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Digital Imaging Review

Digital Imaging Review. RT 255. Display Workstations. Conventional film/screen radiography uses large multiviewer lightboxes. With early PACS, radiologists thought that they needed 4-6 monitors. Now, the number of monitors has dropped to an average of 2.

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Digital Imaging Review

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  1. Digital Imaging Review RT 255

  2. Display Workstations • Conventional film/screen radiography uses large multiviewer lightboxes. • With early PACS, radiologists thought that they needed 4-6 monitors. • Now, the number of monitors has dropped to an average of 2. • Development of viewing software and better hardware.

  3. The CRT is the most popular monitor on the market. It consists of a cathode and anode within a vacuum tube. Cathode boils off a cloud of electrons, and then a potential difference is placed on the tube. A stream of electrons is sent across to the anode, which in the case of the monitor is a sheet of glass coated with a phosphor layer. CRT

  4. MONITORS:Display Workstations • Early PACS reading rooms required supplemental air-conditioning to offset the heat from multiple CRTs. • Resolution and orientation of the monitor is also a factor in determining which type of monitor is to be used. • Most cross-sectional imaging is read on a 1K square monitor. • Most computed radiography (CR) and digital radiography (DR) images are read on at least a 2K portrait monitor

  5. 1.3 megapixels to 5 megapixels. mammography imaging = at least 5 megapixel resolution is required.   LCD

  6. Rads VIEWER • 4K or higher

  7. Physician Review Stations Monitors • Step-down model of the radiologist’s reading station (lower res) • Some functions reduced • One of the most important features • is ability to view current and previous reports with images. • Many vendors are integrating • the RIS functions • with PACS software.

  8. Technologist QC Stations • review images after acquisition but before sending them to the radiologist • May be used to improve or adjust image-quality characteristics • May be used to verify patient demographic information • Placed between the CR and DR acquisition modalities as a pass-through to ensure that the images have met the departmental quality standard

  9. Technologist QC Station • Generally has a 1K monitor • Does not have the resolution capabilities of the radiologist’s reading station • Care required of technologist when manipulating images not to change the appearance too much from original acquired image

  10. Technologist workstation monitors are used in brightly lighted areas. So monitor luminance, the brightness of a monitor display, is an important consideration. Also, the monitor must allow a technologist to visualize enough detail to discern motion and that the recorded lines are sharp and visible.

  11. The File Room Workstation • Workstation may be used to • look up exams for a physician • or to print copies of images • for the patient to take to an • outside physician. • Many hospitals are moving away from printing films because of the cost. • Hospitals are moving toward burning compact disks (CDs) with the patient’s images.

  12. Name the 3 types of monitors • Two major types of monitors with a third type gaining acceptance: • CRT • LCD • Plasma screen

  13. ARRT DEFINITIONS Image Display= MONITORS • viewing conditions • (i.e.,luminance,ambient lighting) • spatial resolution • contrast resolution/dynamic range • DICOM gray scale function • window level and width function

  14. How does this affect viewing images? Surrounding light impacted what was seen on image – now : With different monitors: LCD gives more light. LCD can be used in areas with a high amount of ambient light. “dark rooms” not necessary viewing conditions luminance,ambient lighting

  15. WINDOW LEVEL / WIDTHWhich one controlsDenisty (brightness) ?Contrast What else control these in DIGITAL IMAGING?

  16. The ability to window is a valuable feature of all digital images. Windowing is the process of selecting some segment of the total pixel value range (the wide dynamic range of the receptors) and then displaying the pixel values within that segment over the full brightness (shades of gray) range from white to black. Important point...Contrast will be visible only for the pixel values that are within the selected window.  All pixel values that are either below or above the window will be all white or all black and display no contrast. The person controlling the display can adjust both the center and the width of the window.  The combination of these two parameters determine the range of pixel values that will be displayed with contrast in the image.

  17. Enhanced Visualization Image Processing • Takes image diagnostic quality to a new level • Increases latitude while preserving contrast • Process decreases windowing and leveling • Virtually eliminates detail loss in dense tissues

  18. Nyquist frequency Review • The highest spatial frequency that can be recorded by a digital detector. • is determined by the ________________ • The Nyquist frequency is half the number of pixels/mm.

  19. 10 PIXEL DENSITY WHAT IS THE NYQUIST FREQUENCY= ?

  20. A digital system with a pixel density of 10 pixels/mm would have a Nyquist frequency of 5 line pair/mm. 10 PIXEL DENSITY WHAT IS THE NYQUIST FREQUENCY= ?

  21. Sampling Frequency ? Define ……

  22. Sampling Frequency • The sampling frequency is the rate at • which the laser extracts the image data • from the plate. • What other term does this relate to?

  23. Nyquist Frequency • The Nyquist Frequency will be ½ of the sampling frequency. • A plate that is scanned using a sampling frequency of 10 pixels per millimeter would not be able to demonstrate more than 5 line pairs per millimeter based upon the Nyquist Frequency. • The Nyquist Frequency allows the • determination of the spatial resolution for • a given sampling frequency.

  24. In the typical digital radiographic system, a variety of LUTs are installed.  The appropriate LUT is then automatically selected to give the desired contrast characteristics to match the type of procedure (chest, extremity, etc) that is designated by the operator. LUT a histogram of the luminance values derived during image acquisition used as a reference to evaluate the raw information and correct the luminance values. a mapping function in which all pixels are changed to a new gray value.

  25. LOOK UP TABLE (LUT) Black Saturation White Saturation Linear LUT Black Shirt Facial Tones * No Detail in Black Areas * High Contrast * Only Detail in White Areas can be seen * No Detail in White Areas * Low Contrast * Only Detail in Black Areas can be seen

  26. ARRT definitions Image Receptors digital image characteristics • spatial resolution • sampling frequency • DEL (detector element size) • receptor size and matrix size • image signal (exposure related) • quantum mottle • SNR (signal to noise ratio) or • CNR (contrast to noise ratio)

  27. image signal (exposure related)Exposure Indicators • The amount of light given off by the imaging plate is a result of the radiation exposure that the plate has received. • The light is converted into a signal that is used to calculate the exposure indicator number, which is a different number from one vendor to another.

  28. Digital artifacts • Grid Lines: Appear as grid cutoff. • Moire (Aliasing) • wavy artifact occurs because the grid lines and the scanning laser are parallel. • When the spatial frequency is greater than the Nyquist frequency • Maintenance (e.g., detector fog): When errors occur in equipment performance, corrective action must occur. These corrections will generally be done by service personnel employed by the vendor. • non-uniformity, erasure - blub problems

  29. How else can Morie OCCUR?

  30. ARRT definitions quantum mottle failure of an imaging system to record densities usually caused by a lack of x-ray photons. “PHOTON STARVED” KVP & MAS HOW IS THIS AFFECTED IN DR / CR?

  31. PREPROCESSING ARTIFACTS • dead detector elements • dead columns or rows • nonuniform response • heel effect (fixed systems) • light guide variations • ghosting

  32. ARRT definitions Image Receptors digital image characteristics • spatial resolution • sampling frequency • DEL (detector element size) • receptor size and matrix size • image signal (exposure related) • quantum mottle • SNR (signal to noise ratio) or • CNR (contrast to noise ratio)

  33. Image Acquisition and Readout • PSP (photo-stimulable phosphor) • flat panel detectors • (direct and indirect)

  34. CR Imaging Plate • Construction • Image recorded on a thin sheet of plastic known as the imaging plate - PSP • Consists of several layers: • Phosphor?

  35. Imaging Plate • Phosphor? • BARIUM FLUORO • HALIDE WITH • A EUROPIUM BASE

  36. Digital Radiography • Two types of digital radiography • Indirect capture DR • Machine absorbs x-rays and converts them to light. • CCD or thin-film transistor (TFT) converts light to electric signals. • Computer processes electric signals. • Images are viewed on computer monitor.

  37. Digital Radiography • Direct capture DR • Photoconductor absorbs x-rays. • TFT collects signal. • Electrical signal is sent to computer for processing. • Image is viewed on computer screen.

  38. Image Acquisition and Readout flat panel detectors Phosphors? • direct = Am SELENIUM • indirect = Am SILICON

  39. CR PSL light guide PMT/PD • Indirect CsI/GOS contact layer PD/TFT array • Direct a-Se none TFT array • CCD/CMOS Cs/GOS lens/fiber CCD/CMOS

  40. CMOS ?

  41. CMOS ? • Complimentary metal-oxide semi-conductor (CMOS) – A photographic detector.

  42. CCD and CMOS systems – both use a scintillator. These systems are camera-like, they both use lenses to focus the light onto a detector.

  43. Digital Systems • electronic collimation • grayscale rendition or look-up table (LUT) • edge enhancement/ • noise suppression • contrast enhancement • system malfunctions (e.g., ghost image, banding, erasure, dead pixels, readout problems, printer distortion)

  44. Image Data Recognitionand Preprocessing shutter • Agfa uses the term collimation, Kodak uses the term segmentation, and Fuji uses the phrase “exposure data recognition.” • All systems use a region of interest to define the area where the part to be examined is recognized and the exposure outside the region of interest is subtracted.

  45. Excessive  exposure to receptor without rescaling. Excessive  exposure to receptor with rescaling. • So in essence, rescaling provided an acceptable image, despite an excess level of exposure to the receptor. • What about the dose to the patient?

  46. Rescaling • image pixel values to appear appropriate, display properly, can lead to overexposing a patient. • The visual cue to the technologist that overexposure has occurred isn’t present. • With an analog system, a technologist would have seen the image on the left as it came out of the processor and used the excessive density of the image as a visual cue to repeat the image. • Rescaling forces a technologist to look elsewhere for signs that a proper exposure was used to produce an image.

  47. refers to the potential to gradually increase patient exposure over time. However, a technologist lacks visual feedback that additional radiation is being used to produce the images Dose creep

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