Physics Chapters 13-24 Review

• The physics final review covers chapters 13-24, including material from powerpoint slides and the book. Some exam content might not be included in these slides.

Temporal Resolution and Real Time Imaging (Chapter 13)

• Considers factors affecting frame rate and temporal resolution in ultrasound imaging.
• Frame rate and temporal resolution can be improved with adjustments to the system settings.

Factors Affecting Frame Rate

• The speed of sound in the medium and imaging depth affect frame rate
• Imaging depth and number of pulses per frame
• The shorter the go-return time, the shorter the T frame, resulting in a higher frame rate and superior temporal resolution
• The longer the go-return time, the longer the T frame, resulting in a lower frame rate and inferior temporal resolution
• Deeper imaging requires more listening time, decreased temporal resolution, and lower frame rate
• More listening time is required in deeper imaging
• Shallower imaging requires less listening time, increased temporal resolution, and higher frame rate

Determining Number of Pulses per Frame

• Number of pulses per frame are determined by: the number of focal points, sector size, and line density
• Frame rate decreases with multiple focal points, increased sector size, and increased line density

Single Focus

• One pulse per scan line is required for single focus
• Single focus creates a shorter T frame, with a higher frame rate and superior temporal resolution
• Inferior lateral resolution can result with single focus

Multi Focus

• Many pulses per scan line is required for multi focus
• Multi focus creates a longer T frame, with a lower frame rate and inferior temporal resolution
• Superior lateral resolution can result with multi focus

Narrow Sector

• Fewer pulses per frame are sent in a narrow sector
• Narrow sectoring creates a shorter T frame, with a higher frame rate and superior temporal resolution

Wide Sector

• More pulses per frame are sent in a wide sector
• Wide sectoring creates a longer T frame, with a lower frame rate and inferior temporal resolution

Low Line Density

• Widely spaced lines are a result of low line density
• Fewer pulses per frame are sent in low line density
• Low line density creates a shorter T frame, with a higher frame rate and superior temporal resolution
• Inferior spatial resolution can result with low line density

High Line Density

• Tightly packed lines are a result of high line density
• More pulses per frame are sent in high line density
• High line density creates a longer T frame, with a lower frame rate and inferior temporal resolution
• Superior spatial resolution can result with high line density

Worse Temporal Resolution

• Lower frame rate and greater depth both indicate worse temporal resolution
• More pulses and a wider sector also degrade temporal resolution.
• Multi-focus improves spatial resolution while degrading temporal resolution
• High line density improves spatial resolution, but degrades temporal resolution

Better Temporal Resolution

• Higher frame rates and less depth improves temporal resolution
• Fewer pulses and narrowed sector improves temporal resolution
• Single focus degrades spatial resolution, but improves temporal resolution
• Low line density degrades spatial resolution but increases temporal resolution

Pulse Echo Instrumentation (Chapter 14)

• Explores ultrasound systems, their components, and controls.
• Includes topics such as transducers, the pulser/beam former, receiver, display, storage, synchronizer, and the order of receiver functions

Major Components of Ultrasound Systems

• Transducer: During transmission transforms electrical energy into acoustic energy; during reception converts returning acoustic energy into electrical energy
• Pulser/Beam Former: Creates the pulse and adjusts the voltage during transmission; adjusts electrical spike voltage to reduce lobe artifacts through apodization and distributes delay patterns
• Receiver: Amplifies returning echoes evenly throughout the image
• Display: Presents processed data
• Storage: Archives ultrasound studies using typical storage devices or media
• Synchronizer: Maintains and organizes the proper timing and interaction of the system's components

Receiver Functions

• Amplification: All signals treated identically; adjustable; entire image gets brighter or darker
• Compensation: Signals treated differently based on reflector depth; adjustable; image will be uniformly bright from top to bottom
• Compression: Signals treated differently depending on strength; adjustable; changes grayscale mapping
• Demodulation: Prepares electrical signals to be suitable for CRT display; not adjustable; none
• Rejection: Only weak signals affected, strong signals remain unchanged; adjustable; weak echoes are present or eliminated from the image

Output Power

• Changes brightness of entire image
• Alters signal-to-noise ratio with bioeffect concerns; therefore alters patient exposure
• Should be descreased this first if image is too bright

Receiver Gain

• Changes brightness of entire image
• Does not affect signal-to-noise ratio, therefore does not change patient exposure and does not have bioeffect concerns
• Should be increased first if image is too dark

ALARA Principle

• ALARA (As Low As Reasonably Achievable) principle used to minimize patient exposure while obtaining a clinically relevant image during scanning

Display and Image Storage (Chapter 15)

• Examines the display controls, bistable vs gray scale, and scan converters of ultrasound imaging

Display Controls

• Two user controls alter the characteristics of the image; contrast and brightness

Contrast

• Controls the amount of black and white or grayscale on the monitor

Brightness

• Controls how bright or dark the image is on the monitor

Bistable Image

• Composed of only two shades (black and white)
• Has high contrast and narrow range

Gray Scale Image

• Displays multiple levels of brightness, eg, white, light gray, medium gray, etc
• Has low contrast and a wide range

Scan converters

• Translates information from a spoke format into video format and allows the image to be manipulated
• In early days of two dimensional imaging, scan converters were constructed from analog technology
• Today, scan converters use computer technology and are known as digital scan converters
• First stores information ("writing") then displays the information ("read")

Analog Numbers

• From the real world, contains continuous values with umlimited number of choices

Digital Numbers

• From the computer world, contains discrete values with limited choices

Low Pixel Density

• Few pixels per inch; larger pixels
• Less detailed image and lower spatial resolution

High Pixel Density

• Many pixels per inch; smaller pixels
• More detailed image and higher spatial resolution

Fewer Bits Per Pixel

• Fewer shades of gray and degraded contrast resolution

More Bits Per Pixel

• More shades of gray and improved contrast resolution

Elements of Digital Images

• Pixel: Smallest element of a digital picture
• Bit: Smallest amount of computer memory

Bits and Number of Shades:

• 1 bit represents 2 shades
• 2 bits represents 4 shades
• 3 bits represents 8 shades
• 4 bits represents 16 shades
• 5 bits represents 32 shades
• 6 bits represents 64 shades
• 7 bits represents 128 shades
• 8 bits represents 256 shades
• Four bits are required to store from 9 to 16 shades of gray

Pixel VS Bits

• Pixels are the image element, used for image detail, resulting in spatial resolution
• Bits are computer memory, resulting in gray shades and contrast resolution

Preprocessing

• Time gain compensation
• Log compression
• Write magnification
• Fill-in interpolation

Postprocessing

• Changes that can be made after the freeze frame in terms of contrast variation, read magnification, and black/white inversion

Read Magnification

• Uses old data and is postprocessing
• Larger pixel size with the same numbers of pixels as the original ROI
• Unchanged spatial and temporal resolution

Write Magnification

• Acquires new data and is preprocessing
• Identical pixel size with more pixels than in the original ROI
• Improved spatial resolution with possible improved temporal resolution

Definitions to Review

• Review definitions for: coded excitation, spatial compounding, frequency compounding, fill-in interpolation, elastography, persistence or temporal averaging, PACS, and DICOM

Dynamic Range (Chapter 16)

• Reviews the functions, mathematical equations, and various ranges for ultrasound dynamic range.

Dynamic Range of Ultrasound Components

• Transducer: 120 dB
• Receiver: 100 to 120 dB
• Scan Converter: 40 to 50 dB
• Display: 20 to 30 dB
• Archive: 10 to 30 dB

Narrow Dynamic Range

• Few choices available that includes; figure A with few choices, high contrast, bistables, and black and white images

Wide Dynamic Frame

• Many choices available that includes; figure B with many choices, low contrast, gray scale images, and narrow dynamic range.

Mathematics of Compression

• Mathematics of compression using decibels by adding or subtracting to determine dynamic ranges.

Dynamic Range Compressions

• To compress decibals subtract the compression amount from original range.
• To calculate original range if dynmic range is already compressed, add compression amount to compressed amount.

Harmonics and Contrast Agents (Chapter 17)

• Examines topics on linear vs. nonlinear contrast agents such as microbubbles and mechanical vs tissue harmonics.

Harmonic Imaging

• Creation of an image from sound reflections at twice the transmitted sound's frequency

Linear Behavior

• Even or proportional
• Rhythmic behavior
• The system responds in an even manner

Nonlinear Behavior

• Uneven or irregular
• Arrhythmic behavior
• The system becomes nonlinear when it behaves unevenly

Tissue Harmonics

• Not present when sound leaves the transducer, created in the tissues deeper into the transmission.
• Are caused by nonlinear behavior along the beam's main axis
• Sound travels faster in compressions when compared to rarefactions

Contrast Harmonics

• Stronger harmonics are created thanks to ultrasound pulses interaction with contrast agents.
• Microbubbles act in a nonlinear manner when struck by the sound waves; therefore it expands to a greater extent than they are compressed and produced.
• Contrast harmonics are created during reflection.

Mechanical Index (MI)

• The amount of contrast harmonics produced is estimated by the mechanical index
• Depends on the frequency of the transmitted sound and the pressure of the sound wave.
• Increase with: lower frequency sound and stronger sound waves (high pressure variation)
• Low mechanical index sound beams do not create harmonics because the microbubbles expand and contract evenly
• Creates a linear fashion with more backscatter
• Higher mechanical index sound beams when is less than 0.1

Pulse Inversion Harmonics

• Imaging, instead of only a single pulse, includes two pulses transmitted down each ray line.
• Two consecutive ultrasound pulses are transmitted down each scan line

TIssue Harmonics VS Contrast Harmonics

• Created during transmission in tissue and occurs as sound propagates in tissue.
• Contrast Harmonics created during Reflection off of micro bubble, that occurs with contrast agents in tissue with transmission.

Lower MI

• Les cavitation and pressure with higher frequency and weaker harmonic signal.

Higher MI

• Above 0.1 to 1 results in strong harmonics with resonance non-linear linear behavior with lower frequency and higher beam strength and will disrupt bubbles.

Hemodynamics (Chapter 18)

Pulsatile Flow

• Occurs when blood moves with a variable velocity, accelerates, and decelerates from cardiac contractions
• Commonly appears in arterial circulation

Phasic Flow

• Occurs when blood moves with variable velocity, accelerates, and decelerates from respiration
• Often appears in venous circulation

Steady Flow

• Occurs when fluid moves at a constant speed/velocity
• Example is water flowing through a garden hose
• Present in venous circulation when individuals stop breathing for a moment

Laminar Flow

• Exhibited as Plug Flow and Parabolic Flow
• Plug Flow: when all layers and blood cells travel at the same velocity
• Parabolic flow: has a bullet shaped profile with velocity highest in the center

Turbulent Flow

• Characterized by chaotic flow patterns in many different directions and at many speeds, where streamlines are often obliterated
• Converts flow energy into other forms like sound and vibration
• Sound associated with turbulence called murmur or bruit

Reynolds Number

• Predicts whether flow is laminar or turbulent
• Reynolds number >2,000 indicates turbulent flow
• Reynolds number <1,500 indicates laminar flow

Hemodynamics of Stenosis

• Change in flow direction
• Increased velocity as vessel narrows
• Turbulence downstream from the stenosis
• Pressure gradient across the stenosis
• Loss of pulsatility

Resistance

• Force that opposes the flow of a fluid (blood)
• In blood vessels, resistance is due to vessel length and diameter
• As a vessel diameter decreases, resistance increases
• As a vessel length increases, resistance increases and the flow decreases

Ohm's Law

• For a fluid, pressure gradient = flow x resistance
• For an electrical system, Voltage = current x resistance
• Pressure related to voltage, flow related to current, and resistance related to resistance

Hydrostatic Pressure: Supine

• Measured Pressure = Circulatory Pressure + Hydrostatic Pressure
• With supine, all arteries are at equal level or 0 mmHg.

Hydrostatic Pressure: Standing

• Dependable on the level of measurement and pressure with vessels located at 0mmHg.
• Hydrostatic pressure increases or decreases depending on heart level.

Breathing and Venous Flow

• Inspiration: diaphragm moves downward toward abdomen, thoracic pressure decreases, flow in arms and vena cava increases, abdominal pressure increases, and flow in legs decreases
• Expiration: diaphragm moves upward into thorax, thoracic pressure increases, flow in arms and vena cava decreases, abdominal pressure decreases, and flow in legs increases
• Note: venous flow in legs correlates with movement of diaphragm
• Downward movement of diaphragm (inspiration) decreases venous flow in legs (downward decrease)
• Upward movement of diaphragm (expiration) increases venous flow in legs (upward increase)
• When venous flow in the leg decreases, venous return to heart increases
• When venous flow in the leg increases, venous return decreases

Doppler (Chapter 19 & 20)

• Discusses physics, calculations, and definitions central to Doppler ultrasound.

Doppler Shift

• Measured in Hertz and is directly proportional to velocity and directly proportional to transducer frequency

Angle and Cosine

• 0° has a cosine of 1.0
• 30° has a cosine of 0.87
• 60° has a cosine of 0.5
• 90° has a cosine of 0
• 120° has a cosine of -0.5
• 150° has a cosine of -0.87

Less Aliasing

• Slower blood velocity
• Lower frequency transducer
• Shallow gate (high PRF)

More Aliasing

• Faster blood velocity
• Higher frequency transducer
• Deep gate (low PRF)

Pulsed Doppler

• Has range resolution and sample volume
• Has limited maximum velocity (Nyquist) and is prone to aliasing

CW Doppler

• Has range ambiguity and region of overlap
• Unlimited maximum velocity without aliasing

Pulsed Doppler Transducer

• At least one crystal, with a damped PZT and low-Q factor
• Wide bandwidth with lower sensitivity for imaging at a normal incidence of 90°

CW Doppler Transducer

• At least two crystals, with undampened PZT and a high Q-factor
• Narrow bandwidth with higher sensitivity
• Lower frequency avoids aliasing
• Pulsed or CW
• Minimum 1 (pulsed) or 2 (CW) crystals

Nyquist Limit

• The point at which aliasing occurs.
• Nyquist frequency is equal to ½ the PRF
• Higher frequency transducers cause even more aliasing

To Avoid Aliasing

• Adjust the scale to its maximum
• Select a new view with a shallower sample volume
• Select a lower frequency transducer
• Use baseline shift
• Use continuous wave Doppler.

Doppler Modality Roles: Continuous Wave

• Identifies highest velocity jets anywhere along ultrasound beam
• Presents range ambiguity with the most sensitivity for quick use

Doppler Modality Roles: Pulsed Wave

• Accurately identifies the location of flow
• Presents range resolution with more moderate temporal sensitivities

Doppler Modality Roles: Color Flow

• Accurately identify flow location
• Range Resolution or moderate sensitivity
• Reduced resolution in multiple packets, with velocity or spectral resolution
• Will often alias with spectral analysis.

Doppler Modality Roles: Power Doppler

• Used with small slow volumes and is sensitive with no aliasing or measurements

Gray Shades in a Spectrum

• Related to to the number of reflectors in the wave patterns.

Resistive Index

• A quantitative, Doppler-derived measurement of vascular resistance of a segment in the arterial system
• Useful in diagnosing arterial stenosis
• RI = (Max Velocity – Min Velocity) / Max Velocity

Pulsatility Index

• Quantifies or measure oscillations of the waveform. The index is used best with diastolic reversal flow and is found by the equiation:
• PI = Max Velocity – Min Velocity/ Mean Veolcity
• Velocity Map shows movement or velocity with Variance Mode Map
• Has two map: with either positive doppler shift

Variance Mode Map

• Variance mode maps show the same as the velocity map In addition where color from the left side of the map indicates laminar flow and where colors from the right side of the map indicate turbulent flow

Calculating for Doppler Shift

• Doppler Shift (Hz) = Reflected frequency + Transmitted Frequency
• Doppler Shift = 2 x Speed of Blood x Transducer Frequency x Cos Θ / Propagation Speed
• Measured Velocity = True Velocity x Cos Θ
• Nyquist Limit (Hz) = PRF / 2
• PI = (Velocitymax - Velocitymin) / Velocitymean
• RI = (Velocitymax - Velocitymin) / Velocitymax