## 4 Application
### Cardiac Magnetic Field and MCG
MCG can detect the position, size, and shape of the heart. An individual SAWS is compact in size, facilitating integration. Figure 9a illustrates an integrated MP-SAWS arranged in a square-array format. This array format strengthens the signal, reduces errors, and expands the detection range. In the sensing array, sensing units devoid of magnetostrictive materials are incorporated, serving to determine and sense the temperature. Differential processing is applied to the magnetic field signals from distinct sensing units, effectively eliminating the influence of factors such as heartbeats and other forms of stress. The employment of the control variable method enables the sensor to detect diverse physical quantities. By employing appropriate algorithms, a two-dimensional MCG can be constructed. In practical use, the MP-SAWS is attached to the heart, and the test data are wirelessly transmitted to a computer, with the results displayed as a time-magnetic field relationship curve. In clinical applications, the MP-SAWS combined with a fixed-value constant magnetic field generator can achieve better sensing results. According to the magnetic field sensing curve of MP-SAWS, the sensor exhibits higher sensitivity in areas with strong magnetic fields. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) recommends that human body parts should not be exposed to static magnetic fields above 400 mT in non-work situations [70]. Figure 9: MCG measurement using MP-SAWS. (**a**) Schematic diagram of the MCG measurement; (**b**) The curve between magnetic flux density and resonant frequency for the MCG measurement. Based on the existing magnetic field-sensing relationship curve, a constant magnetic field strength of 200 mT is selected as it meets the safety standards and significantly enhances the sensor's sensitivity. This is due to the mechanical relationship shown in Figure 8b, where the slope of the magnetic field is steeper at higher magnetic fields. The direction of the static magnetic field is chosen parallel to the direction of the measured cardiac magnetic field. Since the amplitude of the cardiac magnetic field is very small, only a small portion of the magnetic field sensing curve is required after adding the constant magnetic field, as depicted in Figure 9b. In simulation software, this can be achieved by setting an additional static magnetic field of 200 mT. The weak cardiac magnetic field and the static magnetic field superimpose and act on the sensor. The cardiac magnetic field is obtained by reading the resonant frequency change in the SAW device. In a clinical setting, an additional magnetic field-generating device is required to provide a 200 mT static magnetic field at the heart's location. Any additional interfering magnetic fields need to be shielded. In this range, the sensing curve is almost linear, reducing the complexity of the sensing relationship. For magnetic field measurements, the environmental temperature is set at 293.15 K, corresponding to the standard room temperature. The enhanced magnetic field sensitivity is 0.171 Hz/uT. To evaluate the effectiveness of the MP-SAWS for detecting the cardiac magnetic field, simulated cardiac magnetic field testing has been employed. According to reference [69], the trend of changes in a cardiac magnetic field time-magnetic field graph is similar to that of the time-electric field graph of an ECG. Using the MIT-BIH arrhythmia database, a 10 s ECG was selected, and a set of simulated cardiac magnetic field time-magnetic field data was generated by scaling the MCG data of the arrhythmia patient [71]. Additionally, using a program to simulate MCG, a set of 10 s MCG data was generated as a healthy person's cardiac magnetic field data [72]. In the simulation program, the two sets of simulated magnetic field data were input, and corresponding time-resonance frequency curves were output. The results for patients with arrhythmia are shown in Figure 10b, while those for healthy individuals are shown in Figure 10d.