Magnetic resonance apparatus and operating method with adjustment of the excitation angle dependent on data acquisition field of view

Abstract

In a magnetic resonance tomography (MRT) apparatus and operating method, a field of view for imaging a target object is acquired. A relative position of this field of view in relation to a receiving space of the MRT scanner, in which the target object is received, is then automatically determined. A radio-frequency (RF) pulse to be used by the MRT scanner for imaging the target object is then automatically adjusted depending on this relative position. An excitation angle produced in the field of view by the RF pulse is changed compared to the use of the corresponding unadjusted RF pulse.

Claims

1. A method for operating a magnetic resonance tomography (MRT) apparatus comprising an MRT data acquisition scanner having a receiving space for receiving a target object from which MRT data are to be acquired by executing an MRT data acquisition protocol, provided to a control computer of said MRT apparatus, said MRT data acquisition protocol comprising at least one radio-frequency (RF) pulse, said method comprising: providing said computer with a specification of a field of view that encompasses a portion of said receiving space from which said MRT data are to be acquired from said target object; in said computer, automatically determining a relative position of the field of view in relation to said receiving space of the MRT scanner; and in said computer, automatically adjusting said at least one RF pulse dependent on the determined relative position of the field of view, so as to change an excitation angle produced by said at least one RF pulse in said field of view, compared to an excitation angle produced by the original RF pulse in said MRT data acquisition protocol.

2. A method as claimed in claim 1 comprising changing an amplitude of said RF pulse while maintaining a duration of said RF pulse unchanged.

3. A method as claimed in claim 1 comprising making a larger change to said RF pulse as said specification of said field of view shows that said field of view is closer to an edge of said receiving space.

4. A method as claimed in claim 1 comprising: from said specification of said field of view, automatically determining a loading that exists in said receiving space; automatically determining an expected field strength of B.sub.1 field produced by said RF pulse in said field of view, dependent on said loading; and changing said RF pulse dependent on a difference between the expected B.sub.1 field strength and a specified target field strength in said MRT data acquisition protocol, so as to align said excitation angle resulting with the expected B.sub.1 field strength with a specified target excitation angle produced by said target field strength.

5. A method as claimed in claim 1 wherein said MRT data acquisition protocol comprises a plurality of pulses, and changing all of said plurality of pulses in a same way as said at least one RF pulse.

6. A method as claimed in claim 1 wherein said MRT data acquisition protocol comprises a plurality of pulses, with only a subset of said plurality pulses being excitation pulses, and changing all excitation pulses in said subset in a same way as said at least one RF pulse.

7. A method as claimed in claim 1 wherein said MRT data acquisition protocol comprises a plurality of pulses, with only a subset of said plurality pulses being refocusing pulses, and changing all refocusing pulses in said subset in a same way as said at least one RF pulse.

8. A method as claimed in claim 1 wherein said MRT data acquisition protocol comprises a plurality of RF pulses, including fat saturation pulses, and changing all of said RF pulses, except said fat saturation pulses, in a same way as said at least one RF pulse is changed.

9. A method as claimed in claim 1 comprising changing said RF pulse according to a specified function selected from the group consisting of a quadratic function that is dependent on a spatial variable in said field of view, a polynomial function that is higher than a second degree, a step function, and an exponential function.

10. A magnetic resonance tomography (MRT) apparatus comprising: an MRT data acquisition scanner comprising a receiving space in which a target object is received; a control computer configured to operate the MRT data acquisition scanner in order to execute an MRT data acquisition protocol so as to acquire MRT data from the target object, said MRT data acquisition protocol comprising at least one RF pulse; a detector that detects and provides said computer with a specification of a field of view that encompasses a portion of said receiving space from which said MRT data are to be acquired from said target object; said computer being configured to automatically determine a relative position of the field of view in relation to said receiving space of the MRT scanner; and said computer being configured to automatically adjust said at least one RF pulse dependent on the determined relative position of the field of view, so as to change an excitation angle produced by said at least one RF pulse in said field of view, compared to an excitation angle produced by the original RF pulse in said MRT data acquisition protocol.

11. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a control computer of a magnetic resonance tomography (MRT) apparatus that comprises an MRT data acquisition scanner having a receiving space in which a target object is received in order to acquire MRT data from the target object according to an MRT data acquisition protocol that comprises at least one RF pulse, said programming instructions causing said control computer to: receive a specification of a field of view that encompasses a portion of said receiving space from which said MRT data are to be acquired from said target object; determine a relative position of the field of view in relation to said receiving space of the MRT scanner; and adjust said at least one RF pulse dependent on the determined relative position of the field of view, so as to change an excitation angle produced by said at least one RF pulse in said field of view, compared to an excitation angle produced by the original RF pulse in said MRT data acquisition protocol.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates an inhomogeneity of the type that may occur in the execution of an MRT procedure.

(2) FIG. 2 shows a first MRT image having an undesirable brightness characteristic.

(3) FIG. 3 shows a second MRT image having an undesirable brightness characteristic.

(4) FIG. 4 is an exemplary flowchart of an embodiment of the method for operating an MRT apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) The exemplary embodiments explained hereinafter are preferred embodiments of the invention. In the exemplary embodiments the described components of the embodiments are each individual features of the invention to be considered independently of each other, which each also develop the invention independently of each other and thereby can also be regarded as components of the invention, individually or in a combination different to that shown. Furthermore, the described embodiments can also be supplemented by further features of the invention already described.

(6) Identical elements, those with the same function or mutually corresponding elements are each identified by the same reference characters in the figures.

(7) FIG. 1 is a schematic diagram to illustrate an inhomogeneity in the case of an MRT procedure and to illustrate a method for operating an MRT scanner 1. The MRT scanner 1 is only schematically indicated as a ring here, which encloses a receiving space 2. In the present case a patient support 3 is arranged in the receiving space 2 as part of the MRT scanner 1. Here, the method can be in the form of a computer program, a computer program product or program code, which is stored on a data carrier that can be loaded into a control computer of the MRT scanner 1. The method is therefore carried out or applied by the computer program or the program code being executed by the control computer of the MRT scanner 1.

(8) In the present case, a patient 4 is arranged on the patient support 3 in the receiving space 2 as a target object for imaging by the MRT scanner 1. The patient 4 has (schematically indicated) a torso 5 and a left arm 6 and a right arm 7. For imaging the patient 4, the receiving space 2 is penetrated by a static magnetic field B.sub.0 and a magnetic alternating field B.sub.1.

(9) A reference point 8 is marked in a center point of the patient support 3. For further illustration, a spatial axis 9 is illustrated on which a spatial coordinate x is plotted, which indicates a position inside the receiving space 2 along the patient support 3 in relation to the reference point 8. An origin or zero point of the spatial axis 9 corresponds here to the location or the position of the reference point 8. An exemplary angular characteristic 10 is schematically illustrated or plotted in relation to the spatial axis 9 and the receiving space 2. This angular characteristic 10 illustrates a possible characteristic of a size of an excitation angle, which would result in the patient 4 when using an unmodified, specified measuring or pulse sequence for imaging the patient 4 over a width of the receiving space 2.

(10) The pulse sequence is specified such that, under the assumption that ideal conditions exist throughout the receiving space 2, so an identical specified target excitation angle would be achieved. In reality, the MRT scanner 1 is subject onto technical limitations, and the patient 4 also can affect the angular characteristic 10, in other words an underlying field strength of the B.sub.1 field. As a result, when using the specified pulse sequence, the angular characteristic 10 would be produced such that, for example, the specified target excitation angle would be achieved only in the region of the reference point 8, and in the direction of a left edge region 11 and in the direction of a right edge region 12 of the receiving space 2, an increasing (larger) excitation angle would occur. In other cases, dependent on the structure of the respective MRT system, an opposite angular characteristic may occur, which, without adjustment, reduces or decreases resulting excitation angles, from the reference point 8 to the edge regions 11, 12.

(11) If, for example, the right arm 7 is to be imaged, then a schematically indicated field of view 13 can be specified for this imaging scan. As can be seen here with reference to the schematic illustration of the angular characteristic 10, a target excitation angle greater than the specified angle would then occur in the field of view 13.

(12) FIG. 2 shows a first MRT image 14, which was acquired by conventional operation of an MRT scanner, using the unmodified specified pulse sequence. Specifically, a B.sub.1 field distribution in the region of an elbow is illustrated in the first MRT image 14. Due to the inhomogeneity of the B1 field in the receiving space 2, a brightness or intensity characteristic between a first section 15 and a second section 16 results, due to the spatial dependency of the excitation angle illustrated by the angular characteristic 10. This brightness or intensity characteristic, in other words a corresponding gradient, is an artifact, therefore does not depict an actual, real property of the imaged object.

(13) FIG. 3 shows a second MRT image 17, in which analogously to the first MRT image 14, a gradient between a first section 15 and a second section 16 of a real homogeneous tissue region is likewise illustrated. Furthermore, a third section 18 different from a fourth section 19 is depicted in the second MRT image 17, although these two sections 18, 19 have the same tissue type, ideally would be imaged identically therefore.

(14) FIG. 4 schematically shows an exemplary flowchart 20 of a method for operating the MRT scanner 1 to achieve an improved image quality.

(15) The method starts in method step S1 wherein the MRT scanner 1 is made ready implement a scan of the patient 4, and the patient 4 is arranged in the receiving space 2. This can include selecting or setting a data acquisition protocol in order to implement the scan, the control protocol including at least one RF pulse. The data acquisition protocol is provided to the control computer that operates the MR scanner 1.

(16) The field of view 13 for imaging and a pulse sequence to be used by the MRT scanner 1 for imaging this field of view 13 or a corresponding section of the patient 4, are specified in method step S2.

(17) Loading of the receiving space 21, here, the patient 4 therefore, is detected in a method step S3 by a detector 21 of the MRT scanner 1. Detection data supplied by the detector 21 are then automatically evaluated by the computer of the MRT scanner 1 in order to determine at least one property of the current loading, here, of the patient 4. For example, the build or body type of the patient 4 can be automatically determined by suitable image processing and execution of an object recognition algorithm. For example, a body fat percentage can be estimated. This can have a direct effect on the B.sub.1 field strength that can be achieved with a specified RF pulse.

(18) A relative position of the field of view 13 in relation to the receiving space 2, in particular in relation to the reference point 8, is determined in method step S4. A B.sub.1 field strength expected in the region of the field of view 13 when using the specified pulse sequence, and a resulting excitation angle, is then determined by taking into account this determined relative position and the previously detected loading.

(19) The expected B.sub.1 field strength in the field of view 13 or the resulting expected excitation angle is compared with a specified B.sub.1 target field strength or with a specified target excitation angle and a corresponding difference determined in method step S5.

(20) An RF pulse of the specified pulse sequence is automatically adjusted by the control computer of the MRT scanner 1 depending on the determined difference—and therefore also depending on the determined relative position of the field of view 13 in relation to the reference point 8, in method step S6 in order to compensate the difference between the expected excitation angle and the specified target excitation angle. Therefore, in the present case a level or amplitude of at least one RF pulse of the specified pulse sequence is increased contrary to the angular characteristic 10.

(21) The RF pulse(s) to be adjusted or the corresponding excitation or flip angles is/are therefore adapted according to the position of the field of view 13 or according to the position of a center of the field of view 13 in the receiving space 12 or on the spatial axis 9. After the adjustment or adaptation of the RF pulse(s), this does not produce the illustrated angular characteristic 10 in the field of view 13, and instead the specified target excitation angle is also achieved in the field of view 13, corresponding to a minimum of the angular characteristic 10 at x=0, therefore at the reference point 8.

(22) Different adjustment or adaptation functions for scaling the RF pulse(s) or the corresponding excitation or flip angles are available for this purpose. Different target excitation angles can be specified for different RF pulses of the specified pulse sequence, so that, accordingly, different pulses of the specified pulse sequence can be scaled or adjusted in different ways in order to achieve the respective target excitation angle. For example, a target excitation angle of 90° can be specified for an excitation pulse of the specified pulse sequence. When using the unadjusted specified excitation pulse, however, this would only be achieved in the region of the reference point 8.

(23) In the present case, the field of view 13 is located in a right-hand edge region 12 of the receiving space 2, however, so that when using the excitation pulse provided according to the specified pulse sequence, for example, an excitation angle of 100° would be achieved here. Depending on the specific MRT scanner 1 or depending on the situation or application, a quadratic function can be specified for adjusting or scaling the RF pulse or the corresponding excitation angle. The RF pulse is then scaled by a factor of 1/x.sup.2, with the determined position of the center of the field of view 13 on the spatial axis 9 being used for x.

(24) A step function can likewise be specified. For this purpose, a boundary value 22 can be specified for x, which divides a region between the reference point 8 and an edge of the receiving space 2 into a first value range 23 and into a second value range 24. If the position x of the center of the field of view 13 is then in the first value range 23, then, for example, the specified pulse sequence can be used. If, on the other hand, the position x of the center of the field of view 13 is in the second value range 24, then the RF pulse to be adjusted in each case can be modified for instance by a specified, in particular constant, factor or offset.

(25) A higher-order function, for example a fourth degree polynomial function, can likewise be specified for adjusting or scaling the respective RF pulse with or depending on the position of the field of view 13 relative to the reference point 8. An exponential function can likewise be specified for this, according to which the respective RF pulse is then modified for instance by a factor |exp(mx)|. In this case m is a scaling or optimization factor, which can be adjusted or adapted for example by a respective user. The scaling factor m therefore provides an optimization option or an optimization criterion in order to further improve the image quality in the respective individual case, in other words, to actually be able to make an optimum adaptation.

(26) A section of the patient 4 located in the field of view 13, in the present case corresponding to the right arm 7, is then imaged in a method step S7 by applying or using the adjusted RF pulse(s), in other words, by the correspondingly adjusted pulse sequence.

(27) In summary, the described examples show how an automatic adaptation of an excitation flip angle can be implemented to improve the image quality. The described method can be applied particularly advantageously to TSE, SPACE, HASTE and SE sequences.

(28) Although modifications and changes may be suggested by those skilled in the art, it is the intention of the Applicant to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the Applicant's contribution to the art.