Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Transducer should be placed perfectly parallel to the blood flow to get an insonation angle of 0 to 180 degrees, at the time of Doppler ultrasonography.
Reason (R): At an insonation angle of 0 to 180 degrees, the transducer receives maximum Doppler's shift from the blood flow.
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:

In Doppler ultrasound, parallel alignment ($\theta = 0^\circ$ or $180^\circ$) yields the maximum Doppler shift because $\cos(0^\circ) = 1$.
Avoid a $90^\circ$ angle (perpendicular), as $\cos(90^\circ) = 0$, which will show no blood flow on the screen even if the vessel is patent.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Doppler ultrasonography is used to measure the velocity and direction of blood flow.
The frequency shift of the reflected ultrasound waves (Doppler shift) is described mathematically by the Doppler equation.
Key Formula or Approach:
The Doppler shift ($\Delta f$) is calculated using the formula:
\[ \Delta f = \frac{2 f_0 v \cos\theta}{c} \]
where $f_0$ is the transmitted frequency, $v$ is the blood flow velocity, $c$ is the speed of sound in tissue, and $\theta$ is the angle of insonation (the angle between the ultrasound beam and the direction of blood flow).

Step 2: Detailed Explanation:
Let us analyze the relationship between the angle of insonation ($\theta$) and the Doppler shift:
The Doppler shift ($\Delta f$) is directly proportional to $\cos\theta$.
The maximum possible value for $\cos\theta$ is $+1$ (when $\theta = 0^\circ$, blood flowing directly toward the transducer) or $-1$ (when $\theta = 180^\circ$, blood flowing directly away from the transducer).
If the ultrasound beam is perpendicular to the blood flow ($\theta = 90^\circ$), $\cos(90^\circ) = 0$, resulting in zero Doppler shift.
Therefore, to receive the maximum Doppler shift and obtain accurate blood velocity measurements, the transducer should be aligned as parallel as possible to the blood vessel ($\theta \approx 0^\circ$ or $180^\circ$).
This makes Assertion (A) correct, and Reason (R) correct.
Since the maximum Doppler shift is the exact reason why we position the transducer parallel, (R) is the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are correct, and (R) is the correct explanation of (A).