Hello, I'm MJ!

1. Types of Displays

1) A-Mode, or Amplitude Modulation [★]

- display of amplitude spikes of different heights

- does not need scan converter

- only 1 line of sight are sampled and displayed

- used for ophthalmology studies to detect finding in the optic nerve

- consists of x-axis (depth/distance) and y-axis (amplitude)

- ex) internal contents of a simple cyst: an area with no spikes

 

2) B-Mode, or Brightness Modulation

- most common form of ultrasound imaging

- display of 2D map of B-mode data

- based on brightness with absence of vertical spikes

- brightness depends upon the amplitude/intensity of echo (image of large/brighter and small dots)

- no y-axis on B-Mode. x-axis (depth) and z-axis (echo intensity or amplitude)

- transducers used for diagnostic ultrasound are damped to improve axial resolution

               : damping – reduce pulse duration and spatial pulse length

               : usually have pulse length of 1-3 cycles

- frequency for doppler is usually lower than for imaging in a given transducer

               : B-mode real-time imaging = 10-50 Hz

- blood vessels usually appear anechoic on B-mode imaging

               : reflection from the RBC is too weak to be displayed

 

3) M-Mode, or Motion Mode (= Time Motion or TM-Mode) [★★]

- display of a one-dimensional image

- used for analyzing moving body parts (commonly in cardiac and fetal cardiac imaging)

               : time, motion pattern, and amplitude

- useful for measuring dimensions of structures

               : produces display more similar to a tracing than actual anatomic picture

- repeatedly measure distance of the object from a single transducer at a given moment

: record the amplitude and rate of motion in real time

- limitations

               : information is obtained along only one line of sight

                              - single sound beam (single line of sight) transmitted

                              - depth of reflectors along a single line of sight vs. time

: displayed as dots of varying intensities

               : motion lateral to the transducer is not displayed

               : 2-D shape of a structure is not shown

2. Terms to Know

 

1) Sector display

- wedge of a circle.  A narrow near field and broader far field.

 - phased array

 

2) Linear display=rectangular

- linear array

 

3) Footprint

- Aperture of the outside of probe. What touches the patient

- small footprint transducers

               : beam diverges very rapidly in the far field → poor lateral resolution

 

 4) Array

- A collection of transducer crystal elements.

 

5) Steering

-  Sending the pulse out in different directions

- Directing the beam

- Electronic steering/Phasing

: steering by using small time delays between excitation pulses to each elements within the array

 

6) Dynamic receive focusing [★]

- rephrase the signals by dynamically introducing electronic delay circuitry upon reception

- curved wavefront cause reflected echoes to be received at varying times at the different elements

- echoes that are received first are held in delay circuitry

: until all of the echoes from the same depth have been received

- time delays before echo signals from array elements are combined

: results in constructive interference of waves

: produces higher amplitude and more focused signal

 

7) Dynamic aperture [★]

- change number of elements in array receiving reflections

: deeper reflections (take longer to return) - more elements used to receive

- minimize the degree to which beam width varies with depth

 

8) f-number

- ratio of focal length to the size of the aperture

               = focal length/aperture

 

* Main advantage of intracavitary probe

- closer to the area of interest: higher frequency transducer can be used

→ superior spatial resolution

 

* Standoff pad

- used to evaluate superficial mass

- Advantage: increases the distance between transducer and the mass

               → mass will be located close to the elevational focus of sound beam

3. Linear array [★★★★★]

1)

2) creates a rectangular image

- same distance between scan lines in both the near and far fields

- best suited to vascular imaging

 

3) beam focusing: electronically

 

4) beam steering: electronically

               : create trapezoidal shape to improve field of view

 

5) slice thickness: determined by the point of mechanical focusing, along the width of array

: with 1D linear array

 

6) Linear Phased Array

- Elements arranged in a line (linear)

- Display=Sector image (fan)

: Greater depth = greater gaps between scan lines (worse Lateral resolution)

- Small footprint

: Compact: good for tight spaces (예: between ribs)

 

7) Linear Sequential Array 

 

- Elements arranged in a line

- Display=Rectangular Image

- Large footprint

- One element damaged → Vertical drop out line directly below damaged element

4. Phased array [★★★★★]

 

1) Sector display

2) beam focusing: electronically

 

3) beam steering: electronically

 

4) mechanically focused along the elevational dimension

 

5) fires all of the elements for each acoustic scan line using small time delays to steer the beam

- compared to non-phased array

               : smaller transducer footprint

               : precise beam focusing and steering

               : higher sensitivity and dynamic range

               : reduce low amplitude echoes, which removes grating and side lobes

               : ideal for cardiac imaging (poor abdominal)

5. Convex/Curved Arrays [★★★]

 

1)

2) Crystals arranged along an arc

 

3) Display= Blunted Sector Image

: Greater depth = greater gaps between scan lines (worse Lateral resolution)

 

4) Large/Long Footprint (Large up to 10cm long)

               : acquire sonogram with the largest possible field of view both in near field an at depth

 

5) Electronic Steering

 

6) Electronic Focusing

* Annular array

 

 

- beam is symmetric about the beam axis

               : lateral resolution = elevational resolution

 

* Advantage of single crystal (pure-wave crystal) transducer

- wide bandwidth

6. Lobes

1) artifact from sound energy transmitted in a direction other than along the beam’s main axis

- Unwanted sound energy

- side lobe: created by a single crystal transducer

- grating lobe: created by array transducers

 

2) produced by all probes

 

3) Side lobe

- far field energy interferes with lateral resolution (degrades lateral resolution)

- <10% of energy of main US beam (lobes are weaker than primary beam)

- minimized in

               : phased array transducers

               : pulsed mode operation, broad bandwidth pulses

- eliminated by

               : apodization

                              - varying the excitation voltage to each element in the group used to form US pulse

- maximizing excitation voltage for elements near the center of the beam

- reducing it toward the periphery

               : use tissue harmonics

 

4) Grating lobes [★]

-

 

- summation of side lobes generated by linear arrays

: cause smearing of ultrasound beam → degrade lateral resolution

- related to the spacing of elements in the array

- if spacing reduced to less than one wavelength: grating lobes are eliminated

               (but elements are always spaced greater than 1 wavelength)

- Subdicing the elements into smaller sub-elements

: decrease effective distance between elements → reduce grating lobes

- relatively small when sound beam is unsteered

: beam steering (trapezoidal display) → grating lobes become prominent

- technique most helpful in reducing grating lobe = tissue harmonic imaging

 

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. Normal Incidence (Perpendicular Incidence)

1) Incident sound beam encounters a boundary between two media at a 0° incident angle

- The sound beam is perpendicular to the boundary.

 

2) Reflected sound returns in the same direction as the incident sound

 

3) Transmitted sound continues on in the same direction as the incident sound

 

4) There is no Refraction (bending of sound)

2. Oblique Incidence [★]

 

 

1) When the incident sound beam encounters the boundary between two media at an angle

- The incident angle is something other than 0°

 

2) The reflected angle is equal to incident angle

 

3) Transmitted sound will also continue on an angle (Refraction)

 

4) The angle of reflection will be oriented away from the transducer

- resulting in decreased visualization of the structure

3. Refraction

1) A change in the direction of sound after encountering a boundary (bending of sound)

 

2) Requirements for refraction

- Oblique Incidence

: Perpendicular/Normal Incidence = No refraction

- Mismatch in Propagating speeds (c) of two media

- M2>M1 than T>I: the transmission angle is greater than the incidence angle

- M2<M1than T< I: the transmission angle is less than the incident angle

 

3) Matching PS and mismatched Impedance causes reflection but not refraction

4. Acoustic Impedance (z)

 

1) physical property of tissue

- Resistance to travel that a sound beam encounters as it passes through a medium [rayls]

Z [rayls] = d [kg/m³] x c [m/s] [★★★★]

- affected by tissue stiffness, density and soundwave speed      

: not) frequency 

               - density = most responsible factor

 

2) An Impedance mismatch determines reflection

- Tissue to Tissue: Reflection ≤2% (mostly transmitted = weakest reflected signal)

- Tissue to Air: mostly reflected without coupling medium

- Diagnostic application to adult brain is limited

: great acoustic impedance mismatch between cranium and soft tissue

: causing most sound to be reflected at interface

- With oblique incidence reflection can occur without refraction

: when there are mismatched impedances, but matching propagating speeds

5. Important Terms

1) Intensity Reflection Coefficient (IRC)

- The fraction of incident intensity that is reflected

: IRC = Ir/Ii (reflected intensity/incident intensity)

- IRC and impedance mismatch: proportional

: ↑z mismatch → ↑IRC

: ↓z mismatch → ↓IRC

- If impedance of the two media are the same: no reflection

 

2) Intensity Transmission Coefficient (ITC)

- The fraction of incident intensity that is transmitted into the second medium

               : ITC = It/Ii (transmitted intensity/incident intensity)

: ITC = 1 – IRC

- IRC and ITC should always equal one

- IRC ↑ → ITC ↓, vice versa.

 

6) Scattering

- The diffusion or redirection of sound in several directions upon encountering a rough surface

- Backscatter: Sound scattered back in the direction from which it originally came

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. Acoustic Waves

 

1) Traveling variation (oscillation) in acoustic variables

- Molecules oscillate back and forth to propagate sound waves

- Do not move from one end of the medium to another

- Acoustic variables

: Temperature

: Pressure - Concentration of force in an area

: Density - Concentration of mass in a volume

: Distance - Measure of particle motion

2) Mechanical longitudinal wave

 

3) Vacuum: a space void of matter

- Sound cannot travel in a vacuum

- Electromagnetic radiation, light/x-ray can travel through a vacuum

2. Mechanical Waves

 

1) require a medium for propagation (gas, liquid, or solid)

- cause motion of the particles they are moving through

- molecules do not travel from one end to the other (it is not a flow of particles). 

: Molecules vibrate back and forth

- can be either Transverse or longitudinal.

 

2) Longitudinal Waves

- particles of medium vibrating in the same direction as the wave propagation direction

- sound: mechanical longitudinal wave

 

3) Transverse Waves/Shear Waves/Stress Waves

- propagates by particles of the medium moving perpendicular to the wave propagation direction.

- Bone: The only biological tissue that can cause the production of transverse waves 

3. Terms Describing Sound Waves/The Properties of Sound [★★]

1) Compression (Compression zone, Peak, Up-hump, Wavefront, Leading portion of a wave)

- High pressure region of the wave form

- Area of maximum particle density

 

2) Rarefaction (Trough)

- Low pressure region of the wave form

- Area of minimum particle density

 

3) Cycle

- one high pressure and one low pressure region of a wave.

 

4) Frequency (f)

- The number of cycles that occur in one second [MHz, kHz or Hz]

: F = 1/p (frequency = 1/period)

: F= c/λ (frequency = propagating speed/wavelength)

- Hertz (Hz): One cycle per second

: MHz = 1,000,000 cycles/second  

: kHz = 1,000 cycles/sec

- Diagnostic ultrasound frequency Range: 1-16MHz

- frequency is important in diagnostic ultrasound: affects penetration and image quality.

 

5) Period (T)

- Time it takes for one cycle to complete itself [seconds(s) or microseconds (μs)]

- Time between two successive compression zones or rarefaction zones

: T = 1/f (Period = 1/frequency)

: frequency and period are reciprocals

 

6) Wavelength (λ)

-

- The distance one cycle takes up [meters(m), centimeter (cm), or millimeter (mm)]

-  The distance between two successive density zones.

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

- wavelength and frequency: inversely proportional

 

7) Propagation

- Changes in pressure conveyed from one location to another                          

 

8) Propagating Speed (Acoustic Velocity)

- speed of sound moving through a medium. [mm/us or m/s]

- c = f (Hz) x λ (m)

- Determined only by the medium. Especially, the density and stiffness of the medium

: Stiffness - the ability of a material to resist compression

- Stiffness has a greater effect on PS than density

- Stiffness and Propagating Speed: proportional

: ↑ Stiffness = ↑ PS

: Density - relative weight of the material

- Density and propagating speed: inversely proportional

: ↑ Density = ↓ PS

 : Not operator adjustable

- Materials that are very stiff but not dense will have the highest propagating speed.

- Materials that are not very stiff but are extremely dense will have the lowest propagating speed.

- Propagating speed in soft tissue: 1.54mm/us or 1540 m/s

- speed used to calibrate range-measuring circuits on diagnostic sonography instruments: 1540 m/s

- Tissue Type & Correlating Speeds

: Propagating speed through gas is low

: Propagating speed through liquid is higher 

: Propagating speed through a solid is the highest.              

- Air: 300 m/s

- Lung: 500 m/s

- Fat: 1,450 m/s

- Water: 1,480 m/s

- Soft Tissue.: 1,540 m/s

- Liver: 1,560 m/s

- Blood: 1,560 m/s

- Muscle: 1,600 m/s

- Tendon: 1,700 m/s

- Bone: 3,500 m/s ~ 4080 m/s

- Metals: 2,000 – 7,000 m/s

 

9) Properties of the medium that effect Propagating Speed

- Elasticity

: the ability of an object to return to its original shape and volume after a force

: Force applied to an object cause a change in its shape or volume (distortion)

- The strength of the force determines the amount of distortion.

- Density (d)

: The mass of a medium per unit volume. 

               : The relative weight of an object.

: d = m/v

- larger mass requires more force to cause motion

- larger mass requires more force to stop molecules already in motion

- Density and propagating speed: inversely proportional

- Stiffness (s, = Bulk Modulus)

: an objects ability to resist compression

: the inverse of compressibility

              : Stiffness and Propagating Speed: proportional

- Compressibility (K)

: The fractional decrease in volume when pressure is applied to the material

: stiffness ↑→ compressibility ↓, acoustic velocity ↑

 

10) The source is able to determine the Period (T), Frequency (f), Amplitude, Power, and Intensity

- The source does not determine the Propagating Speed(c) (the medium does)

- Frequency is not related to propagating speed (propagating speed is a constant in soft tissue)

- Wavelength is determined by both the medium and the source

 

11) Interference [★★]

- algebraic summation of waves leading to patterns of minima and maxima

- interference patterns of reflected waves cause acoustic speckle

               : to reduce speckle

                              - use frame averaging (persistence)

                              - use compound imaging

- two waves overlap at the same location, at the same time

               : combine into a single new wave

- constructive interference: sound waves are in phase and resulting amplitude is increased

- destructive interference: amplitude of new wave is decreased

               : complete destructive interference creates black pixels

4. Terms describing Pulsed Waves [★★★]

 

1) Pulse “A Burst of Cycles”

- collection/group of two or more cycles followed by a resting time.

- We use pulsed waves for diagnostic ultrasound

- pulsed wave US is necessary for real-time imaging

               : depth of interface from which the echo originated can be determined

 

2) Pulse Duration (PD)

- Time from the beginning to the end of a single pulse of ultrasound

- Time it takes for one pulse to occur (excludes the resting time) [μs]

: PD = n x T (Pulse Duration = number of cycles x Period)

- not operator adjustable

 

3) Pulse Repetition Period (PRP)

- The amount of time from the start of one pulse to the start of the next pulse

: includes resting time, sound on and off time [μs]

: PRP = 1/PRF

- operator adjustable, determined by sound source

- unrelated to period

 

4) Pulse Repetition frequency (PRF)

- The number of pulses that an ultrasound system transmits into the body each second [MHz or Hz]

               : rate at which the transmitter applies electronic voltage pulses to the transducer

: PRF = 1/PRP

- Along with PRP, determine the maximum imaging depth (depth of view)

- Determined by

: Sound source (pulser)

: operator adjustable

: Determined by the maximum imaging of the system

- limited by the speed of sound in tissue

               : there must be enough time between pulses for US to travel to and back from the reflector

               : or else, range ambiguity occurs

               : if sound travels faster in tissue, maximum PRF can be increased

- Relationships

: PRF & depth of view – inversely proportional

               - imaging depth ↓ → short listening time, PRP ↓, PRF ↑

- Imaging depth ↑ → longer listening time, PRP ↑, PRF ↓

: PRF & PRP – inversely proportional

: PRF and frame rate - proportional

                              - PRF ↑ → frame rate ↑

- If PRF is too high for the imaging depth: range ambiguity

               : a pulse should be received before the next pulse is transmitted

               : if pulse is transmitted before echoes from first pulse are received

                              → echoes would be misplaced axially on the image

- operator adjustable.

- pulsed-wave doppler: PRF ↑ → acoustic exposure ↑

 

5) Spatial Pulse Length (SPL)

- the length of space over which one pulse occurs [mm]

: SPL = n x λ (Spatial pulse length = number of cycles x wavelength)

: frequency ↑ → wavelength ↓, SPL ↓

- shorter pulses → better images

- pulse duration ↓ → SPL ↓ → better axial resolution

 

6) Duty Factor (DF)

- The fraction of time the transducer is actively transmitting sound

- It compares on and off time.

: DF = PD/PRP

: PRP ↑ → DF ↓.

: PRP ↓ (PRF ↑) → DF ↑

- for sonographic systems: averages between 0.2% - 0.5% (0.1% - 1%)

5. Levels of Sound

-

1) Infrasound

- a frequency of less than 20Hz.

- A sound frequency too low for human hearing

 

2) Audible Sound

- The range of human hearing: 20-20,000Hz

 

3) Ultrasound

- 20,000Hz or higher (high frequency mechanical waves that humans cannot hear)

- A sound frequency too high for human hearing

 

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. Shape of sound beam

1) Sound beam is not uniform as it travels (beam width changes as it travels)

- The beam width is the same as the transducer diameter at the starting point

(beam width=disk diameter)

- The beam narrows as it travels to the focus: smallest diameter at NZL

(beam width= 1/2 disk diameter)

- After the beam reaches the focus it diverges (expands)

(beam width = disk diameter at 2 NZL, then rapidly diverges)

2. Parts of the Beam [★]

 

1) Near Zone (= Fresnel Zone, Near Field)

- Region from the transducer to the focus

- Beam gradually narrows (converges) within the near zone

- At the end of the near zone, the beam narrows to only ½ width of the active element

- Determined by the size and operating frequency of the element

NZL (mm)= Diameter (mm) X Frequency (MHz)

: If aperture increases, near-zone length increases

: If frequency increase, near-zone length increases

 

2) Focal Length/focal Depth

- The distance (length) from the transducer to the focus.

- The length of the Near Zone (Near Zone Length)

 

3) Focus/Focal Point

- Narrowest part of the beam

: Beam width = ½ Disk diameter       

- Located at the end of the near zone

- The starting point of the far zone

- The middle of the focal zone

- point of maximum intensity in a sound beam

 

4) Focal Zone

- Region around the focus

: Region on either side of the focal point where the beam is relatively narrow

- The area that creates more accurate images

 

5) Far Zone (= Fraunhofer Zone, Far Field)

- starts at focus and extends deeper (Region of the beam beyond the NZL)

- Can’t focus in far zone

- At 2 near zone lengths from the transducer: Beam width = Disk diameter

: depth past 2 near zone lengths are wider than disk diameter

3. Beam Diameter

1) Depends on crystal aperture, frequency, and distance from transducer

- Beam diameter = transducer diameter

- Beam diameter = ½ transducer diameter at the focus

- Beam diameter = transducer diameter at 2 near zone lengths

- Beam diameter > transducer diameter deeper than 2 near zone lengths

2) large beam diameter → focus at greater depths 

4. Determining the Focal Depth

1) focal depth [mm] = D²f/6 or D²/4λ

 

2) Transducer/Disk Diameter: directly related to focal depth

- Large diameter = deeper focus  

- small diameter = shallow focus

 

3) Frequency: directly related to focal depth

- Higher frequency = deeper focus

- lower frequency = more shallow focus

 

3) Divergence

- the divergence of the beam in the far field is also determined by disk diameter & frequency

- Disk diameter (inversely proportional to divergence)

: smaller diameters = greater divergence in far field

: large diameters = less divergence in the far field

- Large disk diameter improves Lateral Resolution

- Frequency (inversely proportional to divergence)

: lower frequency = greater divergence in far field

: higher frequency = less divergence in the far field

-High frequency improves Lateral 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

* In order: Pulser → Beam former → Transducer → Receiver → Memory (scan converter) → Display

 

* Analog signal

- does not have discrete steps

- values may vary continuously between minimum and maximum point

               (continuous variation of the signal is possible)

 

* Digital signal

- have discrete values that have fixed steps between values

- bits determine levels in digital system

 

1. Pulser (= Voltage Generator, Transmitter) [★]

1) originates action

 

2) Sends an Electric voltage pulse (EVP) to transducer through the beam former

- EVP (electric) = analog part

- Starting piezoelectric effect

 

3) Output power control

- adjust to increase or decrease the intensity of transmitted pulse

- output power: most closely affects patient exposure

 

4) PRF (Pulse repetition frequency) of Pulser = PRF of Transducer

2. Beam Former [★★]

1) Sends EVP to Transducer.

- EVP (electric) = analog part

 

2) Responsible for

- Aperture control

- Beam steering

- Focusing (shaping of beam)

- Apodization: reducing side lobes, grading lobes, gets rid of additional beams

(improves lateral resolution)

 

3. Transducer [★★★]

1) Converts one form of energy to another

 

2) Change EVP to MVP, vice versa (w/ applied pressure) = The Piezoelectric Effect

- The Piezoelectric Effect

: Mechanical deformation results when an electric voltage is applied to certain crystal materials

: Varying electrical signal is produced when the crystal structure is mechanically deformed

- When electric signal is applied to a piezoelectric element

               : element expands and contracts to produce mechanical vibrations (sound waves)

- EVP and MVP: neither analog nor digital

: Mechanical Voltage pulse (MVP) = sound wave

- Sends long MVP into body. Receives returning MVP

- Converts returning MVP to EVP, sends EVP to receiver

 

4) Matching layer: between piezoelectric element and tissue

- places on face of element

- reduce acoustic impedance mismatch between the element and tissue

               : improve sound transmission, reduces reflection

- multiple matching layers

: increase transducer bandwidth (short duration) → improve axial resolution

- optimal thickness = ¼ wavelength

- not) used for focusing

 

5) Backing material

- Advantage

: dampen US pulse and reduce spatial pulse length → improves axial resolution

: control ringing of piezoelectric element

- Disadvantage

: reduces sensitivity

: decrease overall intensity of sound beam

 

6) Radiofrequency shield

- used around the crystal and backing material

- Purpose

               : reduce noise from electromagnetic interference

               : enhance sensitivity to weak signals

 

7) commonly used material in modern transducer element

- lead zirconate titanate

 

8) Strain relief

- portion of transducer that protects the insertion of the cable into the transducer housing

- area prone to wear and tear with repeated use and bending of the cable

- strain relief area should be regularly inspected for cracks and exposed wiring

- use of a transducer should be discontinued if a crack appears in any area

 

9) Damage to the lens or transducer crystals

- result

               : degradation of image quality

               : underestimation of maximum flow velocity

4. Receiver/Signal Processor

 

1) Receives EVP signals from Transducer

- EVP (electric) = analog part

 

2) Alters the signal to make suitable for processing in memory

- Improves image

- Any pre-processing function = Receiver

- Refines signal through 5 functions of Pre-processing (ACDCR)

 

* Gain

- brightness of entire image changes.

- system control that determines the amount of amplification that occurs in the receiver

 

3) Between Receiver and Memory: Analog to Digital Converter (ADC)

5. Memory/Image Processor/Scan Converter (if digital) [★★]

 

1) Storage for the signals returning

- Digital

: stores signals as numbers

: Only Digital or Scan Converter are currently in use

: Allows more signal storage per location

 

2) Storage components

- Matrix board (checker board) = 1 Bit

: pixel (individual square) = picture element

- each pixel stores a signal (signal location)

- binary number (0-1) stored in pixel

- single bit can represent 2 levels of information

: 8bits (Matrix boards) = 1 Byte

- Matrix boards can be stacked

: Allows multiple signals stored per single location → Better Image

: Usually 6-8 matrix boards

- spatial resolution of scan converter is determined by # of pixels in the matrix

- Between Memory (scan converter) and Display: Digital to Analog Converter (DAC)

6. Display/Cathode Ray Tube (CRT)

 

1) Viewing Tube

- Phosphor covered tube

- the image is made for viewing

- not what we see. inside machine

 

2) Sends EVP to the monitor

- EVP (electrical) = analog

 

3) Three colors used on color monitor to produce range of available colors

- RGB (red, green, blue)

 

7. Five Functions of the Receiver (ACCDR) [★★★]

1) Amplification (= receiver gain) [dB]

- Returning echoes vary in strength

- Each returning signal is amplified uniformly

- purpose of preamplification of incoming signal

               : to increase echo voltages before noise is induced through the cable

- Operator adjustable

 

2) Compensation [=Time Gain Compensation (TGC); Depth Gain Compensation (DGC)] [dB] [★★★★★]

- Compensates for attenuation at different depths

- Produces an image with uniform brightness regardless of the depth of the reflector.

: optimize the image producing very bright echoes on display

: equalize differences in echo amplitudes received from similar structures at different depths

- Operator adjustable

 

3) Compression (=Log Compression; Dynamic Range) [dB]

- 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)

- display cannot accommodate the wide dynamic range of the incoming signals

- Decreases dynamic range of the processed signal (Equalizes difference between signals)

               : Increases smaller signals, reduces larger signals

: Keeps gray scale images within 20 distinguishable shades of gray

- Does not alter the incoming signals; i.e. larger signals are still large and smaller signals remain small.  

- Partially operator adjustable (gray scale adjustments)

 

4) Demodulation (=Detection)

- Changes the form of echo voltages into an appropriate form for the display monitor.

1. Rectification: Turns negative portion of radiofrequency (RF) signal to positive

2. Smoothing (enveloping): even out the rough edges.

- Not operator adjustable (the only function that is not operator adjustable)

 

5) Rejection/ filter (=Threshold; Suppression)

- eliminates non-diagnostically relevant low-level signals/noise → cleaner image.

               : increased rejection (threshold) level → decreased low-level echoes

- Operator adjustable

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