15 - Displays and Image Processing. Understanding Ultrasound Physics

Display Controls

contrast and brightness

Bistable

The prefix "bi" means 2.

Composed of only 2 shades:

-Black

-White

Gray Scale Display

Display in which several values of pixel brightnesses may be displayed.

-Present multiple levels of brightness.

Contrast

Determines the range of brilliancies within the displayed image.

Bistable images, in which only white and black appear, are high contrast.

Brightness

determines the brilliance of the displayed image.

Scan Converters

makes grayscale imaging possible by storing the image data then displays it on the screen.

Storage of the image information in the scan converter is called "writing".

Later, image data is "read" from the scan converter for display on the monitor.

The scan converter translates the information from the spoke format into the video format.

The data stored in the scan converter can be altered between the writing process and the reading process.

The scan converter converts the penetration pattern into a horizontal line pattern.

Analog Numbers

* Real world

* Unlimited choices

* Continuous values

Digital Numbers

* computer world

* Limited choices

*Discrete values

Analog Scan Converter

the first type of scan converter and made gray scale imaging possible.

spatial resolution

Is "image detail"

limitations of analog scan converters

- Image fades over time

- Image flickering

- Instability

- Deterioration

Digital Scan Converter

uses computer technology to convert images into numbers, a process called digitizing.

Advantages of digital scan converters

- Uniformity: consistent gray scale quality throughout the image

- Stability: does not fade or drift

- Durability: not affected by age or heavy use

- Speed: nearly instant processing

- Accuracy: error-free

Two important elements of digital scan converters are:

1. Pixel

2. Bit

Pixel

Short for "picture element" it is the fundamental unit of a digital image, typically a tiny square or dot which contains a single point of color of a larger image.

Pixel density

The number of device pixels on a physical surface; often measured in pixels per inch.

low pixel density

1. few pixels per inch

2. larger pixels

3. less detailed image

4. lower spatial resolution

high pixel density

1. many pixels per inch

2. smaller pixels

3. more detailed image

4. higher spatial resolution

Bit

A contraction of "Binary Digit". A bit is the single unit of information in a computer, typically represented as a 0 or 1.

byte

a group of 8 bits

Calculating the number of gray shades

2^number of bits (ex. 2 bits = 2^2=4)

# of bits: 4 = 16 number of shades = 2^4 (2 to the power of 4)

Always 2 to the power of number of bits

word

Consists of two bytes, or 16 bits.

fewer bits per pixel

fewer shades of gray

low contrast resolution

more bits per pixel

more shades of gray

improved contrast resolution

calculating the number of gray shades

2^number of bits (ex. 2 bits = 2^2=4

pixels

image element

image detail

spatial resolution

bits

computer memory

gray shades

contrast resolution

analog and digital image data

Converting information from analog to digital form is advantageous because digital information is far less susceptible to contamination.

analog to digital converter

to move information form the real world (analog) to the computer world (digital) requires a translator, called an analog-to-digital or A-to-D converter. And, vise-versa

preprocessing

Manipulation of image data before storage in the scan converter.

time gain compensation, log compression, write magnification, persistence, spatial compounding, edge enhancement, fill-in interpolation

postprocessing

Manipulation of image data after storage in the scan converter.

any change after freeze frame, black/white inversion, read magnification, contrast variation, 3-D rendering

magnification

Improve visualization of anatomic detail by enlarging a portion of an image to fill the entire screen.

the selected part of the image is known as the region of interest, or ROI.

2 different forms of magnification exist:

1. read magnification

2. write magnification

Read magnification

Occurs after the image data is stored in the scan converter.

Rad magnification of the ROI creates larger pixels from the information that is already in the scan converter.

The sonographer identifies the ROI, and the system reads and displays only the original data that pertains to the region of interest. The ROI is not rescanned.

write magnification

Applied during data acquisition, before storage in the scan converter.

The ultrasound system then rescans only the ROI and writes new data into the scan converter.

write magnification rescans the ROI and creates a new image with increased spatial resolution.

Hint: With write magnification, temporal resolution may be improved if the bottom of the region of interest (ROI) is shallower than the original image's depth of view.

Read Magnification

uses old data,

postprocessing,

larger pixel size,

same # of pixels as in the original ROI,

unchanged spatial resolution,

unchanged temporal resolution

Write Magnification

acquires new data,

preprocessing,

identical pixel size,

more pixels than in the original ROI,

improved spatial resolution,

may improve temporal resolution

Coded Excitation

Occurs in the pulser.

coded excitation provides

-higher signal-to-noise ratio

-improved axial resolution

-improved spatial resolution

-improved contrast resolution

-deeper penetration

Spatial compounding

technique that eliminates edge shadowing because the object is imaged at different angles.

Reduces shadowing artifacts.

Reduces speckles.

drops framerate. = degraded or more poor temporal resolution

frequency compounding

advanced technique that reduces speckle artifact and noise in ultrasound images.

the reflected signal is divided into sub-bands of limited frequencies, and an image is created fro each sub-band. The images from the sub-bands are then combined, or compounded into a single image.

The reflected sound wave is divided into parts with different frequency ranges, called sub-bands.

edge enhancement

image processing method that makes pictures look sharper.

Increases the image contrast in the area immediately around the edge.

Temporal compounding

Also called Persistence or Temporal averaging is an image processing technique that continues to display information from older images.

Reduction in the displayed frame rate, which in turn, reduces temporal resolution.

Most effective with slowly moving structures.

Superimposes the current frame on previous frames to create a smoother image.

Used to "enhance image"

Higher signal to noise ratio.

fill-in interpolation

-a method of constructing new simulated data points to fill in the gaps of missing data in a way that cannot be detected by the observer.

Form of preprocessing.

Predicts what information lies between the gaps, and improves spatial resolution.

Decreases frame rate = poor temporal resolution

Elastography

A sonographic technique employed to evaluate a mass based on its stiffness, ultimately providing a prediction as to if the mass is more likely malignant or benign.

What happens when compression is applied to the mass. Great for Cancer diagnosis / research.

PACS

Picture Archiving and Communication System

3 major advantages:

-virtually instant access to archived studies

-no degradation of data, which often happens over extended time periods with analog archiving.

-"store and forward" telemedicine, the ability to electronically transmit images and reports to remote sites.

Computer hard drives are the primary digital storage devices used in PACS.

DICOM

Digital imaging and communications in medicine, a standard protocol used for blending a picture archiving and communications system and various imaging modalities

Paper Media

Example:

Charts from pen writers

Advantages:

Portability

Does not require a device to read

Disadvantages:

Bulky, hard to store

Difficult to make copies

Cannot display dynamic images

Magnetic Media

Examples:

Computer discs

computer memory

magnetic tape

video tape

Advantages:

Able to store large amounts of information efficiently

can store and play dynamic images

can record color

Disadvantages:

can be erased by strong magnetic fields

Chemically Mediated Photographs

Examples:

photographs

flat films

multiformat camera film

Advantages:

high resolution

accepted in the medical community

can produce color images

disadvantages:

bulky, difficult to store and retrieve

requires chemical processing

artifacts can arise from dirt or chemical contamination

optical media

examples:

laser discs

compact discs

advantages:

store huge amounts of data

inexpensive

not erased by exposure to magnetic fields

disadvantages:

requires a display system

no standardized format for image display and storage