Microwave Technique Based Noninvasive Monitoring of Intracranial Pressure Using Realistic Phantom Models

The brain is one of the sovereign organs in the human body responsible for a variety of intricate biological processes that affect a person’s overall functioning and well-being. Thus monitoring the different phenomena associated with brain activity and functioning is of paramount importance to doctors and researchers working in the field of medicine and bioengineering [32]. The measurement of Intracranial Pressure (ICP) is one of these phenomena that can give deep insights into brain health and performance. Numerous neurological disorders such as swelling in the brain, intracranial hemorrhage, stroke, brain tumor, traumatic brain injury (TBI), and/or hydrocephalus have an impact on ICP [3, 18, 24, 30]. The human brain is surrounded by a rigid bone structure that maintains a constant pressure inside the skull by optimizing the volume of its contents. ICP is the pressure within the craniospinal compartment constituted of brain, blood, and Cerebrospinal Fluid (CSF) and is governed by the Monro-Kellie doctrine [25]. The mean ICP for human adults is in the range of 5–15 mmHg when the subject is lying down with face and body looking upwards [4].

The current methods for ICP measurement in clinical conditions are mainly invasive and involve inserting either an intraventricular catheter, micro transducer, external ventricular drain (EVD), or lumbar puncture inside the skull [6, 38]. These methods can produce fairly accurate ICP results but are bound by numerous limitations in terms of infections, malposition, time to set, and the requirement of precise neurosurgical expertise. To add to this, the ICP measuring device can cause hemorrhage of its own. Further, the invasive methods cannot be used for a prolonged time and are only suitable for surgical procedures in hospital settings [37]. The field of wearable brain monitoring technologies has seen a massive upsurge in clinical trials and research over the past two decades due to its ease of access and safety. In recent years, a lot of non-invasive methods have been proposed as a solution to the problems of invasive methods. A non-invasive device can eliminate the problems associated with invasive devices and is a suitable candidate for both clinical applications as well as prolonged monitoring of ICP outside hospital settings. The use of current 5G and 6G technologies in healthcare is also supporting the usage of non-invasive devices outside standard hospital settings. Understanding the trend of ICP values can benefit the diagnosis of less critical illnesses such as headaches, migraines, and sight issues, for which ICP readings are typically not considered required [38].

The non-invasive methods for ICP measurements include ultrasound time of flight technique [27, 28], Transcranial Doppler (TCD) ultrasonography [2, 34], otoacoustic emission [8], Magnetic Resonance Imaging (MRI) [11], Electroencephalography (EEG) [7], tympanic membrane displacement [9], acoustic methods [8, 21], optic nerve sheath ultrasonography [20], ophthalmodynamometry [22], optical coherence tomography of retina [35] and jugular vein measurement [36]. Table 1 gives an overview of different techniques for ICP measurements based on the type of sensors and phantom models. Apart from these methods, the microwave technique is also proposed in some of the studies for ICP measurement [1, 12, 13, 16, 26, 29]. When compared with other techniques, the use of microwaves in ICP measurement offers several advantageous features such as safety due to the use of a non-ionizing electromagnetic (EM) field, higher penetration depth compared to optical modalities, mobility of equipment due to low power small size transducers and transceivers, ease of application due to its non-invasive nature and the possibility to be used from a distance in wireless measurements from the bedside without requirements of moving the patient, fast signal acquisition, lower cost of equipment and usage, etc. These benefits make microwaves an ideal solution for the measurement of ICP.

Motivated by the advantages of non-invasive methods for ICP measurement, this paper presents a method for accurately monitoring ICP changes in the head using microwave technology. One unique feature of this work is the testing of the proposed microwave method on realistic head phantom models. Most of the related studies in the literature are based on very simplified phantom models that fail to resemble the actual hydrodynamics of the human head. In this regard, a realistic phantom model is developed in this paper which is more suitable to study the changes in ICP. Another important feature of this work is the comparison between different antenna configurations which aids in selecting the best suitable position for placement of antennas around the skull. Further, unlike most of the previous studies, the present work is not confined to the study of only S11 but also considers other S parameters. This provides a better insight into the relationship between the S parameters and changing ICP values. Measurements made on realistic phantoms and simulation results showcase that the proposed system can be utilized for accurate and efficient non-invasive measurement of ICP.

The rest of the paper is organized as follows: Sect. 2 presents the Material and Methods utilized in the study which constitutes the description of the brain phantom model and microwave method for ICP measurement. The results and discussion is presented in Sect. 3 and Sect. 4 holds the concluding remarks of the paper.

The block diagram depicting the setup utilized in this study for ICP measurements is shown in Fig. 1. The setup comprises two blocks i.e. the phantom system and the system for ICP measurement. The phantom system consists of a head phantom, an electromagnetic dosing pump, and a water container. The ICP measurement system consists of a Vector Network Analyzer (VNA) connected to two microstrip patch antennas using SMA connectors, two pressure sensors, a Data Acquisition (DAQ) System, and a personal computer. A detailed description of both sub-systems is presented in the following subsections.

The microwave technique-based setup explained in Sect. 2 is utilized for measuring ICP in different physiological conditions. One of the primary aims of this study is to test the feasibility of microwave techniques for ICP monitoring in realistic scenarios. The pressure sensors installed in the skull phantom and brain phantom are utilized to extract very precise real-time pressure values. These pressure values are utilized as a reference to verify the results obtained from the proposed microwave method. The relationship between variation in S parameters at different frequencies and ICP values is developed using numerous trials.

Figure 4(a) shows the S11 plot for the first configuration of the antenna (when both antennas are placed on the same side). The measurements are taken with increasing ICP values from 5 mmHg to 26 mmHg. The subfigures are provided in each graph to present a zoomed view of a particular frequency range of interest. It can be visible from the subfigures of Fig. 4(a) that change in ICP has a distinctive effect on the S11 parameter at some specific frequency bands. The results are shown for 3.6–3.8 GHz and 5.05–5.2 GHz. It can be observed that the S11 curves show a distinctive pattern, especially in the 5.05–5.2 GHz band with changing ICP. The S21 results for this antenna configuration are presented in Fig. 4(b). Similar to the case of S11, S21 also shows a cognitive trend at some frequency bands (4.9–5.4 GHz) with increasing values of ICP. The S22 results for microwave setup with both antennas on the same side and increasing values of pressure are shown in Fig. 4(c). An interesting observation from Fig. 4(c) is that the trend of S22 curves is similar to S11 and S21. This gives a better insight into the system response regarding changing ICP values especially when the difference in S parameters is small for minute changes in ICP.

In order to verify the trend and establish a thorough understanding of the relationship between S parameters and ICP values, the investigation carried out for the case described in Fig. 4 is repeated but with decreasing values of ICP. For this case, the balloon was initially filled to achieve the maximum value of ICP, and the pressure was then gradually reduced in small steps to study the trends of S parameters. The results of S parameters for the case of decreasing values of pressure when both antennas are placed on the same side of the skull are shown in Fig. 5. It can be visualized from Fig. 5(a) that the S11 parameters showcase a similar trend as observed during increasing values of ICP but not in the same frequency bands. In this case, a strong trend is visible for the frequency bands of 3.3–3.8 GHz and 4.35–4.75 GHz. Similarly, the results of S22 (dB) v/s frequency for this configuration are shown in Fig. 5(b).

Further, Fig. 6 shows the results of S parameters for another antenna configuration wherein the antennas are placed on opposite sides of the skull 13 cm apart. The S11 v/s frequency plot for this configuration is shown in Fig. 6(a). It can be visualized from Fig. 6(a) that the S11 parameter shows very subtle changes wrt. frequency. The reason for such a response is the penetration losses which become higher in this case due to signal transmission through the complete head phantom. Similar results for S21 and S22 are shown in Fig. 6(b) and (c). It is also observed that the S parameter curves show some outliers in the trend with change in ICP values which is again due to the higher signal losses as compared to earlier cases when antennas were placed on the same side of the skull.

A microwave-based method is proposed in this work for non-invasive monitoring of ICP. The proposed method is based on flexible small-sized antennas and a realistic phantom model. The method is tested for both the cases of increasing as well as decreasing ICP values. Two different antenna configurations have been evaluated and it is observed that placing the antennas on the same side of the skull produces more favorable results in terms of accurate tracking of ICP as compared to when the antennas are placed on opposite sides. The interrelation between the S-parameters and ICP values is visible from the results. However, further research is required to accurately translate the antenna coefficients to the corresponding ICP values. This work can be further extended by testing with different antenna setups to determine the frequency band and optimal distance between the antennas.