Selectively adjustable phantom that is compatible with a magnetic resonance imaging system and environment

A phantom for use in an MRI or MRS system includes an array of subresolvable compartments that are each connected to receive fluid from one of a plurality of fluid reservoirs. The array of compartments is divided into sub-arrays which differ from each other by the mix of compartments in each sub-array receiving fluids from the different reservoirs. Studies can be performed with the phantom by filling the compartments with selected fluids from the reservoirs, disconnecting the reservoirs and placing the phantom in the system bore. The phantom can be reused in other studies by replacing the fluids with different fluids.

BACKGROUND OF THE INVENTION

The field of the invention is nuclear magnetic resonance imaging (MRI) or spectroscopy (MRS) methods and systems. More particularly, the invention relates to a phantom that may be used to study the effectiveness of MRI and MRS methods and materials.

When utilizing these signals to produce images, magnetic field gradients (Gx, Gyand Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.

Phantoms are devices that are placed in the bore of an MRI system to test or calibrate its operation. Phantoms may be made of materials having known magnetic resonance properties or they may contain cavities filled with such materials. The MRI system is operated with the phantom in place to produce a spectrum or an image from which proper operation of the MRI system may be determined. The shape and size of the phantom or its cavities may be designed to measure magnetic field strength or field homogeneity and it may be used in combination with a procedure that enables calibration or trimming of such fields. Phantoms used in this manner are exemplified by those disclosed in U.S. Pat. No. 5,036,280; and published U.S. Pat. Appln. No. 2003/0086535.

Phantoms are also designed to model anatomical structures so that imaging or spectroscopy methods can be developed which accurately depict or represent such structures. As described in U.S. Pat. No. 6,205,871, for example, vascular structures are modeled with a phantom in order to test the efficacy of magnetic resonance angiography methods, and a phantom is disclosed in published U.S. Pat. Appln. No. 2006/0195030 that uses fiber bundles to create phantoms for use in testing diffusion tensor imaging (DTI) methods.

Manufacturing methods enable phantoms to be constructed with subresolvable regions. Three-dimensional fabrication methods such as printing on a thin film sheet or substrate using photo lithography, electrostatic xerographic printing or etching as disclosed in U.S. Pat. No. 6,720,766 may be used to produce cavities and fluid passages in the phantom that are smaller than the voxel size of a high resolution MR image. This technology is used in prior phantoms to model a specific anatomic structure or to achieve a specific effect.

SUMMARY OF THE INVENTION

The present invention is a phantom for use in testing MR methods and materials. More particularly, the phantom is constructed with a plurality of fluid circuits that each includes a reservoir for a fluid that may be forced into an array of compartments formed in the phantom. The compartments in each fluid circuit are preferably subresolvable in size and are located adjacent the subresolvable compartments in other fluid circuits.

A general object of the invention is to provide a phantom that can be used in many different MR studies. To study the effectiveness of a spectroscopy method, for example, fluids containing different metabolites may be employed in the different fluid circuits of the phantom to determine if the method will resolve the different metabolites in a voxel.

Another aspect of the invention is to separate the array of compartments into a plurality of sub-arrays in which the relative amount of fluids from the separate reservoirs differ in each sub-array. For example, in one sub-array the compartments are designed to contain 20% fluid from one reservoir and 80% fluid from another reservoir and in another sub-array the compartments contain 50% fluid from each reservoir. The MR signals from each sub-array of compartments will be affected by the ratio of fluids in each and this enables one to determine what ratio of fluids (such as metabolites or contrast agents) will have a discernable impact on the MR image or spectrum.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring particularly toFIG. 1, the preferred embodiment of the invention is employed in an MRI system. The MRI system includes a workstation10having a display12and a keyboard14. The workstation10includes a processor16which is a commercially available programmable machine running a commercially available operating system. The workstation10provides the operator interface which enables scan prescriptions to be entered into the MRI system.

The workstation10is coupled to four servers: a pulse sequence server18; a data acquisition server20; a data processing server22, and a data store server23. In the preferred embodiment the data store server23is performed by the workstation processor16and associated disc drive interface circuitry. The server18is performed by a separate processor and the servers20and22are combined in a single processor. The workstation10and each processor for the servers18,20and22are connected to an Ethernet communications network. This network conveys data that is downloaded to the servers18,20and22from the workstation10, and it conveys data that is communicated between the servers.

The pulse sequence server18functions in response to instructions downloaded from the workstation10to operate a gradient system24and an RF system26. Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system24which excites gradient coils in an assembly28to produce the magnetic field gradients Gx, Gyand Gzused for position encoding NMR signals. The gradient coil assembly28forms part of a magnet assembly30which includes a polarizing magnet32and a whole-body RF coil34.

RF excitation waveforms are applied to the RF coil34by the RF system26to perform the prescribed magnetic resonance pulse sequence. Responsive NMR signals detected by the RF coil34are received by the RF system26, amplified, demodulated, filtered and digitized under direction of commands produced by the pulse sequence server18. The RF system26includes an RF transmitter for producing a wide variety of RF pulses used in MR pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server18to produce RF pulses of the desired frequency, phase and pulse amplitude waveform. The generated RF pulses may be applied to the whole body RF coil34or to one or more local coils or coil arrays.

The RF system26also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the NMR signal received by the coil to which it is connected and a quadrature detector which detects and digitizes the I and Q quadrature components of the received NMR signal. The magnitude of the received NMR signal may thus be determined at any sampled point by the square root of the sum of the squares of the I and Q components:
M=√{square root over (I2+Q2)},
and the phase of the received NMR signal may also be determined:
φ=tan−1Q/I.

The pulse sequence server18also optionally receives patient data from a physiological acquisition controller36. The controller36receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. Such signals are typically used by the pulse sequence server18to synchronize, or “gate”, the performance of the scan with the subject's respiration or heart beat.

The pulse sequence server18also connects to a scan room interface circuit38which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit38that a patient positioning system40receives commands to move the patient to desired positions during the scan.

The digitized NMR signal samples produced by the RF system26are received by the data acquisition server20. The data acquisition server20operates in response to instructions downloaded from the workstation10to receive the real-time NMR data and provide buffer storage such that no data is lost by data overrun. In some scans the data acquisition server20does little more than pass the acquired NMR data to the data processor server22. However, in scans which require information derived from acquired NMR data to control the further performance of the scan, the data acquisition server20is programmed to produce such information and convey it to the pulse sequence server18. For example, during prescans NMR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server18. Also, navigator signals may be acquired during a scan and used to adjust RF or gradient system operating parameters or to control the view order in which k-space is sampled. And, the data acquisition server20may be employed to process NMR signals used to detect the arrival of contrast agent in an MRA scan. In all these examples the data acquisition server20acquires NMR data and processes it in real-time to produce information which is used to control the scan.

The data processing server22receives NMR data from the data acquisition server20and processes it in accordance with instructions downloaded from the workstation10. Such processing may include, for example: Fourier transformation of raw k-space NMR data to produce two or three-dimensional images; the application of filters to a reconstructed image; the performance of a back projection image reconstruction of acquired NMR data; the calculation of functional MR images; the calculation of motion or flow images, etc.

Images reconstructed by the data processing server22are conveyed back to the workstation10where they are stored. Real-time images are stored in a data base memory cache (not shown) from which they may be output to operator display12or a display42which is located near the magnet assembly30for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage44. When such images have been reconstructed and transferred to storage, the data processing server22notifies the data store server23on the workstation10. The workstation10may be used by an operator to archive the images, produce films, or send the images via a network to other facilities.

The phantom constructed according to the teachings of the present invention is inserted into the bore of the magnet in place of the human subject shown inFIG. 1. A preferred embodiment of the phantom is shown inFIGS. 2-4, where the phantom housing100inFIG. 2is the element actually placed into the MRI system.

Referring particularly toFIG. 3, the phantom includes two fluid reservoirs102and104that store fluids such as contrast agents and metabolites that are to be used in a phantom study. Each fluid reservoir102and104is connected to fluid circuits in the phantom housing100through respective valves106and108. Each fluid circuit includes an array of compartments in the housing100which are filled with fluid from the reservoir102or104to which they connect. The array of compartments indicated schematically inFIG. 3at110is in turn divided into sub-arrays112,114,116and118, and as will be described below, each subarray provides a different mix of the reservoir fluids. The fluid circuits convey fluid from the reservoirs102and104into selected compartments in the array110and the over flow fluid from reservoir102exits the array110through a valve120and is recirculated back to reservoir102. Similarly, over flow fluid from reservoir104exits the array100through a valve122and is recirculated back to the reservoir104.

The phantom is prepared for a study by filling the reservoirs102and104with the fluids to be used, and these fluids are pumped through the compartment array110. When all the compartments are filled with fluid, the valves106,108and120,122are closed and disconnected from the reservoirs102and104. The phantom housing100with enclosed compartment array110is then placed in the bore of the MRI system and a test procedure is run. When the test procedure is finished the phantom housing100is removed from the bore and the valves106,108,120and122are opened to drain the fluids from the compartment array110. The phantom can then be reused in an entirely different test procedure by filling the reservoirs with different fluids.

Referring particularly toFIGS. 3 and 4, the compartment array110is constructed using a three-dimensional fabrication method that enables each compartment to be formed at a subresolvable size. The choice of fabrication method is mainly dependent on the desired structural resolution, as well as the ability to make distinct compartments. The compartment array110is organized as 1 mm cubes126disposed along three Cartesian axes to fill a rectangular space. Each cube126contains five compartments that receive fluid from either of reservoir102or104.

The cubes126in each sub-array112,114,116and118are distinguished from each other by the fluid which fills each of its five compartments. Referring toFIG. 4, the cubes126in the sub-array112have four compartments connected to reservoir102and only one compartment connected to reservoir104. As a result, an MR image or spectrum will “see” each voxel sized cube126in sub-array112as 80% fluid A and 20% fluid B.

The cubes126in the other sub-arrays114,116and118contain a different mix of fluids A and B. Sub-array114contains cubes126in which three compartments receive fluid A and two compartments receive fluid B. The cubes126in sub-array116have two compartments receiving fluid A and three compartments receiving fluid B, and cubes126in sub-array118have one compartment receiving fluid A and four compartments receiving fluid B. As a result, the preferred embodiment of the invention enables four different mixes of the two fluids A and B at a subresolvable scale.

It should be apparent to those skilled in the art that many variations are possible from the above-described preferred embodiment. The number of compartments in each subresolvable cube126can be increased to enable a more highly resolved range of fluid A and B mixes. Also, the compartment array110need not be formed as cubical shaped collections of compartments, but instead may be shaped to model specific tissues or anatomic structures.

While two fluid reservoirs and corresponding fluid circuits are employed in the preferred embodiment, additional fluid reservoirs and associated fluid circuits may be employed. In such case each cube126will contain compartments that are selectively connected to the additional fluid reservoirs and the range of fluid mixes will take on additional dimensions.

An example use of the preferred phantom is to fill the two reservoirs with solutions containing chemicals with nearly overlapping resonances. By acquiring MRS measurements on compartments containing different mixes of the two solutions, it can then be determined at which mixture the MRS measurement method can distinguish between the two metabolites.

While the preferred embodiment is a phantom for MRI studies, the same design principles may be applied for other modalities where a sub-resolution mixture of circuits containing liquids or gas can provide the signal sources. Examples of such other modalities include radionuclide imaging (where the different reservoirs are filled with fluids containing different radionuclide sources) and optical tomography (where the different reservoirs are filled with fluids having different optical properties).

A second preferred embodiment of the phantom is constructed to mimic the diffusion properties of tissue or other homogeneous media. The circuits in this embodiment include numerous tube-like structures such as those indicated inFIG. 5that produce a directional asymmetry in the diffusion of the material contained in the circuit. An individual tube-like structure can have a variety of properties such as diameter, curvature and direction that can vary between neighboring structures as well as varying along its length.

A third preferred embodiment is a phantom whose electrical conductivity is spatially anisotropic. The circuits are filled with a conductive medium, such as saline. By constructing the circuits as shown inFIG. 6with fluid passages of different diameters in different directions, greater conductivity in certain directions is achieved and an isotopic conductivity can be created.