Concepts of Radiographic Image Quality PDF

PRINCIPLES OF IMAGING James Lawrence Aduche, RRT, RSO, MSRT (ip) Associate Professor, Southwestern University – PHINMA License No: 0023749 TOPIC OUTLINE: Radiographic Image Quality Resolution Noise Speed Film Factors Geometric Factors...

PRINCIPLES OF IMAGING James Lawrence Aduche, RRT, RSO, MSRT (ip) Associate Professor, Southwestern University – PHINMA License No: 0023749 TOPIC OUTLINE: Radiographic Image Quality Resolution Noise Speed Film Factors Geometric Factors Subject Factors Tools for Improved Radiographic Image Quality Concepts of Radiographic Image Quality RADIOGRAPHIC IMAGE QUALITY RADIOGRAPHIC IMAGE QUALITY is the exactness of representation of the patient’s anatomy on a radiographic image. High-quality images are required so that radiologists can make accurate diagnoses. 5 RADIOGRAPHIC IMAGE QUALITY To produce high-quality images, radiographers apply knowledge of the three major interrelated categories of radiographic quality: film factors, geometric factors, and subject factors. Each of these factors influences the quality of a radiographic image, and each is under the control of radiologic technologists. 6 RADIOGRAPHIC IMAGE QUALITY Refers to the fidelity with which the anatomical structure that is being examined is rendered on the radiograph. 7 RADIOGRAPHIC IMAGE QUALITY SPATIAL NOISE RESOLUTION IMAGE QUALITY CONTRAST ARTIFACTS RESOLUTION 8 RESOLUTION Resolution is the ability to image two separate objects and visually distinguish one from the other. Spatial resolution refers to the ability to image small objects that have high subject contrast, such as a bone–soft tissue interface, a breast microcalcification, or a calcified lung nodule. 9 RESOLUTION Screen Blur Spatial resolution Motion Blur Geometric Blur 10 CONTRAST RESOLUTION Contrast resolution is the ability to distinguish anatomical structures of similar subject contrast such as liver–spleen and gray matter–white matter. 11 DETAIL Detail / Recorded detail - refer to the degree of sharpness of structural lines on a radiograph. Visibility of detail refers to the ability to visualize recorded detail when image contrast and optical density (OD) are optimized. 12 NOISE Radiographic noise is the random fluctuation in the OD of the image. NOISE CONTRAST RESOLUTION 13 4 COMPONENTS OF NOISE NOISE FILM STRUCTURE QUANTUM SCATTER GRAININESS MOTTLE RADIATION MOTTLE 14 FILM GRAININESS Film graininess refers to the distribution in size and space of silver halide grains in the emulsion. Structure mottle refers to the size and space distribution of phosphor of the radiographic intensifying screen. Film graininess and structure mottle are inherent in the screen-film image receptor. 15 FILM GRAININESS Quantum mottle refers to the random nature by which x-rays interact with the image receptor. – principal contributor to noise The use of high-mAs, low-kVp and of slower image receptors reduces quantum mottle. 16 17 18 19 SPEED It describe the sensitivity of film to x-rays. 20 RADIOGRAPHIC QUALITY RULES Resolution, noise, and speed are interrelated characteristics of radiographic RESOLUTION quality. 1. Fast image receptors have high noise, and low spatial resolution and low contrast resolution. 2. High spatial resolution and high contrast resolution require low noise and slow image receptors. 3. Low noise accompanies slow image NOISE SPEED receptors with high spatial resolution and high contrast resolution. 21  Organization chart of principal factors that may affect radiographic quality. In general, the quality of a radiograph is directly related to an understanding of the basic principles of x-ray physics and the factors that affect radiographic quality. 22 FILM FACTORS FILM FACTORS The study of the relationship between the intensity of exposure of the film and the blackness after processing is called sensitometry. 24  SPEED CONTRAST PROCESSING TIME FILM FACTORS PROCESSING DENSITY TEMPERATURE 25 LATITUDE FILM FACTORS The two principal measurements involved in sensitometry are the exposure to the film and the percentage of light transmitted through the processed film. 26 CHARACTERISTIC CURVE H & D curve Hurter and Driffield It describe the relationship between OD and radiation exposure. 27  Toe – low radiation exposure level Straight line portion – intermediate radiation exposure level – the region in which a properly exposed radiograph appears Shoulder – high radiation exposure level 28 CHARACTERISTIC CURVE Apparatus that are needed to construct a characteristic curve: 1. Optical step wedge / sensitometer 2. Densitometer - a device that measures OD 29  Steps involved in the construction of a characteristic curve. 30 OPTICAL DENSITY The degree of blackening on the radiograph. It is a logarithmic function. Has a precise numeric value that can be calculated if the level of light incident on a processed film (Io) and the level of light transmitted through that film (It) are measured. 31 OPTICAL DENSITY Question: – The lung field of a chest radiograph transmits only 0.15% of incident light as determined with a densitometer. What is the OD? Answer: 32 OPTICAL DENSITY Question: – The OD of a region of a lung field is 2.5. What percentage of visible light is transmitted through that region of the image? Answer: – Reference to Table 10-1 shows that an OD = 2.5 is equal to 2 of every 625 light photons that are being transmitted, or 0.32%. ODs of unexposed film are attributable to base density and fog density 33 BASE DENSITY Base density is attributable to the composition of the base and the tint added to the base to make the radiograph more pleasing to the eye. Base density has a value of approximately 0.1. 34 FOG DENSITY Fog density results from inadvertent exposure of film during storage, undesirable chemical contamination, improper processing, and a number of other influences. Fog density on a processed radiograph should not exceed 0.1. 35 FOG DENSITY Higher fog density reduces the contrast of the radiographic image. 36  The useful range of OD is approximately 0.25 to 2.5. The most useful range of OD is highly dependent on viewbox illumination, the viewing conditions, and the shape of the characteristic curve. Base plus fog OD has a range of approximately 0.1 to 0.3. 37 38 RECIPROCITY LAW The reciprocity law states that the OD on a radiograph is proportional only to the total energy imparted to the radiographic film and independent of the time of exposure. Whether a radiograph is made with short exposure time or long exposure time, the reciprocity law states that the OD will be the same if the mAs value is constant. 39 CONTRAST The difference in optical density. – high contrast - radiograph that has marked differences in OD – low contrast - the OD differences are small and are not distinct 40  CONTRAST FIGURE 10-9 This vicious guard dog is posed to demonstrate differences in contrast. A, Low contrast. B, Moderate contrast. C, High contrast. (CourtesyButterscotch.) Radiographic contrast is the product of image receptor contrast and subject contrast. 41 CONTRAST 42 CONTRAST Image receptor contrast is inherent in the screen-film combination and is influenced somewhat by processing of the film. Subject contrast is determined by the size, shape, and x-ray attenuating characteristics of the anatomy that is being examined and the energy (kVp) of the x-ray beam. 43 44 AVERAGE GRADIENT Used to numerically specify the image receptor contrast. The average gradient is the slope of a straight line drawn between two points on the characteristic curve at ODs 0.25 and 2.0 above base and fog densities. This is the approximate useful range of OD on most radiographs. 45  AVERAGE GRADIENT Most radiographic image receptors have an average gradient in the range of 2.5 to 3.5. 46 QUESTION A radiographic film has a base density of 0.06 and a fog density of 0.11. At what ODs should one evaluate the characteristic curve to determine the film contrast? Answer: – The curve should be evaluated at OD 0.25 and 2.0 above base plus fog densities. – Therefore, at OD of OD1 = 0.06 + 0.11 + 0.25 = 0.42 and OD2 = 0.06 + 0.11 + 2.0 = 2.17 47 AVERAGE GRADIENT Image receptor contrast also may be identified by gradient. The gradient is the slope of the tangent at any point on the characteristic curve. Toe gradient is probably more important than average gradient for general radiography because many clinical ODs appear in the toe region of the characteristic curve. Midgradient or shoulder gradient is more important for mammography. 48 SPEED Fast/high speed IR - more than 100 Par speed - 100 Slow/low/detail IR - less than 100 Slow speed → less noise → more patient dose Fast speed → more noise → less patient dose 49 QUESTION If the ODs of 0.42 and 2.17 on the characteristic curve in the preceding example correspond to LREs of 0.95 and 1.75, what is the average gradient? 50 Question: How much exposure is required to produce an OD of 1.0 above base plus fog density on a 600-speed image receptor? 51 SPEED When image receptors are replaced, a change in the mAs setting may be necessary to maintain the same OD. For example, if image receptor speed is doubled, the mAs must be halved. No change is required in kVp. 52 Question: A PA chest examination requires 120kVp/8 mAs with a 250-speed image receptor. What radiographic technique should be used with a 400-speed image receptor? 53 LATITUDE Latitude refers to the range of exposures over which the image receptor responds with ODs in the diagnostically useful range. Latitude also can be thought of as the margin of error in technical factors. With wider latitude, mAs can vary more and still produce a diagnostic image. 54 LATITUDE Image receptor B responds to a much wider range of exposures than A and therefore has a wider latitude than A. 55 LATITUDE Wide latitude → long gray scale → low contrast Narrow latitude → short gray scale → high contrast 56 FILM PROCESSING Proper film processing is required for optimal image receptor contrast because the degree of development has a pronounced effect on the level of fog density and on the ODs resulting from a given exposure at a given image receptor speed. 57 FACTORS THAT MAY AFFECT THE FINISHED RADIOGRAPH 1 CONCENTRATION OF PROCESSING CHEMICALS DEVELOPMENT TEMPERATURE 4 2 DEGREE OF CHEMISTRY AGITATION DURING DEVELOPMENT DEVELOPMENT TIME 3 58 FILM PROCESSING Because development time is varied, the characteristic curve for any film changes in shape and position along the LRE axis. 59 LATITUDE Analysis of characteristic curves at various development times and temperatures yields relationships for contrast, speed, and fog for 90- second automatically processed film. 60 FILM PROCESSING DEVELOPMENT TIME ↑ Development time ↑ IR speed and fog ↓ IR contrast DEVELOPMENT TEMP IR speed Development temperature Fog 61 GEOMETRIC FACTORS GEOMETRIC FACTORS MAGNIFICATION GEOMETRIC DISTORTION FACTORS FOCAL SPOT BLUR 63 MAGNIFICATION All images on the radiograph are larger than the objects they represent. The smallest magnification possible should be maintained. Quantitatively, magnification is expressed by the magnification factor (MF). 64 MAGNIFICATION For most radiographs taken at a source to-image receptor distance (SID) of: 100 cm the MF is approximately 1.1 180 cm the MF is approximately 1.05 65 66  Question: – If a heart measures 12.5 cm at its maximum width and its image on a chest radiograph measures 14.7 cm, what is the MF? 67  Question: – A renal calculus measures 1.2 cm on the radiograph. The SID is 100 cm, and the SOD is estimated at 92 cm. What is the size of the calculus? 68  Question: – A lateral view of the lumbar spine taken at 100 cm SID results in the image of a vertebral body with maximum and minimum dimensions of 6.4 cm and 4.2 cm, respectively. What is the object size if the vertebral body is 25 cm from the image receptor? 69  The SID is standard in most radiology departments: – 180cm (72’’) for chest imaging – 100cm (40”) for routine examinations – 90cm (35”) some special studies, such as mobile radiography and trauma radiography. – 50 to 70 cm for dedicated mammography imaging systems 70 DISTORTION Three conditions contribute to image distortion: 1. Object thickness 2. Object position 3. Object shape 71 DISTORTION Object Thickness – Thick objects are more distorted than thin objects. – With a thick object, the OID changes measurably across the object. Consider, for instance, two rectangular structures of different thicknesses 72 DISTORTION When positioned on the central axis, the images of both objects appear as circles. The image of the sphere appears less distinct because of its varying thickness, but it does appear circular. 73 DISTORTION If the object plane and the image plane are parallel, the image is not distorted. However, distortion is possible in every radiographic examination if the patient is not properly positioned. 74 DISTORTION Object Position – If the object plane and the image plane are not parallel, distortion occurs. – Foreshortening - reduction in image size; related to the angle of inclination of the object – Elongation - image appear longer than it really is 75 DISTORTION If an inclined object is not located on the central x-ray beam, the degree of distortion is affected by the object’s angle of inclination and its lateral position from the central axis. 76 DISTORTION With multiple objects positioned at various OIDs, spatial distortion can occur. Spatial distortion – Is the misrepresentation in the image of the actual spatial relationships among objects 77 FOCAL SPOT BLUR Focal-spot blur occurs because the focal spot is not a point. Focal-spot blur is the most important factor for determining spatial resolution. 78 FOCAL SPOT BLUR Focal-spot blur occurs because the focal spot is not a point. Focal-spot blur is the most important factor for determining spatial resolution. 79 FOCAL SPOT BLUR If an arrowhead were positioned near the x-ray tube target, the size of the focal spot blur would be larger than that of the effective focal spot. In general, the object is much closer to the image receptor; therefore, the focal-spot blur is much smaller than the effective focal spot. 80  Question: – An x-ray tube target with a 0.6-mm effective focal spot is used to image a calcified nodule estimated to be 8 cm from the anterior chest wall. If the radiograph is taken in a PA projection at 180 cm SID with a tabletop to image receptor separation of 5 cm, what will be the size of the focal-spot blur? 81 FOCAL SPOT BLUR § To minimize focal-spot blur, you should use: Focal Spot OID SID § High-contrast objects that are smaller than the focal-spot blur normally cannot be imaged. 82 HEEL EFFECT Described as varying radiation intensity across the x-ray field in the anode–cathode direction caused by attenuation of x-rays in the heel of the anode. Another characteristic of the heel effect is unrelated to x-ray intensity but affects focal- spot blur. An x-ray tube said to have a 1-mm focal spot, has a smaller effective focal spot on the anode side and a larger effective focal spot on the cathode side. 83 HEEL EFFECT The FSB is small on the anode side and large on the cathode side of the image. Images toward the cathode side of a radiograph have a higher degree of blur and poorer spatial resolution than those to the anode side. This is clinically significant when x-ray tubes with small target angles are used at short SID’s 84 HEEL EFFECT This variation in focal-spot size results in variation in focal-spot blur. Consequently, images toward the cathode side of a radiograph have a higher degree of blur and poorer spatial resolution than those to the anode side. This is clinically significant when x-ray tubes with small target angles are used at short SIDs. 85 HEEL EFFECT Patient Positioning for Examinations That Can Take Advantage of the Heel Effect 86 SUBJECT FACTORS SUBJECT FACTORS TISSUE MASS DENSITY EFFECTIVE SUBJECT ATOMIC NUMBER CONTRAST KILOVOLTAGE PEAK PATIENT THICKNESS OBJECT SHAPE 88 SUBJECT CONTRAST The contrast of a radiograph viewed on an illuminator is called radiographic contrast. Radiographic contrast is a function of image receptor contrast and subject contrast. In fact, radiographic contrast is simply the product of image receptor contrast and subject contrast. 89  SUBJECT CONTRAST Question: – Screen film with an average gradient of 3.1 is used to radiograph a long bone with subject contrast of 4.5. What is the radiographic contrast? 90 PATIENT THICKNESS Given a standard composition, a thick body section attenuates a greater number of x-rays than does a thin body section. The degree of subject contrast is directly proportional to the relative number of x- rays leaving those sections of the body. 91 TISSUE MASS DENSITY Different sections of the body may have equal thicknesses yet different mass densities. Tissue mass density is an important factor that affects subject contrast. These materials have the same thickness and chemical composition. However, they have slightly different mass density from water and therefore will be imaged. 92 TISSUE MASS DENSITY The effect of mass density on subject contras 93 EFFECTIVE ATOMIC NUMBER When the effective atomic number of adjacent tissues is very much different, subject contrast is very high. Subject contrast can be enhanced greatly by the use of contrast media. The high atomic numbers of iodine and barium result in extremely high subject contrast. Contrast media are effective because they accentuate subject contrast through enhanced photoelectric absorption. 94 OBJECT SHAPE The shape of the anatomical structure under investigation influences its radiographic quality, not only through its geometry but also through its contribution to subject contrast. Structure that has a form that coincides with the x-ray beam has maximum subject contrast 95 OBJECT SHAPE This characteristic of the subject that affects subject contrast is sometimes called absorption blur. It reduces the spatial resolution and the contrast resolution of any anatomical structure, but it is most troublesome during interventional procedures in which vessels with small diameters are examined. 96 kVp The absolute magnitude of subject contrast, however, is greatly controlled by the kVp of operation. kVp also influences film contrast but not to the extent that it controls subject contrast. kVp is the most important influence on subject contrast. 97 kVp ↑ kVp ↓ subject contrast - Long gray scale ↓ kVp ↑ subject contrast - Short gray scale Low kVp radiography has two major disadvantages: 1. As the kVp is lowered for any radiographic examination, the x-ray beam becomes less penetrating, requiring a higher mAs to produce an acceptable range of ODs. The result is higher patient dose. 98 kVp 2. A radiographic technique that produces low subject contrast allows for wide latitude in exposure factors. Optimization of radiographic technique by mAs selection is not so critical when high kVp is used. 99 MOTION BLUR Movement of the patient or the x-ray tube during exposure results in blurring of the radiographic image. The radiographer can reduce motion blur by carefully instructing the patient, “Take a deep breath and hold it. Don’t move.” 100 MOTION BLUR Patient motion of two types may occur. 1. Voluntary motion of the limbs and muscles is controlled by immobilization. 2. Involuntary motion of the heart and lungs is controlled by short exposure time. 101 PROCEDURES FOR REDUCING MOTION BLUR Use the shortest Restrict patient motion by Use a large SID Use a small OID possible exposure providing instruction or time. using a restraining device. 102  TOOLS FOR IMPROVED RADIOGRAPHIC IMAGE QUALITY PATIENT POSITIONING Proper patient positioning requires that the anatomical structure under investigation be placed as close to the image receptor as is practical and that the axis of this structure should lie in a plane that is parallel to the plane of the image receptor. The central ray should be incident on the center of the structure. Finally, the patient must be immobilized effectively to minimize motion blur 104 IMAGE RECEPTORS A standard type of screen-film image receptor is used throughout a radiology department for a given type of examination. In general, extremity and soft tissue radiographs are taken with fine- detail screen-film combinations. Most other radiographs use double-emulsion film with screens. The new, structured-grain x-ray films used with high-resolution intensifying screens produce exquisite images with limited patient dose. 105 PRINCIPLES TO BE CONSIDERED WHEN PLANNING A PARTICULAR EXAMINATION: 1 Use of intensifying screens decreases patient dose by a factor of at least 20. 2 As the speed of the image receptor increases, radiographic noise increases, and spatial resolution is reduced. 3 Low-contrast imaging procedures have wider latitude, or margin of error, in producing an acceptable radiograph. 106 SELECTION OF TECHNIQUE FACTORS Before each examination, the radiologic technologist must select the optimum radiographic technique factors, that is, kVp, mAs, and exposure time. Many considerations determine the value of each of these factors, and they are complexly interrelated. One generalization that can be made for all radiographic exposures is that the time of exposure should be as short as possible. Image quality is improved by short exposure times that cause reduced motion blur. 107 SELECTION OF TECHNIQUE FACTORS One of the reasons why three-phase and high-frequency generators are better than single-phase generators is that shorter exposure times are possible with the former. 108 Thank You! James Lawrence Aduche, RRT, RSO, MSRT (ip) Associate Professor, Southwestern University – PHINMA Jbaduche.swu.phinaed.com