Sonography Physics and Instrumentation

Sound wave requirements

Sound is a mechanical wave that requires a medium to travel and cannot travel in a vacuum.

Movement of sound through matter

Sound waves cause molecules to vibrate successively along their path.

Sound wave energy transport

Sound waves carry energy from one point to another without carrying matter or particles along the waveform.

Piezoelectric effect (production)

Electricity is applied to a material which vibrates (expands and contracts) to produce mechanical sound or pressure waves.

Piezoelectric effect (reception)

Returning sound waves cause mechanical vibrations of the material that are converted into electrical signals.

Reflected wave amplitude factors

Amplitude is proportional to the incident angle and acoustic impedance mismatch.

Reflection brightness on display

The brightness is proportional to the amplitude of the reflected wave.

Reflection location determination

The location is determined by the time the sound wave took to travel to the reflector and back.

Sound travel time for deeper structures

Deeper structures require longer travel times for sound waves.

Single element transducer focusing

Requires mechanical focusing and steering; electronic focusing and steering is not possible.

Phased array element usage

Multiple elements are used to create an image by varying the timing of electrical pulses (firing).

Transducer frequency and penetration

Transducer frequency is inversely proportional to penetration.

Transducer frequency and resolution

Transducer frequency is directly related to resolution.

High frequency probe characteristics

Reduced penetration with improved spatial and contrast resolution.

Frequency and scatter

Scatter increases with increasing frequency.

Transmitted frequency and beam width

Beam width is inversely proportional to the transmitted frequency.

Transmitted frequency and Spatial Pulse Length (SPL)

Spatial pulse length is inversely proportional to the transmitted frequency.

Transmitted frequency and pulse duration

Pulse duration is inversely proportional to the transmitted frequency.

Average transducer frequency range

$$2.5 - 4\,MHz$$

Frequency for thin, easy to image patients

$$5\,MHz$$

Frequency for large habitus or COPD patients

$2\,MHz$ (or lower)

Purpose of acoustic gel

Used to eliminate the air between the skin and the face of the transducer.

Acoustic gel impedance

The impedance is between that of the matching layer and the skin to reduce acoustic mismatch.

Effect of gel on sound transmission

Reduces reflection and improves sound transmission at the skin boundary.

Sector Phased Array elements

Contains 100-300 rectangular piezoelectric elements arranged in a linear pattern.

Sector Phased Array image shape

Wedge-shaped or pie wedge-shaped image.

Steering in Sector Phased Arrays

Electronic steering is used to create the wedge-shaped image.

Acoustic line transmission in Sector Arrays

Each acoustic line is transmitted at an angle different from the previous line.

Sector Array line spacing

The distance between individual acoustic lines in the far field is greater than in the near field.

Sector Phased Array focusing

Allows multiple focal points at varied depths (variable focusing).

Sector Array field-of-view (near field)

Most limited field-of-view in the near field compared to all transducer types.

Sector Array field-of-view (far field)

Greater field-of-view in the far field than a linear image.

Sector Phased Array applications

Echocardiography, abdominal, pelvic, vascular, transcranial, and neonatal brain imaging.

Vector Array image format

Trapezoidal imaging format.

Vector Array technology combination

Combines sequenced linear array and phased array techniques.

Vector Array footprint size

Slightly larger footprint than a sector phased array transducer.

Vector Array near field FOV

Sector image has a flat top with a wider field-of-view in the near field than a sector array.

Vector Array steering and focusing

Employs electronic steering and focusing.

Sector image line density

The number of scan lines per degree of the sector.

Rectangular image line density

The number of scan lines per $cm$ (\text{# of scan lines/cm}).

Effect of increasing line density on pulses

Increases the number of pulses per frame.

Effect of increasing line density on spatial resolution

Improves spatial resolution (axial and lateral).

Effect of increasing line density on frame rate

Decreases the frame rate.

Effect of increasing line density on temporal resolution

Decreases temporal resolution.

Sector Angle definition

Also known as Field of View (FOV) or Sector Width.

Sector Angle and frame rate relationship

Decreasing the angle decreases the line density and increases the frame rate.

Half-size sector resolution trade-off

If a sector with 100 lines is halved to 50 lines, it results in degraded spatial resolution but improved temporal resolution.

Double-size sector resolution trade-off

Increasing sector from 100 to 200 lines improves spatial resolution but degrades temporal resolution.

Narrow sector characteristics

Lower line density, increased frame rate, increased temporal resolution, and degraded spatial resolution.

Fundamental Imaging frequency

Transmits at the same frequency as the received frequency processed for the image (e.g., transmit $2\,MHz$, receive $2\,MHz$).

Tissue Harmonic Imaging (THI) basis

Non-linear behavior of a sound wave results in the production of harmonic frequencies.

Harmonic imaging resolution improvement

Uses frequency compounding and higher frequency reflections to create the image.

Harmonic imaging frequency example

System transmits a low frequency (e.g., $2\,MHz$) but displays double the frequency (e.g., $4\,MHz$).

THI penetration and resolution benefit

Allows deeper penetration with low transmitted frequency while improving image resolution via harmonic processing.

THI and artifact reduction

Provides narrower beam width to reduce side lobe and grating lobe artifacts.

THI and lateral resolution

Improved lateral resolution due to a narrower beam width.

Clinical applications of THI

Improved endocardial border delineation, detection of wall motion abnormalities, and contrast visualization.

Required uses for THI

Calculation of the EF% and 3D image acquisition.

Spatial resolution: Axial resolution

Ability to resolve two separate structures that lie parallel to the ultrasound beam.

Axial resolution improvement factors

Higher transducer frequency, wider bandwidths, and shorter pulse lengths.

Axial resolution formula

Axial resolution = $1/2 \times \text{spatial pulse length}$.

Axial resolution for $3\,MHz$ probe

Usually $1\,mm$.

Axial resolution for $7.5\,MHz$ probe

Usually $0.5\,mm$.

Spatial resolution: Lateral resolution

Ability to resolve two separate structures that lie perpendicular to the ultrasound beam.

Lateral resolution improvement factors

Higher frequency, harmonic imaging, beam focusing, wider bandwidths, and more superficial structures.

Contrast resolution

Ability to differentiate two structures with varied echogenicity.

Image matrix and contrast resolution

A larger image matrix on the monitor displays more pixels and shades of grey, improving contrast resolution.

Bits per pixel and contrast resolution

The more bits per pixel, the more shades of grey and the better the contrast resolution.

Dynamic range and contrast resolution

Higher dynamic range settings demonstrate more shades of gray.

Compression and dynamic range relationship

Increasing compression decreases dynamic range and reduces the number of shades of grey displayed.

Decreasing compression effect

Increases dynamic range and the number of shades of grey displayed.

Compression control 'up' or right

Increases the number of shades of grey displayed (actually decreasing compression of the dynamic range).

Compression control 'down' or left

Reduces the number of shades of grey displayed (actually increasing compression of the dynamic range).

Rejection settings and contrast

Lower rejection settings will demonstrate more shades of gray.

Elevational resolution

Also known as slice thickness resolution; ability to resolve structures in the slice thickness.

Elevational resolution improvement

Improves with thinner image slices, increased frequency, and focusing.

Tools for focusing slice thickness

Curved elements and lenses improve focusing and decrease slice thickness.

Temporal Resolution

The ability to detect that an object has moved over time.

Frame rate for temporal resolution

Higher frame rates equal improved temporal resolution.

Low temporal resolution appearance

Causes the image to 'drag' with a visible difference between probe motion and structure display.

Factors improving temporal resolution

Decreased number of focal zones, smaller field of view, decreased image depth, and decreased line density.

Temporal resolution comparison

PW Doppler and M-mode have better temporal resolution than 2D and color Doppler.

Minimum Frame Rate

Should be at least $30\,Hz$ to avoid flicker visible to the human eye.

Frame rate and scan lines

As the number of scan lines increases, the frame rate decreases.

Benefit of low frame rates

Increased number of acoustic lines improves image quality (spatial resolution); good for non-mobile structures.

Echocardiography frame rate requirement

Higher frame rates must be used due to movement of cardiac structures.

Echocardiography resolution trade-off

Higher frame rates used for heart movement cause reduced spatial resolution (image quality).

Parallel processing

Refers to processing multiple lines of signals at the same time to improve frame rates.

Transmit focusing

Varied timing of the pulser voltage used to vary the firing of elements in an array to focus the beam.

Receive focusing

Performed by varying the timing of the processing of reflected sound waves.

Primary benefit of focusing

Focusing improves lateral resolution.

Electronic Beam Steering purpose

Uses varied timing of pulser voltage to steer the beam and adjust sector angle.

Small sector resolution tradeoff

Improved temporal resolution but degraded spatial resolution.

Pulse Repetition Frequency (PRF)

The number of pulses emitted per second.

Pulse Repetition Period (PRP)

The time from the beginning of one pulse to the beginning of the next pulse.

PRF and PRP relationship

PRF is inversely related to the PRP.

Image depth and PRF relationship

Image depth is inversely related to PRF.

Why PRF decreases with depth

Pulses take longer to reach deep structures and return, so fewer pulses can be emitted per second.

Range Equation purpose

Used to calculate the time of flight (go-return time) to determine the distance to a reflector.

Range Equation formula

$$\text{distance to reflector} = 1/2 \times \text{speed of sound} \times \text{time of flight}$$