UTC Ultrasound Physics & Instrumentation 2 — Questions and Answers
Question 1: In pulsed wave Doppler, the Nyquist limit represents which value?
- Maximum imaging depth achievable
- Minimum frequency required for tissue imaging
- Half the pulse repetition frequency (PRF/2), the maximum detectable Doppler shift without aliasing (Correct answer)
- Maximum acoustic power output of the transducer
Correct answer: Half the pulse repetition frequency (PRF/2), the maximum detectable Doppler shift without aliasing
The Nyquist limit equals PRF/2; Doppler shift frequencies exceeding this limit result in aliasing, where high-velocity flows are misrepresented as flowing in the opposite direction.
Question 2: Which transducer type fires elements sequentially to produce a rectangular field of view, making it ideal for vascular and musculoskeletal imaging?
- Phased array
- Curvilinear (convex) array
- Linear array (Correct answer)
- Annular array
Correct answer: Linear array
Linear array transducers fire elements sequentially to produce a flat, rectangular image, providing excellent near-field resolution for superficial structures such as vessels and tendons.
Question 3: The Doppler effect used in ultrasound refers to which phenomenon?
- Attenuation of sound waves as they travel through tissue
- Change in received frequency caused by relative motion between the sound source and a moving reflector (Correct answer)
- Reflection of sound waves at tissue boundaries
- Focusing of the sound beam by the transducer lens
Correct answer: Change in received frequency caused by relative motion between the sound source and a moving reflector
The Doppler effect is the change in received frequency compared to transmitted frequency due to relative motion between the transducer and moving reflectors such as blood cells.
Question 4: What is the primary advantage of tissue harmonic imaging over fundamental (conventional) imaging?
- Increased penetration depth for deep structures
- Higher frame rates for moving structures
- Reduced near-field artifacts and improved contrast resolution (Correct answer)
- Better sensitivity for low-velocity Doppler flows
Correct answer: Reduced near-field artifacts and improved contrast resolution
Tissue harmonic imaging uses harmonics generated within tissue (at twice the transmitted frequency) rather than the fundamental frequency, reducing artifacts and clutter while improving contrast resolution.
Question 5: A phased array transducer is most commonly used for which clinical application?
- Peripheral vascular imaging of leg veins
- Cardiac (echocardiographic) imaging through narrow intercostal windows (Correct answer)
- Musculoskeletal imaging of superficial tendons
- Breast mass characterization
Correct answer: Cardiac (echocardiographic) imaging through narrow intercostal windows
Phased array transducers steer the beam electronically through a small footprint, allowing cardiac imaging through the narrow intercostal spaces between the ribs.
Question 6: Aliasing in Doppler ultrasound is best described as occurring when:
- The overall system gain is set too high
- The Doppler angle exceeds 60 degrees from the direction of flow
- The detected Doppler shift frequency exceeds the Nyquist limit (PRF/2) (Correct answer)
- The transducer frequency is too low for the depth of interest
Correct answer: The detected Doppler shift frequency exceeds the Nyquist limit (PRF/2)
Aliasing occurs when the Doppler shift exceeds the Nyquist limit (PRF/2), causing high-velocity flow signals to wrap around and be displayed as flowing in the opposite direction.
Question 7: What is the primary purpose of time gain compensation (TGC) in ultrasound imaging?
- To increase the frame rate for real-time imaging
- To reduce the depth of the displayed image
- To amplify returning echoes from deeper tissues and compensate for acoustic attenuation (Correct answer)
- To narrow the beam width at a specific focal depth
Correct answer: To amplify returning echoes from deeper tissues and compensate for acoustic attenuation
TGC applies progressively increasing amplification to echoes from deeper structures, compensating for the natural attenuation of sound as it travels deeper into tissue and ensuring uniform image brightness.
UTC Ultrasound Physics & Instrumentation 2
In pulsed wave Doppler, the Nyquist limit represents which value?