International Journal of Bioelectromagnetism Vol. 12, No. 2, pp. 81 - 84, 2010
A XY Magnetic Scanning Device for Magnetic Tracers: Preliminary Results Pacheco AHa, Cano MEb, Palomar-Lever Eb, Córdova-Fraga Ta, De la Roca JMc, Hernández-Sámano Aa, Felix-Medina Rd. a
División de Ciencias e Ingenierias de la Universidad de Guanajuato, León, Gto. México. Centro Universitario de la Ciénega de la Universidad de Guadalajara, Ocotlán, Jal. México c Asociación Cultural Nueva Acrópolis México, León, Guanajuato. México d Universidad Autonoma de Sinaloa, Culiacán, Sinaloa, México
Contact: Cano ME, Universidad de Guadalajara, Centro Universitario de la Ciénega, Av. Universidad edificio B #1115, col. Linda Vista, Ocotlán, Jalisco, México. E-mail: [email protected]
Abstract. This paper presents the maps of magnetic field obtained in phantoms magnetically marked, resulting from laboratory research. A mobile automatic two-way device was used, which was developed to detect changes in magnetic flux based on an array of magnetoresistive sensors designed to detect magnetic fields ranging from 100 mT to 10 nT. Experimental development stages and future applications for the device are discussed. Keywords: magnetic, magnetorresistive, scanning, tissue.
1. Introduction There are several devices in the market used to obtain medical images, commonly used in laboratories, to help medical doctors with clinical diagnostics. X-rays and radiographs devices are good examples. Their physical principle is based on the emission of X-rays through a material. Images using Nuclear Magnetic Resonance (NMR) is another example. Its physical principle is based on quantum mechanics and it is based on the detection of the radio waves emitted by atomic nucleus. They were previously aligned using an intense uniform magnetic field and small localized field gradients subsequently the nucleus are perturbed using radiofrequency waves. Ultrasound images, whose reconstruction were based on the detection of acoustic waves using small arrangements of piezoelectric transducers, which recorded the sound wave that bounces in the material medium (echo). There are other types of images based on the detection of X-ray, known as Computed Axial Tomography (CT scan), which is based on the detection of radiation absorbed by a material medium, similar to a radiograph. The main difference is that a CT scan, the detector makes a scan around a circle in the axial plane to the sample under study, while recording images. Some of the above techniques can be supplemented by substances that improve the quality of an image called contrast media. These substances can be radio-opaque elements such as barium or highly paramagnetic heavy elements such as gadolinium. However, there are other techniques to obtain medical images such as the Nuclear Medicine (NM) and Positron Emission Tomography (PET) using cameras for detection and short-lived radioactive isotopes. The purpose of this paper is to expound a novel device based on magnetoresistive sensors designed to obtain two-dimensional maps of the magnetic flux in phantoms prepared in laboratory using a mixture of Vaseline and magnetite (Fe3O2) as a magnetic tracer. This kind of magnetic tracer has been used in some biomagnetic studies developed in humans. [Córdova et al., 2004] and [Carneiro et al., 1999] carried out studies of gastric motility, for which it was necessary that the tracer was ingested by the volunteers. They underscore the fact that this substance is not absorbed by the body. This same type of iron oxide was also used for studies of magnetically cell induced hyperthermia as a tracer for tumour tissue. See examples [Bahadur et al., 2003; Hergt et al., 2006]. Some similar measurement devices that use transducers of the same nature have been developed, such as [Cano et al., 2005], who developed a magnetic 16-channel scanner for imaging magnetic 81
surfaces, and the one developed by [Leyva et al, 2006], which was used to acquire images of ferromagnetic tracers. In these devices (as opposed to ours) the array of sensors remains static while the sample moves in a plane above them. It is known that the spatial distribution of magnetic fields leads to a mathematical problem known as the inverse problem, which involves more than one solution to the distribution of dipoles resulting in the spatial form of field measured. There are different methods for solving the inverse problem which can be seen in references [Roth et al. 1991, Shaofen et al. 1996, Jukka et al. 1987, Bradshaw et al. 2001, Kanta et al. 1995, Jannetta et al. 2004]. The solution to this problem is beyond the reach of this paper.
2. Material and Methods The magnetic scanner requires three sensors. The magnetoresistive Honeywell HMC1001 was used in praxis. Each sensor consists of an array of three resistors and a ferromagnetic component in a Wheatstone bridge configuration. The sensor has an offset resistor and another one for set/reset purposes. The bridge generates a voltage difference, using the magnetoresistive effect, based on the fact that the material (permalloy) of the ferromagnetic component changes its resistance in response to the presence of an external magnetic field. Fig. 1 shows a block diagram of the XY scanner. A: Signals Conditioning. B: Power Supply for the Automatic XY Positioner. C: XY Positioner. D: Sensing plate. E: Acquisition Card. F: Laptop connected through a USB port. G: Phantom with magnetic marker.
Figure 1. Experimental Setup. Since the average sensitivity of the sensor is pretty low (3.2 mV / V / gauss), a voltage amplifier is needed. An instrumentation amplifier (AD620) followed by a passive low pass filter with a cut-off frequency at 10 Hz was used, followed by another amplification stage. Amplifier OPAM LT1028 was used in its non-inverting configuration. It is a low noise chip, having a gain of 10,000, which allows the sensor to capture differences in the magnetic field. The circuit used is shown in figure 2.
Figure 2. Diagram of the interface stage using the HMC1001 amplifier and sensors. The sensors were placed on a tablet mounted on an XY positioner, consisting of two perpendicular rails. Each rail is coupled to a unipolar step motor, which moves in minimal steps of one millimeter in either direction. The motors are connected to a power stage through transistors and optoisolators. Each motor is controlled using 4 digital signals. 82
Record acquisition and positioning of the sensors are carried out using a USB 2.0 port of a laptop computer. The latter is in turn connected to the analogue inputs and digital outputs of an analogue card. The National Instruments NI USB-6008 card was used for this purpose. It provides 12 bit resolution, and it was used with LabVIEW 8.2.1. The phantoms were fabricated using 0.5 grams of magnetite powder homogeneously diluted in 50 ml of Vaseline. A portion of this mixture was placed in clay figures made with some form defined. Fig. 3 depicts one. Phantoms were magnetized using an array of Helmholtz coils, which have the capacity to generate pulses of a magnetic field of about 30 mT. This methodology was used by [Cano et al., 2005; Leyva et al. 2006].
Figure 3. Laboratory obtained Phantoms. The procedure to obtain an image is the following: place the sample on the sensors and scan it, defining a rectangular maximum area of 10 cm x 20 cm. Do breaks of 10 ms to store the records of magnetic field in a computer file. The records obtained after a complete sweep are normalised and plotted using 255 shades of gray. The stages of the electronics used are shown in the figure 4.
Figure 4. Electronic stages used in the XY scanning device.
3. Results In figs. 5, (a) y (b) show the unprocessed images of the magnetic field maps of the phantom such as the Fig. 3.
Figures 5 (a) and (b). Magnetic field map of the phantom with the shape of a kid in 3D and 2D respectively. 83
4. Conclusions. This novel technique for mapping magnetic field using magnetic tracers is presented. Twodimensonal maps of objects using magnetic field tracers can be obtained. Good consistency is observed between the form of the phantom and the map measured. This is the starting point to obtain magnetic images of non-invasive of magnetic tracers in marked tissues, which lead to two immediate problems: to solve the inverse problem and to bring the magnetic marker functionalized into the region of interest through the bloodstream. In this point we requires our marker to be biocompatible, so that, when in contact with biological tissue does not cause adverse reactions that can alter the physiology of the cells surrounding, at the same time that their magnetic properties are not altered drastically. However adding one more degree of freedom and image processing (similar at CT scan), it will be possible to obtain three-dimensional images and to determinate tissue volumes. Acknowledgements. This work is supported by PROMEP_103.5/08/4722, SNI-ESTUDIANTES-2008-01 and CONACYT J50182. The authors wish to thank Noriega JM and Martinez R for their help. References Córdova-Fraga T, Bernal-Alvarado JJ, Gutierrez-Juarez G, Sosa M, Vargas-Luna M. Gastric activity studies using a magnetic tracer, Physiol. Meas., 25, 1261-1270, 2004. Carneiro A., Baffa O., and Oliveira R. B., Study of stomach motility using the relaxation of magnetic tracers, Phys. Med. Biol. 44: 1691–1697, 1999. Bahadur D, Jyotsnendu Giri. Biomaterials and magnetism, Sadhana 28: 3-4: 639–656, 2003. Hergt Rudolf, Dutz Silvio, Muller Robert and Zeisberger Matthigas. Magnetic particle hyperthermia: nanoparticle magnetism and materials development for cancer therapy. J. Phys.: Condens. Matter 18: S2919–S2934, 2006. Cano ME, Martínez JC, Bernal-Alvarado J, Sosa M, Córdova T. 16-channel magnetoresistive scanner for magnetic surface imaging Review of Scientific Instruments 76: 086106-1 - 086106-3, 2005. Leyva Juan A, Carneiro Antonio AO, Murta Luís O and Baffa O. Imaging Ferromagnetic Tracers with a Magnetoresistive Sensors Array. AIP Conf. Proc., 854: 167-169, 2006. Roth JR, Sepulveda NG and Wikswo JP. Using a magnetometer to image a two-dimensional current distribution. J. Appl. Phys., 65(1): 361 (1989).Hillery AD and Chin RT. Iterative Wiener filters for image restoration. IEEE Trans. Signal Processing, 39: 1892-1899, 1991. Shaofen Tan, Yu Pei Ma, Ian M. Thomas and Wikswo JP. Reconstruction of tow-dimensional magnetization and susceptibility distributions from the magnetic field pf Soft magnetic materials. IEEE Transactions on Magnetics 32(1): 230-235, 1996. Jukka Sarvas. Basic mathematical and electromagnetic concepts of the Biomagnetic inverse problem. Phys. Med. Biol. 32(1): 11-22, 1987. Bradshaw L. A., Wijesinghe R. S., and. Wikswo J. P, JR. Spatial Filter Approach for Comparison of the Forward and Inverse Problems of Electroencephalography and Magnetoencephalography. Annals of Biomedical Engineering, (29): 214–226, 2001. Kanta Matsuura, Yoichi Ocabel. Selective minimum-norm solution of the Biomagnetic inverse problem. IEEE Transactions on Biomedical Engieneering. 42(6): 608-615, 1995. Jannetta A et al. Mammography image restoration using maximum entropy deconvolution. Phys. Med. Biol. 49: 4997–5010, 2004.