Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-02-22
2002-10-08
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S320000, C324S309000
Reexamination Certificate
active
06462543
ABSTRACT:
The invention is directed to a method for performing verification measurements and corrections of magnetic fields in magnets for Nuclear Magnetic Resonance imaging machines, comprising the following steps:
making a test member having such characteristics as to generate predetermined and known Nuclear Magnetic Resonance images;
obtaining a Magnetic Resonance image from said test member;
comparing the known theoretical image with the detected image and/or the received signals wherefrom the detected image is reconstructed with the corresponding set of signals, related to the known theoretical image in any of their processing or imaging steps;
determining the differences between the detected image and the known image or between the set of detected signals and the set of signals corresponding to the known image;
determining correction parameters, i.e. the correcting magnetic charge and/or the number of correcting magnetic charges having a predetermined value, or the volumes of ferromagnetic material and their position on the magnetic structure.
Nuclear Magnetic Resonance imaging machines use electromagnetic echoes of previously excited nuclei to retrieve therefrom information for imaging. In order to obtain echo signals allowing to reconstruct images sufficiently corresponding to reality, at least a considerable number of the nuclei of the material being examined must be oriented with substantially parallel spins. To this end, static magnetic fields are used which must be comparatively intense and constant within a predetermined volume corresponding to a body part to be examined.
At the same time, additional magnetic fields are applied, the so-called gradients, which are used to select certain sections of the body part to be examined and to create a parameter for identifying the signal received from individual portions of the section under examination so that signals can be ordered for two- or three-dimensional imaging.
The tolerances required for a good correspondence between reality and the reconstructed image are very small, being of the order of a few millionths deviations from nominal values. Moreover, since the magnetic fields in use are relatively intense, the constructions of magnets have a considerable size, whereby the physically huge structure hinders the required construction accuracy. This is particularly relevant for the so-called permanent magnets, in which the magnetic field is generated by permanently magnetized materials and not by induction generators.
After manufacture, magnets are submitted to fine calibration of the magnetic field, the so-called shimming, which is designed to correct construction inaccuracies of the individual parts of the magnet, and includes measuring the static field by a probe, detecting aberrations with respect to nominal values and disposing magnetized or ferromagnetic correcting elements in several appropriate areas of the magnet structure as determined from the differences between the nominal values and the actual values of the field within the volume designed to accommodate the body or the body part under examination. Shimming is performed during fabrication at the manufacturer's site. This technique is described in greater detail, for instance, in patent application SV98A000015 filed by the applicant hereof.
Nevertheless, frequently and unavoidably, upon installation at the customer's site, manufacture settings appear to be changed or perturbed. This is especially caused by stresses acting on the big metal masses which form the magnetic structure, especially as this structure has a modular construction, or made of several parts fastened together.
Further, the personnel using the machines is not sufficiently qualified to verify their operating accurateness during use, except in a few exceptional cases. Hence, little aberrations might remain unnoticed.
Due to this, a problem arises to make functional testing of machines easier, at least below a certain accuracy degree, in an easy and safe manner, even for personnel less qualified than the personnel charged of fabricating and testing the machines at the manufacturer's site.
Document EP-A-0230027 discloses a method and a device according the above described method and which attempts to solve the above disclosed problems. Also U.S. Pat. No. 5,545,995 and U.S. Pat. No. 5,055,791, disclose method of the kind described above.
The known disclosed methods, suggest solutions to the problem of testing and correcting imaging aberrations and anomalies in which the image of the phantom is determined by a combination of several coefficients of the theoretical mathematical function which describes the field and which combination of coefficients includes coefficients of low and high order. Carrying out the correction of the field under these circumstances requests very long calculations which are not necessary to achieve a practical level of precision of the field. Indeed in the disclosed solutions the structure of the phantom is not suited to the structure of the function describing the field being normally a spherical polynomial expansion. The images of the phantom and the aberrations thereof from the theoretical image are not determined only by a small number of low order coefficients of the polynomial expansion. Thus as stated above the mathematical comparison of the real image of the phantom and of the theoretical image and the calculation of the correction to be applied to the field are very complicated and time consuming.
The invention has the main object to allow fast functional testing of the magnetic structure of Nuclear Magnetic Resonance machines, particularly after installation, in a relatively fast and simple manner.
The invention has the further object to allow an at least partly automatic correction of aberrations and anomalies occurring after installation without requiring special laboratory equipment, which is complex and costly and may be only used by very highly knowledgeable and specialized personnel.
The invention also aims at setting the bases for implementation of several automation degrees of the correction to be performed, with the smallest number of operation steps which might be carried out by an average specialized technician, specifically trained in the maintenance of one or more specific machines.
The invention achieves the above purposes with a method for performing verification measurements and corrections of magnetic fields in magnets for Nuclear Magnetic Resonance imaging machines, in which
A mathematical theoretical function describing the field is choosen, particularly a polynomial expansion, preferably a spherical polynomial expansion,
A phantom is provided with a structure suited to the said function describing the field and which produces one or more selected images, each of these images being correlated to one or to a limited number of selected low order coefficients of the function describing the field, such that the differences between a theoretical image of the phantom and the real image of the phantom depends only to the one or to the limited number of selected coefficients of the function describing the magnetic field.
Thus by selecting a specific mathematical structure for describing the field, for instance and preferably but without limitation, a representation of the field by spherical harmonics, the above method may be refined so that several orders and degrees of harmonics, and hence of field coefficients may be examined individually.
In this case, the method provides that the test member has elements which do not emit Nuclear Magnetic Resonance signals and are defined transparent in the following text and claims , which are related to a definite harmonic or to precise coefficients of a certain order of a mathematical description of the field by a polynomial expansion, preferably by spherical harmonics, there being provided means for application of so-called reading gradients of the magnetic field, which only detect the echo signal along certain directions, said directions being selected in such a manner as to suppress the contribution
Carlini Davide
Pittaluga Stefano
Trequattrini Alessandro
Esaote S.p.A.
Lefkowitz Edward
Shrivastav Brij B.
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