Boards Flashcards
Axial resolution
SPL/2
Axial resolution in soft tissue
.77 x #of cycles in pulse/ frequency
Axial resolution is best with
Short SPL
Short PD
High frequency
Fewer cycles/pulse
Lower numerical values
Intensity
Power/beam area
If amplitude doubles, intensity increases
By factor of 4
Focal length
Transducer diameter squared x frequency/6
CW beam diameter
2NZL
Frequency in PW
Prop speed/2x thickness
With oblique incidence, angle of reflection
Equals incident angle
Time of Flight
1.54/2
13 per cm reflector depth w/total distance 2 cm
Focal depth
Diameter squared x frequency/6
Or
Diameter squared/4 x wavelength
Aperture
Beam width/beam diameter
Two things that determine frequency in PW
Speed of sound in PZT
Thickness of PZT
INVERSELY RELATED
Frequency
Sound speed in PZT/2 x thickness
QF
Resonant frequency/bandwidth
Imaging probes have
- Pulses w/short length and duration
- Backing material
- Reduced sensitivity
- Wide bandwidth
- Lower QF
- Improved axial resolution
Dampening Material
- Decreases sensitivity
- Wide bandwidth
- Low QF
1/4 wavelength thick
PRF in soft tissue
77,000/imaging depth
As depth increases PRF decreases
Snell’s law
Refraction
1. If media 1 speed = media 2, no refraction
2. If media 1 is less than media 2 transmission angle is greater than incident
3. If media 2 is faster than media 1, transmission angle is less than incident angle
Transmission w/oblique incidence and different prop speeds
Refraction
Incident intensity
Reflected intensity + transmitted intensity
Sound waves initial intensity before it strikes a boundary
How much gets reflected at soft tissue
1%
How much gets reflected at air-tissue
99%
How much gets reflected at bone- tissue
50%
ITC
Transmitted intensity/incident intensity
X 100
99% transmitted at soft tissue
In clinical imaging what percent of incident sound wave is reflected?
1% or less
Normal incidence
Strikes at 90°
Perpendicular
Right angle
Orthogonal
IRC
% of intensity that bounces back when sound hits boundary between media
Transmitted intensity
Incident intensity x ITC
Matching layer
1/4 wavelength thick
PZT
Active element
1/2 wavelength thick
Distance to boundary
Go-return x speed/ 2
In soft tissue distance=time x .77
Pressure
Force/area
Power
Amplitude squared
If amp is tripled, power increases by 9
Duty factor
PD/PRP
In imaging DF = .2% (small time transmitting, long time receiving)
SPTA
most related to tissue heating
If CW and PW have same intensity SPTP - CW has higher SPTA
Attenuation coefficient
Frequency/2
In soft tissue .5dB/cm/MHzh
Attenuation
Requires 2 intensities
*more attenuation, longer distance, higher frequency
Reflection
Specular - smooth
Diffuse - irregular
Half value layer thickness
Distance sound travels in tissue that reduces intensity to 1/2 its original value
Thin 1/2 layer = high frequency
Depends on media and frequency
Reflection Angle aka
Incident angle
Incident angle
Angle at which wave strikes boundary
Pixel size
Total length of picture edge/ # of pixels in that length.
Byte
8 bits or 2x2x2x2x2x2x2x2 or 256 shades
Word
2 bytes
Huygen’s principle
The minimum distance that two structures positioned front to back can be apart and show 2 images
Total attenuation
Path length x attenuation coefficient
Impedance
Density x speed Rayls
Noise
Increasing output power is most common way to get rid of noise
Rayleigh Scattering
RBC
Hitting much smaller than beam’s wavelength
Increases with increasing frequency
Scattering
Random direction of sound in different directions
High frequency, more scattering
W/fixed focus transducer focal depth depends on
- Transducer diameter
- Frequency of sound
Shallow w/smaller PZT diameter and lower frequency
Adjustable focus
Phased array
Pulse duration
of cycles in pulse/frequency
Determined by source NOT adjustable
Shorter PD - better image
Focusing techniques
- Lens (external) fixed, conventional, mechanical
- Curved active element - (internal) - fixed
- Electronic (phased array) - adjustable
Lateral resolution
1/2 transducer diameter
Best at 1 NZL (at focus)
Changes with depth
The smaller the number the better
Slice thickness or elevational resolution
Deals with 3D shape
1. Shallow to deep
2. Side to side
3. Above and below imaging plane
Thick slice structures above and below create reflections
Side lobes
Created by single element transducer
Degrades lateral resolution
Grating lobes
Array transducers
Degrade lateral resolution which can be fixed by apodization (alters electrical spike voltages and reduces lobe strength
Reduced by subdicing
Temporal resolution
of pulses x PRP
Best with high FR
Impacted by speed of sound in medium and imaging depth
Frame rate
Decreased by multi focus
Inversely related to depth
Pulsed & beam former
Pulser - determines amplitude, PRP, PRF
Former - responsible for firing delay patterns
Receiver
Analog to digital
Display
Presents processed data
Storage
Archives
Master Synchronizer
Maintains and organizes proper timing
Pulser Voltage
Output gain, acoustic power, Pulser power, energy output, transmitter output, power, gain
*changes brightness of entire image
PRP
Determines maximum imaging depth
As depth increases, PRP increases
Depth x 13usec
Shallow imaging
- less listening time
- shorter PRP
- higher PRF
- higher DF
Channel
A single PZT element, the electronics in the beam former/Pulser and connecting wire
*most systems between 32 and 256 channels
Receiver
- Amplification
- Compensation
- Compression
- Demodulation
- Reject
Adjustable
Amplification
Receiver gain, image becomes brighter
Compensation
TGC, corrects attenuation
Compression
Modifies gray scale mapping 20 shades
Demodulation
Rectification and smoothing
Reject
Controls low level gray scale info aka threshold or suppression
Dynamic frequency tuning
Only uses high freq part of pulse to create superficial image and lower freq to create deeper parts
Analog
Best for spatial resolution
Limits are image fade, image flicker, instability, deterioration
Digital
Benefit - uniformity, stability, durability, speed, accuracy
Pixel and bit (digital)
Low pixel density means less detail, larger pixels, lower spatial resolution
Calculating shades of gray
If 5 bits of memory 2x2x2x2x2=32
Bit versus pixel
Bit - shades of gray, computer memory and contrast resolution.
Pixel - image element, image detail, spatial resolution
Preprocessing
- TGC
- Log compression
- Write magnification
- Persistence
- Spatial compounding
- Edge enhancement
- Fill in interpolation
Write magnification
Rescans ROI creates new image w/increased spatial resolution
Post processing
- Any change after freeze frame
- Black/white inversion
- Read magnification
- Contrast variation
- 3D rendering
Read magnification
Creates larger pixels from info already in scan converter
Coded excitation
Higher signal to noise ratio
Improved axial, spatial, and contrast resolution
Deeper penetration
*uses a series of pulses to create wider range of frequencies
Spatial compounding
*reduces shadowing artifact
Method of using sono info from several different imaging angles to produce a single image
Frequency compounding
Reduces speckle artifact and noise in images
Temporal compounding
Persistence
*reduces temporal resolution, used to better fill a vessel with color
Less effective w/slow flow
Fill in interpolation
*improves spatial resolution by increasing line density, improves ability to precisely visualize boarders of round structures
MI
Peak rarefaction pressure/square root of frequency
*low MI - small pressure variation and higher frequency
Reynolds’s number
Predicts if flow is laminar or turbulent
Less than 1,500 laminar
2,300 or more turbulent
Stenosis effects
- Changes direction as blood flows in and out
- Increased velocity in stenosis
- Post stenotic turbulence
- Pressure gradient across stenosis (pressure downstream less than upstream)
- Conversion of pulsatilla flow patterns to steady flow
Bernoulli’s principle
Describes relationship between velocity and pressure in moving fluid
Pressure gradient
Flow x resistance
Hydrostatic pressure
Related to weight of blood above or below heart
Supine pressure is 0mm/Hg
With inspiration flow to legs
Decreases
*flow to heart increases
With expiration
Reduces venous return to heart and increases flow to legs
Doppler Shift
=reflected freq - transmitted freq
Directly related to velocity and frequency of transmitted sound
Angles and Cosin
0°. 1
60° .5
90° 0
Doppler performed w/a 2MHz transducer and the Doppler shift is 3 kHz. Same study with 4 MHz transducer - shift becomes 6kHz
Nyquist limit
PRF/2
Highest frequency or velocity that can be measured w/o aliasing
Aliasing happens
When sample volume is deep, PRF is low, and Nyquist limit is low
Avoid Aliasing by
- Adjust scale to maximum
- Select a new shallower view
- Lower frequency transducer
- Baseline shift
- Use CW Doppler
Increasing Nyquist limit
Adjust scale to max
Select shallower view
Doppler artifacts
Ghosting (color bleed out of vessel)
Clutter
Eliminate Doppler artifacts
With wall filter - eliminate low frequency Doppler shifts around baseline (color from slow velocity reflectors - ie, movement)
Crosstalk
Special form of mirror image but w/spectral Doppler
Happens when
1. Doppler gain is set too high
2. Incidence angle is near 90° between sound beam and flow direction
Spectral Analysis
2 forms
1. FFT
2. Autocorrection
Spectral measures peak velocity
FFT
Digital processing, very accurate, displays all individual velocities
Autocorrection
Digital technique used to analyze color flow Doppler
Color Flow Doppler
- PW
- range resolution
- subject to aliasing
- Presence of flow
- Direction
- Average velocity
- Character of flow
Measures average
Power Doppler
Non directional
- picks up low flow
- unaffected by angles
- no aliasing
BUT
- no measurement of velocity or direction
- lower frame rate
- susceptible to motion of transducer/patient (flash artifact)
Flash artifact
Caused by motion of patient or transducer
Enhancement
W/abnormally low attenuation
Hyperechoic line between tissues with abnormally low attenuation
Focal enhancement
Side by side, most prominent at focud
Reverberation
Multiple equally spaced echoes
Shadowing
Too much attenuation
Edge shadow
Created as sound beams refract and diverge along the edge of a curved structure
Comet tail
Polyps
Created by gas bubble that resonates and produces its own sound wave
Attenuation coefficient
Frequency/2
.5 dB/cm/MHz
In soft tissue
2D imaging Mechanical Transducer
Single, circular active element
- fan or sector
- fixed focal depth
1. Internal focus (active element)
2. External focus (lens)
Entire image lost when crystal malfunctions
Array transducer
- Linear ——
- Annular (circles in circles)
- Convex
Linear Array
Steered and focused by phasing
- fan or sector
- electronic steering
- if element damaged, inconsistent steering and focusing
Annular Phased array
Mechanical steering
- multi focal zones
- fan or sector shaped like spokes on bike wheel
- when one ring malfunctions only portion of image lost
Linear Sequential Array
- sector shaped images, large acoustic footprint, rectangle shaped
120-250 strips of PZT
Steering - small group fired simultaneously
Convex or Curvilinear array
120-250 elements
Focus is electronic
Shape is blunted sector
Dynamic receive focusing
In convex/curvilinear is achieved by phase delays in signals returning to transducer
Vector Array
Small footprint
Focusing is electronic
Trapezoidal image
PD
Period x # cycles in pulse
