Class 7: Two-Dimensional and Real-Time Imaging

Flashcards covering two-dimensional imaging, various transducer types, electronic steering and focusing, and temporal resolution concepts based on the lecture notes.

Frame

A complete sweep of the ultrasound beam.

Real-time sonography

Displays images (frames) in rapid sequential order.

Automatic scanning

The electronic steering of the sound beam rapidly and repeatedly through tissues using array transducers.

Sequencing

A transducer operation where voltage pulses are applied sequentially to groups of elements, with each group acting as a single large element.

Phasing

A transducer operation where voltage pulses are applied to all elements with small timing differences (typically less than $1\,ms$).

Phase delay

The delay between pulses in phasing, typically ranging from $10\,ns$ to $60\,ns$.

Mechanical transducer

A transducer containing a single circular, disc-shaped active element that is physically moved to create a scan plane.

Mechanical steering

The process of rotating a crystal using a motor to produce a two-dimensional image.

Fixed focusing

Also known as conventional or mechanical focusing, where the focal depth cannot be changed.

Internal focusing

A method of fixed focusing achieved through the use of a curved active element.

External focusing

A method of fixed focusing achieved through the use of an acoustic lens.

Linear phased array

Also known as a sector array, it produces a fan-shaped image and uses electronic steering and focusing.

Annular phased array

A disc-shaped transducer with crystals arranged in a bull’s-eye pattern that is mechanically steered and electronically focused.

Linear sequential array

A transducer that produces parallel scan lines and a rectangular image shape through electronic steering and focusing.

Convex sequenced array

A transducer with non-parallel scan lines that produce a blunted sector image shape.

Vector array

A hybrid transducer combining linear sequential and phased array techniques to produce a flat-top sector (trapezoidal) image.

Electronic Slope pattern

The timing delay pattern used to achieve beam steering in an array transducer.

Electronic Curvature

The phasing pattern used to focus the beam; greater curvature creates a shallower focus.

Multiple focal zones

The use of multiple pulses per scan line, each focused at a different depth, to improve lateral resolution.

Temporal resolution

Resolution related to time, representing the ability to accurately display rapidly moving structures in milliseconds.

Frame rate

The number of images stored per second, determined by imaging depth, number of focuses, and scan lines.

Frame Rate Formula

$$FR = 77 / (depth \times \text{number of focal zones} \times \text{number of scan lines})$$

Frame Rate Unit

$$\text{frames per second (Hz)}$$

PRF (Pulse Repetition Frequency) calculation

$$PRF (Hz) = \text{number of focuses} \times \text{number of the scan line} \times \text{frame rate (Hz)}$$

Elevational Resolution

Also known as slice thickness resolution; it is best when using disc-shaped elements.

1.5D array

A newer transducer with multiple crystals in the up-and-down direction to create thinner beams and improve slice thickness resolution.

Vertical dropout line

The result in the image when an element is defective in a linear phased or linear sequenced array.

Blunted sector image

The specific image shape produced by a convex sequenced array.

Trapezoidal display

The flat-top sector image shape characteristic of vector arrays.

Disc-shaped active element

Found in mechanical and annular phased array transducers, providing the best elevational resolution.

Shallow focus

Result of greater electronic curvature with larger time delays between element firings.

Deep focus

Result of less electronic curvature with smaller time delays between element firings.

Lateral resolution trade-off

Improved by multiple focuses at the expense of temporal resolution.

Curvilinear

One of the two different transducer shapes used in automatic scanning, featuring a curved footprint.

Linear

One of the two different transducer shapes used in automatic scanning, featuring a straight footprint.

Acoustic lens

A tool used to achieve external fixed focusing in mechanical transducers.

Bull’s-eye pattern

The specific arrangement of crystals in an annular array.

Sector shape

The image shape produced by mechanical, linear phased, and annular arrays.

PZT crystal

The active element from which ultrasound pulses radiate.

Scan line

The individual path of an ultrasound pulse; parallel in linear sequential arrays but not in convex arrays.

10 ns to 60 ns

The specific range of phase delay used during electronic steering.

77

The constant value used in the numerator of the Frame Rate equation provided in the text.

Fast moving structures

An example being the heart, where the use of multiple focuses is not permitted due to temporal resolution requirements.

Electronic focusing

The method of narrowing the beam using phasing and curvature rather than physical lenses or curved crystals.

Electronic steering

The method of sweeping the beam using phasing (timing delays) without moving parts.

Number of pulses per scan line

A factor that determines frame rate; multiple focuses require more pulses.

Imaging depth

A primary determinant of frame rate; as depth increases, frame rate decreases.

Transducer function

To emit ultrasound pulses and receive returning echoes.

Fan shape

A description of the sector image produced by mechanical and linear phased arrays.

Pulse Repetition Frequency (PRF) Equation

$FR = \frac{PRF}{\text{number of scan lines}}$.