Atb Magnetocardiographs

Advanced Technologies Biomagnetics (AtB) s.r.l.

The Italian company AtB (Advanced Technologies Biomagnetics) has developed MCG as well as MEG devices (www.atb-pro.com). The basic component of all magnetic sensors is a fully integrated planar SQUID magnetometer based on Niobium technology. The sensing area is a square of 8 mm side length. This SQUID element is the modular basis for a flexible construction of complex sensor systems, including arrays of triaxial vector magnetometers with an intrinsic noise level better than 7 f T/pHz at 10 Hz.

The noise cancellation strategy is based on a combination of cost-optimized shielded environment and real-time compensation (software gradiometers) using reference magnetometers measuring the environmental disturbances. The liquid helium dewars are built on the basis of monolithic structures of compound materials for maximization of safety and reliability; that is, it is composed of only one entire inner and one entire outer container made of fiberglass composite, which results in low-noise dewars. The distance at the dewar bottom [''warm'' (300 K) to

''cold'' (4.2 K)] is typically about 18 mm. The typical filling volume for MCG-dewars is 62 L, and the stationary evaporation rate is about 8 L per day. On this basis, the ARGOS family with several sensor array configurations has been developed, optimized for different applications as MCG, MEG or fMCG recordings. The systems operate in AtB-magnetically shielded rooms most with three layers of m-metal and one rf shield.

The ARGOS 50 MCG is a planar biomagnetometer designed especially for magnetocardiography, and contains 77 SQUID magnetometers. The sensor array (lowest level) consists of 55 SQUID sensors recording Bz (i.e., the magnetic field component perpendicular to the chest and the Earth's surface, respectively). They are located in a hexagonal grid with a grid constant of 3.2 cm covering a circular planar surface with a diameter of 23.0 cm. Some 7.0 cm above these is the corresponding second level, with 19 magnetometers acting as compensation coils. The third level, located 14.0 cm above the first one, contains a triplet of magnetometers oriented in x-, y-, and z-directions. Software gradiometers (first-order gradiometers with 7.0-cm or 14.0-cm baseline or second-order gradiometers) can be used digitally to reduce the quasi-homogeneous ambient noise (Erne et al., 1999).

The multichannel-vector-magnetocardiograph ARGOS 200 is a biomagnetometer specially designed for magnetocardiography and magnetoneurography (see Fig. 2.24a). The main idea of the ARGOS 200 is to form a triplet of SQUIDs to allow recording of the magnetic field vector (i.e., Bx, By and Bz). The triplets are distributed over four levels (Nowak et al., 2003). The lower level, the main measurement plane, is a planar sensor array consisting of 56 SQUID sensor triplets covering a circular planar surface with a diameter of 23.0 cm. The reference array consists of seven SQUID sensor triplets located in the second level in a plane which is positioned parallel to the measurement plane at a distance of 9.8 cm. The third or fourth level with one triplet, are located at distances of 19.6 cm and 25.4 cm, respectively, above the measurement plane. The center of the triplets in the third and fourth levels are located at the axis (measurement plane's position 0-0) of the insert. In total, 195 SQUID sensors are used. The position plot of a recorded vector MCG is shown in Figure 2.24 (b).

The ARGOS 150 is a helmet-shaped system for magnetoencephalography. The sensor array consists of 165 SQUID sensors located in 165 independent measurement sites, with an inter-sensor separation less than 3.0 cm for appropriate spatial sampling.

The ARGOS 500 is a helmet-shaped biovector magnetometer specially designed for magnetoencephalography (Pasquarelli et al., 2004a) (Fig. 2.25). The large vertical-oriented dewar is localized and fixed on the basement with a helmet-shaped cavity to accommodate the patient's head. The sensor array consists of 165 SQUID sensors triplets located in 165 independent measurement sites, with an inter-sensor separation of less than 3.0 cm (Pasquarelli et al., 2004b). The ARGOS 500/PL is a system derived from the ARGOS 500, but with standard, quasi-radial, nonvectorial sensor distribution. This device resembles the ARGOS 500.

The bioelectrical amplifiers are low-noise, high-impedance, differential-input bio-preamplifiers, modularly expandable between 32 and 128 channels. The maximum

Patient Vector Plots
Fig. 2.24. (a) Multichannel vector-magnetocardiograph ARGOS 200. (b) Position plot of a vector-MCG-recording by ARGOS 200.

number of electrical channels usable simultaneously with the magnetic recordings depends on the chosen configuration of magnetic sensor and data acquisition system. The data acquisition system is modularly expandable until 644 channels. The resolution of conversion is better then 18 bits. A fixed sampling rate can be chosen either by 8.4 kHz for a maximum of 330 channels or alternatively by 4.2 kHz for a maximum of 660 channels. DC-coupled or AC-coupling at hardware level and acquisition bandpass is determined via digital filters.

An overview of AtB-ARGOS systems, together with installation year and location, is provided in Table 2.5.

Magnetic Shielding Room Installation

Fig. 2.25. Argos 500 with patient bed inside a magnetically shielded room. 2.2.7.5

CardioMag Imaging™

Fig. 2.25. Argos 500 with patient bed inside a magnetically shielded room. 2.2.7.5

CardioMag Imaging™

CardioMag ImagingTM (CMI) (www.cardiomag.com) was incorporated in 1999. The company introduced a line of commercially available cardiac diagnostic devices called MagnetoCardioGraphs (e.g., a 9- or a 36-channel system) which record the electrical activity of the heart using SQUID-sensors with the capability of operating in unshielded environments. Therefore, a second-order gradiometer (diameter 2.0 cm) is the base antenna for these devices. An intrinsic noise level is obtained with 10 f T/\fHz between 1 and 10 Hz, which is normally superimposed by the clinical environmental disturbances. A total balancing of the measuring channels is achieved with less than 10~4 with an additional Electronic Noise Suppression System using three background environment reference channels. The nine-channel device covers an area of 8.0 x 8.0 cm2, and the 36-channel device an area of 20.0 x 20.0 cm2. The measurement system is installed in a dewar with a capacity of 13.5 L for liquid helium. The helium boil-off rate is less than 2.5 L per day.

An installed nine-channel CMI-MagnetoCardioGraph is shown in Figure 2.26 (a) in a clinical environment (Chest Pain Unit).

Having received regulatory approvals such as FDA, UL, CE Mark, and GMP certifications, the CMI-systems are now being used to collect clinical data in accordance with a number of diagnostic protocols aimed at rapid, real-time cardiac functional testing. Patients undergo a safe and silent, resting 6- to 10-minute non-invasive test procedure, during which time a series of cardiac signals are displayed and subsequently analyzed to determine the existence or absence of heart disease (Brazdeikis et al., 2002; Brisinda et al., 2003).

2.2.7 Commercial Biomagnetic Measurement Devices | 143 Table 2.5. Installation year and location of AtB-ARGOS systems.

Year No. of channels Affiliation Europe

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