POI-MIDTERM Principles of Imaging PDF

LUTERTE, JEANNE BSRT-II PRINCIPLES OF IMAGING 1ST SEMESTER-MIDTERM CONTRAST:KVP kvp=morescatteredradiation=morera d i o g r a p h i c f o g = m o r e IR e x p o s u r e c o n t r a s t ( l o w d e g r e e o f d i ff e r e n c e b e t w e entwotones) CONTRAST: beam restriction beam restriction =...

LUTERTE, JEANNE BSRT-II PRINCIPLES OF IMAGING 1ST SEMESTER-MIDTERM CONTRAST:KVP kvp=morescatteredradiation=morera d i o g r a p h i c f o g = m o r e IR e x p o s u r e c o n t r a s t ( l o w d e g r e e o f d i ff e r e n c e b e t w e entwotones) CONTRAST: beam restriction beam restriction = less scattered radiation = less unwanted IR exposure contrast (high degree ofdifference between two tones) III. RADIOGRAPHIC DENSITY OID = less scattered rad = Less IR exposure contrast (high degree of difference between two tones) III.TECHNIQUECONVERSION FACTORS (PARTII) FACTOR OF MOTION From a radiographic standpoint, motion can be classified into two groups a) physiological or involuntary b) accidental or voluntary. The former can be controlled by employing high milliamperages and short exposures  density/IR exposure when one or more technical factors are altered. TAKE NOTE: Film density should remain unchanged as long as the total exposure remains unchanged. If the mAs used to create one image is the same as the mAs used to create a second image of the same structure, then both images should have very close to the same film density. As long as mAs is constant, any combination of mAs and exposure time values will create the same film density and IR exposure. TAKE NOTE: The general rule of thumb for mAs changes is to make adjustments in increments of doubles or halves. For example, a repeated image that is underexposed at 10 mAs should be repeated at 20, 40, or 80 mAs, depending on the circumstances. Of course, determining which circumstances require 20 mAs, which require 40 mAs, and which require 80 mAs is something that is acquired only through experience and further study. TAKE NOTE: The relationship of a film’s reaction to exposure was mAs and Density described in 1875 by Bunsen and Roscoe, who studied the reaction of photographic film to light and stated If the exposure to a film is increased, the density to that the reaction of a photographic film to light is that film will increase until the point where the film equal to the product of the intensity of the light and reaches its maximum density (Dmax). Because density the duration of the exposure. This concept is known as is primarily determined by the amount of exposure a the reciprocity law. When applied to x-rays, the film receives, and because exposure is directly reciprocity law can be restated as the density/IR proportional to mAs, mAs is used as the primary exposure should remain unchanged as long as the controller of radiographic film density and image intensity and duration of the x-ray exposure receptor exposure. As mAs increases, xray exposure (controlled by mAs) remains unchanged increases proportionally and radiographic density also increases. The direct proportional relationship between mAs and exposure is used to calculate mAs changes necessary to maintain consistent film  change in kVp in the lower ranges (30–50 kVp), but an 8–9 percent change is required in the middle ranges (50–90 kVp), and a 10–12 percent change in the higher ranges (90–130 kVp). Focal Spot Size Large focal spots tend to bloom more at higher milliamperages and may occasionally reach a point where they alter the IR exposure. Blooming occurs with large milliamperages because the incident electron beam is not as easily focused by the focusing cup. It is rare for blooming to cause a visible density/IR exposure difference. Because properly calibrated units will not exhibit FACTORS AFFECTING RADIOGRAPHIC density/IR exposure changes when focal DENSITY: spots are changed, differences of this type should be reported as a quality control procedure. If density/IR exposure differences are perceived due to focal spot blooming, replacement of the tube may be indicated. Influencing factors of Radiographic density KVP Kilovoltage alters the intensity of the beam Anode Heel Effect reaching the IR in two ways. Kilovoltage The anode heel effect alters the intensity controls the energy and therefore the of radiation, and therefore the density, strength of the electrons striking the target of between the anode and cathode ends of the x-ray tube for any given mAs. the x-ray tube. Depending on the angle Both the quantity and quality of the x-ray of the anode, this effect can cause an IR beam will vary significantly with changes in exposure variation of up to 45 kilovoltage. As a result, kVp has a tremendous impact on density/IR exposure. percent between the anode and Research has been done to determine a cathode ends of the image. practical formula that takes both the IR exposure is always greater quantity and quality factors into account. at the cathode end. The anode The primary finding is that there are too heel effect is more pronounced many variables to be quantified into a reliable when the collimator is open formula. A change in film density can wide than when it is closed usually be detected with a 4–5 percent because a greater portion of the  peripheral beam, and therefore a greater portion of the intensity difference, reaches the IR when the collimator is wide open. The anode heel effect is also more significant when using extremely small angle anodes (12° or less). However, the most common situation in The anode heel effect may be radiography is a need to maintain an converted to an advantage in examinations of objects with acceptable IR exposure/density while greater subject density at one end changing the distance. To maintain IR than at another. The advantage is exposure, mAs (or an influencer) must utilized by placing the portion of be changed to compensate for the the object with the greatest subject exposure change. density toward the cathode end of the tube. This utilizes the greater intensity for the greater subject density and leaves the lesser intensity of the anode end of the tube for the lesser subject density. Object-to-image-receptor distance (OID) has an effect on density/IR exposure. Theair-gap technique uses an increased OID to prevent scatter radiation from reachingthe IR. This scatter radiation would normally cause a visible increase Distance (SID and OID) in density/ IRexposure when Source-to-IR distance (SID) alters the radiographing large patients. By intensity of the beam reaching the increasing OID using the IR,according to the inverse square law. air-gaptechnique, scatter that would The law states that the intensity normally strike the IR will miss the (exposure) varies inversely with the receptor, causing adecrease in density/IR square of thedistance. exposure The inverse square law formula expresses the change in intensity when the distancechanges. Density/IR Exposure Relationship to Distance Because distance has an effect on x-ray intensity, it will in turn affect density/IR exposure. As the distance increases, intensity decreases, which causes a decrease in IR exposure. For film/screen image receptors, this will decrease film density. The reverse is also true. As the distance decreases, intensity and IR exposure increases, which in turn causes an increase in film density high distance = less IR exposure = less density The formula is sometimes known as the exposure maintenance formula and is based on the principle of the inverse square law. This example 2: formula is a direct square law. A direct relationship is necessary to compensate for the An acceptable chest image results from an changes in intensity and IR exposure. exposure taken using 4 mAs at 100 kVp at a DIRECT SQUARE LAW (aka exposure 72-inch distance. A second image must be maintenance formula) states that it is necessary taken supine at a 36-inch distance. If the same to compensate for the changes in mAs and film technical factors are used, the inverse square density with varying SID. law tells us that when the distance is decreased - DSQ describes a relationship between by a factor of 2, the intensity (exposure) will optical density and distance from the source of increase by a factor of 4. The second image radiation will be overexposed if the technical factors are. - The Direct Square Law is used to adjust the not adjusted. The mAs can be adjusted to exposure factors when changing the distance compensate for the distance change. The between the X-ray tube and the image receptor exposure maintenance formula can be used to (or patient) determine the compensation necessary for any change in distance. Filtration  Filtration and its ability to alter beam Grids with high ratios, low intensity affect IR exposure and frequency, and dense interspace density/IR exposure. All types of material; moving grids; and filtration—inherent, added, and improperly used grids (incorrect focal total—alter density/IR exposure. distance, etc.) all reduce density/IR Density/IR exposure decreases when exposure. filtration is increased. Beam Restriction Restricting the beam, collimating, or reducing the primary beam field size reduces the total number of photons available. This reduces the amount of scatter radiation and therefore reduces the overall IR exposure and density/IR exposure. DISTORTION Anatomical Part Distortion is the second of There is an inverse relationship between tissue thickness/type and density/IR the two geometric properties exposure. In other words, as tissue thickness, affecting radiographic image average atomic number of the tissue, and/or quality. tissue density increases, density/IR exposure Distortion is a misrepresentation of decreases. the size or shape of the structures NOTE: being examined. It creates a This is not a linear relationship because of the misrepresentation of the size multitude of variations in tissue composition. and/or shape of the anatomical part Depending on the type of contrast media or the type being imaged. This of pathology, an inverse or a direct relationship may misrepresentation can be classified exist as either size or shape distortion. Grid Construction Grids absorb scatter, which would otherwise add exposure to the IR and density to the film. The more efficient the grid, the less will be the density/IR exposure.  Calculating Size Distortion Size distortion is present in any radiographic image and can be measured very accurately by using simple geometry. Magnification, or size distortion, can be assessed by calculation of the magnification factor. The magnification factor is the degree of magnification and is calculated by: FACTORS AFFECTING SIZE DISTORTION Shape distortion displaces the projected image of an object from its actualposition and can be described as either elongation or foreshortening. Elongation projects the object so it appears to be longer than it really is,whereas foreshortening projects it so it appears shorter than it really is. ELONGATION VS. FORESHORTENING Elongation occurs when the tube or the image receptor is improperly aligned. Foreshortening occurs only when the part is improperly aligned. Changes in the tube angle cause elongation, never foreshortening. FACTORS AFFECTING SIZE DISTORTION 1.Alignment Shape distortion can be caused or avoided by careful alignment of the central ray with the anatomical part and the image receptor. Proper positioning is achieved when the central ray is at right angles to the anatomical part and to the image receptor. NOTE: This means the part and the image receptor must be parallel.  Alignment adjustments involve bringing the tube central ray, the part, and the image receptor back into their correct relationship—part and image receptor parallel to one another with the central ray perpendicular to both. Incorrect centering may occur from off-centering the tube (misalignment of the central ray), incorrectly positioning the part, or off-centering the image receptor. Incorrect centering may occur from off-centering the tube (misalignment of the central ray), incorrectly positioning the part, or offcentering the image receptor Central Ray The central ray is the theoretical photon that exits from the exact center of the focal spot. Ideally the central ray is intended to be projected perpendicular to both the anatomical part and the image receptor. Whenever the central ray is not perpendicular, some degree of shape distortion will result. Any structure that is not positioned at the central ray will be distorted because of the divergence of the beam— the farther from the central ray, the greater the distortion. When the part is superimposed over other structures, central ray angulation can be a useful tool to provide a projection that would otherwise be impossible to differentiate from overlying structures. Anatomical Part The long axis of the anatomical part, or object, is intended to be positioned perpendicular to the central ray and parallel to the image receptor. When these positions are incorrect, distortion may occur. The image receptor is intended to be positioned perpendicular to the central ray and parallel to the anatomical part. As long as the image receptor plane is parallel to the object, the only result of off-centering of the image receptor is the clipping of a portion of the area of interest. However, when the image receptor plane is not parallel to the object, or if the central ray is not centered to the part, serious shape distortion results exactly as if the object were not parallel. ANGULATION Angulation refers to the direction and degree the tube is moved from its  normal position perpendicular to the image receptor. Numerous radiographic projections utilize angulation to avoid superimposition of parts. The semiaxial AP projection of the cranium, tangential calcaneus, and axial clavicle are all examples. The angulation of the tube is designed to cause a controlled or expected amount of shape distortion to avoid superimposition. As long as the specified angulation is applied, the image is comparable to norms and is of diagnostic quality. Luterte, jeanne Once the PSP is exposed, the BSRT-II cassette is taken to a reader to Brightness -digital display, process the plate and create the Density -film image. Digital radiography (DR) systems typically have the detector and reader that are a permanent part of a table or wall unit; therefore, a cassette is not needed. With DR systems, the image is acquired and sent directly to the display monitor without the need for the radiographer to physically move the detector for the image to be processed. This makes these systems faster and more efficient at creating an image. IR exposure - digital Digital radiography imaging systems replace traditionalfilm with a reusable detector. These systems are divided into two types generally known as computed DR SYSTEM radiography (CR) and digital There are a number of detector radiography (DR). configurations used in DR that Computed radiography systems employ either direct conversion (without scintillator) uses a photostimulable storage indirect conversion (with phosphor imaging plate (PSP or scintillator). Indirect conversion IP) typically inside a cassette. This detectors use a two-part process cassette can be used in a bucky or involving a scintillator (which for portable exams, similar to converts incoming x-ray traditional film/screen systems. photons to light) and a photodetector (which converts IR exposure - digital light into an electronic signal).  Direct conversion detectors Each box of an image matrix will directly convert incoming x-ray display a numerical value that can be photons to an electronic signal. transformed into a visual brightness These systems use amorphous or density level. selenium and a TFT (thin film transistor) DIGITAL IMAGE FORMATION A digital image is one that has been converted into numerical values for transmission or processing. Digital radiography systems have replaced traditional film with a reusable detector. Computers operate from a binary machine language. Just as English has a The individual matrix boxes are 26-letter-symbol alphabet, the binary known as picture elements or pixels. system operates with a two- symbol Each pixel location is determined by alphabet. Because electrical currents are its address. most easily understood as being either on The total number of pixels in the or off, the binary system consists of matrix would be calculated by information recorded as either a 0 for off multiplying the number of boxes in or a 1 for on. the row by the number of boxes in Each binary number is called a bit, for the column. binary digit. For example, a matrix of 512 x 512 would have a total of 262,144 pixels that form the image. In medical imaging, each pixel represents a three-dimensional volume of tissue known as a voxel. DIGITAL IMAGE CHARACTERISTIC Computerized digital images are described in terms of the number of values displayed per image. A matrix is a series of boxes laid out in rows and columns that gives form to the image  The overall dimension of the image matrix is called the field of view (FOV). In digital radiography systems, the FOV is determined by the size of the detector, whereas in other modalities, such as CT and MRI, the operatorcan select the FOV for a particular study. Each pixel in the matrix is capable of representing a wide range of different shades of gray from pure white through total black. Windowing is a point CR SYSTEM(PSP) processing operation that Computed radiography (CR) uses a changes the contrast and photostimulable storage phosphor brightness of the image on the imaging plate (PSP or IP), typically monitor. inside a cassette. The brightness (density) and CR first became available in the early 1980s when contrast of the digital image it was introduced by Fuji, but, as happens with depends on the shades of gray, many new technologies, there were problems which are controlled by varying with high cost and poor image quality the numerical values of each The cassette-based CR with the PSP pixel. requires a reader to process the plate and create the image and is a two-step process because the radiographer must move the detector between image acquisition and display.  layer holds the photostimulable phosphor, which is the active component in the plate. The support layer is simply a base on which to coat the other layers. The conductor layer grounds the plate to eliminate electrostatic problems and absorb light to increase sharpness. Finally, the light-shielding layer prevents light from erasing data on the imaging plate or leaking through the backing, decreasing the spatial resolution. The imaging Photostimulable Imaging Plates plate is loaded into a cassette that A photostimulable phosphor looks much like a radiographic imaging plate is a rigid sheet with film and intensifying screen several layers that are designed to cassette. record and enhance transmission Consequently, computed of the image from a beam of radiography cassettes are ionizing radiation. sometimes referred to as “filmless The layers include a protective cassettes. layer, a phosphor layer, a support In order for CR to function, the layer made of polyester, a imaging plate material must have conductor layer, and a lightshield the ability to store and release the layer image information in a usable form. The most common phosphors with characteristics favorable for CR are barium fluorohalide bromides and iodides with europium activators Image acquisition The protective layer simply Image acquisition begins with insulates the imaging plate from x-ray exposure to the imaging handling trauma. The phosphor  plate. Because the imaging plate is placed in a cassette, it can be used tabletop or with a grid, similar to the use of film/screen. It is this stored energy that is used to create an image during reading and processing. Some of the electrons, which are excited by the absorbed energy, are trapped in the crystal structure of the phosphor at higher energy levels. Thus a latent image is stored in the imaging plate, similar to a latent image on film. Latent Image The imaging plate then needs to be read to release the stored information, which can be manipulated by the computer and used in either soft- or hard-copy form. The latent image will lose about 25 percent of its energy in 8 hours, so it is important to process the cassette shortly after exposure. Cassettes stored for several days after exposure and before processing lose most of their latent image. The latent image is processed by loading the cassette into an image reader device (IRD) where the imaging plate is scanned by a helium-neon laser beam. Reading These laser beam scans cause the phosphors to emit the stored latent image in the form of light photons, which are detected by photosensitive receptors and converted to an electrical signal, which is in turn converted to a unique digital value for that level of luminescence.