Spectroscopic imaging method

Details Spectroscopic imaging method Spectroscopic imaging method

2001-03-28

2003-10-28

Robinson, Daniel (Department: 3742)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

Reexamination Certificate

active

06640125

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a spectroscopic imaging method.
2. Description of the Prior Art
For more than four decades, magnetic resonance spectroscopy has been used in fundamental research in physics, chemistry, and biochemistry, for example as a technique of analysis or for the structural clarification of complex molecules. Clinical magnetic resonance spectroscopy refers to magnetic resonance spectroscopy using clinical magnetic resonance tomography apparatuses. The methods of localized magnetic resonance spectroscopy are distinguished from those of magnetic resonance imaging essentially only in that in spectroscopy chemical displacement is also resolved, in addition to tomographic spatial resolution. In tomographic imaging, for example multi-spin echo methods, such as the RARE (Rapid Acquisition with Relaxation Enhancement) method are known, in which, in contrast to a single spin echo pulse sequence, additional spin echoes are produced by adding additional 180° radio-frequency pulses. For multi-spin echo methods in tomographic imaging, improvements are known that seek to achieve a high signal level for a largest possible number of additional spin echoes. Here, reference is made to, among other sources. For example, A. A. Maudsley, “Modified Carr-Purcell-Meiboom-Gill Sequence for NMR Fourier Imaging Application,” Journal of Magnetic Resonance 69, 1986, pp.488-491, discloses producing a 90° phase offset from pulse to pulse for radio-frequency pulses following one another immediately in time.
Currently, in clinical applications two localization methods are dominant for magnetic resonance spectroscopy. One type includes individual volume techniques based on echo methods, in which a spectrum of a target volume selected beforehand on the basis of proton images is recorded. Another type are spectroscopic imaging methods, known as CSI methods (Chemical Shift Imaging), that simultaneously enable the recording of spectra of a multiplicity of spatially contiguous target volumes.
The single-volume techniques standardly used today are based on an acquisition of a stimulated echo or of a secondary spin echo. In both cases, a spatial resolution takes place by successive selective excitation of three orthogonal layers. The target volume is defined by the slice volume of these three layers. Only the magnetization of the target volume experiences all three selective radio-frequency pulses and thus contributes to the stimulated and secondary spin echo. The spectrum of the target volume is obtained by one-dimensional Fourier transformation of a time signal corresponding to the stimulated echo or to the secondary spin echo.
Spectroscopic imaging methods are used both in clinical phosphorus spectroscopy as well as in proton spectroscopy. A 3D CSI pulse sequence has, for example, the following steps: After a non-layer-selective 90° radio-frequency pulse, a combination of magnetic phase-coding gradients of the three spatial directions is activated for a defined time duration, and subsequently the magnetic resonance signal is read out in the absence of all gradients. This procedure is repeated as often as necessary with different combinations of phase-coding-coding gradients until the desired spatial resolution has been achieved. A four-dimensional Fourier transformation of the magnetic resonance signals supplies the desired spatial distribution of the resonance lines. If the above-described non-selective radio-frequency pulse is replaced by a layer-selective excitation, consisting of a frequency-selective radio-frequency pulse and a corresponding magnetic gradient, one phase-coding direction can be omitted, and in a 2D CSI pulse sequence of this sort the measurement time is reduced in relation to the 3D CSI pulse sequence.
In clinical proton spectroscopy, the intensive water signals are often suppressed by means of water suppression techniques. One such technique for water suppression is, for example, the CHESS technique, in which the nuclear spins of the water molecules are first selectively excited by narrowband 90° radio-frequency pulses, and their cross-magnetization is subsequently dephased through the switching of magnetic field gradients. For an immediately subsequent spectroscopic imaging method, in the ideal case no detectable magnetization of the water molecules is therefore available. In methods using a suppression of a dominant resonance line, however, lines adjacent to the dominant resonance line are also at least partially saturated as well, so that, disadvantageously, these lines appear only weakly, or not at all, in the associated spectrum.
In general, fast CSI methods are based on multiecho sequences. Besides the one desired echo per readout interval, secondary echoes and stimulated echoes also occur in multiecho sequences. In connection with the large offset frequencies due to the chemical displacement, this leads to the formation of two echo groups, known as an even echo family and an odd echo family.
One of the fast CSI methods is known as the CSI-U-FLARE method. Here a distinction is made between variants known as coherent, phase-cyclical, and pushing-apart. In the coherent CSI-U-FLARE method, for the suppression of the above-cited formation of two echo groups an attempt is made, inside an acquisition window, to superimpose the even and the odd echoes in phase-coherent fashion by carrying out a fine adjustment of gradients and sequence parameters. Because the above-cited superimposition succeeds only for a single resonance line, and because slight de-adjustments already cause significant artefacts, the coherent CSI-U-FLARE method has not achieved significance in spectroscopic imaging.
In the phase-cyclical CSI-U-FLARE method, two complete measurements are carried out that are distinguished from one another only in that the refocusing radio-frequency pulses have respective phase angles that are offset by, for example, 90°. By means of a corresponding subsequent processing of the two measurement results, an unambiguous identification of the two echo families can be achieved. The measurement results are thereby separated in the time domain, and for the two echo families the corresponding spectra are reconstructed and the two spectra are added, with a mirroring of the spectrum for one of the echo families being necessary before the addition. The necessary mirroring of the spectra has, for example, the result that, given an incomplete separation of the measurement results, artefacts arise in the two spectra. In relation to the theoretically ideal coherent method, in the phase-cyclical method the overall measurement time is doubled due to the two measurements, and the signal-noise ratio for a comparable measurement time is reduced to approximately 71%.
In the pushing-apart variant of the CSI-U-FLARE method, the two echo families are purposely pushed apart in such a way that either only one of the echo families is detected in an acquisition window, or both are sufficiently distant from one another that they can be acquired individually. For this purpose, gradient time surfaces are intentionally misadjusted. In relation to the theoretically ideal coherent method, the signal-to-noise ratio for a comparable measurement time is thereby reduced to 50 percent. Further explanation of the CSI-U-FLARE method can be found in the article by W. Dreher et al., “Improved Proton Spectroscopic U-FLARE Imaging for the Detection of Coupled Resonances in the Rat Brain in Vivo,” Magnetic Resonance Imaging, volume 17, no. 4, 1999, pp. 611-621.
A general disadvantage of the CSI-U-FLARE methods is that an excited transversal magnetization during the measurement time is partially converted into a longitudinal magnetization, and thus is not available for signal acquisition. A further disadvantage of the above-cited methods is that the amplitudes of the various echo families during the first echo first must be stabilized in order to ensure a uniform distribution of both echo families and a subsequent signal curve that decreases monotonically. Due

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