Bridge member for a magnetic resonance examination system

Abstract

A bridge member containing MR responsive material is provided in an open space between body parts to establish a correspondence between the body parts. The MR responsive material generates magnetic resonance signals in response the RF excitation, so that between the separate body parts via the bridge member magnetic resonance signal are obtained from positions between which there is at most a limited spatial variation of the main magnetic field, so that phase ambiguities between the signals from these positions are avoided. Thus, chemical shift separation, notably water-fat separation though a region-of-interest containing several (both) body parts may rely on a smoothness condition imposed on the spatial distribution of the main magnetic field. This avoids artefacts, such as water-fat swaps when separating water and fat contributions in the reconstructed magnetic resonance image.

Claims

1. A bridge member for use in a magnetic resonance examination system and to establish a correspondence between MR signals from separated body parts of a patient to be examined, the bridge member containing a MR responsive fat-like material that has a magnetic resonance response that in response to RF excitation emits magnetic resonance signals having an appreciable signal-to-noise ratio (SNR) and said response resembles fat-like tissue's magnetic resonance response, wherein the bridge member is formed by a patient support and contains the MR responsive material at least along the surface to face the patient to be examined, such that the separated body parts and the bridge member form a path between the separated body parts along which path the fat-like MR response is emitted.

2. A bridge member configured for use in a magnetic resonance examination system and to establish a phase correspondence between magnetic signals emitted from separated body parts of a patient to be examined, the bridge member containing a MR responsive fat-like material that has a magnetic resonance response that in response to RF excitation emits magnetic resonance signals having an appreciable signal-to-noise ratio (SNR) and said response resembles fat-like tissue's magnetic resonance response, wherein the bridge member is formed by a stabilisation bag that is shaped and/or is deformable to keep the separate body parts in position and provide a magnetic signal emitting path connecting the separated body parts.

3. A method of using a bridge member in a magnetic resonance examination system to establish a correspondence in a phase of MR signals from separated body parts of a patient to be examined, the method comprising positioning a bridge member comprising a MR responsive fat-like material in response to RF excitation emits magnetic resonance signals having an appreciable signal-to-noise ratio (SNR) such that said response resembles fat-like tissue's magnetic resonance response along a path of the bridge member connecting the separated body parts.

4. The bridge member as claimed in claim 1, wherein the bridge member includes a flexible member configured to be placed on a surface of a patient carrier and be flexibly deformed by weight of the patient to be examined into a shape that contacts or is in close proximity to each of the separated body parts and conforms with surface portions of each of the separated body parts.

5. The bridge member as claimed in claim 1 wherein the MR responsive material is disposed in at least part of (i) an outer layer or (ii) along at least part of the bridge member's outline.

6. The bridge member as claimed in claim 1, wherein the fat-like material consists mainly of saturated —CH2- chains.

7. The magnetic resonance examination system provided with bridge member of claim 1.

8. The bridge member as claimed in claim 1, wherein in water-fat separation imaging, the fat-like material generates a bright fat signal and does not generate a significant water signal.

9. The bridge member as claimed in claim 1, wherein in angiography, a level of generated MR signal from the fat-like material is less than a level of MR signals from blood vessels.

10. The bridge member as claimed in claim 1, wherein magnetic resonance signals across the bridge member between one of the separated body parts and another undergo a phase shift of 2π or less.

11. The bridge member as claimed in claim 2, wherein in water-fat separation imaging, the fat-like material generates a bright fat signal and does not generate a noticeable water signal.

12. The bridge member as claimed in claim 2, wherein in angiography, a level of generated MR signal from the fat-like material is less than a level of MR signals from vessels.

13. The bridge member as claimed in claim 2, wherein magnetic resonance signals across the bridge member between one of the body parts and another undergo a B.sub.0 induced phase shift of 2π or less.

14. The method of using the bridge member as claimed in claim 3, further including: generating separate water and fat images of the separated body parts and the bridge member, wherein the bridge member is visible in the fat image forming a path between the separated body parts in the fat image and not in the water image.

15. The method of using the bridge member as claimed in claim 3, further including: from phase shifts along the path from a first of the separated bod parts to a second of the separated body parts, determining a phase correspondence in MR signals from the first and second body parts.

16. The method of using the bridge member as claimed in claim 15, wherein a phase shift across the bridge member between the first and second body parts is 2π or less.

17. The method of using the bridge member as claimed in claim 3, further including: generating angiographic images such that MR signals from vessels are stronger than MR signals from the fat-like material in the bridge member.

18. The method of using the bridge member as claimed in claim 3, wherein the bridge member includes a flexible mattress, and wherein the method further includes: placing the mattress on a patient support, positioning a patient on the mattress such that weight of the patient causes the mattress to deform into conformity with the separated body parts and into a space between the separated body parts forming a path of the fat-like material between the separated body parts.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a diagrammatic representation of a magnetic resonance examination system in which the invention is employed;

(2) FIG. 2 shows a first example of a bridge member of the invention;

(3) FIG. 3 shows a second example of a bridge member of the invention;

(4) FIG. 4 shows a third example of a bridge member of the invention;

(5) FIG. 5 shows a fourth example of a bridge member of the invention incorporated in the patient support;

(6) FIG. 6 shows a fifth example of a bridge member of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) FIG. 1 shows diagrammatically a magnetic resonance imaging system in which the invention is used. FIG. 1 shows a diagrammatic representation of a magnetic resonance examination system in which the invention is employed. The magnetic resonance imaging system includes a main magnet with a set of main coils 10 whereby the steady, uniform magnetic field is generated. The main coils are constructed, for example in such a manner that they from a bore to enclose a tunnel-shaped examination space. The patient to be examined is placed on a patient carrier which is slid into this tunnel-shaped examination space. The magnetic resonance imaging system also includes a number of gradient coils 11, 12 whereby magnetic fields exhibiting spatial variations, notably in the form of temporary gradients in individual directions, are generated so as to be superposed on the uniform magnetic field. The gradient coils 11, 12 are connected to a gradient control 21 which includes one or more gradient amplifier and a controllable power supply unit. The gradient coils 11, 12 are energised by application of an electric current by means of the power supply unit 21; to this end the power supply unit is fitted with electronic gradient amplification circuit that applies the electric current to the gradient coils so as to generate gradient pulses (also termed ‘gradient waveforms’) of appropriate temporal shape. The strength, direction and duration of the gradients are controlled by control of the power supply unit. The magnetic resonance imaging system also includes transmission and receiving antennae (coils or coil arrays) 13, 16 for generating the RF excitation pulses and for picking up the magnetic resonance signals, respectively. The transmission coil 13 is preferably constructed as a body coil 13 whereby (a part of) the object to be examined can be enclosed. The body coil is usually arranged in the magnetic resonance imaging system in such a manner that the patient 30 to be examined is enclosed by the body coil 13 when he or she is arranged in the magnetic resonance imaging system. The body coil 13 acts as a transmission antenna for the transmission of the RF excitation pulses and RF refocusing pulses. Preferably, the body coil 13 involves a spatially uniform intensity distribution of the transmitted RF pulses (RFS). The same coil or antenna is generally used alternately as the transmission coil and the receiving coil. Typically, a receiving coil includes a multiplicity of elements, each typically forming a single loop. Various geometries of the shape of the loop and the arrangement of various elements are possible. The transmission and receiving coil 13 is connected to an electronic transmission and receiving circuit 15.

(8) It is to be noted that is that there is one (or a few) RF antenna elements that can act as transmit and receive; additionally, typically, the user may choose to employ an application-specific receive antenna that typically is formed as an array of receive-elements. For example, surface coil arrays 16 can be used as receiving and/or transmission coils. Such surface coil arrays have a high sensitivity in a comparatively small volume. The receiving coil is connected to a preamplifier 23. The preamplifier 23 amplifies the RF resonance signal (MS) received by the receiving coil 16 and the amplified RF resonance signal is applied to a demodulator 24. The receiving antennae, such as the surface coil arrays, are connected to a demodulator 24 and the received pre-amplified magnetic resonance signals (MS) are demodulated by means of the demodulator 24. The pre-amplifier 23 and demodulator 24 may be digitally implemented and integrated in the surface coil array. The demodulated magnetic resonance signals (DMS) are applied to a reconstruction unit. Note that the reconstructing function to reconstruct the magnetic resonance image from the acquired magnetic resonance signal may also be performed at a remote location, even outside of the building or the organisation where the magnetic resonance examination system is set up or controlled by. The reconstruction may be done by processing capabilities elsewhere, e.g. in ‘the cloud’ and the data files representing the reconstructed magnetic resonance image returned to the magnetic resonance examination system. The demodulator 24 demodulates the amplified RF resonance signal. The demodulated resonance signal contains the actual information concerning the local spin densities in the part of the object to be imaged. Furthermore, the transmission and receiving circuit 15 is connected to a modulator 22. The modulator 22 and the transmission and receiving circuit 15 activate the transmission coil 13 so as to transmit the RF excitation and refocusing pulses. In particular the surface receive coil arrays 16 are coupled to the transmission and receive circuit by way of a wireless link. Magnetic resonance signal data received by the surface coil arrays 16 are transmitted to the transmission and receiving circuit 15 and control signals (e.g. to tune and detune the surface coils) are sent to the surface coils over the wireless link.

(9) The reconstruction unit derives one or more image signals from the demodulated magnetic resonance signals (DMS), which image signals represent the image information of the imaged part of the object to be examined. The reconstruction unit 25 in practice is constructed preferably as a digital image processing unit 25 which is programmed so as to derive from the demodulated magnetic resonance signals the image signals which represent the image information of the part of the object to be imaged. The signal on the output of the reconstruction is applied to a monitor 26, so that the reconstructed magnetic resonance image can be displayed on the monitor. It is alternatively possible to store the signal from the reconstruction unit 25 in a buffer unit 27 while awaiting further processing or display. The reconstruction unit may be located remotely from the magnetic resonance examination system and have a data-connection to the magnetic resonance examination system's host computer.

(10) The magnetic resonance imaging system according to the invention is also provided with a control unit 20, for example in the form of a computer which includes a (micro)processor. The control unit 20 controls the execution of the RF excitations and the application of the temporary gradient fields. To this end, the computer program according to the invention is loaded, for example, into the control unit 20 and the reconstruction unit 25.

(11) The bridge member 51 is placed on the patient support 14.

(12) FIG. 2 shows a first example of a bridge member of the invention. The bridge member 51 is formed as a single member that may be placed between the body parts 41,42. The bridge member 51 is made of the fat-like material, so that magnetic resonance signal resembling those of human fat may be produced throughout the volume of the bridge member. This enables robust water fat-separation in the magnetic resonance image showing both body parts and in which the bridge member will show up only in the fat-image and not in the water-image.

(13) FIG. 3 shows a second example of a bridge member of the invention. The bridge member 51 is formed from several bridge units 511 that contain fat-like material. The neighbouring bridge unit are at a distance of at most a distance over which the main magnetic field varies appreciably. Accordingly, magnetic resonance signal from neighbouring bridge units 511 may have a phase-shift of at most the predetermined threshold of 2π or a smaller value.

(14) FIG. 4 shows a third example of a bridge member of the invention. In the example of FIG. 4, the bridge member 51 is a single member which has a layer 512 of fat-like material disposed along its outline. This layer of fat-like material may be just below the outer surface of the bridge member 51.

(15) Accordingly, magnetic resonance signal from positions spaced apart less than the typical distance over which the main magnetic field varies appreciably have phase-shift of at most the predetermined threshold of 2π or a smaller value.

(16) FIG. 5 shows a fourth example of a bridge member of the invention incorporated in the patient support 14. A layer of fat-like material 512 is disposed at or just below the surface to face the patient to be examined. When in use the separate body parts, e.g. patient's legs or torso and arms make contact or are in close proximity with the surface of the patient carrier and are close to the layer of fat-like material such that the magnetic resonance signals from the body part and from the layer of fat like material may be less than the predetermined threshold of 2π or less. Further, the layer 512 of fat-like material allows a path through the layer of positions that generate magnetic resonance signals and B0-induced phase shifts between adjacent position along the path (shown as a drawn line) are less than the predetermined threshold. Accordingly, along the path there are no large B0-induced phase shifts between adjacent positions so that phase ambiguities along the path from one body part to the other cannot occur.

(17) FIG. 6 shows a fifth example of a bridge member of the invention. The bridge member 51 of FIG. 6 is formed as a flexible member that can be placed on the surface of the patient carrier. When the patient to be examined is place on the patient support the body parts are supported by the flexible bridge member. Hence, owing to the deformation of the flexible bridge member, relatively wider contact or close proximity areas are formed between the flexible bridge member and the respective body parts. These wider contact or close proximity areas ensure that that the magnetic resonance signals from the body part and from the layer of fat like material may be less than the predetermined threshold of 2π or less. Further, the bridge member containing the fat-like material allows a path through the bridge member of positions that generate magnetic resonance signals and B0-induced phase shifts between adjacent position along the path (shown as a drawn line) are less than the predetermined threshold. Accordingly, phase ambiguities between magnetic resonance signals from ten respective body parts are avoided.

(18) The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.