1725 Viewing the Medical Image

Photometric Qualities

• Human vision is a complex response that involves psychology, physiology and physics

• First attempt to quantify human vision was in 1924 by the Commission Internationale de l’Eclairage (CIE)

• Recognized difference between photopic (bright light) and scotopic (dim light) vision

Photometric Units

• The basic unit is Lumen

  • Luminous flux describes the light intensity

  • Illumination describes the amount of light incident on a surface

  • Luminance intensity is a property of the light source. It describes the light emitted

  • Luminance is a measure of the brightness

• Inverse Square Law

• Cosine Law

• Hard Copy vs. Soft Copy

Active Matrix Liquid Crystal Display

• LCD have structure of a crystal with viscosity of a fluid (liquid)

• They are aligned by the action of electric fields

• They function with filters (color) and films (intensity) to allow the image to be displayed

• The brightness of the image and the ambient light affect the visualization of the image

• LCDs reduce the affect of ambient light

Light Emitting Diode Display

• LED emits light when electrically stimulated

  • Electroluminescence

• In radiography, LEDs are found in digital display devices and video monitors

  • Provides the backlight

Pre-Processing

• The ability to manipulate the image before it is displayed

  • Largely automatic

• It is designed to prevent artifacts caused by the image receptor

  • Electronic calibration to reduce pixel-pixel, row-to-row, and column-to-column response differences

• Defective pixel

  • Interpolate (Average) adjacent pixel signals

• Image Lag

  • Offset correction

• Line Noise

  • Corrected from a dark reference zone

Defective Pixel

• A pixel in the digital detector that does not respond correctly to x-ray exposure due to manufacturing defects, aging or damage

• Effects: Can appear as bright (hot) or dark (dead) spots in the image

• Correction: These pixels are identified through calibration and their values are corrected using interpolation techniques

Interpolate Adjacent Pixel Signals

• A method used to estimate and replace missing or defective pixel values based on the values of surrounding (adjacent) pixels

• How it works: If a pixel is defective, its intensity is calculated by averaging the signals of nearby functional pixels, ensuring a smooth transition without abrupt artifacts

• Application: Used in bad pixel correction to maintain image quality

Image Lag

• The residual signal or “ghost image” that appears in a subsequent X-ray exposure due to incomplete charge transfer in digital detectors

• Causes: Long exposure times; High detector sensitivity; Incomplete charge clearing from previous exposures

• Effects: Can create ghost images, reducing image contrast and causing diagnostic errors

• Correction: Proper detector calibration, adequate time between exposures, and optimizing detector readout settings

Offset Correction

• A process used to remove background electronic noise from the image by subtracting a “Dark image” (captured without exposure)

• Purpose: Ensures that the detector’s inherent noise does not affect the final x-ray image

• How it works: A baseline image is taken with no x-ray exposure. This baseline noise pattern is subtracted from all future images

• Benefit: Enhances image clarity by eliminating unwanted noise

Line Noise

• Unwanted horizontal or vertical streaks in the image caused by interference in the detector’s electronic readout system

• Causes: Electrical interference; Variations in detector readout circuits; malfunctioning detector rows/columns

• Correction: Applying software based noise reduction algorithms; using shielded cables and proper grounding to minimize electrical interference; Replacing faulty detector components if needed

Post-Processing

• This is the strength of the digital imaging systems

• Refers to anything that can be done to a digital image after it is acquired

• Requires action by the technologist

• Allows for a complete evaluation of the data

Annotation

Label the image

Window and Level

Expand the digital grayscale

Magnification

Improves visualization and spatial resolution

Image Flip

Reorient the image

Image Inversion

Reverse blacks and whites

Subtraction

Improve image contrast by removing certain information

Pixel Shift

Reregister an image to correct for patient motion

Region of Interest

Determines average pixel value to display

Window Level and Window Width

• User is able to manipulate quickly through use of the mouse

• Movement of the mouse in one direction (vertical or horizontal) controls brightness and the other direction controls contrast

Window Level

• Brightness

• Controls how light or dark the image is

• A higher window level darkens the image, while a lower brightens it

Window Width

• Contrast

• Controls the ratio of black to white or contrast

• A narrow window width increases contrast (useful for bone imaging) while a wider width decreases contrast (useful for soft tissue evaluation)

Background Removal or Shutter

• A post-processing technique used unnecessary areas from an image

  • This helps to reduce glare, improve contrast, and enhance visualization of the area of interest

  • Unexposed border around the collimation edges allow excess light to enter the eye

Veil Glare

• Causes over sensitization of a chemical within the eye called rhodopsin

  • This results in temporary white light blindness

• Eye recovers quickly enough so that viewer recognizes only that the light is very bright

  • Veil glare is a great distraction that interferes with image reception by the eye

Shutter

• In digital imaging, automatic shuttering is used to blacken out the white collimation borders

  • Shuttering is a viewing technique only

  • Shuttering should never be used to mask poor collimation practices

  • Removal of the white unexposed border results in an overall smaller number of pixels

  • This reduces the amount of information to be stored

Image Orientation

• The way the anatomy is oriented on the imaging plate

  • The image reader must be informed of the part location

    • Head vs. feet

    • Left vs. right

  • The image is displayed exactly as read unless the reader is informed otherwise

• The cassette must be oriented so the image is processed to display as expected

  • Vendors mark the cassettes in different ways

    • Fuji uses a tape-type orientation marker

    • Kodax Carestream uses a sticker

• With FPD systems the position of the part should correspond with the marked top and sides of the detector

Flip and Rotate

• Used to orient the image in the correct anatomic hanging position

  • Tool is usually a left-to-right flip and 90 degree clockwise and counterclockwise icon

• Use of lead markers is important to ensure that the radiologist is reading the correct side

  • Digital markers may be a legal liability

Image Stitching

• Used for anatomy or areas of interest too large to fit on one cassette

  • Multiple images can be “Stitched” together

  • Sometimes special cassette holders are used and positioned vertically, corresponding to foot to hip or entire spine studies

  • Images are processed in computer programs that nearly seamlessly join the anatomy

  • Computer displays one single image

  • Process eliminated the need for large (36”) cassettes previously used in film/screen radiography

Image Annotation

• Information other than standard identification must be added to the image

• In screen/film radiography additional information is marked by the following

  • Time and date stickers

  • Grease pencils

  • Permanent markers

• Function

  • Allows selection of preset terms or manual text input

  • Can be useful when such additional information is necessary

  • Overlay the image as bitmap images

  • May not transfer to PACS

• Input of annotation for identification of the patient’s left or right side should never be used as a substitute for the technologist’s anatomy markers

Pan, Zoom, and Magnify

• Used primarily by the radiologist to increase the size of an area on the image

  • Magnify usually enlarges a square area of the image

    • The square can be moved around the image to quickly see various areas magnified

  • Pan and zoom function are usually used together

    • Image is first zoomed up to the desired magnification level

    • Pan icon is activated so that the zoomed image can be moved around to view the different areas of the image

Magnification

• Two basic types of magnification techniques are standard with digital systems

  • One technique functions as a magnifying glass:

    • A box is placed over a small segment of anatomy on the main image

    • The box shows a magnified version of the underlying anatomy

    • The size of the magnified areas and the amount of magnification can be made larger or smaller

  • The other technique is zoom

    • Zoom allows magnification of the entire image

    • Image can be enlarged enough that only parts of it are visible on the screen

    • Those parts can be seen through mouse navigation

Zoom

Adjusts the overall size of the image proportionally without changing resolution. Enlarges or reduces the image for better visualization while maintaining the entire image

Magnify

Increases the size of a specific region rather than the whole image. Focuses on fine details in a small area. Often used with a magnification box or tool

Pan

Moves the image across the screen without resizing it. Used when zoomed in to navigate different parts of the image. Helps examine large images without losing context

Measurements

• Most common are distance and angle

  • Distance measurement

    • Software uses pixel size to measure structures on the image

  • Angle measurement

    • Can give an angle measurement between two structures

    • Commonly used when reading spine studies

Region of Interest (ROI)

• Another common measurement tool used

• ROI determines the pixel intensity of a certain area

  • Different tissue or fluid has a different intensity reading

  • Radiologist uses this to determine whether something is solid or fluid

Image Management

• Patient demographic input

• Manual send

• Archive query

Patient Demographics Input

• Proper identification of the patient is even more critical with digital imaging

  • Retrieval can be nearly impossible if the image is not properly and accurately identified

• Demographic information about the patient includes

  • Name

  • Health care facility

  • Patient identification number

  • Date of birth

  • Exam date

  • Other pertinent information

• Information should be input or linked via a barcode label scans before the start of the exam and before the processing phase

• Function should be tracked to link the technologist with alterations for accuracy and accountability

• The patient’s name must be entered the same for each visit or exam

Problems with patient demographics

• Problems occur if name is entered differently

  • Suppose a patient gives a middle name or initial and previous exams have only his or her first name

    • The system will save each version of the name as a different patient

    • Merging of files can be difficult

    • Retrieval of previous files will be difficult

  • The right images must be placed in the correct data files

PACS

• Picture Archiving and Communication System

• Storage and transmitting images without “Hard copy”

• The projected efficiency increase from avoiding the hard copy is substantial

Principal Components of PACS System

• Image Acquisition

• Display System

• The Network

  • Teleradiology - The process of remote transmission and viewing the image

• Storage

  • Huge savings in terms of space and reliability of the records

• PACS is the electronic versions of the radiologist’s reading room and the file room

Electronic Programs

• Network

• Storage

• Teleradiology - Process of remote transmission and viewing of images; transferring digitized images for delivery at a distance to radiologists

RIS

• Radiology Information System

• Holds all radiology specific patient data

• Patient rad exam scheduling, to the radiologist’s dictated and transcribed report

EMR

• Electronic Medical Record

• An Electronic version of a patient’s medical history

HIS

• Hospital Information System

• Integrated with other hospital systems for information transfer and storage

Network

• DICOM

  • Digital Imaging and Communication in Medicine

  • Set of instructions (Standards) to manufactures that allows different modalities within radiology or medicine to communicate with each other

  • Standards that describe a file format

DICOM

• First version - 1985

• Universally accepted standard for exchanging medical images among modality, viewing stations, and the archive

• Groundwork for future integrated PACS development

Storage

• Determined by the number of images and the image data file size

  • A product of the the matrix size and bit depth

  • Answer in bytes or megabytes

  • How much computer capacity is required to store an image

  • Calculated by multiplying matrix size, dividing bit depth by 8 (convert bit to byte), multiply matrix x bytes, to convert bytes to megabytes divide by 1,000,000

Manual Send

• Because the quality control workstation is networked to the PACS, it also has the capability to send images to local network workstations

• The manual send function allows the quality control technologist to select one or more local computers to receive images

Archive Query

• PACS archive can be queried for historical images

• Function allows retrieval of images from the PAC system based on the following:

  • Date of exam

  • Patient name or number

  • Exam number

  • Pathologic condition

  • Anatomic area

• Ex: Technologist could query PACS to retrieve all chest radiographs for a particular date or range of dates. Technologist could query retrieval of all of a certain patient’s images

• Multiply combinations of query fields can generate reports from storage that include many categories of information or a few specific ones