LECTURE NOTES

ts off moving red blood cells.

📌 If there is no relative motion, there is no Doppler shift.

🔁 DOPPLER SHIFT (DOPPLER FREQUENCY)

Doppler shift is the difference between transmitted and reflected frequencies.

Doppler shift = reflected frequency − transmitted frequency

• It is a low-frequency signal riding on a high-frequency carrier wave

• Extracting this low frequency is called demodulation

• Doppler shifts fall in the audible range (20 Hz–20 kHz)

📌 Transducer frequencies are in MHz, Doppler shifts are in Hz

➕ POSITIVE vs ➖ NEGATIVE DOPPLER SHIFTS

Positive Doppler Shift

• Blood moving toward the transducer

• Reflected frequency is higher

• Displayed above the baseline

Negative Doppler Shift

• Blood moving away from the transducer

• Reflected frequency is lower

• Displayed below the baseline

🚗 SPEED vs 🎯 VELOCITY

Speed

• Magnitude only

• Distance per time (cm/s)

Velocity

• Magnitude and direction

• Doppler measures velocity, not speed

📌 Doppler always includes directional information

🧮 THE DOPPLER EQUATION (CONCEPTUAL)

Doppler shift is directly related to:

• Blood velocity

• Transducer frequency

• Cosine of the insonation angle

Doppler shift is inversely related to:

• Propagation speed of sound in tissue

📌 The “2” in the equation exists because Doppler shift occurs twice:

1. When sound hits moving blood

2. When reflected sound returns to the transducer

⚡ DOPPLER SHIFT & VELOCITY RELATIONSHIP

• Velocity ↑ → Doppler shift ↑

• Velocity ↓ → Doppler shift ↓

• If velocity doubles → Doppler shift doubles

📌 Doppler shift is proportional to velocity

🎚️ DOPPLER SHIFT & TRANSDUCER FREQUENCY

• Higher transducer frequency → higher Doppler shift

• Lower transducer frequency → lower Doppler shift

📌 Even though Doppler shift changes, calculated velocity remains the same

📐 ANGLE DEPENDENCE (COSINE θ)

The Doppler measurement depends on the angle between blood flow and the sound beam.

Measured velocity = true velocity × cos θ

Key Angles

• 0° or 180° → 100% of true velocity

• 60° → 50% of true velocity

• 90° → 0 velocity measured (cos 90° = 0)

📌 Doppler cannot measure velocity at 90°

🔄 BIDIRECTIONAL DOPPLER

Bidirectional Doppler distinguishes flow direction:

• Toward transducer → positive shift

• Away from transducer → negative shift

Displayed as:

• Above baseline (toward)

• Below baseline (away)

📌 Uses phase quadrature detection

🌊 CONTINUOUS WAVE (CW) DOPPLER

How it works

• Two crystals: one transmits continuously, one receives continuously

Advantages

• Measures very high velocities

• No aliasing

• High sensitivity

Disadvantages

• No range resolution

• Measures velocities along entire beam

• Range ambiguity

• No TGC

📌 Best for severe stenosis

📍 PULSED WAVE (PW) DOPPLER

How it works

• One crystal alternates between transmit and receive

• Uses a sample volume (gate)

Advantages

• Exact location of velocity measurement

• Range resolution

Disadvantages

• Limited maximum velocity

• Aliasing occurs

📌 Used in duplex imaging

🚨 ALIASING

Aliasing

• False display of high velocity in the opposite direction

• Occurs only with pulsed Doppler

Nyquist Limit

• Maximum Doppler frequency without aliasing

• Nyquist limit = PRF ÷ 2

📌 Aliasing occurs when Doppler shift exceeds Nyquist limit

🛠️ METHODS TO REDUCE ALIASING

1. Increase Doppler scale (↑ PRF)

2. Use a shallower sample volume

3. Use a lower frequency transducer

4. Apply baseline shift (display fix only)

5. Use continuous wave Doppler

🎨 COLOR FLOW DOPPLER

• Displays mean velocity

• Superimposed on B-mode image

• Direction and flow presence

• Subject to aliasing

• Uses pulsed ultrasound

📌 Color ≠ anatomy, grayscale = anatomy

🗺️ COLOR MAPS

Velocity Mode

• Up/down color change

• Shows direction and speed

Variance Mode

• Up/down = direction

• Left/right = laminar vs turbulent

📌 Left = Laminar
📌 Right = Turbulent

📦 DOPPLER PACKETS (ENSEMBLES)

Packets = multiple pulses per line

Larger packets

• Better accuracy

• Better low-flow sensitivity

• Lower frame rate

📌 Balance accuracy vs temporal resolution

🔋 POWER DOPPLER

• Displays presence of flow only

• No velocity or direction

• Based on signal amplitude

Advantages

• Very sensitive to low flow

• No aliasing

• Angle independent (except 90°)

Disadvantages

• Motion sensitive

• Lower frame rate

• No velocity data

⚠️ DOPPLER ARTIFACTS

Clutter / Ghosting

• From slow-moving tissue

• Removed using wall filters

Crosstalk

• Mirror spectrum above and below baseline

• Caused by high Doppler gain or near-90° angle

📊 SPECTRAL ANALYSIS

Used to break complex Doppler signals into velocity components.

FFT

• Used for PW and CW Doppler

• Displays full velocity range

• Shows spectral broadening

Autocorrelation

• Used for color Doppler

• Faster but less accurate

📌 Spectral broadening = turbulence

🎯 SECTION 19 – SPI EXAM TAKEAWAYS

• Doppler measures velocity, not speed

• Angle correction is critical

• Aliasing = pulsed Doppler only

• CW Doppler never aliases

• Color Doppler = mean velocity

• Spectral Doppler = peak velocity

• Wall filters remove clutter