Patent Application
20080214924
The invention relates to a magnetic resonance (MR) imaging system comprising a magnet system for acquiring MR spectroscopic data from an object of interest comprised within a polarizing magnetic field.
The invention further relates to a method of acquiring MR spectroscopic data, the method comprising the steps of generating a polarizing magnetic field and acquiring MR spectroscopic data from an object of interest comprised within the polarizing magnetic field.
The invention further relates to a computer program product to be loaded by a computer arrangement comprising a processing unit and a memory, the computer program product comprising instructions for acquiring MR spectroscopic data from an MRI system and, after being loaded, enabling the MRI system to carry out said method.
Magnet systems can be broadly categorized into “bore-type” and “gap-type” systems, also referred to as “cylindrical” and “open” systems, respectively. In a bore-type system, a solenoid magnet generates a polarizing magnetic field, B0, along its own longitudinal axis and the common longitudinal axis of the cylindrical bore of the system. In a gap-type system, the B0 field is generated in an open region between a pair of magnetic pole faces. In either case, the direction of the B0 field is commonly denoted as the z-axis. The polarizing magnetic field B0 polarizes the nuclear magnetic spin system of an object placed within the field. To generate an MR signal, the polarized spin system is first excited by applying an MR excitation signal or radio frequency (RF) magnetic field B1, perpendicular to the z-axis. The RF excitation pulse tips the magnetization out of alignment with the z-axis and causes its macroscopic magnetic moment vector to precess around the z-axis. The precessing magnetic moment, in turn, generates an RF MR signal in a pick-up or receiver coil placed perpendicular to the z-axis.
To generate an MR image, gradient pulses are typically applied along the x, y and z-axis directions to localize the spins along the three spatial dimensions, and MR signals are acquired in the presence of one or more readout gradient pulses. An image depicting the spatial distribution of a particular nucleus in a region of interest of the object is then generated, using known post-processing techniques. Typically, the hydrogen nucleus (1H) is imaged, though other MR-detectable nuclei may also be used to generate images.
In the case of MR spectroscopy, MR signals are acquired in the absence of readout gradient pulses, though other gradient pulses may be used to prepare the spins prior to signal acquisition. The acquired MR signals are processed and normally displayed in the form of spectra that contain information based on the differences in the resonance frequencies of different MR-detectable nuclei. In practice, different signals from MR-detectable nuclei of a single species or element resonating at slightly different resonance frequencies are acquired to generate the spectra. The slight difference in resonance frequencies arises from the nuclei existing in different chemical environments. This frequency difference is measured in units of parts per million (ppm), as it is of the order of a few Hertz (Hz) as compared to the MR signals which precess at the rate of a few million Hz.
To be able to delineate the various MR signals arising a few ppm apart, MR spectroscopic systems benefit from higher magnetic field strengths. Such systems also impose more stringent requirements on the homogeneity of the B0 field. Bore-type magnets typically have higher field strengths and a better magnetic field homogeneity as compared to gap-type systems. Hence, there is a prejudice in the prior art against the use of gap-type systems for MR spectroscopy. In addition, there is also a prejudice against the use of MR imaging systems with a polarizing field strength of less than 1.5 T for MR spectroscopy.
Prejudice against the use of gap-type magnet systems and low-field and mid-field MRI systems for MR spectroscopy may be found in an article by Melissa Minkin, entitled “Open vs. Closed MR: Options for Brain Tumor Patients”, in the magazine “Search”, Issue No. 47, Spring 2001, which states: “ . . . ” open MRI still lacks the ability to perform advanced techniques like a Functional MRI exam”. The next paragraph continues with: “Magnetic resonance spectroscopy (MRS) is another technique reserved for high field systems”.
Additional evidence of prejudice is found in the publication, “Susil R C, Menard C, et al., Transrectal Prostate Biopsy and Fiducial Marker Placement in a Standard 1.5 T MRI Scanner, Journal of Radiology 2004 (in review)”, which states: “Prior work has been performed on low field strength (e.g. 0.2 or 0.5 T) open scanner architectures (6-8). While these systems provide easier access to the patient, they do not produce the highest quality MR images, have limited potential for functional and spectroscopic imaging, and are less widely available.”
The Scottish Health Guidance Note on Magnetic Resonance Imaging, by the NHSScotland Property & Environment Forum, June 2001, states: “High field MRI systems may also be capable of performing magnetic resonance spectroscopy, which provides in-vivo biochemical information on cell metabolism”. At a later point in the document, “high field” is defined as “1.5 T or above”.
Further evidence as to the prejudice regarding the requirement of high field strengths for MR spectroscopy, clearly defined as 1.5 T or higher, is disclosed in EP085240A1.
EP085240A1 describes an embodiment of a main field magnet having a large clear material section and a low main magnetic field strength, wherein the main field magnet is combined with a small magnet which is capable of generating a high magnetic field. The small magnet has means for making it mobile so that it can be transported between an inoperative position outside the large magnet and an operative position inside the large magnet. The main field magnet is used for interventional MR imaging, while the small magnet is used for MR spectroscopy.
A problem with the prior art is that the small magnet, which is capable of MR spectroscopy, is not large enough to facilitate interventional procedures to be performed inside the magnet.
It is therefore an object of the invention to provide an MRI system which is capable of MR spectroscopy and allows interventional procedures to be performed inside the MRI system.
This object is achieved by an MRI system as described in the opening paragraph, wherein the MRI system comprises multiple pole faces for generating the polarizing magnetic field. The multiple pole faces provide a gap into which the object of interest, for example, a human patient or a dummy or an imaging phantom may be introduced. The gap is typically large enough to permit interventional procedures to be performed in between the pole faces of the magnet.
These and other aspects of the invention will be elaborated on the basis of the following preferred embodiments, which are defined in the dependent claims.
Since the magnetic field homogeneity requirements for MR spectroscopy are more stringent as compared to those for MRI, one preferred embodiment of the MRI system comprises an indicator for indicating a region for acquiring MR spectroscopic data. This region will hereinafter be referred to as the spectroscopic volume. The indicator may be, for example, an optical means, such as a set of laser beams, or a mechanical means, such as physical markers on the patient table, etc.
Another preferred embodiment comprises a positioner placed within the magnet system, to position the object of interest, for example, a selected part of a patient's body, an imaging phantom, an animal, or a dummy, in the spectroscopic volume. The positioner is preferably a constraining device that immobilizes the object of interest. The positioner may be alternatively a device that allows movement of or around the object of interest, for example, the neck, a knee, an elbow, or other parts of the body, while still retaining the object of interest within the spectroscopic volume. The positioner may be alternatively a resistance device that enables load-bearing or stress studies of the object of interest.
It is a further object of the invention to provide a method of acquiring MR spectroscopic data from an MRI system that allows interventional procedures to be performed in the MRI system.
This object is achieved by a method as described in the opening paragraphs, wherein the polarizing magnetic field is generated between multiple pole faces of the MRI system.
Further embodiments of the method are defined in the dependent claims 5 and 6.
It is a further object of the invention to provide a computer program product to be loaded by a computer arrangement, the computer program product comprising instructions for acquiring MR spectroscopic data from an MRI system that allows interventional procedures to be performed in the MRI system.
This object is achieved by a computer program product as described in the opening paragraphs, wherein the polarizing magnetic field is generated between multiple pole faces of the MRI system.
These and other aspects of the invention will be described in detail hereinafter, by way of example, on the basis of the following embodiments, with reference to the accompanying drawings, wherein
It should be noted that corresponding reference numerals used in the various Figures represent corresponding structures in these Figures.
The pole faces 104 generate a static magnetic field strength of, for example 1.0 T, in the air gap. The object of interest 103, for example, a human or an animal or an imaging phantom, is placed in the air gap. To enable MR imaging, temporally variable magnetic field gradients superimposed on the static magnetic field are generated by the multiple gradient systems 114 in response to currents supplied by the gradient driver unit 115. The control unit 117 controls the characteristics of the currents, notably their strengths, durations and directions, flowing through the gradient coils. The RF coils 113 generate RF excitation pulses in the object of interest 103 and receive MR signals generated by the object of interest 103 in response to the RF excitation pulses. For MR spectroscopy, free induction decay (FID) signals are typically received by the RF coil 113, while for MR imaging, gradient recalled echoes or spin echoes are typically received. The RF coil driver unit 116 supplies current to the RF coil 113 to transmit the RF excitation pulse, and amplifies the MR signals received by the RF coil 113. The characteristics of the transmitted RF excitation pulses, notably their strength and duration, are controlled by the control unit 117. It is to be noted that, although the transmitting and receiving coil are shown as one unit in this embodiment, it is also possible to have separate coils for transmission and reception, respectively. It is further possible to have multiple RF coils 113 for transmission or reception, or both. The RF coils 113 may be integrated into the magnet 102, or may be separate surface coils. The received signals are reconstructed by the reconstruction unit 118 and displayed on the display unit 119. FID signals are typically Fourier-transformed in one dimension by the reconstruction unit 118 and displayed as spectra, while echoes are Fourier-transformed in two dimensions and displayed as two-dimensional images.
An advantage of the gap-type system is that it enhances patient comfort due to the fact that somebody can be close to the object of interest, such as a patient, to give the patient a secure feeling during the data acquisition. This is especially advantageous while studying pediatric patients.
It is known that MR spectroscopy requires more homogeneous fields when compared to MR imaging. For example, MR imaging typically requires magnetic fields with a homogeneity of around 10 ppm, peak-to-peak, in a volume of typically around 50 cm3. MR spectroscopy typically requires a homogeneity of less than 1 ppm, peak-to-peak, although it may be in a smaller volume. This volume of optimal homogeneity for either spectroscopy or imaging is called the “sweet spot” for that particular application. It is known that the magnetic field homogeneity deteriorates when one moves away from the sweet spot. The indicators 206, 207 thus indicate the limits of the spectroscopic volume, or the “sweet spot” for spectroscopy. Such an indicator can be useful to an operator, for example, to confirm that, before starting data acquisition, the region of interest is contained within the spectroscopic volume.
The indicators 206, 207 are also moveable and provide a feedback of their current position to the MRI system 101, such that the system software can warn the operator if the region of interest of the object of interest 103 extends beyond the spectroscopic volume. When the operator positions the region of interest within the magnet 102, using available landmarking or referencing facilities, the optical, mechanical or other indicators 206, 207 can be moved to the limits of the region of interest, from which MR spectroscopic data is to be acquired. If the limits of the desired region of interest as specified by the indicators 206, 207 fall outside the spectroscopic volume, the system software can warn the operator to adjust the region of interest. Alternatively, the software can control the patient table 208 in such a way that the region extending beyond the spectroscopic volume is automatically brought within the spectroscopic volume before MR spectroscopic data are acquired from the region.
The order of the described embodiments of the method of the invention is not mandatory. A person skilled in the art may change the order of steps or perform steps concurrently, using threading models, multi-processor systems or multiple processes without departing from the concept as intended by the invention.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. Use of the indefinite article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the system claims enumerating several means, several of these means can be embodied by one and the same item of computer-readable software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
It is to be further noted that the term “MRI system” as used in the application is not limited to MR imaging systems but also covers MR systems in general.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05106623.1 | Jul 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2006/052468 | 7/19/2006 | WO | 00 | 1/17/2008 |
