Temporal resolution (ultrasound)

Temporal resolution (ultrasound) represents the extent to which an ultrasound system is able to distinguish changes between successive image frames over time (i.e. movement).

Temporal resolution is chiefly determined by the image frame rate of the system (measured in hertz), which may vary depending on a number of factors. Overall, an increased frame rate equates to an increased likelihood of discerning fast movements (e.g. valve leaflets in echocardiography), and thus improved temporal resolution ¹.

Factors which increase frame rate, and hence improve temporal resolution include ¹:

• Increased propagation speed of sound waves through tissue: ultrasound instruments assume a constant 1540m/s
• Reduced depth of field: shorter pulse travel distance
• Reduced number of beam lines per field
• Reduced width of field: in many instruments, a narrowed field equates to fewer beam lines per field
• Reduced number of focal points^1,2: limits beam line duplication 

In practice, optimum temporal resolution may achieved by limiting the depth and width of field such that the desired object or region is tightly captured within the field.

Explanation

Ultrasound images are generated by sending high-frequency pulses of sound along a set number of beam trajectories (lines) into the tissue beneath the transducer. For each beam line in turn, the transducer sends a pulse, and then awaits any reflected echoes from the tissue below, down to the maximum depth of field setting. Each individual beam is addressed in sequence; the transducer will not move to the next beam line until echoes from the maximum depth of field in the previously fired beam have been received ^1,3.

An entire image (frame) is only generated once all beam lines have been completed ^1,3. Thus, factors which increase the amount of time required for a beam line to be completed will increase the amount of time required to generate the entire image. This, in turn, reduces the maximum number of images able to be generated in one second (i.e. the frame rate). Increasing the number of beam lines in the field will also have this effect. To confirm this, consider the following calculation ³.

Calculation

Time = Distance / Speed

If:

• D = depth of field setting
• C = speed of sound in tissue 
• N = number of beam lines in the field
• F = number of focal points being examined

Then the time for pulses of the maximum depth to be propagated and reflected, for a single beam is:

T(beam) = 2D / C

To generate an entire image frame from the data of N beam lines, the time required is:

T(frame) = N ⨉ T(beam)               = 2ND / C (seconds per frame)

Each beam line will also need to be replicated for each focal point examined. Hence:

T(frame) = F ⨉ N ⨉ T(beam)               = 2FND / C (seconds per frame)

Inverting both sides of the equation allows calculation of the maximum number of frames able to be generated per second (i.e. frame rate in hertz):

Frame rate (Hz) = 1 / F ⨉ N ⨉ T(beam)                           = C / 2FND

Plugging in C = 1540 m/s:

Frame rate (Hz) = 770 / FND, where D is measured in metres.

Converting to centimetres:

Frame rate (Hz) = 77,000 / FND, where D is measured in centimetres ¹.

This relationship determines that frame rate, and hence temporal resolution, is inversely proportional to the number of focal points examined, the number of beam lines in the field, and the depth of field setting.

See also

• axial resolution (ultrasound)
• lateral resolution (ultrasound)