physics 2 final overall

Pulse-echo technique

The foundational method of ultrasound where the system sends out a pulse and receives an echo back, using round-trip timing to determine depth.

Artifact

Anything on an ultrasound image that does NOT represent a real anatomical structure.

Confirming Artifact

Four steps: Change scanning plane, change acoustic window, use color Doppler, and check for presence with different angle/window.

Cyst vs Blood Vessel in Transverse Scan

A cyst remains circular; a blood vessel elongates.

Six Machine Assumptions

Important to understand as every artifact is a violation of one assumption.

Straight Path Assumption

Assumes sound travels in a straight line from transducer to reflector; violation causes mirror image or refraction.

Straight Return Assumption

Assumes sound returns along the same path; violation causes artifacts such as mirror images.

Speed Accuracy Assumption

Assumes sound travels at exactly 1540 m/s in soft tissue; violation leads to propagation speed errors.

Thin Imaging Plane Assumption

Assumes imaging plane is infinitely thin, which leads to slice thickness artifact if violated.

Attenuation Correlation Assumption

Assumes echo amplitude relates directly to tissue characteristics; violation causes shadowing and enhancement.

13 Microsecond Rule

Assumes 13 μs of round-trip time equals 1 cm of depth in soft tissue.

Spatial Resolution

Overall detail of the ultrasound image, considering axial, lateral, and contrast resolution.

Axial Resolution

Ability to distinguish structures parallel to the sound beam, determined by spatial pulse length.

Lateral Resolution

Ability to distinguish structures perpendicular to the sound beam, determined by beam width.

Elevational Resolution

Related to section thickness and slice thickness artifact, affecting echoes collapsed into a 2D image.

Classic Elevational Resolution Example

False echoes mistaken for sludge in the gallbladder.

Parallel vs Perpendicular Trick

Parallel to sound beam is axial;

perpendicular to sound beam and scan plane is lateral;

perpendicular to scan plane is elevational.

Acoustic Speckle

Caused by constructive and destructive interference of returning echoes, resulting in a grainy appearance.

Reverberation Artifact

Multiple equally spaced echogenic lines caused by sound bouncing between strong reflectors.

Classic Reverberation Example

Imaging a needle creates strong reflections causing reverberation.

Ring Down Artifact

Reverberation caused by gas or air bubbles, appearing as a continuous bright echogenic stream.

Comet Tail Artifact

Single long hyperechoic line parallel to the beam caused by high propagation speed media.

Distinction Between Artifacts

Reverberation has distinct spaces, ring down is continuous and widens, comet tail tapers.

Mirror Image Artifact

A second copy of a structure placed deeper than the original due to reflection.

Most Common Cause of Mirror Image

Diaphragm, which can cause mirrored appearances of structures.

Crosstalk

A form of mirror imaging seen on spectral Doppler with duplicated waveforms.

Color Mirroring Artifact

Duplication of a vessel and color flow below the real structure due to a strong reflector.

Refraction Artifact

Side-by-side duplication of a structure at the same depth caused by oblique sound trajectory.

Distinction Between Mirror vs Refraction

Mirror artifact creates deeper duplicates; refraction creates side-by-side duplicates.

Side Lobes

Extra acoustic energy outside the main beam produced by single-element transducers.

Grating Lobes

Off-axis sound beams produced by array transducers that degrade lateral resolution.

Compensation for Grating Lobes

Subdicing and apodization reduce side and grating lobes.

Apodization

Process reducing the strength of side and grating lobes by varying signal strength between crystals.

Propagation Speed Error

Artifact due to incorrect medium speed, misplacing structures axially.

Effect of Sound Speed on Structure Placement

Slower speed places structure too deep; faster speed places structure too shallow.

Range Ambiguity

Echo from a previous pulse arrives after the next pulse, misplacing structures shallowly.

Speed Error vs Range Ambiguity

Speed error places structures too deep; range ambiguity places them too shallow.

Shadowing Artifact

Weakening of echoes distal to a strongly attenuating structure.

Clean vs Dirty Shadowing

Clean shadowing is anechoic; dirty shadowing appears cloudy.

Edge Shadowing

Occurs at edges of curved reflectors due to refraction.

Enhancement Artifact

Strengthening of echoes distal to a weakly attenuating structure.

Focal Enhancement Artifact

A bright horizontal band across the image at the focal zone level.

Spatial Compounding Use

Reduces shadowing and enhancement by averaging scan lines from multiple angles.

Limitation of Spatial Compounding

Useful artifacts like shadowing and enhancement should not always be suppressed.

Noise in Ultrasound Image

Small, low-level echoes that do not represent real anatomy.

Causes of Electrical Interference

Due to other electronic devices in the room causing specific patterns on the screen.

Aliasing in Doppler

Improper representation of velocity when blood velocity exceeds sampling ability.

Sample Volume Depth Effect on Aliasing

Shallow depth increases PRF and Nyquist limit, reducing aliasing; deep depth decreases them, increasing aliasing.

Nyquist Limit Formula

The maximum Doppler frequency shift measurable without aliasing: Nyquist limit = 0.5 × PRF.

Calculating Aliasing with PRF

If PRF = 6, aliasing occurs at velocities exceeding 3.

Doppler Modalities That Alias

Pulsed Doppler and Color Doppler both alias.

Doppler Modalities That Do Not Alias

Continuous Wave (CW) Doppler and Power Doppler do not alias.

Disadvantage of CW Doppler

Range ambiguity due to no depth resolution.

Power Doppler Functionality

Shows amplitude of moving red blood cells but not velocity or direction.

Speed vs Velocity

Speed is magnitude; velocity is speed with direction.

Doppler Shadowing Artifact

Weakening of Doppler signals posterior to an attenuating object.

Clutter Artifact in Doppler

Low-frequency shifts on spectral display obscuring the baseline.

Ghosting Artifact in Color Doppler

Low-frequency shifts from vessel wall motion appearing outside vessel boundaries.

Fixing Clutter and Ghosting

Use the wall filter to eliminate low-frequency signals.

Flash Artifact Definition

Sudden burst of color due to rapid motion, not blood flow.

Ureteral Jet Usefulness

Confirms patency of ureters; indicates obstruction if absent.

Echo Path in Ultrasound System

Transducer → Beam Former → Signal Processor → Image Processor → Display.

Seven Components of Beam Former

Pulser, pulse delays, T/R switch, transmit/receive amplifiers, ADC, echo delays, summer.

Function of Pulser

Creates electronic signals to excite the probe’s crystals; adjustable output strength.

Other Names for Pulser

Output power, acoustic power, pulsing power.

Measurements Related to Pulser Output

Mechanical Index (MI) and Thermal Index (TI); both increase with output power.

Pulse Delays Function

Control beam steering, focusing, and aperture size through timing coordination.

Channel Definition

One piezoelectric crystal element and its electronic connection.

Functions of T/R Switch

Routes signals between pulser and transducer during transmit and protects sensitive components.

Receive Amplifiers Function

Amplify weak returning echo signals affected by attenuation.

Need for Amplifiers

Compensate for attenuation by amplifying both signal and noise.

Improving Signal-to-Noise Ratio

Coded excitation, harmonic imaging, and persistence improve SNR.

ADC Function

Converts analog signals into digital format for computer processing.

Echo Delays and Summer Function

Accomplish dynamic focus and steering, and combine channels to produce one scan line.

Line Density Definition

The number of scan lines fitting in an image; more lines = better detail.

Three Jobs of Signal Processor

Filtering, detection/demodulation, and compression of the signal.

Filtering in Signal Processor

Band-pass filtering allows specific frequency ranges to pass, filtering out others.

Harmonic Frequencies Explanation

Even multiples of the operating frequency used for imaging.

Benefits of Harmonic Imaging

Improves resolution, SNR, and reduces artifacts.

Detection (Demodulation) Process

Converts echo voltages into video form for display while preserving amplitude.

Compression (Dynamic Range) Purpose

Combines wide-ranging raw signals into manageable groups for processing.

Dynamic Range Examples

Narrow range yields fewer gray shades and high contrast; wide range yields many shades and low contrast.

Dynamic Range Clinical Use

Helps differentiate structures like heterogeneous liver due to metastatic cancer.

Image Processor Function

Converts digitized echo info into a displayable image using memory.

Pre-processing Explanation

Actions taken before the freeze button is pressed.

Pre-processing Features

TGC, dynamic range compression, write magnification, edge enhancement, pixel interpolation.

Edge Enhancement Function

Sharpens edges of structures to enhance clarity and definition.

Pixel Interpolation Definition

Fills in missing echo info by considering surrounding pixel brightness.

Persistence Alternative Names

Temporal compounding or frame averaging, enhancing image quality.

Persistence Pros and Cons

Increases SNR but degrades temporal resolution, not ideal for moving structures.

Ideal Use for Persistence

Stationary structures, like the abdomen or liver.

Panoramic Imaging Description

Creates an extended image by sliding the transducer across a wide area.

Uses for Panoramic Imaging

Measuring large structures that don't fit on standard screens.

Spatial Compounding Technique

Sends scan lines from multiple angles to reduce artifacts.

Spatial Compounding Eliminates

Shadowing and enhancement allowing for clearer visualization of structures.

Spatial vs Temporal Compounding Difference

Spatial uses multiple angles; temporal uses multiple frames.

3D Imaging Explanation

Pieces together 2D images for a still 3D result.

4D Imaging Explanation

Real-time 3D imaging adding the dimension of time.

Post-processing Description

Manipulation of stored image data after the freeze button is pressed.

Post-processing Features

Read magnification, black-and-white inversion, contrast variation.