Fiducial marker system for subject movement compensation during medical treatment

The present invention in general relates to non-optical fiducial markers deployed as part of a system for subject movement during a medical treatment and in particular, to a fiducial marker system that compensates for subject movement in real time.

The delivery of high intensity radiation to cancerous body tissue with a highly focused delivery is complicated by subject movement during the radiation administration session. While bolting a subject skull into a frame has proven to be partially successful, cervical flexion and respiration still contributes to radiation delivery defocus. While anesthesia has proven partly successful in compensating for cervical flexion and respiration, the difficulties and possible complications associated with anesthesia make this an unattractive option.

Treatment of a lesion located in the thoracic or abdominal cavity exacerbates the problem associated with patient movement. With a diaphragm traveling about 10 cm in an adult human, dynamic positional change of organs is observed during a respiration cycle. The result of organ movement is a radiation delivery defocus with tissue surrounding a lesion being subjected to unintended dosing.

Prior art attempts to address physiological movement during radiation dosing have met with limited success. Typical of these systems is the establishment of a camera array around the three-dimensional subject volume. A series of blinking lights secured to subject skin are tracked by the cameras and through geometric triangulation, the location of a surgical tool, catheter tip, or fiducial marker is noted. However, owing to the slow speed associated with such a system, the scans are typically performed prior to a surgical procedure or during an interruption in the procedure and, as such, lack real-time responsiveness needed for radio-dosing. A variation on such a system uses reflective spheres secured to the subject with pulsing lights proximal to the cameras in order to approximate volume through triangulation. These methodologies have met with limited acceptance owing to the inability of the optical system to simultaneously detect a fiducial marker or medical instrument internal to the subject volume while computing volume changes associated with respiratory physiology.

A more sophisticated prior art approach to this problem achieves a five second lag time relative to subject motion and is available under the trade name CyberKnife®. This method uses a series of magnetic resonance imaging or computed-aided tomography images to compute hundreds of planar x-ray images prior to a procedure. The procedure occurs on a fluoroscopy table with fluoroscopy images being compared by a computer to the computed x-ray images to ascertain biplanar fluoroscopy image pattern match with the computed x-ray images so as to determine subject position. This process has met with limited acceptance owing to a five second lag being a considerable time period as compared to a respiratory cycle. Additionally, a subject must be semi-restrained in order to derive a therapeutic effect.

To further improve the compensation for respiratory physiology, a constellation of radio-opaque fiducials are implanted within the subject volume that is to be the subject of the therapy and the procedure repeated of collecting MRI or CT scans from which biplanar x-ray images are derived prior to a therapeutic session. The computed biplanar x-rays are compared with fluoroscope images collected prior to or during a procedure, which still further reduces the lag time during the computed respiratory cycle position and the actual body position. While a constellation of fiducials made up of skin marks or markers placed on the chest wall afford a timing of respiratory physiology-related movement, a time lag still persists.

Thus, there exists a need for a fiducial marking system capable of calculating a target movement within a subject related to subject movement on a greater precision than has been heretofore available. Additionally, there exists a need for a fiduciary marking system capable of predicting periodic subject movement so as to further define radiation dosing.

A system is provided for subject movement compensation during a medical procedure. The system uses passive radiofrequency identification tags associated with a subject, and at least one active RFID reader is positioned to interrogate the position of at least three such passive radiofrequency identification tags so as to triangulate the geometric position of a subject body tissue. The reader generates an output signal corresponding to a distance between said reader and a tag. A microprocessor calculates a displacement of a tag relative to the reader from said output signal to yield a value corresponding to subject movement. A medical device operates in synchronicity with the calculation corresponding to subject movement to compensate for subject movement during a treatment process in essentially real time.

A process for subject movement compensation during a medical procedure is also provided. The process involves scanning a subject to compute a vectoral distance between a passive radiofrequency identification tag secured to the subject and a target tissue within the subject. The vectoral distance is stored in a digital memory accessible by a microprocessor. A radiofrequency reader is then activated to generate an output signal corresponding to a distance between the tag and the reader. A microprocessor then computes a position of the target tissue based on the output signal, with the coordinates of the target tissue being communicated to a medical device by way of the output signal.

FIG. 1 is a partial cutaway perspective view of an inventive fiducial marker system for calculating positional changes in a subject;

FIG. 2 is a transverse planar view along the axis A-A′ of FIG. 1 ; and

FIG. 3 is a schematic block diagram of procedural steps according to the present invention.

The present invention has utility in compensating for subject movement during the course of a medical procedure. The present invention achieves a superior movement compensation scheme through the application of passive radiofrequency identification tags (RFIDs). Through the implantation or skin adherence of multiple fiducial RFID markers, the movement of a subject is measured with shorter time intervals as compared to the prior art, or even continuously. As a result, radiotherapy treatments are delivered with greater efficacy and less incidental exposure of surrounding tissues.

At least one RFID fiducial marker is implanted or otherwise secured in a fixed positional relationship relative to a target. As the present invention monitors target position through triangulation, the present invention in its simplest embodiment implants a passive RFID tag in proximity to the target and monitors the position of the RFID tag and therefore the target through triangulation signal intercept by two active radiofrequency readers. It is appreciated that while two readers monitoring a single RFD tag affords displacement information in a plane defined by the two readers and the RFID tag, the inclusion of a third reader outside of this plane affords three-dimensional displacement information. While the theory and the practice of distance monitoring between a passive RFID tag and an RFID reader are well known, in brief, with a known interrogation frequency emitted from the reader, the time after which a return signal is received from the RFID tag is directly proportional to the distance therebetween.

Alternatively, an inventive system implants or otherwise secures multiple passive RFID tags to a subject as fiducial markers in the general area of the target. A multi-channel reader or multiple readers of single or multi-channel design calculates the relative position of the constellation of RFID fiducial markers and through vector geometry, the position of the target. Preferably, multiple markers define a triangle about a target tissue. More preferably, four or more markers bound a target tissue to define a tetrahedron. It is appreciated that target positional correlation relative to a fiducial marker is facilitated by calculating movement ranges associated with extreme positions associated with a respiratory cycle.

Referring now to FIGS. 1 and 2 , the inventive system is depicted generally at 10 . A system 10 has at least one passive RFID fiducial marker 12 positioned relative to a target mass T while a subject rests on a treatment platform 13 . It is appreciated that a marker 12 placed adjacent to, or within a target T, is sufficient to measure the position of target T in three dimensions with at least two readers 14 and 16 exterior to the subject. Fiducial markers 18 according to the present invention optionally decorate an elastomeric belt 20 adapted to secure about the thoracic or abdominal cavity of an individual. Through a simultaneous reading of the movement of fiducial markers 18 decorating the belt 20 , it is possible to accurately predict whether the chest cavity of a subject will continue to expand or deflate based on measurement of one or more previous breathing cycles. Additionally, in instances where a radioactive seed is threaded by a catheter into an individual, optionally, an inventive RFID fiducial marker 21 is secured proximal to the seed or catheter tip.

Optionally, upon completion of at least one full respiratory cycle, predictive coordinate data is fed to the radiation source based on the period and chest cavity expansion associated with a prior respiratory cycle so as to afford a regular motion of the radiation source. Such predictive positional data is updated by actual measured positional signal from the reader. The use of a belt 20 is appreciated to facilitate predictive movements based on the respiratory cycle.

Owing to the small dimensions of an RFID tag, typically on the order to tens to hundreds of microns, and the exogenous nature of the tag requires an inventive tag to be encapsulated with a biocompatible coating. Suitable coatings for an RFID fiducial tag illustratively include surgically implantable polymers, such as polyesters, such as nylon; fluoropolymers, such as perfluoroalkalenes; metals, such as titanium; and combinations thereof. It is appreciated that the exact placement of a given RFID fiducial tag is immaterial so long as the dynamic vectoral separations between a given marker and a target are known throughout the course of a given subject movement, such as a respiratory cycle.

Referring now to FIG. 3 , the inventive process of monitoring subject movement in the course of a medical procedure is depicted generally at 100 . The implantation or a fixation of multiple fiducial markers in a subject, an imaging scan 102 is collected to identify position of target T relative to the markers. Imaging scans are collected as MRI or CT scans. From the scans, vectoral distances between markers and a target mass are computed. The vectoral distances are stored in a digital memory and made available to a microprocessor 104 . With a subject placed on a treatment platform as depicted in FIG. 1 , a reader external to the subject is activated, the gain adjusted until a signal is received from fiducial marker 106 .

The microprocessor having access to the vectoral distances between markers and target receives an electronic signal from the reader distance measurements between each fiducial marker and a reader 108 . A baseline position is determined by comparing a reader signal to the vectoral distances stored in the digital memory 104 and serves to index the target position 110 . A radiation source is positioned relative to the target tissue through provision of coordinate data 112 from the microprocessor. Radiation sources are those known in the art and include synchrotrons, cathode ray tubes, and robotic arms carrying radioactive sources. Subsequent movement of the subject, whether the motion is translational, rotational or respirational, is then compensated for based on the relative movement of each fiducial marker and radiation source coordinate adjustments are fed to the radiation source 114 .

As an alternate to movement of a radiation source relative to a target, it is appreciated that the microprocessor instead controls a radiation source shutter, such that radiation is only emitted from the source onto the target upon coincidence between the target and a fixed radiation source.

By way of example, compared to a fixed reader, a parallel distance displacement of all responding fiducial markers is indicative of subject translation, such as that associated with sliding onto a treatment platform. Motion of distant markers towards a reader, as depicted in FIGS. 1 and 2 , towards the fixed reader while more proximal fiducial markers become more distant by an approximately equal amount is indicative of subject rotation. Movement of an abdominal or thoracically mounted fiducial marker relative to the lumbar of fixed marker is associated with respiration. It is appreciated that actual data collected from a reader, while predominantly corresponding to a respiratory mode, often has rotational or translational mode components, especially as a radiation session continues and subject muscles are required to retain a particular position begin to fatigue. As a radiofrequency reader is capable of collecting thousands of data points relating to the position of each fiducial marker per second, and matrix algebra to compute the position of a target can occur on a gigahertz time scale, effective real-time translation of a radiation source is provided to maintain exposure cross-section of the target.

Patents and publications mentioned in the specification are indicative of the levels of those skilled in the art to which the invention pertains. These patents and publications are incorporated herein by reference to the same extent as if each individual patent or publication was explicitly and individually incorporated herein by reference.

One skilled in the art will readily appreciate that the present invention is described herein with specific examples representative of preferred embodiments. Changes with respect to the present invention and other uses therefor will occur to those skilled in the art. These changes are encompassed within the spirit of the invention as defined by the scope of the appended claims.