RADIOGRAPHIC IMAGING INTRODUCTION TO RADIOLOGUC TECHNOLOGY.pptx

Radiographic imaging is an essential but complicated subject. There are many concepts that one must not only understand but also apply in the clinical environment. It will present many basic concepts that will be expanded in other courses. It is important to grasp the basic definitions and concepts before moving on to the more involved topics.

1. INTRODUCTION TO RADIOLOGIC TECHNOLOGIST

2. Radiographic Imaging

3. Radiographic imaging is an essential but complicated subject. There are many concepts that one must not only understand but also apply in the clinical environment. It will present many basic concepts that will be expanded in other courses. It is important to grasp the basic definitions and concepts before moving on to the more involved topics. Image Production A beam of x-rays, mechanically produced by passing high voltage through a cathode ray tube, traverses a patient and is partially absorbed in the process. A device called an image receptor (IR) intercepts the x-ray photons that are able to exit the patient. Multiple different IR systems are used in radiography, including film-screen systems, computed radiography (CR) cassette-based systems, digital radiography (DR) cassette-less systems, and fluoroscopic imaging systems. 7/1/20XX 3

4. Basically, the following four requirements exist for the production of x-rays: 1. Vacuum (tube housing) 2. Source of electrons (filament) 3. Method to accelerate the electrons (voltage) rapidly 4. Method to stop the electrons (target) In describing the relative ease with which x-ray photons may pass through matter of different types, two terms are commonly used. Radiolucent materials allow x-ray photons to pass through comparatively easily; translucent panes of glass allow the passage of light. Radiopaque materials are not easily traversed by x-ray photons, just as panes of tinted glass do not allow the full amount of light to pass through. Thus bone is described as a relatively radiopaque tissue, whereas air is described as relatively radiolucent.

5. The exposure factors under the control of the radiographer, often referred to as technique or prime factors, include the following: Milliampere-seconds ( mAs ) is the parameter that controls the amount of x-radiation produced by the x-ray tube Kilovolt peak ( kVp ) is a measure of the electrical pressure (potential difference) forcing the current through the tube 3. Source-to-image distance (SID) is the distance between the point of x-ray emission in the x-ray tube (the focal spot) and the IR. Other factors that can be controlled by the radiographer include focal spot size, primary beam configuration, quantity and quality of scatter, and speed of the IR.

6. Image Receptor Systems Once the attenuated beam has exited the patient as remnant radiation, the information it carries about the types of tissue the beam has traversed must be translated from an energy message to a visual image that can be viewed and stored. X-ray photons have the ability to produce changes in photographic film, photostimulable phosphors, and photoconductor materials, such as amorphous selenium (a-Se). Various IR systems exist, including film-screen systems, digital cassette-based systems, and digital cassette-less systems. Historically, film was the primary recording medium; however, almost all film-based systems have now been replaced with CR and DR systems.

7. Film-Screen Systems A useful analogy is that of regular photographic film, in which the camera is loaded with the film, which then receives the light reflecting off the subject. When the roll of film is finished, it is rewound into a light-tight canister, developed, and printed. The printed image can then be viewed and stored. The image is not visible before processing because it is stored in a form that is not visible. The image is stored in the emulsion until it is processed. This invisible image is called the latent image. Once the film has been processed, a visual image appears. The correct term to describe an image produced by x-ray photons on a piece of film is a radiograph. Intensifying screens are thin layers of polyester plastic coated with layers of luminescent (light-emitting) phosphor crystals. The screens are mounted in a cassette, and the film is placed inside. Typically, radiographic film has an emulsion coating on both sides and is known as duplitized or double-emulsion film.

8. Digital Cassette Systems Photostimulable Phosphor Systems Also known as CR or cassette-based DR, photostimulable phosphor systems make use of the digital acquisition modality in which photostimulable storage phosphor (PSP) plates are used to produce radiographic images. PSP plates are also referred to as imaging plates (IPs). CR can be used in standard radiographic rooms just like film-screen systems; therefore, no special changes are needed to the x-ray rooms. The new equipment that is required includes the CR cassettes and phosphor plates, the CR readers, and the image-display workstations. In CR, the radiographic image is recorded on a thin sheet of plastic known as the imaging plate. The IP consists of several layers, including the protective layer, phosphor layer, conductive layer, light-shield layer, support layer, and backing layer. The cassette also contains a barcode label or barcode sticker on the cassette or on the IP (viewed through a window in the cassette), which allows the technologist to match the image information with the patient identifying barcode on the examination request.

9. Digital Cassette-less Systems Direct Capture In direct capture, x-ray photons are absorbed by the coating material and immediately converted into an electrical signal. The DR detector has a radiation conversion material or scintillator, typically made of amorphous selenium. This material absorbs x-rays and converts them to electrons, which are stored in the thin-film transistor (TFT) detectors. The TFT is an array of small (approximately 100 to 200 μm ) pixels. A pixel is a single picture element, and a matrix is a rectangular series of pixels. The degree of accuracy of the structural lines recorded, known as spatial resolution is determined by the individual size of each pixel in a digital image.

10. Digital Cassette-less Systems Indirect Capture Indirect capture detectors are similar to direct detectors in that they may use TFT technology. Unlike direct capture, indirect capture is a two-step process: x-rays photons are first converted to light using a scintillator, and that light is then converted to an electric signal. The scintillation layer in the IP is excited by x-ray photons, and the scintillator reacts by producing visible light. This visible light then strikes the amorphous silicon, which conducts electrons down into the detector directly below the area where the light struck. There are two types of indirect conversion devices: the charge-coupled device (CCD) and the TFT array. The CCD uses a chip to convert light photons to electrical charge. The TFT array isolates each pixel element and reacts like a switch to send the electrical charges to the image processor. As with direct capture, more than 1 million pixels can be read and converted to a composite digital image in less than 1 second

11. Image Quality Factors The acceptance characteristics of a diagnostic-quality image, termed image quality factors, fall into two main categories: (1) photographic qualities affecting the visibility of the image and (2) geometric qualities affecting the sharpness and accuracy of the image . There are four primary image quality factors. Two of these are photographic in nature: IR exposure and contrast. Radiographic density is defined as the overall blackening of film emulsion in response to this exposure. With digital systems, this important image quality factor has not changed but can be expressed simply as IR exposure, because film is no longer the receptor of the image. In the digital environment, brightness is a monitor control function that can change the lightness and darkness of the image, but it is not related to IR exposure. Contrast is the visible difference between adjacent IR exposures, or the ratio of black to white. The two geometric quality factors are spatial resolution (also known as sharpness or recorded detail), the distinct representation of an object’s true borders or edges, and distortion , the misrepresentation of the true size or shape of an object.

12. Photographic Qualities Image Receptor Exposure Radiographic density can be described technically as a comparison of the light incident on the film to the light transmitted through the film. If a digital image is printed to hard-copy film, the traditional term density can still be used. The darker areas that block the transmission of light are said to have greater radiographic density. Although it can be easily measured scientifically with a densitometer, density is often a subjective measurement, judged by the human eye. A radiograph must possess the proper IR exposure to present adequate visibility of detail to the viewer in the same way that a photograph should not be overexposed or underexposed to do justice to its subject. In many instances, a radiologist’s use of the term density refers to anatomic density and not to radiographic density. A report noting an increased density in the right lung field should be interpreted to mean that the lung tissue is denser than other tissues. The IR exposure, or radiographic density on a film, in such an area would therefore be decreased because the denser tissue would absorb more of the x-ray beam than the tissue that is less dense. Many variables can affect IR exposure, including mAs , patient factors, kVp , distance, beam modification, grids, and IRs

13. Photographic Qualities

14. Photographic Qualities Milliampere-Seconds The number of electrons that flow from cathode to anode in the x-ray tube is controlled by mAs . This process in turn controls the number of x-ray photons produced. The greater the number of x-ray photons generated, the greater will be the resultant IR exposure. Increasing the number of x-ray photons produced increases the exposure (in milliroentgens [ mR ]) in a directly proportional relationship, and this results in an overall increase in IR exposure. mAs is the product of mA and time. Any combination of mA and time producing equivalent mAs values should produce equivalent IR exposures. This process is known as mAs reciprocity. mA x time (s) = mAs The mAs , mA, and time factors are all directly related to image receptor exposure, as well as patient exposure. These effects can also be stated as follows: Increasing mAs increases IR exposure and patient exposure. Decreasing mAs decreases IR exposure and patient exposure

15. Photographic Qualities Patient Factors Various patient factors affect IR exposure. Patient size and thickness, the predominant atomic numbers of the materials (which may include contrast media intentionally introduced into the body), pathologic conditions, anomalies, temporarily compressed tissues, and a number of other techniques all change the subject density of the object being examined. As subject density increases, IR exposure decreases, and vice versa. Kilovolt Peak An x-ray photon of very low energy would have difficulty passing through dense body tissue. Conversely, this same low-energy photon would pass easily through less dense tissue. This characteristic is referred to as the penetrating ability of an x-ray beam. Each average body part can be shown to best advantage by using an optimal kVp setting as a guideline. The relationship between kVp and exposure is not as simple as that of mAs . As kVp increases, IR exposure increases but not in direct proportion. The general rule of thumb to account for the change in IR exposure relative to change in kVp is called the 15% rule. Increasing kVp 15% will approximately double IR exposure. Decreasing kVp 15% will approximately halve IR exposure.

16. Photographic Qualities Kilovolt Peak Using this rule to change kVp while maintaining the same IR exposure is also possible. This process is done by changing the mAs to compensate for the exposure change caused by the change in kVp . When this adjustment is made, the change in kVp does not change the quantity of the exposure—only the spectrum or energy of the photons. kVp can be changed while maintaining the same IR exposure as follows: Increase kVp 15% and halve mAs . Decrease kVp 15% and double mAs Example:To maintain the original image receptor exposure, what new value of mAs is necessary when changing from 75 kVp and 50mAs to 86 kVp ? 75 x 0.15 (15%) = 11.25 ( ~ 11) 75 + 11 = 86 Because the kVp increased 15%, the mAs must be halved to maintain the original image receptor exposure: 25mAs

17. Photographic Qualities

18. Photographic Qualities Distance A beam of radiation obeys many of the same laws that light does. If a flashlight beam is projected onto a wall, the relative intensity of the light increases as it is moved closer to the wall. The intensity increases as the distance decreases. As the flashlight is moved farther from the wall, the intensity of the light decreases as the distance increases. This characteristic is described as an inverse relationship. The relationship between distance and exposure is described by the inverse square law: the intensity of radiation (as measured in mR ) is inversely proportional to the square of the distance from the source. In other words, as an example, if the distance is doubled, the intensity decreases to one-fourth of the original.

19. Photographic Qualities Distance The mathematic expression of the inverse square law is as follows: = Where in: = Original intensity ( mR ) = New intensity ( mR ) = Original distance = New distance Example:If the intensity of the beam is 40 mR at the original distance of 40 cm, what will the intensity be if the new distance is 20 cm? x=160mR  

20. Photographic Qualities =   Distance Note that decreasing the distance by half causes the intensity to increase by a factor of 4. The inverse square law describes the effect of a change in distance on beam intensity, but the radiographer would frequently like to be able to compensate for a necessary change in distance. This compensation may be accomplished by using a conversion of the inverse square law known as the exposure maintenance formula. This formula is actually a direct square law. The mathematic expression of the exposure maintenance formula is as follows: Where in: mAs1 = Original mAs value mAs2 = New mAs value D1 = Original distance D2 = New distance Because mAs , mA, and time are all directly proportional to beam intensity, this formula may be used to derive any of these three factors.

21. Photographic Qualities Beam Modification Anything that changes the nature of the radiation beam, apart from the factors already discussed, is referred to as beam modification. The beam may be modified before it enters the patient, in which case it is called primary beam modification, or after it exits the patient, in which case it is generally known as scatter control. The primary beam can be adjusted by changing filtration and beam limitation. Filtration is the use of attenuating material, usually aluminum, between the x-ray tube and the patient. This substance mainly removes very-low-energy nondiagnostic x-ray photons in the primary beam to decrease patient exposure The amount of attenuating material required to reduce the intensity of a beam to half the original value is referred to as the half-value layer. Because aluminum is the most common material used for filtration in diagnostic radiography, half-value layer is usually expressed in terms of millimeters of aluminum equivalency (mm Al/Eq).

22. Photographic Qualities Beam Modification Beam limitation is the use of devices, such as a collimator, to confine the x-ray beam to the area of interest, thereby reducing exposure to body parts other than those under examination. In addition to patient protection, beam limitation dramatically affects radiographic quality. During the transit of an x-ray photon through matter, the probability that the photon will collide with an atom is high. This collision may result in a change in direction, as well as a decrease in the energy of the photon. This scattered photon is virtually useless from a diagnostic standpoint and contributes only to patient dose. This type of photon is usually described as scatter radiation . If scatter radiation reaches the IR, it is not carrying useful information. Scattered photons that strike the IR degrade the quality of the image by contributing unwanted exposure known as fog .

23. Photographic Qualities Grids Despite the careful use of primary beam modification, once the beam enters the patient, scatter radiation is produced. As stated previously, the more scatter that reaches the IR as fog, the poorer the appreciation of the details. A grid is a device that is designed to remove as many scattered photons exiting the patient as possible before they reach the IR. A grid consists of thin radiopaque lead strips interspersed with radiolucent spacing material. The grid is placed between the patient and the IR to intercept scattered photons, which, by definition, have been diverted from their original paths. Increasing the lead in a grid increases its ability to remove scatter from the remnant beam. Decreasing the amount of scatter enhances the radiographic contrast. However, the additional lead in the grid also requires increased exposure factor settings, which increases the radiation dose to the patient

24. Photographic Qualities Image Receptors With digital IRs, there are several key factors that must be considered when evaluating proper IR exposure. These include exposure latitude , the exposure indicator (EI), automatic rescaling , and window leveling. Exposure latitude refers to the range of exposures that can be used and still result in the capture of a diagnostic-quality image. The exposure latitude is greater for digital imaging receptors than for film-screen exposures. The greater exposure latitude is a result of the higher dynamic range of the receptors. Dynamic range refers to the IR’s ability to respond to the exposure. With film screen systems, overexposure and underexposure are quite evident and are reflected in an image on a film that is too light or too dark. In digital radiography systems, this difference, which is easy to see on film, is not evident on the display monitor. In CR and DR, if the exposure is more than 50% below the ideal exposure, quantum mottle results. The biggest difference between digital and film-screen radiography lies in the ability to manipulate the digitized pixel values, which allows for greater exposure latitude.

25. Photographic Qualities Contrast The second photographic image quality factor is contrast. Contrast is the visible difference between adjacent IR exposures. An object may be accurately represented on an image, but if it cannot be distinguished from the objects surrounding it, then the eye will not adequately appreciate the object. Proper contrast enhances the visibility of detail. Contrast can be understood by recalling the story of the little boy who was asked to draw a picture in art class. After laboring for some time, he presented a sheet of completely white paper to the teacher. The puzzled teacher asked what the picture was supposed to represent, to which the little boy replied, “It’s a white horse eating marshmallows in a snowstorm.” Of course, because no contrast existed between the different densities, the teacher failed to see the image the child described.