1. Correct Answer: B. 2 to 5 MHz

Rationale: In order to evaluate cardiogenic causes of her symptoms, a transthoracic echo would be the most appropriate first step. This would most commonly be performed with a phased array probe, with a frequency of about 2 to 5 MHz. A 1 to 2 MHz probe is more likely to be used for deeper structures, with less movement, like a curvilinear probe used for abdominal imaging, although commonly these probes also have a wider range, like 2 to 5 MHz. A higher frequency probe, like 5 to 12 MHz, would be used to image more superficial structures, for example, vasculature with a linear array probe. A 12 to 15 MHz probe is not likely to be used for clinical imaging.

1. Soni NJ, Arntfield R, Kory P. Point of Care Ultrasound. Elsevier; 2019.

2. Correct Answer: D. 4-mm-thick crystal

Rationale: A 4-mm-thick crystal will have the best lateral resolution. The Fraunhofer zone, or far zone, is the area deep to the focal zone where ultrasound beams diverge. Divergence describes the spread of the ultrasound beam and is determined by the transducer diameter and ultrasound frequency. Sound beams with more divergence spread over a larger area, decreasing lateral resolution. Higher frequencies and larger transducer diameters decrease divergence, improving lateral resolution. Ultrasound frequency is inversely proportional to the piezoelectric crystal thickness. Therefore, the 4-mm-crystal will have the highest frequency, least divergence, and best lateral resolution in the Fraunhofer zone.

1. Edelman SK. Ultrasound Physics and Instrumentation. Education for the Sonographic Professional; 2011.

2. Hoskins P, Martin K, Thrush A. Diagnostic Ultrasound: Physics and Equipment. Cambridge University Press; 2019.

3. Correct Answer: B. 7.5 MHz, 8 mm crystal diameter

Rationale: The focal depth is the area with the best lateral image resolution and depends on ultrasound frequency and the diameter of the active element. Less divergence produces a narrower beam in the far field, also improving lateral resolution. Transducers with a larger active element and higher frequency will have a deeper focal length and less divergence, and therefore improved lateral resolution in the far field.

1. Edelman SK. Ultrasound Physics and Instrumentation. Education for the Sonographic Professional; 2011.

2. Hoskins P, Martin K, Thrush A. Diagnostic Ultrasound: Physics and Equipment. Cambridge University Press; 2019.

4. Correct Answer: B. The phased array transducer uses delays in electronic impulses to electronically steer and focus the ultrasound beam.

Rationale: Phased array probes are designed with the unique ability of electronic focusing and steering. They are composed of an array of hundreds of individual crystal elements. Electronic signals from the ultrasound machine excite the elements, creating a sound pulse, but with variable time delays of approximately 10 ns between each element. These time delays are called phasing and act as an electronic lens that “steers” and focuses the beam. Linear sequential arrays are linear arrangements of piezoelectric elements that sequentially fire. Linear sequential arrays use either a curved crystal (option A) or an external lens to generate fixed focusing (option D). A convex or curved array (e.g., curvilinear probe) uses piezoelectric crystal elements arranged in a curve to provide a natural sector image (option C).