Patent ID: 12222412

DETAILED DESCRIPTION

FIG.1depicts a representation of a pulse sequence1for the diffusion measurement containing radiofrequency pulses RF and a readout window AF in which radiofrequency measurement signals are received, and containing a series of monopolar gradients Gdiff,x, Gdiff,y, Gdiff,zaccording to an embodiment. The pulses and signals are each plotted over time t. In the diffusion measurement are used series of gradient pulses of very high amplitude in order to imprint in the shortest possible time on nuclear spins their current spatial position in the form of a signal phase, and later to detect a change in the position. Short encoding times are needed here in order to reduce signal loss resulting from relaxation effects, for example caused by transverse relaxation, and hence to acquire images of sufficient quality for the diagnosis as rapidly as possible.

Depending on the clinical issue and the body region, different series of diffusion gradient pulses Gdiffmay be used here, for instance a pulse sequence with “monopolar” encoding (having gradient pulses of the same polarity), as is shown inFIG.1. Such a series with a specific encoding may be, for example, the gradient pulse sequence that forms the basis of a measurement protocol for an actual measurement. Although the full measurement process, or the actual pulse sequence associated with this measurement process, may contain further gradients, in many cases it is the diffusion gradients that determine the stimulation potential, and therefore it may be sufficient to consider this sub-segment.FIG.1depicts in the first line of the mapped diagram an RF excitation pulse RF1 and an RF refocusing pulse RF2. Also shown in the first line is an acquisition window AF in which RF signals are received from a region of interest as echo signals. In the second line are shown diffusion gradients Gdiff,xencoded in a monopolar manner in the x direction, of which a first gradient is switched after the RF excitation pulse RF1, and a second gradient is switched after the RF refocusing pulse RF2. In the third and fourth lines inFIG.1are switched diffusion gradients Gdiff,y, Gdiff,zin the y direction and in the z direction. Between the diffusion gradients in one direction there is a time interval in which the RF refocusing pulse RF2 is output. The gradients in different directions each have the same duration but some have a different amplitude. For the sake of clarity, further gradient pulses for the spatial encoding have been omitted from the diagram.

FIG.2depicts a representation2of a pulse sequence for the diffusion measurement having a series of bipolar diffusion gradients according to an exemplary embodiment. Each of the diffusion gradients Gdiff,x, Gdiff,y, Gdiff,z, or the amplitude thereof, therefore alternates in sign.

FIG.3depicts a representation of a pulse sequence for the diffusion measurement having a series of gradients Gdiff,x, Gdiff,y, Gdiff,zencoded in an oscillating manner according to an exemplary embodiment. In this variant, the diffusion gradients Gdiff,x, Gdiff,y, Gdiff,zchange their sign alike multiple times after the excitation and after the refocusing.

It is evident from the pulse sequences shown inFIG.1toFIG.3and that form the basis of an actual measurement protocol that a shorter duration of the gradient ramps, i.e., a higher slew rate S, allows a shorter time interval between the excitation module and the readout module AF. Since the diffusion gradients have high amplitudes, the stimulation potential rises considerably with shorter ramps. Therefore when preparing for MR imaging, the stimulation potentials and permitted maximum gradient slew rates S are calculated for representative pulse sequence segments associated with these pulse sequences. It is also possible to determine from the permitted maximum gradient slew rates permitted ramp durations given by R=G/S, where R is the ramp duration, G the gradient amplitude, and S the gradient slew rate. In addition, the rise time T=1/S may also be calculated as an alternative to the gradient slew rate S.

FIG.4depicts a representation4of a representative pulse sequence segment of gradients of the pulse sequence shown inFIG.1having the selectable parameters duration D, time interval A, and gradient amplitude Gref.

A brief illustration of a pre-calculation of the aforementioned permitted ramp parameters or ramp parameter values is given for the monopolar representative pulse sequence segment shown inFIG.4. At the time of this pre-calculation, neither the exact timing of the two gradients nor their actual amplitude are normally known. For this reason, slew rates S are determined for different gradient amplitudes G, and, for a number of possible timing options, a lower limit min of the maximum permitted slew rate of the individual series is determined as the maximum slew rate Smaxof an entire group of possible series. During later editing of an actual measurement protocol, a representative pulse sequence segment may then be picked from a database having the parameters of the actual measurement protocol, and its permitted slew rate Smaxused as a limit for specifying the actual measurement protocol.

For example, it is known for the diffusion measurement that the duration of diffusion encoding gradients lies in the range of 5 ms to 50 ms. In addition, for a particular type of gradient coils, the time constants of the SAFE model are known, that lie in the range of 1 ms to 10 ms. Hence for this scenario, it may be sufficient, for example, to consider gradient pulse durations of 1 ms to 30 ms and gradient pulse intervals of 0 ms to 30 ms. This parameter range captures cumulative effects of successive ramps as well as possible compensatory effects.

With regard to contributions to the stimulation that depend solely on the magnitude of the gradient rise, then series of rising and falling ramps at short time intervals may make a strong contribution. This cumulative effect is captured by gradient pulses of short duration and short interval. Such an effect exists, for example, for a series of monopolar gradient pulses of short duration D and short interval A.

With regard to contributions to the stimulation for which the sign of the rise is included in the calculation, a preceding falling ramp may reduce the magnitude of a subsequent rising ramp. This compensation effect, however, is reduced by longer gradient pulses of larger time interval. Such a contribution thus has an effect in a series of gradient pulses of long duration D and long interval A.

The pre-calculation calculates in advance for a particular gradient amplitude G, for different combinations of duration D and time interval A of the monopolar gradient pulses, the maximum gradient slew rate Smax(G,D,A) in each case. For a particular gradient amplitude G=Grefis then obtained as the lower limit for any values of the durations D and time intervals A of the gradients a common upper limit for the gradient slew rate Smax(G)=min D,A (Smax(G,D,A)). During the subsequent editing of the measurement protocol, in the simplest case, a pulse sequence for imaging may then be planned on a conservative basis using the maximum slew rate for the maximum gradient amplitude Gmaxof the system Smax(Gmax), or using the value relevant to the actual gradient amplitude G, for which value is stored in the database a maximum gradient slew rate Smax(G), or for which value the maximum gradient slew rate Smax(G) is calculated by interpolation of two adjacent tabulated values of maximum gradient slew rates.

For the two representative pulse sequence segments shown inFIG.5andFIG.6, that are bipolar and oscillating respectively, the procedure may be very similar to that for the representative monopolar pulse sequence segment shown inFIG.4.

As illustrated in the representation5inFIG.5, for the bipolar encoding, a gradient pair of time duration D and time interval A of the gradients, and an amplitude that may equal +Grefor −Gref, is specified as a representative pulse sequence segment of a bipolar pulse sequence.

As illustrated in the representation6inFIG.6, for an oscillating encoding, a series of alternating gradient pulses of duration D without any break, i.e., with the time interval A=0, is considered. In this case, the number of gradients to be taken into account for determining a stimulation potential SP of a representative pulse sequence segment is based on the relevant time constant r of the model used as the basis for calculating the stimulation potential, in this exemplary embodiment the SAFE model. For example, as many gradient pulses may be considered as may be applied within three times the longest time constant r.

FIG.7depicts a representation7of a representative pulse sequence segment of a pulse sequence for the diffusion measurement having an additional imaging gradient (shown dashed).

The imaging gradient shown inFIG.7is inserted in a monopolar pulse sequence and has an opposite polarity to the diffusion gradients (shown by continuous lines) of the pulse sequence and an amplitude Gimgthat differs from the amplitude Grefof the diffusion gradients. The pre-calculation of maximum gradient slew rates Smaxmay also take into account a modification of this type by generating representative pulse sequence segments containing additional imaging gradients, that are included in the pre-calculation of the stimulation and of the maximum gradient slew rates.

For example, an imaging gradient for slice selection may be applied between the two diffusion gradients. Depending on the image orientation, this gradient may have a different sign from a subsequent or preceding diffusion gradient, and hence increase the stimulation potential.

If this actual pulse sequence were to be assigned that ofFIG.4as the “representative” pulse sequence segment, then the stimulation potential might be underestimated using this “representative” pulse sequence segment, and therefore the representative pulse sequence segment shown inFIG.4would no longer be “representative”.

Thus for this case, it would be better to assign the representative pulse sequence segment shown inFIG.7as the “representative pulse sequence segment”.

FIG.8depicts a flow diagram800, that illustrates a method for preparing magnetic resonance imaging of an object under examination according to an embodiment.

In step 8.I, a plurality of representative pulse sequence segments R-PSA are generated, each of which is associated with a reference gradient amplitude Grefof the gradient having the highest stimulation potential SP of the particular representative pulse sequence segment R-PSA, and the stimulation potential SP of which is representative of a group of partially different pulse sequences.

In step 8.II, a maximum gradient slew rate Smaxfor which a permitted maximum value of the stimulation potential SP is not exceeded is determined for each of the representative pulse sequence segments R-PSA and for at least one reference gradient amplitude. Usually, however, an entire table of different gradient amplitude values G is generated, for each of which a maximum gradient slew rate Smax(G) is determined.

Steps 8.I and 8.II are carried out once as part of the pre-calculation of the representative pulse sequence segments R-PSA and of their maximum gradient slew rates Smax.

In step 8.III, a measurement protocol P is opened for editing by a user or for automatic adjustment by a magnetic resonance imaging system. In step 8.III, the number and extent of the slices of a field of view (FoV) needed to cover a region of interest ROI are specified, if applicable in an automated or semi-automated manner. In addition, the position and orientation of the region of interest ROI may be adjusted in an automated manner on the basis of landmarks.

In step 8.IV, one of the representative pulse sequence segments R-PSA for a measurement protocol P to be planned for a magnetic resonance imaging actually to be performed is determined and selected according to a characteristic or the type of the pulse sequence PS on which the measurement protocol is based. Different types of representative pulse sequence segments R-PSA may be selected according to a setting of the measurement protocol. For example, for a first setting of the measurement protocol P, a representative pulse sequence segment R-PSA of identical polarity may be selected (seeFIG.1,FIG.4). For a second setting of the measurement protocol, on the other hand, a representative pulse sequence segment R-PSA of alternating polarity is selected (seeFIGS.2,3,5,6). The tables pre-calculated for the representative pulse sequence segment R-PSA may then be used to determine the maximum permitted gradient slew rate Smax(G, R-PSA) according to the gradient amplitude G of a gradient pulse belonging to the pulse sequence PS on which the measurement protocol P is based.

In step 8.V, a pulse sequence segment M-PSA of the pulse sequence of the planned magnetic resonance imaging is adjusted in such a way that the determined maximum gradient slew rate Smaxis not exceeded.

FIG.9depicts a schematic representation of a preparation device90according to an embodiment.

The preparation device90has a representative-data generation unit91. The representative-data generation unit91is configured to generate a plurality of representative pulse sequence segments R-PSA containing at least one gradient pulse and usually a plurality of gradient pulses. The amplitude of at least one of the gradient pulses that is responsible for the highest stimulation potential is scaled by a reference gradient amplitude Grefdefined by an input parameter.

The preparation device90also includes a rise determination unit92for determining, for each of the representative pulse sequence segments R-PSA, a maximum gradient slew rate Smaxfor which a permitted maximum value of the stimulation potential SP is not exceeded.

The preparation device90also has a database92a, that stores the generated representative pulse sequence segments R-PSA with each of their associated maximum gradient slew rates Smaxfor different reference gradient amplitudes Gref, and retains for later planning of a specific measurement protocol P for magnetic resonance imaging.

Part of the preparation device90is also a selection unit93for determining and selecting for a measurement protocol P to be planned for a magnetic resonance imaging actually to be performed one of the representative pulse sequence segments R-PSA according to the highest gradient amplitude G of the pulse sequence PS on which the measurement protocol P is based.

The preparation device90also has an adaptation unit94for adjusting a protocol parameter PP of the measurement protocol P to be planned. The adjustment is made in such a way that a maximum gradient slew rate Smaxassociated with the selected representative pulse sequence segment R-PSA is not exceeded.

FIG.10depicts a sketch of a magnetic resonance imaging system100(referred to below as “MR system” for short). It includes the actual magnetic resonance scanner102containing an examination space103or patient tunnel, into which may be moved on a couch108an object under examination O, or rather in this case a patient or person under examination, in whose body is located, for instance, a specific organ to be mapped.

The magnetic resonance scanner102is equipped with a main magnetic field system104, a gradient system106and also an RF transmit antenna system105and an RF receive antenna system107. In the embodiment shown, the RF transmit antenna system105is a body coil that is fixed in the magnetic resonance scanner102, whereas the RF receive antenna system107consists of local coils (represented inFIG.10by a single local coil) to be arranged on the patient or person under examination. In principle, however, the whole-body coil105may also be used as the RF receive antenna system, and the local coils107may be used as the RF transmit antenna system, provided these coils may each be switched into different operating modes.

The MR system100also includes a central control device110, that is used to control the MR system100. This central control device110includes a sequence control unit114for controlling the pulse sequence. This is used to specify the timing of radiofrequency pulses (RF pulses) and gradient pulses according to a selected imaging sequence PS. Such an imaging sequence PS may be specified, for example, in a measurement or control protocol P. Different control protocols P for different measurements are typically stored in a memory119, and may be selected by an operator (and possibly modified if required), and then used to perform the measurement.

For the output of the individual RF pulses, the central control device110includes a radiofrequency transmit device115, that generates and amplifies the RF pulses and feeds the RF pulses into the RF transmit antenna system105via a suitable interface (details not presented). The control device110includes a gradient system interface116for controlling the gradient coils of the gradient system106. The sequence controller114communicates in a suitable manner, for example by sending out sequence control data SD, with the radiofrequency transmit unit115and the gradient system interface116for the emission of the pulse sequences PS. The control device110also includes a radiofrequency receive device117(likewise communicating with the sequence control unit114in a suitable manner) for the purpose of coordinated acquisition of magnetic resonance signals received by the RF receive antenna system107. A reconstruction unit118receives the acquired data after demodulation and digitization as raw data or k-space data RD and reconstructs the MR image data BD therefrom. This image data BD may then be stored in a memory119, for example, or displayed via a display unit109.

The central control device110may be operated by a terminal having an input unit111and the display unit109, via which an operator may hence also operate the entire MR system100. MR images may also be displayed on the display unit109, and the input unit111, if applicable in combination with the display unit109, may be used to plan and start measurements, and for example to select and, if applicable, modify suitable control protocols containing suitable measurement sequences, as described above.

FIG.10also depicts the preparation device90already illustrated inFIG.9. The preparation device90is configured to modify a measurement protocol P having protocol parameters PP in such a way that a maximum gradient slew rate Smaxassociated with a selected representative pulse sequence segment R-PSA is not exceeded. For adjusting the measurement protocol P, first a value range of one or more protocol parameters is determined such that in the value range the previously determined maximum gradient slew rate Smaxis not exceeded by the pulse sequence segment M-PSA being considered and to be modified.

The previously determined value range is presented before or during the change to a relevant protocol parameter.

Furthermore, the MR system100and for example the control device110may also include a multiplicity of further components, that are not presented here in detail but are typically present in such equipment, for instance components such as a network interface, in order to connect the entire system to a network and to be able to transfer raw data RD and/or image data BD or parameter maps, but also further data such as patient-related data or control protocols, for example.

The principles of how suitable raw data RD may be acquired by applying RF pulses and generating gradient fields, and how MR images BD may be reconstructed from the raw data, are known to a person skilled in the art and are not explained further here.

Embodiments provide an effective improvement in, in terms of the length of time needed, a method for preparing magnetic resonance imaging of an object under examination and a magnetic resonance imaging method.

Finally, it shall be reiterated that the detailed methods and assemblies described above are merely exemplary embodiments, and that the basic principle may also be modified in many aspects by a person skilled in the art without departing from the scope of the invention insofar as this is defined by the claims. It is mentioned for the sake of completeness that the use of the indefinite article “a” or “an” does not rule out the possibility of there also being more than one of the features concerned. Likewise, the term “unit” does not exclude the possibility that the unit consists of a plurality of components, that may also be spatially distributed if applicable. Independent of the grammatical term usage, individuals with male, female or other gender identities are included within the term.

It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present disclosure. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that the dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.

While the present disclosure has been described above by reference to various embodiments, it may be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.