Ultrasound Image Quality and Modes

Ultrasound Imaging

• Ultrasound image quality is determined by a compromise between good resolution and deep penetration.

Reflection

• Perpendicular reflection originates the echo signal, while non-perpendicular reflection causes an intensity loss in the echo signal.
• Smooth surface → low scattering → good image
• Rough surface → high scattering → bad image

A-Mode (1D)

• Used to obtain diagnostic information about the depth of structures (image with 1-dimension).
• Sends US waves into the body and measures the time required to receive the reflected sound (echoes) from the interface between different tissues.
• Depth = Velocity x time
• Used to detect brain tumors and eye diseases.

Applications of A-Mode Scan

• Echo Encephalography: • Used in the detection of brain tumors. • Pulses of ultrasound are sent to a thin region of the skull above the ear, and the echoes from different structures are displayed on the oscilloscope. • Compare the echoes from the left side of the head to the right side, and find the shift in the middle structure. • If the shift > 3 mm for an adult (abnormal) or > 2 mm for a child (abnormal).
• Ophthalmology: • Used in diagnosis of eye diseases. • Used in biometry (measurement of distance in the eye, such as lens thickness, depth from cornea to lens, distance to the retina, and thickness of the vitreous tumor).

US Image Modes

• B-Mode (2D): • Used to obtain 2D images of the body. • Principle is the same as in A-mode, except that the transducer is moving. • Provides information about the internal structure of the body, such as size, location, and change with time of the eye, liver, breast, heart, and fetus.

• M-Mode (2D + motion): • Used to study motion, such as that of the heart and heart valves (image with 2D + motion). • Combines features of A- and B-mode. • Used in diagnostic information about the heart (mitral valve) and detection of pericardial effusion.

• D-Mode (3D + motion; or 4D): • Takes 3-dimensional US images and adds the element of time to the process (image with 3D with motion).

Physiological Effects of Ultrasound

• Various physiological and chemical effects occur when ultrasonic waves pass through the body, and they can cause physiological effects.
• The magnitude of physiological effects depends on the frequency and amplitude of the sound.

Low Intensity US (~ 0.01 W/cm²)

• No harmful effects are observed → used for diagnostic work (as in sonar).

Continues US (~1 W/cm²)

• Deep heating effect (diathermy) → temperature rise due to the absorption of acoustic energy in the tissue.

Continues US (1-10 W/cm²)

• Sound moves through → region of compression and rarefactions → pressure differences in adjacent regions of tissues (micromassage).

Continues US (~ 35 W/cm²)

• Tissue destroying effect → rupture DNA molecules.

Continues and focused US (~ 10³ W/cm²)

• Selective destroying of deep tissue using a focused ultrasound beam.

Ultrasound Generation

• Ultrasound is sound with a frequency of 20 kHz to 1 GHz (for medical applications).
• It is greater than the upper limit of human hearing.

Piezoelectric Principle

• The ultrasound signal is generated and detected by the sensor.
• The transducer is based on the piezoelectric principle.
• Many crystals can be used so that AC voltage (electrical energy) across the crystal will produce a vibration of the crystal (mechanical energy), thus generating an ultrasound wave and vice versa.

SONAR

• It is a device that uses an US wave to generate an image of a particular soft tissue structure in the body.

Image Production

• Three concepts that affect US image production: • Focal zone • Acoustic impedance • Refraction