Temporal resolution (ultrasound)

Last revised by Craig Hacking on 21 May 2020

Temporal resolution in 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 rapid movements (e.g. valve leaflets in echocardiography), and thus improved temporal resolution 1.

Ultrasound images are generated by sending high-frequency pulses of sound along with a set number of beam trajectories (lines) into the tissue beneath the transducer. For each beamline, 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 beamline 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 beamlines have been completed 1,3. Thus, factors which increase the amount of time required for a beamline 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 beamlines in the field will also have this effect.

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

  • increased propagation speed of sound waves through the tissue
  • reduced depth of field (as it shortens pulse travel distance)
  • reduced number of beamlines per field
  • reduced width of field
    • in many instruments, a narrowed field equates to fewer beamlines per field
  • reduced number of focal points
    • limits beamline duplication. Some new systems can utilize a broad array of focal points for a single field, without significantly restricting temporal resolution 2

Investigating the mathematical relationship between the above factors demonstrates that frame rate, and hence temporal resolution is inversely proportional to the number of focal points examined, the number of beamlines in the field, and the depth of field setting 3.

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

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