The spatial resolution of any imaging system is defined as its ability to distinguish two points as seperate in space. Spatial resolution is measured in units of distance such as mm. The higher the spatial resolution, the smaller the distance which can be distinguished.
Spatial resolution is commonly further sub-categorized into axial resolution and lateral resolution.
Axial resolution, also known as longitudinal resolution or azimuthal resolution is resolution in the direction parallel to the ultrasound beam. The resolution at any point along the beam is the same; therefore axial resolution is not affected by depth of imaging.
Axial resolution = spatial pulse length/2 or (# cycles in the pulse x wavelength)/2
Clearly, from the above equation we can see that any measure that shortens the length of the ultrasound pulse will improve axial resolution. For example, decreasing the number of cycles in the pulse or increasing the frequency of the pulse should improve axial resolution.
By way of contrast, lateral resolution is defined as the ability of the system to distinguish two points in the direction perpendicular to the direction of the ultrasound beam. Lateral resolution is affected by the width of the beam and the depth of imaging. Wider beams typically diverge further in the far field and any ultrasound beam diverges at greater depth, decreasing lateral resolution. Therefore, lateral resolution is best at shallow depths and worse with deeper imaging.
Temporal resolution is the ability to detect that an object has moved over time. For the purposes of medical ultrasound, temporal resolution is synonymous with frame rate. Typical frame rates in echo imaging systems are 30-100 Hz. The temporal resolution or frame rate = 1/(time to scan 1 frame). The time to scan one frame is equal to the pulse repitition period x number of scan lines per frame.
Common means of improving frame rate include 1) narrowing the imaging sector, which decreases the time it takes to scan one frame 2) decreasing the depth which decreases the PRP 3) decreasing the line density, which requires fewer lines to scan one frame (at the cost of spatial resolution) 4) turning of multifocus, which decreases the number of pulses needed per line. See some examples below: