New Developments in Medical Imaging Technology—Multi-Physics Coupled Imaging

Medical imaging technology has become an important research area in clinical diagnosis, treatment, and medical research. A wide variety of medical imaging equipment has emerged, leading to a qualitative leap in the level of disease diagnosis. In recent decades, the main medical imaging equipment used in hospital clinical applications has been X-ray Computed Tomography (X-CT), Ultrasonic Tomography, and Magnetic Resonance Imaging (MRI). Their widespread application has played a universally recognized role in the diagnosis of patients' conditions. At the same time, they themselves are constantly developing and improving, with their performance and functions constantly being improved and enhanced, allowing medical images to develop rapidly from morphology to function, from static to dynamic, from planar to three-dimensional, and from local to global.

Things in the world are always dichotomous. No imaging technology is omnipotent. While each of the above widely used imaging technologies demonstrates its advantages, it also has certain defects and shortcomings. In particular, they are powerless against early lesions when tissue morphology has not yet undergone significant changes. In recent years, scientists have, through unremitting exploration and research, developed medical imaging methods that utilize the coupling of multiple physical fields such as electricity, magnetism, sound, and heat, which can compensate for the shortcomings of the above traditional imaging methods in diagnosing early lesions. This article discusses the development status and prospects of these medical imaging technologies.

There are two performance indicators for medical imaging diagnostic instruments: resolution and contrast.

The resolution of medical imaging instruments has two meanings:

(1) Performance indicator of the display screen: Refers to the number of pixels contained per unit length on the instrument's display screen. Its unit is usually pixels per inch (ppi). Resolution determines the fineness of image detail. Generally, the higher the image resolution, the more pixels it contains, and the clearer the image. For a screen with a resolution of 1024×768, each horizontal line contains 1024 pixels, with a total of 768 lines, i.e., 1024 columns and 768 rows.

(2) Ability to identify the minimum size of human tissue: Refers to the ability of electromagnetic waves or ultrasound used for imaging to identify the smallest unit of human tissue. The higher the resolution, the smaller the smallest unit identified, and the higher the ability to identify human tissue diseases. The actual resolution of the instrument depends on the wavelength of the high-frequency electromagnetic wave or ultrasound; the size of the smallest unit identified is equal to 1/2 of the wavelength used. Based on the wavelength of the electromagnetic wave used by the medical imaging instrument, the high or low ability to identify human tissue can be determined.

The resolution of several commonly used medical imaging diagnostic instruments is as follows:

X-ray Imaging Instrument: The frequency range of X-rays is 3×10^16 to 3×10^20 Hz, and the wavelength range is 10nm to 0.001nm. The maximum wavelength of the X-ray imaging instrument is 10nm, so the largest minimum unit of human tissue identified is 5nm, and its resolution is quite high.

Ultrasound Imaging Diagnostic Instrument: The resolution of an ultrasound diagnostic instrument depends on the wavelength of the ultrasound used. The frequency of medical ultrasound diagnostic instruments is mostly 1.25MHz to 20MHz, with a wavelength range of 1.2mm to 0.075mm. Its minimum resolution unit is 0.075mm/2=0.00375mm=3.75μm, so its resolution is also quite high. From the perspective of imaging resolution, ultrasound is extremely suitable for medical imaging technology.

Magnetic Resonance Imaging Instrument (MRI): The resolution of magnetic resonance imaging mainly depends on the gradient of the external magnetic field used. Different magnetic field gradients have different resolutions. However, the magnetic resonance imaging widely used in hospitals currently has a certain range in magnetic field gradient design, so its resolution is mostly maintained at around 1 millimeter. In addition, the scanning field of view of the patient's diseased area in magnetic resonance imaging is also one of the factors related to resolution. Under the condition that the external magnetic field gradient remains unchanged, the smaller the scanning field of view, the higher the resolution. For example, when the scanning field of view is 16cm*16cm and the resolution is 1mm, when the scanning field of view is 8cm*8cm, the resolution can be increased to 0.5mm.

Contrast refers to the ratio of the brightness of the brightest (white) and darkest (black) points on the screen. Contrast is the degree of contrast between colors. The higher the contrast, the more obvious the difference between the colors, especially the more obvious the contrast between black and white.

The contrast of medical imaging instruments also has two meanings:

(1)Performance indicator of the display screen: That is, the display screen's ability to display images. It represents the ability to distinguish and display different colors, especially the difference in brightness between the brightest white and darkest black in the light and dark areas, and whether the expected value can be achieved.

(2)Ability to identify the color of human tissue: Different tissues inside the human body have different colors, and the colors of normal and diseased tissues will also be different. The detection substances of imaging equipment with high contrast will have a high ability to identify different colors of human tissues, which will then be displayed on the display screen of the imaging equipment. If the contrast is low, the diseased tissue and normal tissue will be mixed together and difficult to distinguish. The higher the contrast, the easier it is to identify the mixed diseased and normal tissues.

Among several medical imaging diagnostic instruments widely used in hospitals, X-ray fluoroscopy and X-CT have relatively high contrast, ultrasound imaging diagnostic instruments have relatively low contrast, and magnetic resonance imaging instruments have relatively high contrast.

Their common disadvantage is that they all belong to morphological anatomical imaging techniques and can only be diagnosed after morphological changes occur in human tissues and organs. They cannot perform early diagnosis of patients' conditions.

Pathology shows that the lesions of human tissues and organs, their functional lesions precede morphological lesions. In order to change this status quo, enabling medical imaging instruments to perform early diagnosis of human diseases and create conditions for early treatment, imaging technologies with functional imaging characteristics have entered the research field of scientists.

Electrical impedance imaging technology was the first functional imaging technology to Enter people's field of vision. The electrical characteristics (electrical impedance or conductivity) of human tissues are closely related to the structure, function, physiology, and pathology of the tissues. Different tissues and organs have different impedance characteristics. More importantly, the electrical impedance characteristics of some tissues and organs change with their functional state. Electrical impedance tomography (EIT) is a medical imaging technology that uses the distribution of electrical resistance (conductivity) inside the human body as the imaging target. Its basic principle is: an electrode array is arranged on the surface of the human body, current (or voltage) is applied to the human body through the electrodes, and the voltage (or current) on the body surface is measured. Through a certain algorithm, the image of the internal tissue impedance distribution of the human body can be reconstructed.

Since a professor at the University of Wisconsin in the United States proposed the concept of electrical impedance imaging in the 1970s, many universities in the United States have carried out research in this area and established corresponding physical model experimental systems. The United Kingdom and Belgium have also carried out research in this area. In China, the Fourth Military Medical University, Chongqing University, and the Chinese Academy of Medical Sciences also started research in this area in the 1990s. Among them, the Fourth Military Medical University has done a better job and has made experimental prototypes for clinical trials in hospitals.

The advantages of electrical impedance imaging are that it can perform functional imaging of tissues or organs inside the human body for early diagnosis of diseases; it has no ionizing radiation, low equipment cost, small size, and easy operation. However, this technology also has obvious shortcomings: due to the limited number of available electrodes, the resolution is low, and the ability to identify human diseases is low; because current always needs to be injected into the human body through electrodes during imaging, if not controlled well, there are also safety hazards. These shortcomings and deficiencies constitute an important obstacle to the practical application of EIT imaging technology.

Electrical impedance imaging has the ability to perform early diagnosis of human diseases, but its low resolution makes it difficult to be practical. In view of these problems, combined with the advantages of high resolution of ultrasound imaging, scientists have proposed magnetic acoustic imaging technology that combines electromagnetic fields and ultrasonic fields.

The rapid development of sensor technology, array measurement technology, and information processing technology has driven the development of weak signal detection technology. The development of modern computer technology has also made multi-field coupling analysis and rapid solution of inverse problems possible, which has led to the rapid development of multi-physical field imaging technology. A series of medical multi-physical field coupling imaging methods have been developed, such as microwave thermoacoustic imaging, magnetic acoustic imaging, and magneto-thermoacoustic imaging.
 

Microwave thermoacoustic imaging combines the advantages of ultrasound imaging and microwave imaging. Scientists proposed the concept of microwave imaging in 1978. It has the advantages of high contrast and safe non-destructive imaging, but due to its relatively high technical difficulty, mainly reflected in three aspects: multi-channel detection of microwave signals, shallow penetration depth, and low spatial resolution, it is difficult to be practical.

Microwave-stimulated biotissue thermoacoustic imaging (i.e., microwave thermoacoustic imaging) technology is based on a new principle—the microwave-stimulated thermoacoustic effect. When a biological body is subjected to microwave radiation, part of the microwave energy is rapidly absorbed by the tissue and converted into heat, causing the internal temperature of the tissue to rise and forming a temperature gradient relative to the tissue surface. Because the electromagnetic wave propagation speed is much greater than the sound wave propagation speed, the thermal expansion caused by microwave irradiation occurs instantaneously, causing the biological tissue to produce strain, thus generating outward propagating thermoacoustic waves. The excited thermoacoustic waves carry information about the microwave absorption characteristics of the irradiated tissue. This thermoacoustic signal is closely related to the contrast of the thermoacoustic wave's effect on biological tissues, and it can truly reflect a wide range of electromagnetic wave frequencies. Ultrasound sensors detect these thermoacoustic signals, which are quantified and collected by the computer's data acquisition system. Imaging algorithms are used to process the data to reconstruct the image of the biological tissue.

Although microwave thermoacoustic imaging has achieved the complementary advantages of high contrast and high resolution of microwave imaging and ultrasound imaging, the disadvantage of shallow penetration depth still cannot be eliminated. Because microwaves are electromagnetic waves above 300 MHz, they have a strong thermal effect on biological tissues, and the penetration depth of biological tissues with high water content is less than 3.2 cm, which makes it impossible to image deep tissues. This disadvantage also makes it difficult to be practical.

Inductive magnetoacoustic imaging technology was first proposed by Bin He, Yuan Xu, et al. at the University of Minnesota in 2005. It is a new type of imaging technology that combines traditional electrical impedance imaging technology, magnetic induction imaging technology, and ultrasound imaging technology. It uses conductivity as the imaging parameter and has the advantages of good contrast and high resolution. According to the detection method, it can also be divided intoUltrasonic Detection TypeandElectromagnetic Detection Typetwo categories:

Ultrasonic Detection TypeThe principle of magnetoacoustic imaging is shown in Figure 1. An alternating electromagnetic excitation is applied to an object placed in a static magnetic field, and an alternating current is injected into the object using electrodes, or a current is induced inside the object using a coil. The injected or induced current interacts with the static magnetic field to produce a Lorentz force, forming a sound source in the object and radiating ultrasound outward. The ultrasound signals are measured by several transducers around the object, and the sound source image is reconstructed using the time reversal algorithm. Further, the conductivity image of the object is reconstructed using the electromagnetic field inverse problem algorithm.

Electromagnetic Detection TypeThe principle of magnetoacoustic imaging is shown in Figure 2. An ultrasonic excitation is applied to an object placed in a static magnetic field, and several transducers around the object emit ultrasound in turn. The ions inside the object vibrate with the propagation of ultrasound. Under the action of the static magnetic field, they are subjected to the Lorentz force and positive and negative charges are separated, forming a local motional electromotive force, generating a current distribution in the object. The electrical signals are measured through electrodes or coils, and the conductivity image of the object is reconstructed using the electromagnetic field inverse problem algorithm.

From the imaging principle, the two types of imaging are anti-modes. The former uses a static magnetic field and an alternating electromagnetic field to generate ultrasound signals, while the latter uses a static magnetic field and ultrasound to generate electrical signals. From the perspective of energy conversion, the former converts electrical energy into mechanical energy, which can be regarded as a generalized electric motor, while the latter converts mechanical energy into electrical energy, which can be regarded as a generalized generator.

Magnetoacoustic imaging is an intersection of acoustics and electromagnetics in the field of imaging. As an emerging detection technology, for biological media, which can be approximated as low-conductivity fluids or quasi-fluids, it involves the conductive and field coupling characteristics of the medium. The acoustic field and electromagnetic field are weakly coupled, and the fluid properties and conductive characteristics of the medium determine the acoustic field propagation characteristics and electromagnetic field distribution characteristics, respectively.

Magnetoacoustic imaging has broad application prospects in the biomedical field. Ultrasound-based magnetoacoustic imaging can obtain high-resolution conductivity images of diseased tissues, enabling early diagnosis of diseases. Electromagnetic detection-based magnetoacoustic imaging can be combined with ultrasound elastography to form a dual-modality imaging method that simultaneously acquires the viscoelastic coefficients and conductivity parameters of biological tissues.

The main advantages of magnetoacoustic imaging are: it retains the functional imaging capabilities of electrical impedance imaging, enabling early diagnosis of human diseases; it retains the high resolution of ultrasound imaging and the high contrast of electrical impedance imaging, resulting in high identification ability for human diseases; it uses an inductive method, which is relatively safe; compared to magnetic resonance instruments, it has low equipment cost, small size, and easy operation.

Magnetoacoustic imaging technology is currently still under development and cannot be clinically applied.

After Bin He, Yuan Xu, and others at the University of Minnesota proposed inductive magnetoacoustic imaging technology in 2005, in 2006, Researcher Liu Guoqiang of the Institute of Electrical Engineering, Chinese Academy of Sciences, began research on this technology. Over the past ten years, with the support of the National Natural Science Foundation of China (including one key project and one instrument project), significant progress has been made in the technology, and it has reached an internationally advanced level. Currently, research is focused on the practical application of early diagnosis technology for human diseases. In addition, the Institute of Medical Engineering of the Chinese Academy of Medical Sciences and Nanjing Normal University have also conducted research, obtaining a considerable amount of experimental data, which has driven the development of this technology.

In order to solve the shortcomings of shallow penetration depth and impracticality of microwave photoacoustic imaging, the Zheng research group at Nanyang Technological University in Singapore proposed an inductive photoacoustic imaging method in 2013. This method applies a radio frequency electromagnetic field below 20 MHz to high-conductivity metallic copper, studying the feasibility of imaging high-conductivity targets using this method. The principle diagram of this method is shown in Figure 3. It is also a type of electrical impedance imaging method based on the photoacoustic effect. The photoacoustic effect utilizes electromagnetic wave radiation on biological tissues. The biological tissues absorb electromagnetic energy and generate thermal expansion, and ultrasound signals are generated along with the thermal expansion. The intensity of the ultrasound signal depends on the amount of electromagnetic energy absorbed by the biological tissue. Due to the difference in electromagnetic energy absorption characteristics between normal tissues and diseased tissues, the intensity of the generated ultrasound signals is also different, therefore it can be used to distinguish normal tissues from diseased tissues.

Figure 3 Principle of Inductive Photoacoustic Imaging

In 2014, the Engineering Electromagnetic Field and Its Application Research Department of the Institute of Electrical Engineering, Chinese Academy of Sciences, began research on the inductive photoacoustic imaging method. After researching the principle of the method, they built the experimental device shown in the experimental principle diagram (Figure 5) and conducted a series of experiments. Using a pulse excitation source below 20 MHz, experiments were conducted on biological targets close to the human body. Experimental results showed that the penetration depth of the experiment on biological tissue was significantly deeper than microwave photoacoustic imaging, reaching 15 cm. This experiment demonstrated the feasibility of early diagnosis and detection of lesions in deep biological tissues (such as lungs and liver). Magneto-photoacoustic imaging technology integrates photoacoustic imaging technology and electrical impedance imaging technology, combining the advantages of high resolution of photoacoustic imaging and high contrast of electrical impedance imaging. Therefore, once magneto-photoacoustic imaging is put into practical use in the field of medical imaging, its application prospects are excellent.

Figure 4 Experimental Principle Diagram of Magneto-photoacoustic Imaging

Figure 5 Experimental Photo of Magneto-photoacoustic Imaging

Medical multi-physical field coupling imaging technology is a new type of medical imaging technology developed in recent decades. It has the ability of functional imaging and makes up for the shortcomings of traditional morphological anatomical imaging technologies such as X-ray imaging, ultrasound imaging, and magnetic resonance imaging, helping in the early diagnosis of human diseases. Inductive magnetoacoustic imaging technology and photoacoustic imaging technology have good development prospects. Once these two new technologies are put into practical use, they can be used for early diagnosis of human diseases, providing a reliable basis for early treatment of human diseases and bringing great benefits to human health.

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