# Ultrasound Transducer and System

Ultrasound Transducer and System

Christopher R. Tainter

Cameron Smyres

Michael Self

Raghu Seethala

1. A 37-year-old woman who is 34 weeks pregnant presents with acute shortness of breath. In order to evaluate for cardiogenic causes of her symptoms, which of the following transducers would be most appropriate?

A. 1 to 2 MHz

B. 2 to 5 MHz

C. 5 to 12 MHz

D. 12 to 15 MHz

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.

Selected Reference

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

2. You want to get the best quality image of a deep structure while performing echocardiography with a phased array transducer. Which transducer crystal will have the best lateral resolution in the Fraunhofer zone?

A. 10-mm-thick crystal

B. 8-mm-thick crystal

C. 6-mm-thick crystal

D. 4-mm-thick crystal

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.

Selected References

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. A 65-year-old man with long-standing hypertension is being evaluated for severe chest pain radiating to his back. Point-of-care ultrasound is being used to identify the descending aorta in a long-axis view. Which of the following transducers will provide the best resolution of the descending aorta?

A. 4 MHz, 4 mm crystal diameter

B. 7.5 MHz, 8 mm crystal diameter

C. 4 MHz, 8 mm crystal diameter

D. 7.5 MHz, 4 mm crystal diameter

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.

Selected References

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. A 28-year-old man is admitted to the intensive care unit (ICU) in cardiogenic shock from myocarditis. You prepare to perform transthoracic echocardiography to assess his response to resuscitation. The ultrasound machine is equipped with a linear sequential transducer and a phased array transducer. Which of the following options best describes how the phased array transducer differs from the linear transducer?

A. The phased array transducer uses a curved crystal to generate fixed focusing.

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

C. The phased array transducer uses piezoelectric crystals arranged in a curve to provide a natural sector image.

D. The phased array transducer uses an external lens to generate adjustable focusing.