Ultrasound imaging is a non-invasive imaging method that is used in many medical fields. From diagnosing cardiovascular diseases and examining fetal growth during pregnancy to evaluating tumors and examining joints, ultrasound is considered an extremely valuable tool for doctors; Especially since ultrasound does not use X-rays, it is a more suitable method for patients of any age group, including pregnant women.

During World War I, French scientists used ultrasonic waves to locate enemy submarines. The use of ultrasound waves was considered a great achievement at that time. In 1956, Professor Ian Donald (Glasgow Royal Maternity Hospital) was the first to use ultrasound for medical diagnosis and measurement of fetal head diameter.

This act was a turning point in the development of ultrasound as a practical tool in medical diagnosis, pregnancy care, and fetal health. Since then, ultrasound has become one of the most common and safest methods in diagnosing and monitoring the condition of the fetus during pregnancy. In ultrasound, we use ultrasound waves to image the internal organs of the body.

In this article, we will first get acquainted with the physics governing the ultrasound machine, then we will talk about imaging with it.

Let us explain this process with a simple example to better understand the propagation of sound waves in the environment. Suppose you have a speaker or a microphone and you are giving a speech to a crowd. When playing sound from the speaker, its diaphragm moves forward and backward. The vibrating movement of the diaphragm causes compression and condensation of air molecules near it. This means that the vibration of the diaphragm also vibrates the air molecules and this vibration will be transmitted to the nearby molecules.

As you can see in the image above, the air molecules near the speaker (sound source) oscillate in a sinusoidal fashion. As we said, the ultrasound machine takes pictures of the internal organs of the body with the help of ultrasound waves. Ultrasound waves are sound waves that oscillate at frequencies higher than 20,000 Hz; That is, higher than the range of frequencies that the human ear can detect (between 20 and 20,000 Hz). Some animals, such as owls, can naturally produce ultrasonic waves and use them for communication and echolocation.

Echolocation is a method that bats use to find their way and identify objects or other animals in the environment. By producing an ultrasonic wave, the bat sends it to its surroundings. After hitting different objects, the sent wave becomes an echo and reaches the bat. After receiving the reflected wave, the bat identifies the location and distance of the objects by analyzing the return time and sound intensity.

Suppose we have two different environments, such as air and water, and the ultrasonic wave, moving from air to water, hits their separating boundary. what happens A part of the wave is reflected from the boundary separating the two environments and a part of it enters the water after crossing the boundary. This is called partial reflection because only part of the radiated wave is reflected. Therefore, whenever the ultrasonic wave hits the boundary separating two environments, part of it is reflected and another part enters the second environment.

Now suppose we shine ultrasound waves on an object that has different boundaries, therefore, a part of the waves will return to us after hitting each boundary. By knowing the speed of sound waves and the duration of their reflection, we can get the distance of each border. This mode is similar to when a bat uses ultrasound waves to determine the distance between different objects. By collecting and examining all the reflected waves, we get information about the different boundaries inside the object and its structure.

The important question is what factors determine the speed of sound waves in the environment. The density of the environment is one of the factors that determine the speed of sound. The denser the environment, the slower the sound travels in it. Also, the more compressible the medium, the slower the speed of sound in that medium. In addition to density and compressibility, the hardness of the environment also affects the speed of sound movement in the environment; More difficulty, more speed.

Here we come to a paradox. At the beginning of this section, we mentioned that sound travels faster in a solid than in a gas or liquid. But solid density is higher than liquid and gas! We must pay attention to the fact that the effect of each factor on the speed of sound is not the same and the effect of density on speed is less than the effect of compressibility. For example, although the density of mercury is 13.5 times that of water, sound travels at about the same speed in water and mercury. The reason for this is due to the compressibility of water 13.4 times compared to mercury.

This is very helpful in ultrasound imaging. When imaging soft tissues inside the body, we assume that sound travels at a constant speed inside all body tissues (such as the liver and kidney); Of course, the speed of sound is different in human tissues. For example, its amount in breast tissue is about 1430 m2 and in muscles about 1647 m2. Despite these differences, ultrasound machines usually use a fixed speed (1540 m/s) to reconstruct the images.

So far, we have learned about the physics governing the ultrasound machine. Next, we will talk about imaging with this device.

In the ultrasound machine, we use a device called “Transducer” for imaging. A transducer is a device that converts energy from one form to another. In ultrasound, with the help of transducers, we send sound waves into the body and receive reflected waves from internal tissues. These returning waves are converted into images that doctors use to examine the organs and internal tissues of the body.

For imaging, the doctor places the transducer on the skin as shown in the image below. Next, the transducer receives electrical energy and converts it into sound energy. After reaching the tissues, a part of the sent sound wave is reflected from them and hits the transducer. In other words, after sending a sound wave (sound pulse) into the body, the ultrasound device waits for the return or echo of that wave from the body tissues.

Based on the time required for the reflected waves to reach the transducer, the device calculates the exact location of the reflected waves (distance or depth from where the wave comes) and creates a corresponding pixel in the image. By repeating this process continuously, a two-dimensional image of the cross-section of tissues is formed.

The device simply calculates the depth or distance. We assumed the speed of sound to be constant in different tissues. Therefore, by measuring the time of sending, reflecting a part of the sound wave from the tissue, and reaching it again to the transducer, we can easily obtain the distance from which the wave was reflected. According to the intensity of the wave reaching the transducer, we can determine the brightness of the pixel.

As we mentioned in the previous section, part of the ultrasound waves are reflected after reaching the boundary separating the two environments. This happens when this wave hits the internal tissues of the body. The texture border separates two different textures from each other. Ultrasound waves can reach fat after passing through muscle. We call the border between muscle and fat, tissue border.

When ultrasound waves hit the boundary between tissues, any of the following situations may occur:

• A part of the wave radiated to the boundary passes through it and the rest is reflected from the boundary.
• All the wave that hits the boundary is reflected from it in the direction of the radiated wave.
• The radiated wave is reflected from the boundary of two tissues in a mirror or non-mirror form.
• The wave radiated to the boundary of the tissue is scattered inside it. This state occurs when the constituent units of the tissue are smaller than the wavelength of the irradiated ultrasound. In this case, most of the wave energy radiated to the tissue is lost and a part of it is spread in all directions.

What determines the type of interaction of ultrasound waves at the tissue boundary is called “acoustic impedance with acoustic resistance” (Acoustic Impedance or Z). Acoustic impedance is an inherent property of every tissue and is obtained from the product of tissue density and the speed of sound movement inside that tissue. In simple words, acoustic impedance indicates the resistance of a material to the passage of sound waves through it.

Suppose we send the ultrasound pulse to a specific tissue inside the patient’s body. When the pulse reaches the boundary of the tissue, it encounters another tissue with a different acoustic impedance. The value of the acoustic impedance difference between the two tissues determines the value of the ultrasound wave passing through the boundary and the value of the reflected wave (echo). The table below shows the sound impedance of different environments.

As you can see in the table above, the amount of sound impedance in water and bone is significantly different from each other. If the acoustic impedance values ​​of two tissues are very close to each other (such as fat and water), most of the ultrasound wave will pass through the boundary of the two tissues, and a small amount of it will be reflected as an echo. On the other hand, if the acoustic impedance values ​​of two tissues are very different from each other (such as muscle and bone), most of the ultrasound wave will be reflected from the boundary of the two tissues and a small amount of it will pass through the boundary.

As we mentioned in the previous section, due to the difference in acoustic impedance between the two tissues, part of the ultrasound waves pass through the border and another part is reflected perpendicularly to the border, in a mirrored, regular, or irregular way. Vertical reflection occurs when the boundary of the tissue on the radiating ultrasound wave is perpendicular and the boundary is also large and flat. This condition can occur in the renal capsule. Due to the difference in the acoustic impedance of the fat around the kidney and the kidney tissue, some of the radiated ultrasound waves is reflected and reach the transducer.

In some cases, all the ultrasound waves radiated to the tissue are reflected from it. This case occurs in a case where the difference in acoustic impedance between the two environments is very large. For this reason, we cannot take pictures of the lungs with the help of ultrasound waves, because the air inside the lungs has a much lower acoustic impedance compared to the surrounding soft tissue.

In some cases, instead of vertical radiation, the ultrasonic wave hits a flat and large surface with a specific angle. In this case, regular or mirror reflection occurs as shown below. When a surface is smooth (such as a mirror or the surface of a bone), sound waves are reflected in a specific and regular direction instead of being scattered in different directions. This is similar to the reflection of light from a mirror, where the light is reflected directly at a certain angle and not diffusely.

In most cases in regular irradiation, the reflected ultrasound beam does not return to the transducer. Fortunately, most large surfaces inside the body are not perfectly flat and smooth and do not reflect ultrasound waves in a mirror-like fashion. As a result, irregular or non-specular reflection occurs most of the time. In this case, the reflected wave is reflected in different directions. The intensity of the vertically reflected wave is much stronger than the irregularly reflected wave.

About 59% of the ultrasound waves radiated to the bone pass through it and 41% are reflected, but we cannot see the images of the tissues behind the bones with the help of ultrasound waves. The reason for this is that the amplitude of ultrasound waves decreases after entering the bone and moving through it.

In the previous section, we learned about the reflection of ultrasound waves from body tissues. If the ultrasound wave hits the boundary of two tissues with a certain angle, after interacting with the tissue boundary, part of it will be reflected from it and another part of the boundary will pass and enter the second environment with a different angle (larger or smaller) than the radiation wave. . In this case, instead of the value of the wave transmitted or reflected from the boundary, we obtain the angle change by changing the speed of the ultrasonic wave in each environment.

When ultrasound waves interact with internal body tissues, they are reflected, scattered, or passed through them. Also, the amplitude of ultrasound waves decreases when entering bone or other tissues. Scattering occurs when the wavelength of the wave when hitting an object or part of it is smaller than the dimensions of that object.

When the ultrasonic wave passes through an environment with dimensions larger than its wavelength, it will pass without scattering. If the environment consists of components with dimensions smaller than the wavelength of the ultrasound beam, scattering occurs. As a result, the amplitude and intensity of ultrasound waves are reduced when passing through such environments (such as bone).

The lower the density of areas with dimensions smaller than the ultrasonic wavelength, the greater the scattering. Scattering is one of the main factors in creating the “echogenicity” of the tissue or the ability of the tissue to reflect sound waves. The greater the dispersion, the greater the echogenicity. In other words, the structures inside the tissue that cause the scattering of the waves help create a clearer image in the ultrasound.

Unlike ultrasound imaging, in X-ray imaging, we are not interested in scattering. When the ultrasound wave passes through a tissue such as the kidney, it is scattered by its smallest constituent units and a part of it reaches the transducer. Although the waves reaching the transducer are noisy and not very regular, they give us information about the kidney or any other tissue.

In addition to scattering, heat generation in tissues can also reduce the amplitude of ultrasound waves. Ultrasound waves may be absorbed by tissues as they pass through them, leading to heat generation. The attenuation of ultrasound waves varies from one tissue to another and depends on the attenuation coefficient of each tissue and the frequency of the ultrasound wave.

Suppose we send an ultrasound wave with a frequency of 4 MHz from the transducer to one of the internal tissues of the body. After traveling a distance of 5 cm, this wave reaches the boundary of the tissue shown in the image below. The intensity of this wave after traveling a distance of five centimeters and reaching the border of the tissue becomes about one tenth. After reaching the boundary, the wave reflects and moves towards the transducer.

The intensity of the wave after reflecting from the boundary and reaching the transducer also decreases by another tenth. As a result, the wave reaching the transducer is about one hundredth of the original ultrasonic wave. Of course, this is the case if the ultrasonic wave is perpendicular to the boundary and all of it is reflected.

So far, we have learned about the interaction of ultrasound waves with the internal tissues of the body. In the ultrasound machine, the transducer receives the reflected waves by producing ultrasound waves and irradiating them to the tissues. The reflected waves are converted to a digital signal by the transducer and then analyzed.

The piezoelectric material in the transducer is responsible for creating ultrasound waves and sending these waves to the body tissues. Also, this part receives reflected and scattered ultrasound waves from tissues and converts them into electrical signals. As you can see in the picture above, the piezoelectric material is located in front of the transducer.

The piezoelectric material has a unique feature; Squeezing this material induces an electric current and the current passing through it causes the material to vibrate and move. In the ultrasound machine, sound waves hit the piezoelectric material between two electrodes and compress it. This compression creates an electric current corresponding to the intensity and frequency of the sound waves, which will be transmitted as a signal to the ultrasound machine.

At the same time, the ultrasound device can cause the piezoelectric material to vibrate and produce sound waves by applying an electric current. This feature allows the ultrasound device to convert signals received from the body into images and send sound waves to examine body tissues. Electrodes also play an important role in enhancing the piezoelectric effect and conducting electric current.

The speed of sound in piezoelectric material is much higher than the speed of its movement in body tissues. As a result, these two environments (piezoelectric material and tissue boundary) have a high acoustic impedance difference. The large difference in acoustic impedance between these two environments causes a large part of the energy of sound waves to be wasted when passing through the crystal to the body. The adaptation layer in the transducer prevents the loss of energy of the sound waves during transmission from the piezoelectric material to the internal tissues of the body.

As you can see in the image above, the matching layer is located on the front of the transducer and protects the piezoelectric crystals and internal body tissues. Also, the acoustic impedance of this layer is a number between the acoustic impedance of the piezoelectric material and the internal tissues of the body. The adaptation layer, by adjusting and adjusting the audio impedance, increases the transmission of waves and improves the quality of the ultrasound image.

Our skin is not a perfectly smooth surface, so the matching layer can’t sit on it very well. Hence, some air is trapped between the matching layer and the texture. The acoustic impedance of air is very small, as a result, the acoustic impedance difference becomes very large and the sound waves cannot be easily transmitted from the transducer to the tissue. “Coupling Gel” is used to solve this problem.

Coupling gel is a water-based material with acoustic impedance close to the tissue, which is placed between the transducer and the patient’s skin, and its main function is to remove air between the probe and the skin. By removing the air, the gel causes better transmission of sound waves from the device to the body and vice versa. This helps create clearer and more detailed images of the inside of the body.

So far, we got acquainted with the piezoelectric material and the matching layer that is placed in front of the transducer. With a little distance from the front of the transducer and before the piezoelectric material, we reach a part called damping. The main purpose of attenuation is to improve image quality and reduce noise. As we said, ultrasound waves are produced by the vibration of piezoelectric material with a specific frequency. The vibration of the piezoelectric material continues for a certain period of time, but the damping part can shorten the period of its oscillation.

When the piezoelectric material contracts and expands, part of the generated waves move towards the patient and another part returns to the transducer. The damping piece prevents the generated waves from returning to the transducer and directs them to the target tissue.

So far, we have learned about the interaction of ultrasound waves with the internal tissues of the body and the internal structure of the transducer. In the following, we will talk a little about the details of imaging with ultrasound waves in the ultrasound machine.

The ultrasound image is a cross-section of the patient’s body, which is formed line by line in front of the transducer. The transducer sends ultrasound waves to the internal tissues of the body. After hitting different tissues, the sent waves are reflected from them and hit the transducer. Each line of the image is created based on these reflections and in this order, with the movement of the transducer on the body, successive lines are formed and the final image is made of the internal structure of the body.

In the following, we will examine together how the image of the kidney (right kidney) is formed in the ultrasound machine. First, an electric current is applied to the transducer and the piezoelectric material, and an ultrasonic wave is created. As we said, the ultrasound wave moves at a speed of about 1540 meters per second inside the body tissues. Part of the ultrasound waves, upon reaching the internal structure of the kidney and the tissue border, are reflected and move toward the transducer. The black-and-white image of the kidney is produced after processing the reflected waves. Because sound travels at a certain speed, the ultrasound machine can interpret the received signals and show the structures exactly where they are located on the monitor.

In ultrasound, the depth of the structures in the tissue is directly related to their position in the image. Ultrasound waves reflected from structures close to the surface of the tissue reach the transducer earlier, so they are displayed in the upper parts of the image. In contrast, deeper structures need more time to send and receive sound waves, so they will appear in the lower parts of the image. This basic principle in ultrasound allows us to view a cross-section of the body and estimate the depth of each structure relative to the surface of the skin.

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As we mentioned in the previous sections, tissues are heated by absorbing ultrasound waves, and the amplitude of these waves decreases with increasing depth of penetration. Therefore, it is more difficult to see deeper structures on ultrasound, because the ultrasound waves are too weak to reach them. To compensate for this attenuation, most ultrasound systems use a feature called “Time Gain Compensation” (TGC). TGC amplifies the return signals from deeper layers so that we can have better and clearer images of deeper structures.

Some structures, such as bones, are strong reflectors of ultrasound waves, and most of the ultrasound waves irradiated to them are reflected. In this case, the acoustic shadow is created as shown in the image below.

As we mentioned in the previous sections, ultrasound waves are scattered after hitting a heterogeneous surface. Dispersion of ultrasound waves in the body tissues leads to the appearance of speckles in ultrasound images. These stains may reduce image resolution and make it harder to accurately identify structures.

Frequency greatly affects the resolution of images taken with ultrasound waves. Ultrasound waves with a smaller frequency can travel a greater distance inside the body and penetrate the deep structures of the body better than ultrasound waves with a larger frequency. In contrast, higher-frequency ultrasound waves are suitable for imaging surface structures. As a result, to get the best image of the internal organs of the body, we must choose a transducer with a suitable frequency.