Displays and Image Processing

Displays and Image Processing

Bistable vs. Gray Scale Displays

• Bistable Images:
  • Composed of only two shades: black and white.
• Gray Scale Displays:
  • Present multiple levels of brightness.
  • Include black, white, and many shades of gray in between.
  • Allow assigning different gray shades to different echo amplitudes.
  • Help differentiate biologic tissues of different reflectivity.

Display Controls

• Two user controls on video monitors:
  • Contrast
  • Brightness

Contrast

• Determines the range of brilliancies within the displayed image.
• Bistable images are high contrast because they only have black and white.

Brightness

• Determines the brilliance of the displayed image.

Scan Converters

• Function:
  • Store information and then later display it.
  • Storing information is called "writing."
  • Retrieving information for display is called "reading."
• Evolution:
  • Originally used analog technology.
  • Now use digital scan converters with computer technology.
• Translation:
  • Translate ultrasound information from multiple scan lines (spokes) into video format (horizontal lines).
• Data Alteration:
  • Data stored in the scan converter can be altered between the writing and reading processes.

Analog vs. Digital Numbers

• Analog Numbers:
  • Real-world numbers.
  • Have an unlimited and continuous range of values.
• Digital Numbers:
  • Associated with computer devices.
  • Have discrete values.

Analog Scan Converter

• Construction:
  • Funnel-shaped vacuum tube with an electron gun at the smaller end.
  • Larger end contains a dielectric matrix or silicon wafer.
• Process:
  • Electrons containing image information are shot from the electron gun.
  • Electrons strike and are stored in the matrix.
  • The matrix is divided into millions of tiny storage elements.
  • Stored electrons are read to retrieve image information.
• Advantages:
  • Has excellent spatial resolution (image detail).
• Limitations:
  • Image fade: stored charges on the silicon wafer dissipate over time.
  • Image flicker: caused by switching between read and write modes.
  • Instability: picture quality depends on many factors.
  • Deterioration: image degrades as the device ages.
• Obsolescence:
  • Due to limitations, analog converters are now obsolete.

Digital Scan Converter

• Process:
  • Uses computer technology to convert images into numbers (digitizing).
  • Image is stored in computer memory as a series of zeroes and ones.
  • Numerical representation is processed and retranslated into an image for display.
• Advantages:
  • Uniformity: Consistent gray scale quality.
  • Stability: Does not fade or drift.
  • Durability: Not affected by age or heavy use.
  • Speed: Nearly instant processing.
  • Accuracy: Error-free.
• Key Elements:
  • Pixel
  • Bit

Pixel

• Definition:
  • "Picture Element."
  • Smallest building block of a digital picture.
  • The entire pixel is a single shade of gray at any instant in time.
• Pixel Density:
  • Number of pixel elements per inch.
  • Higher pixel density = more detail and improved spatial resolution, smaller pixels, and more pixels per inch
  • Lower pixel density = less detailed image, larger pixels, fewer pixels per inch, and lower spatial resolution

Bit

• Definition:
  • "Binary Digit."
  • Smallest amount of computer memory.
  • Bistable: has a value of either 0 or 1.
• Binary Number:
  • A group of bits; a series of zeroes and ones (e.g., 0101010011).
• Byte:
  • Group of eight bits of computer memory (e.g., 10011111).
• Word:
  • Consists of two bytes (16 bits).
• Gray Shades:
  • Each pixel's shade of gray is determined by the cluster of bits assigned to it.
  • Fewer bits per pixel: fewer shades of gray, degraded contrast resolution.
  • More bits per pixel: more shades of gray, improved contrast resolution.

Calculating Number of Gray Shades

• To determine the number of gray shades that can be represented by a cluster of bits, multiply the number 2 by itself the same number of times there are bits:
• $2^{\text{number of bits}}$

Analog and Digital Image Data

• Benefits of Digital Conversion:
  • Digital information is less susceptible to noise contamination.
• Process:
  • Information is taken from the transducer in analog form, changed to digital form for computer storage, and then turned back into analog form for display.
  • This is a 5-step process.

Analog -> Digital -> Analog

1. Electrical signals from the transducer are converted from analog to digital by the analog-to-digital (A-to-D) converter.
2. Digital information is stored in the scan converter’s computer memory. Processing before storage is called preprocessing.
3. The image information (digital form) is processed by the ultrasound system’s computer. Processing after storage is called postprocessing.
4. Digital signals are translated back into analog form by a digital-to-analog (D-to-A) converter for analog display devices.
5. The signal is now in analog form and presented on the analog video display for interpretation.

• Modern Flat-Panels:
  • Digital display devices, so step 4 is unnecessary in these cases.

Pre- and Post-processing

• Preprocessing:
  • Manipulation of image data before storage in the scan converter.
  • Controlled by the sonographer.
  • Alters image data permanently and cannot be reversed.
  • Examples:
    • Time gain compensation
    • Log compression
    • Write magnification
    • Persistence
    • Spatial compounding
    • Edge enhancement
    • Fill-in interpolation
• Post-processing:
  • Manipulation of image data after storage in the scan converter.
  • Controlled by the sonographer.
  • Changes can be reversed.
  • Alterations to a frozen image = postprocessing.
  • Examples:
    • Any change after freeze frame
    • Black/white inversion
    • Read magnification
    • Contrast variation
    • 3-D rendering

Magnification (Zoom)

• Enlarging a portion of the image to fill the entire screen for improved visualization of anatomic detail.
• The selected part of the image is called the region of interest (ROI).
• Two types of magnification:
  • Read magnification
  • Write magnification

Read Magnification

• Occurs after the image data is stored in the scan converter.
• Three steps:
  1. The US system scans the anatomy.
  2. Image is converted from analog to digital form and stored in the scan converter.
  3. Sonographer identifies the ROI, and the system reads and displays only the original data that pertains to that area (ROI is not rescanned).
• Characteristics:
  • The number of pixels or scan lines in the magnified image is the same as the original image.
  • Spatial resolution does not change.
  • Pixels are larger in the zoomed image.
  • Can be used on a frozen image.
  • Postprocessing.

Write Magnification

• Applied during data acquisition, before data is stored in the scan converter.
• Four steps:
  1. The US system scans the anatomy and creates an image.
  2. Image is converted from analog to digital and stored in the scan converter.
  3. Sonographer identifies the ROI. The US system discards all the existing data in the scan converter.
  4. The US system rescans only the ROI and writes new data into the scan converter.
• Characteristics:
  • The image used to identify the ROI is discarded, and all new image information is acquired.
  • The number of pixels or scan lines in the ROI image is greater than that in the ROI’s portion of the original image.
  • Increased number of pixels improves spatial resolution for the ROI.
  • Cannot be used on a frozen image.
  • Preprocessing.

Comparison of Read and Write Magnification

Coded Excitation

• Sophisticated method of improving image quality.
• Developed within the context of bioeffects due to high-intensity levels of short sound pulses.
• Creates very long sound pulses containing a wide range of frequencies.
• Long pulses distribute energy over a broad frequency range.
• Improves penetration.
• Occurs in the pulser.
• Special mathematical techniques alter the long reflected pulses into a form suitable for high image quality.
• The coding of the excitation shortens the pulse mathematically and improves resolution.

Advantages of Coded Excitation

• Higher signal-to-noise ratio
• Improved axial resolution
• Improved spatial resolution
• Improved contrast resolution
• Deeper penetration

Spatial Compounding

• A method of using sonographic information from several different imaging angles to produce a single image.
• The number of frames and steering angles varies.
• More frames = better compound image quality.
• Reduces speckle and shadowing artifacts.
• Reduced frame rate/temporal resolution.
• Uses electronic steering – phased arrays only.

Frequency Compounding

• Reduces speckle artifact and noise in ultrasound images.
• All reflected sound pulses contain a large range of frequencies.
• Reflected signal is divided into sub-bands of limited frequencies, and an image is created from each sub-band.
• Images from the sub-bands are then combined into a single image.
• Noise components of the reflections are different for each sub-band, and noise is reduced when they are combined.

Edge Enhancement

• Image processing method that makes pictures look sharper.
• The computer identifies and emphasizes sharp edge boundaries in the image.
• Increases the image contrast in the area immediately around the edge.
• Creates subtle bright and dark highlights on either side of these boundaries to make them appear more defined.

Temporal Compounding

• Also known as persistence or temporal averaging.
• An image processing technique that continues to display information from older images.
• A number of previous frames are superimposed on the most current frame.
• Creates a smoother image with reduced noise and improved image quality.
• Reduction of frame rate/temporal resolution.
• Most effective with slowly moving structures.

Fill-in Interpolation

• With sector-shaped images, the scan lines separate at increasing depths, creating gaps.
• Interpolation constructs new simulated data points to fill in the gaps.
• The known gray-scale level of neighboring pixels are used to predict the missing information.
• Preprocessing.
• Spatial resolution is improved.

Elastography

• Dynamic technique that produces images from sound reflections in an entirely new way.
• Related to the mechanical properties of the tissue – tissues will deform differently following the application of a force.
• Estimates of tissue stiffness are obtained and combined with ultrasound reflections into images called elastograms.
• Potential as a complementary tool for the diagnosis of cancer.

PACS

• Picture Archiving and Communications System.
• Digital ultrasound lab where images and additional medical information are digitized and stored on a computer network.
• Three major advantages:
  • Virtually instant access to archived studies.
  • No degradation of data.
  • “Store and forward” telemedicine – can electronically transmit images and reports to remote sites.
• Computer hard drives are the primary digital storage devices used in PACS.

DICOM

• Digital Imaging and Computers in Medicine.
• A set of rules that allows imaging systems to share information on a network.
• If an US system adheres to DICOM standards, the system can successfully connect to a PACS network.
• Guarantees that all devices developed in the future will work with PACS as well.

Recording and Archiving Techniques