DOI 10.1007/s10527-015-9475-0 Biomedical Engineering, Vol. 48, No. 6, March, 2015, pp. 305309. Translated from Meditsinskaya Tekhnika, Vol. 48, No. 6, Nov.Dec., 2014, pp. 1923. Original article submitted October 17, 2014.
Magnetic Field Sensors in Medical Diagnostics L. P. Ichkitidze1*, N. A. Bazaev1, D. V. Telyshev1, R. Y. Preobrazhensky1, and M. L. Gavrushina2
Magnetic field sensors are considered promising in medical diagnostics. They are grouped into two types: type I – operating at room temperature; type II – requiring cryogenic cooling. It is noted that among type I sensors, laserpumped atomic magnetometers are suitable, and among type II – SQUIDs (Superconducting Quantum Interference Devices). Also particularly promising are combined sensors consisting of a superconducting film with a nanostructured active band serving as a magnetic field hub and a structure with magnetoresistance as a mag netically sensitive element.
Highly sensitive magnetic field sensors (MFS) are now used in many fields of human activity, e.g. electron ic compasses, archaeological research, space devices, and in medical diagnostic systems [1]. In the latter case, high sensitivity is required, as many organisms generate weak but measurable magnetic field B 10 nT. These biomagnetic signals could be static fields caused by direct currents or small magnetic particles in tissues and oscil lating electrical activity. MFS are used in many areas of medicine, such as clinical diagnosis [2], gastroenterolo gy [3], recognition of biomolecules [4], magnetocardio graphy (MCG) [5], magnetoencephalography (MEG) [6], etc. Two moststudied sources of biomagnetic signals in the human body are the brain and the heart. MEG and MCGsignals are caused by electrical currents flow ing in nerve cells of the brain and the heart muscles, respectively. Timing accuracy of these methods is very high (in the millisecond range), and the recording of sig nals in several spatial positions usually allows locating their sources. Biomagnetic fields generated by the brain and heart mainly lie in the range B from 10 pT to 1 fT. Modern medicine practice employs numerous pas sive (e.g. bone tissue substitutes) and active implants (cir culatory assist devices, artificial hearts, various stimu lants, etc.). Noninvasive inspection of their functional 1
National Research University of Electronic Technology MIET, Zelenograd, Moscow, Russia; Email:
[email protected] 2 Base Technologies Co, Zelenograd, Moscow, Russia. * To whom correspondence should be addressed.
characteristics, resources and other properties can be per formed by highly sensitive magnetic field sensors. This article discusses the most promising lowfre quency (1 kHz) MFS in the field of medical diagnos tics. MFS are systematized into two types: operating at room temperature – type I, and requiring cryogenic cooling – II type. It briefly describes physical principles of operation and possibilities of using the devices, as well as provides some numerical and experimental data: reso lution of magnetic field δB, resolution of magnetic flux δφ, energy resolution ε, dynamic measurement range Dr, etc.
Type I 1.1. Magnetoresistance – the property of materials that change their resistance when exposed to an external magnetic field. The most significant magnetoresistive effects are anisotropic magnetoresistance, giant magne toresistance (GMR), extraordinary magnetoresistance (EMR), and tunneling magnetoresistance. Numerous commercial MFS are based on these effects. Magneto resistive MFS can record magnetic fields in range from 1 mT to 1 nT. For example, they are used in gastroen terology when magnetoresistive MFS systems and mag netic field concentrators (MFC) track magnetic markers in the esophagus [3]. GMR effectbased sensors are used for identification of biomolecules [4]. Figure 1 shows a magnetoresistive MFS for studying DNA hybridization.
305
00063398/15/48060305 © 2015 Springer Science+Business Media New York
306
Ichkitidze et al.
Magnetic markers Target DNA
Magnetic moment of the marker
Focusing
DNAprobes Hybridization of DNAprobe and target
Passivation Substrate Structure of magnetic marker transport
Spintronic transducer
Fig. 1. Example of chemical identification of biomolecules using magnetoresistive MFS [4].
Magnetoresistive sensors are also used in clinical diagnostics. Sensors track the position of functionalized magnetic microgranules [6]. An EMRbased sensor is demonstrated whose main noise sources are thermal Johnson noises only [7]. These sensors are of size less than 50 × 50 μm2, their resolution is δφ ~ 10–6 φ0 and δl < 50 μm, where φ0 ≈ 2⋅10–15 Wb –
Distance between sensor center and skin of head
magnetic flux quantum. Selfnoise can be reduced to the level of Bn 1 pT/Hz1/2, which would allow using these sensors as MFS in magnetocardiography. 1.2. In the beginning of this century, production of laserpumped atomic magnetometers (LPAM) began; the volume of their working cell was 1 cm3. LPAM with cell volume of 4 × 19 × 40 mm3 containing potassium atoms
Number of sensors in an array: 306
< 4 mm Sensor head
Sensor price:
Sensor cell MEG Elekta Neuromag
Price of MEG Elekta Neuromag:
Cooling liquid consumption: 12 liters/day
SQUID array in MEG Elekta Neuromag
Fig. 2. LPAM sensor head location near human skull [4].
Fig. 3. MEG Elekta Neuromag. The helmet contains a SQUID array; the head of the patient is located inside the helmet [16].
Magnetic Field Sensors in Medical Diagnostics
307
TABLE 1. Some Parameters of Commercial SQUIDs Name
Tristan LSQ/20 [14]
Tristan HTM8 [14]
Cryo GA1165 [15]
Cryo M1000 [15]
