Patent Application
20110030698
Implementations generally relate to methods and devices for medical imaging and, more particularly, methods and devices for magnetic resonance imaging of joints.
In a magnetic resonance imaging (MRI) system, when a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
In MRI systems, the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is near the Larmor frequency, and its initial amplitude, A0, is determined by the magnitude of the transverse magnetic moment Mt. The amplitude, A, of the emitted NMR signal decays in an exponential fashion with time, t. The decay constant 1/T2* depends on the homogeneity of the magnetic field and on T2, which is referred to as the “spin-spin relaxation” constant, or the “transverse relaxation” constant. The T2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B1 in a perfectly homogeneous field. The practical value of the T2 constant is that tissues have different T2 values and this can be exploited as a means of enhancing the contrast between such tissues.
Another important factor that contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process that is characterized by the time constant T1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T1 time constant is longer than T2, much longer in most substances of medical interest. As with the T2 constant, the difference in T1 between tissues can be exploited to provide image contrast.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy and 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 MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Many patients suffer from knee disorders such as tumors or osteoarthritis. Osteoarthritis (OA), for example, is a degenerative disease of articular cartilage that afflicts more than 21 million people and is a leading cause of disability for adults in the US. OA decreases the load bearing capabilities of tissue and leads to impaired joints. MRI is a powerful tool for non-invasively evaluating OA and joint pathology in-vivo. MRI studies of diarthrodial joints can evaluate qualitative measures of OA within a joint. Examples of quantitative measures include minimum joint space width (JSW), cartilage volume, cartilage thickness, cartilage T2 values, and T1 values of cartilage in the presence of the contrast agent GD-DTPA. However, it is difficult to measure joint changes over time using these techniques due to variations in subject positioning and image slice definition. This is especially problematic for the knee, which can be difficult for technologists to consistently position for different MRI scans.
It would therefore be desirable to have a system and method to reduce intra- and inter-technologist variability between scans and allow consistent quantitative measurement of a characteristic of the knee.
A knee positioning device allows quantitative measurements of knee characteristics to be repeated with consistency. This consistency in repeated measurements can reduce both intra- and inter-technologist variability.
In one implementation a device is used to position the knee of a subject for an MRI scan. The device includes a local coil apparatus configured to position a knee of a subject, a thigh support proximal to a first end of the local coil apparatus, a foot positioning apparatus proximal to a second end of the local coil apparatus, and a user control. The thigh support apparatus has an adjustable medial thigh support and adjustable lateral thigh support configured to position the subject's thigh. The foot support is configured to position a foot of the subject within a foot positioning apparatus. The user control is configured to position each of the foot positioning apparatus, the thigh positioning apparatus, and the local coil apparatus to allow repeated quantitative measurements of a property of the knee.
In another implementation, a device is used to position a knee of a subject for a Magnetic Resonance Imaging (MRI) scan. The device includes a MRI local coil apparatus and a leg stabilizer. The leg stabilizer is configured to position a thigh joint and an ankle joint of a leg of a subject to a predetermined configuration to allow repeated quantitative measurements of a property of the knee.
In yet another implementation, repeated quantitative measurements of a property of a knee of a subject are conducted. The knee is positioned with a MRI local coil apparatus, the thigh is positioned with an adjustable medial thigh support and an adjustable lateral thigh support each coupled to the MRI local coil apparatus, and the foot is positioned with a foot positioning apparatus that is also coupled to the MRI local coil apparatus. At least one parameter is recorded that is usable to reposition each of: the thigh with the adjustable medial and lateral thigh supports; the knee with the MRI local coil; and the foot with the MRI local coil apparatus.
Implementations will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like elements bear like reference numerals.
Referring to
The pulse sequence server 18 functions in response to instructions downloaded from the workstation 10 to operate a gradient system 24 and an RF system 26. Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system 24 that excites gradient coils in an assembly 28 to produce the magnetic field gradients Gx, Gy and Gz used for position encoding MR signals. The gradient coil assembly 28 forms part of a magnet assembly 30 that includes a polarizing magnet 32 and a whole-body RF coil 34.
RF excitation waveforms are applied to the RF coil 34 by the RF system 26 to perform the prescribed magnetic resonance pulse sequence. Responsive MR signals detected by the RF coil 34 or a separate local coil (not shown in
The RF system 26 also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the MR signal received by the coil to which it is connected and a detector that detects and digitizes the I and Q quadrature components of the received MR signal. The magnitude of the received MR 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 MR signal may also be determined:
φ=tan−1Q/I.
The pulse sequence server 18 also optionally receives patient or subject data from a physiological acquisition controller 36. The controller 36 receives 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 server 18 to synchronize, or “gate”, the performance of the scan with the subject's respiration or heart beat.
The pulse sequence server 18 also connects to a scan room interface circuit 38 that receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit 38 that a patient positioning system 40 receives commands to move the patient to desired positions during the scan by translating the patient table 41.
The digitized MR signal samples produced by the RF system 26 are received by the data acquisition server 20. The data acquisition server 20 operates in response to instructions downloaded from the workstation 10 to receive the real-time MR data and provide buffer storage such that no data is lost by data overrun. In some scans the data acquisition server 20 does little more than pass the acquired MR data to the data processor server 22. However, in scans that require information derived from acquired MR data to control the further performance of the scan, the data acquisition server 20 is programmed to produce such information and convey it to the pulse sequence server 18. For example, during prescans, MR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server 18. 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 server 20 may be employed to process MR signals used to detect the arrival of contrast agent in an MRA scan. In all these examples the data acquisition server 20 acquires MR data and processes it in real-time to produce information that is used to control the scan.
The data processing server 22 receives MR data from the data acquisition server 20 and processes it in accordance with instructions downloaded from the workstation 10. Such processing may include, for example, Fourier transformation of raw k-space MR data to produce two or three-dimensional images, the application of filters to a reconstructed image, the performance of a backprojection image reconstruction of acquired MR data; the calculation of functional MR images, the calculation of motion or flow images, and the like.
Images reconstructed by the data processing server 22 are conveyed back to the workstation 10 where they are stored. Real-time images are stored in a data base memory cache (not shown) from which they may be output to operator display 12 or a display 42 that is located near the magnet assembly 30 for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage 44. When such images have been reconstructed and transferred to storage, the data processing server 22 notifies the data store server 23 on the workstation 10. The workstation 10 may be used by an operator to archive the images, produce films, or send the images via a network to other facilities.
As shown in
Referring particularly to
The magnitude of the RF excitation pulse produced at output 205 is attenuated by an exciter attenuator circuit 206 that receives a digital command from the pulse sequence server 18. The attenuated RF excitation pulses are applied to the power amplifier 151 that drives the RF coil 152A.
Referring still to
Referring to
The thigh positioning device 305 is configured to position a subject's thigh and includes a medial support 310 and lateral support 312, which are positioned at opposite ends of the support table's width and together confine the thigh in the intervening space therebetween. Thus, medial support 310 and lateral support 312 are configured to constrain subject motion in the medial/lateral direction. It is contemplated that the medial support 310 is cylindrical and extends in an anterior direction upwards from the plane of the support platform 302. The lateral support 312 may be substantially planar, extending lengthwise in anterior direction and buttressed by a secondary support 313 to prevent distortion in the plane parallel to the support platform 302. Both of these supports, may for example, utilize a peg-board of holes drilled into the support platform 302, which may be made of Plexiglas materials for example, to enable the repositioning and adjustment of the thigh positioning device 305 according to subject size. The knee positioning device 308 is configured to mount an MRI local coil apparatus, such as a MRI knee coil 315, by having a surface shaped to firmly envelop the underside of the knee coil 315. The tightness of fit of the knee coil 315 to this surface, in combination with the weight of the subject's knee, helps ensure that the knee is fixed in a selected location and will not move significantly during scanning.
The foot positioning device 306 includes an ankle foot orthosis 316 having a hinge 317 and a toe strap 318 to secure the subject's foot. The components of the foot positioning device 306 can be moved to provide incremental changes in the anterior/posterior (transverse plane), medial/lateral (coronal plane), and superior/inferior (sagittal plane) positions of the patient's foot. The hinge 317 allows incremental changes of foot rotation in the dorsal/plantar and inversion/eversion directions. Accordingly, the subject's foot may be moved (e.g., translated and/or rotated) to position the knee in a desired pose at the magnetic iso-center of the magnet. Remaining degrees of freedom of the device 300 can be individually positioned to a given subject's comfort. The thigh positioning apparatus and foot positioning apparatus may collectively be referred herein as a “leg stabilizer.”
A health care provider, such as a clinician, may employ the above-described device and process to perform consistent repeat measurements of joint characteristics to, for example, evaluate quantitative changes of joint anatomy or pathology over time. In one implementation, the device 300 is used to evaluate pathology such as tumors or knee OA. Other uses of the device 300 are also contemplated such as repeat measurements of joint anatomy to evaluate the structural integrity or health of the underlying imaged tissues. Each measurement typically begins with the positioning of a subject within the device 300, wherein the position and angle of the device components are adjusted based on the subject size and imaging task or to conform to settings used for a previous scan. To quantitatively measure minimum JSW in the patello-femoral joint or the tibio-femoral joint, for example, the subject's thigh may be positioned in the thigh positioning device 305 and secured firmly between the medial thigh support 310 and the lateral thigh support 312. The subject's leg can be positioned so that the knee rests on the bottom portion of and through the knee coil 315, which rests firmly within the knee positioning device 308, and the foot is secured in the foot positioning device 306. The top portion of the knee coil 315 can then be secured to the bottom of the knee coil 315 and the positioning devices can then be individually translated and/or rotated to position the subject's knee within the “sweet spot” of the MRI bore. Slice profiles can then be defined along an axis-of-interest, for example, along the line connecting the most posterior aspect of the medial and lateral femoral condyles seen in scout images, where the central most slice is positioned through the widest portion of the patella. An MRI can then be performed to obtain a plurality of image slices, which can subsequently be analyzed by the clinician to determine the minimum JSW. By repeating such scans over time using similar position and rotation settings for the knee positioning device 300, the clinician can observe changes in minimum JSW that may be indicative of OA. Since JSW measurement variability between scans is reduced via the use of the knee positioning device 300, the accuracy of the clinician's observations may increase significantly.
Referring to
In some implementations, the positioning of the knee, thigh, and foot are done manually by a technician, for example. In other implementations, the positioning is done automatically with a mechanical device that is controlled by a processor executing a computer program. For example, the executed program may drive motors and gears that position each of the knee, joint, and foot in a predefined configuration.
At step 408, a user control receives a user-selection of a relative position of at least one of the foot positioning apparatus, the MRI local coil, and the thigh positioning apparatus. For example, in the example above, the user control may receive a location of the pegs in the plane of the peg-board for each of the lateral support 312 and the medial support 310 of the thigh positioning device 305 from a user. Similarly, the user control may receive each of the angles of the knee joint positioned in the MRI local coil apparatus and the ankle joint positioned in the ankle foot orthosis. Further at step 408, the user control provides an at least one positional parameter indicating the user-selected, relative position of each of the foot positioning apparatus and the thigh positioning apparatus, designed to facilitate repeated quantitative measurements of a property of the knee across a plurality of medical imaging scans. In the peg-board example above, the user control may provide an indication of location of the pegs when the subject is positioned in the leg stabilizer. In one implementation, the indication may be recorded and stored in a database or displayed via a user interface. For example, in the automated implementations described above, information about the amount of revolutions of the each of the motors that produced the configuration of the leg, for example, may be electronically recorded and stored in a database.
A first MRI scan is taken including at least one MRI image of the knee at step 410. The subject may be scanned in a relaxed state, such that muscle contraction is negligible. Alternatively, the subject may be scanned while at least some of the muscles of the leg are under isometric contraction. The subject, or the technician, then removes the leg from the knee positioning device 300.
At step 412, a quantitative measurement of a property of the knee is determined. In one implementation, the property of the knee is JSW determined using an algorithm. A processor executes a computer program that utilize data obtained from the MRI scan. For example, the executed program may receive as input a series of oblique, sagittal, spiral, fast spoiled gradient recalled (SPGR) images with frequency selective fat suppression. The executed program assigns each pixel in the images a value based on the ratio of signal intensity differences in the local 8-pixel neighborhood to maximal signal intensity differences in the image. Next, the user of the program defines a seed point to initialize the program to search for the cartilage-bone interface. This seed point is placed on the anterior or posterior margins of the femur and tibia for calculating tibio-femoral JSW or on the proximal or distal margin of the patella and corresponding region on the femur for calculating patello-femoral JSW, for example. The program then performs a semi-automated line search on the image, starting at the seed point and follows along a calculated path of maximal signal intensity differences to determine the edges of the joint space, from which the minimum JSW is calculated. In another implementation, the property of the knee may be cartilage volume, cartilage thickness, cartilage T2 values, or T1 values of cartilage in the presence of the contrast agent, such as gadolinium diethylenetriamine penta-acetic acid (Gd-DTPA), that employ 2D or 3D MRI images.
At step 414, the thigh, the knee, and the foot of the subject are repositioned using the parameter recorded at step 408. For example, the subject may return for a second scan a month after the first scan. The leg of the subject is repositioned to its previous configuration, repeating steps 402 through 406 in process 400. To illustrate, the pegs in the peg board may be repositioned to their previous location such that the lateral support 312 and the medial support 310 of the thigh positioning device 305 are identical or about identical to their previous respective positions. Similarly, the knee angle may be repositioned with the MRI local coil and the foot angle may be repositioned with the foot positioning apparatus.
At step 416, a second, subsequent MRI scan of the knee is taken of the leg that has been repositioned in step 414. At step 418, a subsequent quantitative measurement of the property of the knee is determined based on the repositioned thigh, knee, and foot. For example, the JSW may be recalculated using a similar method described in step 412. Here, however, the executed program may receive as input a series of SPGR images taken of the repositioned leg.
At step 420, the quantitative measurement of the property of the knee is compared with the subsequent quantitative measurement of the property. For example, the JSW determined for the initially positioned leg is compared to the JSW determined for the subsequently repositioned leg. The result of the comparison may be usable in a diagnosis. For example, if the JSW has significantly changed in the one month span between the first MRI scan and the second MRI scan, then the OA of the patient may have caused degeneration of the knee joint.
Thus, implementations provide multiple advantages over prior devices for positioning the knee within an MRI system 5. That is, prior devices were generally designed for kinematic studies in which the knee is intentionally moved during or between scans, for example, to study patellar tracking abnormalities or the real-time motion of the patella during knee flexion and extension. Prior knee positioning devices were designed to reduce subject motion during the scan, as some MRI acquisition sequences are sensitive to motion. However, these devices were not designed to secure the knee in a consistent position throughout a series of different scans that are potentially performed by different technologists. Consequently, these prior devices do not lead to a reduction in scan variability and are unsuitable for situations in which repeated quantitative measurements of a characteristic of the knee, such as minimum JSW, are desired.
It should be understood that implementations can be implemented in the form of control logic, in a modular or integrated manner, using software, hardware or a combination of both. The steps of a method, process, or algorithm described in connection with the implementations disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. The various steps or acts in a method or process may be performed in the order shown, or may be performed in another order. Additionally, one or more process or method steps may be omitted or one or more process or method steps may be added to the methods and processes. An additional step, block, or action may be added in the beginning, end, or intervening existing elements of the methods and processes. Based on the disclosure and teachings provided herein, a person of ordinary skill in the art will appreciate other ways and/or methods to implement various implementations.
It is understood that the examples and implementations described herein are for illustrative purposes only and that various modifications, equivalents, alternatives, variations, or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
This application is based on, claims the priority of, and incorporates herein by reference U.S. Provisional Application Ser. No. 61/231,940, entitled “MRI Compatible Knee Positioning Device,” filed Aug. 6, 2009.
This invention was made with government support under Grant No. NIAMS R01AR048768 (KRK, KKA) and F32AR053430-01 (MFK). The United States Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61231940 | Aug 2009 | US |
