Physics and Knobology

What is sound?

Human hearing- 20 to 20kHz

Sound is a vibration that moves through a medium like a gas, liquid, or solid as a sound wave.

What is ultrasound?

Sound waves with frequencies above 20kHz. This is above the range of human hearing.

Diagnostic - 2MHz

Ultrasound
> 20kHz

How does ultrasound work?

Ultrasound makes pictures of the inside of the body by using sound waves with a high frequency. A transducer, which is a hand-held device that is put on the skin, sends out the sound waves. The transducer picks up the sound waves that bounce off the cells and organs inside the body. The transducer then changes the sound waves into electrical signals, which are then turned into pictures that are shown on a computer screen.

The physics of ultrasound waves are based on the principle of reflection. When a sound wave encounters a boundary between two different materials, some of the sound waves are reflected back and some of the sound waves are transmitted into the second material. The amount of sound waves that are reflected depends on the difference in acoustic impedance between the two materials. Acoustic impedance is a measure of how much sound waves are reflected by a material.

The higher the acoustic impedance of a material, the more sound waves are reflected. For example, bone has a high acoustic impedance, so most of the sound waves that are transmitted into bone are reflected back. This is why ultrasound images of bone are very bright.

Soft tissues, on the other hand, have a lower acoustic impedance than bone. This means that less of the sound waves that are transmitted into soft tissues are reflected back. This is why ultrasound images of soft tissues are not as bright as images of bone.

The physics of ultrasound waves

The following are some of the important physics concepts related to ultrasound waves:

Frequency: The frequency of an ultrasound wave is the number of waves that pass a given point in a second. Ultrasound waves used for medical imaging have frequencies in the range of 2 to 15 MHz.

Wavelength: The wavelength of an ultrasound wave is the distance between two consecutive peaks of the wave. The wavelength of an ultrasound wave is inversely proportional to the frequency.

Propagation speed: The propagation speed of an ultrasound wave is the speed at which the wave travels through a medium. The propagation speed of an ultrasound wave in soft tissue is about 1,540 m/s.

Attenuation: Attenuation is the loss of energy of an ultrasound wave as it travels through a medium. The attenuation of an ultrasound wave is affected by the frequency of the wave, the density of the medium, and the presence of impurities in the medium.

Ultrasound transducers

How do transducers work?

A transducer is a device that converts one form of energy into another form. In the case of ultrasound transducers, they convert electrical energy into sound energy and vice versa.

The most common type of ultrasound transducer for medical imaging is the piezoelectric transducer. These transducers are made of materials such as quartz or PZT (lead zirconate titanate). When an electrical signal is applied to the transducer, it causes the material to vibrate. The vibrations create sound waves that are transmitted into the body.

The sound waves reflect off of the tissues and are then received by the transducer. The transducer converts the sound waves back into electrical signals, which are then displayed on the ultrasound monitor.

The performance of an ultrasound transducer is affected by a number of factors, including the frequency, the size, and the shape of the transducer. The frequency of the transducer determines the resolution of the image. A higher frequency transducer will produce a higher-resolution image, but it will also have a shorter range. The size of the transducer determines the depth of field of the image. A larger transducer will have a deeper depth of field, but it will also be less portable. The shape of the transducer can be designed to focus the sound waves in a particular direction.

The choice of transducer depends on the specific application. For example, a high-frequency transducer would be used for imaging superficial structures, such as the skin and muscles. A lower-frequency transducer would be used for imaging deeper structures, such as the liver and kidneys.

Here are the steps on how a transducer works:

An electrical signal is applied to the transducer.
The transducer vibrates, creating sound waves.
The sound waves are transmitted into the body.
The sound waves reflect off of the tissues.
The reflected sound waves are received by the transducer.
The transducer converts the sound waves back into electrical signals.
The electrical signals are displayed on the ultrasound monitor.

Ultrasound transducers are a safe and effective way to image the body. They do not use ionizing radiation, so they do not pose a risk of cancer. Ultrasound transducers are also relatively inexpensive and easy to use.

Different types of transducers

There are two main types of ultrasound transducers:

Piezoelectric transducers: These transducers use the piezoelectric effect to convert electrical energy into sound energy and vice versa. The piezoelectric effect is the property of certain materials to generate an electric current when they are deformed, or to deform when an electric current is applied to them.
Micromachined ultrasonic transducers (MUTs): These transducers are made of silicon and are etched using micromachining techniques. They are smaller and lighter than piezoelectric transducers, but they have a lower frequency range.

Here are some of the most common types of piezoelectric transducers:

Linear transducers: These transducers have a rectangular shape and are used to create a two-dimensional image.
Convex transducers: These transducers have a curved shape and are used to create a three-dimensional image.
Phased array transducers: These transducers have multiple elements that can be electronically controlled to focus the sound waves in a particular direction.
Interchangeable transducers: These transducers have multiple heads that can be attached to the same machine. This allows the user to choose the best transducer for the specific application.

MUTs are not as common as piezoelectric transducers, but they are becoming increasingly popular due to their small size and light weight. MUTs are often used in portable ultrasound machines and in minimally invasive procedures.

The choice of transducer depends on the specific application. For example, a linear transducer would be used for imaging superficial structures, such as the skin and muscles. A convex transducer would be used for imaging deeper structures, such as the liver and kidneys. A phased array transducer would be used for imaging moving structures, such as the heart. And an interchangeable transducer would be used for a variety of applications.

Choosing the right transducer

Here are some factors to consider when choosing the right transducer for ultrasound imaging:

Frequency: The frequency of the transducer determines the resolution of the image. A higher frequency transducer will produce a higher-resolution image, but it will also have a shorter range.
Depth of field: The depth of field of the image is the distance over which the image is in focus. A larger transducer will have a deeper depth of field, but it will also be less portable.
Shape: The shape of the transducer can be designed to focus the sound waves in a particular direction.
Application: The type of application will determine the specific features that are needed in the transducer. For example, a transducer for imaging the heart will need to be able to image moving structures.
Cost: The cost of the transducer will vary depending on the features and specifications.

Here are some additional tips for choosing the right transducer:

Consult with an expert: If you are not sure which transducer is right for you, consult with an expert who can help you choose the best option for your needs.
Consider the patient's condition: The patient's condition may also affect the choice of transducer. For example, a patient with a large amount of body fat may require a lower-frequency transducer to achieve a good image.
Consider the portability of the transducer: If you need to move the transducer around frequently, you will need to choose a transducer that is lightweight and portable.
Consider the warranty: Make sure to choose a transducer that comes with a good warranty in case something goes wrong.

Ultrasound imaging modes

There are four main ultrasound imaging modes:

A-mode (amplitude mode): This mode displays the amplitude of the reflected sound waves as a function of depth. A-mode is used to measure the thickness of tissues and to identify structures that reflect sound waves well, such as bone.

A-mode (amplitude mode) ultrasound imaging

B-mode (brightness mode): This mode displays the brightness of the reflected sound waves as a function of depth. B-mode is the most common ultrasound imaging mode and is used to create two-dimensional images of tissues.

B-mode (brightness mode) ultrasound imaging

M-mode (motion mode): This mode displays the motion of tissues as a function of time. M-mode is used to study the movement of structures, such as the heart valves.

motion-m-mode-echocardiography-ultrasound-image-echo-1.jpg

M-mode (motion mode) ultrasound imaging

Doppler mode: This mode measures the frequency of the reflected sound waves to determine the velocity of blood flow. Doppler mode is used to study blood flow in vessels and to diagnose conditions such as heart disease and vascular occlusion.

1-Various-display-modes-in-Doppler-ultrasound-a-spectrogram-time-varying-velocity.png

Doppler mode ultrasound imaging

There are also other ultrasound imaging modes, such as three-dimensional (3D) ultrasound and elastography. 3D ultrasound creates three-dimensional images of tissues, which can be used to visualize structures that are not visible in two-dimensional images. Elastography is a technique that uses ultrasound to measure the stiffness of tissues. Elastography can be used to diagnose conditions such as cancer and fibrosis.

The choice of ultrasound imaging mode depends on the specific application. For example, A-mode is used to measure the thickness of tissues, B-mode is used to create two-dimensional images of tissues, M-mode is used to study the movement of structures, and Doppler mode is used to measure the velocity of blood flow.

Ultrasound artifacts

What are ultrasound artifacts?

Ultrasound artifacts are distortions of ultrasound images that are caused by factors other than the actual structures being imaged. They can be caused by the properties of the tissues being imaged, the settings of the ultrasound machine, and the technique used by the sonographer. Artifacts can make it difficult to interpret ultrasound images, but they can often be recognized and corrected.

Common ultrasound artifacts

Here are some of the most common ultrasound artifacts:

Reverberation: This artifact occurs when sound waves are reflected multiple times between two interfaces. This can create a series of parallel lines on the ultrasound image. Reverberation artifacts can be caused by gas bubbles, bone, or other highly reflective structures.

Reverberation ultrasound artifact

Shadowing: This artifact occurs when sound waves are blocked by a highly reflective structure. This can create a dark area on the ultrasound image behind the reflective structure. Shadowing artifacts can be caused by bone, gas bubbles, or other highly reflective structures.

Shadowing ultrasound artifact

Edge artifacts: This artifact occurs at the edges of structures. This can create a bright or dark line on the ultrasound image. Edge artifacts can be caused by the finite size of the ultrasound beam and by the difference in acoustic impedance between tissues.

Edge artifacts ultrasound artifact

Attenuation: This is the loss of sound energy as it travels through tissue. This can cause the ultrasound image to become darker with increasing depth. Attenuation artifacts can be caused by the frequency of the sound waves, the density of the tissue, and the presence of impurities in the tissue.

Attenuation ultrasound artifact

Doppler artifacts: These artifacts can occur in Doppler ultrasound images. They can be caused by the movement of the transducer, the movement of the patient, or the presence of artifacts in the ultrasound beam. Doppler artifacts can make it difficult to interpret Doppler images.

Doppler artifacts ultrasound artifact

Mirror imaging: This artifact occurs when sound waves are reflected back from a curved surface. This can create a mirror image of the structure behind the curved surface. Mirror imaging artifacts can be caused by the curvature of the transducer or by the curvature of a structure in the body.

Mirror imaging ultrasound artifact

Ultrasound artifacts can be caused by a variety of factors, including the properties of the tissues being imaged, the settings of the ultrasound machine, and the technique used by the sonographer. Artifacts can make it difficult to interpret ultrasound images, but they can often be recognized and corrected.

Here are some tips for avoiding ultrasound artifacts:

Use the correct transducer for the application.
Adjust the settings of the ultrasound machine to match the tissues being imaged.
Use a technique that minimizes artifacts.
Be aware of the common types of artifacts and how to recognize them.

If you are concerned about ultrasound artifacts, you should consult with a sonographer or radiologist. They can help you to interpret the images and to determine if any artifacts are present.

Ring-down artefacts : are made when tiny crystals, like cholesterol or air bubbles, vibrate at the frequency of ultrasound and make sound.

Because the sound comes out after the transducer gets the first reflection, the system thinks that the sound is coming from structures deeper in the body.

Ring-down artefacts

How to avoid ultrasound artifacts

Here are some tips on how to avoid ultrasound artifacts:

Use the correct transducer for the application. The transducer is the device that converts electrical energy into sound energy and vice versa. The type of transducer you use will depend on the tissues you are imaging. For example, a higher frequency transducer will be needed to image superficial structures, while a lower frequency transducer will be needed to image deeper structures.
Adjust the settings of the ultrasound machine to match the tissues being imaged. The settings of the ultrasound machine can affect the appearance of artifacts. For example, the gain setting can affect the brightness of the image, and the depth setting can affect the depth of field.
Use a technique that minimizes artifacts. There are a number of techniques that can be used to minimize artifacts, such as:
- Using a standoff pad to create a larger distance between the transducer and the skin.
- Using a water bath to image structures that are close to the surface of the body.
- Moving the transducer slowly and smoothly.
- Avoiding contact with bone or other highly reflective structures.
Be aware of the common types of artifacts and how to recognize them. Once you are familiar with the common types of artifacts, you will be better able to recognize them and avoid them.

It is important to note that it is not always possible to avoid ultrasound artifacts completely. However, by following these tips, you can minimize the occurrence of artifacts and improve the quality of your ultrasound images.

Point-of-care ultrasound (POCUS) is the use of ultrasound by non-radiologists in the clinical setting to rapidly image patients for diagnostic and/or therapeutic purposes. POCUS is becoming increasingly popular in a variety of medical specialties, including emergency medicine, critical care, cardiology, obstetrics and gynecology, musculoskeletal, vascular, and surgery.

Applications of point-of-care ultrasound

Here are some of the applications of POCUS in each of these specialties:

Emergency medicine: POCUS can be used to quickly assess patients for a variety of conditions, including:
- Chest pain: POCUS can be used to assess for pericardial effusion, pneumothorax, and cardiac tamponade.
- Abdominal pain: POCUS can be used to assess for free fluid, masses, and organ injury.
- Stroke: POCUS can be used to assess for stroke and to guide the placement of a clot-busting medication.
- Trauma: POCUS can be used to assess for hemothorax, pneumothorax, and other injuries.
Critical care: POCUS can be used to monitor critically ill patients for a variety of conditions, including:
- Heart failure: POCUS can be used to assess left ventricular function and to guide the management of fluid therapy.
- Pneumonia: POCUS can be used to assess for consolidation and to guide the placement of a chest tube.
- Acute kidney injury: POCUS can be used to assess for volume status and to guide the management of fluids.
Cardiology: POCUS can be used to diagnose and manage a variety of cardiac conditions, including:
- Heart failure: POCUS can be used to assess left ventricular function and to guide the management of fluid therapy.
- Myocardial infarction: POCUS can be used to assess for wall motion abnormalities and to guide the placement of a coronary stent.
- Valve disease: POCUS can be used to assess the function of the heart valves.
Obstetrics and gynecology: POCUS can be used to diagnose and manage a variety of obstetric and gynecological conditions, including:
- Pregnancy: POCUS can be used to assess the location of the placenta, to measure the fetal heart rate, and to diagnose complications of pregnancy, such as ectopic pregnancy and preterm labor.
- Ovarian cysts: POCUS can be used to diagnose and characterize ovarian cysts.
- Endometriosis: POCUS can be used to diagnose endometriosis.
Musculoskeletal: POCUS can be used to diagnose and manage a variety of musculoskeletal conditions, including:
- Tendinitis: POCUS can be used to assess for tendon tears.
- Muscle strain: POCUS can be used to assess for muscle tears.
- Joint effusion: POCUS can be used to assess for joint effusions.
Vascular: POCUS can be used to diagnose and manage a variety of vascular conditions, including:
- Deep vein thrombosis: POCUS can be used to diagnose deep vein thrombosis.
- Arterial occlusion: POCUS can be used to diagnose arterial occlusion.
- Vascular grafts: POCUS can be used to assess the patency of vascular grafts.
Surgery: POCUS can be used to guide a variety of surgical procedures, including:
- Laparoscopic surgery: POCUS can be used to guide the placement of trocars and to identify structures during surgery.
- Endoscopic surgery: POCUS can be used to guide the placement of endoscopes and to identify structures during surgery.
- Vascular surgery: POCUS can be used to guide the placement of stents and grafts.

POCUS is a rapidly evolving field with new applications being developed all the time. It is a valuable tool for healthcare providers in a variety of settings.

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