ULTRASOUND

RATIONALE:

In this chapter, we will explain the basic physics of how sound waves can produce images of the human body. Start from the idea that all the various techniques of diagnostic ultrasound involve the detection and display of the acoustic energy reflected off different tissues in the body.

CORE IDEA

Ultrasound uses high-frequency sound waves (2–20 MHz) to produce images based on echoes from tissue interfaces — NO ionizing radiation.

⚖ VERY IMPORTANT (Board Comparison)

Ultrasound

Nuclear Medicine

Sound waves

Gamma rays

No radiation

Uses radioactive tracers

Real-time imaging

Functional imaging

Operator dependent

Less operator-dependent

1. PROPERTIES OF ULTRASOUND (ULTRA HIGH-YIELD)

Key Equation:

λ=vf\lambda = \frac{v}{f}λ=fv​

👉 Meaning:

  • ↑ Frequency → ↓ Wavelength → ↑ Resolution

  • BUT ↓ Penetration ❗

MUST MEMORIZE:

  • Speed in soft tissue = 1540 m/s (BOARD FAVORITE)

2. TYPES OF WAVES

  • Longitudinal → main in soft tissue

  • Transverse → only in solids (bone)

👉 Exam trap:
Ultrasound in body = LONGITUDINAL

3. PIEZOELECTRIC EFFECT (VERY IMPORTANT)

Concept:

  • Converts electrical → sound (transmit)

  • Converts sound → electrical (receive)

👉 Crystal = heart of the ultrasound machine

Keywords:

  • PZT (Lead Zirconate Titanate)

  • Discovered by the Curie brothers (1880)

4. HOW IS AN IMAGE FORMED

  1. The transducer emits a pulse

  2. Sound hits tissue

  3. Echo returns

  4. Converted to an electrical signal

  5. Image formed

👉 Bright = strong echo
👉 Dark = no echo (fluid)

5. ULTRASOUND SYSTEM COMPONENTS

  • Transducer (MOST IMPORTANT)

  • Pulse generator

  • Amplifier (TGC ⚠)

  • Scan converter

  • Image processor

  • Display

👉 BOARD TIP:
TGC = adjusts brightness based on depth

6. TYPES OF TRANSDUCERS (SUPER COMMON IN EXAMS)

Type

Use

Frequency

Linear

Superficial

High

Sector

Cardiac

Low

Convex

Abdomen

Medium

👉 Pattern:

  • High freq → shallow

  • Low freq → deep

7. MODES OF ULTRASOUND

  • Pulsed → imaging (MOST COMMON)

  • Continuous wave → Doppler (blood flow)

👉 If the question mentions velocity of blood → DOPPLER

8. DOPPLER (IMPORTANT LINK TO CLINICAL)

  • Measures:

    • Blood flow

    • Direction

    • Velocity

👉 Uses Doppler Principle

9. IMAGE QUALITY FACTORS

Resolution depends on:

  • Frequency (↑ freq = ↑ resolution)

  • Wavelength (shorter = better)

Penetration depends on:

  • Frequency (↓ freq = deeper)

10. LIMITATIONS (COMMON MCQ)

  • Cannot pass through:

    • Bone

    • Air

👉 Result:

  • Acoustic shadowing

11. INTENSITY & SAFETY

  • Measured in:

    • W/cm²

  • Effects:

    • Thermal

    • Mechanical (cavitation)

👉 BUT:
Ultrasound is generally SAFE

🧠 ULTRA-SHORT SUMMARY (FOR LAST-MINUTE REVIEW)

  • No radiation

  • 2–20 MHz

  • Speed = 1540 m/s

  • Piezoelectric crystals

  • Pulse-echo principle

  • High freq = better resolution, less penetration

  • Doppler = blood flow

⚠ HONEST NOTE (since you said NUCMED focus)

This topic is a lower priority compared to NUCMED on the boards.

👉 NUCMED usually includes:

  • Radiopharmaceuticals

  • Gamma camera

  • Half-life

  • Decay

  • PET / SPECT

ULTRASOUND MODES & DOPPLER — CORE UNDERSTANDING

🔹 1. A-MODE (Amplitude Mode)

Concept:

  • Simplest and oldest ultrasound technique (now mostly obsolete).

  • Produces a 1D graph:

    • X-axis = time/depth

    • Y-axis = amplitude (echo strength)

How it works:

  • Echoes appear as spikes/peaks

  • Taller peak = stronger reflection

Clinical use (limited):

  • Ophthalmology

  • Detecting midline brain shift

Limitations:

  • No image (only spikes)

  • No direction

  • Cannot identify structures clearly

👉 Think: “Just peaks, no picture.”

🔹 2. B-MODE (Brightness Mode) ⭐ MAIN MODE

Concept:

  • Converts echo amplitude into brightness (dots)

  • Produces a 2D grayscale image

How it works:

  • Each echo = a dot

  • Strong echo → bright (white)

  • Weak echo → dark (gray/black)

  • Many scan lines → form an image

Key directions:

  • Axial = along the beam

  • Lateral = across the beam

Why important:

  • Foundation of modern ultrasound

  • Real-time imaging

👉 Think: “Dots become a picture.”

🔹 3. M-MODE (Motion Mode)

Concept:

  • Shows motion over time (1D + time)

How it works:

  • Repeated scanning of one line only

  • Displays movement as a waveform

Best use:

  • Echocardiography (heart valves, walls)

Limitation:

  • Hard to position the beam accurately

👉 Think: “Motion tracker of one line.”

🔹 4. 2D B-SCAN

Concept:

  • Multiple A-lines combined → 2D image

Types of transducers:

  • Linear array → rectangular image

  • Curved array → curved image

  • Phased array → sector image

Key idea:

  • Image built line-by-line rapidly → real-time

👉 Think: “Many lines = full image.”

🔹 5. 3D & 4D ULTRASOUND

3D:

  • Combines multiple 2D images

  • Produces depth + volume

4D:

  • 3D + real-time motion

Uses:

  • Obstetrics (fetal face, anomalies)

  • Interventional guidance

👉 Think:

  • 3D = photo

  • 4D = video

🔹 6. HARMONIC IMAGING

Concept:

  • Uses harmonic frequencies (2× original frequency)

Advantage:

  • Better image quality

  • Less noise/artifacts

👉 Think: “Cleaner image using echoes’ harmonics.”

🔹 7. B-FLOW

Concept:

  • Direct visualization of blood flow without Doppler

Features:

  • Shows blood as moving dots

  • Simultaneous vessel + flow visualization

Limitation:

  • No velocity measurement

👉 Think: “See flow, but not speed.”

🩸 DOPPLER ULTRASOUND

🔹 DOPPLER EFFECT (CORE IDEA)

Definition:

  • Change in frequency due to motion

Key rules:

  • Toward probe → higher frequency

  • Away from probe → lower frequency

👉 Used to measure:

  • Blood flow velocity

  • Blood flow direction

🔹 DOPPLER EQUATION (VERY IMPORTANT)

fD=2ftvcos⁡(θ)cf_D = \frac{2 f_t v \cos(\theta)}{c}fD​=c2ft​vcos(θ)​

Meaning of variables:

  • fDf_DfD​ = Doppler shift

  • ftf_tft​ = transmitted frequency

  • vvv = blood velocity

  • θ\thetaθ = angle

  • ccc = speed of sound

Key insight:

  • Smaller angle → more accurate velocity

  • At 90° → no Doppler signal

👉 Think: “Align beam with flow.”

🔹 TYPES OF DOPPLER

1. CONTINUOUS WAVE (CW)

Features:

  • Continuous transmit + receive

  • Uses 2 crystals

Advantage:

  • Measures very high velocities

Disadvantage:

  • No depth information

👉 Think: “Fast but blind to location.”

2. PULSED WAVE (PW)

Features:

  • Sends pulses

  • Measures at specific depth (range gating)

Advantage:

  • Precise location

Disadvantage:

  • Cannot measure high velocity → aliasing

👉 Think: “Precise but limited.”

🔹 NYQUIST LIMIT & ALIASING ⚠

Definition:

  • Maximum measurable Doppler shift

Nyquist Limit=PRF2\text{Nyquist Limit} = \frac{\text{PRF}}{2}Nyquist Limit=2PRF​

Aliasing occurs when:

  • Velocity > Nyquist limit

Appearance:

  • “Wrap-around” signal

  • Wrong direction display

👉 Think: “Too fast = wrong reading.”

🔹 COLOR FLOW MAPPING

Concept:

  • Combines B-mode + Doppler

Output:

  • Color-coded flow:

    • Red → toward probe

    • Blue → away

👉 Think: “Color shows flow direction.”

🔹 KEY COMPARISON (HIGH-YIELD)

Feature

A-mode

B-mode

M-mode

Dimension

1D

2D

1D + time

Output

Spikes

Image

Motion graph

Use

Rare

Main imaging

Heart motion

🔥 FINAL MEMORY ANCHOR

  • A-mode → spikes

  • B-mode → image ⭐

  • M-mode → motion

  • CW Doppler → high velocity

  • PW Doppler → precise location

  • Aliasing → velocity too high

  • 3D/4D → depth + real-time