Displays and Image Processing Study Guide

Display Systems and Image Characteristics

• Role of the Display:

  • Serves as the vital link between the ultrasound system and the sonographer or medical staff.

  • Visual presentation directly governs diagnostic interpretation: what is seen determines the clinical conclusion.

• Modern Display Technology:

  • Modern flat panel computer monitors have replaced older cathode ray tube displays.

  • Provide higher frame rates and support increased line density (more scan lines per image).

  • Feature physical advantages including being lightweight and thin.

• Image Contrast and Brightness:

  • Bistable Images:

  • The prefix bi signifies two.

  • Composed of only two discrete shades: purely black and purely white.

  • Exhibit very high contrast (narrow dynamic range).

  • Gray Scale Displays:

  • Present multiple levels of brightness ranging from black, through varying shades of gray, to white.

  • Low contrast displays present a broad spectrum of gray shades, allowing fine differentiation of tissue parenchymal textures.

  • Brightness:

  • Determines the overall brilliance of the displayed image.

  • Controlled via system gain adjustments (for instance, an image adjusted with a gain of $20\,\text{dB}$ displays significantly brighter than an image adjusted at $11\,\text{dB}$, while operating at the same ultrasound frequency such as $4.0\,\text{MHz}$ and depth such as $140\,\text{mm}$).

Scan Converters: Analog vs. Digital

• Function of the Scan Converter:

  • Operates by first storing image data and subsequently displaying it.

  • Writing: The process of storing incoming acoustic information in the scan converter.

  • Reading: The process of retrieving stored image information from the scan converter to display on a monitor.

  • Converts the format of acoustic data: ultrasound data is acquired along radial spoke penetrations into the body (penetration lines), which the scan converter translates into horizontal video lines suitable for display screens.

• Analog Numbers vs. Digital Numbers:

  • Analog Numbers: Represent real-world values that exist on a continuous continuum without discrete steps (e.g., human weight, length of a string, exact voltages down to infinitely divisible decimal points).

  • Digital Numbers: Computer-based values limited to discrete, fixed steps or whole values without continuous fractions or half options.

• Analog Scan Converter:

  • Historically made gray scale imaging possible by using a silicon wafer/dielectric matrix to store electrical charges.

  • Now obsolete due to inherent technological shortcomings:

  • Image Fade: Charges stored on the dielectric matrix dissipate over time.

  • Image Flicker: Caused by constant switching between read and write modes.

  • Instability: Image quality drifts based on system operating temperature and duration of use.

  • Deterioration: Device components physically age, degrading picture fidelity over time.

• Digital Scan Converter:

  • Employs microprocessor technology to convert acoustic images into numerical binary values (digitization).

  • Advantages of Digital Systems:

  • Uniformity: Consistent gray scale quality across the entire image field.

  • Stability: Eliminates image drift and susceptibility to component aging or temperature changes.

  • Durability: Performance does not degrade with prolonged use.

  • Speed: Instantaneous image processing and display transfer.

  • Accuracy: Error-free representation without signal fade.

Digital Representation: Pixels and Bits

• Pixel (Picture Element):

  • The smallest individual building block of a digital picture.

  • When a digital picture is divided into a grid, each individual square is a pixel.

  • Each individual pixel can only represent one single shade of gray at any given time.

• Pixel Density:

  • The number of picture elements per inch.

  • Relationship to Spatial Resolution: Higher pixel density is achieved with smaller pixels, yielding a greater number of pixels per unit area.

  • Spatial Resolution: Directly improves with higher pixel density because finer anatomical detail and smaller structures can be accurately resolved.

• Bit (Binary Digit):

  • The smallest amount of computer memory.

  • Bistable in nature, holding an exclusive value of either $0$ or $1$.

  • Binary Number: A sequence or group of bits (e.g., $0001010100011101$).

• Byte and Word:

  • Byte: A collection of $8$ bits of computer memory (e.g., $00011010$).

  • Word: Consists of $2$ bytes, which equals $16$ bits of computer memory.

• Shades of Gray Calculation:

  • The number of discrete gray shades that an ultrasound system can display is determined by the number of bits allocated per pixel in the scan converter memory.

  • Mathematical rule: The number of shades is equal to $2$ raised to the power of $n$, where $n$ is the total number of bits: $\text{Number of Shades} = 2^n$.

  • An increase in the number of bits per pixel produces a greater number of gray shades, improving contrast resolution.

• Mathematical Matrix of Bits and Shades of Gray:

  • $1\,\text{bit}: 2^1 = 2\,\text{shades}$

  • $2\,\text{bits}: 2^2 = 2 \times 2 = 4\,\text{shades}$

  • $3\,\text{bits}: 2^3 = 2 \times 2 \times 2 = 8\,\text{shades}$

  • $4\,\text{bits}: 2^4 = 2 \times 2 \times 2 \times 2 = 16\,\text{shades}$

  • $5\,\text{bits}: 2^5 = 2 \times 2 \times 2 \times 2 \times 2 = 32\,\text{shades}$

  • $10\,\text{bits}: 2^{10} = 1024\,\text{shades}$

  • $n\,\text{bits}: 2^n\,\text{shades}$

Five-Step Signal Conversion Process

• Signal Conversion Pathway:

  • During reception, piezoelectric transducer elements generate analog electrical voltages that are weak and vulnerable to electrical noise and distortion.

  • To process, store, and present these signals on traditional displays, the system executes a mandatory five-stage conversion sequence:

  1. Analog-to-Digital (A-to-D) Conversion: Electrical signals generated by the transducer are translated from real-world continuous analog signals into digital binary numbers (bits).

  2. Digital Storage: The digital bits are loaded and stored within the digital memory matrix of the scan converter.

  3. Digital Processing: While residing in digital format, image data undergoes digital manipulation and processing by the ultrasound computer processor.

  4. Digital-to-Analog (D-to-A) Conversion: Since analog video display monitors cannot process or project discrete digital bits, the digital information is converted back into an analog voltage stream via a digital-to-analog converter.

  5. Analog Display: The converted analog signal is routed to the video screen to render the final visual image.

Image Processing: Preprocessing and Postprocessing

• Preprocessing:

  • Involves the manipulation and modification of echo data before it is stored in the scan converter memory.

  • Performed on live, real-time imaging data under sonographer control.

  • Permanence: All modifications are permanently written into the scan converter memory and cannot be undone or reversed once saved.

• Postprocessing:

  • Involves the alteration and optimization of image data after it has been stored in the scan converter memory.

  • Can be applied to frozen or static frames.

  • Reversibility: Fully reversible under sonographer control because the underlying stored digital memory values can be restored to their original status.

• Categorization of Processing Functions:

  • Preprocessing Modalities:

  • Time gain compensation (TGC)

  • Log compression (dynamic range adjustment)

  • Write magnification

  • Persistence (temporal compounding)

  • Spatial compounding

  • Edge enhancement

  • Fill-in interpolation

  • Postprocessing Modalities:

  • Any modification executed after activating freeze frame

  • Black/white inversion

  • Read magnification

  • Contrast variation adjustments

  • 3-D rendering

Magnification (Zoom): Read vs. Write

• General Principles:

  • Magnification permits clinical visualization of a localized region of interest (ROI) enlarged across the display monitor.

  • Divided into two distinct mechanisms based on when the zoom is applied relative to scan converter storage.

• Read Magnification:

  • Applied after the image data has entered and been stored in the scan converter (postprocessing).

  • Utilized on frozen images.

  • Operational Mechanism:

  • The sonographer identifies a specific ROI.

  • The system reads only the stored original data corresponding to that ROI and displays it enlarged.

  • No rescanning of patient tissue occurs.

  • The pixel size increases (pixels appear enlarged and coarse).

  • The number of pixels and scan lines within the magnified display is identical to the number in the original ROI.

  • Resolution Impacts:

  • Spatial resolution remains entirely unchanged.

  • Temporal resolution remains unchanged.

• Write Magnification:

  • Applied before image data is permanently stored in the scan converter (preprocessing).

  • Utilized during active, real-time live scanning.

  • Operational Mechanism:

  • The sonographer selects the target ROI on the screen.

  • The ultrasound system discards all existing information stored in the scan converter.

  • The system directs the transducer to rescan exclusively the designated ROI, collecting brand new acoustic data.

  • The pixel size remains identical to the original image pixels.

  • The number of pixels within the ROI increases significantly over the original display area.

  • Resolution Impacts:

  • Spatial resolution is substantially improved due to the increased pixel density within the ROI.

  • Temporal resolution may improve if the designated ROI has a shallower imaging depth than the initial scanning field.

• Direct Comparison: Read vs. Write Magnification:

  • Data Source: Read magnification uses old data; write magnification acquires new data.

  • Processing Phase: Read magnification is postprocessing; write magnification is preprocessing.

  • Pixel Size: Read magnification produces larger pixel sizes; write magnification maintains identical pixel size.

  • Pixel Count: Read magnification maintains the same number of pixels as the original ROI; write magnification yields more pixels than in the original ROI.

  • Spatial Resolution: Read magnification maintains unchanged spatial resolution; write magnification provides improved spatial resolution.

  • Temporal Resolution: Read magnification maintains unchanged temporal resolution; write magnification may improve temporal resolution.

Advanced Image Processing and Acquisition Techniques

• Coded Excitation:

  • Developed within the pulser to overcome the physical limitations of peak acoustic power.

  • Physical Trade-off: Traditional ultrasound imaging relies on short sound pulses to achieve superior axial resolution; however, driving short pulses at high amplitudes generates extremely high peak intensity, raising bioeffects concerns regulated by FDA thresholds.

  • Mechanism: Generates very long sound pulses that carry a complex mathematical pattern and a broad band of frequencies.

  • The acoustic energy is distributed across a longer duration, keeping peak intensities safely within allowable FDA ranges while maintaining high signal energy.

  • Complex mathematical deconvolution and decoding algorithms in the receiver compress the long reflected echo into an ultra-short, high-fidelity pulse.

  • Performance Benefits of Coded Excitation:

  • Higher signal-to-noise ratio

  • Improved axial resolution

  • Improved spatial resolution

  • Improved contrast resolution

  • Deeper acoustic penetration

• Spatial Compounding:

  • An acquisition technique that steers sound beams electronically in multiple directions from different angles.

  • Multiple individual frames obtained from different imaging angles are averaged together into a single composite image.

  • Hardware Requirement: Can only be performed with phased array transducers that possess electronic beam steering capabilities; mechanical transducers cannot perform spatial compounding.

  • Clinical Benefits:

  • Substantially reduces acoustic speckle.

  • Minimizes and eliminates shadowing artifacts.

  • Limitations:

  • Requires multiple pulses per line of sight across various angles, which significantly reduces the frame rate.

  • Reduced frame rate degrades temporal resolution.

• Frequency Compounding:

  • Traditional imaging creates a frame using the entire frequency spectrum of the returning echo pulse.

  • Frequency compounding divides the wide bandwidth of the returning reflected sound wave into several sub-bands of smaller frequency ranges.

  • Separate images are created simultaneously from each distinct sub-band.

  • The sub-band images are compounded (averaged) together into a single image.

  • Diagnostic Impact: Significantly diminishes acoustic speckle and noise, as noise artifacts in one sub-band do not correlate with noise in another.

• Edge Enhancement:

  • A preprocessing computational filter designed to increase the apparent sharpness of an image.

  • Identifies sharp brightness transitions representing tissue boundaries (such as the reflecting interface between two distinct media).

  • Emphasizes borders by creating subtle bright and dark highlight bands immediately adjacent to the edges, making boundaries appear more distinct, crisp, and defined.

• Temporal Compounding (Persistence / Temporal Averaging):

  • Continues to superimpose a specific number of previously acquired frames over the most current real-time frame.

  • Generates a composite running average of acoustic information over time.

  • Clinical Advantages:

  • Produces a smoother image display.

  • Reduces acoustic noise and speckle.

  • Enhances the signal-to-noise ratio.

  • Improves overall image quality and luminal filling in vascular color Doppler.

  • Limitations:

  • Because it blends temporal frames, persistence causes motion blur and reduces temporal resolution.

  • Best suited for slowly moving or stationary organs (e.g., abdominal parenchyma, vascular flow).

  • Unsuitable for rapidly moving structures such as cardiac valves.

• Fill-in Interpolation:

  • Ultrasound scan lines diverge with increasing depth, creating widening triangular or pie-shaped physical gaps with zero acoustic data (especially pronounced in sector, curved, and vector formats).

  • Fill-in interpolation is a preprocessing algorithmic function that constructs new, simulated pixel data points to fill in the missing tissue gaps.

  • Evaluates the gray scale levels of neighboring scan lines and calculates mathematical averages to interpolate the intervening empty pixels.

  • Operates imperceptibly to the observer, increasing line density and improving spatial resolution.

Elastography

• Physical Principle:

  • Grounded in the biomechanical principle that different tissue types deform differently following the application of mechanical force.

  • Stiff, rigid tissues deform minimally, whereas soft, compliant tissues deform extensively under stress.

• Image Generation:

  • In modern systems, deformation is applied automatically using the acoustic radiation force of the sound beam.

  • The system computes local tissue stiffness estimates and superimposes these mechanical elasticity properties directly onto the conventional B-mode ultrasound reflections.

  • The resulting composite image is termed an elastogram.

• Diagnostic Utility:

  • Provides enhanced tissue characterization that conventional sonograms cannot resolve alone.

  • Proven effective in differentiating benign tissue from malignant tumors (e.g., ductal carcinoma of the breast presents as a hard, non-deformable, stiff mass clearly demarcated on an elastogram relative to adjacent soft parenchyma).

Image Archiving and Communication: PACS, DICOM, and Media Types

• Picture Archiving and Communication System (PACS):

  • The comprehensive digital network and archival system responsible for the electronic storage, retrieval, distribution, and presentation of medical imaging data.

  • Primarily utilizes computer hard drive arrays for digital data storage.

  • Three Major Advantages:

  • Instant Access: Immediate access to archived current and historical imaging studies.

  • Data Integrity: No degradation of data over time, eliminating analog image fade.

  • Electronic Transmission: Facilitates rapid electronic transmission of images and clinical reports to remote sites and external consulting physicians.

• Digital Imaging and Computers in Medicine (DICOM):

  • A standardized set of universal rules, communication protocols, and file formats that allow diverse medical imaging systems and computer networks to communicate and share data seamlessly.

  • Contemporary ultrasound systems incorporate built-in DICOM compliance to ensure seamless interoperability with institutional PACS networks.

• Storage and Archiving Media:

  • Paper Media:

  • Examples: Hard-copy charts produced by pen-writers, paper printouts.

  • Advantages: Highly portable; does not require specialized hardware or a playback device to read.

  • Disadvantages: Extremely bulky; difficult to store and index; copying is labor-intensive; completely incapable of displaying dynamic cine clips.

  • Magnetic Media:

  • Examples: Computer hard discs, solid-state computer memory, magnetic tape, video tape.

  • Advantages: Stores massive amounts of information efficiently; capable of recording and playing dynamic imaging loops; preserves color Doppler data.

  • Disadvantages: Highly vulnerable to irreversible erasure and corruption if exposed to strong magnetic fields.

  • Chemically Mediated Photographs:

  • Examples: Photographic paper prints, transparent flat films, multiformat camera film.

  • Advantages: High optical resolution; historically accepted standard across the global medical community; capable of rendering color images.

  • Disadvantages: Bulky; difficult to catalog, store, and retrieve; mandates wet chemical processing; vulnerable to image artifacts from physical dirt, scratches, and chemical contamination.

  • Optical Media:

  • Examples: Laser discs, compact discs (CDs/DVDs).

  • Advantages: Capable of storing massive quantities of digital data; inexpensive; completely immune to erasure from magnetic fields.

  • Disadvantages: Requires an external optical display/reading system; historically lacked standardized formats for image display and long-term storage.

Questions & Discussion

• Question: What is the total number of shades of gray that can be represented by $3\,\text{bits}$ of memory?

  • Response: Multiplying $2$ by itself $3$ times yields: $2^3 = 2 \times 2 \times 2 = 8$. Therefore, $3\,\text{bits}$ can represent exactly $8$ different shades of gray.

• Question: Which of the following digital scan converters will provide the best spatial resolution?

  • A. $256 \times 256$

  • B. $512 \times 512$

  • C. $128 \times 128$

  • D. $1000 \times 1000$

  • Response: Option D ($1000 \times 1000$). The grid containing the highest number of pixels per unit area provides the highest pixel density, which yields the best spatial resolution.

• Question: Which of these statements regarding a bit of computer memory is false?

  • A. It is the smallest element of an image

  • B. It is bistable

  • C. Eight bits combine to make a byte

  • D. It is a component of a digital scan converter

  • Response: Option A is false. A pixel is the smallest element of an image, whereas a bit is the smallest element of computer memory.

• Question: Where does coded excitation take place within the ultrasound instrumentation?

  • A. In the transducer

  • B. In the receiver

  • C. In the pulser

  • D. In the PACS system

  • Response: Option C. Coded excitation takes place directly in the pulser of the ultrasound system.

• Question: What is the primary disadvantage of video tape and computer disc methods of image archiving?

  • A. These methods are very costly

  • B. The information is vulnerable to erasure when exposed to strong magnetic fields

  • C. Only low resolution images are stored

  • D. Only black and white images are stored

  • Response: Option B. Because video tape and computer discs rely on magnetic media, their primary vulnerability is accidental erasure when placed in proximity to strong magnetic fields.

• Question: Which of the following statements is not true regarding spatial compounding?

  • A. Data is collected from different viewing angles

  • B. Frame rates are reduced

  • C. Shadowing artifact can be eliminated

  • D. Can be performed with a mechanical transducer

  • Response: Option D is not true. Spatial compounding requires electronic steering of the ultrasound beams, which is only possible with phased array transducers, not mechanical transducers.

• Question: Which of the following technologies uses long sound pulses containing a complex pattern of frequencies to create an image?

  • A. Coded excitation

  • B. Harmonics

  • C. Pulse inversion

  • D. Rendering

  • Response: Option A. Coded excitation generates long sound pulses containing a complex range of frequencies to distribute energy safely while maximizing image penetration and resolution.

• Question: In modern elastography, how are clinical tissues deformed?

  • A. By pressing the transducer into the skin

  • B. With the use of a second probe

  • C. Automatically

  • D. As a result of respiration

  • Response: Option C. In modern elastographic imaging systems, tissue deformation is produced automatically via the acoustic radiation force generated by the sound beam itself.