Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): In ultrasonography, the axial resolution and the maximum available depth of examination is higher for higher frequency transducers.
Reason (R): The high frequency transducers produce shorter pulses with longer pulse interval, and are attenuated early in the tissues due to higher number of interactions.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

Remember the golden rule of ultrasound frequencies:
- High Frequency (e.g., 10-15 MHz): Excellent resolution, poor penetration (used for superficial organs like tendons, eyes, or cat abdomens).
- Low Frequency (e.g., 3-5 MHz): Poor resolution, deep penetration (used for large dogs or transabdominal equine scans).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Ultrasonic resolution and tissue penetration are determined by the frequency of the transducer.
There is a fundamental clinical trade-off between image resolution (ability to distinguish closely spaced structures) and penetration depth.

Step 2: Detailed Explanation:
Let us analyze Assertion (A):
Axial resolution is the ability to distinguish two structures lying along the path of the ultrasound beam.
Axial resolution is calculated as:
\[ \text{Axial Resolution} = \frac{\text{Spatial Pulse Length (SPL)}}{2} \]
Higher frequency transducers produce shorter wavelengths ($\lambda$), which decreases the SPL and significantly improves (highers) axial resolution.
However, high-frequency sound waves undergo greater attenuation (energy loss) as they travel through tissue.
Therefore, the maximum available depth of examination is lower (not higher) for higher frequency transducers.
Because depth of examination decreases with higher frequency, Assertion (A) is incorrect.
Let us analyze Reason (R):
High-frequency transducers produce shorter pulses.
These high-frequency waves undergo rapid attenuation early in tissues due to a higher frequency of interactions (absorption and scattering) per unit distance.
Therefore, Reason (R) is correct in stating that high-frequency waves attenuate early due to these interactions.

Step 3: Final Answer:
Since Assertion (A) is not correct but Reason (R) is correct, the correct option is (D).