Image Quality Geometry 1. 130 kVp chest radiographs differ from those made at 80 kVp because: A. Those at 130 kV have more contrast. B. Those at 130 kV have less quantum mottle. C. Grids are not necessary at high kVp. D. The difference in mass attenuation coefficients of tissue and bone is smaller at 130 kV. E. All of the above. 2. Changing the focal spot from 0.6 mm to 1.0 mm has the following effect: A. Heat loading capacity of the anode is decreased. B. Geometric unsharpness of the image is improved. C. High contrast spatial resolution of the image is improved. D. Available exposure times at a given mA station are increased. E. Patient dose decreases. 3. Changes in subject contrast are affected by changes in: 1. Film-screen contact A. 1 only 2. Tissue atomic number B. Both 3 and 5 3. Object-film distance C. 2 only 4. Beam quality D. Both 2 and 4 5. Development time and temperature E. All of the above 4. The lower left rib area is consistently blurry on images from a dedicated chest unit. The most probable cause is: A. Patient motion. B. Anode heel effect. C. Screen-film contact. D. Off-focus radiation. E. Bad processor rollers. 5. A phototimed chest unit consistently produces dark PA radiographs. The most likely reason for the change is: A. The back-up timer is set to a short time. B. Patient is mispositioned off the AEC ion chambers. C. The calibration of the left/right phototimer detectors has changed. D. The back-up timer is set for too long a time. E. The processor temperature has decreased. 6. All of the following factors cause non-uniform radiographic film density, except: A. Heel effect with 14” x 17” film, 11˚ anode angle, and 40”SID. B. Use of high ratio parallel grids for short SIDs. C. Inverted focused grids. D. Worn cassette screens. E. Phototimer drift. 7. If an imaging system has a resolving power of about 5 lp/mm, this represents system MTF response at about: A. 90%. B. 75%. C. 50%. D. 20%. E. 10%. 8. The sharpness of a radiograph will decrease when: A. Switching from a large to a small focal spot when magnification = 1.5. B. Moving the x-ray tube of a C-arm closer to the patient. C. Changing from single to three-phase using the same kVp and mA. D. Changing from a dual screen to a thin single screen system for extremity examination. 9. Which of the following steps will decrease noise in the resultant image? A. Increasing the intensifying screen conversion efficiency. B. Increasing the film processor temperature. C. Changing the CT reconstruction algorithm from “soft tissue” to “bone”. D. Frame averaging in digital subtraction angiography. E. None of the above. 10. The following change is least likely to reduce motion unsharpness in a radiograph: A. Increasing the mA and maintaining the mAs. B. Decreasing the focal spot to 0.1 mm and maintaining the mAs. C. Changing to a faster screen and maintaining the image density. D. Changing to a faster film and maintaining the image density. E. Using appropriate patient restraints. 11. A measurement of the cardiac dimension is obtained from a chest film. The SID is 72” and the heart is midway in a 14” thick chest. The distance between the chest changer surface and the film is 3”. The dimensions on the radiographs are ______ than the actual anatomy. A. 32% larger B. 16% larger C. 8% smaller D. 16% smaller E. 32% smaller 12. In a diagnostic radiograph the process mostly responsible for differential attenuation is: A. Coherent scatter. B. Compton interaction. C. Photoelectric interaction D. Pair production. 13. A radiograph has little contrast from one region to the next. Which would improve contrast in a “retake” film? 1. Change to higher ratio grid. A. 1,3 2. Move the film closer to the patient. B. 1.4 3. Collimate the beam to as small a field as possible. C. 2.3 4. Raise the kV to lower the exposure time. D. 1,2,4 E.. 1,2,3,4 14. Low contrast detectability refers to the ability of a system to distinguish (as an example): A. A calcified lung nodule. B. A non-calcified lung nodule. C. Between overlying and underlying tissues. D. The size of a small fracture. E. Vessels during the arterial phase of a normal angiogram. 15. The modulation transfer function (MTF) is a tool for describing the: A. Properties of the H&D curve of an imaging system. B. Noise content of an imaging system. C. Latitude of an imaging system. D. Effect of blurring on the by imaging system components. 16. L-4 is radiographed at 100 cm SID and an object-to-image distance (OID) of 20 cm. The width of L-4 measured on the radiograph is 35 mm. The true width is: A. 25 mm B. 28 mm C. 30 mm D. 35 mm E. 44 mm 17. A resolution test pattern in the center of the x-ray field and 4.5 cm from the image receptor shows a limiting resolution of 11 lp/mm; what resolution is expected if the pattern is moved to the anode-side edge of the field? A. 1 lp/mm B. 10 lp/mm C. 11 lp/mm D. 12 lp/mm E. 20 lp/mm 18-22. Match the test tool with the appropriate test: 18. Aluminum filters A. Focal Spot Size 19. Line pair or mesh pattern B. Mr/mAs 20. Lead markers or coins C. HVL 21. Ion chamber dosimeter D. Light/X-ray field congruence 22.. Slit camera or star pattern E. Fluoroscopic resolution 23. Breast microcalcifications are seldom seen on routine chest radiographs because: A. The film size is too large. B. Of an increased mass attenuation coefficient for soft tissue at chest radiographic kVps. C. Of a decreased mass attenuation coefficient for soft tissue at chest radiographic kVps. D. Of an increased mass attenuation coefficient for Calcium at chest radiographic kVps. E. Of a decreased mass attenuation coefficient for Calcium at chest radiographic kVps. 24. Geometric magnification can improve detection of high contrast objects. The limit on useful magnification is: A. Blurring due to focal spot size. B. Blurring due to removal of the grid. C. H&D curve of the image receptor. D. MTF of the image receptor. E. Size of the image receptor. 25. In nuclear medicine, the modulation transfer function (MTF) represents: A. The energy resolution of the imaging system. B. The sensitivity of the imaging system. C. The overall spatial resolution characteristics of the system. D. The full width at half maximum of the line spread function. E. The temporal resolution of the system. 26. Methods of assessing spatial resolution of an imaging system include all of the following except: A. Bar patterns. B. Step wedges. C. Wire mesh pattern. D. Hold pattern. E. Wire impulse response. 27. Which imaging system has the highest spatial resolution? A. Direct film extremity radiograph. B. TV of an R/F unit. C. Digital Angiography. D. Screen-film radiography E. CT 28. Regarding geometric blur, which of the following is false? A. Inversely proportional to focal spot size. B. Directly proportional to object-film distance. C. Inversely proportional to focal spot-object distance. D. Characterized by penumbra width. 29. Noise in an x-ray image is: A. Increased by increasing the film speed in a screen-film cassette. B. Decreased by increasing the film speed in a screen-film cassette. C. Increased by decreasing the focal-spot size. D. Decreased by decreasing the focal-spot size. E. Mainly determined by imperfections in the image receptor. 30. The penumbra associated with the image of an object edge placed 50 cm above the film with an SID of 100 cm and a focal spot size of 1.0 mm is ______ mm. A. 0.01 B. 0.1 C. 1.0 D. 10 31. Concerning linear blurring tomography, which of the following is false? A. It uses a conventional grid. B. It achieves a section thickness of about 2 mm using a wide angle. C. It requires a higher patient x-ray exposure than a plain film. D. It requires long exposure times for thin sections. E. It yields very high contrast images for thin sections. Image Quality Geometry – answers 1. D At relatively high kVp, the photoelectric attenuation is minimal and the Compton attenuation completely dominates; thus differential attenuation is less. 2. D A larger focal spot distributes the instantaneous heat load over larger a area, increasing the anode heat loading capacity and thus the available exposure times at a given mA station. Unsharpness and resolution are degraded. The dose remains the same since the total mAs required is uncharted. 3. D Changes in subject contrast are a function of variations in tissue and beam quality. Lower subject contrast is expected at increased beam energy (higher quality, less photoelectric effect), smaller tissue thickness, lower tissue density, and lower atomic number. Other film/screen and processor variation will affect radiographic contrast (overall image contrast) but not subject contrast. 4. C Consistent blurriness in the same area is from poor screen-film contact in that area of the screen. 5. C A and E would result in decreased film density. B would not yield consistently dark images. D should have no effect, unless the phototimer malfunctions. Calibration drifts are the most common source of consistently dark or light phototimed radiographs. 6. E The heel effect causes a lowered film density toward the anode side of the film that can be dramatic near the edge of large films at shorter SID if the anode angle is small enough. High ratio parallel grids produce film density which decreases away from the center due to grid cut-off. Inverted grids only have density in the central region. Worn screens produce spots of low density in the worn regions. Phototimer drifts uniformly increase or decrease the film density. 7. E The 10% point on the MTF curve represents the limit of resolution that the eye observes with a lead line pair test pattern, the typical method of determining resolving power. 8. B Geometric unsharpness = (focal spot size) x (magnification – 1). If the C-arm x-ray tube is closer to the patient, the magnification increases causing more unsharpness. 3-phase generators allow increased x-ray output per mA, resulting in shorter exposure times and less motion blur. In contact radiography (no magnification), screen determines image sharpness. Single thin screen is sharper. 9. D Frame averaging in DSA decreases noise by a factor of N 1/2, where N is the number of averaged frames. Increased conversion efficiency means more light is produced from fewer x-rays, and hence more noise. A higher processor temperature means faster film speed, fewer x-rays needed; hence more noise. A bone algorithm is a sharpening filter, which increases noise. 10. B B will generally require a decrease in current, which will require a longer exposure time. A, C, D will allow a reduction in exposure time. 11. B The heart is situated 10” from the film. The magnification factor is equal to (72/62) = 1.16. The image on the film is larger than the object. 12. C The probability of a photoelectric interaction is proportional to Z3 whereas Compton scatter is approx independent of Z. Thus, small differences in Z between materials cause large differences in the number of photoelectric interactions occuring at low energies where they are most common. 13. A Radiographic contrast can be improved by use of a higher ratio grid to reduce scatter components reaching the film. Similarly, reducing the primary beam size to a minimum encompassing the anatomy to be imaged further reduces the scatter by lowering the volume in which it can occur. 14 B This term represents the ability of a system to reproduce an object whose linear attenuation coefficient does not vary greatly from the surrounding material. 15. D 16. B The magnification is M = SID / (SID – OID), or 100 / (100 – 20) = 1.25. The image will be magnified by M = 1.25 on the radiograph, so the true size is 35 / 1.25 = 28. 17. D The effective focal spot will be smaller at the anode-side of the x-ray field (line-focus principle), Therefore, the limiting resolution should be somewhat greater at the anode side (but not doubled). 18. C HVL is measured by adding enough aluminum in the beam to decrease its exposure rate to 1/2. 19. E Fluoroscopic resolution is measured in terms of line pairs per mm with a lead bar pattern, or in holes per inch using copper mesh. 20. D Coins or markers are lined up at the edges of the light field and an exposure made to see if the x- ray field edges are well aligned with the light field. They are hardly ever perfect. 21. B At a given kVp, the output is measured with an ion chamber and divided by the mAs. This number should be constant for all mA and mAs stations if the generator is well calibrated. This is important because the technologist assumes linearity when setting patient techniques. 22. A Focal spot size can be measured with a pinhole camera, slit camera, or star test pattern. 23. E Mammograms and chest x-rays are typically performed at 25 kVp and 100 kVp respectively. The mass attenuation coefficient of Ca decreases rapidly over this energy range due to the decline in its photoelectric component. The mass attenuation coefficient of soft tissue does not change dramatically over this range. 24. A Penumbra, caused by a finite focal spot, increases with magnification. Eventually this dominates the image. The grid, H&D curve, and size have no effect on magnification. The receptor’s MTF becomes less important as magnification increases. 25. C In nuclear medicine it is usual to state the MTF of the system. Temporal resolution is known as the “dead time.” Sensitivity is a function of crustal thickness and collimator septa size. Energy resolution is related to the FWHM of the photopeak. The line spread function is related to the MTF of the crystal (intrinsic spatial resolution) or the crystal plus collimator (extrinsic resolution). 26. B Step wedges are used to create a gray scale used to evaluate contrast of the image receptor system. 27. A Typical resolutions are: CT = 0.5 to 1.0 lp/mm 525 line TV with 6” II = 1.8 to 2.0 lp/mm DVI is usually equal to but possibly less than TV Screen-film is typically 4 to 10 lp/mm Direct film exposures resolution can exceed 50 lp/mm. 28. A Geometric blur is reduced by minimizing magnification and increases with focal spot size. 29. A Increasing the film speed in a screen-film cassette reduces the number of x-ray photons absorbed by the screens to produce the same density on the film. Thus, quantum noise increases. Noise is due to the statistics of x-ray detection; it is therefore independent of focal-spot size. 30. C The penumbra = focal spot size, since source-object and object-film distances are equal. 31. E Wide-angle tomography produces thin sections which have low contrast. This is because all the anatomy above and below the section in focus is blurred across the image with an effect similar to fog. The patient exposure is higher because most of the time the x-ray beam is passing through the patient at an angle, i.e., the patient is effectively thicker.