Method and magnetic resonance system for functional MR imaging of a predetermined volume segment of the brain of a living examination subject

Abstract

In a method and a magnetic resonance (MR) system for functional MR imaging of a predetermined volume segment of THE brain of a living examination subject, an RF excitation pulse is radiated into the subject and at least one magnetic field gradient is activated, and MR data of the predetermined volume segment is acquired beginning at a predetermined echo time after the RF excitation pulse. The echo time is in a time period of 10 μs to 1000 μs.

Claims

1. A method of acquiring functional magnetic resonance (MR) data from a predetermined volume segment in the brain of an examination subject, comprising: from a control computer, operating an MR data acquisition unit, comprising an RF radiator and a gradient system, in order to execute a functional MR data acquisition sequence; from said control computer, in said functional MR data acquisition sequence, activating at least one pre-pulse selected from the group consisting of a T1-selected pre-pulse and a T2-selected pre-pulse that induce relaxation of nuclear spins in a predetermined volume segment of an examination subject, said relaxation having a relaxation duration, and during said relaxation duration, radiating an RF excitation pulse from said RF radiator that excites said nuclear spins in said predetermined volume segment of the examination subject, and operating said gradient system in order to activate at least one magnetic field gradient and acquiring functional MR data produced by excitation of the nuclear spins in said predetermined volume segment, during each of a plurality of predetermined echo times that occur after said RF excitation pulse and during said relaxation duration; and from said control unit, operating said MR data acquisition unit in order to cause each of said plurality of predetermined echo times that occur after said RF excitation pulse to be in a time period in a microsecond range between 10 μs and 1,000 μs.

2. A method as claimed in claim 1 wherein said plurality of predetermined echo times is 2 to 500 echo times.

3. A method as claimed in claim 1 comprising, from said control unit, operating said gradient coil system in order to ramp up multiple magnetic field gradients for spatial coding of said functional MR data, simultaneously with beginning acquisition of said functional MR data.

4. A method as claimed in claim 1 comprising, from said control unit, operating said gradient coil system in order to ramp up multiple magnetic field gradients before radiating said RF excitation pulse, in order to acquire said functional MR data.

5. A method as claimed in claim 4 comprising entering said functional MR data into an electronic memory representing k-space, at respective k-space points in k-space, by entering said functional MR data into k-space points in a middle region of k-space individually so that said functional MR data are entered into only one k-space point in said middle region of k-space per each RF excitation pulse.

6. A method as claimed in claim 1 comprising entering said functional MR data into an electronic memory representing k-space, comprising a plurality of k-space points, and dividing said k-space points in k-space into a middle region of k-space and an outer region of k-space, k-space consisting of said middle region and said outer region, and entering said functional MR data into k-space points in said middle region more often than entering said functional MR data into k-space points in said outer region.

7. A method as claimed in claim 6 comprising: entering said functional MR data into k-space repeatedly in multiple, successive time windows respectively coinciding with said plurality of predetermined echo times; for each time window, reconstructing multiple MR images of said predetermined volume segment of the brain of an examination subject from the functional MR data acquired in that time window; during each time window, entering said functional MR data into k-space points in a predetermined number of lines in said middle region of k-space, said predetermined number of lines corresponding to the number of said multiple MR images reconstructed from each time window; and during each time window, entering functional MR data into k-space points in said outer region once, and reconstructing thereafter each of said multiple MR images of said predetermined volume segment of the brain of an examination subject, using a respective time window from the functional MR data entered into the middle region and the outer region while the respective time window is occurring.

8. A magnetic resonance (MR) system comprising: an MR data acquisition unit comprising an RF radiator and a gradient system; a control computer configured to operate said MR data acquisition unit in order to execute a functional MR data acquisition sequence; said control computer, in said functional MR data acquisition sequence, being configured to operate said RF radiator to activate at least one pre-pulse selected from the group consisting of a T1-selected pre-pulse and a T2-selected pre-pulse that induce relaxation of nuclear spins in a predetermined volume segment of the brain of an examination subject, said relaxation having a relaxation duration, and to radiate during said relaxation duration, an RF excitation pulse that excites said nuclear spins in said predetermined volume segment of the brain of the examination subject, and being configured to operate said gradient system in order to activate at least one magnetic field gradient and acquire functional MR data produced by excitation of the nuclear spins in said predetermined volume segment of the brain of an examination subject, during each of a plurality of predetermined echo times that occur after said RF excitation pulse and during said relaxation duration; and said control computer being configured to operate said MR data acquisition unit in order to cause each of said plurality of predetermined echo times that occur after said RF excitation pulse to be in a time period in a microsecond range between 10 μs and 1,000 μs.

9. A method as claimed in claim 8 wherein said plurality of predetermined echo times is 2 to 500 echo times.

10. A non-transitory, computer-readable data storage medium encoded with programming instructions, said data storage medium being loaded into a computerized control system of a magnetic resonance (MR) apparatus, said MR apparatus also comprising an MR data acquisition unit comprising an RF radiator and a gradient system, and said programming instructions causing said control system to: operate said functional MR data acquisition unit in order to execute a functional MR data acquisition sequence; in said functional MR data acquisition sequence, operate said RF radiator in order to activate at least one pre-pulse selected from the group consisting of a T1-selected pre-pulse and a T2-selected pre-pulse that induce a relaxation of nuclear spins in a predetermined volume segment of the brain of an examination subject, said relaxation having a relaxation duration, and in order to radiate during said relaxation duration an RF excitation pulse from said RF radiator that excites said nuclear spins in said predetermined volume segment of the brain of the examination subject, and to operate said gradient system in order to activate at least one magnetic field gradient and acquire functional MR data produced by excitation of the nuclear spins in said predetermined volume segment of the brain of an examination subject, during each of a plurality of predetermined echo times that occur after said RF excitation pulse and during said relaxation duration; and operate said MR data acquisition unit in order to cause each of said plurality of predetermined echo times that occur after said RF pulse to be in a time period in a microsecond range between 10 μs and 1,000 μs.

11. A method as claimed in claim 10 wherein said plurality of predetermined echo times is 2 to 500 echo times.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates a magnetic resonance system according to the invention.

(2) FIG. 2 shows a UTE sequence according to the invention.

(3) FIG. 3 depicts k-space trajectories for the sequence shown in FIG. 2.

(4) FIG. 4 depicts a sequence according to the invention for the radial acquisition of an outer region of k-space.

(5) FIG. 5 depicts a sequence according to the invention for single point acquisition of the middle region of k-space.

(6) FIG. 6 depicts the k-space acquisition scheme for a slice according to the sequences shown in FIGS. 4 and 5.

(7) FIG. 7 depicts k-space subdivided into a middle region and an outer region.

(8) FIG. 8 schematically shows from which MR data the MR images are reconstructed according to the invention.

(9) In FIG. 9 illustrates the present invention in comparison to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) FIG. 1 is a schematic representation of a magnetic resonance system 5 (a magnetic resonance imaging or nuclear magnetic resonance tomography apparatus for fMRI). A basic field magnet 1 generates a temporally constant, strong magnetic field for polarization or alignment of the nuclear spins in a volume segment of a subject O, for example of a brain of a human body lying on a table 23 that is driven into the magnetic resonance system 5 for examination or measurement. The high homogeneity of the basic magnetic field that is required for the magnetic resonance measurement (data acquisition) is defined in a typically spherical measurement volume M into which the parts of the human body that are to be examined are introduced. Shim plates made of ferromagnetic material are attached at suitable points to support the homogeneity requirements, and in particular to eliminate temporally invariable influences. Temporally variable influences are eliminated by shim coils 2 operable by a shim coil amplifier 27.

(11) A cylindrical gradient coil system 3 having three sub-windings is inserted into the basic field magnet 1. Each sub-winding is supplied with current by an amplifier to generate a linear (also temporally variable) gradient field in a respective direction of a Cartesian coordinate system. The first sub-winding of the gradient field system 3 generates a gradient G.sub.x in the x-direction; the second sub-winding generates a gradient G.sub.y in the y-direction; and the third sub-winding generates a gradient G.sub.z in the z-direction. Each amplifier has a digital/analog converter that is operated by a sequence controller 18 for accurately timed generation of gradient pulses.

(12) Located within the gradient field system 3 are one or more radio-frequency antennas 4 that convert the radio-frequency pulses emitted by a radio-frequency power amplifier into an alternating magnetic field for excitation of the nuclei out of the basic filed alignment of the nuclear spins of the subject O to be examined, or of the region of the subject O that is to be examined. Each radio-frequency antenna 4 has one or more RF transmission coils and multiple RF reception coil elements in the form of an annular, linear or matrix-like arrangement of component coils. The alternating field emanating from the precessing nuclear spins—i.e. normally the nuclear spin echo signals caused by a pulse sequence composed of one or more radio-frequency pulses and one or more gradient pulses—is also converted by the RF reception coil elements into a voltage (measurement signal) that is supplied via an amplifier 7 to a radio-frequency reception channel 8 of a radio-frequency system 22. The radio-frequency system 22 furthermore has a transmission channel 9 in which the radio-frequency pulses are generated for the excitation of magnetic resonance signals. The respective radio-frequency pulses are digitally represented in the sequence controller 18 as a series of complex numbers based on a pulse sequence predetermined by the system computer 20. This number sequence is supplied as a real part and imaginary part to respective digital/analog converters (DAC) in the radio-frequency system 22 via respective input 12, and from the digital/analog converters to the transmission channel 9. In the transmission channel 9 the pulse sequences are modulated on a radio-frequency carrier signal having a base frequency that corresponds to the center frequency, and are supplied to an RF power amplifier 28.

(13) The switching from transmission operation to reception operation takes place via a transmission/reception diplexer 6. The RF transmission coil of the radio-frequency antenna 4 radiates the radio-frequency pulses, supplied by the RF power amplifier 28, into the measurement volume M in order to excite nuclear spins in the subject located in the measurement volume M, and receives (detects) the resulting echo signals via the RF reception coils. These acquired magnetic resonance signals are phase-sensitively demodulated to an intermediate frequency in a reception channel 8′ (first demodulator) of the radio-frequency system 22 and are digitized in an analog/digital converter (ADC). This signal is further demodulated to a frequency of zero. The demodulation to a frequency of zero and the separation into a real part and an imaginary part occur in a second demodulator 8 after the digitization in the digital domain. An MR image or three-dimensional image data set is reconstructed by the image computer 17 from the measurement data acquired in such a manner. The administration of the measurement data, the image data and the control programs takes place via the system computer 20. Based on a specification with control programs, the sequence controller 18 monitors the generation of the respective desired pulse sequences and the corresponding scanning of k-space. The sequence controller 18 thereby controls the accurately timed switching of the gradients, the emission of the radio-frequency pulses with defined phase amplitude and the reception of the nuclear magnetic resonance signals. The time base for the radio-frequency system 22 and the sequence controller 18 is provided by a synthesizer 19. The selection of corresponding control programs to generate an MR image (which control program are stored on a DVD 21, for example) and the presentation of the generated MR image, take place via a terminal 13 that includes a keyboard 15, a mouse 16 and a monitor 14.

(14) A UTE sequence to acquire MR data with an ultrashort echo time for a functional MR imaging is shown in FIG. 2. It can be seen that a slice selection gradient G.sub.Z is activated while the RF excitation pulse 31 is radiated, causing nuclear spins in a slice to be excited that is perpendicular to the z-direction. The magnetic field gradients G.sub.x and G.sub.y are ramped up with the beginning of the MR data acquisition 32.

(15) The mathematical domain that is known as k-space 33 is realized as an electronic memory composed of locations at which data entries are made. The trajectories for data entry according to the sequence shown in FIG. 2 is presented in FIG. 3, these trajectories extending radially from a center. According to the invention, the points shown on the trajectories or, respectively, spokes corresponding to scanned k-space points 34. Since, according to the invention, only one slice is excited with the RF excitation pulse 31 and the simultaneously switched magnetic field gradient G.sub.z, the embodiment shown in FIGS. 2 and 3 is a two-dimensional MR measurement.

(16) An additional sequence according to the invention for the acquisition of MR data for functional MR imaging is shown in FIGS. 4 and 5. In this embodiment, the magnetic field gradients G have already been activated when the RF excitation pulse 31 is radiated, as is shown in FIG. 4. The MR data acquisition time period 32 begins an ultrashort echo time TE.sub.1 after the RF excitation pulse 31. The echo time TE.sub.1 designates the time period from the middle of the RF excitation pulse 31 up to the beginning of the MR data acquisition or, respectively, the MR data acquisition time period 32. During the first MR data acquisition time period 32, k-space points are acquired along half of a radial trajectory 41 which begins at the edge of the middle region 35 and ends at the end of the outer region 36. In contrast to this, k-space points along a full trajectory are acquired with an echo time TE.sub.2 during the chronologically following second MR data acquisition time period 32.

(17) A sequence according to the invention for scanning the middle region 35 is shown in FIG. 5. It can be seen that only a short MR data acquisition time period 32′ in which respectively only one k-space point is acquired is present after each RF excitation pulse 31. The magnetic field gradient G.sub.x changes incrementally from one MR data acquisition time period 32′ to the next MR data acquisition time period 32′, such that a k-space line is essentially scanned point by point along the X-axis.

(18) The k-space acquisition scheme for a slice through the middle of three-dimensional k-space 33 is shown in FIG. 6. While the k-space points 34 in the middle region 35 are scanned in a Cartesian manner by means of single point imaging (with the use of the sequence shown in FIG. 5), the k-space points in the outer region 36 are scanned radially (with the use of the sequence shown in FIG. 4).

(19) In FIG. 8 it is schematically shown from which MR data 38.sub.x, 39.sub.x the MR images 37.sub.x are reconstructed.

(20) For example, the first MR image 37.sub.1 is reconstructed from the first MR data 38.sub.1 of the middle region 35 and the first MR data 39.sub.1 of the outer region 36. The second MR image 37.sub.2 is reconstructed from the second MR data 38.sub.2 of the middle region 35 and from the same first MR data 39.sub.1 of the outer region 36. The third (fourth) MR image 37.sub.3 (37.sub.4) is reconstructed from the third (fourth) MR data 38.sub.3 (38.sub.4) of the middle region 35 and likewise from the same first MR data 39.sub.1 of the outer region 36. In other words: the first four MR images 37.sub.1 through 37.sub.4 are reconstructed from the same MR data 39.sub.1 of the outer region 36, wherein the MR data of these MR images 37.sub.1 through 37.sub.4 differ only with regard to their MR data 38.sub.1 through 38.sub.4 from the middle region 35.

(21) In a similar manner, the last MR images 37.sub.5 through 37.sub.8 shown in FIG. 8 are reconstructed from the same MR data 39.sub.2 of the outer region 36 and from respective individual MR data 38.sub.5 through 38.sub.8 of the middle region 35.

(22) FIG. 9 schematically shows the acquisition with a sequence according to the invention, with an ultrashort echo time TE.sub.1 in comparison to an acquisition according to the prior art, wherein a spin echo sequence with an echo time TE in the millisecond range is used.

(23) While the MR data acquisition according to the invention takes place in an MR data acquisition time period 32 which begins an ultrashort echo time TE.sub.1 after the RF excitation pulse 31, the MR data acquisition according to the prior art takes place markedly later. According to the prior art, at a time period T after the RF excitation pulse an additional RF pulse 42 for rephasing, wherein the acquisition of the MR data during the MR data acquisition time period 32″ takes place only markedly after this second RF pulse 42. While the acquisition of the MR data according to the invention takes place in a time period of 10 to 1000 μs after the RF excitation pulse 31, the acquisition of the MR data according to the prior art takes place milliseconds after the RF excitation pulse 31.

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