1. Field of the Invention
The present invention concerns a radio-frequency acquisition device for a magnetic resonance tomography apparatus with at least one reception antenna to acquire magnetic resonance signals and with an amplifier device to amplify the acquired magnetic resonance signals.
2. Description of the Prior Art
For examination of a human body part it is known to introduce the body part to be examined into a homogeneous magnetic field, known as the basic field. The magnetic field causes an alignment of the nuclear spins of atomic nuclei in the body part, in particular of hydrogen atomic nuclei (protons) bound to water. These nuclei are excited to a precession movement by irradiated radio-frequency excitation pulses. After the end of an excitation pulse, the atomic nuclei precess with a frequency that depends on the strength of the basic field and, due to their spins, settle again into the preferred direction predetermined by the basic field after a predetermined relaxation time. The atomic nuclei thereby emit radio-frequency signals, what are known as magnetic resonance examination signals. An image can be generated from the spatial spin density or the distribution of the relaxation times within a body slice by computational or measurement analysis of the magnetic resonance signals. The association of the magnetic resonance signals (detectable as a result of the precession movement) with the respective location of their formation ensues by the application of linear gradient fields. For this purpose, suitable gradient fields are superimposed on the basic field and are controlled such that an excitation of the nuclei ensues only in a slice to be imaged. An image depiction based on these physical effects is known as magnetic resonance tomography (MRT).
The known design of an acquisition system of such a magnetic resonance tomography apparatus is shown substantially in FIG. 3. There are systems that have a number of additional components, for example detuning units arranged on the reception antenna in order to decouple the antennas from one another given use of a second antenna (as described in DE 298 04 339 U1) or adaptation circuits in order to compensate for incorrect adaptation of the antenna caused by the patient body (as described in DE 40 35 994 A1). For better clarity, however, only the components essential for later explanation of the invention are shown in FIG. 3.
Usually local surface coils (local coils), known as loop antennas, or array arrangements constructed from multiple loop antennas are used as a reception antenna 31 to acquire the magnetic resonance signals from the examination subject.
The magnetic resonance signals generated by the excited atomic nuclei induce a voltage Uind in the acquisition antenna 31 that is then amplified in a low-noise preamplifier 32 (LNA, Low Noise Amplifier) and conducted via a bed cable 33 to an additional amplifier device 34. The magnetic resonance signals (which are thus amplified twice) are then relayed via a further conductor 35 to an acquisition electronic 36 wherein which they are processed further.
Systems known as high field systems operated with basic field strengths at three Tesla and more, and are used to improve the signal-to-noise ratio, in particular to generate high-resolution slice representations.
Theoretically, a quadrupling of the power of the receivable magnetic resonance signals results by a doubling of the basic field strength. For example, typically maximum signal powers of −27 dBm occur at the input of the preamplifier given a basic field strength of 1.5 Tesla; the maximum signal power at the input of the preamplifier is typically −21 dBm given a basic field strength of 3 Tesla.
The value range of the signal powers of the acquired magnetic resonance signals or the amplitudes of the acquired magnetic resonance signals present at the input of the preamplifier thus increases due to the increase of the maximum basic field strengths used.
The demands on the preamplifier increase due to the increase of the value range of the signal powers present at the input of the preamplifier. This should be able to operate optimally without distortion to amplify magnetic resonance signals across the entire possible signal power value range, thus from thermal noise up to the maximum signal power. This is no longer ensured in a satisfactory manner given the high maximum magnetic fields (and the high maximum signal powers associated therewith) in modern high field systems.
To mitigate the problem, in particular to increase the dynamic range of the acquisition system, it is known to design the additional amplifier device 34 shown in FIG. 3 such that it can be switched by a control signal, such that magnetic resonance signals with low signal power (in particular low maximum power) are more strongly amplified (high gain) than magnetic resonance signals with relatively high signal power, in particular high maximum power.
This solution using the preamplifier has previously not been considered to lead to the desired goal. As shown in FIG. 4, in an input stage of the preamplifier 42 the source impedance Zloop of the reception antenna 41 is transformed by a transformation device 47 into a source impedance adapted to the input transistor 49 of the preamplifier 42. Zin is the amplifier input impedance.
The transformation device 47 has a capacitor C and an inductor L. The amplification unit 48 of the preamplifier 42 can have additional elements (in particular transistors) in addition to the input transistor 49.
The source impedance Zloop of the reception antenna 41 is transformed by the transformation device 47, in particular into the optimal source impedance Zopt for the low-noise input transistor 49. Under this boundary condition, the voltage rise UGS is present at the gate-source path of the input transistor 49 given a predetermined source power (and therefore also the maximum allowable source power from the reception antenna).
In order to now increase the dynamic range of the preamplifier, the rest current in the low noise input transistor (typically a GaAs field effect transistor) can be increased. This measure, however, runs against a limit when the power loss is significantly increased that the input transistor is damaged or the heat development leads to a noticeable negative effect on the patient comfort. Although the possible damage to the input transistor could be prevented by the use of multiple individual semiconductors, the patient stress due to the heat input would still exist.
As an alternative, the acquired magnetic resonance signal can be attenuated by an attenuation element between the reception antenna and the preamplifier, but this would lead to an unwanted, strong increase of the noise ratio of the acquisition chain.