim currents (of first order or higher) for each slice to be acquired using the imaging sequence. Then the imaging sequence containing the optimized shim currents is started. In this process, the user must pay very close attention to consistency between imaging sequence and field determination sequence in order to prevent inconsistencies resulting in poorer image quality. Thus a field determination sequence must be created anew for every imaging sequence and for every change to such a sequence, and this sequence must be run before the measurement with the imaging sequence. This method is thus very complex for the user and difficult to combine with other e.g. static adjustments, because interactions between different parameters cannot be taken into account or only taken into account to a limited extent. If statically adjusted parameters are changed, this can have implications for the optimum dynamic adjustments of the shim currents and it would be necessary to run the field determination sequence again and to recalculate the optimized shim currents. Furthermore, optimization is limited in such a conventional procedure to slices in the imaging sequence. The method does not consider smaller volumes, e.g. regional saturation volumes.
DE 10 2009 020 661 B4 describes a method that is used to adapt setting parameters of a measurement sequence, e.g. in magnetic resonance technology, while the measurement sequence is running. This document also notes that different functional sub-sequences are normally assigned different effective volumes, which means that a different sub-volume of the total measurement volume is relevant for each sub-sequence. Control signals defining optimized setting parameters are determined for each sub-sequence and each effective volume and used for dynamic control. This approach is developed by the methods described in DE 10 2014 219 778.3, DE 10 2014 219 785.6, DE 10 2014 219 784.8, DE 10 2014 219 782.1 and DE 10 2014 219 779.1, which concern variations relating to the time at which the control signals defining the setting parameters are determined and to the variables to be included in the optimization.
Problems still exist, however, when combining such dynamic adjustments, i.e. using setting parameters that vary while the measurement sequence is in progress, with static adjustments, i.e. of setting parameters that do not change while the sequence is running, and in particular when there is interaction between the statically adjusted and the dynamically adjusted setting parameters.
Although dynamic adjustments normally facilitate better image quality, it can still be practical, depending on the measurement situation, to adjust setting parameters statically. For instance this makes sense if dynamic adjustments to certain setting parameters require the acquisition of base data, for example B0 maps or B1 maps, and the time needed for this is not available. Other instances are when there are no dynamic switching capabilities available for certain setting parameters (for example with higher-order shim currents for which the power supply only allows slow switching operations) or when the static adjustment data are needed for certain calibration purposes, for example for SAR monitoring.
If static and dynamic adjustments are used simultaneously, dependencies between the setting parameters currently being considered can be a factor, as already mentioned. For example, if static adjustments set new values for the higher-order shim currents for a measurement, previously acquired B0 maps (as base data for a dynamic frequency correction, for instance) are thus no longer valid and must be acquired again. This is explained in more detail using the following practical example.
I. Perform a first measurement using a measurement sequence, which measurement is meant to capture a stack of three-dimensional slices as sub-volumes. The center frequency and the first-order shim settings, i.e. the gradient offset currents as setting parameters, are meant to be optimized dynamically for each slice. For the second-order shim, a static compromise is meant to be set for the entire slice stack, because in the example case, the switching capability is not fast enough for the second-order shim coils.
II. Perform a second measurement using a second measurement sequence, which relates to a different slice geometry, for example to a larger imaging volume or a different slice orientation. Again in this case the center frequency and the offset gradient currents are meant to be dynamically adjusted whereas the setting parameters for the second-order shim are adjusted statically.
The known procedure in the prior art leads in total to the following exemplary, sequence, which is intended to be limited to the steps relevant here.                1) Static frequency adjustment                    a) Measure the resonant frequency in the first slice stack            b) Determine a static center frequency            c) Adjust the static center frequency                        2) Static shim adjustment                    a) Measure the field homogeneity in the first slice stack            b) Determine static first-order and second-order shim currents            c) Adjust the static first-order and second-order shim currents                        3) Preparation for dynamic adjustment of center frequency and first-order shim                    a) Measure the absolute field distribution at least in the first slice stack            b) Determine optimized settings for the center frequency and the first-order shim for each relevant sub-volume for the first measurement using the first measurement sequence, as described in DE 10 2009 020 661 A1 for example.                        4) Implementation of the first measurement using dynamic adjustments        5) Static frequency adjustment                    a) Measure the resonant frequency in the second slice stack            b) Determine a static center frequency            c) Adjust the static center frequency                        6) Static shim adjustment                    a) Determine static first-order and second-order shim currents, with recourse to data from step 2a also being possible if there is no change in position of the patient and/or of the patient table.            b) Adjust the static shim currents                        7) Preparation for dynamic adjustment of center frequency and first-order shim                    a) Measure the absolute frequency distribution in the second slice stack (this measurement is needed because the field distribution changes as a result of steps 5c and 6b)            b) Determine optimized settings for center frequency and first-order shim for each relevant sub-volume in the second measurement using the second measurement sequence                        8) Implementation of the second measurement using dynamic adjustments        
It should also be noted that step 3b can also be integrated in step 4, which similarly applies to step 7b and step 8.
This example sequence shows that a new measurement of the absolute field distribution as the basis for the dynamic adjustment is needed at the latest after each change in static higher-order shim currents. Such a measurement can take 10 to 60 seconds in the prior art, with the total measurement time increasing significantly accordingly. In fact, however, the data of a relative field distribution can already be considered out of date, and thus necessitating a new measurement, just with the change in the static center frequency in step 5c.
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