Display Mode, Real Time Imaging and Transducers

Cathode Ray Tube (CRT)

(displays)

• vintage TV

• emits electrons, screen coated with phosphorescent layer that glows when excited by electrons

Liquid Crystal Display (LCD)

(display)

• works with light source: fluorescent or light emitting diode (LED)

• lights are positioned behind two polarizing filters with liquid crystals

Principle Display Modes

the basic modes of display formats in ultrasound:

(1) A-mode

(2) B-mode

(3) M-mode

(4) C-mode

(1) A-mode

Amplitude

• early systems did not have scan converters

• A-mode appears as a series of upward spikes of different amplitudes

• NO 2D IMAGES

• x-axis represents reflector depth (and/or time)

• y-axis represents amplitude or strength of reflected signal

  • the height of upward deflection

  • strong echoes create tall spikes

  • weak echoes create short spikes

• non-scanned modality- sends pulse to same spot over and over again, NOT moving around

(2) B-mode (Brightness) / B-scan

Brightness

• appears as a line of bright dots of varying brightness

• brightness of a dot indicates the strength of the reflection

  • BRIGHT DOTS = strong reflection

  • DARKER GRAY DOTS = weaker reflection

• x-axis represents reflector depth (time of flight of the sound pulse)

• z-axis measures reflection amplitude

• B-scan- basis for all other grayscale imaging

• gray scale images = B-mode or B-scan

• creates 2D black and white images

B-mode to A-mode

*10*

(3) M-mode

Motion

• group of horizontal wavy lines

• 1- dimensional display used to investigate moving structures

• shows motion over time

  • echocardiography and 1st trimester OB

• x-axis represents time

• y-axis represents the depth of the reflector

• non-scanned modality

• lines represent motion of reflectors as they occur over time

  • NOT RELATED TO ECHO AMPLITUDE

(4) C-mode

Color Doppler

• provide blood flow average velocities over time

  • provides direction of flow

• pulsed wave ultrasound technique

• has range resolution

• gate selects selects data from a specific depth from an A-mode line

• image is formed in a plane normal to a B-mode image

Spatial Resolution

related to the overall detail in an image, determined by:

(1) Line Density

(2) Axial Resolution- reflectors PARALLEL to sound beam

• SPL/2 and improved by damping

(3) Lateral Resolution- reflectors PERPENDICULAR to sound beam

• improved by focusing in lateral dimension or using multiple foci

(4) Elevation Resolution- slice thickness plane

• improved by focusing on the elevation plane (with a lens)

(5) Contrast Resolution- varying shades of gray

• related to dynamic range

• HIGH contrast res- more shades of gray

• LOW contrast res- less shades of gray

optimizing spatial resolution

(1) # of bits per pixel and dynamic range

(2) Matrix size: ↑ # of columns and rows = ↑ spatial resolution

• 512×512 matrix has better spatial resolution than a 256×256 matrix

(3) Pixel density: ↑ pixel density = ↑ spatial resolution

(4) # of display raster lines: ↑ resolution monitors have ↑ raster lines

(5) Size of field of view: ↓ field of view = ↑ spatial AND temporal resolution

scanning speed restrictions

• real time displays multiple 2D frames per second (FR)

• every frame has many scan lines (LPF- lines per frame)

  • more scan lines = better spatial resolution and ↓ temporal resolution

• generating a scan line: system transmits pulse and waits for all echoes to be received before transmitting next pulse

• system assumes c = 1540 m/sec

• to avoid depth ambiguity:

  • Penetration Depth (cm) x # of foci x FR = < 77000

altering the Frame Rate (FR)

(1) Imaging Depth- FR suffers if it has to scan beyond what you are scanning (ex: 32cm deep), FR improves when depth is decreased (ex: 16cm)

• ↑ depth = ↑ PRP = ↓ PRF

(2) Focal Zones-

• ↑ focal zones = extra pulses per scan line → # of frames per second drops and takes longer

(3) # of scan line/frame/line density

• more scan lines = more time to take image

• ↓ lines = ↓ image quality BUT ↑ FR

• ↑ lines = ↑ image quality BUT ↓ FR

Depth and PRF

• ↑ depth = longer PRP = ↓ FR & ↓ PRF (# pules per sec)

• ↑ # of focus = ↓ FR & ↓ PRF

• ↑ LPF (lines per frame) = ↓ FR

• FR = PRF/LPF

  • LPF↑ —> FR↓

  • PRF↓ —> FR↓

factors affecting FR

• Frame Rate (FR)- the # of frames per second

• human eye sees flicker at FR <15 to 20 fps

• typical US FR are 30-60 HZ

• FR = PRF/LPF

• FR can be optimized by operator by:

  • ↓ depth

  • ↓ sector width

  • ↓ LPF (lines per frame), smaller image width or sector angle

Temporal Resolution

• Temporal Resolution- time taken to acquire an image, ability to display structures in real time

• ↑ FR = improved temporal resolution

  • needed in cardiac imaging

• Factors affecting TR:

(1) Depth

(2) Image Size- width of display or sector angle

(3) Packet Size (for color)- big color box —> slows FR

(4) Number of focus

(5) Line Density (scan lines/frame)

(6) Persistence/Frame Averaging- avg multiple frames to create an image

(7) Parallel Processing

(8) Compound Imaging

(1) (2) (3) Depth, Image, and Packet Size

*23*

(4) Multiple Foci

improves lateral resolution

• each display line is made up of multiple acoustic lines with each acoustic line having a different depth of focus

• multiple focus can ↓ FR

• composite time = (13 μsec/cm x depth of 1st scan line) + (13 μsec/cm x depth of 2nd scan line)…

(5) Line Density (scan lines/frame)

• the spacing between sound beams

• low line density = spaced far apart, high line density = closely packed

• ↓ line density —> fewer pulses used to create image, FR ↑ and TR ↑

• ↑ line density —> more pulses used to create image, FR ↓ and TR ↓

Sector Size/FOV (Field of View)

controls angle of sector displayed on monitor

• affects FR:

  • WIDER sector size —> ↓ FR and bad TR

  • use a SMALLER sector size to optimize FR when evaluating the heart

(6) Persistence/Frame Averaging

combines/”averages” multiple image frames into a single image

• LOWERS FR

Spatial Compounding

combines images from different angles (viewpoints) to reduce speckle and artifacts

• eliminates edge shadowing

• improves margin delineation

• reduces reverberation leading to a smoother image and improved resolution

• steers the ultrasound beam to view the same area from multiple angles and averages the results

Transducer Arrays (Types)

(1) Blind CW Doppler (non-imaging)

(2) Mechanical Rotating

(3) Mechanical Oscillating

(4) Linear Switched Sequential Array

(5) Linear Phased Array

(6) Curved Convex Switched Array

(7) Curved Convex Phased Array

(8) Annular Mechanical Array

(9) Annular Phased Array

(10) Sector Phased Array

(11) Vector Phased Array

(1) Blind CW Doppler Probe

• produces continuous electrical signals

• contains 2 crystals: 1 continuously transmits signal, 1 continuously listens

• highly sensitive probe bc it has NO damping

  • ↑ Q factor: very sensitive

  • narrow bandwidth: operates at 1 freq.

• used in cardiac, OB, and vascular to listen to pulses

  • produces no image and listens to Doppler flow

    • no image bc of range ambiguity (picks up on ANYTHING, not depth specific)

  • can produce tracings

• operating freq. = drive voltage freq.

Mechanical Transducer

Multiple Element (Crystal) Transducers

Linear Array- crystals arranged in a straight line

Curved Array- crystals arranged in a curved/arched line (convex)

Annular Array- crystals are arranged as circular rings

Multidimensional Array- 1D or 2D multiple elements are arranged in a series of rows

Electronic Operation Sequential

Electronic Operation Segmental

Sequencing