Chapters 16, 17 & 18: Patient Image Optimization, Viewing the Digital Image, PACS and Communication Systems

Chapter 16: Patient Image Optimization

Patient Factors

• Radiographic technique charts are based on sthenic patients.
• Adjustments may be needed for:
  • Thickness
  • Composition
  • Pathology
• Patient Mass Density/Composition
  • Thorax vs. abdomen
  • Subject contrast
    • ST low subject contrast
      • Low kVp due to low subject contrast
    • Bone high subject contrast
      • Low kVp due to decreased part thickness
    • Thorax high to moderate subject contrast
      • High kVp due to high contrast and thick part
• Habitus:
  • Sthenic: strong/active
  • Hyposthenic: thin/healthy
  • Hypersthenic: big frame/usually over weight
  • Asthenic: small/frail/emaciated (often elderly)

Terminology and Charts

• Radiopacity
• Radiolucency
• Charts:
  • Define:
    • kVp
    • mA
    • Time
    • SID
    • Grid use
    • Focal spot size
    • Filtration
  • May be kVp adjustable or mAs adjustable

Pathology

• Pathology may change the normal effective atomic number of tissue resulting in differences in attenuation from the normal
• Technique factors depend on:
  • Composition
  • Destructive/subtractive (e.g., Osteoporosis, necrosis)
  • Constructive/additive (e.g., Ascites, pleural effusion, Paget’s disease)
  • Size
  • 5% rule
    • Increase kVp 5% accompanied by mAs reduction of 30% maintains the same IR response
• Examples of Destructive vs. Constructive Pathologies:
  • Destructive: TB, Atrophy, Bowel Obstruction, Cancer, emphysema, Pneumothorax, sclerosis
  • Constructive: Ascites, Atelectasis, Cirrhosis, Hypertrophy, Pneumonia

Image-Quality Factors

• The language of radiographers
• Image detail
  • Sharpness and accuracy of image detail is best measured by spatial resolution.
  • Determined by pixel size and geometrics (SID, OID, Beam angulation, part angulation)
  • Visibility of image detail is best measured by contrast resolution.
    • Degraded by any noise
    • Maintained by use of scatter prevention and appropriate technique
    • Limited by bit depth
• Distortion:
  • Misrepresentation of anatomy in size or shape
  • Reduced by positioning the anatomical part of interest in a plane parallel to that of the image receptor with the CR perpendicular to the part/IR
  • Magnification/elongation/foreshortening
• Image artifacts

Magnification Radiography

• Improves the visualization of small structures
• Decreases spatial resolution
• Magnification factor = $SID/SOD$

Geometrics of Radiography

• [Diagram of geometric relationships]

Summary

1. The language of Radiology can assist in effective communication and instruction
2. Patient body type, composition and pathology can direct the use of technical factors
3. Utilization of technique charts or Automatic Programmed Consoles can assist in consistency between images and are developed with kVp, mA, time, Grid use, SID, OID, receptor speed and patient composition in mind
4. Technical adjustments must be made for additive or subtractive pathologies
5. Technologists must understand the relationship of the part to image receptor and CR and evaluate how that may affect spatial resolution and image quality factors

Chapter 17: Visualizing the Image

Vision and Photometry

• Vision relies on psychology, physiology, physics and physiology
• Cones of the eye are involved in bright light vision (photopic)
• Rods are involved in dim light vision (scotopic)
• Photometry is the measurement of light:
  • Basic unit of photometry is lumen
  • Nanometers is a measurement of light wavelength
  • Maximum eye response is at 555nm (nanometers)
    • $Nano=10^{-9}$
  • Best vision is between 555-505nm

Photometry Terminology

• Luminous Flux:
  • Total intensity of light from a source
  • Described in lumens
  • Watts is the amount of power consumed; lumens is the amount of light released
• Illuminance:
  • Intensity of light incident on a surface
  • One lumen of luminous flux on a square foot surface is called a foot candle (English System)
  • Metric version is 1 lumen per square meter, known as Lux
  • $1fc=10.8Lux$
• Luminance intensity:
  • The luminous flux emitted from a source into the entire viewing area
  • Measured in lumens per steradian or candela.
• Luminance:
  • Measure of brightness emitted from a source such as a digital display
  • Measured in candela per square meter (English) or nit (SI)

Laws of Photometry

• Inverse Square Law:
  • Luminance decreases as distance increases according to the inverse square law, just like radiation!
• Cosine Law:
  • When a monitor is viewed from directly in front, it demonstrates maximum luminance
  • When viewed from an angle, luminance is decreased according to the cosine law
  • Luminous intensity falls off rapidly as one views the device from larger angles

Hard vs. Soft Copy

• Hardcopy: Images presented on film
• Softcopy: viewing images from a digital display device
  • Usually LCD (Liquid Crystal Display) or LED (Light Emitting Diode Display)
  • Formerly Cathode Ray Tubes (CRT)

Displays

Cathode Ray Tube (CRT):

• Glass envelope tube containing an electron gun directed at a fluorescent screen with graphite lining
• Fluorescent screen is made of linear phosphor, needle like crystals backed by aluminum
• How it works:
  • Tube receives signal proportional to light intensity
  • The electron gun focuses an electron beam onto the output fluorescent screen
  • Gun motion is controlled by external coils
  • Electrons interact with output phosphors causing a burst of light
• Raster Pattern
  • Like a typewriter, writes a line left to right, shuts off and returns to start (called horizontal retrace)
  • Motion repeats until image is written top to bottom
  • Vertical retrace occurs top to bottom, running from left to right on the screen skipping areas that were filled by horizontal retrace (called interlacing)
  • 1 horizontal or vertical retrace occurs per electronic cycle, so in US, field rate will be 60 Hz and frame rate will be 30 Hz
  • Most CRT’s are 525-1049 line systems

Liquid Crystal Display (LCD):

• Liquid crystal is a linear organic molecule in a solid/liquid form with structure and viscosity.
• They are electrically charged linear molecules which have a dipole, allowing them to be aligned through manipulation of an electric field
• Each display pixel is backlit by an intense white lumination
• Each pixel has 2 glass plate substrates sandwiching the liquid crystals
• Each pixel has a TFT to control it and provide signal
• Each pixel also has light polarizing filters and films to control color and transmission of illumination
• B&W and color only differ due to design of filters and films used in display
• Color monitors have red, green and blue filters acting as subpixels of each pixel
• These films and filters are manipulated to block or transmit the backlighting to create the image
• Resolution is controlled by pixel size
• High resolution systems usually have 5 megapixels (2000x2500 matrix)
• Inefficient
  • 10% of backlight is transmitted
  • Due to light absorption in the filters and blockage of portion of pixel by TFT
  • Portion of pixel that can transmit light is the “aperture ratio” similar to fill factor (typically 50-80%)
• Have good Gray Scale and not effected by Veiling glare

Light Emitting Diode (LED) Display

• Diode: electronic device that allows current to flow in one direction
• LED’s: a solid state semiconductor diode that emits light
  • Light emission is caused by outer shell electrons being stimulated into a metastable state, when they drop back in the vacant shell the emit light
  • Fluorescence, Phosphorescence, electroluminescence
    • Flourescnece: light emitted only during stimulation
    • Phosphorescence: light emitted after stimulation
    • Electroluminescence: light emitted when stimulated by electrical current (How LED’s operate)

LED Display:

• Provides backlighting for LCD displays
• Is not replacing LCD
• LED backlighting is replacing fluorescent backlighting because:
  • They are thinner
  • Larger active area
  • Allow for concave and larger video screens
  • 2x longer life
  • Lower power consumption
  • Produce less heat

The Reading Room

• LCD screens: less glare from ambient lighting
  • Angle of visualization dependent
• Ergonomics:
  • Matching the worker to the environment for maximum comfort and efficiency
  • Includes ambient lighting, seating, angle of visualization of monitors

Preprocessing

• Manipulation of image before display
• Typically to reduce artifact and improve contrast
• Largely automatic
• Includes:
  • Electronic calibration to reduce variation between pixels of different rows and columns
    • Equipment uses Offset images and Gain images that are automatically calibrated images designed to make response of the image receptor uniform to maintain preprocessing standards.
  • Pixel interpolation: “covers up” defective pixels by averaging surrounding pixels
  • Lag correction: utilizes offset voltage between images to remove any lagging image information from previous latent image
  • Noise correction: corrects line noise by applying a voltage correction from a row or column of pixels in a dark area of the IR
    • Line noise is a linear artifact on an image caused by electrical variations in the buses of a pixel
  • Flatfielding: calibration techniques including offset images and gain images
    • Corrects defects from anode heel effect and other image receptor issues that cause nonuniform IR response
    • Helps overcome perception of anode heel effect

Post Processing

• Requires action by technologist or radiologist
• Any action that can be done to change the digital image after it is displayed on the workstation monitor
• Types of post processing:
  • Annotation
  • Window width (gray scale) and level (brightness) adjustment
  • Typical digital bit depth is 16
  • Magnification of ROI
  • Image flipping
  • Image inversion
  • Image subtraction
  • Pixel shift (correcting misregistration of image when patient moves during serial imaging)
  • Quantitative imaging (using the numeric value of a pixel to help in diagnosis): requires selection of an ROI and its mean pixel value
  • Edge Enhancement
  • Highlighting
  • Pan, Scroll and zoom

Summary - Chapter 17

• Photometry is the measurement of light, which is essential to understanding the viewing processes of digital imaging
• Common digital displays such as CRT and LCD displays utilize various luminescent qualities to produce an image
• CRT displays images that are created by directing electrons toward a fluorescent screen in a raster pattern
• LCDs utilize TFT controlled pixels filled with a liquid crystal material and illuminated posteriorly
• LCD crystals are manipulated with the TFT controlled electrical signal due to their dipole properties- when lined up, they let light through when close off the filter the light
• Digital systems have significantly reduced the need for storage and expedited the process of image acquisition, access to images and transferability of care
• HIS, RIS, PACS and DICOM are essential components of the digital imaging healthcare system

Chapter 18: Picture Archiving and Communication System (PACS)

Digitization

• “scanning” and digitizing analog images

Picture Archiving and Communication System (PACS)

• Allows acquisition, interpretation and storage of digital images
• Allows easy viewing, storage and recall of images

PACS 4 Components:

• Acquisition component
• Display
• Network
• Storage

Network:

• Connection of many computers to interact with one another
• Each workstation is microprocessor controlled and interacts with each imaging system and a central computer
• Often connected to storage, devices for retrieval and viewing, PACS workstations, Remote PACS wrokstations, Department and Hospital Mainframes
• All components of the network is called a “client”
• Clients are interconnected by telephone, cable or television lines or microwave or satellite transmission

Storage:

• PACS systems usually have many workstations connected to a storage device

The Process

• Image obtained at digital imaging system
• Image reconstruced or processed at workstation
• Image transmitted to PACS
• Image retrieved from PACS for radiologist
• Read Images retrieved and transferred as necessary from PACS system

Teleradiology

• Remote transmission and viewing of images
• To ensure communication between different systems the ACR and the National Electrical Manufacturers Association (NEMA) developed and interface format called DICOM (Digital Imaging and Communications in Medicine)
• Requires fast networks with broad bandwidth

Radiology Information System (RIS)

• Database portion of PACS
• Provides:
  • Messaging and mail function
  • Tracking
  • Reporting data
  • Storage of text data (radiology reports, histories, etc.)
  • Financial accounting and planning

Storage Systems

• Storage of an exam depends on the number of images and the amount of data within each image
• File size is a product of matrix x bit depth
• Estimated storage sizes
• File rooms are replaced by optical memory devices
  • Optical discs hold tens of GB
  • These can be stored in a juke box to hold tarabytes

File Size

• Physical file size is now replaced with large data file size
• File size is calculated by multiplying the matrix times the bit depth
• Largest to smallest files are as follows:
  • Mammo
  • DR radiography
  • CT
  • MRI
  • Sonography
  • Nuclear medicine

Electronic Medical Records (EMR)

• Stored in the Hospital Information System (HIS)
  • Includes the basic patient information:
    • Insurance
    • Demographics
    • MR number
    • Billing information
    • Advanced directives
• HIS is integrated with all the hospital, including the RIS system
  • RIS is database for radiology department
    • Tracks patient radiology exams
    • Reports
    • Accession numbers
    • Times of exams, etc.
    • Feeds the Modality worklist

Electronic Programs

• Network connects imaging systems to a central computer to interact with one another
• Clients are interconnected by wire among buildings, and by microwave or satellite transmission to remote facilities.
• Allows for teleradiology
  • Remote transmission and viewing of medical images
  • Controlled by DICOM
• Storage system
• Electronic Medical Record (EMR):
  • Incorporated with HIS
  • Communicates with RIS and PACS
• Informatic Communications
  • DICOM

Informatics Communication

DICOM

• Rules for digital imaging
• Requires header metadata
• Specific to each type of imaging modality
• Automatic
• May require documents to link to images, such as dose reports, BI RADS reports, OB measurements, etc.
• Often utilizes AI algorithms

HL7 (Health Level 7) Standard:

• Rules for how health information is shared

IHE (Integrating Healthcare Enterprise)

• Manges data flow from DICOM to PACS

Medical Information Organizations

• The Technical Standards for Electronic Practice of Medical Imaging
  • Developed by the ACR the AAPM and the Society for Imaging Informatics in Medicine (SIIM)
  • Identifies medical imaging team responsibilities
  • Radiologist has ultimate responsibility of all steps and personnel
• American Medical Association (AMA)
  • Developed Current Procedural Terminology (CPT codes)
  • Codes over 15,000 items for clinical situations
• Center for Medicare and Medicaid Services (CMS)
  • International Classification of Diseases (ICD codes)
  • Codes to identify disease and symptoms
• RadLex developed by the RSNA (Radiolgical Society of North America)
  • Language of image descriptors of imaging modalities to help radiologists more clearly state findings.

Summary Chapter 18

• PACS, HIS and RIS are computer systems often integrated through networks that are responsible for patient processes and storage
• Changes to digital imaging has greatly disrupted the traditional film screen work flow processes
• Society and organization standards, along with local, state and federal requirements have been integral in keeping patient information safe and transferrable
  • DICOM
  • HL7
  • HIPAA
• Radiographers now must be familiar with the digital imaging processes in addition to image acquisition