1. Correct Answer: C. M-mode ultrasound

Rationale: A-mode ultrasound is the mode used for single-dimensional imaging. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumor or ophthalmologic ultrasound uses A-mode applications when the beam can be directed at a known target. In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image, giving a grayscale image of several contiguous beams (similar to several A-mode images placed next to each other). In M-mode (motion mode), a defined line is plotted over time to provide a rapid sequence of B-mode scans only capturing the structures on that line (see Figure 11.10). This is the most accurate way to measure movement within a single plane over time, as in diaphragmatic excursion. Doppler ultrasound uses the Doppler effect to measure velocities of structures reflecting ultrasound waves (e.g., tissue movement or blood flow) along the line of insonation.

1. Carovac A, Smajlovic F, Junuzovic D. Application of ultrasound in medicine. Acta Inform Med. 2011;19(3):168-171. doi:10.5455/aim.2011.19.168-171.

2. Sarwal A, Walker FO, Cartwright MS. Neuromuscular ultrasound for evaluation of the diaphragm. Muscle Nerve. 2013;47(3):319-329. doi:10.1002/mus.23671.

2. Correct Answer: B. Pulsed-wave Doppler

Rationale: Spectral Doppler uses the Doppler principle to convert frequency changes generated by reflections from moving red blood cells into velocities and displays a “spectrum” of these frequencies as a waveform (at the bottom of the image). Spectral Doppler waveforms can be acquired by pulsed-wave or continuous-wave Doppler depending on sample selection. Continuous-wave Doppler simultaneously transmits and receives sound waves with different receivers and transmitters and samples the entire range of returning frequencies along its beam path. Assessment of velocities along the valves to calculate the valvular gradient utilizes continuous Doppler to ensure capture of all range of velocities encountered. In pulsed-wave Doppler, only the velocities from a user-defined sample within the B-mode image are displayed in the spectral waveforms (the “sampling gate,” which is placed within the blood vessel in the image). Most vascular ultrasound utilizes pulsed-wave Doppler waveforms because of its precision and ability to localize the sample site for low-velocity flow. Higher velocity flow (e.g., valvular stenosis or regurgitation) is limited by aliasing and is more appropriate for continuous-wave Doppler. The conventional color flow mode utilizes two special colors (red and blue) and indicates the direction of the blood in relation to the probe. In power Doppler (or color power Doppler, CPD) mode, the direction of blood flow is disregarded, and a single color is used, with the gradient reflecting the strength of the Doppler signals. Power Doppler may be useful for lower-flow states, for example, to distinguish between near occlusion or total occlusion in blood vessels like the carotid artery.

1. Mitchell C, Rahko PS, Blauwet LA, et al. Guidelines for performing a comprehensive transthoracic echocardiographic examination in adults: recommendations from the American Society of Echocardiography. J Am Soc Echocardiogr. 2019;32(1):1-64. doi:10.1016/j.echo.2018.06.004.

2. Oglat AA, Matjafri MZ, Suardi N, Oqlat MA, Abdelrahman MA, Oqlat AA. A review of medical Doppler ultrasonography of blood flow in general and especially in common carotid artery. J Med Ultrasound. 2018;26(1):3-13. doi:10.4103/JMU.JMU_11_17.

3. Correct Answer: B. Aliasing caused by velocities in blood flow higher than color scale specified

Rationale: Aliasing is an artifact produced by attempting to measure velocities exceeding the Nyquist limit. This is a limiting factor for all forms of pulsed-wave Doppler functions, including color flow Doppler. When a velocity exceeds the Nyquist limit, the signal may appear to flow in the opposite direction (similar to the “wagon wheel effect” when the frame rate is below the rotational speed of a wagon wheel, and it appears to rotate backward). In color flow Doppler, aliasing can be seen as color signals corresponding to the extremes of the color scale (yellow and aqua blue, in this case), in contrast to the red and blue that classically represent directional shifts. M-mode, B-mode, or continuous-wave Doppler do not manifest aliasing.

1. Kremkau FW. Doppler color imaging. Principles and instrumentation. Clin Diagn Ultrasound. 1992;27:7-60.

4. Correct Answer: B. Increase in gain

Rationale: Increasing gain in this instance will enhance the visualization of the insonated structures and may help optimize the image prior to altering other parameters like depth or changing the probe position and direction. Mechanical and thermal index are preset dependent parameters that can help increase the penetration and output of ultrasound signals. While increasing these may improve imaging in some cases, it would result in more energy transmission to the patient, and adjusting gain would be a more appropriate first step. Additional gel would be unlikely to improve the quality of this image, because there appears to be adequate transmission of the ultrasound signal.

1. Hangiandreou NJ. AAPM/RSNA physics tutorial for residents. Topics in US: B-mode US: basic concepts and new technology. Radiographics. 2003;23(4):1019-1033. doi:10.1148/rg.234035034.

5. Correct Answer: A. Depth

Rationale: Adjusting the depth to <3 cm will allow a closer look at the pleural space to assess for any possible fluid collections. Given adequate visualization of lung pleura, associated comet tails (B-lines), and diaphragm (left side of Figure 11.4, flush with the pleura), enhancing the gain may not help detect fluid and could obscure anechoic fluid if the gain is too high. Power can be adjusted indirectly by changing imaging modes, but would not help enhance the area of interest in this image. Mechanical index is a dependent parameter and is not adjusted directly, but rather is a consequence of the power delivered.

1. Patel CJ, Bhatt HB, Parikh SN, Jhaveri BN, Puranik JH. Bedside lung ultrasound in emergency protocol as a diagnostic tool in patients of acute respiratory distress presenting to emergency department. J Emerg Trauma Shock. 2018;11(2):125-129. doi:10.4103/JETS.JETS_21_17.

6. Correct Answer: B. Decrease the baseline

Rationale/Critique: The pulsed-wave Doppler scale is represented on the right lower part of Figure 11.5. Because the measured velocity exceeds the upper limit of the display, “aliasing” appears, as the higher velocities are displayed as negative values at the bottom of the scale. This artifact can be reduced by lowering the baseline or by increasing the scale of the pulsed-wave Doppler spectrum (increasing the pulse rate frequency). Because we are interested in velocity in only one direction, lowering the baseline to focus on the signal “above the line” should be the first step in optimization. The scale can then be adjusted if necessary, but appears to be appropriate for this image after the baseline is adjusted. Adjusting gain on the pulsed-wave Doppler measurement would merely make the waveform appear brighter and not correct the aliasing artifact. Decreasing depth would not affect the Doppler aliasing, but may allow for more accurate measurement.

1. Oglat AA, Matjafri MZ, Suardi N, Oqlat MA, Abdelrahman MA, Oqlat AA. A review of medical Doppler ultrasonography of blood flow in general and especially in common carotid artery. J Med Ultrasound. 2018;26(1):3-13. doi:10.4103/JMU.JMU_11_17.

7. Correct Answer: D. Mirror artifact produced by the reflective surface of the pericardium

Rationale/Critique: A-lines are the horizontal artifacts arising from and parallel to the pleural line, generated by strong reflections from subpleural air. Because an echodensity is apparent at the bottom of the image, acoustic shadowing does not explain the hypoechoic appearance below the pericardium. Figure 11.11A shows B-lines that are reverberation artifacts produced by a thickened serosal surface (e.g., pleura) or interlobular septae, usually in the setting of pulmonary edema. The B-lines here are produced by the excursion of the lung pleura next to the pericardium, not by the septum. The hyperechoic pericardium can serve as a reflector in some cases and produce a mirror image of the heart as seen above it (Figure 11.11B).

1. Patel CJ, Bhatt HB, Parikh SN, Jhaveri BN, Puranik JH. Bedside lung ultrasound in emergency protocol as a diagnostic tool in patients of acute respiratory distress presenting to emergency department. J Emerg Trauma Shock. 2018;11(2):125-129. doi:10.4103/JETS.JETS_21_17.

8. Correct Answer: B. THI is useful for depicting cystic lesions and those containing echogenic tissues such as fat, calcium, or air.