Detection unit including an RF transceiver system and a pet detector

A detection unit is disclosed for arrangement in the main magnet of an MR device, which has both an RF transceiver system and a PET detector. In at least one embodiment the RF transceiver system is divided into two parts and the two parts are arranged upstream and downstream of the PET detector in the longitudinal direction of the patient tunnel. The RF transceiver system and PET detector are applied to the same image volume. In at least one other embodiment, an MR device is equipped with the detection unit, and in at least one other embodiment, a method operates the detection unit.

PRIORITY STATEMENT

The present application hereby claims priority under 35 U.S.C. §119 on German patent application number DE 10 2006 045 399.9 filed Sep. 26, 2006, the entire contents of which is hereby incorporated herein by reference.

FIELD

Embodiments of the invention generally relate to the combination of the medical imaging methods of MRT (magnetic resonance tomography) and PET (positron emission tomography) in one device. Embodiments may relate, for example, to a detection unit for arrangement in the magnetic field of a main magnet of an MR device that includes both an RF transceiver system for transmitting RF pulses and/or receiving MR signals, and/or a PET detector for detecting gamma rays, the RF transceiver system and PET detector being arranged radially around a patient tunnel.

BACKGROUND

Magnetic resonance tomography (MR or MRT) is an imaging method for displaying tissue in the human or animal body. MRT is based on the principle of nuclear spin resonance, in accordance with which atomic nuclei such as the hydrogen nuclei present in large numbers in the body exhibit a magnetic moment. They can thereby be excited with electromagnetic radiation in the radio frequency region (RF radiation) in an applied external magnetic field, and emit this radiation shortly thereafter. This RF radiation is detected with an antenna that mostly also generates the exciter pulse; this is why use is made of the term RF transceiver system, or RF coil, for short.

The magnetic field is mostly generated by a superconducting main magnet that is integrated in a field generating unit that encloses a horizontal patient tunnel into which the patient to be examined is pushed. The main magnetic field then runs parallel to the longitudinal direction of the tubes, in the so called z direction.

The resonant frequency of the atomic nuclei is directly proportional to the applied main magnetic field. Consequently, the spatial coding inside an image volume is achieved by virtue of the fact that so called gradient fields are applied in addition to the main magnetic field during the measurement; these are briefly applied magnetic fields with as linear as possible a gradient in the x, y or z directions. The gradient fields are mostly generated by specific gradient coils that are arranged inside the field generating unit.

A further medical imaging method is positron emission tomography (PET). As a nuclear medicine method, PET is suitable, in particular, for displaying biochemical processes in the body, for example for finding tumors and metastases. In this case, the patient is administered a tracer with a radionuclide that is distributed in the body and emits radioactive radiation in the form of positrons in the process. After a short time, the positrons decay into two opposite gamma quanta that are captured by suitable detectors. These are mostly arranged around the body as an annular PET detector. For example, the photons are captured by a matrix, made from scintillation crystals, in which each photon produces a scintillation upon striking. Said scintillation is, in turn, captured and amplified by photodetectors, for example by photomultiplier tubes or avalanche photodiodes.

Interest has recently been taken in combining MRT and PET with one another in one device in order to be able to apply the two imaging modalities simultaneously or shortly after one another to the same patient. This requires arranging the two units of MR-RF transceiver system and PET detector required for data acquisition inside the (mostly superconducting) main magnet and MR gradient coil.

In an obvious solution, to this end a PET detector is inserted into the patient tunnel of the field generating unit of an MR device, and, in turn, an RF transceiver unit is inserted into the PET detector ring. However, this arrangement is problematic, since the RF coil and PET detector exert a negative mutual influence: the currents inside the PET detector generate interference fields that are captured by the RF coil and can lead to interference signals. Since it is arranged between the examination region and the PET detector, the RF coil, in turn, can lead to scattering of gamma quanta and thus reduce the sensitivity of the PET detector. Moreover, nesting the RF coil and PET detector ring from the inside to the outside strongly reduces the inside diameter remaining for the patient inside the main magnet, and this can, in particular, result in the measurement being incapable of being carried out given a patient who is claustrophobically disposed or overweight.

An example of an MR device in which a PET detector is arranged between the gradient coil and the RF transceiver system is illustrated inFIG. 2.FIG. 2shows a longitudinal half of the field generating unit9of an MR device, with further components integrated therein, in cross section. The dashed and dotted line2represents the middle line of the substantially tubular field generating unit9. Permanently integrated in the field generating unit9is a gradient coil3that is likewise approximately tubular and that has coils for generating x, y and z gradients.

Inserted into the gradient coil3is a PET detector ring4, an RF shield6, a support tube5and an RF transceiver system7. The RF shield6ensures that the RF fields are shielded against the PET ring during excitation of the RF coil7. The RF coil7is provided with a cladding8against the examination region and/or patient tunnel13.

The design illustrated inFIG. 2therefore has the advantage that a whole body examination is thereby possible. The designation “body coil” is also used for an RF coil7, integrated permanently in the main magnet9, according toFIG. 2, which permits excitation and detection over the entire examination region.

On the other hand, the arrangement illustrated inFIG. 2has the abovementioned disadvantages, for example it diminishes the onion skin type design of the patient tunnel. In order still to enable a sufficiently large inside diameter, the distance between the RF shield6and the RF conductor structures that is required for the formation of the field return space and thus for a good quality of the RF coil7must be strongly reduced inside the coil7.

SUMMARY

In at least one embodiment of the invention, a detection unit includes an RF transceiver system and a PET detector that can be inserted into a main magnet of an MR device and at least one of lessens and does not have at least one of the above designated disadvantages, and/or in the case of which, in particular, the RF coil and PET detector exert the smallest possible mutual negatives.

In at least one embodiment, the RF transceiver system has an antenna system divided into two in the longitudinal direction, of which the first part is arranged in the longitudinal direction upstream, and the second part is arranged in the longitudinal direction downstream of the PET detector, and images of spatially at least partially overlapping image volumes can be acquired with the aid of the RF transceiver system and the PET detector. The RF transceiver system and the PET detector are thus arranged one behind another in the longitudinal direction of the patient tunnel.

This has an advantage, for example, that the two detector units are not arranged concentrically, but one beside another, and can therefore also be better shielded against one another. There is, for example, no need for the gamma quanta to traverse the RF transceiver system in order to reach the PET detector. On the other hand, the currents of the PET detector interfere less with the RF transceiver system.

Nevertheless, images of spatially at least partially overlapping image volumes can be acquired with the aid of the RF transceiver system and the PET detector, that is to say it is possible to record images of the same region in the body with both modalities without repositioning the patient located in the patient tunnel. It is particularly preferred, in at least one example embodiment, for the image volumes (also known as fields of view) of the RF transceiver system and PET detector to be arranged in the center of the patient tunnel and an MR field generating unit, as well as inside the PET detector ring.

The RF transceiver system is preferably arranged in the longitudinal direction symmetrically around the PET detector. This renders possible an antenna configuration that generates as homogeneous an RF field as possible in the center, that is to say in the region of the PET detector. This also includes a mirror symmetric or point symmetric arrangement around the imaging center in the center of the field of view.

In accordance with an example embodiment, the RF transceiver system includes at least two antennas or RF coils, of which at least one is arranged in the longitudinal direction upstream, and at least one is arranged in the longitudinal direction downstream of the PET detector. The RF transceiver system therefore encloses the PET detector, which is preferably arranged in the middle of the patient tunnel in the longitudinal direction, and therefore in the region in which the MR imaging is also operated. The bipartite RF transceiver system is preferably selected such that an adequate homogeneity of the RF pulse is achieved by superposing the fields of the individual antennas within the central region of the patient tunnel (for example z=−20 cm to +20 cm). For example, the bipartite antenna system comprises two so called semi-birdcage antennas. A birdcage antenna is in the form of a cage and is known as an RF coil for whole body and head imaging.

The RF transceiver system and the PET detector are preferably fastened on the inner side of a support tube. The cables for connecting the PET detector to a signal processing unit situated outside the main magnet can, for example, be integrated in this support tube. Furthermore, the support tube can also include a cooling system for cooling the PET detector and/or the RF transceiver system. The support tube can then be pushed as a whole into the gradient coil of an MR device.

It is particularly preferred that the RF transceiver system can be removed from the support tube. Since the RF transceiver system preferably takes the form of two rings that are arranged upstream and downstream of the PET detector ring, these can therefore be removed on both sides, whereas the PET detector ring remains in the center.

Owing to the now no longer critical conditions of space inside the patient tunnel, it is possible to arrange the RF transceiver system at a certain distance from the patient. This contributes to keeping the local SAR (Specific Absorption Rate) in the patient low. To this end, it is particularly preferred to provide the RF transceiver system and the PET detector, facing inward toward the patient tunnel, with a cladding whose side pointing at the patient tunnel is arranged at a greater distance from the RF transceiver system than from the PET detector. The minimum distance between the patient and RF coil is thereby increased.

Because of the larger inside diameter of the PET detector ring that is possible with the invention, it is further possible to equip the PET detector at its end faces with so called end rings. These are as far as possible opaque to gamma rays, and thus effect a shielding against scattered radiation from outside the PET detector.

The support tube preferably also comprises an RF shield both between the PET detector and RF coil, and between the gradient coil and RF coil. Moreover, the PET detector is preferably equipped with an RF shield that can be removed in relation to the patient tunnel. The PET detector is in this way accessible from the patient tunnel, for example for maintenance purposes.

At least one embodiment of the invention is also directed at a field generating unit of an MR device, in which an above described detection unit is preferably integrated inside the gradient coil. It is preferred here to leave an air gap between the detection unit or the support tube thereof and the gradient coil.

Furthermore, at least one embodiment of the invention is directed to a method for operating an MR device in which the above described detection unit is integrated. In the method of at least one embodiment, a patient is laid in the patient tunnel and both MR data and PET data are thereupon acquired, it being possible for this to happen both sequentially and simultaneously. In the case of a bipartite RF transceiver system, the MR data are preferably acquired by using a SENSE method. The so called SENSE methods can be applied in the case of multipartite RF transceiver systems, and make careful use of the various spatial sensitive regions of the plurality of RF coils in order to reduce the acquisition time in conjunction with the same signal-to-noise ratio. The basics of SENSE methods are described in K. P. Pruessmann, M. Weiger, M. B. Scheidegger, P. Boesiger: “Sense: Sensitivity Encoding for Fast MRI”, Magnetic Resonance in Medicine 42: 952 to 962, 1999, the entire contents of which are hereby incorporated herein by reference. Since sense methods are therefore familiar to the person skilled in the art, it is not intended to examine them in more detail at this juncture.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

Referencing the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, example embodiments of the present patent application are hereafter described.

FIG. 1shows an illustration that corresponds in part toFIG. 2, identical components being marked with identical reference numerals. In accordance with one embodiment of the invention, the device ofFIG. 1differs from the conventional arrangement in accordance withFIG. 2in that the RF transceiver system is divided into two and comprises two RF coils7aand7bthat are arranged upstream and downstream of the PET detector4in the longitudinal direction. The PET detector4has an end ring20at each of its end faces.

Both detection apparatuses are shielded from the gradient tube3by an RF shield6. The latter preferably has the form of a slotted copper sheet. Furthermore, a shield is also provided between the RF coils7a,7band the PET detector4. It is particularly preferred for the PET detector to be completely surrounded by an RF shield in the manner of a Faraday cage.

As becomes clear fromFIG. 1, the inside diameter of the patient tunnel13is less restricted by this arrangement than in the case of the concentric variant in accordance withFIG. 2. Furthermore, both systems (RF coils and PET detector) are largely decoupled from one another and can be better optimized with reference to their image quality. Finally, the attenuation of the gamma radiation by resonator structures lying inside the PET detector ring is also eliminated. Consequently, the complex measures (such as attenuation correction) otherwise required for detecting and correcting structures lying inside the PET detector ring can be eliminated. This saves examination time and computing time in the image reconstruction.

The support tube5with the detection systems4and7afastened thereon together form a detection unit1with which conventional MR devices can also be equipped. In this case, the support tube5is merely pushed into the gradient coil3and/or the field generating unit9of the MR device.

To the inside, a cover8protects the patient in the examination space13against direct contact with the RF coils7a,7band the PET detector4.

The RF coils7aand7bare illustrated in greater detail in the perspective illustration of the detection unit1inFIG. 3, and in this case they are two semi-birdcage antennas. These have an annular conductor18from which a row of conductors19aligned in the longitudinal direction emanate. If the coils7aand7bare excited with an RF current, the longitudinal conductors19generate an alternating field perpendicular to the direction z of the main magnetic field. Because of the symmetrical design of the RF transceiver system around the center z=0, it is possible to attain a spatial field distribution that is homogeneous over an examination region from approximately z=−20 cm to z=+20 cm.

The part antennas7aand7bupstream and downstream, respectively, of the PET ring4can be used both individually and also together for transmitting and/or receiving MR data. A SENSE method with the factor2can then be applied when acquiring MR data, in order to reduce the measuring time.

As indicated inFIG. 3, the part antennas7aand7bcan also be divided into further segments in the circumferential direction. In this case, further reductions in the measuring time are possible through the use of SENSE. Methods such as SENSE can also be used when transmitting the RF pulses, in order to achieve improvements in the image quality.

The graph inFIG. 4shows the field profile of the divided antenna arrangement7a,7b.Here,14aillustrates the field profile generated by the antenna7a,as a function of the z direction, and the graph14billustrates the field profile generated by the antenna7b,as a function of the z direction. The graph16is the sum of the graphs14aand14b.As may be seen fromFIG. 4, the superposed field profile16exhibits a good homogeneity in a range from approximately −100 mm to +100 mm. The strength of the RF field drops outside this range, but can still be used up to a range from approximately −200 mm to +200 mm.

As shown inFIG. 5, the field distribution16of such a divided birdcage antenna is even better than the field distribution17of a conventional birdcage antenna.

A further example embodiment of an inventive detection unit is illustrated in greater detail inFIG. 6. Here, in turn, two part antennas7aand7barranged upstream and downstream of a PET detector ring4. Facing toward the patient tunnel13, these are provided with a cladding8that is thicker in the region of the RF transceiver system7a,7bthan in the region of the PET detector, in order to keep the local SAR of the RF radiation to a minimum.

The PET detector4is provided with an RF shield10. In the example illustrated, the PET detector is shielded only toward the patient tunnel13and toward the RF coils7a,7b, although it would also be possible to conceive a complete enclosure in the manner of the Faraday cage.

The RF coils7aand7bare likewise provided with an RF shield6both toward the PET detector4and toward the support tube5. The purpose of the RF shield6is to provide the RF coils7a,7bwith a suitable environment, for example for the magnetic return path. The two RF shields6and10can also be combined to form a single shield in the region between the PET detector4and RF coil7aor7b.The support tube5is also preferably provided with an RF shield on the outside of the longitudinal sides (not illustrated).

FIG. 6shows more accurately the arrangement of the PET detector4and RF coils7aand7bon the support tube5. Accordingly, the PET detector4is permanently fastened on the support tube, while the aim is to be able to remove the RF coils7aand7bas easily as possible. The cables leading to the PET detector are, for example, integrated in a channel or a cable conduit11in the wall of the support tube5. Furthermore, cooling tubes for a coolant for cooling the PET detector4can also be integrated in the support tube5.

The support tube5is preferably configured such that it can be inserted as easily as possible into the gradient tube3of an MR device. In the example illustrated, an annular air gap12of a thickness of approximately 4 mm is left between the support tube5and gradient coil3.

As an optional feature,FIG. 6shows a further RF shield6bon the inner side of the gradient coil3. This is not mandatory, at any rate if the channel11is shielded from the outside. The RF shield6bhas, however, the advantageous effect that electric lines possibly guided in the channel11are shielded against interference from both sides.

Including cladding8, RF coils7a,7b,PET detector4and support tube5, the detection unit illustrated has a thickness of only approximately 60 to 80 mm, preferably approximately 70 mm.