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1. Resolution [★]

1) The ability to separate closely spaced objects

 

2) Types of Resolution

- Detail resolution: relating to the transducer

: Axial Resolution

: Lateral resolution

: Elevational Resolution

- Contrast Resolution: relating to the instrument

- Spatial Resolution: relates to instrument

- Temporal Resolution: Relating to the instrument

2. Axial Resolution= Longitudinal, Axial, Range/Radial Depth (LARD) [★★]

 

 

1) Accuracy in imaging parallel to beams axis. 

- The ability to accurately distinguish two reflectors as two reflectors parallel to the beam

 

2) Is a Number

- Depicts minimum separation that can be resolved

: Lower number = more accurate image (better axial resolution)

: Space between 2 reflectors less than AR = unresolved

· Structures will not be shown as individual structures 

: Space between two reflectors greater than AR = resolved

· Structures will be shown as individual structures

- AR [mm] = SPL/2 (best axial resolution with given transducer)

: spatial pulse length (SPL, [mm]) = # of cycles in a pulse x wavelength

: λ = c/f (wavelength = Propagating speed / frequency)

- propagating speed: determined by medium

- frequency: determined by source

: f↑- λ↓ - SPL↓ - AR↓(better axial resolution)

: pulse duration ↓ (bandwidth ↑) → SPL ↓ → better axial resolution

 

3) Superior AR

- AR depends on [★★★★]

: frequency (wavelength) - f↑ → better AR

: pulse duration (bandwidth) – PD ↓ → better AR

: higher bandwidth, increased damping → better AR

: AR is determined chiefly by pulse duration

- Increased focusing → beam width ↓ → better LR but pulse length ↑ (poorer AR)

4) Best measure of resolution for modern-day ultrasound: axial resolution

- cf) worst measure of resolution: elevational resolution

3. Lateral Resolution = Lateral, Angular, Transverse, Azimuthal (LATA) [★★]

-

 

 

1) Accuracy in imaging perpendicular to beam axis

- The ability to separate two reflectors as two reflectors perpendicular to the beam axis

- The minimum separation of two structures positioned side by side

 

2) Is a Number

- Depicts minimum separation that can be resolved

: Lower number = more accurate image (better lateral resolution)

: Space between 2 reflectors less than LR = unresolved

· Structures will not be shown as individual structures

: Space between two reflectors greater than LR = resolved

· Structures will be shown as individual structures

- LR [mm] = Beam diameter (beam width) [mm]

: Narrow Beams = Better LR. 

: Best at beam’s focus (end of near zone) 

 

3) Superior LR

- 3 things affect LR [★★]

1) Beam Aperture (diameter): Narrow/small is better

2) Distance from the transducer (T): Beam diameter varies with depth

3) Frequency: Higher frequency (narrower beam) is better

- to improve LR in far field: ↑frequency pulses diverge less in far field    

- primary method of improving LR: focusing, dynamic aperture, ↑ line density [★★★★★]

               : increase the number of transmit focal zones and optimize their location

 

4) Axial resolution is better than Lateral resolution for imaging

- US pulses are shorter than they are wide

- Higher frequency improves both

4. Slice Thickness/Elevational Resolution

 

 

1) The third dimension of the imaging plane, section thickness

- It is not thin or uniform

- True reflectors lie above/below assumed imaging plane but displayed on image                                                                                                                                                                           

 

2) Related to dimension of the beam perpendicular to the imaging plane [★★]

- Elevational resolution is worst with one-dimensional linear array transducer

- Elevational resolution is most affected by mechanical focus on linear array transducer

 

3) Contributes to image Artifacts

- Thicker portions: worse ER (worse artifact of image)

- Thinner portions: better ER (less artifact of image)

 

4) If using poor elevational resolution

- inability to clearly demonstrate small cystic structures

5. Contrast Resolution [★★★★]

 

 

1) ability of gray scale display to distinguish echoes of slightly different amplitudes and intensities

- ability to differentiate between two regions at different depths having similar echogenicity

- Better Contrast Resolution = Better detail

- improper adjustment → operator will likely over-gain or under-gain the image

 

2) Digital Displays [shades of gray]

- Pixel = picture element

: Smallest building block of image

- Entire pixel = single shade of gray

: Shade of gray determined by bits

- The more bits per pixel, the more shades of gray = better CR

               : 8 bit system produces a maximum of 2⁸ (256) possible shades of gray

               : n bit system → 2ⁿ shades of gray

- Dynamic Range/Shades of Gray

               : ratio of the largest to the smallest signal that a system can handle

: Determines the extent a signal can vary and maintain accuracy

: Narrow dynamic range = fewer shades of gray (High contrast)

: Wide dynamic range = many shades of gray (Low contrast)

- contrast resolution is improved by changing the gray-scale map

 

3) to improve contrast resolution: use 2D or matrix array transducer

- 2D (matrix array) transducer

               : have both rows and columns of elements

               → electronic focusing in the out-of-plane dimension possible (slice thickness is thinner)

               : contrast resolution is improved due to decreased volume averaging

6. Spatial Resolution

 

 

1) The overall detail of an image

- Greater detail images =↑Spatial Resolution

 

2) Determining factors [★]

- Line density: More scan lines = Better SR

: High line density (Sound pulses are closely packed) = better SR

: Low line density (Wider gaps between sound pulses) = worse SR

- reduce beam width (better SR) by focusing

- Axial resolution

- Lateral resolution

 

3) Digital Displays (pixel density)

- Pixel density: #of pixels per inch

: High pixel density = ↑SR (better image detail)

- Smaller pixel size

: Low pixel density = ↓SR (worse image detail)

- Large pixel size

 

4) higher frequency → better spatial resolution but greater attenuation

- to gain penetration: use lower frequency, sacrifice some spatial resolution

- for improved spatial resolution: use higher frequency, sacrifice penetration

7. Temporal Resolution

 

 

1) The ability of the display to distinguish closely spaced events in time

- Accuracy of displaying structures as they pertain to time

- The ability to precisely position moving structures from instant to instant

 

2) Determined by frame rate

- Frame Rate: # of frames (still images) displayed per second [★]

: Frame Time x Frame Rate = 1

(FR = 1/FT)

- More frames per second (higher frame rates) = Improved TR

 

3) Determining Factors of frame rate / Temporal resolution [★★★★★]

- Number of Pulses per frame

: Greater Line Density = ↑Frame Time, ↓Frame Rate (Worse TR)

- Multi focus: ↑ pulses per scan line, ↑Frame Time, ↓Frame Rate (Worse TR)

a. ↓pulses ↑ Frame Rate (Improved TR)

b. ↑pulses↓Frame Rate (Worse TR)

: Increase PRF = faster frame rate (improved TR)

: sector width ↑ → ↓Frame Rate (Worse TR)

- Depth of image

: Time of flight/Frame Time is directly related to depth

: Shallower Imaging

- shorter go return time, shorter Frame Time, higher Frame Rate (Superior TR)

: Deeper Imaging

- Longer go return time, longer Frame time, Lower Frame Rate (Inferior TR)

8. Frame Rate

1) Frame rate [★]

- Image formation

: Many single images are displayed in 1 second to produce “motion”

- How many frames pass in front of you per second

- similar to motion picture, multiple still images flashed in a rapid succession

- US examinations requiring highest frame rate: cardiac

- Determined by two factors

: Sound’s speed in the medium

: Depth of the image

- System settings that affect frame rate [★★]

: Image depth

: Number of pulses per frame (proportional)

               - PRF ↑ → frame rate ↑

- if scanning depth ↑, system automatically decreases PRF to avoid range ambiguity

- sector width ↑ → frame rate ↓

               : system fires more scan lines for each imaging frame

               → increase the length of time it takes to create each frame

- Determines Temporal Resolution

: Accuracy of displaying structures as it pertains to time

: Ability to precisely position moving structures from instant to instant

: Excellent when a system produces many frames per second

: Substandard when displays few images per second

 

2) Relationship between FR and Time

- FR = 1/FT

 

* Multiple transmit focusing [★]

1) beam must be fired once for each zone on each line of sight → reduce frame rate

- other types of focusing do not affect frame rate

2) Parallel processing (co-processing)

- method for improving frame rates with multizone electronic focusing

               : simultaneously acquiring data for multiple acoustic scan lines

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler

1. Reflection [★]

1) Must have an Impedance (z) mismatch to have any reflection

- All of the sound will be transmitted if two media have the same impedance.

 

2) the sound traveling back towards the source (transducer) after encountering a boundary

- Speed it travels back is based on medium it is traveling through

- Sound-tissue interaction necessary to form an ultrasound image

- Reflections from different frequencies have identical transit times

 

3) ex: shadow of a renal stone is a result of reflection

2. Specular Reflection [★★]

1) Must have mismatch of impedance (Z)

 

2) Mirror-like (reflection comes right back)

- No refraction

- Returned from large flat surfaces

- Boundary is larger than beam (Beam is smaller than boundary)

- Boundary is larger than wavelength (wavelength smaller than boundary)

- Most angle-dependent:  Must be 90⁰ angle (Perpendicular/Normal Incidence)

- Strength of received signal depends on

               : Difference in acoustic impedance

               : Angle of incidence

 

3) ex: diaphragm, pericardium are specular reflectors

 

4) Good reflectors: Bright smooth reflections                                                                                    

 

5) Limitation: if not normal incidence not as nice of reflection

- Operator dependent

 

6) When sound strikes specular reflector at an oblique angle

- Angle of reflection equals to the angle of incidence

 

3. Non-Specular Reflection (=Diffuse reflection, Scattering) [★]

-

 

1) Must have mismatch of impedance (Z)

 

2) Not straight reflections (often): Defuses all over (scatter)

- Does not come right back

- refraction (+), scatter (+), absorption (+)

- Rough irregular surfaces

- Beam larger than Boundary (Boundary smaller than beam)

- Boundary smaller than wavelength (Wavelength larger than boundary)

- Not angle-dependent   

 

3) ex: Liver tissue

- Scattering: primarily responsible for imaging internal structure of organs

 

4) Directly related to frequency

- Higher frequency (f↑) = more scatter (scatter ↑)

 

5) Backscatter

- Scatter that returns back to source

- directly related to frequency

- These will be low amplitude reflections

 

6) Rayleigh scatter [★]

- reflector is smaller than the wavelength of sound beam

 - Scattering intensity is proportional to frequency raised to the fourth power

-ex) RBC

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler

1. Preprocessing [★]

 

1) Manipulation of scan data before storage in the memory after scan conversion

- Any function performed by receiver is preprocessing

 

2) Write Magnification (preprocessing)

- Before storage in the scan converter

: Scans anatomy creates image

: Image converted from analog to digital

: Sonographer Identify ROI (region of interest). 

: System discards existing data in scan converter.

: US rescans only the ROI and writes new data into scan converter (All new info acquired)

- # of pixels, scan lines in ROI is greater than ROI of original image

· More pixels = better spatial resolution

· Pixels are the same size as original

 

3) Pixel Interpolation/Fill-in Interpolation (Preprocessing) [★★]

- A way of filling in gaps of data undetected by the observer

- With a sector shape image: scan lines separate more at increasing depths

: Interpolation fills in the data missed due to increased space between scan lines

: without interpolation, image results in a series of scan lines with blank data between the lines

- Uses gray scale of surrounding pixels to predict missing information

: Increases line density

: Improves spatial resolution

2. Postprocessing

1) Data manipulations performed with the receiver

- Manipulates data after it has been stored in scan converter, prior to display

- Processing a frozen image, after memory

- Operator controllable

- Improves contrast resolution

 

2) Read magnification (post processing) [★]

- After image information is stored in the scan converter

               : performance on frozen image

: Scans anatomy and creates image

: Image converted from analog to digital and stored

: Identify region of interest

: Pixels are enlarged, ROI fills screen

               - Resolution loss

- Reads & displays only original data from ROI

- gray-scale map assignment

- # of scan lines & pixels are same as original

· Just magnification causes larger pixels

· Spatial resolution remains the same because same # of pixels

: Undergoes digital to analog conversion for the display

3. Persistence (Frame averaging) [★]

1) Continues to display information from older images

2) Number of previous frames are superimposed on the most current frame

- Positives: Improves image quality during real-time acquisition

: Smoother image w/ reduced noise (Higher signal to noise ratio)

- Limitation

: Reduces frame rate

: Reduces temporal resolution

- Most effective with slow moving structures

4. Edge Enhancement

1) Image processing method that makes image look sharper

2) Increases image contrast in area immediately around sharp edges

- Creates subtle bright and dark highlights on either side of those boundaries

- Makes boundaries more defined

5. Spatial compounding [★★]

1) uses information obtained from several imaging angles to create one image

- Averages frames obtained from different angles due to steering by employing time delays

- Overlap frames to form single real-time image

2) help demonstrate tissue boundaries that are not perpendicular to sound beam

               (improve border definition)

3) More frames = better image quality (improve border definition)

- Reduces speckle, reduce refraction and enhancement

- Minimizes shadowing, artifacts

4) Limitations

- Reduces frame rate (Reducing temporal resolution)

- electrical steering is used → only phased array Transducers

6. Panoramic Imaging

1) Expands images beyond the normal limits of the transducer field of view

- Retains echo information from previous frames and new echoes are added

2) Limitation

- Decreases frame rate (Reduces temporal resolution)

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler

1. Characteristics of blood

1) Density = Mass/unit volume [g/ml]

- Blood is dense than water

- Density ↑, Propagating Speed ↓(stiffness ↑, propagating speed ↑)

 

2) Viscosity

- Resistance to flow by fluid in motion.

 

3) Flow Volume Rate

- Rate at which a certain amount of blood is moving [L/min]

- A pressure difference is needed for flow to occur (No pressure change = No flow)

- Flow Volume Rate

= pressure difference / flow resistance

= Average Flow x Area of tube

(Volume flow = mean velocity x area of vessel)

- Pressure Difference↑, Flow Rate↑

- Flow Resistance↑, Flow Rate↓

 

4) Flow Resistance

- Depends on the viscosity of a fluid, the tube length, and radius.

: ↑Viscosity, ↑Flow Resistance, Flow Rate↓

: ↑Length, ↑ Flow Resistance, Flow Rate↓

: ↑Radius, ↓ Flow Resistance, Flow Rate↑

 

5) 3 Categories of Blood Flow

- Pulsatile flow

: Movement with variable velocity

: Blood accelerates & decelerates from cardiac contraction (arterial circulation)

- Phasic flow

: Movement with variable velocity.

: Blood accelerated and decelerates due to respiration (Venous Circulation)

- Steady flow

: Fluid movement at a constant speed/velocity

: Brief moment when hold breath

2. Flow types (Laminar)

 

1) Laminar Flow = Smooth (normal physiological states), silent flow

 

 

- flow streamlines are aligned & parallel

- Layers traveling at individual speeds

- < 1500 Reynolds #

 

2) 3 forms of laminar flow [★]

 

 

 - Plug Flow

: Uniform flow traveling at a constant speed

: laminar. individual layers with parallel aligned streamlines

: ex) Large vessels, Entrance of a vessel

- Parabolic Flow (bullet)

 

 

: Layers have individual speeds

: Velocity is the greatest in the center of the lumen (center = fast)

: Velocity is minimum at vessel walls (sides = slow)

: ex) Smaller vessels

- Disturbed Flow (between Laminar & Turbulent)

: Flow is altered from straight line but remains going forward

 (not turbulent because still forward motion)

: Cells move in different directions

: 예) Stenosis, Bifurcation

: 1600-2000 Reynolds #

3. Flow types (Turbulent)

1) Turbulent flow (pathology) [★]

- Chaotic flow patterns. Flow in many different directions

- Affected by velocity

- No streamlines

- Hurricane like swirling = Vortex / Whirling, circular = Eddy Current

: Little to no forward motion

- Not in normal physiological state= Pathology & elevated blood velocity

: Usually seen downstream from a significant stenosis “post-stenotic turbulence”

· Flow in a stenosis is greater than proximal and distal to it.

               : pressure is reduced distal/downstream to stenosis

- Converts flow energy into other forms of energy

: Sound vibration = murmur/bruit

: Tissue vibration = thrill

- bruit

               : seen as bright echoes near the zero-baseline located underneath the systolic peak

- > 2000 Reynolds # (turbulent flow can be predicted) [★★]

- Spectral broadening: ass/w turbulent flow [★★★]

 

 

 

2) Reynolds Number

- Predicts the onset of turbulent flow

- Reynolds # = Avg. flow speed x tube diameter x density / Viscosity [★★]

: Flow Speed ↑, Reynolds #↑

: Tube diameter ↑, Reynolds #↑

: Density↑, Reynolds #↑

: Viscosity↑, Reynolds #↓

4. Terms to Know

 

1) Pressure Energy

- A form of potential energy

- A major form of energy for circulating blood

: A pressure difference (pressure gradient) is needed for blood flow

- Blood gets Energy through Contraction of heart (systole)

- Energy loss

: Inertia

- Energy lost due to change in speed

- Stenosis

- energy loss is greatest in tortuous vessel with multiple obstructions

: Due to friction

- Energy turned into heat (like absorption in sound waves)

- Blood sliding across vessel walls causes friction

: Due to Viscosity (thickness)

- ↑ hematocrit (RBC’s) =↑ Viscosity

- Anemia = ↓ Viscosity

 

2) Flow in stenosis

 

 

 

- Reversed flow in doppler

- d/t pressure drop caused by high-grade proximal stenosis

 

3) Continuity Rule “volumetric flow rate” “stenosis” [★]

- flow rate must remain constant throughout a vessel (all regions)

               : proximal, at, and distal to stenosis

- Volumetric flow rate is constant

: Blood/fluid is neither created nor destroyed as it flows through vessel/tube

- Flow speed is increased with smaller diameter (stenosis)

 

4) Poiseuille’s Law “average flow speed” “entire vessel”

- Poiseuille’s equation: Flow rate = πPR⁴ / 8VL [★]

               P = Pressure difference

               R = radius

               V = viscosity

               L = length

- ↑ pressure difference → ↑ flow rate

- Flow speed decreases with smaller vessel diameters

 

5) Bernoulli Effect “Pressure” “stenosis” [★]

- Decreased pressure in high flow regions

- Pressure is less within a stenosis

: Pressure is greater proximal & distal to the stenosis.

 

6) Stenosis

- Effects of stenosis

: Change in flow direction

: Increased velocity as vessel narrows (continuity rule)

: Turbulence downstream from stenosis

: Pressure gradient across stenosis

: Loss of pulsatility

- 50% diameter stenosis ≈ 75% area stenosis

- 75% diameter stenosis ≈ 90% area stenosis

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler

1. Doppler shift

 

1) Doppler effect is used non-invasively to detect blood flow & the motion of body structures

 

2) Frequency of the reflected beam will be different than the initial frequency

- doppler shift = difference between the transmitted and received frequencies [★★]

 

3) Equations

: fd = fr- ft

fd= Doppler shift

fr= reflected frequency

ft= transmitted frequency

2. Doppler effect

 

1) A change in reflected frequency caused by reflector motion [Hz]

- Positive (toward) / Negative (away)

3. Doppler angle

 

 

1) The angle that the ultrasound beam makes with the direction of flow

 

2) Doppler Shift signal is largest when the blood flow is directly toward or away from transducer

- Parallel transducer orientation is not possible

- Ultrasound beam will be at angle with respect to the vessel

 

3) Doppler Equation [★★★★★]

: Δf = 2 • F_T • V • cosϴ / C

Δf = Doppler shift

F_T = transmitted frequency (doppler frequency)

               ↑ doppler frequency → ↑ doppler shift

V = reflector/RBC speed (velocity of the interface)

               ↓ reflector speed → ↓ doppler shift

ϴ = doppler angle of incidence

               cosϴ: smaller the angle, larger the Δf (↑ doppler angle → ↓ doppler shift)

               : cosine changes rapidly at large angles

                              - angles of 60⁰ or less are recommended to reduce error

C = propagating speed (velocity of the medium)

 

4) Velocity estimation in doppler US

- based on measurement of doppler angle of incidence

- angle correct cursor should be adjusted parallel to the vessel wall

 

* system control on doppler that adjusts PRF: spectral velocity scale

 

5) Angle should be between 30 -60

 

- Above 60⁰: too little f shift

- Below 30⁰: increase beam attenuation due to longer path lengths

- Image is best obtained at 90⁰

               : cannot detect frequency shift at incident angle of 90⁰ (perpendicular)

- Doppler is most accurate at 0⁰

               : maximum frequency shift will be obtained at 0⁰

4. Continuous Wave Doppler [★]

 

 

1) continuously transmitting and receiving an ultrasound signal

 

2) Transducer

- 2 crystal elements, one transmits and one receives

: One crystal in continuously transmitting

: The other is continuously receiving

- Advantage

: High velocities are accurately measured

: crystals overlap to produce a region of maximum sensitivity

→ most accurate Doppler shift info

: higher sensitivity and ease in detecting small doppler shifts

- Disadvantage: range ambiguity

- need to change angle with change in frequency (increase your f = greater beam attenuation)

- no damping is applied

 

3) Receiver

- detects the differences between f when there is a reflector motion (Doppler shift)

- demodulation of the signal

- Phase quadrature detection determines the direction of the shift

: Bi directional systems: determine motion and flow

: Uni or non-directional systems: only detect motion

- Threshold function eliminates noise and weaker signals

- CW detects flow anywhere within the sensitive region, regardless of depth

→ can be confusing with 2 vessels at once

 

4) CW doppler does not provide Range resolution

- range resolution

: ability to determine depth from which an echo has arrived

: sound must be pulsed

                              - echo arrival time from each pulse can be measured

5. Pulsed Wave Doppler

 

 

 

1) designed to overcome the lack of range resolution in continuous wave

 

2) Transducer

- Number of Crystals: One crystal, alternates between sending and receiving

: low quality factor, low sensitivity, wide bandwidth

- Advantage

: Echoes arise only from the area of interrogation (sample volume or Gate)

: greatest advantage being able to select the exact location

- Disadvantage

: Aliasing, error in measuring high velocities

 

3) Receiver

- detects the Doppler shift

- Range Gating

: allows to get depth information

: receives velocity from small regions along ULS beam

6.Color Flow Doppler [★★]

1) Doppler shifts are coded into colors and superimposed on the existing B-mode image

- color and B-mode images are formed from separate pulses

               : frequency used for color doppler is generally lower than B-mode image

- color threshold (priority control) [★★★]

: controls gray-scale brightness at which color will be displayed

: ↓ color threshold - color will overwrite vessel or cardiac wall

: spatial resolution (axial & latera) resolution in color doppler: always poorer than in B-mode

- Determining spatial resolution of color image: frequency & line density

- frame rate decrease when color doppler is activated

               : more pulses are fired on each line of sight

- once color doppler is selected, the system automatically turns off all but 1-2 focal zones

               : 1-2 pulses per scan line are used to create the underlying B-mode image

 

2) Color doppler is based on PW: subject to range resolution and aliasing

 

3) Velocity Mode: displays average values

 

 

 

- Colors = flow direction [★★★]

: Black = no Doppler shift (flow perpendicular to sound beam)

: Above black region (red) = flow toward the transducer (+ doppler shift)

: Below black region (blue) = flow away from the transducer (- doppler shift)

: highest positive doppler shift (yellow/orange)

 

4) Variance Mode (Variance) [★★]

 

 

 

- Velocity info

: + shift = colors in top half

: -shift = colors in bottom half

- Distinguishes laminar from turbulent flow

- Variance maps display different colors from side to side

: Left side = laminar flow

: Right side = turbulent flow (yellow: turbulent toward, green: turbulent away)

 

5) Packet size (= ensemble length, shots per line, dwell time) [★★]

- # of pulse (listen cycles) per acoustic scan line

               : for color doppler, each line of sight must be pulsed multiple times.

- ↑ packet size → ↓ frame rate, improved signal-to-noise ratio

 

6) Doppler signal spectral display

- depicts relative signal power (amplitude) at each frequency in the doppler signal

(depicts the frequency bandwidth, range of amplitude in reflected signal)

- z-axis (brightness) on doppler spectrum = amplitude

               : adjust gain to increase amplification

 

9) Color sample gate

- parameter to describe axial length of sampling volume for a color pixel

7. Power Doppler (Energy mode, Color Angio)

 

 

1) Doppler shift colorized without consideration of direction or speed

- non-directional, not angle dependent

- will only show that flow is present

- colorized amplitude of reflected doppler signal

 

2) Advantage

- Increased sensitivity to low flows (Venous flow, Flow in small vessels)

- Not affected by doppler angles

- No aliasing

 

3) Limitations

- No measurements of velocity or direction

- Lower FR, (reduced Temporal resolution) when compared to conventional color flow doppler

- Motion sensitive

8. Spectral Analysis

 

1) determine the distribution and magnitude of frequency shifts in the reflected doppler signal

 

2) Current Methods

- For PW or CW doppler = Fast Fourier Transform (FFT)

- For color doppler = Auto-correlation function

 

3) Fast Fourier Transform (FFT)

- processes PW & CW Doppler

- very accurate

- display all individual velocities

- distinguishes laminar (similar velocities) from turbulent flow (chaotic)

 

4) Auto-correlation function

- analyzes color flow

- faster than FFT, not as accurate as FFT

- used when larger amounts of data need to be processed

 

5) spectral window

 

 

- area underneath the systolic peak on the doppler waveform that is absent of echoes

-filled in when

: the doppler sample volume size is large compared to the size of the vessel

: turbulent flow is present

: doppler gain is set too high

: position of doppler sample volume is not centered within the vessel

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler