This application claims the benefit of DE 102014207843.1, filed on Apr. 25, 2014, which is hereby incorporated by reference in its entirety.
The disclosed embodiments relate to devices and methods for magnetic resonance imaging (MRI) imaging of a knee.
Magnetic resonance imaging (MRI) devices for examining objects or patients by magnetic resonance tomography are known from, e.g., DE 103 14 215 B4.
The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary.
The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, the disclosed embodiments may optimize MRI imaging of a knee.
In order to use a magnetic resonance imaging device MRI device 101 to examine a body 105 (an examination object or a patient) via magnetic resonance imaging, different magnetic fields, which are precisely matched to one another in terms of their temporal and spatial characteristics, are emitted onto the body 105. A strong magnet (often a cryomagnet 107) in a measurement cabin with an opening 103, which is tunnel-shaped in this case, generates a strong static main magnetic field B1, which has a strength of, e.g., 0.2 Tesla to 3 Tesla or more. A body 105 to be examined is, while supported by a patient couch 104, driven into a region of the main magnetic field B1, which is approximately homogeneous in the observation region (also referred to as “field of view” or FoV). The nuclear spins of atomic nuclei of the body 105 are excited by magnetic radiofrequency excitation pulses B1(x,y,z,t), which are emitted in via a radiofrequency antenna (and/or, optionally, a local coil arrangement), which is depicted here in a simplified manner as (e.g., multi-part=108a, 108b, 108c) body coil 108. By way of example, radiofrequency excitation pulses are generated by a pulse generation unit 109, which is controlled by a pulse sequence control unit 110. After amplification by a radiofrequency amplifier 111, the excitation pulses are conducted to the radiofrequency antenna 108. The radiofrequency system shown here is merely indicated schematically. In a magnetic resonance imaging device 101, use may also be made of more than one pulse generation unit 109, more than one radiofrequency amplifier 111 and several radiofrequency antennas 108a,b,c.
The MRI device 101 furthermore includes gradient coils 112x, 112y, 112z, by which magnetic gradient fields BG(x,y,z,t) are emitted in during a measurement for selective slice excitation and for spatial encoding of the measurement signal. The gradient coils 112x, 112y, 112z are controlled by a gradient coil control unit 114 (and optionally via amplifiers Vx, Vy, Vz), which, like the pulse generation unit 109, is connected to the pulse sequence control unit 110.
Signals emitted by the excited nuclear spins (of the atomic nuclei in the examination object) are received by the body coil 108 and/or at least one local coil arrangement 106, amplified by associated radiofrequency preamplifiers 116 and processed further and digitized by a reception unit 117. The recorded measurement data are digitized and stored as complex numbers in a k-space matrix. An associated MR image may be reconstructed from the k-space matrix filled with values via a multidimensional Fourier transform.
For a coil that may be operated both in transmission mode and in reception mode, such as, e.g., the body coil 108 or the local coil 106, the correct signal transmission is regulated by an upstream transmission/reception switch 118.
An image processing unit 119 generates an image from the measurement data. The image is displayed to a user via an operating console 120 and/or stored in a storage unit 121. A central computer unit 122 controls the individual installation components.
In magnetic resonance (MR) imaging, images with a high signal-to-noise ratio (SNR) may be recorded using so-called local coil arrangements (coils, local coils). The local coil arrangements are antenna systems attached in the direct vicinity on (anterior) or under (posterior) or at or in the body 105. During an MR measurement, the excited nuclei induce a voltage in the individual antennas of the local coil, which is then amplified using a low-noise preamplifier (e.g. LNA, preamp) and transmitted to the reception electronics. In order to improve the signal-to-noise ratio, e.g., in the case of high-resolution images, use is made of so-called high-field installations (1.5 T-12 T or more). If more individual antennas may be connected to an MR reception system than receivers are available, a switching matrix (also referred to as RCCS), for example, is installed between reception antennas and receivers. The switching matrix routes the currently active reception channels (usually those that currently lie in the field of view of the magnet) to the available receivers. As a result, more coil elements may be connected than receivers are available because, in the case of a whole body cover, it is only useful to read the coils that are disposed in the FoV or in the homogeneous volume of the magnet.
By way of example, an antenna system, which may, e.g., include a single antenna element or, as an array coil, several antenna elements (e.g., coil elements), may be referred to as local coil arrangement 106. By way of example, these individual antenna elements are configured as loop antennas (loops), butterfly coils, flex coils or saddle coils. By way of example, a local coil arrangement includes coil elements, a preamplifier, further electronics (standing wave traps etc.), a housing, supports and usually a cable with plug, via which the local coil arrangement is connected to the MRI installation. A receiver 168 attached to the installation side filters and digitizes a signal received from a local coil 106, e.g., by radio link etc., and transmits the data to a digital signal processing device, which may derive an image or a spectrum from the data obtained by a measurement and makes the image available to the user, e.g., for the subsequent diagnosis by the user and/or for storing.
In MR knee imaging, the knee K1 to be examined of an examination subject or patient 104 is placed into a local coil 106 for improving the image quality. The second knee K2 is placed next to it. In order to save measuring time, e.g., the field of view (FoV) is selected to be as small as possible (and, to the extent that this is possible, only the knee K1 to be examined is acquired). Although the local coil 106 is significantly more sensitive to signals Si from within the local coil than to signals Si from outside the local coil, the antenna elements At3, which (e.g., in
This situation is addressed via technical measures achieving that only the knee to be examined is excited by the radiofrequency signal. This may be brought about in the following way:
1. A knee coil is equipped with a local transmission function (TX/RX coil).
2. In addition to the reception antennas, a knee coil also includes an antenna structure, e.g., in the form of a birdcage 108a,b,c, which resonates freely in the case of transmission and concentrates the transmission field of the body coil in the knee K1. If protons largely only in the knee K1 to be examined are excited in the process, it is also only these protons that supply signals Si in the reception case. As a result, the folding-in artifacts are suppressed.
According to some embodiments, excitement of only the knee to be examined may be achieved by applying an RF shield S, which may have a high shielding effect in relation to signals Si and/or Bi(x,y,z,t) in the frequency range of the MR signal Bi(x,y,z,t), but may be transmissive to frequencies in the frequency range of the gradient currents BG(x,y,z,t). By way of example, this may be achieved by a slotting of the shield S. This shield may either be purely passive or else be operated in an actively switchable manner (in accordance with US Patent Publication No. 2012/0187950 A1, the entire disclosure of which is incorporated into this application by reference). The image region is excited by transmission by the body coil 108a,b,c. With respect to placement of the shield S, the two following examples may be used.
1. The shield S is placed around the neighboring knee K2 (as in
The following three examples of the shield S may be used.
A: Passive Shield S:
A simple, passive, slotted shield S is either placed around the opening 02 of the neighboring knee K2 (in respect of the currently examined knee K1) (in accordance with
This shield S shields the neighboring knee K2 in, e.g., both the transmission and reception case. In the case of the partial (i.e., only lateral) shield (in accordance with
B: Active Shield S:
The active shield S may be closed (less transmissive) for RF radiation/RF signals (to radio frequencies or only to, e.g., RF excitation pulses and/or RF signals emitted by the knee) in the transmission case (which is also referred to as TX case) and open (i.e., more transmissive and/or, e.g., non-conductively interrupted by, e.g., PIN diodes) in the reception case (RX case).
By way of example, an active slotted shield S is placed around the opening 02 of the neighboring knee K2 (in accordance with
C: Active Shield S:
An active shield S is, e.g., closed in the RX case and open in the TX case.
By way of example, an active slotted shield S is either placed around the opening O2 of the neighboring knee K2 (in accordance with
A switchable shield S around the reception elements At1, At2, At3 of the knee K1 to be examined:
An active slotted shield S is placed around the reception elements At1, At2, At3 of the knee K1 to be examined (in accordance with
The case 1,A has been tested in a machine.
In
The right-hand bottle may no longer be identified on the right-hand side of the image. Accordingly, no folding-in is identifiable anymore in the case of a reduced field of view in
Conversely,
A cost-effective alternative to TX/RX coils for avoiding aliasing effects in knee imaging is provided. Embodiments are implementable in a purely passive manner. In the case of an active embodiment, only a few PIN diodes and a switchable DC current supply may be used for the switchable shield (the switchable DC current supply is used in a local coil and for detuning the antenna elements).
It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.
While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.
Number | Date | Country | Kind |
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102014207843.1 | Apr 2014 | DE | national |