Ultrasound / Diagnostic Application

Ultrasound / Diagnostic Application

Ultrasound

The Heart, as seen w/Ultrasound Probe (B-Mode)

Ultrasound refers to sound waves (sound wave = a type of mechanical wave) with frequencies above the upper limit of human hearing, typically greater than 20 kHz. This threshold lies just above the audible range for young, healthy adults.

Ultrasound has a wide range of applications in medicine, scientific research, and industry. It is commonly used for medical imaging and diagnostics, as well as for measuring distances, detecting objects, and in industrial processes such as cleaning, mixing, and accelerating chemical reactions.

Ultrasound in Medicine

Ultrasound is a widely used diagnostic imaging method that uses high-frequency sound waves to visualize structures inside the body. It is safe, non-invasive, and easily accessible, making it common in many medical fields.

Main Applications

• Ultrasound imaging (Ultrasonography) - creates images based on differences in tissue properties
• Doppler ultrasound - measures and visualizes blood flow
• Elastography - assesses tissue stiffness
• Bone densitometry - evaluates bone density (less accurate than X-ray methods)

Advantages and Limitations

Advantages

• Non-invasive and no ionizing radiation
• Real-time imaging
• Portable and relatively inexpensive
• Safe for repeated use (e.g pregnancy)

Limitations

• Operator dependant (requires skill)
• Limited penetration through:
  • Bone
  • Air (lungs, bowel gas)
• Lower image resolution compared to CT or MRI for deep structures

Clinical Uses

• Obstetrics – fetal monitoring
• Cardiology – echocardiography
• Abdominal imaging – liver, gallbladder, kidneys
• Vascular medicine – blood flow, thrombosis
• Emergency medicine – FAST exam (trauma assessment)

Physical Principle

Piezoelectric Effect visualized

Piezoelectric Effect

Ultrasound is generated using the piezoelectric effect, where certain materials produce sound waves when exposed to an electric field and generate electrical signals when mechanically deformed. This allows a device called a transducer to both send and receive ultrasound waves.

The materials used (usually a crystal or ceramic) have the property that:

• When an electric voltage is applied, they change shape (mechanical deformation)
• When they are mechanically deformed, they produce an electric voltage

These two processes are called:

• Direct piezoelectric effect – mechanical stress → electrical signal
• Inverse piezoelectric effect – electrical signal → mechanical vibration

Wave Propagation

Ultrasound travels through tissues as longitudinal waves. When it reaches boundaries between different tissues, part of the wave is reflected back as an echo, which is used to form an image.

The depth of structures is calculated based on the time it takes for echoes to return. The average speed of ultrasound in soft tissue is about 1540 m/s.

Frequency and Resolution

• Higher frequency → better image detail, lower penetration
• Lower frequency → deeper penetration, lower resolution

Thus:

• Low frequencies are used for deep organs
• High frequencies are used for superficial structures

Diagnostic Applications

Imaging Modes

• A-mode (Amplitude) – one-dimensional signal display; rarely used today
• B-mode (Brightness) – standard 2D imaging method
• M-mode (Motion) – used to study movement, especially in the heart
• 3D/4D imaging – provides volumetric and real-time images

Ultrasound Probes

Different probes are used depending on the application:

• Linear probe – for superficial structures
• Convex probe – for abdominal imaging
• Sector probe – for heart and deep structures
• Pencil probe – for Doppler measurements
• Circular probe – for internal examinations (e.g., prostate)
• Array probe – for 3D imaging

Doppler Ultrasound

Doppler techniques measure blood flow using changes in frequency caused by moving red blood cells.

Types

• Continuous Doppler – measures flow velocity but not location
• Pulsed Doppler – measures both velocity and location
• Duplex ultrasound – combines imaging and flow measurement
• Color Doppler – shows flow direction and velocity using color:
  • Red → flow toward probe
  • Blue → flow away from probe

Sources:

LF1 Biophysics Lectures & Protocols