Week 9 PRE II Review 1 PDF

Final Review Week 9 Review CAHE Radiography Program PRE II Yana Strochkova MHS, LRT, RT(R) [email protected]  The Imaging Process Regardless of the system used the imaging process alwa...

Final Review Week 9 Review CAHE Radiography Program PRE II Yana Strochkova MHS, LRT, RT(R) [email protected]  The Imaging Process Regardless of the system used the imaging process always consists of 5 specific, well established phases Image acquisition Image processing Image archiving Image display Image analysis The x-ray production is the same for all the systems, however, the phases entitle very unique, system dependent sequences 1. Image Acquisition Results of interaction b/w X-ray beam and IR Creating “latent”, invisible image 2: Image Processing Images can be processed by either hard or soft copy film/hard copy can be digitized into DICOM-format images monitor/soft copy can be produced as film/hard copy by a laser or dry imaging system 3: Image Archiving Storing images for reference Hard copies or electronically  4: Image Display Critical element in both conventional and digital imaging Monitor resolution is the weakest point 5: Image analysis  Radiographic Image Quality Fidelity with which the anatomical structure being imaged is rendered on the radiograph. Factors Affecting Image Quality  Image Quality The most important characteristics of radiographic image quality are spatial resolution, contrast resolution, detail, visibility of the detail, noise and artifacts Resolution – ability to image two separate objects and visually distinguish one from another Spatial resolution – ability to image small object that have high subject contrast (bone-soft tissue) (screen –film radiography has excellent spatial resolution) Spatial resolution improves with reduction in screen blur, motion blur and geometric blur Contrast resolution – is ability to distinguish anatomical structures of similar subject contrast (spleen-liver, gray matter-white matter) Detail and recorded detail sometimes are used instead of spatial resolution and contrast resolution Visibility of detail – refers to the ability to visualize recorded detail when image contrast and optical density are optimized Noise – unwanted fluctuation of the optical density on the image An artifact is a structure or an appearance that is not normally present on the radiograph, and is produced by artificial means. It’s presence may be due to technical errors, processing errors (related to all aspects of processing) or positioning  Noise Can be inherent in the system = structure mottle Under control of a radiographer = quantum mottle Use of high mAs, low kV and slower IR reduces quantum mottle Resolution – ability of an image receptor to reproduce separate images of closely spaced small objects and visually distinguish them. Resolution and Noise are intimately connected with the speed of IR The faster the speed of the IR – the higher the noise level, the worse the resolution  Saturation Extreme overexposure to the IR can overwhelm the digital detection system, causing lost of data that results in the flat black appearance of the overexposed portion of the image Saturation ≠ image fog If image appears saturated(overexposed) and some details of the black area can be retrieved through image manipulation → it is not saturation! Saturation can not be altered It takes 8-10 times normal exposure to reach saturation on most sensitive system (IE# 3000 for Kodak and S# 25 for Fuji  Principal Factors That may Affect Image Quality Increase Pt Dose Magnif F.Spot blur Motion blur Absorpt. OD Contrast blur RS Grid Ratio Pt. thickness Field size Use of CM F.Spot size SID OID mA Time kVp Filtration Voltage ripple  The Image Diagnostic Process It includes four major steps: Narrowing the search field Hypothesis activation Information seeking Hypothesis evaluation If density needed to be change → half or double your mAs (30% change is not enough) If contrast should be changed → adjust kV at least by 4% (1 or 2 kV will not make it!!!) Acceptance Limits When acceptance limits are very narrow → high repeat rate and production of optimal images Distortion Distortion – any type of misrepresentation of the shape or the size of an anatomy Size distortion: Magnification!!!! Shape distortion: Elongation and foreshortening Size distortion controlled by radiographic distances (OID has bigger effect than SID) Shape distortion controlled by alignment of the IR, CR and the body part Foreshortening → the body part is not parallel to the IR Elongation → the CR is not perpendicular QA/QC Quality Assurance – works on the process of identifying problem, monitoring it and than resolving it Asses everything that concerns patient care and technical aspect (equipment) Quality Control – aspect of QA that deals with the equipment, QC is the prime responsibility of the radiologic technologist Purchasing equipment Identification of the Imaging Requirements Development of equipment specifications Selection of Involves radiologists, medical physicist equipment New equipment must be demonstrated to at least 2 persons CE must be an ongoing process Installation and acceptance testing In-service education and initial training demonstration Continuing Monitoring of education equipment performance External beam Processing system evaluation  Computers in Medicine I generation – vacuum tubes II generation – transistors III generation – silicon chip IV generation – large scale integration (silicon chip) V generation – very large scale integration (silicon chip) Analog refers to the continuously varying quantity; Digital system uses only two values that vary discreetly through coding  Computers in Medicine Hardware – physical components of the computer Software – programs that “tell” the hardware what to do, how to store and manipulate data All computer languages translate what the user input into a series of “0” and “1” for the computer to understand – it operates in binary system The binary number system has only two digits – 1 (on) and 0 (off) single binary digit (0 or 1) is called bit Bits are grouped into group of 8, called bytes Computer word = 2 bytes Digital images are made of discrete picture element – pixel, arranged in matrix  Computers in Medicine The sequence of instructions developed by a software programmer → computer program Operating system – is the program that most closely related to the hardware Application programs allow users to perform multiple functions Primary element that allows computer to manipulate data and carry out software instructions – CPU (central processing unit); it consists of Control unit Directs data to ALU or to memory Controls data transfer between the main memory and input and output hardware ALU (arithmetic logic unit) Components are connected by bus  Computers in Medicine Computer memory ≠ storage (storage is the archival form of memory) RAM (Random Access Memory) – erasable ROM (read only memory) – contains info supplied by the manufacturer – non- erasable Output Devices: Display Screens and Printers Soft copy - refers to the output seen on the display screen Input Devices: keyboard, mouse, trackballs, touchpads and source data entry devices (scanners, bar code readers, imaging systems, audio and video devices, electronic cameras, voice recognition systems, sensors and biologic input devices)  Computed Radiography Terminology Computed radiography is the form of IP – imaging plate digital radiography PD – photodiode Luminescence – ability of a material to emit light PMT – photomultiplier tube Phosphorescence – ability of a material to emit PSL – photostimulable light during and after interaction with x-ray photons luminescence (afterglow) PSP – photostimulable Fluorescence – ability of a material to emit light phosphor only (!!!!!!) during interaction with x-ray photons SP – storage phosphor Photostimulable luminescence – material will SPS – storage phosphor emit light after being exposed to a different light screen source  Computed Radiography The CR image receptor made of Barium Fluoride with Europium BaFl – phosphorus material that is phosphorescent, and it also has photostimulable luminescence (PSL) The Eu – activator (without it the latent image would not be possible) Computed Radiography = PSP technology Latent Image occurs in a form of metastable electrons in the storage phosphor screen (SPS) The diameter of the laser beam determines the Emitted light spatial resolution of the → the signal CR imaging system Stimulating Small laser beam light → the diameter is critical for noise ensuring high spatial  Computed Radiography Sources of Image Noise in CR Imaging System Mechanical defects Computer defects Slow scan driver Electronic noise Fast scan driver Inadequate sampling Optical defects Inadequate Laser intensity control quantization Scatter of stimulating beam Light quanta emitted Light quanta collected CR systems have lower noise levels and therefore allow additional reduction of the patient dose 20  Computed Radiography In Computed Radiography CR is faster IR than film → less mAs needed → lower patient dose possible However: low exposure to the CR image receptor produces higher noise levels “kVp controls contrast”, “mAs controls OD” → use with caution when referring to CR imaging: KVp – controls subject contrast, however the radiographic contrast is controlled by the software (LUT) mAs – controls the IR exposure rather than OD of the image Lets have a small break!!!  Digital Imaging Film digitizers: Convert radiographs recorded on conventional film into digital images with use of ADC Down fault: suboptimal image quality In DR there are two main classes of the IR 1 – PSP plate (storage phosphor imaging plate) used in CR radiography 2 – direct digital (DR) used in direct digital radiography, referred as cassetteless digital IR direct conversion IR indirect conversion IR  Digital Imaging Digital IR is sensitive to all exposure intensities → very wide dynamic range Moderately underexposed or overexposed images still fall into the acceptable diagnostic quality range The response to the exposure of the digital IR is linear Digital IR can display greater range of radiographic densities Different anatomical areas are easy visualized  Digital Imaging Digital image is recorded as a matrix (combination of rows and columns ► array)of pixels (picture element) Each pixel is recorded as single numerical value which is represented as brightness level on a display monitor Location of the pixel within the image matrix corresponds to the area within the patient or volume of tissue Spatial resolution is improved with the larger matrix size (greater number of smaller pixels) Spatial resolution is improved with the smaller FOV for the same size matrix because the pixel size will decrease (pitch) Spatial resolution is improved for a larger matrix size with smaller pixels  Digital Imaging Matrix size = rows of pixels × columns of pixels Pixel size = FOV (image size) ÷ Matrix size FOV = Pixel size × Matrix size If FOV is constant, the larger the matrix size → the smaller the pixel size → lesser signal received by individual pixel (higher exposure factors to achieve adequate IR exposure is required)→ higher the patient dose → the better the spatial resolution (image detector not the monitor!)  Digital Imaging The numerical value assigned to each pixel is determined by the relative attenuation of x-rays passing through the corresponding volume of tissue Pixels corresponding to the high attenuating area (bones) will be assigned low value → higher brightness, lower radiographic density Pixels corresponding to low attenuating area (air) will be assigned high value → lower brightness, higher radiographic density Each pixel also has a bit depth → number of bits that determines the precision with which remnant radiation is recorded and controls exact pixel saturation A larger bit depth allows a larger number of shades of gray to be displayed on a grayscale monitor A system that can display a greater number of shades of gray has better contrast resolution Down fault: increased the computer processing time, network transmission time and digital storage space 27  Digital Imaging Digital Image Spatial Resolution Pixel Size – measured from side to side of the pixel Pixel Pitch – measured from the center of one pixel to the center of an adjacent one Pixel Density – measures the number of pixels contained within a unit area DEL – detector element – its size contributes to the image blur The larger the DEL → more blur (flash light and laser pointer) Spatial resolution is determined by pixel size, pixel pitch, pixel density and DEL 28  Digital Imaging Direct digital radiography employs array of x-ray detectors that receive the remnant radiation and convert varying x-ray intensities into the electronic signal that are processed by a computer workstation Direct Digital Radiography CCD (Charged-Coupled Devices) Solid state array of light sensitive sensor elements Each pixel functions as a capacitor → stores a charge proportional to the intensity of incident light, coupled with a phosphor type screen (scintillator) to receive a remnant radiation and turn it into varying intensities light FPDCS (Flat Panel Direct Capture Systems) DR constructed using the TFT (thin-film transistor) array below a layer of attenuating material, which ultimately converts x-rays into electrical charge Signal storage, read-out, and digitizing electronics are integrated into the flat panel device After exposure the digital image almost immediately available to be viewed on a monitor  Digital Imaging Direct Digital Radiography - Indirect Conversion Detectors Indirect conversion = two step process of conversion x-ray intensities into light first and than to electronic charge during image acquisition Detectors use scintillator (cesium iodide) to convert remnant radiation into visible light Then it converted into electronic signal by a layer of Amorphous Silicone The electronic charge temporary stored in a TFT array before it is digitized and processed by a computer  Digital Imaging Direct Digital Radiography - Direct Conversion Detectors Amorphous Selenium coated detector converts remnant radiation into electric charge Selenium limits lateral electron diffusion as they migrate toward TFT array → excellent spatial resolution is maintained Electronic charge is stored in a TFT array before it is amplified, digitized, and then processed in the computer Regardless of the type of the digital imaging system the electrical signals are sent to the ADC for conversion into the digital data The digitized pixel intensities are patterned in the computer to form the image matrix  Digital Imaging The digital data are used to construct a histogram Histogram – graphic display of the distribution of the pixels values to evaluate and determine overall intensity of the x-ray exposure reaching the IR LUT – Look Up Table – preprogrammed, anatomy specific reference point. Provides method of altering the raw image to change the display of the digital image  Digital Imaging Visualizing a digital image requires that the display monitor has high spatial resolution capabilities and specific luminance or brightness to see the grayscale Required resolution: Min ► 1024 (1K) Preferred ► 2048 (2K) Postprocessing image manipulation: There are 5 common post-processing techniques Subtraction Contrast enhancement Edge enhancement Black/white inversion Smoothing (noise suppression)  Digital Imaging Organizational scheme for digital radiography CR – computed Radiography SPR – scanned projection radiography Digital Imaging Cesium Iodide Amorphous Silicon CsI is used to capture x-ray and to transmit the resultant scintillation light to collection element Silicone (collection element) is sandwiched as TFT CsI/a-Si – is an indirect DR image receptor tat converts x-ray into light first, and than into the electronic signal DR image receptor constructed into individual pixels Each pixel has a light sensitive face (capture element ) of a a-Si with a capacitor and TFT embedded Each pixel occupied by conductors, TFT and a capacitor → it is not totally sensitive to the image forming x-ray beam The percentage of the pixel face that is sensitive to x-ray is the fill factor The fill factor of a pixel describes the ratio of light sensitive area versus total area of a pixel The fill factor in DR is about 80% ►20% of x-ray beam does not contribute to the image Smaller pixel = reduction in a fill factor. Causes in increase demand for the higher intensity beam to fulfil the maintenance of the adequate signal strength Digital Imaging Amorphous Selenium (a-Se) DR modalities identified as direct DR → it does not involves the scintillation phosphor The image forming X-ray beam interacts directly with the a-Se → the charged pairs are produced a-Se assumes role of both the capture element and the coupling element In a DR process the x-rays are converted to electronic signal X-ray incidents on the a-Se create an electron-hole pair through the direct ionization of the selenium Created charge is collected by a storage capacitor and remains there until signal is read by switching action of the TFT  Digital Imaging Compare the line spread function of different types of the digital imaging systems The least amount of spread is with use of direct DR IR The largest amount of spread happen with film/screen IR The efficiency of an X-ray detector system can be described by its detective quantum efficiency (DQE). This is a parameter that reflects the efficiency of photon detection and the noise added to the detected signal. If every X-ray photon is recorded in the image with no additional noise, then DQE would be 100%. DQE for DR systems may be as high as 65%, whereas for CR and film–screen systems it is closer to 30%.  Digital Imaging Data manipulation 1. Exposure field recognition – recognizes clinically useful area (Eliminates signals outside the collimated area) The best image will result if body part is centered, aligned parallel to the long axis of the IR and at least 2 sided collimation that is parallel and equidistant form sides of the IR is present 2. Histogram analysis Appropriate anatomical menu should be selected Eliminates unnecessary info (scattered radiation) Once true information is being identified, the information is rescaled and LUT is applied to stabilize image density and contrast 3. Grayscale analysis  Digital Imaging Image Reprocessing Raw data - Stored by CR system workstation Contrast requirements Similar to DlogE curves of different types of radiographic film Scale of contrast or the slope of the DlogE curve can be adjusted (Window width) Spatial Frequency Processing – Affects image sharpness Edge enhancement Unsharp mask technique Low-pass filter High spatial frequency signal remains High spatial frequency signal is amplified and added back into the image Increases noise resulting in lower quality images Lower contrast and higher base fog levels  Digital Imaging Selecting the proper body part and position is important for the proper conversion to take place If the improper part and/or position is selected, the computer will misinterpret the image It is not acceptable to select a body part or position different from that actually being performed simply because it looks better Appropriate technical factors should be selected at all the times kVp is selected according to the desired penetration of the body part Milliampere-second is selected according to the number of electrons needed for a particular part. Too few electrons and no matter what level of kilovoltage peak is chosen, the result will be a lack of sufficient phosphor stimulation. When insufficient light is produced, the image will be grainy, a condition known as quantum mottle or quantum noise. (photon starvation) Grid selection factors are frequency, ratio, focus, and size Digital Imaging Grid frequency refers to the number of grid lines per centimeter or lines per inch The closer the grid frequency is to the laser scanning frequency, the greater likelihood of frequency harmonics or matching and the more likely the risk of moiré effects The relationship between the height of the lead strips and the space between the lead strips is known as grid ratio The higher the ratio, the more scatter radiation is absorbed. The higher the ratio, the more critical the positioning Digital Imaging shuttering - applying black background around the original collimation Shuttering ≠ collimation!!!!!! Shuttering is an image aesthetic only and does not change the amount or angles of scatter created  Digital Imaging Each medical image has multiple characteristics that will categorize quality of the image The two most important are the spatial resolution and contrast resolution properties Spatial Frequency – is expressed in line pair per millimeter (lp/mm) The fundamental concept of spatial frequency refers to the line pair, and not the size Each line pair consists of a line and interspace of the same width as a line Spatial frequency relates to the number of line pairs in a given length (cm, mm, or inches) As spatial frequency becomes larger, the objects become smaller → higher spatial frequency = better spatial resolution Digital Imaging Digital Imaging Human anatomy: Soft tissue have low spatial frequency (liver, kidneys, brain) they have the same consistency Bone trabeculae, breast micro-calcifications and contrast filled vessels are high spatial frequency objects is very intricate in their appearance Different imaging systems have different spatial frequencies Imaging system with higher spatial frequency has better spatial resolution Digital Imaging MTF – description of the ability of an imaging system to render objects of different sizes onto image Small objects are harder to image For the purpose of the MTF evaluation the object is considered to be high contrast (B&W), regardless of its size The ideal imaging system would be the one that produces an image as an exact replica of the object; (MTF would be = 1) MTF can be viewed as the ratio of image to object as a function of spatial frequency With increasing spatial frequency – line pairs become progressively more blurry due to decreasing amplitude of the represented frequency No DR imaging systems can resolve an object smaller than a pixel size  Digital Imaging Contrast resolution The principal descriptor for contrast resolution is gray scale, also called dynamic range Dynamic range is the number of gray shades that an imaging system can reproduce, identified by a bit depth It might have 8-bit depth (256 shades) or up to 16-bit depth (65,536 shades) Digital Imaging Signal – to – Noise Ratio In digital radiography the signal - is that portion of x-ray interaction with IR that represent anatomy Signal represents the difference between x-rays transmitted to the image and absorbed photoelectrically SNR – is a generic term which, in radiology, is a measure of true signal (i.e. reflecting actual anatomy) to noise (e.g. random quantum mottle) Dose Creep: Unnoticed Variations in Diagnostic Radiation Exposures → which is a pattern of radiation exposure levels (ie, dose) being increased by clinicians over time in an attempt to achieve better image quality in diagnostic radiography. Technique Creep: attempt to reduce patient dose by using higher kV and lower mAs.  Digital Imaging Viewing of digital Images Photometric quantities The basic unit of photometry is the lumen Luminous flux – total intensity of light from the source; expressed in lumens = lm Illuminance – intensity of the light incident on the surface; expressed in foot-candle = fc, or lux = lx (in metric system) Luminance intensity – property of the source of light; is the luminous flux is emitted into the entire viewing area; measured in candela Luminance – quantity that is similar to luminance intensity, it is another measure of the brightness of the source such as digital display device expressed as units of candela per square meter = nit  Digital Imaging Fundamental laws of photometry There are two fundamentals laws that associated with photometry Inverse Square Law – luminous intensity decreases in proportion to the inverse square distance from the source Cosine Law – the best viewing of the digital device is straight on. Digital Imaging Postprocessing is designed to optimization of the digital image for viewing Annotation Image inversion Window and Level adjustment Magnification and Zooming Scroll and Pan Image flip Image subtraction Pixel Shift Digital Imaging Preprocessing is designed to produce artifact free digital images It provides electronic calibration to reduce pixel-to-pixel, row-to-row, and column-to-column response differences Process of pixel interpolation, lag correction and noise correction are automatically applied in most of the digital systems flat-fielding - Averaging techniques are used to reduce noise signal interpolation – correction of defective or dead pixel offset voltage – to prevent image lag  PROCESSED VERSUS RAW IMAGES Raw images: Actual data that are captured, pixel by pixel, from imaging detector Processed images: Information captured by pixels with mathematical algorithms applied The applied algorithms are designed to optimize the visualization of the image in a manner pleasing for the radiologist to view Radiologist views processed images Expensive to store both If only the processed images are saved, some of the original information is lost forever. If the raw images are saved, when the image is processed again at another time, there could be a difference in what is visualized, causing medico-legal repercussions. Each facility must decide which image to store