Define:

Real-time imaging

The production of a motion picture

A series of frames displayed in a rapid succession to give the impression of constant motion.

The most important operational parameter associated with an ultrasound "movie" is the system's ability to create numerous frames each second. This parameter is called?

Frame rate

Frame rate is determined by what two factors?

1. sound's speed in a medium

2. the depth of imaging

The speed of sound in soft tissue is considered to be constant at 1.54 km/s. In clinical u/s, the maximum imaging depth determines frame rate.

What is the measuring units utilized for frame rate?

Frame rate is measured in hertz (Hz), or "per second." The units of frame rate are "images per second."

Define:

Temporal Resolution

The ability to accurately locate moving structures at any particular instant in time.

What is temporal resolution determined by?

Frame rate only.

The faster the frame rate, the better the temporal resolution.

What are the 4 factors that are under the control of the sonographer that can affect temporal resolution?

1. maximum imaging depth

2. # of pulses per scan line (multi-focus systems)

3. sector size

4. line density (lines per degree of sector)

To determine the effect on temporal resolution, what 2 questions should the sonographer ask?

1. Is the image shallower?

2. Is the image comprised of fewer pulses?

With regard to maximum imaging depth, what will create a frame in less time?

Shallow depth of view makes a frame faster and improves temporal resolution.

Imaging to a shallow depth requires less listening.

Imaging to a greater depth of view requires longer listening.

If imaging depth is double (6 cm to 12 cm), the frame rate will be halved.

With regard to pulses per scan line, what will create a frame with fewer pulses?

Single focus systems use fewer pulses, and have better temporal resolution.

Single focus systems use only 1 pulse per scan line.

Multi-focus systems, such as annular phased arrays, use multiple pulses per scan line. This lowers the frame rate and reduces temporal resolution.

What do you usually lose if you improve image quality?

Improving image quality often degrades temporal resolution.

Multi-focus generally improves lateral resolution but reduces frame rate; thus, degrading temporal resolution.

With regards to sector size, what will create a frame with fewer pulses?

Smaller sector angle creates image with few pulses, improving temporal resolution.

Creating an image with a 30° sector requires a certain number of u/s pulses. Whereas, a 90° sector image uses three times as many pulses.

With regard to line density, what will create a frame with fewer pulses?

Low line density images use fewer pulses, and have better temporal resolution. However, lower line density degrades spatial resolution (called detail resolution.)

Line density in a sector image is the number of scan lines per degree of sector. Line density in a rectangular image is the number of scan lines per centimeter. When the line density is low, temporal resolution is high.