# Longitudinal, Lateral, and Temporal Resolution

Longitudinal, Lateral, and Temporal Resolution

Michael J. Lanspa

1. Which of the following changes will improve the axial (longitudinal) resolution of an ultrasound image?

A. Decrease the image sector depth

B. Narrow the image sector width

C. Use a higher frequency probe

D. Change the imaging from fundamental to harmonic (tissue harmonic imaging [THI])

1. Correct Answer: C. Use a higher frequency probe

Rationale: Axial resolution (also called longitudinal, range, radial, or depth resolution) is the ability to resolve two structures that are close to each other along the beam’s main axis. Axial resolution is determined by the frequency and spatial pulse. Axial resolution improves with higher frequency sound, which has a shorter wavelength. Axial resolution may also improve with fewer cycles per pulse (short pulse length or short pulse duration). The frequency of the transducer is determined by the thickness of the piezoelectric crystals and the damping material behind them, which shortens the pulses of sound waves emitted. Changing these settings requires changing the probe; neither frequency nor pulse duration can be adjusted by the sonographer. Changing sector depth or width may improve temporal resolution, but not axial resolution. Axial resolution is identical throughout an image. While harmonic imaging may improve black-white border definition and reduce some artifacts, it will decrease axial resolution due to its narrowed bandwidth.

Selected References

1. Anvari A, Forsberg F, Samir AE. A primer on the physical principles of tissue harmonic imaging. Radiographics. 2015;25:1955-1964.

2. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2000.

3. Kremkau FW. Diagnostic Ultrasound: Principles and Instruments. WB Saunders Company; 2005.

2. Which of the following statements is most true regarding lateral resolution?

A. Lateral resolution is best at the focus.

B. Lateral resolution worsens with a smaller beam diameter.

C. Lateral resolution worsens as scan line density increases.

D. Lateral resolution is typically better than axial resolution.

2. Correct Answer: A. Lateral resolution is best at the focus.

Rationale: Lateral resolution (also called angular, transverse, or azimuthal resolution) is the minimum distance between two reflectors that can be distinguished when they are located perpendicular to the ultrasound beam (Figure 6.1). The ultrasound beam width determines the lateral resolution, and a narrower beam produces improved lateral resolution (Figure 6.2). Unlike axial resolution, which is uniform throughout the image, the lateral resolution varies with depth, because the ultrasound beam width changes with depth. The lateral resolution is the smallest (best) at the focus, or one near zone length from the transducer (focal depth) because the sound beam is narrowest at that point (Figure 6.1). Option B is incorrect, as the lateral resolution is equal to the beam diameter, meaning it improves with smaller diameter beams. Option C is incorrect because the lateral resolution is improved by increasing the number of scan lines per image, although increasing the scan line density may worsen the temporal resolution (frame rate). Axial resolution is usually much better than lateral resolution, because spatial pulse length, which determines axial resolution, is typically much smaller than the ultrasound beam width, which determines lateral resolution.

Selected References

1. Edelman SK. Understanding Ultrasound Physics. 4th ed. E.S.P. Ultrasound; 2000.

2. Ng A, Swanevelder J. Resolution in ultrasound imaging. Contin Educ Anaesth Crit Care Pain. 2011;11(5):186-192.

3. Which of the following will increase temporal resolution?

A. Changing from B-mode to color flow Doppler imaging

B. Increasing the frequency from 3 MHz to 5 MHz

C. Increasing the scan line density

D. Increasing the pulse repetition frequency