**Question:** In colour Doppler the color depends upon:

A. Resistance index
B. Velocity index
C. Spectral waveform
D. Rendezvous point

**Core Concept:**

Color Doppler ultrasound is a technique used in medical imaging that combines the principles of conventional ultrasound and Doppler ultrasound to visualize blood flow within blood vessels. The color in color Doppler represents the direction and velocity of blood flow. Color is generated based on the frequency shift of the reflected ultrasound waves due to the motion of red blood cells.

**Why the Correct Answer is Right:**

The correct answer, C. Spectral waveform, explains that the color in color Doppler ultrasound is determined by the spectral waveform. A spectral waveform is generated for each blood vessel when blood flow is detected. This waveform represents the frequency shift of the reflected ultrasound waves due to the motion of red blood cells. The color on the image corresponds to the direction and velocity of blood flow.

**Why Each Wrong Option is Incorrect:**

A. Resistance index (RI) is a numerical value that indicates the resistance of blood flow in a vessel. It is not the correct basis for color in color Doppler ultrasound.

B. Velocity index (VI) is a numerical value that represents the average velocity of blood flow in a vessel. It is not the correct basis for color in color Doppler ultrasound.

D. Rendezvous point refers to a point where two blood vessels meet or converge. It does not describe the basis of color generation in color Doppler ultrasound.

**Clinical Pearl:**

Understanding the basis of color in color Doppler ultrasound is essential for interpreting the images accurately. The spectral waveform provides important information about blood flow, including direction, velocity, and blood flow characteristics, allowing healthcare professionals to diagnose and monitor various cardiovascular and vascular disorders.