DIGITAL IMAGING

DIGITAL IMAGING

TERMINOLOGY Film digitizer Digital radiography Digital fluoroscopy Dynamic range Matrix Pixel Imaging plate Histogram Algorithms Window level Window width TFT CCD Post-processing image enhancement Latitude Analog Digital Image acquisition Image processing Image display Raster pattern Laser Photomultiplier tube a-Selenium ADC

ANALOG VS. DIGITAL Discrete bits of information-digital Information along a continuum

ANALOG VS. DIGITAL

SIMILARITIES AND DIFFERENCES FILM-SCREEN IMAGING DIGITAL & CR IMAGING Need to select exposure factors Accurate positioning Use of accessory devices IR receives radiation after passing thru patient Latent image is produced and enhanced by the use of phosphorescence Latent image is chemically processed Limited dynamic range (30:1) No post processing possible Storage and retrieval issues Need to select exposure factors Accurate positioning Use of accessory devices IR receives radiation after passing thru patient Latent image is produced (CR) and enhanced by the use of phosphorescence Wide dynamic rage (1000:1) Post Processing Enhancement is possible Processing time reduced Storage and retrieval easier You should be able to state several differences between film/screen, computed radiography and digital radiography. You should be able to note advantages and disadvantages to each system.

COMPUTED RADIOGRAPHY Image acquisition Image processing Image display

IMAGING PLATE Protective layer: Thin clear plastic that protects the phosphor layer Phosphor layer: this active layer contains the photo-stimuable-phosphor (barium fluorohalide phosphors) that react to x-ray exposure Reflective layer: reflects light forward when the plate is in the reader Conductive layer: absorbs the electrons released during exposure and reduces static electricity Color layer: absorbs stimulating light but reflects emitted light Support layer: semi- rigid layer that provides support Barcode label: allows technologist to use patient/exam identifying information You should know all of the layers in a imaging plate and their function. Photostimulated luminescence (PSL) is the release of stored energy within a phosphor by stimulation with visible light, to produce a luminescent signal. Commonly this is used to allow the storage of a weak radioactive signal in a phosphor plate that takes the place of X-ray film, potentially over a prolonged period, which PSL transforms to obtain a highly sensitive image of the original pattern of radiation. Also known as an image plate, a photostimulable phosphor (PSP) plate can be used record a two-dimensional image of the intensity short-wavelength (typically, X-ray) electromagnetic radiation. The device to read such a plate is known as a phosphorimager (occasionally abbreviated to phosphoimager, perhaps reflecting its common application in molecular biology of detecting radiolabeled phosphorylated proteins and nucleic acids).- Wikipedia

PSP IMAGING PLATE(CR) After the initial exposure, excited electrons in the phosphor material remain 'trapped' in 'colour centres' in the crystal lattice until stimulated by the second illumination. For example, Fuji's photostimulable phosphor is deposited on a flexible polyester film support with grain size about 5 micrometers, and is described as "barium fluorobromide containing a trace amount of bivalent europium as a luminescence center". Europium is a divalent cation that replaces barium to create a solid solution. When Eu2+ ions are struck by ionizing radiation, they lose an additional electron to become Eu3+ ions. These electrons enter the conduction band of the crystal and become trapped in the bromine ion empty lattice of the crystal. This metastable state is higher in energy than the original condition, so a lower-frequency light source that is insufficient in energy to create more Eu3+ ions can return the trapped electrons to the conduction band. As these mobilized electrons encounter Eu3+ ions, they release a blue-violet 400 nm luminescence.[1] This light is produced in proportion to the number of trapped electrons, and thus in proportion to the original X-ray signal. It can be collected (often by a photomultiplier tube), enabling the resulting signal to be converted into a digital image. This process is also known as Photostimulable Luminescence (PSL). Unlike film, a PSP plate can be reused: plates can be "erased," by exposing the plate to room-intensity white light.

CR Phosphor Plates ABSORPTION EMISSION X-RAY LIGHT LASER STIMULATION ELECTRON TRAP ELECTRON TRAP X-RAY LIGHT Incomng xray reacts with the PSP plate…..the barium fluorohalide phosphors….and traps electrons. The number of electrons trapped is related to the amount of xray absorbed by the PSP. This creates the LATENT IMAGE

LATENT IMAGE (CR) Formed by x-ray interaction with PSP Ionizes phosphors, releasing electrons Electrons trapped in crystal lattice of phosphor Latent image is formed Remains until processed by a reader But does begin to decay so must be “read” in a timely fashion After the initial exposure, excited electrons in the phosphor material remain 'trapped' in 'colour centres' in the crystal lattice until stimulated by the second illumination. For example, Fuji's photostimulable phosphor is deposited on a flexible polyester film support with grain size about 5 micrometers, and is described as "barium fluorobromide containing a trace amount of bivalent europium as a luminescence center". Europium is a divalent cation that replaces barium to create a solid solution. When Eu2+ ions are struck by ionizing radiation, they lose an additional electron to become Eu3+ ions. These electrons enter the conduction band of the crystal and become trapped in the bromine ion empty lattice of the crystal. This metastable state is higher in energy than the original condition, so a lower-frequency light source that is insufficient in energy to create more Eu3+ ions can return the trapped electrons to the conduction band. As these mobilized electrons encounter Eu3+ ions, they release a blue-violet 400 nm luminescence.[1] This light is produced in proportion to the number of trapped electrons, and thus in proportion to the original X-ray signal. It can be collected (often by a photomultiplier tube), enabling the resulting signal to be converted into a digital image. This process is also known as Photostimulable Luminescence (PSL). Unlike film, a PSP plate can be reused: plates can be "erased," by exposing the plate to room-intensity white light.

IMAGE PROCESSING

IMAGE PROCESSING PSP plate exposed to radiation Electrons are trapped in phosphor layer Plate is exposed to a red laser light As electrons are released, a blue light is emitted Blue light is captured and recorded by PMT Image is sent to monitor for display PSP plate is exposed to intense white light for erasure

IMAGE PROCESSING Photomultiplier tube- do not memorize this diagram, only here as added information. Know the function of the PMT.

IMAGE DISPLAY

An increasingly common method is the use of photostimulated luminescence (PSL), pioneered by Fuji in the 1980s. In modern hospitals a photostimulable phosphor plate (PSP plate) is used in place of the photographic plate. After the plate is X-rayed, excited electrons in the phosphor material remain 'trapped' in 'colour centres' in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube and the resulting signal is converted into a digital image by computer technology, which gives this process its common name, computed radiography (also referred to asdigital radiography). The PSP plate can be reused, and existing X-ray equipment requires no modification to use them.

Computed Radiography The CR system is not much faster than film screen cassettes. It is really not faster it is just different. We can manipulate the image and it is better quality images.

CR Imaging-Image Acquisition summary

DIRECT DIGITAL IMAGING Cassette-less imaging Uses TFT-Thin Film Transistor No film, so no developing, No PSP, so CR reader needed Immediate image viewing Post Processing capabilities Multiple viewing stations

TYPES OF DIRECT DIGITAL IMAGING Two types of direct digital imaging…..direct conversion and indirect conversion. Either way, no latent image

DIRECT DIGITAL IMAGING Matrix can be considered as a chart with columns and rows. Each cell can have various values which relate to the amount of xray absorbed in that cell. In turn, this value is convert to a gray scale value and converted into an image. Each cell is termed a pixel….or a picture element.

DIRECT DIGITAL IMAGING Flat panel detector consists of plate covered with amorphous selenium (a-Selenium). This material absorbs x-rays and converts them to electrons. These electrons are stored in the TFT This is a one step process. X-rays interact with the phosphor screen (a-Selenium) and release electrons. The TFT capture the electrons and convert into digital signal. Image is displayed on monitor.

DIRECT DIGITAL IMAGING Phosphor based scintillator is commonly Cesium Iodide- CsI In simple terms, a CCD acts similar to a camera that transfers light signals into electrical charges. The charges are stored and sent to a ADC (analog to digital converter) Charged-Coupled Device From Wikipedia, the free encyclopedia A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins. The CCD is a major piece of technology in digital imaging. In a CCD image sensor, pixels are represented by p-doped MOS capacitors. These capacitors are biased above the threshold for inversion when image acquisition begins, allowing the conversion of incoming photons into electron charges at the semiconductor-oxide interface; the CCD is then used to read out these charges. Although CCDs are not the only technology to allow for light detection, CCD image sensors are widely used in professional, medical, and scientific applications where high-quality image data is required. In applications where a somewhat lower quality can be tolerated, such as webcams, cheaper active pixel sensors (CMOS) are generally used.

TECHNIQUE CONSIDERATIONS kVp Dependent, need mAs to saturate optimally Now COMPUTER controls CONTRAST Higher kVp to stimulate electron traps

80 kVp 200mAs 10 mAs 80 kVp Note Quantum Mottle Latitude refers to the amount of error that can be made and still result in the capture of a quality image. Algorithms are mathematical formulas applied to the raw data to enhance edges, manipulate contrast.

Histograms are used to plot density of data, and often for density estimation: estimating the probability density function of the underlying variable. The total area of a histogram used for probability density is always normalized to 1. If the length of the intervals on the x-axis are all 1, then a histogram is identical to a relative frequency plot. For x-ray purposes, a histogram tells how often a certain degree of gray is seen in the image. Example of histogram for photography Exposure indicators will help determine whether an image has been over, under or optimally exposed. This varies from each manufacturer. The number is called the exposure index.

Algorithms are mathematical formulas applied to the raw data to enhance edges, manipulate contrast.

To Produce Quality Images For Film/Screen Radiography or Digital/CR Radiography: The same rules, theories, and laws still apply and can not be overlooked SID, Inverse Square Law, Beam Alignment, Tube-Part-Film Alignment, Collimation, Grid, Exposure Factors: kVp, mAs, Patient Positioning PATIENT POSITIONING Accounts for 85% of the total number of repeat exposures. Has a direct affect on exposure technique.

Patient positioning Quality Images Accounts for 85% of the total number of repeat exposures. Has a direct affect on exposure technique.

COLLIMATION CRITICAL As the computer reads the density value of each pixel- it is averaged into the total Close collimation= Better contrast Bad collimation= more grays and less detail

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