1. Field of the Invention
The present invention relates in general to magnetic resonance tomography (MRT) as used in the medical field for the examination of patients. This invention relates in particular to a process for avoiding peripheral interference signals in spin-echo images such as e.g. the ambiguity artifact.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear spin resonance and has been successfully implemented in the medical field and in biophysics for more than 15 years. In this examination method, the object is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the object, which were previously randomly oriented, are thereby aligned. Radio-frequency energy can now excite these “ordered” spins to a specific oscillation. This oscillation creates in MRT the actual measurement signal, which is detected by means of suitable receiving coils. By the use of non-homogeneous magnetic fields, created by gradient coils, the measurement object in each area of interest—also called FOV (field of view)—can be encoded in all three spatial directions, which in general is called “spatial encoding.”
The recording of the data in MRT occurs in k-space (frequency domain). The MRT image in the image domain is linked with the MRT data in k-space by Fourier transformation. The spatial encoding of the object, which spans k-space, takes place by means of the gradients in all three spatial directions. In this process a differentiation is made between the slice selection (determines the exposure slice in the object, normally the z axis), the frequency encoding (determines a direction in the slice, normally the x axis), and the phase encoding (determines the second dimension within the slice, normally the y axis).
Thus, a slice e.g. in the z direction is first selectively excited by a slice selection gradient Gs or Gz. The encoding of the location information in the slice takes place by combined phase and frequency encoding by means of both of the previously mentioned orthogonal gradient fields Gr and Gp, which are created in the example of a slice excited in the z direction by the aforementioned gradient coils in the x and y directions.
One known form for recording the data in an MRT measurement (scan) is shown in FIGS. 2A and 2B. The sequence used is a spin-echo sequence. In this sequence, the magnetization of the spins is flipped in the x-y plane by a 90° excitation impulse (with defined amplitude and bandwidth). Over time, the result is a dephasing of the magnetization components, which together form the transverse magnetization in the x-y plane Mxy. After a certain period of time (e.g. ½ TE, TE is the echo time), a 180° pulse (also with defined amplitude and bandwidth) is emitted in the x-y plane so that the dephased magnetization components are mirrored without changing the precession direction and precession speed of the individual magnetization components. After another period of time (½ TE), the magnetization components point in the same direction again, i.e. the result is a regeneration of the transverse magnetization, called “rephrasing,” which is appropriately read-out. The complete regeneration of the transverse magnetization is called the spin-echo.
In order to measure an entire slice of the object to be examined, the imaging sequence is repeated N times with different values of the phase-encoding gradient GP or Gy, whereby the frequency of the magnetic resonance signal (spin-echo signal) is scanned, digitized, and stored with each sequence repetition by the Δt-clocked ADC (analog digital converter) N times in equidistant time increments Δt in the presence of the selection gradients GR or GX. In this manner, a numerical matrix is created line by line as per FIG. 2b (matrix in k-space or k matrix) with N×N data points (a symmetrical matrix with N×N points is only one example; asymmetrical matrices also can be created). An MR image of the observed slice with a resolution of N×N pixels can be reconstructed directly from this data record via a Fourier transformation.
The scanning of the k matrix (k matrices when recording several slices) for spin-echo sequences with diagnostically usable image quality typically requires several minutes of measurement time, which represents a problem for many clinical applications. For example, patients might not be able to remain immobile for the required period of time. For examinations in the thorax or the pelvic area, body movement is generally unavoidable (heart and breathing movement, peristalsis). One way to accelerate spin-echo sequences was published in 1986 as the turbo-spin-echo sequence (TSE sequence) also known by the acronym RARE (Rapid Acquisition with Relaxation Enhancement) (J. Hennig et al. Magn. Reson, Med. 3, 823–833, 1986). In this imaging procedure, which is much faster compared to the conventional aforementioned spin-echo procedure, several (multiple) echoes are created after a 90° excitation pulse, and each of these echoes is individually phase-encoded. A pulse sequence is shown in FIG. 3A for the case when seven echoes are created. The phase-encoding gradient corresponding to the Fourier line to be selected must be switched before and after each echo. In this manner, a linear scanning of the k matrix as shown in FIG. 3B takes place after a single RF excitation pulse (90°). The necessary total measurement time is shortened in this example by a factor of 7. The signal progression in FIG. 3a is shown in an idealized manner. In reality, the later echoes have increasingly smaller amplitudes due to the T2 decay of the transverse magnetization.
An even faster imaging sequence is a combination of RARE with the Half-Fourier technique, which was introduced in 1994 as the so-called HASTE sequence (Half Fourier Acquired Single Shot Turbo Spin Echo) (B. Kiefer et al., J. Magn. Reson. Imaging, 4(P), 86, 1994). HASTE uses the same basic technique as RARE, but only half of the k matrix is scanned. The other half of the k matrix is reconstructed mathematically by means of the Half-Fourier algorithm. This takes advantage of the fact that the data points of the k matrix are arranged mirror-symmetrical to the center point of the k matrix. For this reason, it is sufficient to measure only the data points of a k-matrix half and to mathematically complete the raw data matrix by mirroring with respect to the center point (and complex conjugation). In this manner, the measurement time can be reduced by half. The reduction of the recording time, however, degrades the signal-to-noise ratio (SNR) by a factor of √2.
A general problem with spin-echo sequences (SE sequences) is that the resonance condition during the radio-frequency excitation by the RF pulse is determined not only in the FOV (field of view, characterized by homogeneity of the basic field as well as linearity of the gradient fields) but also in the non-homogeneous border area of the FOV. Due to the actual inhomogeneity of the basic field and the nonlinearity of the gradient fields in the border area of the MRT device, the principle of a unique reversible assignment of each spatial point to one specific magnetic field strength is violated. This means that a generally interfering image from the inhomogeneity area in the form of an artifact superimposes the image of actual measurement field. This unwanted artifact is known as a “double-entendre artifact” and occurs in a pronounced form in particular in spin-echo sequences as a result of the spin refocusing. The “double-entendre artifact” becomes the more pronounced the shorter the extent of the basic field magnet in the z-direction. Thus, future MRT systems will tend toward shorter magnets, intensify this problem, and it will no longer be solvable with the previous measures for suppressing this artifact.
Prior strategies for reducing this type of artifact are hardware measures and pulse-sequence modifications.
Hardware measures for the RF system include determining the spatial positions with field double entendres outside the useful volume for the given magnet and gradient design. The design of the RF coils is subject to the restriction of sufficiently minimizing its sensitivity for these critical spatial positions so that a significant artifact formation is prevented. RF field distributions, however, are not always able to be appropriately designed. As an unwanted side effect, RF field inhomogeneities that impair the image quality are also created within the useful volume.
The main focuses for the design of future MR devices are shorter magnets, spaciousness, and the largest possible patient accessibility (e.g. for surgical intervention). With this type of magnetic field geometry, the necessary reversible unique correspondence between space and field are violated such that the previous hardware measures fail. For shorter magnets with a large diameter, a suitable RF coil design thus is not possible.
Pulse sequence modifications (new approaches to sequence formation) often represent the only practical solution if the direct avoidance of the drawback (here the non-monotonous magnetic field progression) reaches theoretical or technological limits or requires disproportionately high effort, which would compromise the economic efficiency of the product.
A possible form of the pulse sequence modification according to U.S. Pat. No. 6,486,668 is to bring about an artifact suppression by emitting additional so-called preparation pulses. A disadvantage of this approach is the clear reduction in the time efficiency as well as the simultaneous creation of other image quality problems in the form of parasite spin-echo signal components due to the preparation pulses.
Another form of pulse sequence modification is implemented in U.S. Patent Application Publication No. 2002/0101237, wherein artifact suppression takes place by switching the polarity switch of the selection gradients between the RF excitation pulse and the RF refocusing pulse. In contrast to conventional slice excitation of an SE sequence, the slice selection gradient, which is switched during the slice excitation by the (90°-) RF pulse, compared to the slice selection gradient, which is switched during the (180°-) refocusing pulse, is inverted as to its algebraic sign or its polarity. This causes selection of the (90°-) RF pulse as well as the (180°-) refocusing pulse to occur in the spatial domain in different non-overlapping areas. In this manner, no interfering echo signals are created. This procedure, however, has the disadvantage that it places increased technical demands on the exact temporal synchronization of RF pulses and gradient pulses as well as on the system shimming. Moreover, the simultaneous representation of different chemical components (e.g. fat and water) is only possible with a notable signal loss.