**3. Technical means of magnetocardiography and procedure of patients examination**

Measurement of ultra-weak magnetic fields that occur during the work of the human heart and are almost a million times smaller than the magnitude of the Earth's magnetic field (≈10<sup>4</sup> Tl), requires very sensitive equipment. A significant increase in the sensitivity of biomagnetic measurements was achieved with the introduction of SQUID magnetometers, which operate on the basis of the stationary Josephson effect at a temperature of liquid helium (4.2 K). Beginning in 1970, when the SQUID magnetometer was first used [6], the MCG registration procedure became available for medical research and clinical practice.

Each of the currently known MCG systems can be divided into three functional modules. The first module (measuring) contains a registration part, which consists of sensors, antenna systems, and electronics for reading sensor signals. The second module (control) includes electronic units and microprocessor control of the entire system. The third (software module) provides computer processing of signals and their display using an application package with a high level of intelligent software. Additional hardware and software for protection against magnetic interference are used in each module.

Our MCG-system CARDIOMOXMCG 9 is installed in an unshielded clinical setting and during normal daytime operation, environmental noise was relatively constant. During acquisition, power lines represent the most dominant source of high amplitude noise (**Figure 4A**).

The position of the examined object is idle on the back. The MCG detections are held inside a six-by-six rectangular nettings that create an area (4 cm<sup>2</sup> ) over the precordial area making nine prethoracic sites. The sensor is moved to the thorax as

*Unshielded Magnetocardiography in Clinical Practice: Detection of Myocardial Damage… DOI: http://dx.doi.org/10.5772/intechopen.104924*

**Figure 4.**

*Samples of the MCG signals. (A) Initial channel 21 output (power noise included), (B) Same channel: 50 Hz notch filtered data, (C) Same channel: averaged signal, DC offset corrected prior to the P-wave, (D) Averaged signal in at all 36 registration sites.*

close as possible, right above the heart, starting from the jugulum (**Figure 5**). All points are aligned with this reference point with the help of a rigid grid pitch.

Having the SQUID detector in an unmovable position over the adjustable table for examinations, the patient was moved to each designed grid position (9 in total) without leaving the idle lying state.

The above-described measuring grid is the most widely used. There are other types of grids, in some grids, each point is set taking into account its own anatomical landmark.

The registration records were taken to collect data from each site for 30 seconds with 1 kHz sampling frequency while a 0.1–120 bandpass filter was applied. At the same time, the surface ECG lead II was registered. All the obtained data was written onto memory devices for later processing. It took from 7 to 8 minutes to measure each intended location.

**Figure 5.**

*A typical grid applied for getting the records, the precordial area covered has the dimensions 20 cm\* 20 cm. The starting point for the sensor is just higher the jugulum.*

## **4. The role of Electrophysiology for magnetocardiography**

The ion current at cellular membranes in cardiac muscle cells defines their depolarisation and repolarisation. The latter also depends on single ions' temporally different permeability. Due to this, a shift arises in the membrane followed by changes in both the intra and extracellular volume currents. The spreading of these volume currents throughout the body causes the potential to alter the surface of the skin, so an electrocardiograph is able to detect the electrical potential changes once again. The nature and the functionality of the heart's specific cardiac conductivity system work in a way that it is electrically induced from the bottom state to the apex state. To apply the modeling for the heart's electrical activity, it can be substituted by a current dipole (also known as an equivalent dipole). Having the dipole with a distributed electrical field around it, the magnetic field should exist around it as well. The Biot-Savart approach helps to calculate the spatial dispersion induced by the dipole respectively. Here we define the magnetocardiogram as a recording of the frequent alternations in the magnetic field raising due to the cardiac cycle.

## **5. The MCG and the ECG key differences**

Of course, MCG has similar morphological properties likewise ECG: we set a P-wave, a QRS complex, and T- and U- waves. Temporal correspondence between them is also most similar to ECG [7]. The majority of MCG devices make the magnetic field components measurement in a perpendicular way (radial or z-component) to the anterior chest (Bz). The key difference between dimensional layouts for ECG and MCG is their spatial alignment by 90° (**Figure 6**).

MCG is more sensitive to currents tangential to the chest surface, whereas ECG is more sensitive to radial currents.

*Unshielded Magnetocardiography in Clinical Practice: Detection of Myocardial Damage… DOI: http://dx.doi.org/10.5772/intechopen.104924*

#### **Figure 6.**

*Dimensional correspondence of electrical and magnetic fields.*

Moreover, MCG is affected by isolated (vortex) current sources, which do not prevent any drops in potential on the body surface and thus the ECG is not able to detect them [8]. On the other hand, MCG is less influenced by conductivity alternations in the body (lungs, muscles, skin) than ECG. The MCG method does not invade the body at all, making the issues in the skin to electrode contact neglectable while being faced during the ECG. The switching in the ischemic diastolic TP and "true" ST switching is detected separately from each other by means of the direct-current MCG, due to the fact of missing potentials originating from the skin-electrode area.
