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.

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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. 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.

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

3. Correct Answer: D. Increasing the pulse repetition frequency

Rationale: Temporal resolution depends only on the frame rate (the number of images per second). The frame rate is determined by two factors: the imaging depth and the number of pulses per frame. Shallower image depth and fewer pulses will increase the frame rate and lead to improved temporal resolution. The frame rate is calculated as 77,000 cm/s divided by the pulses/frame and sector depth in centimeters. Pulse repetition frequency (the reciprocal of the pulse repetition period) is the number of pulses created by the system in 1 s. It is inversely related to imaging depth, which can be changed by the sonographer. Decreasing the imaging depth will increase the pulse repetition frequency and will also improve temporal resolution because less time will be required for waves to return to the transducer. Changing from B-mode to color imaging will decrease the frame rate. Any sort of Doppler imaging requires multiple ultrasound pulses to accurately determine velocities. The frame rate in color imaging depends on several factors, including the width and depth of the color box. The wider the box, the more scan lines are required, and the longer it will take to acquire the data to produce the image. Increasing the transducer frequency is not something adjustable by the sonographer, unless the probe is exchanged for a higher frequency probe, and this alone will not change the frame rate (although a different probe may use a different sector type). Increasing the scan line density will result in more scan lines and more pulses per frame, which will decrease the frame rate. Additionally, although not a choice, increasing the image sector width would increase the number of pulses per frame and decrease temporal resolution, while narrowing the sector width would increase temporal resolution.