Fundamentals of Digital Mammography Imaging
Fundamentals of Digital Mammography Imaging
Discuss terminology associated with digital imaging. Differentiate between the response of screen-film imaging systems and digital imaging systems. Discuss the fundamentals of digital imaging characteristics. Describe the composition and mechanism by which an amorphous selenium flat panel detector works. State why higher kVp levels are possible with digital imaging. Describe the various pre- and post image processing algorithms. Fundamentals of Digital Mammography Lauren Noble, Ed.D., RT(R) University of North Carolina at Chapel Hill School of Medicine Division of Radiologic Science Digital Systems Overview Cassette-based (CR) Cassette-less (DR) Workflow similar to screen-film daylight processing Uses PSP plates ˜New workflow paradigm Flat panel detectors CCD Comparison of Screen-Film Imaging to Digital Imaging Radiographic Process Screen-Film Response Digital Response Why Digital? Screen-Film Digital Image Acquisition Screen-film Technical factors Cassette-based Cassette-less Technical factors Image Processing Processor Chemicals Darkroom Daylight ˜Computer / electronics Image Display Hard Copy Soft Copy Hard Copy Screen-film imaging Image acquisition and image display are coupled No manipulation of the final image Hard copy image Digital imaging Image acquisition and image display are decoupled Manipulation of the final image Soft copy E – Under exposure Image is “clear” Eo Optimal exposure Region which produces quality image at appropriate patient dose E+ Over exposure Region where image is dark Hot light ? Patient over exposed E++ Gross over exposure Image is totally black Patient grossly over exposed Limited dynamic range Much of the anatomical data is beyond the acquisition range of film. ˜ Display is limited to captured data E – Under exposure Region where signal is too low Image exhibits excessive noise Eo Optimal exposure Region which produces quality image at appropriate patient dose E+ Over exposure Region where image quality is good, but patient receives higher dose than necessary E++ Gross over exposure Region where contrast degraded by extra scatter generated Patient grossly over exposed E+++ ˜Saturation Region where sensor unable to respond All pixels at maximum value Patient grossly over exposed → Wide Dynamic Range Digital systems capture more data due to larger dynamic range ˜ ˜Raw data has very low contrast Processing algorithm applied Includes LUT (Look-Up Table) LUT applied ˜Displays an image with specific contrast Dynamic range The acquisition or capture range of a detector Exposure latitude The exposure that yields a quality image while keeping patient exposure to a minimum It is important to note that just because a digital imaging system has the ability to produce an image from gross underexposure or gross overexposure, it does not equate to a greater exposure latitude The reason the system is capable of producing an image when significant exposure errors occur is through the process of automatic rescaling Decouples image acquisition and display Allows for image processing optimization Pre-processing algorithms Post processing algorithms Higher kVp techniques Allows for lower patient exposure Increased throughput No darkrooms Digital systems have higher contrast resolution than screen-film systems ˜The ability to distinguish low contrast objects from one another Advantageous with mammography where breast tissue and tumors may be of similar composition Loss of contrast for portions of film image with exposure near toe and shoulder. Digital provides a greater range of contrast compared to film. Screen-Film Digital Higher DQE (Detective Quantum Efficiency) A measure of the detector’s efficiency Higher DQE means less exposure necessary to achieve the appropriate signal to the detector DQE is an indicator of the dose required to produce a quality image Digtal Image Characteristics Pixel Matrix Bit Pixel Pixel size Matrix Matrix size Bit Byte Bit depth Digital receptors Converts x-rays (data) to numerical values Pixels Exhibited as discrete gray shades for each pixel in the image after processing algorithm applied Pixel - A two dimensional square containing a discrete gray shade. Matrix - a two dimensional array of pixels or picture elements Pixel size Dimension of the pixel Micron NovationDR Mammo Unit 70 µm (0.07 mm) Micron = .001 mm - symbol = µ (.00004 inches) Pixel size and Spatial Resolution Smaller pixel size = increased spatial resolution NovationDR Mammo Unit - 7 lp/mm Spatial Resolution Matrix size - The number of rows and columns of pixels in an image NovationDR - 3328 X 4084 Bit Comes from the words “binary digit” Binary digits have two possible numerical values - “0” or “1” “0” = on (light) “1” = off (dark) Byte Bits are bundled together into 8-bit groups 1 byte = 8 bits Each of the 8 bits can be “0” or “1” Bit depth The number of gray shades available for image display (grayscale) The number of bits available for each pixel Represented by “n” Since digital deals with the binary number system, the number of gray shades is 2n Bit depth Greater number of bits = more values More data Higher contrast resolution More storage space 10, 12, 14 bit used in digital mammography to be comparable to the grayscale available with screen-film Digital Detector Systems TFT Flat Panel Detector Exposure TFT (thin-film transistor) Transistor – a semiconductor device that amplifies signals A “transparent” transistor whose active, current-collecting layer is a thin layer of selenium with solid state connections coated on glass substrate In flat panel DR image detectors, the TFT collects the charge representing the signal intensity per DEL (detector element) The TFT is then “read” by columns and rows Charge Collecting Detector Element (DEL) Light sensitive area (active area) Readout connection Direct acquisition / capture How x-ray signals are converted to digital signals Direct: x-rays → electrons Amorphous selenium Direct TFT Detector Detector is read by rows and columns to provide unique value (quantization) to each detector element. DEL Digital Value Bit depth (grayscale) 10 bit 1 – 1024 12 bit 1 – 4096 14 bit 1 – 16,384 DEL values with 10 bit system DEL value assigned gray value based on processing algorithm (LUT) Exposure and image appearance are decoupled With digital imaging, image processing and image display have more of an impact on contrast than kVp Higher energy photons may be used More photon transmission through dense glandular tissue Higher kVp also reduces patient exposure Raw “for processing” image – 26 kVp Dose Reduction By changing gradient processing LUT slope 30 kVp image exhibits the appropriate contrast and reduces patient exposure Further Dose Reduction Tungsten (W) target with Rhenium (Rh) filter ↑ Production efficiency 80% @ 30 kVp Shorter exposure times Less patient exposure Image Processing Initial Image Processing Post Image Processing Initial image processing Responsible for the initial image display Computer software driven Processing algorithms Applied to numerical data (digital values – the collected data) Post image processing Image manipulation after initial image display Computer software driven Processing algorithms Applied to numerical data (digital values – the collected data) Discuss terminology associated with digital imaging. Differentiate between the response of screen-film imaging systems and digital imaging systems. Discuss the fundamentals of digital imaging characteristics. Describe the composition and mechanism by which an amorphous selenium flat panel detector works. State why higher kVp levels are possible with digital imaging. Describe the various pre- and post image processing algorithms.
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