Ultrasound Physics: Real-Time Imaging and Temporal Resolution

These flashcards cover the fundamental principles of real-time imaging in ultrasound, including frame rate, temporal resolution, and the physical trade-offs involved in image optimization.

What is the difference between static scanning and real-time imaging?

Static scanning involves displaying one frame at a time, similar to an ultrasound "photograph," while real-time imaging creates and displays frames very quickly to provide the impression of constant motion, like an ultrasound "movie."

What is the definition of frame rate, and what are its units?

Frame rate is the system's ability to create numerous frames each second and is measured in hertz ($Hz$) or "images per second."

What two factors determine the frame rate of an ultrasound system?

Frame rate is determined by the speed of sound in the medium and the imaging depth.

What is the assumed constant speed of sound in soft tissue for clinical ultrasound?

The speed of sound in soft tissue is considered to be constant at $1.54\,km/s$.

What is temporal resolution?

Temporal resolution pertains to "accuracy in time" and describes the ability to precisely position moving structures from instant to instant.

What is the mathematical relationship between frame rate and the time required to make a single image ($T_{frame}$)?

They are inversely related and are reciprocals: $T_{frame} \times \text{frame rate} = 1$.

If an ultrasound system creates an image in $1/50$ of a second, what is the frame rate?

The frame rate is $50\,frames/second$ or $50\,Hz$.

How does imaging depth affect temporal resolution?

Shallow imaging increases frame rate and improves temporal resolution, while deeper imaging decreases frame rate and degrades temporal resolution.

What is the formula to calculate the time needed to make a single frame ($T_{frame}$) based on pulses and pulse repetition period ($PRP$)?

T_{frame} = \text{# pulses} \times PRP.

What three factors determine the number of pulses needed to create each image?

The number of pulses per scan line (multi-focus vs. single focus), sector size, and line density (lines per angle of sector).

What is the trade-off of using multi-focus imaging?

Multi-focusing improves lateral resolution because the beam is narrow over a wide range of depths, but it decreases the frame rate and diminishes temporal resolution.

How does sector size affect frame rate?

Expanding the sector size (field of view) requires more pulses to create an image, which decreases frame rate and temporal resolution.

What is line density and how does high line density affect image characteristics?

Line density is the spacing between sound beams. High line density improves spatial (detail) resolution due to smaller gaps between lines but decreases frame rate and temporal resolution.

What system settings are ideal for imaging a stationary structure like the gallbladder?

Multi-focusing, a wide field of view, and high line density are appropriate because they provide excellent image quality despite poor temporal resolution.

What system settings are used to achieve exceptional temporal resolution for moving structures like a fetal heart?

Single focus, a narrow field of view, and low line density (widely spaced scan lines) result in a very high frame rate.

According to the transcript, what is the critical factor in determining frame rate, line density, and imaging depth?

The critical factor is time.

What parameters determine the pulse repetition frequency ($PRF$)?

The number of lines per frame and the frame rate determine the pulse repetition frequency ($PRF$).

If the imaging depth is doubled from $5\,cm$ to $10\,cm$, what happens to the frame rate?

The frame rate is halved because frame rate and imaging depth are inversely related.