Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
  • G01R33/56563—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0

Definitions

• Pulse sequences used today are generally based on the so-called "SPIN-ARP" method, as described, for example, in US Pat. No. 4,706,025 .
• a nuclear magnetic resonance signal is phase-coded in at least one direction before reading and frequency-coded in a further direction by a reading gradient during reading.
• a number of nuclear-magnetic resonance signals which are phase-coded differently in the phase coding direction are obtained.
• the core resonance signals are sampled, digitized on a raster in k-space and entered in a raw data matrix in k-space.
• a Fourier transformation is carried out in the raw data matrix for image acquisition both in the phase coding direction and in the frequency coding direction.
• Inhomogeneities of the basic magnetic field in the phase coding direction are relatively uncritical, since it is only a question of signal differences between the individual phase coding steps.
• the overlay of the readout gradient with basic field inhomogeneities leads to distortions.
• EP 0 492 706 AI describes a method for compensating image errors, which e.g. external magnetic fields, movement of the examination object (e.g. due to breathing) and drift in amplifiers or permanent magnets due to temperature influences. It is suggested to use overlapping data lines in the Fourier domain to estimate these errors. This estimate is used to correct the data sets obtained before the image reconstruction.
• image errors e.g. external magnetic fields, movement of the examination object (e.g. due to breathing) and drift in amplifiers or permanent magnets due to temperature influences. It is suggested to use overlapping data lines in the Fourier domain to estimate these errors. This estimate is used to correct the data sets obtained before the image reconstruction.
• EP 0 585 973 describes a method in which only a magnetic field gradient is applied in the slice selection direction before the actual imaging, but no magnetic field gradients in the phase coding direction and frequency coding direction.
• an MR signal is generated, detected and Fourier-transformed.
• Information about the intensity of the static magnetic field is obtained from the resulting frequency spectrum. This information is used as a parameter in imaging in order to obtain an MR image free of position errors.
• the object of the invention is to provide an image reconstruction method which, in the case of basic magnetic fields of known inhomogeneity, delivers images which are largely free of distortion, that is to say images without geometric distortions and intensity errors.
• 7 shows the relationship between location and resonance frequency for a linear magnetic field gradient
• 8 shows the relationship between location and resonance frequency, taking magnetic field inhomogeneities into account
• 9 and 10 show the comparison of an idealized SPIN distribution and an SPIN distribution actually measured in the inhomogeneous field for two different gradient slopes
• Pulse sequences can be used, in which nuclear magnetic resonance signals are phase-coded before reading and frequency-coded during reading.
• a frequency-selective high-frequency pulse RF is first radiated in under the action of a slice selection gradient Gg.
• a slice selection gradient Gg nuclear spins are only excited in one slice of the object under examination.
• the dephasing caused by the positive partial pulse of the slice selection gradient Gg is reversed by a negative partial pulse Gg ⁇ .
• a phase coding gradient Gp is also irradiated.
• a negative readout gradient GR ⁇ is also switched on.
• the pulse sequence shown is repeated N times with different values of the phase coding gradient Gp, so that a total of one measurement matrix with N lines is obtained.
• the phase coding gradient is switched from pulse sequence to pulse sequence in the same steps from the highest positive to the highest negative value or vice versa.
• the raw data matrix RD can be regarded as a measurement space, in which, in the exemplary embodiment, the present two-dimensional case is considered as the measurement data level. This measurement data space is referred to as "K space" in magnetic resonance imaging.
• the information about the spatial origin of the signal contributions necessary for image generation is coded in the phase factors, the relationship between the location space and the K space being mathematically related via a Fourier transformation. The following applies:
• each line corresponds to an individual nuclear magnetic resonance signal S.
• the phase coding gradient Gpc is incremented, the scanning in K space takes place in successive lines.
• a phase coding gradient Gp is switched on in front of the first nuclear resonance signal S ⁇ [, the gradient amplitude of which gradually increases step by step from partial sequence to partial sequence. If, for example, each nuclear magnetic resonance signal is sampled with 256 measuring points and a 256 phase coding step is carried out, a raw data matrix with 256 rows and 256 columns, ie 256 x 256 measured values in k-space, is obtained.
• the analog measurement signals obtained in the pulse sequence according to FIGS. 1 to 5 are digitized on a raster in k-space.
• the aim of the invention is now to find a method in which the distortions shown are already avoided during image reconstruction.
• the minimum resolution in the image is given by the smallest slope of the effective gradient, i. H. here there are optimization possibilities between inhomogeneity and the linear gradient. For example, ambiguities can be avoided by increasing the linear readout gradient.
• distortion is not only understood to mean geometrical shifts, but also intensity errors.
• the basic magnetic field B total ( - (x) is composed of the homogeneous basic magnetic field B and a portion of the inhomogeneity B (x).
• the inhomogeneity can also be regarded as a gradient which, together with the linear readout gradients g, is an effective gradient g e ff results in:
• This location dependence of the effective gradient means in particular that the resolution and bandwidth are not necessarily constant for the entire image.
• the effect of inhomogeneities is shown again in FIGS. 9 and 10.
• Bg in each case represents the intensity distribution in the image in the case of a one-dimensional, step-shaped spin distribution without the effect of an inhomogeneity. If, due to inhomogeneities, the effective gradient is no longer linear, but instead has a quadratic position dependency, the image B actually obtained becomes either to the left as shown in FIG. 9 (
• Image pixels x m are obtained using the following formula from the measurement data previously Fourier-transformed in the phase coding direction for one row of the raw data matrix:
• I (x) is an intensity correction factor with which the above-mentioned intensity errors are to be corrected. This intensity correction factor I (x) is determined as follows:
• the following measurement signal s (t) is obtained in a spin-echo sequence.
• t n the time grid of the scanning and designated by g the gradient in the x direction.
• Equation 6 Equation 6
• w (k) can be done by weighting the measurement data with a window function w (k), which adaptively reduces the resolution for each pixel exactly to the extent that would have been obtained in a homogeneous magnetic field under the same conditions.
• a weighting function w (k n ) can be introduced into the reconstruction formula according to equation 1.
• FIG. 11 shows the time dependence on nuclear magnetic resonance signals s (t) schematically, curve a representing a signal under a smaller gradient and curve b representing a signal under a larger gradient.
• the weighting function w (k n ) could, for example, be implemented as a window function in the form of a rectangle F as in FIG. 11, the width of the window being adapted to the width of the echo signal s (t).
• the weighting function w (t) could have the following form:
• the weighting function does not necessarily have to have a rectangular shape.
• the window function improves the signal-to-noise ratio. Artifacts that are otherwise caused by the varying resolution can be achieved due to the locally different effective gradient.
• the described method does require a significantly higher computing time than conventional FFT algorithms. However, since the computing power available is constantly increasing, this disadvantage is becoming less important.

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• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Radiology & Medical Imaging (AREA)
• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• High Energy & Nuclear Physics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

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Cited By (7)

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• 1994
  • 1994-05-09 DE DE19944416363 patent/DE4416363C2/de not_active Expired - Fee Related
• 1995
  • 1995-05-05 JP JP7528597A patent/JPH09512723A/ja active Pending
  • 1995-05-05 WO PCT/DE1995/000592 patent/WO1995030908A1/de active Application Filing

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