Chapter 4: Photostimulable Phosphor (PSP) - Digital Image Acquisition PDF

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AUTHOR QUERY FORM Please e-mail your responses and any corrections to: E-mail: [email protected] Book: Carter-9780323826983 Chapter: 4 Dear Author, Any queries or remarks that have arisen during the processing of your manuscript are listed below and are highlighted by flags in the proof. (AU indicates author queries; ED indicates editor queries; and TS/TY indicates typesetter queries.) Please check your proof carefully and answer all AU queries. Mark all corrections and query answers at the appropriate place in the proof using on-screen annotation in the PDF file. For a written tutorial on how to annotate PDFs, click http://www.elsevier.com/__data/assets/pdf_file/0016/203560/Annotating-PDFs- Adobe-Reader-9-X-or-XI.pdf. A video tutorial is also available at http://www.screencast.com/t/9OIDFhihgE9a. Alternatively, you may compile them in a separate list and tick off below to indicate that you have answered the query. Please return your input as instructed by the project manager. Location in Chapter Query/Remark: AU1: page 43 You are responsible for the accuracy of all drug dosages contained in this chapter. Please confirm that all drug dosages are correct. AU2: page 43 mAs is given as ‘milliampere-seconds’ in previous chapter – check if this should be consistent and amend if necessary AU3: page 44 please confirm the headings hierarchy AU4: page 56 please confirm cross reference to chapter 2 is correct. AU5: page 56 Both bucky and Bucky were used in the chapter and also in the previous edition. This has been made consistent to bucky (lower case). However, please indicate if the distinction is required. AU6: page 60 Please check the callouts for Fig. 4.30 A and C match the images and add callout for 4.30B if necessary.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 4 Photostimulable Phosphor tit0005 Image Capture [AU1] st0010 OBJECTIVES p0005 On completion of this chapter, you should be able to: Discuss the importance of matching the body part Describe the basic construction of a photostimulable being examined to the examination menu. phosphor (PSP) cassette and imaging plate. Discuss the selection of technical factors for density, Describe the purpose of each layer of the imaging plate. contrast, and penetration. Explain the process of photostimulation in the Describe the imaging plate and grid selection imaging plate. process. Explain the process of reading and erasing the Discuss the importance of preprocessing collimation imaging plate. and image marking. Compare conventional radiographic screen and film Compare exposure indicators for the major speed to PSP systems. computed radiography manufacturers and vendors. st0020 OUTLINE o0005 Photostimulable Phosphor Equipment, 44 Side/Position Markers, 57 o0050 o0010 Cassette, 44 Image Data Recognition and Preprocessing, 58 o0055 o0015 Imaging Plate, 44 Artifacts, 58 o0060 o0020 The Reader, 46 Imaging Plate Artifacts, 58 o0065 o0025 Exposure, 53 Image Processing Artifacts, 59 o0070 o0030 Part Selection, 53 Plate Reader Artifacts, 59 o0075 o0035 Technical Factors, 54 Printer Artifacts, 60 o0080 o0040 Equipment Selection, 55 Operator Errors, 60 o0085 o0045 Collimation, 57 Summary, 63 o0090 st0005 KEY TERMS: key0001 Artifacts Grid ratio key0012 key0002 Backing layer Imaging plate key0013 key0003 Barcode label Kilovoltage peak (kVp) key0014 key0004 Bit depth Laser key0015 key0005 Cassette Milliamperage seconds (mAs) [AU2] key0016 key0006 Color layer Moiré key0017 key0007 Collimation Phosphor center key0018 key0008 Conductive layer Phosphor layer key0019 key0009 Fast scan direction Photodetector key0020 key0010 Focused grid Photostimulable phosphor key0021 key0011 Grid frequency Protective layer key0022 43 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 44 PART 3 Digital Image Acquisition key0023 Reflective layer Shuttering key0026 key0024 Quantum mottle Slow scan direction key0027 key0025 Quantum noise Support layer key0028 p0155 The phrase digital radiographic image acquisition and to protect against static electricity buildup, dust collec- processing is used in this book to categorize the different tion, and mechanical damage to the plate (Fig. 4.3). ways of acquiring and processing digital radiographic images. One way to do this is through photostimulable phosphor (PSP) systems. These systems can be either Imaging Plate st0040 cassette based or cassette-less in design. This chapter in- Construction st0045 troduces the process of acquiring an image using PSP In PSP systems, the radiographic image is recorded on p0180 technology. Key topics include technical factors, equip- a thin sheet of plastic known as the imaging plate. The ment selection, exposure indicators, image data recog- imaging plate consists of several layers (Fig. 4.4): nition, and artifacts. A protective layer. This is a very thin, tough clear u0055 p0160 The term radiographic refers to general X-ray proce- plastic that protects the phosphor layer. dures as distinct from other digital modalities such as A phosphor layer (or active layer). This is a layer u0060 computed tomography (CT), magnetic resonance imag- of photostimulable phosphor that “traps” electrons ing (MRI), and ultrasound (US). during exposure. It is usually made of phosphors p0165 This chapter introduces the basic principles of PSP from the barium fluorohalide family (e.g., barium and discusses how PSP equipment works. Some simi- fluorohalide, chlorohalide, or bromohalide crystals). larities between PSP and conventional radiography are This layer may also contain a dye that differentially discussed. A basic understanding of how PSP works pre- absorbs the stimulating light to minimize the light pares the technologist to make sound ethical decisions spread as much as possible and functions similar to when performing radiographic examinations. the dye added to conventional radiographic screens. p0170 Cassette-based PSP systems differ from conventional A reflective layer. This is a layer that sends light in u0065 radiography in that the cassette is simply a lightproof a forward direction when released in the cassette container that protects an imaging plate from light and reader. This layer may be black to reduce the spread handling. The imaging plate takes the place of radio- of stimulating light and the escape of emitted light. graphic film and is capable of storing an image formed Some detail is lost in this process. by incident X-ray photon excitation of phosphors. The A conductive layer. This is a layer of material that u0070 cassette-less systems using PSP technology function in absorbs and reduces static electricity. a similar fashion but without the need for a cassette. A color layer. Newer plates may contain a color layer u0075 During the reading process, the phosphor releases the between the active layer and the support layer, which stored light and converts it into an electrical signal, absorbs the stimulating light but reflects emitted which is then digitized. light. A support layer. This is a semirigid material that u0080 gives the imaging sheet some strength. st0030 PHOTOSTIMULABLE PHOSPHOR A backing layer. This is a soft polymer that protects u0085 [AU3] EQUIPMENT the back of the cassette. Cassette-based PSP systems contain a window with p0220 st0035 Cassette a barcode label or barcode sticker on the cassette that p0175 The PSP cassette looks like a conventional screen-film allows the technologist to match the image information cassette. It consists of a durable, lightweight plastic with the patient-identifying barcode on the examina- material (Fig. 4.1). The cassette is backed by a thin sheet tion request (Fig. 4.5). For each new examination, the of aluminum or lead that absorbs backscatter X-ray patient-identifying barcode and the barcode label on the photons (Fig. 4.2). In addition to holding the PSP plate, cassette must be scanned and connected to the patient the cassette contains an antistatic material (usually felt) position or examination menu. In cassette-less systems, 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 45 A B f0005 Fig. 4.1 Front (A) and back (B) views of a photostimulable phosphor cassette. the image must be matched with the examination worklist image will need to be rotated or flipped on the screen to on the computer and there will not be a paper-type sticker display the image in correct anatomic orientation. for the plate. The cassette-based system may also have a label such as a colored mark or sticker where applicable, Acquiring and Forming the Image st0050 to indicate the appropriate orientation of the cassette in With PSP systems, the patient is X-rayed exactly the p0225 relation to the patient (Fig. 4.6). With correct cassette ori- same way as in conventional radiography. The patient entation, less image manipulation is required after pro- is positioned using appropriate positioning techniques, cessing. When the examination type is associated with and the body part is aligned with the image receptor. The the cassette, an automatic screen orientation of the image patient is then exposed using the proper combination of is built within the software. If the cassette was correctly kilovoltage peak (kVp), milliamperage seconds (mAs), oriented, the image will be displayed correctly; if not, the and distance. The difference lies in how the exposure 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 46 PART 3 Digital Image Acquisition is recorded. In PSP, the remnant beam interacts with electrons in the barium fluorohalide crystals contained within the imaging plate. This interaction stimulates, or gives energy to, electrons in the crystals, trapping them in an area of the crystal known as the color or phosphor center. This trapped signal will remain for hours, even days, although deterioration begins almost immediately. In fact, the trapped signal is never completely lost. That is, a certain amount of an exposure remains trapped so that the imaging plate can never be completely erased. How- ever, the residual trapped electrons are so few in number that they do not interfere with subsequent exposures. The Reader st0055 There are two types of PSP reader: point scan and line p0230 scan. Point scan readers have an optical stage, a scan- ning laser beam, translation mechanics, a light pickup guide, a photomultiplier, a signal transformer/amplifier, and an analog-to-digital converter (ADC). At any point in time, only a single laser point radiates the imaging plate (Fig. 4.7). Line scan readers are based on simul- taneous stimulation of the imaging plate one line at a time, and with line scan readers, the acquisition of the f0010 Fig. 4.2 Aluminum Absorber in Cassette. photostimulated luminescence (PSL) occurs with a charge-coupled device (CCD) linear array photodetec- tor. PSL refers to the emission of light from the phos- phor layer after stimulation by the relevant light source. Instead of a single laser beam, there is a scanning mod- ule that contains several linear laser units and optical light-collection lenses. The line scan system requires a lens array to focus each laser beam to a corresponding point on the CCD array (Fig. 4.8). f0015 Fig. 4.3 Antistatic Felt in Cassette. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 47 A Protective layer Phosphor layer Light reflective layer Conductive layer Support layer Light shielding layer Backing layer Barcode label B f0020 Fig. 4.4 (A) Imaging plate. (B) Construction. With PSP systems, no chemical processor or dark- p0235 room is necessary. Instead, following exposure, the cassette is fed into a reader (Fig. 4.9) that removes the imaging plate and scans it with a laser to release the stored electrons. When referring to the PSP interaction within the reader, a technologist will often note two scan directions. The fast scan direction is the movement of the laser across the imaging plate (also known as the “scan”) and the slow scan direction is the movement of the imaging plate through the reader (also known as the “translation” or “subscan direction”). The Laser st0060 A laser, or light amplification of stimulated emission of p0240 radiation, is a device that creates and amplifies a narrow, intense beam of coherent light (Fig. 4.10). The atoms or molecules of a crystal such as a ruby or garnet, or of a gas, liquid, or other substance are excited so that more of them are at high energy levels than low energy lev- els. Surfaces at both ends of the laser container reflect f0025 Fig. 4.5 Barcode Identification Labels. energy back and forth as atoms bombard each other, 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 48 PART 3 Digital Image Acquisition A B f0030 Fig. 4.6 (A) Fuji cassette orientation stickers. (B) Kodak orientation label. Scann Light ing m collec odule Laser tion o ptics n ctio ire ed ss ett Ca f0035 Fig. 4.7 Line Reader. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 49 Laser Beam deflector Beam-shaping optics Point Reader (laser irradiates one single point at a time) Sc an dir ec tio n n irectio ette d Cass f0040 Fig. 4.8 Point Reader. stimulating the lower-energy atoms to emit second- ary photons in the same frequency as the bombarding atoms. When the energy builds sufficiently, the atoms discharge simultaneously as a burst of coherent light; it is coherent because all of the photons are traveling in the same direction at the same frequency. The laser requires a constant power source to prevent output fluctuations. The laser beam passes through beam-shaping optics to an optical mirror that directs the laser beam to the sur- face of the imaging plate (Fig. 4.11). Using the laser to read the imaging plate. During p0245 the reading process, the imaging plate is scanned with a helium laser beam or, in more recent systems, solid-state laser diodes. This beam, about 100 micrometers (µm) wide with a wavelength of 633 nanometers (nm) (or 670 to 690 nm for solid state), scans the plate with red light in a raster pattern and gives energy to the trapped elec- trons. The red laser light is emitted at approximately 2 electron volts (eV), which is necessary to energize the trapped electrons. This extra energy allows the trapped electrons to escape the active layer where they emit visi- ble blue light at an energy of 3 eV as they relax into lower energy levels (Fig. 4.12). As the imaging plate moves f0045 Fig. 4.9 Fuji SmartPSP Photostimulable Phosphor Reader. through or remains stationary in the reader, the laser 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 50 PART 3 Digital Image Acquisition Anode Cathode Laser output Helium-neon gas reservoir Laser bore tube Output Glass envelope High coupler reflector f0050 Fig. 4.10 Laser Construction. Laser Beam deflector Beam-shaping optics Sc an dir ec tio n e ction ette dir Cass f0055 Fig. 4.11 Photostimulable Phosphor Reader Laser Optics. Scanning laser (Arrows represent emitted blue light) Imaging plate direction of travel (translation) f0060 Fig. 4.12 The laser scans the imaging plate, releasing stored energy as blue light (arrows). 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 51 Laser Beam deflector Beam-shaping optics Light collection optics Photodetector Optical filter Sc an dir ec tio n ction s s e tte dire Ca f0065 Fig. 4.13 Laser Optics. scans across the imaging plate multiple times. The plate energy to an optical filter and then to the photodetector movement through the scanner is known as translation (Fig. 4.13). because it moves in a parallel manner at a certain rate Although there will be variances among manufactur- p0255 through the reader. This scan process produces lines of ers, the typical throughput is 50 cassettes per hour. Some light intensity information that are detected by a pho- manufacturers claim that a rate of up to 150 cassettes per todetector. The photodetector amplifies the light and hour is possible, but based on average hospital depart- sends it to an ADC. The translation speed of the plate ment work flow, 50 cassettes per hour is a more realistic must be coordinated with the scan direction of the laser, expectation. or the spacing of the scan lines will be affected. Digitizing the signal. When we talk about digitizing p0260 p0250 The action of moving the laser beam across the imag- a signal, such as the light signal from a photodetector, ing plate is much like holding a flashlight at the same we are talking about assigning a numerical value to height and moving it back and forth across a wall. The each light photon. As humans, we experience the world more angled the beam is, the more elliptical the shape analogically. We see the world as infinitely smooth gra- of the beam. The same thing happens with the reader's dients of shape and color. Analog refers to a device or laser beam as it scans. If this change in the beam shape system that represents changing values as continuously is ignored, the output of the screen will differ from the variable physical quantities. A typical analog device is middle to the edges, resulting in differing spatial res- a watch: the hands move continuously around the face olution and inconsistent output signals, depending on and are capable of indicating every possible time of day. the position and angle of the laser beam. To correct In contrast, a digital clock is capable of representing only this, the beam is “shaped” by special optics that keep a finite number of times (e.g., every tenth of a second). the beam size, shape, and speed largely independent of The scanning process results in the conversion of the the beam position. A beam deflector moves the laser light emitted from the storage phosphor into an electri- beam rapidly back and forth across the imaging plate cal signal. The electrical signal is sampled and digitized to stimulate the phosphors. Mirrors are used to ensure to represent a specific location within the image matrix that the beam is positioned consistently. Because the and displays as a specific brightness. A matrix is the type of phosphor material in the imaging plate has an group of squares that make up the image information. effect on the amount of energy required, the laser and Each square is called a pixel or picture element. The p0265 the imaging plate should be designed to work together. typical number of pixels in a matrix ranges from about The light-collection optics direct the released phosphor 512 × 512 to 1024 × 1024 for CT but can be as large as 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 52 PART 3 Digital Image Acquisition 2500 × 2500 for radiography. The more pixels there are, the greater the image resolution for a fixed field of view. The image is digitized both by position (spatial loca- tion) and by intensity (gray level). Each pixel contains bits of information; the number of bits per pixel define the shade of each pixel, which is collectively known as bit depth. If a pixel has a bit depth of 8, for example, then the number of gray tones that pixel can produce is 2 to the power of the bit depth, or 28, or 256 shades of gray. Therefore the number of photons detected within the pixel will determine the amount of gray level or bit depth within that pixel. Some PSP systems have bit depths of 10 or 12, resulting in more shades of gray. Each pixel can have a gray level between 1 (20) and 4096 A (220). The gray level is a factor in determining the quality of the image. p0270 Spatial resolution. The amount of detail present in any image is known as its spatial resolution. Just as the crystal size and thickness of the phosphor layer deter- mine resolution in film/screen radiography, phosphor layer thickness and pixel size help determine resolution in PSP. The thinner the phosphor layer, the higher the resolution. In film/screen radiography, resolution at its best is limited to approximately 10 line pairs (lp) per millimeter (mm). In general projection radiography PSP imaging, resolution is approximately 2.55 to 5 lp/ mm, resulting in less detail. Resolution detail is also affected by the laser beam spot size (the smaller the B diameter of the laser beam, the higher the spatial resolu- tion), translation speed (slower speed allows more expo- Fig. 4.14 (A) Film/screen elbow radiograph. (B) f0070 sure to be detected), sampling frequency (the higher Photostimulable phosphor elbow image. Note the ability the sampling frequency, the more exposure detected), to better visualize the fat pads (arrows) on the lateral and the laser beam sweep in point beam readers (the view. ([A] Courtesy Dr. Loren Yamamoto. Appeared in tighter the sweep or the better shaping of the beam, the Inaba, AS. Radiographic examination of the elbow: the higher the resolution). However, because the bit depth, hourglass sign. In: Radiology cases in emergency med- icine. Vol. 1.) or the number of available shades of gray that can be displayed, is much higher, the difference in resolution is more difficult to discern. More tissue densities on the digital radiograph are seen, giving the appearance of Erasing the image. The process of reading the image p0275 more detail. For example, the fat pads on a lateral elbow returns most but not all of the electrons to a lower are difficult to discern on a film image (Fig. 4.14A). Fat energy state, effectively removing the image from the pads are a very important sign for the radiologists to see plate. However, imaging plates are extremely sensitive in pediatric elbow fractures. In the digital image, the fat to scatter radiation and should be erased to prevent a pads are easily seen (see Fig. 4.14B). This is because of buildup of background signal. At least once a week the the ability to display more shades of gray, thereby mak- plates should be run through an erase cycle to remove ing it possible to visualize more tissues of varying densi- background radiation and scatter. PSP readers have an ties; this does not, however, mean that the digital image erasure mode that allows the surface of the imaging plate contains additional detail. to be scanned without recoding the generated signal. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 53 Systems automatically erase the plate by flooding it with the technologist may choose the appropriate body part light to remove any electrons still trapped after the ini- automatically. Always check to make sure the appropri- tial plate reading (Fig. 4.15). Cassettes should be erased ate part has been selected. When using a cassette-based before using if the last time of erasure is unknown. system, the selection of the body part is usually done st0085 Preprocessing, processing, and forwarding the after exposure and it is imperative that cassettes are p0280 image. Once the imaging plate has been read, the sig- kept apart so that the technologist knows which cas- nal is sent to the computer where it is preprocessed. The sette goes with which body part. For example, if a skull data then go to a monitor where the technologist can examination is to be performed, the technologist would review the image, manipulate it if necessary (postpro- choose “skull” from the workstation menu (Fig. 4.16). cessing), and send it to the quality control (QC) station Selecting the proper body part and position is import- and ultimately to the picture archiving and communica- ant for the proper conversion to take place. Image rec- tion system (PACS). ognition is accomplished through complex mathemati- cal computer algorithms, and if the improper part and/ or position is designated, the image may be processed st0090 EXPOSURE incorrectly and fail to display properly. For example, if a knee examination is to be performed and the exam- st0095 Part Selection ination selected is for skull, the computer will interpret p0285 Depending on the type of system being used, the tech- the exposure for the skull, resulting in improper image nologist will choose the body part imaged either prior display (Fig. 4.17). The resultant image might appear to exposure of the image receptor or after exposure. If too dark or too light and it might appear grainy or as the examination room has a PSP housed in the detec- if it was underexposed. It is not acceptable to select a tor in the table or a wall stand, the patient worklist will body part or position other than that being performed most likely be in the room's workstation, which means simply because it provides a better image. If the proper Strong light source Imaging plate f0075 Fig. 4.15 A fluorescent floodlight is used to remove any remaining trapped energy. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 54 PART 3 Digital Image Acquisition f0080 Fig. 4.16 Workstation Menu Skull Selection. examination/part selection results in a suboptimal However, exposures outside that range are widely used image, then service personnel should be notified of and will depend on the image quality desired. Remem- the problem so it can be corrected as soon as possible. ber, the process of attenuation of the X-ray beam is Improper menu selections may lead to overexposure of exactly the same as in conventional film/screen radiog- the patient and/or repeated exposure. raphy. It takes the same kVp to penetrate the abdomen with PSP systems as it did with a film/screen system. It is vital that the proper balance between patient dose st0100 Technical Factors and part penetration be achieved. A major difference st0105 Kilovoltage Peak Selection between film/screen and digital receptors is that digital p0290 Kilovoltage peak (kVp), milliamperage seconds image contrast is no longer dependent on kVp. Suffi- (mAs), and distance are chosen in exactly the same cient kVp is needed to penetrate the body part; however, manner in digital projection systems as in conventional higher kVp values can be used, allowing for lower mAs film/screen radiography. Traditionally, kVp was chosen values. The dynamic recording range of digital receptors for penetration and tissue type and mAs according to the is much higher than those used in film/screen systems number of photons required for that body part. kVp val- and inherently produces a wide variety of gray values. ues range from 45 to 120 on most digital projection sys- Contrast is determined by computer processing. tems. It is not recommended that kVp values lower than 45 or greater than 120 are used because those values Milliamperage Seconds Selection st0110 may be inconsistent and produce too little or too much Again, mAs selection is based on the number of pho- p0295 excitation of the phosphors. The k-edge of phosphor tons needed for a particular body part. If there are too imaging plates ranges from 30 to 50 kiloelectron volts few photons, no matter what level of kVp is chosen, the (keV), so exposure ranges of 60 to 110 kVp are optimum. result will be a lack of sufficient phosphor stimulation. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 55 A B f0085 Fig. 4.17 (A) Anteroposterior (AP) knee with proper menu selection. (B) AP knee with AP skull selected. When insufficient light is produced, the image is grainy, a condition known as quantum mottle or quantum noise (Fig. 4.18). PSP systems typically use automatic exposure controls (AECs), just as many film/screen sys- tems do. When converting from film/screen systems to a PSP system, it is critical that the AECs be recalibrated to the desired exposure indicator. st0115 st0120 Equipment Selection Imaging Plate Selection p0300 Two important factors should be considered when selecting the PSP imaging cassette: type and size. Most manufacturers produce two types of imaging plates: standard and high resolution. Cassettes should be marked on the outside to indicate high-resolution imaging plates. High-resolution imaging plates con- Fig. 4.18 Grainy Appearance Because of Insufficient Light f0090 tain a thinner phosphor layer compared to the standard Produced in Imaging Plate. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 56 PART 3 Digital Image Acquisition f0095 Fig. 4.19 Pixel Matrix. plates. The thinner layer results in greater image sharp- ness because of the reduced amount of light spreading Fig. 4.20 Lateral Skull Image Displaying Moiré Pattern f0100 in more lateral directions. When light spreads laterally, Artifact Caused by Incorrect Grid Alignment with Laser it causes the images to appear somewhat blurry. This Scan Direction. occurs with any image capture system that involves the release of light. Typically, high-resolution imaging plates are limited to the smaller cassette sizes and are most lines and eliminates the interference. Because the PSP often used for extremities, mammography, and other systems are more sensitive to low levels of radiation, the examinations requiring increased detail. use of a grid is much more critical than in film/screen p0305 PSP digital images are displayed in a matrix of pixels radiography. Appropriate selection of stationary grids (Fig. 4.19), and the pixel size is an important factor in also reduces this interference. Grid selection factors are determining the resolution of the displayed image. As a frequency, ratio, and focus. [AU4] review from Chapter 2, if the matrix of an imaging sys- Frequency. Grid frequency refers to the number of p0315 tem remains constant, as the field of view decreases, the grid lines per centimeter or lines per inch. The higher pixel size also decreases and the spatial resolution of the the frequency or the more lines per inch, the finer the image increases. Therefore the technologist should use grid lines in the image and the less they interfere with the smallest field appropriate for the body part being the image. Typical grid frequency is between 80 and imaged so that the pixel size will be at its smallest and 152 lines/inch. Some manufacturers recommend no the spatial resolution will be at its highest. Appropriate fewer than 103 lines/inch and strongly suggest grid fre- image plate selection for the examination also elimi- quencies greater than 150. The higher-frequency grids nates scatter outside the initial collimation by reducing require more X-ray photons to produce an image than the amount of imaging plate not being exposed, and this lower-frequency grids require, and therefore the patient increases contrast resolution. will receive a higher dose. In addition, the closer the grid frequency is to the laser scanning frequency, the greater st0125 Grid Selection likelihood of frequency harmonics or matching and the p0310 Digital images are displayed in tiny rows of picture ele- more likely the risk of moiré effects. ments or pixels. Grid lines that are projected onto the Ratio. The relationship between the height of the lead p0320 imaging plate when using a stationary grid can interfere strips and the space between the lead strips is known with the image. This results in a wavy artifact known as as the grid ratio. The higher the ratio, the more scatter a moiré pattern, which occurs because the grid lines and radiation is absorbed. However, the higher the ratio, the the scanning laser are parallel (Fig. 4.20). The oscillat- more critical the positioning is, so a high grid ratio is [AU5] ing motion of a moving grid, or bucky, blurs the grid not a good choice for mobile radiography. A grid ratio 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 57 of 6 : 1 would be proper for mobile radiography, whereas can be added around the original collimation edges, a 12 : 1 grid ratio would be appropriate for departmental virtually eliminating the distracting white or clear areas grids that are more stable and less likely to be misposi- (Fig. 4.21). However, this technique is not a replacement tioned, causing grid cutoff errors. for proper preexposure collimation. It is an image aes- p0325 Focus. Most grids chosen by radiography depart- thetic only and does not change the number or angles of ments are parallel and focused. Parallel grids are less scatter. There is no substitute for appropriate collimation critical to beam centering but should not be used at because collimation reduces patient dose. distances less than 48 inches. Focused grid consists of lead strips angled to coincide with the divergence of the Side/Position Markers st0150 X-ray beam and must be used within specific distances Anyone who has used digital image processing equip- p0335 using a precisely centered beam. ment knows that it is very easy to mark images with left- and right-side markers or other position or text markers st0145 Collimation after the exposure has been made. However, we strongly p0330 When exposing a patient, the larger the volume of tis- advise that conventional lead markers be used in the sue being irradiated, the more scatter will be produced. same way they are used in film/screen systems. Mark- Although the use of a grid absorbs the scatter that exits ing the patient examination at the time of exposure not the patient and affects latent image formation, properly only identifies the patient's side but also identifies the used collimation reduces the area of irradiation and the technologist performing the examination. This is also an volume of tissue in which scatter can be created. Colli- issue of legality. If the examination results are used in mation is the reduction of the area of beam that reaches a court case, the images that include the technologist's the patient through the use of two pairs of lead shutters markers allow the possibility of technologist testimony encased in a housing attached to the X-ray tube. Colli- and lend credibility to his or her expertise. mation results in increased contrast resolution because When all the appropriate technical factors and equip- p0340 of the reduction of scatter. Through postexposure image ment have been selected, the image receptor may be manipulation known as shuttering, a black background exposed and then subjected to the reading process. The A B f0105 Fig. 4.21 (A) Lateral ankle without shuttering. (B) Lateral ankle with shuttering. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 58 PART 3 Digital Image Acquisition image will then be displayed. The radiographer must st0160 ARTIFACTS then consider a number of factors: image exposure indi- cators, image processing modes, and image processing As with film/screen systems, artifacts can degrade p0350 parameters. images in digital systems. Artifacts are any undesir- able densities on the processed image other than those st0155 Image Data Recognition and Preprocessing caused by scatter radiation or fog. There are four com- p0345 The image recognition phase is extremely important in mon types of artifacts (in addition to operator errors establishing the parameters that determine collimation that may cause artifacts): imaging plate artifacts, plate borders and edges, and histogram formation. A histo- reader artifacts, image processing artifacts, and printer gram is a graphic representation of the numerical tone artifacts. values of an X-ray exposure. All PSP systems have image recognition, and each has a specific name for this pro- Imaging Plate Artifacts st0165 cess. Agfa uses the term collimation, Carestream uses As the imaging plate ages, it becomes prone to cracks p0355 the term segmentation, and Fuji uses the phrase exposure from the action of removing and replacing the imag- data recognition. All systems use a region of interest to ing plate within the reader. Cracks in the imaging plate define the area where the part to be examined is recog- appear as areas of radiolucency on the image (Fig. 4.22A nized, and the exposure outside the region of interest is and B). The imaging plate must be replaced when cracks subtracted. Each vendor has a specific tool for different occur in clinically useful areas. If static exists because of situations such as neck, breasts, pediatrics, and hips in low humidity, hair can cling to the imaging plate, creat- which the anatomy requires some special recognition. ing another type of image plate artifact (Fig. 4.23). Refer to each vendor's manual to learn their proprietary Backscatter created by X-ray photons transmitted p0360 recognition and processing parameters. The science through the back of the cassette can cause dark line arti- behind each of these is beyond the scope of this book. facts. Areas of the lead coating on the cassette that are A B f0110 Fig. 4.22 (A) Cracks or scratches in the imaging plate which produce areas of radiolucency. The black arrows are pointing to numerous scratches on the surface of the imaging plate. (B) The black oval is surrounding an area with dirt or debris on the imaging plate. This debris is seen as small white specks on the image. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 59 A B f0115 Fig. 4.23 (A) Something as small as a coarse hair can be imaged. (B) The black arrows are pointing to the coarse hair seen in this posteroanterior wrist image. worn or cracked allow scatter to image these weak areas. Proper collimation and regular cassette inspection help to eliminate this problem. st0170 Image Processing Artifacts p0365 Processing artifacts can occur for many different rea- sons, such as choosing the incorrect processing param- eter for a particular body part or incorrect sampling of the image file. As discussed previously, it is very important to set appropriate technical factors and choose the correct body part so that the software algo- rithms will produce the desired image. Poor technique (collimation, grid selection, mAs, kVp, etc.) and posi- tioning can cause these algorithms to misrepresent the image. The technologist must remain aware of all fac- tors that can influence change in the final processed image. st0175 Plate Reader Artifacts p0370 The intermittent appearance of extraneous line patterns can be caused by problems in the plate reader's elec- Fig. 4.24 Extraneous Line Patterns Caused by Noise in the f0120 tronics (Fig. 4.24). Reader electronics may have to be Plate Reader Electronics. (Courtesy Carestream Health, replaced to remedy this problem. Inc.) p0375 White lines that are parallel to the direction of plate travel are caused by dirt, dust, or scratches on the light Insufficient erasure after an overexposure may result p0385 guide. Service personnel will need to clean or replace in residual image information being left in the imaging the light guide. plate before the next exposure. This may also occur if p0380 A rare but possible artifact can occur when multiple the erasure lamp is in need of repair. The results will imaging plates are loaded into a single cassette. In this vary depending on how much residual image is left and instance, usually only one of the plates will be extracted, where it is located. which leaves the other plate to be exposed multiple The orientation of a stationary grid relative to the p0390 times. The result is similar to a conventional film/screen direction of the laser scan is critical to reduce the like- double-exposed cassette. lihood of the moiré artifact (Fig. 4.25). The grid lines 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 60 PART 3 Digital Image Acquisition A B f0125 Fig. 4.25 (A) An exposure of a correctly oriented grid with the grid lines perpendicular to the plate reader's scan lines. (B) A moiré pattern caused by an incorrectly oriented grid, with the grid lines parallel to the plate reader's scan lines. must be perpendicular to the laser scan direction. cassettes. Care should be taken to expose only the tube Moving grids generally do not demonstrate the moiré side of the cassette (Fig. 4.27). artifact because the grid lines are blurred out. Vendors Underexposure produces quantum mottle, and over- p0410 have identified the specific grid frequency for station- exposure reduces contrast. The proper selection of tech- ary grids that will prevent the moiré artifact with their nical factors is critical for both patient dose, image qual- equipment. ity, and to ensure the appropriate production of light from the imaging plate (Fig. 4.28). st0180 Printer Artifacts In Fig. 4.29A, the technologist failed to remove the p0415 p0395 Fine white lines may appear on the image because of patient's elastic-waist shorts. The elastic appears as small debris on the mirror in the laser printer. Service person- radiopaque lines running through the wings of the nel will need to clean the printer. pelvis. Owing to the sensitivity of most digital image receptors, clothing, especially bulky clothing, should be st0185 Operator Errors removed if it is in the anatomy of interest. Fig. 4.29B is a p0400 Insufficient collimation can result in an improper cal- pediatric femur and the patient has a wet diaper. In this culation of the exposure indicator. This may result in a instance, the diaper is not obscuring any pertinent anat- misrepresentation of the displayed image (Fig. 4.26). omy. Care must also be taken when imaging a patient p0405 If the cassette is exposed with the back of a cassette in a t-shirt that has heavy graphics (Fig. 4.30A), beads toward the source, the result will be artifacts from any (Fig. 4.30C), or metallic paint. These can show artifacts [AU6] hinges or other hardware present on the back of the within the chest area and be mistaken for pathology. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTER 4 Photostimulable Phosphor Image Capture 61 S160 S127 A B f0130 Fig. 4.26 Insufficient Collimation Error. (A) Properly collimated lateral ankle. (B) Improper collimation resulting in poor histogram analysis. Note the differences in contrast and exposure indices. f0135 Fig. 4.27 This Anteroposterior Ankle Was Exposed Through the Back of a Cassette. Artifact remedy: be sure radiographers are well educated about how to use the entire computed radiography system. 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 62 PART 3 Digital Image Acquisition A B C f0140 Fig. 4.28 (A) Optimal image. (B) Underexposed image caused by insufficient milliamperage seconds (mAs), resulting in quantum mottle (best demonstrated in soft tissue between metacarpals) (high kilovoltage peak [kVp], low mAs). (C) Overexposed image caused by insufficient kVp, resulting in contrast loss (low kVp, high mAs). 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. CHAPTERFig 4.30(A(B )an ,) d(C T)heseimagesshow theimportanceofremoving anyshirt thathasheavygraphics bea , ds, metallicprint. PhotostimulablePhosphoIm or r ageCapture 63 A B f0145 Fig. 4.29 (A) The black arrows are pointing to the elastic bands in the shorts left on a patient for this exam. (B) The black oval is circling a wet diaper as seen on this anteroposterior femur image. A B C f0150 Fig. 4.30 (A), (B) and (C) These images show the importance of removing any shirt that has heavy graphics, beads, or metallic print. st0190 S U M M A RY u0090 A PSP cassette-based imaging system has a specially A photodetector collects the light and sends it to a u0115 designed cassette made of durable, lightweight plastic. signal digitizer. u0095 The imaging plate is multilayered with protective, The ADC assigns a numerical value to each pixel in u0120 phosphor, reflective, conductive, color, support, and a matrix according to the intensity of the detected backing layers. light. u0100 Barcodes are used to identify the cassette or imaging Spatial resolution of the digital image is determined u0125 plate and examination request to link the imaging by the thickness of the phosphor layer and the size of plate with the patient examination. the pixels. The thinner the phosphor layer, the greater u0105 Barium fluorohalide crystals in the imaging plate the sharpness of the image, and the smaller the pixel release light energy, which is then stored in the con- size, the higher the spatial resolution. ductive layer. The contrast resolution of a digital projection imag- u0130 u0110 The imaging plate reader uses a laser or a module of ing system is higher because the bit depth, or the lasers to scan the imaging plate, releasing the energy number of available shades of gray that can be dis- stored in the active layer as blue light. played, is higher than in conventional film/screen 00004-Carter-9780323826983 These proofs may contain color figures. Those figures may print black and white in the final printed book if a color print product has not been planned. The color figures will appear in color in all electronic versions of this book.  To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), ed- itor(s), reviewer(s), Elsevier and typesetter Aptara. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 64 PART 3 Digital Image Acquisition systems. Because energy stored in the imaging Images are sent to the QC station where they are ana- u0135 plate dissipates over time, imaging plates should be lyzed and sent to the PACS for long-term storage. read as quickly as possible to avoid losing image Imaging plates are erased by exposing them to bright u0140 information. light such as fluorescent light. st0195 REVIEW QUESTIONS o0095 1. The active layer in a PSP plate is usually made of 8. Too much mAs will cause quantum mottle. o0260 phosphors from what family group? a. True o0265 o0100 a. Barium sulfate b. False o0270 o0105 b. Barium fluorohalide 9. If grid lines are present and run parallel to the scan- o0275 o0110 c. Cesium iodide ning laser, the image will not have the moiré pattern o0115 d. Amorphous selenium present. o0120 2. Which layer of the PSP plate sends light in a more a. True o0280 forward direction when released in the reader? b. False o0285 o0125 a. Reflective 10. Overexposure will reduce the contrast resolution of o0290 o0130 b. Conductive the image. o0135 c. Phosphor a. True o0295 o0140 d. Support b. False o0300 o0145 3. Which layer of the PSP plate reduces static electricity? 11. After processing the image, strange artifacts are o0305 o0150 a. Reflective present that look metallic but are not present on the o0155 b. Conductive