(2) Sonography Physics and Instrumentation

What is required for a mechanical sound wave to travel?

A medium

Can sound waves travel in a vacuum?

No

How do sound waves move through matter?

By causing molecules to vibrate successively along their path

What do sound waves carry from one point to another?

Energy

Does matter or particles carry along with the waveform in sound propagation?

No

What effect describes the conversion of electricity into mechanical sound waves?

Piezoelectric Effect

In the piezoelectric effect, what happens when electricity is applied to the material?

The material vibrates (expands and contracts) to produce mechanical sound or pressure waves

How are returning sound waves converted into an electrical signal for display?

Returning sound waves cause mechanical vibrations (acoustic pressure) of the piezoelectric material which are converted into electrical signals

The amplitude of a reflected wave is proportional to which two factors?

Incident angle and acoustic impedance mismatch

What determines the brightness of a reflection on the ultrasound display?

The amplitude of the reflected wave

What determines the location of a reflection on the image display?

The time the sound wave took to travel to the reflector and back

What is the relationship between travel time and structure depth in sonography?

Deeper structures require longer travel times for sound waves

What type of focus and steering is required for a single element transducer?

Mechanical focusing and steering

Is electronic focusing and steering possible with a single element transducer?

No

How does a phased array transducer create an image?

By varying the timing of the electrical pulses (firing) to the different elements in the array

What is the relationship between transducer frequency and penetration?

Transducer frequency is inversely proportional to penetration

What is the relationship between transducer frequency and resolution?

Transducer frequency is directly related to resolution

What are the characteristics of high frequency probes regarding penetration and resolution?

Reduced penetration and improved spatial and contrast resolution

What happens to scatter as frequency increases?

Scatter increases

What three parameters are inversely proportional to the transmitted frequency?

Beam width, spatial pulse length, and pulse duration

What is the average transducer frequency range for echocardiography?

$$2.5 - 4\,MHz$$

Which transducer frequency is ideal for thin, easy to image patients?

$$5\,MHz$$

Which transducer frequency is used for patients with large habitus or lung disease/COPD?

$2\,MHz$ (or lower)

What is the purpose of using acoustic gel between the skin and transducer?

To eliminate the air between the skin and the face of the transducer

How does the impedance of acoustic gel compare to the matching layer and skin?

The gel impedance is between the impedance of the matching layer and the skin

What are the results of reducing the acoustic mismatch between the matching layer and the skin using gel?

Reduced reflection and improved sound transmission at the skin boundary

What elements are contained in a sector phased array and how are they arranged?

A large number of rectangular piezoelectric elements arranged in a linear pattern

How many elements typically make up the small footprint of a sector phased array?

$100 - 300$ elements

What is the shape of the image produced by a sector phased array?

Wedge-shaped or pie wedge-shaped

What method is used to create the wedge-shaped image in a sector phased array?

Electronic steering

How do acoustic lines in a sector phased array relate to each other's angle?

Acoustic lines are transmitted at an angle that is different from that of the previous acoustic line

Where is the distance between individual acoustic lines greater in a sector phased array?

In the far field

What is variable focusing in a sector phased array?

It allows multiple focal points at varied depths

Which transducer has the most limited field-of-view in the near field?

Sector Phased Array

Does a sector phased array have a greater or lesser FOV in the far field compared to a linear image?

Greater field-of-view in the far field

What is transmit focusing in a phased array?

Electronic focusing and steering by firing groups of elements to produce one pulse at a time

What are common imaging applications for Sector Phased Arrays and Vector Arrays?

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

What techniques does a Vector Array combine?

Sequenced linear array and phased array techniques

What is the image format shape produced by a Vector Array?

Trapezoidal imaging format

How does the footprint of a Vector Array compare to a sector phased array?

It has a slightly larger footprint

What defines the near field field-of-view of a Vector Array compared to a sector array?

It has a flat top with a wider field-of-view in the near field

What determines line density in a sector image?

The number of scan lines per degree of the sector

What determines line density in a rectangular image?

The number of scan lines/cm

What effect does increasing line density have on the number of pulses per frame?

Increases the number of pulses per frame

How does increasing line density affect spatial resolution?

It improves spatial resolution (axial and lateral)

How does increasing line density affect frame rate and temporal resolution?

It decreases both frame rate and temporal resolution

What is another name for Sector Angle?

Field of View (FOV) or Sector Width

What happens when you decrease the sector angle?

Line density decreases and frame rate increases

If a sector angle containing 100 scan lines is halved, how many scan lines will the new sector have?

$50$ scan lines

What is the trade-off of narrowing a sector angle regarding resolution?

Improved temporal resolution but degraded spatial resolution

Define the relationship between transmit and receive frequencies in fundamental imaging.

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

What allows for frequency compounding and improved 2D image resolution in THI?

Non-linear behavior of a sound wave

In Harmonic Imaging, if the transmitted frequency is $2\,MHz$, what is the displayed frequency?

$$4\,MHz$$

What are the penetration and resolution benefits of THI?

Allows deeper penetration with lower transmitted frequency while improving image resolution with harmonic display

How does THI affect side lobe or grating lobe artifact?

It provides a narrower beam width to reduce these artifacts

Which resolution is improved in THI due to a narrower beam width?

Lateral resolution

What specific heart features are better detected using THI?

Endocardial border delineation and wall motion abnormalities

For which procedures should THI always be used?

Calculation of the EF% and 3D image acquisition

What is axial resolution?

The ability to resolve two separate structures that lie parallel to the ultrasound beam

What three factors improve axial resolution?

Higher transducer frequency, wider bandwidths, and shorter pulse lengths

What is the formula for Axial Resolution?

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

What is the typical axial resolution for a $3\,MHz$ probe?

$$1\,mm$$

What is the typical axial resolution for a $7.5\,MHz$ probe?

$$0.5\,mm$$

What is lateral resolution?

The ability to resolve two separate structures that lie perpendicular to the ultrasound beam

What factors improve lateral resolution?

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

What is contrast resolution?

The ability to differentiate two structures with varied echogenicity

How does a larger image matrix on a monitor affect contrast resolution?

It allows more pixels and more shades of grey, improving contrast resolution

How does the number of bits per pixel relate to contrast resolution?

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

What dynamic range setting demonstrates more shades of gray?

Higher dynamic range settings

How does increasing compression functionally affect dynamic range?

It decrease dynamic range and the number of shades of grey displayed

How does decreasing compression functionally affect dynamic range?

It increases dynamic range and the number of shades of grey displayed

What happens to the number of shades of grey when the compression control knob is turned 'up' (right)?

The number of shades of grey displayed increases

In what scenario is compression actually decreasing when a knob is turned 'up'?

When the equipment set-up is like a volume knob where 'up' increases the feature (shades of grey)

What rejection setting demonstrates more shades of gray?

Lower rejection settings

What is another name for elevational resolution?

Slice thickness resolution

How can elevational resolution be improved?

Thinner image slices, increased frequency, focusing, and using curved elements/lenses

What is temporal resolution?

The ability to detect that an object has moved over time

What frame rate setting improves temporal resolution?

Higher frame rates

What is the visual indicator of decreased temporal resolution?

The image will 'drag' with a visible difference in motion between the probe and display

What factors improve temporal resolution?

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

Which modes have better temporal resolution than 2D and color Doppler?

PW Doppler and M-mode

What is the minimum frame rate required to avoid visible flicker to the human eye?

$$30\,Hz$$

What variables cause frame rates to change?

Transducer frequency, display depth, and focal-zone settings

What is the relationship between the number of scan lines and frame rate?

As scan lines increase, frame rate decreases

What are low frame rates good for observing?

Non-mobile structures (because they allow higher spatial resolution and line density)

Why are higher frame rates necessary for echocardiography?

Due to the movement of cardiac structures

What technique can improve frame rates by processing multiple signal lines at once?

Parallel processing

What is transmit focusing?

The use of varied timing of the pulser voltage to vary the firing of elements in an array to focus the beam

What resolution is primarily improved by focusing?

Lateral resolution

What is receive focusing?

Focusing performed by varying the timing of the processing of reflected sound waves

What is electronic beam steering used for?

To steer the beam and adjust the sector angle

Define Pulse Repetition Frequency (PRF).

The number of pulses emitted per second

Define Pulse Repetition Period (PRP).

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

What is the relationship between PRF and PRP?

PRF is inversely related to the PRP ($PRF = \frac{1}{PRP}$)

What is the relationship between image depth and PRF?

Image depth is inversely related to PRF

Why can fewer pulses be emitted per second as a pulse travels deeper?

Because of the increased distance and travel time required for the pulses to go and return

What is the range equation used to calculate?

Time of flight or go-return time

What is the Range Equation formula for distance to reflector?

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

List five common 2D artifacts in sonography.

Posterior Enhancement, Edge Shadowing, Mirror Image, Side Lobes, and Valve Masking

How can beam width artifact be reduced?

By narrowing the width of the beam with focusing