IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 4, NO. 4, AUGUST 2010 223 Magnetic Tracking System for Radiation Therapy Wing-Fai Loke, Student Member, IEEE, Tae-Young Choi, Member, IEEE, Teimour Maleki, Member, IEEE, Lech Papiez, Babak Ziaie, Senior Member, IEEE, and Byunghoo Jung, Member, IEEE Abstract—Intensity-modulated radiation therapy (IMRT) re- quires precise delivery of the prescribed dose of radiation to the target and surrounding tissue. Irradiation of moving body anatomy is possible only if stable, accurate, and reliable infor- mation about the moving body structures are provided in real time. This paper presents a magnetic position tracking system for radiation therapy. The proposed system uses only four transmit- ting coils and an implantable transponder. The four transmitting coils generate a magnetic field which is sensed and measured by a biaxial magnetoresistive sensor in the transponder in the tumor. The transponder transmits the information back to a computer to determine the position of the transponder allowing it to track the tumor in real time. The transmission of the information from the transponder to the computer can be wired or wireless. Measurements using a biaxial sensor agree well with the field strength calculated from the ideal equations. The translation from the measurement data to the 3-D location and orientation requires a numerical technique because the equations are in nonclosed forms. The algorithm of tracking is implemented using MATLAB. Each calculation of the position along the target trajectory takes 30 ms, which makes the proposed system suitable for real-time tracking of the transponder for radiation assessment and delivery. An error of less than 2 mm is achieved in the demonstration. Index Terms—Magnetic tracking, radiation therapy, tumor tracking. I. INTRODUCTION R ADIATION therapy is an effective therapeutic modality for combating malignant tumors using ionizing radiation to kill cancerous cells [2]. This technique can be divided into two categories depending on how it is administrated—internal beam therapy (brachytherapy) and external beam therapy. In brachytherapy, the radiation source is encapsulated and then im- planted near the cancerous cells to irradiate them [3]. On the other hand, an external source of radiation is required in external beam therapy to irradiate the cancerous cells. Intensity-mod- ulated radiation therapy (IMRT) is one of the most advanced treatment methods in external beam therapy. In IMRT, the radi- ation is broken into thousands of tiny pencil-thin beams using a multileaf collimator [4], [5]. These beams enter the body from various angles and focus on the cancerous cells. This allows a high dosage to focus on tumor and reduce damage to the sur- rounding healthy tissue. In order to maximize the amount of Manuscript received July 31, 2009; revised November 14, 2009. First pub- lished June 01, 2010; current version published July 28, 2010. This work was supported by NIH under Grant 1R21EB007256. This paper was recommended by Associate Editor E. Lam. W.-F. Loke, T.-Y. Choi, T. Maleki, B. Ziaie and B. Jung are with the School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907 USA (e-mail: jungb@purdue.edu). L. Papiez is with the School of Biomedical Sciences, UT Southwestern Med- ical School, Dallas, TX 75390 USA. Digital Object Identifier 10.1109/TBCAS.2010.2046737 radiation focused on the tumor and minimize the damage on the surrounding healthy tissue, knowledge of the position of the tumor is necessary during treatment. Respiration, circulation, and peristalsis in the patient during radiation therapy can cause movement and change the position of the tumor [6]. Hence, a real-time and accurate knowledge of the position of the tumor is necessary to ensure the optimum treatment outcome by deliv- ering an accurate amount of radiation to the body organs [1]. To collect real-time information on the position of the tumor, transponders can be implanted in the tumor and their vicinity. These transponders contain magnetic sensors which are used to measure the strength of the magnetic field generated by external transmitting coils located next to the patient. The position of the transponder can be found by mapping the field strength mea- sured by the magnetic sensor and the direction of the transmit- ting coils. Several tracking systems for radiation therapy have been pre- sented in various literature [7]–[9]. In [7], a field generator con- sisting of six differential coils is used to generate a magnetic field. Each differential coil is made up of two coils of opposite polarization. A uniaxial wired sensor coil, which is implanted into the tumor, senses the magnetic field and generates signals that are digitized into time-discrete numerical values for the dig- ital signal processor (DSP) to determine the position and orien- tation of the sensor. In [8], a transmitting array consisting of 64 uniaxial coils and a uniaxial receiving coil are used. All 64 coils in the transmitting array are sequentially activated at the initiation stage to compute the initial position of the receiving coil. During subsequent tracking stages, eight coils in the trans- mitting array are activated. In [9], an array of four source coils generates an oscillating field which induces a resonance in the coil in the implanted transponder. When the source coils are turned off, the transponder transmits a signal that is measured by receiver coils to establish the position of the transponder. With uniaxial sensing coils in these systems, the structure of the transponder can be simple and compact but only 1-D infor- mation can be obtained from the transponder. Hence, iterative calculations can take a long time, prohibiting real-time tracking with high measurement frequency. Furthermore, these methods employ a large number of transmitting or receiving coils which can make the system bulky and difficult to use in a clinical en- vironment. Other configurations include using a triaxial sensor with bi or triaxial transmitting coils [10], and biaxial sensor with triaxial transmitting coils [11]. In these configurations, it is pos- sible to obtain noniterative solutions. However, using the tri- axial sensor or transmitting coil is complicated and may be dif- ficult to implement because precise orthogonality between coils is required. In this paper, we present a tracking system based on a biaxial sensor and an external planar array of four transmitting coils 1932-4545/$26.00 © 2010 IEEE