Unit 5 US physics

Ultrasound system

sophisticated device

2 major functions

• prepare, transmit elelectrical signals to transducer

• receive, process, analyze signals to create images

6 major components

1. Transducer

2. Pulser and beam former

3. Receiver

4. Dispay

5. Storage

6. Master Synchronizer

Transducer

transmission- reverse piezoelectric effect (electricity to vibration/sound)

reception- piezoelectric effect (convert sound waves back to electrical signals using pressure.)

Pulser

creates the electrical pulse that ring crysal during transmission

Beam former

• controls and focuses the US beam

• tells the transducer when to send signals and helps combine the echoes they receive

• determines firing delay pattern for phased array systems

• operates during transmission and reception

Pulser and beam former

controls electrical signals sent to transducer

receiver

recieves reflected sound and transforms returning signals for display

Display

visual presentation device (monitor, CRT, TV)

Storage

Archives U/S data (computer hard drives, CD, DVD)

Master synchronizer

• Communicates with ALL individual components

• organizes proper timing and interactions

• prepares to operate as a single integrated system

(like an conductor of an orchestra, making sure all the different parts of the machine work together)

Transducer output

last adjustment because of bioeffects

low voltange= gentle vibration= weak reflections

other names for pulser voltage

• Output gain

• acoustic power

• pulser power

• energy output

• transmitter output

• power

• gain (too vague)

TI

Thermal index (evaluating heat)

MI

Mechanical index (evaluating gas bubbles)

ALARA

as low as reasonably achievable

Noise

random and persistent disturbance that obscures or reduces signal clarity

Signal to Noise Ratio (SNR)

• amount of contamination

• Don’t increase your power

• 5:1 ratio is stronger than 2:1

• better the output better the quality but not good for patient

What determines PRP?

Pulser

PRP

pulse repetition frequency, the time of the start of a pulse to the next pulse

PRF

Pulse repetition Frequency - number of pulses transmitted per second in ultrasound

What kind of relationship does PRP and PRF have

inverse

Pulser: PRP snd PRF determines

depth of view or depth

Shallow imaging requires more or less listening time?

less

Increased depth increases or decreases PRF?

decreases

Increased depth increases or decreases PRP?

increases

Apodization

reduces side lobes by adjusting voltages

dynamic aperture

changes number of crystals used based on depth

dynamic receive focus

changes focus with depth

Digital beam formers

produce signals in digital format

• advantages: updates software only, stable systems, versatile range of transducers

Switch

• component of beam former

• like railroad tracks (goes to the correct direction)

• protects the receiver, and directs returning signals

• transmission- pulser to switch to transmission

• reception- transducer to switch to receiver

what is a channel composed of?

• single crystal (PZT element)

• Wire

• Electronics in the beam former/pulser

(# channels = # elements in array transducer)

5 functions of the receiver in order

1. amplification

2. compensation

3. comression

4. demodulation

5. reject

Amplification

• each returning signal is made larger, EQUALLY

• also called receiver gain (overall gain)

• controlled by sonographer

• units: 60-100dB

• not used for top to bottom uniform brightness

• can be adjusted by sonographer

• all signals treated the same

• brighter or darker

Does amplification improve SNR?

no

pre-amplification

• during reception

• improves signal quality before amplified

• helps prevent electronic noise

Compensation

• corrects for attenuation

• variable amplification (shallower echoes need less amplification)

• Goal: uniform grayscale brightness throughout

• treats echoes DIFFERENTLY based on reflection

Other names for compensation

• time gain compensation (TGC)

• depth gain compensation (DGC)

• swept gain

Higher frequency transducers attenuate more or less?

more, which requires more TGC, improved resolution in imaging.

delay

starts compensation

slope

extra amplification for ever-increasing depths

knee

depth at max amplification is on

far gain

max amount of amplitude

near gain

doesn’t need many adjustments because it is shallow

Compression

• receiver function

• alters the grayscale map

• taking all the shades and compressing them to less shades

• Sonagraphers can adjust one of two types

• signals treated differently depending on strength

which has the highest frequency?

green

What are other names for compression?

log compression or dynamic range

Dynamic fTuning

• higher f reflections for shallow images

• lower f relections for deep

Increasing compression/dynamic range does what?

more shades of gray (high contrast means more blacks and whites)

Demodulation

• cannot be altered by sonographer

• changes electrical signal for TV display

• two steps- Rectification and smoothing

• no affect on image

• prepares the signals

Does demodulation improve resolution

no, it happens automatically in the background

rectification

• turning all negative voltages into positives

• can not be adjusted by sonographer

Smoothing/ enveloping

• puts an envelope around the bumps to even out rough edges (ex. TVs)

• cannot be adjusted by sonographer

Reject

• may eliminate or include low level signals (possibly noise)

• sonographer can adjust

• affect low level signals does not affect brightness

other names for reject

suppression and threshold

what causes an increase in bandwidth

damping

When do you decrease output power and increase receiver gain?

decrease output- when image is to bright

increase receiver gain- if image is dark

The power of a sound beam from a transducer is determined by the:

voltage of the pulsers signal

Which has a greater # of shades of gray?

left

Delay would be deeper with a 3 or 7 MHz TRX?

3

Bistable

-two choices black or white

how ultrasound started

Grayscale

• different levels of brightness

• white- strong (bones)

• gray- intermediate (liver, kidney, spleen)

• black- weak

• helps distinguish tissue

brightness

• determines brilliance of displayed signals

• higher overall gain, overall image brighter

contrast

• determines range of brilliance

• high contrast is dark blacks and whiter whites

• most extreme contrast is bistable

Scan converter

• makes gray scale possible

• stores the data and displays on monitor

• now digital

Analog numbers

• real world numbers

• continuous, no limits

digital numbers

• discrete values

• associated with computer devices

• limited choices of numbers

disadvantages of Analog scan converters

• image fade

• image flicker- switching between read and write modes

• instability- room temp, length of use

• deterioration

Digital scan converters advantages

• uniformity/ consistant gray scale

• stable

• durable

• fast processing

• accurate

digitizing

converts images into numbers then re-translates for display

Pixels

• picture element

• smallest building block of a digital picture

• divide picture into a grid, each box is a pixel

• more pixels per inch the better the spatial resolution

PPI and DPI meanings

PPI- pixels per inch

DPI-dots per inch

if you increase the pixel density what happens to the spacial resolution?

it’s improved

What happens to the pixel size if you increase the pixel density?

the size decreases because there is better resolution

BIT

• smallest amount of computer memory

• binary digit

• bistable value of zero or one

Binary number

2 choices 0-1

Byte

8 bits

word

• 2 bytes

• 16 bits

Entire pixel is how many shades of gray?

one shade

• each pixel is assigned a group of bits

Shades of gray calculation

• 2^ number of bits

• ex. 2^5th

  • 2×2×2×2×2=32

• 5 bits= 32 shades of gray

How many shades of gray is in 4 bits?

• 16 shades of gray

• 2^4 = 2×2×2×2

How many bits are needed to represent 6 shads of gray?

3 because 2³ = 8

more bits means what?

• more shades of gray

• improves contrast resolution

What happens if you increase pixel density?

• more pixels per square inch

• smaller pixels

• increase the overall image detail/ resolution

• spacial resolution

Voxel

• unit of graphic info that defines a point in 3D space

• smallest unit in volume rendering of ultrasound data

when choosing an US system to display subtle differences in grayscale (greater contrast resolution) what would you choose?

a system with more bits

When choosing an ultrasound system with better spacial resolution what would you choose?

a system with more pixels

Analog to Digital

(a to d converter, preprocessing, digital memory storage, post processing, d to a converter, video dispay)

1. convert analog to digital (A to D)

2. preprocessing happens before digital information is stored in the scan converters

3. info is still in digital form, continues to be processed by the US computer

4. scan converters are digital (tv monitors are analog (not flat screens) signals must be re-converted to analog for display using D to A converter

5. signal is now analog (flat screens dont have to do the last step)

preprocessing

any processing of the reflected signals before storage

post processing

any processing after storage in the digital scan converter or changes after you freeze your image

Pre and post processing are determined my what?

whether data has been stored in scan converter

Functions that are preprocessing

• TGC

• log compression

• write magnification

• persistence

• spacial compounding

• edge enhancement

• fill-in interpolation

functions that are post processing

• any changes after freeze frame

• black/white inversion

• read magnification

• contrast variation

• 3-D rendering

Magnification

• zoom function (greater detail)

• two different methods: read and write

• Region of interest magnified to fill the entire screen

• used to evaluate anatomic detail

Read magnification

• after information has been stored in scan converter (post processing)

• scan anatomy, freeze and store into converter, then magnify

• like when you take a picture on your phone and zoom in, it doesn’t make the image clearer because its the same image just zoomed in

• same # of scan lines and same # of pixels just magnified

• does not temporal resolution

Write magnification

• before data is stored or frozen (pre proocessing)

• system zooms live over what you want to magnify and dit rewrites/rescans the information

• more scan lines, and more pixels better resolution

• only scans the area you ask it to

• may improve temporal resolution

if ROI depth is less than original depth temporal resolution improved?

yes

what happens to the frame rate with less depth

it increases

Coded excitation

• creates a longer pulse, spreading intensity overtime to keep the value within the FDAs limits

• improves image quality

• variety of frequencies

• occurs in pulser

• improves SNR, axial, spacial, contrast resolution

• deeper penetration

spatial compounding

• taking multiple frames from different angles and combining them to get a better image

• phased array TRX

• reduces graininess

• minimizes shadowing

• slows down the frame rate and temporal resolution (more pulses slow things down)

• (angles space)

frequency compounding

• taking US signals with different frequencies and combining them into one clearer image

• reflected signals are divided into sub bands of frequencies and combined

• reduces: noise and speckle artifacts (less grainy)