Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-06-01
2004-07-27
Gutierrez, Diego (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S423000, C600S412000, C324S315000
Reexamination Certificate
active
06768917
ABSTRACT:
The invention relates to a magnetic resonance imaging method. In order to form a magnetic resonance image of an object, the object is arranged in a steady, as uniform as possible magnetic field. Often only a part of the object is imaged; to this end, the part of the object to be imaged is then arranged in the steady magnetic field. The steady magnetic field orients spins in the object to be examined predominantly in the direction of the steady magnetic field. According to such a magnetic resonance imaging method, spins in an object to be examined are excited. Relaxation of the excited spins produces magnetic resonance signals which are acquired. A magnetic resonance image is reconstructed from the magnetic resonance signals acquired.
A magnetic resonance imaging method of this kind is known from U.S. Pat. No. 5,378,987.
The known magnetic resonance imaging method is dedicated notably to measurement, on the basis of the magnetic resonance signals, of a temperature distribution in the object to be examined. The cited United States patent deals with the problems caused by displacements of the object to be examined. The cited United States patent mentions notably that the measured temperature distribution may be spoiled by displacement of the object to be examined. The known magnetic resonance imaging method offers a rather cumbersome, time-consuming solution to this problem. The known magnetic resonance imaging method notably necessitates the execution of separate magnetic resonance excitation sequences for the detection of displacements of the object and for the measurement of the frequency shift due to variation of the temperature, referred to as “chemical shift data”, respectively. According to the known magnetic resonance imaging method, such magnetic excitation sequences must both be repeated for different values of the echo time in the measurement of the chemical shift.
It is an object of the invention to provide a magnetic resonance imaging method wherein it is comparatively simply achieved that hardly any disturbances occur due to motions of the object to be examined.
This object is achieved by means of a magnetic resonance imaging method according to the invention wherein
magnetic resonance signals are acquired,
the position of a measuring site is determined, and
the magnetic resonance image is reconstructed from the magnetic resonance signals and on the basis of the position of the measuring site.
In accordance with the invention, the position of a measuring site is separately determined, such a position can be separately measured. Furthermore, a predetermined geometrical relationship exists between the measuring site and the region reproduced in the magnetic resonance image. On the basis of the position determined for the measuring site, disturbances due to motion of the object to be examined can be avoided in the magnetic resonance image on the basis of the predetermined geometrical relationship between the measuring site and the region to be imaged (
FIG. 3
step
320
). The object to be examined is a patient. During the acquisition of the magnetic resonance signals, the patient is liable to move and/or motions are liable to occur within the body of the patient, due to the respiration and/or the heartbeat. The magnetic resonance imaging method according to the invention notably ensures that hardly any disturbances which are due to such motions in and/or of the patient occur in the magnetic resonance image.
The invention is implemented in such a manner that a selected slice of the object to be imaged contains the measuring site. Notably when images of the same slice are repeatedly formed it is achieved that the same slice is always accurately reproduced. It is then ensured that the selected slice always extends through the measuring site. The selection of such a slice is performed on the basis of an RF excitation in combination with a selection gradient. Such a selection gradient is superposed on the steady magnetic field.
The magnetic resonance image can also be accurately corrected for motion in and/or of the object on the basis of the measured position of the measuring site and the predetermined geometrical relationship between the measuring site and the region to be imaged.
It has been found that the position of the measuring site can be readily determined. As a result, disturbances in the magnetic resonance image which are due to motion can be very simply counteracted.
These and other aspects of the invention will be elaborated on the basis of the following embodiments which are defined in the dependent claims.
Preferably, a clearly recognizable detail of the object to be examined and an indication of the measuring site are reproduced in the magnetic resonance image. This is realized by reproducing the relevant detail and the measuring site together in the magnetic resonance image (
FIG. 3
step
380
). On the basis of the predetermined geometrical relationship between the measuring site and the relevant detail, the correct position of the reproduction of the detail relative to the indication of the measuring site in the magnetic resonance image can also be derived (
FIG. 3
step
390
). On the basis of the derived correct position of the detail it can then be readily checked whether the position of the detail has shifted due to motion in and/or of the object and, if desired, the position of the detail in the magnetic resonance image can be corrected.
The magnetic resonance imaging method according to the invention is particularly suitable for accurately deriving the local temperature distribution in the object to be examined by means of the magnetic resonance imaging method. To this end, reference magnetic resonance signals are first acquired at a predetermined reference temperature, after which measuring magnetic resonance signals are acquired at a locally increased temperature in the object to be examined. A reference magnetic resonance image of the part of the object to be examined is reconstructed from the reference magnetic resonance signals. A measuring magnetic resonance image of the part of the object to be examined is reconstructed from the measuring magnetic resonance signals for which the temperature has locally been varied (
FIG. 3
step
370
). The temperature variation causes a frequency shift of the measuring magnetic resonance signals relative to the reference magnetic resonance signals; this frequency shift will be referred to as “temperature dependent chemical shift”. The measuring site is reproduced in the reference magnetic resonance image as well as in the measuring magnetic resonance image and the position of the measuring site is separately reproduced so as to be suitably recognizable in the reference magnetic resonance image and the measuring magnetic resonance image, or is separately measured (
FIG. 3
step
380
). Furthermore, the predetermined geometrical relationship between the reproduction of the detail and the indication of the measuring site in the reference magnetic resonance image is also determined on the basis of the reference magnetic resonance image. As a result, the measuring magnetic resonance image and the reference magnetic resonance image can be made to register, the same details in both images then being situated in the same position in the images relative to the indication of the measuring site in both images. It is thus achieved that the local temperature variation can be accurately derived from the frequency shifts of the measuring magnetic resonance signals relative to the reference magnetic resonance signals while avoiding disturbances due to motion. The determination of the local variation of the temperature on the basis of the temperature dependent chemical shift (
FIG. 3
step
350
) per se is rather sensitive to motion, because the measuring magnetic resonance signals are spatially encoded on the basis of the frequencies of these signals. Because a separate determination or measurement of the position of the measuring site is available according to the invention, the effect of the temperature dependent chemic
Fetzner Tiffany A.
Gutierrez Diego
Koninklijke Philips Electronics , N.V.
Vodopia John
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