Acquisition, Processing and Display

Transcription

1 Acquisition, Processing and Display Terri L. Fauber, R.T. (R)(M) Department of Radiation Sciences School of Allied Health Professions Virginia Commonwealth University

2 Topics Image Characteristics Image Receptors Image Acquisition Digital Image Processing & Display Exposure Technique Factors

3 Image Production Primary beam attenuated by tissues. Exit or remnant radiation composed of varying intensities. Image receptor receives or captures the remnant radiation and Creates a latent or invisible image that needs processing.

4 Image Formation Image Formation for digital imaging differs from filmscreen. Image receptors respond differently to the remnant radiation. Digital image stored and displayed as computer data visible on a monitor as a range of brightness levels Film image is processed to display a range of densities on a polyester sheet

5 Image Receptors Types Film Film-screen Digital Computed radiography (CR) Direct radiography (DR)

6 Digital vs. Film- Screen Radiography Film-Screen Limitations Superimposition of anatomical structures Difficult to visualize widely varying types of tissue Poor soft tissue differentiation Delay in viewing image No changes in image following processing Lack of quantitative analysis of attenuation characteristics Storage and retrieval costs Digital Advantages Post-processing can alter the image to remove superimposing anatomical structures Allows visualization of widely varying types of tissue Excellent soft tissue differentiation Minimal delay in viewing image Allows manipulation of image postprocessing Provides quantitative data analysis of tissue attenuation characteristics No hard copy storage and retrieval costs

7 Image Characteristics Visualize the area of interest Brightness/Density Differentiate among the anatomic details Contrast Resolution/Contrast Maximize sharpness Spatial Resolution/Recorded Detail Minimize distortion Minimize Quantum Noise

8 Density/Brightness The range of image densities is created by the variation in x-ray absorption and transmission as the x-ray beam passes through anatomic tissues. Low density = High brightness High density = Low brightness

9 Digital Image Characteristics A digital image is recorded as a combination of rows and columns known as matrix The smallest component of the matrix is the pixel (picture element); measured in microns (0.001 mm); and recorded as a single numerical value The location of the pixel within the image matrix corresponds to an area within the patient or volume of tissue referred to as voxel

10 Matrix Size For a given field of view (FOV), a larger matrix size includes a greater number of smaller pixels. A matrix size of 1024 x 1024 has 1,048,576 individual pixels; a matrix size of 2048 x 2048 has 4,194,304 pixels.

11 Image Characteristics Each pixel is assigned a numeric value (quantization) that represents a shade of gray or brightness level based on the attenuation characteristics of the volume of tissue imaged When displayed on a computer monitor, high density is low brightness and low density is high brightness

12 Pixel Bit Depth The number of bits determines the number of shades of gray the digital system is capable of displaying (2 n ). 12- and 14- bit pixel can display 4096 and shades of gray, respectively. Increasing pixel bit depth improves image quality

13 Signal-to-Noise Ratio (SNR) SNR is a method of describing the strength of the radiation exposure compared with the amount of noise apparent in a digital image. Because the photon intensities are converted to an electronic signal that is digitized, the term signal refers to the strength or amount of radiation exposure captured by the IR to create the image SNR Image Quality

14 Noise Quantum Noise- low photon energy used to record the image. Electronic or system noise- random effect that degrades the quality of the image. A result of the use of electronic components in digital imaging. When the digital image displays increased noise, regardless of the source, anatomic details have decreased visibility.

15 Digital Imaging Process Unlike film-screen, where acquisition, processing and display are all in one (film), digital imaging separates these three processes: Image Acquisition Image Processing Image Display

16 Digital Radiography Computed Radiography (CR) X-ray photons exiting pt. Photostimulable phosphor Reader (laser) Digital data Memory storage Display monitor Laser printer Direct Radiography (DR) Memory storage X-ray photons exiting pt. Digital data Flat panel detector Display monitor Laser printer

17 Digital Image Receptors (DR) Flat Panel Detectors Exit radiation absorbed by phosphor Laser extracts stored energy as light Light convert to electric signal Electric signal digitized Exit radiation converted to light then electrons or directly to electrons Electric signal digitized CR and DR differ in the method of image acquisition Seibert, AJ. Digital Radiography: Image Quality and Radiation Dose. Health Physics, 2008;95(5):

18 Cross Section of PSP screen The CR IR includes a cassette that houses the imaging plate (IP). The radiation exiting the patient interacts with the IP, where the photon intensities are absorbed by the phosphor. The phosphor layer is composed of barium fluorohalide crystals doped with europium, referred to as the photostimulable phosphor (PSP). This type of phosphor emits visible light when stimulated by a high-intensity laser beam, called photostimulable luminescence.

19 PSP Latent Image Formation CR emits light first during x- ray exposure Carroll, Q Practical Radiographic Imaging, 8 th ed, 2007 The Latent image is formed when some of the freed electrons become trapped Following laser light stimulation, the trapped electrons return to normal state and release visible light.

20 CR Image Acquisition Exit radiation interacts with imaging plate composed of barium fluoride bromide crystals coated with europium; Absorbed energy stored in photostimulable phosphor material. Some energy is released as visible light, but most results in electrons released in phosphor layer due to photoelectric interactions Electrons are trapped in phosphor layer until light energy is released during laser scanning (electrons are trapped in an excited state)

21 CR Latent Image Digitization Three stages Scanning Sampling Quantization

22 CR Image Acquisition: Scanning A continuous pattern of light intensities are sent to the Photomultiplier Tube (PMT) The PMT collects, amplifies, and converts the visible light to an electrical signal proportional to the range of energies stored in the IP Electrical signal is directed to the ADC for sampling and quantization

23 CR Image Acquisition: Sampling Analog to Digital Conversion accomplished by sampling Sampling Frequency for CR Image Receptors Increasing the sampling frequency will increase the spatial resolution

24 Sampling Frequency Nyquist Theorem: you must sample for each pixel at least 2 times to achieve the desired level of spatial resolution. Increasing the sampling frequency will more accurately reflect the analog signal, increase spatial resolution, and improve the quality of the digital image.

25 Sampling & Spatial Resolution In computed radiography (CR) some manufacturers fix the sampling frequency to maintain a fixed spatial resolution while others vary the sampling frequency to maintain a fixed matrix size. If the spatial resolution is fixed, the image matrix size is simply proportional to the imaging plate (IP) size. A larger IP will have a larger matrix to maintain spatial resolution If, on the other hand, the matrix size is fixed, changing the size of the IP will affect the spatial resolution of the digital image.

26 Fixed Sampling Frequency In CR, increasing the IP size for a fixed sampling frequency will maintain the pixel size for a fixed spatial resolution (larger matrix).

27 Fixed Matrix Size In CR, increasing the IP size for a fixed matrix size will increase the pixel size and decrease spatial resolution.

28 Image Acquisition: Quantization During quantization, each pixel, representing a brightness value, is assigned a numerical value that reflects the precision with which each sampled point is recorded. The pixel bit depth determines the system s ability to display a range of shades of gray to represent anatomic tissues. Pixel bit depth (2 n ) is fixed by the choice of ADC and digital systems manufactured with greater bit depth can display more shades of gray which improves the contrast resolution of the digital image.

29 Direct Radiography Flat Panel Detectors (FPD) are solid-state IRs employing a large area active matrix (array) of electronic components ranging in size from 17 x 14 inches to 17 x 17 inches. FPDs are constructed with layers in order to receive the x-ray photons and convert them to electrical charges for storage and readout. Signal storage, signal readout, and digitizing electronics are integrated into the FPD.

30 Flat Panel Detector- Mobile

31 DR Image Acquisition Indirect conversion Flat panel detector array exposed to exit radiation Scintillator converts x-rays to light Light energy converted to electric signals Electric signals digitized Direct conversion Flat panel detector array exposed to exit radiation Selenium converts x-rays to electric signal Electric signals digitized

32 Direct Radiography Image Receptors Indirect Conversion Direct Conversion

33 Flat Panel Detector Systems Indirect Conversion Scintillator Direct Conversion Electronic charge Visible light Electronic charge Digital Image Digital Image

34 Detector Element Size (DEL) Because the pixel detector is built into the DR flat panel type of image receptor, the size and pitch of the pixel is determined by the DEL and fixed. The spatial resolution is limited to the DEL. A system that uses a smaller DEL size will have improved spatial resolution.

35 Sampling Pitch/ Pixel Pitch The distance between the midpoint of adjacent pixels (pixel pitch) will also affect the spatial resolution of the digital image. Decreasing the pixel or sampling pitch will increase spatial resolution and image quality.

36 Film-Screen Image Receptors Prior to digital radiography, film was used to acquire, process, and display the radiographic image. An emulsion layer is adhered to a polyester sheet and serves as the radiation-sensitive and light sensitive layer of the film. Before the development of intensifying screens, film was exposed to the primary beam and then hand processed to produce the desired image. In an effort to reduce patient exposure, film is placed in a cassette that houses two intensifying screens. The film is exposed to the light emitted from the screen in proportion to the amount of radiation exposure.

37 Latent Image Formation-Film Crystals in the emulsion absorb radiant energy Gurney Mott Theory Sensitivity speck entraps free electrons Silver ions collect at speck Chemical processing converts exposed crystals to black metallic silver and viewed as densities

38 Dynamic Range The range of exposure intensities an image receptor can accurately detect. A- Film-screen receptors of narrow dynamic range (latitude) B- Digital image receptors have wide dynamic range (latitude)

39 Dynamic Range Wide Dynamic Range Digital image receptors can detect a greater range of x-ray intensities than film. Dynamic Range Low and high x-ray intensities exiting the patient can be detected and displayed.

40 Film-Screen Image Receptor Exposure Optimum Exposure Insufficient Exposure Excessive Exposure

41 A 14 pixel bit depth system has 16,384 shades of gray or brightness levels available to display the wide range of radiation exposures the image receptor captures.

42 Digital Image Receptor Exposure Insufficient Exposure Optimum Exposure Excessive Exposure

43 Digital Image Processing-CR/DR Following acquisition and digitization of the latent image, various computer manipulations are applied to the digital data for the purpose of optimizing the appearance of the image. Histogram Analysis Values of interest Automatic rescaling Exposure indicator Lookup Tables

44 Processing: Histogram Analysis Graphic representation of a data set or all the pixel values within the exposed image. The histogram represents the number of digital pixel values versus the relative prevalence of the pixel values in the image. X-axis: amount of exposure Y-axis: the incidence of pixels for each exposure level The computer software has stored histogram models, each having a shape characteristic of the selected anatomic region and projection. These stored histogram models have values of interest (VOI), which determine what range of the histogram data set should be included in the displayed image.

45 Processing: Histogram Analysis A histogram is the graphic display of the distribution of pixel values. Each image has its own histogram and is compared to the preestablished histograms for each anatomic projection

46 Processing: Histogram Analysis Histogram analysis is also employed to maintain consistent image brightness despite overexposure or underexposure of the IR, known as automatic rescaling The computer rescales the image based on the comparison of the histogram, which is actually a process of mapping the grayscale to the VOI to present a specific display of brightness

47 Processing: Exposure Indicator During digital processing and as a result of the histogram analysis a numerical value is displayed to indicate the level of x-ray exposure received to the image receptor. Computed Radiography (CR) Exposure Indicators are vendor specific and not currently standardized. Vendor Exposure Indicator Value = 1 mr exposure 2 x Exposure 1/2 Exposure Fuji and Konica Sensitivity (S) Carestream (Kodak) Exposure Index (EI) Agfa Log Median Value (lgm)

48 Processing: Automatic Rescaling

49 Standard Exposure Indicator Exposure Index (EI) an index of the exposure at the detector in the relevant image region Target Exposure Index (EI T ) the target reference obtained when the image receptor is exposed properly Deviation Index (DI) measures how far the actual EI value deviates from the projection-specific EI T DI provides immediate feedback to the radiographer regarding proper exposure Important to note that these values do not provide information about patient exposure and dose.

50 Processing: Look Up Tables (LUTs) Following the histogram analysis, LUTs are a method of altering the image to change the display of the digital image in a variety of ways. Because digital IRs have a linear exposure response and a very large dynamic range, raw data images exhibit low contrast and must be altered to improve visibility of anatomic structures. Image brightness and grayscale can be manipulated to alter how the area of interest is visualized and thus improving the quality of the image.

51 Dynamic Range The range of exposure intensities an image receptor can accurately detect or capture. A- Film-screen receptors of narrow dynamic range (latitude) B- Digital image receptors have wide dynamic range (latitude)

52 Processing: Look Up Tables Graphic display comparing original pixel values and pixel values of a processed image. Straight line indicates the raw data has not been altered. A 14 pixel bit depth system has 16,384 shades of gray or brightness levels available to display the wide range of radiation exposures the image receptor captures.

53 Processing: Look Up Tables Altering the raw data from the original image and comparing the resultant graph resembles a characteristic curve.

54 Processing: Look Up Tables New pixel values applied to the original pixel values can change the contrast of the displayed image.

55 Image Display Soft copy viewing refers to the display of the digital image at a computer workstation. The quality of the digital image is also affected by important features of the display monitor. Primary display monitors used by radiologists for diagnostic interpretation must be of higher quality than those used for routine image review (secondary). Display monitors used for diagnostic interpretation are typically monochrome high-resolution monitors and can be formatted as portrait or landscape and configured with one, two, or four monitors.

56 Image Display Display Monitors Cathode Ray tube (CRT) Liquid Crystal Display (LCD) Viewing conditions Luminance- amount of light emitted from the monitor Ambient lighting- room illumination, natural light, fluorescent, High Resolution 5-megapixel (2,048 x 2,560 pixels) recommended Post-Processing Capabilities Electronic Collimation Window Width Window Level Edge Enhancement

57 Viewing Conditions Placement of display monitors Positioning away from any direct light source will reduce the amount of reflection on the faceplate of the monitor. Ambient lighting: level of light in room Maintaining a low level of ambient lighting can help to enhance the viewer s perception of the image brightness and contrast displayed on the monitor CRTs reflect more ambient lighting due to thicker faceplate

58 Luminance A measurement of the light intensity emitted from the surface of the monitor. The amount of brightness emitted from the display monitor. Luminance affects the quality of the displayed image. Measured in units of candela per square meter (cd/m 2 ).

59 Luminance A ratio of the maximum to minimum luminance is evaluated as a part of display monitor quality control. A digital system capable of displaying 16,384 shades of gray (14-bit) requires a monitor capable of displaying a great grayscale range. Monitors that have a higher luminance ratio are capable of displaying a greater grayscale range.

60 Image Display: Post Processing Electronic Masking Once the image is processed, regions viewed on the image can be altered, also known as shuttering or electronic collimation Remove the increased brightness surrounding the radiation exposed field Remove regions within the radiation exposed field that provide no useful information Electronic masking has no effect on overall image quality or patient exposure

61 Image Display: Brightness Changing the window level will change the image brightness viewed in the area of interest

62 Image Display: Contrast A narrow window width (decreased) will increase image contrast A wide window width will decrease image contrast. An image with increased contrast resolution, when viewed optimally, increases the visibility of very subtle anatomic features.

63 Image Display: Image Visibility The center or midpoint of the window level and the width of the window will demonstrate density and contrast of the displayed image.

64 Contrast Enhancement Post Processing Adjustment

65 Post Processing A major advantage of digital imaging is the ability to alter image quality after the image is processed. In addition to altering the density and contrast, a variety of image enhancements can be performed such as edge enhancement smoothing

66 Exposure Technique Digital imaging systems are capable of producing optimum brightness regardless of exposure errors Radiographers are responsible for selecting exposure techniques that maintain patient exposure As Low AS Reasonable Achievable (ALARA) Exposure Dose Creep

67 Exposure Techniques mas & kvp Both affect exposure to the image receptor. Higher kvp can be used with lower mas to maintain adequate exposure. Scatter control Digital image receptors are sensitive to scatter radiation. The use of grids and appropriate collimation are important. Speed class Digital image receptors can respond to low and high exposures due to their dynamic range and the computer can rescale the brightness of the image.

68 Exposure Techniques Judiciously evaluate APR exposure techniques. Select exposure techniques that reduce patient exposure while maintaining diagnostic image quality. Higher kvp /lower mas; Manually collimate; Shield Monitor exposure factors and image quality for each patient. Use AEC consistently and accurately. Develop and use exposure technique charts.

69 Never underestimate the role of the radiographer in reducing patient radiation exposures. As the complexity of equipment increases, radiographers must be properly educated and trained. It is very easy to relinquish control to the automation of the equipment.