📘 SECTION 01: THE BASICS (DETAILED – SPI FOUNDATION)
🧠 CORE CONCEPTS USED THROUGHOUT ULTRASOUND PHYSICS
📊 GRAPHS IN DIAGNOSTIC ULTRASOUND
In diagnostic ultrasound, information is often displayed in graphical form to show how one variable changes in relation to another (for example, blood velocity over time in Doppler).
AXES OF A GRAPH
-
X-AXIS (Horizontal axis): Runs side to side
-
Y-AXIS (Vertical axis): Runs up and down
📌 The independent variable is usually on the x-axis, and the dependent variable is usually on the y-axis.
🔗 RELATIONSHIPS BETWEEN VARIABLES
Understanding relationships is critical for SPI physics questions.
❌ UNRELATED VARIABLES
Two items have no association with each other.
Examples:
-
Shoe size and IQ
-
Hair color and temperature
-
Day of the week and body weight
📌 A graph of unrelated variables shows no predictable pattern.
➕ DIRECTLY RELATED (DIRECTLY PROPORTIONAL)
When one variable increases, the other also increases.
Graph direction: lower left → upper right
Examples:
-
Practice ↑ → Skill ↑
-
Age ↑ → Experience ↑
-
Weight ↑ → Clothing size ↑
📌 Direct relationships move together.
🔁 RELATED / PROPORTIONAL (GENERAL)
Variables are associated, but the exact pattern is not specified.
Examples:
-
Studying and exam score
-
Flossing and dental health
-
Dieting and weight
📌 May be direct or inverse, but some connection exists.
➖ INVERSELY RELATED (INVERSELY PROPORTIONAL)
When one variable increases, the other decreases.
Graph direction: upper left → lower right
Examples:
-
Exercise ↑ → Weight ↓
-
Alcohol intake ↑ → Sobriety ↓
-
Skill ↑ → Golf score ↓
📌 Inverse relationships move in opposite directions.
🔄 RECIPROCAL RELATIONSHIP
A special inverse relationship where two values multiply to equal 1.
Examples:
-
2 and ½
-
10 and 1/10
-
Period and Frequency (very important in ultrasound)
📌 If one increases, the other must decrease.
📐 UNITS IN ULTRASOUND PHYSICS
A number without units is meaningless.
Example:
“6” is incomplete → 6 seconds? 6 cm? 6 years?
COMMON UNITS
-
Length: cm, mm, meters
-
Area: cm²
-
Volume: cm³
-
Time: seconds (s)
-
Frequency: hertz (Hz)
📌 A percentage (%) is unitless.
FACTOR CHANGES
-
Increase by a factor of 10 → multiply by 10
-
Decrease by a factor of 5 → divide by 5
🔄 UNIT CONVERSION
Unit conversion changes how a value is expressed, not its actual size.
Examples:
-
12 inches = 1 foot
-
1 inch = 2.54 cm
-
1 dollar = 10 dimes
📌 Treat conversion ratios as fractions equal to 1 and carry the units through.
🔢 POWERS OF TEN (SCIENTIFIC NOTATION)
Used to express very large or very small numbers.
RULES
-
Positive exponent: value > 1
-
Negative exponent: value < 1
-
Exponent of zero: value between 1 and 10
Examples:
-
1,000,000 = 1.0 × 10⁶
-
0.000001 = 1.0 × 10⁻⁶
📏 METRIC SYSTEM (POWERS OF TEN)
Common prefixes used in ultrasound:
-
Giga (G): 10⁹
-
Mega (M): 10⁶
-
Kilo (k): 10³
-
Milli (m): 10⁻³
-
Micro (μ): 10⁻⁶
-
Nano (n): 10⁻⁹
📌 As the prefix gets smaller, the number gets larger.
🔁 COMPLEMENTARY METRIC PAIRS (SPI FAVORITE)
These pairs are commonly used together:
-
MHz ↔ μs (frequency ↔ period)
-
kHz ↔ ms
-
GHz ↔ ns
📌 Millions of cycles ↔ millionths of seconds.
🔊 INTRODUCTION TO SOUND (LINK TO SECTION 02)
In diagnostic ultrasound:
-
Sound pulses are generated by a transducer
-
Sound travels through biologic tissue
-
Echoes return and are processed into images
Sound:
-
Is a mechanical wave
-
Requires a medium
-
Travels in a straight line
-
Is a longitudinal wave
🧠 KEY SPI TAKEAWAYS – SECTION 01
✔ Understand graph axes
✔ Recognize direct, inverse, and reciprocal relationships
✔ Always include correct units
✔ Master unit conversions
✔ Know metric prefixes and powers of ten
✔ Recognize complementary metric pairs
✔ Sound concepts begin here
📘 SECTION 02: SOUND (DETAILED – SPI CORE PHYSICS)
🧠 FOUNDATION OF ALL ULTRASOUND IMAGING
🔊 WHAT IS SOUND IN ULTRASOUND?
In diagnostic ultrasound, sound is the tool used to create images.
The transducer sends sound pulses into the body, and echoes returning from tissue boundaries are processed into an image.
📌 Key idea: Ultrasound imaging is entirely based on how sound travels, reflects, and interacts with tissue.
🌊 SOUND WAVES – BASIC CONCEPT
A wave is a method of transferring energy, not matter, from one place to another.
Examples of waves:
-
Sound
-
Light
-
Heat
-
Magnetic waves
📌 Sound is a mechanical wave, meaning it requires a medium to travel.
🚫 SOUND CANNOT TRAVEL THROUGH A VACUUM
Sound must travel through a medium (such as air, water, or tissue).
Reason:
-
Sound depends on particle motion
-
No particles → no sound
This is why ultrasound works in tissue but not in empty space.
🔁 COMPRESSION AND RAREFACTION
Sound travels through tissue by causing particles to vibrate back and forth.
-
Compression:
-
Particles are squeezed together
-
High pressure
-
High density
-
-
Rarefaction:
-
Particles are spread apart
-
Low pressure
-
Low density
-
📌 Ultrasound images are formed by detecting echoes created from these pressure changes.
➡️ SOUND TRAVELS IN A STRAIGHT LINE
Ultrasound systems assume:
-
Sound travels in a straight line
-
Sound travels directly to a reflector and back
📌 Many artifacts occur when these assumptions are violated (important later in SPI).
📐 TYPES OF WAVES
🔀 TRANSVERSE WAVES
-
Particle motion is perpendicular to wave direction
-
Example: shaking a rope up and down
📌 Ultrasound is NOT transverse
➡️ LONGITUDINAL WAVES
-
Particle motion is parallel to wave direction
-
Sound waves move this way
📌 Ultrasound is a longitudinal wave
🧠 ACOUSTIC VARIABLES (IDENTIFY SOUND WAVES)
A wave is classified as a sound wave only if one or more acoustic variables oscillate rhythmically.
THE THREE ACOUSTIC VARIABLES
1️⃣ Pressure
-
Concentration of force over an area
-
Units: pascals (Pa)
2️⃣ Density
-
Mass per unit volume
-
Units: kg/cm³
3️⃣ Distance (Particle Motion)
-
Back-and-forth movement of particles
-
Units: cm, mm
📌 If pressure, density, or distance oscillates → the wave is sound.
⚠️ ACOUSTIC VARIABLES ≠ ACOUSTIC PARAMETERS
-
Acoustic variables → identify sound
-
Acoustic parameters → describe sound
This distinction is heavily tested on SPI.
📏 ACOUSTIC PARAMETERS (INTRODUCTION)
Once a wave is confirmed to be sound, it is described using seven acoustic parameters:
-
Period
-
Frequency
-
Amplitude
-
Power
-
Intensity
-
Wavelength
-
Propagation speed
📌 These are explained in detail in Section 03.
🎵 IN-PHASE AND OUT-OF-PHASE WAVES
✔️ IN-PHASE WAVES
-
Peaks occur at the same time and place
-
Troughs align
-
Waves are “in step”
📌 Like a synchronized marching band
❌ OUT-OF-PHASE WAVES
-
Peaks and troughs do not align
-
Waves are “out of step”
➕➖ INTERFERENCE
When two or more waves arrive at the same location at the same time, they combine → interference.
TYPES OF INTERFERENCE
➕ CONSTRUCTIVE INTERFERENCE
-
Occurs with in-phase waves
-
Amplitudes add together
-
Resulting wave is larger
📌 Stronger echo
➖ DESTRUCTIVE INTERFERENCE
-
Occurs with out-of-phase waves
-
Amplitudes partially or completely cancel
-
Resulting wave is smaller or zero
📌 Weaker or absent echo
❌ COMPLETE DESTRUCTIVE INTERFERENCE
-
Occurs when out-of-phase waves have equal amplitude
-
Result = no wave
🔄 INTERFERENCE WITH DIFFERENT FREQUENCIES
When waves have different frequencies:
-
Sometimes they are in-phase → constructive interference
-
Sometimes out-of-phase → destructive interference
📌 This creates alternating bright and dark patterns (important for speckle and noise later).
🧠 SPI CLINICAL CONNECTION
-
Constructive interference → bright echoes
-
Destructive interference → weak or missing echoes
-
Speckle is caused by interference of many small wavelets
-
📘 SECTION 02: SOUND (DETAILED – SPI CORE PHYSICS)
🧠 FOUNDATION OF ALL ULTRASOUND IMAGING
🔊 WHAT IS SOUND IN ULTRASOUND?
In diagnostic ultrasound, sound is the tool used to create images.
The transducer sends sound pulses into the body, and echoes returning from tissue boundaries are processed into an image.📌 Key idea: Ultrasound imaging is entirely based on how sound travels, reflects, and interacts with tissue.
🌊 SOUND WAVES – BASIC CONCEPT
A wave is a method of transferring energy, not matter, from one place to another.
Examples of waves:
-
Sound
-
Light
-
Heat
-
Magnetic waves
📌 Sound is a mechanical wave, meaning it requires a medium to travel.
🚫 SOUND CANNOT TRAVEL THROUGH A VACUUM
Sound must travel through a medium (such as air, water, or tissue).
Reason:
-
Sound depends on particle motion
-
No particles → no sound
This is why ultrasound works in tissue but not in empty space.
🔁 COMPRESSION AND RAREFACTION
Sound travels through tissue by causing particles to vibrate back and forth.
-
Compression:
-
Particles are squeezed together
-
High pressure
-
High density
-
-
Rarefaction:
-
Particles are spread apart
-
Low pressure
-
Low density
-
📌 Ultrasound images are formed by detecting echoes created from these pressure changes.
➡️ SOUND TRAVELS IN A STRAIGHT LINE
Ultrasound systems assume:
-
Sound travels in a straight line
-
Sound travels directly to a reflector and back
📌 Many artifacts occur when these assumptions are violated (important later in SPI).
📐 TYPES OF WAVES
🔀 TRANSVERSE WAVES
-
Particle motion is perpendicular to wave direction
-
Example: shaking a rope up and down
📌 Ultrasound is NOT transverse
➡️ LONGITUDINAL WAVES
-
Particle motion is parallel to wave direction
-
Sound waves move this way
📌 Ultrasound is a longitudinal wave
🧠 ACOUSTIC VARIABLES (IDENTIFY SOUND WAVES)
A wave is classified as a sound wave only if one or more acoustic variables oscillate rhythmically.
THE THREE ACOUSTIC VARIABLES
1️⃣ Pressure
-
Concentration of force over an area
-
Units: pascals (Pa)
2️⃣ Density
-
Mass per unit volume
-
Units: kg/cm³
3️⃣ Distance (Particle Motion)
-
Back-and-forth movement of particles
-
Units: cm, mm
📌 If pressure, density, or distance oscillates → the wave is sound.
⚠️ ACOUSTIC VARIABLES ≠ ACOUSTIC PARAMETERS
-
Acoustic variables → identify sound
-
Acoustic parameters → describe sound
This distinction is heavily tested on SPI.
📏 ACOUSTIC PARAMETERS (INTRODUCTION)
Once a wave is confirmed to be sound, it is described using seven acoustic parameters:
-
Period
-
Frequency
-
Amplitude
-
Power
-
Intensity
-
Wavelength
-
Propagation speed
📌 These are explained in detail in Section 03.
🎵 IN-PHASE AND OUT-OF-PHASE WAVES
✔️ IN-PHASE WAVES
-
Peaks occur at the same time and place
-
Troughs align
-
Waves are “in step”
📌 Like a synchronized marching band
❌ OUT-OF-PHASE WAVES
-
Peaks and troughs do not align
-
Waves are “out of step”
➕➖ INTERFERENCE
When two or more waves arrive at the same location at the same time, they combine → interference.
TYPES OF INTERFERENCE
➕ CONSTRUCTIVE INTERFERENCE
-
Occurs with in-phase waves
-
Amplitudes add together
-
Resulting wave is larger
📌 Stronger echo
➖ DESTRUCTIVE INTERFERENCE
-
Occurs with out-of-phase waves
-
Amplitudes partially or completely cancel
-
Resulting wave is smaller or zero
📌 Weaker or absent echo
❌ COMPLETE DESTRUCTIVE INTERFERENCE
-
Occurs when out-of-phase waves have equal amplitude
-
Result = no wave
🔄 INTERFERENCE WITH DIFFERENT FREQUENCIES
When waves have different frequencies:
-
Sometimes they are in-phase → constructive interference
-
Sometimes out-of-phase → destructive interference
📌 This creates alternating bright and dark patterns (important for speckle and noise later).
🧠 SPI CLINICAL CONNECTION
-
Constructive interference → bright echoes
-
Destructive interference → weak or missing echoes
-
Speckle is caused by interference of many small wavelets
🧠 SECTION 02 – HIGH-YIELD SPI POINTS
✔ Sound is a mechanical, longitudinal wave
✔ Sound requires a medium
✔ Ultrasound travels by compressions and rarefactions
✔ Acoustic variables identify sound
✔ Acoustic parameters describe sound
✔ Interference explains echo strength and speckle
✔ Sound energy moves — matter does not
📘 SECTION 02: SOUND (DETAILED – SPI CORE PHYSICS)
🧠 FOUNDATION OF ALL ULTRASOUND IMAGING
🔊 WHAT IS SOUND IN ULTRASOUND?
In diagnostic ultrasound, sound is the tool used to create images.
The transducer sends sound pulses into the body, and echoes returning from tissue boundaries are processed into an image.📌 Key idea: Ultrasound imaging is entirely based on how sound travels, reflects, and interacts with tissue.
🌊 SOUND WAVES – BASIC CONCEPT
A wave is a method of transferring energy, not matter, from one place to another.
Examples of waves:
-
Sound
-
Light
-
Heat
-
Magnetic waves
📌 Sound is a mechanical wave, meaning it requires a medium to travel.
🚫 SOUND CANNOT TRAVEL THROUGH A VACUUM
Sound must travel through a medium (such as air, water, or tissue).
Reason:
-
Sound depends on particle motion
-
No particles → no sound
This is why ultrasound works in tissue but not in empty space.
🔁 COMPRESSION AND RAREFACTION
Sound travels through tissue by causing particles to vibrate back and forth.
-
Compression:
-
Particles are squeezed together
-
High pressure
-
High density
-
-
Rarefaction:
-
Particles are spread apart
-
Low pressure
-
Low density
-
📌 Ultrasound images are formed by detecting echoes created from these pressure changes.
➡️ SOUND TRAVELS IN A STRAIGHT LINE
Ultrasound systems assume:
-
Sound travels in a straight line
-
Sound travels directly to a reflector and back
📌 Many artifacts occur when these assumptions are violated (important later in SPI).
📐 TYPES OF WAVES
🔀 TRANSVERSE WAVES
-
Particle motion is perpendicular to wave direction
-
Example: shaking a rope up and down
📌 Ultrasound is NOT transverse
➡️ LONGITUDINAL WAVES
-
Particle motion is parallel to wave direction
-
Sound waves move this way
📌 Ultrasound is a longitudinal wave
🧠 ACOUSTIC VARIABLES (IDENTIFY SOUND WAVES)
A wave is classified as a sound wave only if one or more acoustic variables oscillate rhythmically.
THE THREE ACOUSTIC VARIABLES
1️⃣ Pressure
-
Concentration of force over an area
-
Units: pascals (Pa)
2️⃣ Density
-
Mass per unit volume
-
Units: kg/cm³
3️⃣ Distance (Particle Motion)
-
Back-and-forth movement of particles
-
Units: cm, mm
📌 If pressure, density, or distance oscillates → the wave is sound.
⚠️ ACOUSTIC VARIABLES ≠ ACOUSTIC PARAMETERS
-
Acoustic variables → identify sound
-
Acoustic parameters → describe sound
This distinction is heavily tested on SPI.
📏 ACOUSTIC PARAMETERS (INTRODUCTION)
Once a wave is confirmed to be sound, it is described using seven acoustic parameters:
-
Period
-
Frequency
-
Amplitude
-
Power
-
Intensity
-
Wavelength
-
Propagation speed
📌 These are explained in detail in Section 03.
🎵 IN-PHASE AND OUT-OF-PHASE WAVES
✔️ IN-PHASE WAVES
-
Peaks occur at the same time and place
-
Troughs align
-
Waves are “in step”
📌 Like a synchronized marching band
❌ OUT-OF-PHASE WAVES
-
Peaks and troughs do not align
-
Waves are “out of step”
➕➖ INTERFERENCE
When two or more waves arrive at the same location at the same time, they combine → interference.
TYPES OF INTERFERENCE
➕ CONSTRUCTIVE INTERFERENCE
-
Occurs with in-phase waves
-
Amplitudes add together
-
Resulting wave is larger
📌 Stronger echo
➖ DESTRUCTIVE INTERFERENCE
-
Occurs with out-of-phase waves
-
Amplitudes partially or completely cancel
-
Resulting wave is smaller or zero
📌 Weaker or absent echo
❌ COMPLETE DESTRUCTIVE INTERFERENCE
-
Occurs when out-of-phase waves have equal amplitude
-
Result = no wave
🔄 INTERFERENCE WITH DIFFERENT FREQUENCIES
When waves have different frequencies:
-
Sometimes they are in-phase → constructive interference
-
Sometimes out-of-phase → destructive interference
📌 This creates alternating bright and dark patterns (important for speckle and noise later).
🧠 SPI CLINICAL CONNECTION
-
Constructive interference → bright echoes
-
Destructive interference → weak or missing echoes
-
Speckle is caused by interference of many small wavelets
🧠 SECTION 02 – HIGH-YIELD SPI POINTS
✔ Sound is a mechanical, longitudinal wave
✔ Sound requires a medium
✔ Ultrasound travels by compressions and rarefactions
✔ Acoustic variables identify sound
✔ Acoustic parameters describe sound
✔ Interference explains echo strength and speckle
✔ Sound energy moves — matter does not
-
