Based on an anatomical similarity of each human being, approximative placement of electrodes could be considered. Locations of electrodes are defined in respect to geometrical relations between anatomical features of human body, while proportional differences between specific individuals are taken into account (4, 12, 26, 44–48). The same approach is used also for other standard diagnostic methods, for example, electrocardiography or electroencephalography. The approach could be considered as the fastest and with potential appropriate use in clinical practice as it does not have any other technical requirements and can be done without attendance of physician. An example of anatomical approximation layout is shown in Figure 4.

The stomach location could be also identified using radiography methods which may consist of ingestion or injection of contrast substance such as iodine or barium meal. An x-ray image showing exact position of the stomach in the abdominal cavity is the result of afterward X-ray examination (49, 50). The method is very precise and provides the location of the stomach of an individual subject, especially if taken shortly before actual measurement. However, the subject must undergo another examination, and moreover, he or she must be in contact with ionizing radiation, therefore it is not very suitable for use in clinical practice. An example of an X-ray image is shown in Figure 5.

Other possible approach could be usage of ultrasound. Ultrasonography is a fast and non-invasive method which allows specification of precise stomach location right on the spot just before the measurement is done (52–56). In comparison with the radiography, the ultrasonography is much safer and more comfortable for examined subjects. The example of an ultrasound image of the stomach is shown in Figure 6, where the stomach is visible as dark area in the middle of the image which is taken from transversal plane. The only problem is that a doctor or at least a well-trained technician is necessary to perform and evaluate such an exam, what drastically increases costs in human resources.

There are also alternative methods such as CT or MRI, which can accurately locate the stomach. However, in comparison to the previously described methods, they demand significantly more time, effort, and financial resources, making them economically inefficient (59–61).

Historically, zinc electrodes coated with zinc sulfate (Zn/ZnS) were used for measurement of electrical activity of the stomach (1, 3). Technological advancements and inventions of new materials resulted in discovery of silver electrodes coated with silver chloride (Ag/AgCl), and nowadays, this type of electrodes is considered as primary electrodes for the measurement of bioelectric signals. The Ag/AgCl electrodes are also used for the measurement of electrical activity of the stomach (11, 13, 47, 62–64). In most cases, the adhesive floating Ag/AgCl electrodes prefilled with conductive gel are used as they are cheap and easy to use.

Before the electrodes are attached, to increase reliability of electrode contact and reduction of skin-electrode junction impedance, the skin in location of electrode placement should be shaved to remove all hair and gently abraded by sandpaper, and in order to remove grease, it should be cleaned by alcohol at the end (65–68). Measurement should not start immediately after the attachment of electrodes because of the required adaptation of electrode to be fully polarized, which could cause an artifact in other cases (26).

Electrical connection of leads determines the quality and type of measured signals. Signals could be acquired in unipolar (monopolar) way when electrical potentials are measured between active electrode and reference electrode (47, 64, 69–71). Another way is bipolar acquirement when electrical potentials are measured between two active electrodes with or without use of reference electrode (52, 72–75). Example of one channel bipolar electrode layout with reference electrode is shown in Figure 4. The number of leads used for measurement varies from 1 to an unspecified high number of leads required for multichannel or matrix measurement.

Because of missing standard for measurement, neither of the standard measuring devices is available. There are several commercial systems (Figure 7) or amplifying modules for the acquirement of electrical activity of the stomach. Such systems are from companies such as Synectics Medical, 3CPM, Biopac (Figure 8), Plux, Alimetry, and others. Many authors used commercial systems to acquire gastric signals for their research. For measurement, devices that especially constructed to acquire electrogastrogram (46, 76–79), multipurpose polygraphs (54, 80), or general bioamplifiers (34, 81, 82) are used. Many other authors were also interested in the design and creation of custom systems for measurement of electrogastrogram (10, 11, 13, 83–89). A very important parameter of measurement device is its sampling rate. Sampling rate value may vary from 1 Hz as very sufficient sampling rate regarding the frequency characteristics of signal up to 500 Hz used for high-resolution measurements of stomach electrical activity. However, a higher sampling rate leads to the acquisition of a larger amount of noise component and therefore requires more complex pre-processing (71, 72, 74, 90).

The quality of EGG signals is paramount for accurate interpretation and subsequent clinical or research conclusions. Various studies have employed different methodologies to evaluate and compare the quality of EGG signals. A critical initial step in EGG studies is the acquisition and preprocessing of signals. The quality of EGG signals is heavily dependent on these processes. Authors often compare signal quality based on electrode placement, signal amplification, and filtering techniques. For instance, some studies emphasize the importance of standardized electrode placement on the abdomen to minimize noise and maximize signal integrity. Other researchers focus on the effectiveness of various filtering techniques, such as band-pass filters, to reduce artifacts. The presence of artifacts, such as motion artifacts and respiratory influences, can significantly degrade EGG signal quality. Comparative studies often assess the effectiveness of different noise reduction methods. Studies have mostly evaluated the efficacy of advanced signal processing techniques, such as independent component analysis (ICA) and wavelet transform, in enhancing signal quality (46, 47, 64, 77, 83, 90–93).

Quantitative analysis of EGG signals involves calculating various parameters, such as dominant frequency, power, and the percentage of normal slow waves. Authors compare these parameters to assess the quality and reliability of the EGG recordings. Ultimately, the quality of EGG signals is evaluated based on their clinical relevance and diagnostic utility. Studies often compare the sensitivity and specificity of EGG parameters in diagnosing gastrointestinal disorders. The comparison of EGG signal quality in scientific studies involves a multifaceted approach, considering signal acquisition, preprocessing, noise reduction, quantification, and clinical relevance. Despite the variability in methodologies, a growing consensus on best practices is emerging, which is driven by comparative analyses and reviews. Ensuring high-quality EGG signals is essential for accurate diagnosis and research in gastrointestinal motility disorders. The quality of the acquired signals can be also represented by the signal-to-noise ratio (SNR), which compares the level of the useful signal with its noise. However, this parameter is strongly dependent on the measurement conditions, such as ambient noise and electrode contact. Most authors did not report the actual value of the SNR, although they usually report it as poor. In experimental conditions, it is also used as the main parameter to determine the best location of the measuring electrodes (14, 55, 83, 90–93).

Measured subjects are selected in compliance with specific research; however, to perform statistical comparison, the control group of healthy (26, 94) and pathological group (25, 27, 95) of subjects is needed. Many authors were interested in measurement of electrical activity of the stomach in animals, especially in dogs (96–99), pigs (81, 84, 91, 100–104), and rabbits (105, 106). Measurement with human subjects was usually aimed to pathological cases of adults (107–109) or children (66, 75, 110, 111). Most observed pathology in reviewed articles was gastroparesis and pathologies that required partial or total gastrectomy, e.g., stomach cancer (18, 25, 27, 53, 95).

The procedure of measurement consisted of several steps which can include ingestion of specific meal. The electrical activity of stomach is usually measured in the fasting condition (preprandial activity) first and then in the fed condition (postprandial activity). Ingested meal is used as stimuli for stomach activity, and at the same time, it is an experimental intervention required to perform statistical analysis. Many authors do measurements in both conditions with previously defined circumstances and specifically defined experimental meals (65–67, 112, 113). Because of very low frequency of gastric electrical waves, the time of measurement varies from the order of tens of minutes (50, 75, 108, 114) to few hours (94, 114–116). The stomach has a delay after ingestion of experimental meal; therefore, there should be also a pause between measurement of preprandial and postprandial activity. There were also research studies aimed to full-day measurement of electrogastrogram (9, 48, 92, 117). An example measurement procedure consists of a 60-min preprandial measurement, followed by 10 min of food consumption, a 10-min response delay, and finally a 60-min postprandial measurement.

Experimental diet represents testing substance served to subjects as stimulus for the stomach. It could be yogurt (10, 68, 110), scrambled eggs (66, 79, 94, 118), or a type of pastry (10, 67, 119, 120). Liquids, such as water (10, 121, 122) or milk (115, 118, 123), were also examined in some research studies. However, no study defines what specific parameter of experimental diet is most significant, e.g., consistency or density. Examples of experimental diet are shown in Figure 9.