Image Quality Geometry - DOC by malj


									Image Quality Geometry

1.   130 kVp chest radiographs differ fro m 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. Geo metric unsharpness of the image is imp roved.
     C. High contrast spatial resolution of the image is imp roved.
     D. Available exposure times at a g iven 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 nu mber                                   B.       Both 3 and 5
     3. Object-film distance                                    C.       2 only
     4. Beam quality                                            D.       Both 2 and 4
     5. Develop ment time and temperature                       E.       All of the above

4.   The lower left rib area is consistently blurry on images fro m 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 rad iographs. 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 calibrat ion 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 rad iographic 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 imag ing 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 fro m a large to a small focal spot when magnificat ion = 1.5.
     B.   Moving the x-ray tube of a C-arm closer to the patient.
     C.   Changing fro m single to three-phase using the same kVp and mA.
     D.   Changing fro m a dual screen to a thin single screen system for extremity examination.
9.   Which of the follo wing 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 algorith m fro m “soft tissue” to “bone”.
     D.  Frame averag ing in dig ital 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 maintain ing the mAs.
    B.    Decreasing the focal spot to 0.1 mm and maintaining the mAs.
    C.    Changing to a faster screen and maintain ing the image density.
    D.    Changing to a faster film and maintain ing the image density.
    E.    Using appropriate patient restraints.

11. A measurement of the cardiac d imension is obtained fro m 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% s maller
    D. 16% smaller
    E. 32% smaller

12. In a diagnostic radiograph the process mostly responsible for differential attenuation is:
    A. Coherent scatter.
    B. Co mpton interaction.
    C. Photoelectric interaction
    D. Pair production.

13. A radiograph has little contrast from one region to the next. Which would imp rove 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 examp le):
    A. A calcified lung nodule.
    B.   A non-calcified lung nodule.
    C.   Between overly ing and underlying tissues.
    D.   The size of a small fracture.
    E.   Vessels during the arterial phase of a normal angiogram.

15. The modu lation transfer function (MTF) is a tool for describ ing the:
    A. Properties of the H&D cu rve of an imaging system.
    B.   Noise content of an imaging system.
    C.   Latitude of an imag ing system.
    D.   Effect of b lurring on the by imaging system co mponents.

16. L-4 is rad iographed 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 fro m the image receptor shows a limiting
    resolution of 11 lp/mm; what resolution is expected if the pattern is mo ved 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. A lu minu m 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 fo r 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 fo r Calciu m at chest radiographic kVps.
    E. Of a decreased mass attenuation coefficient for Calciu m at chest radiographic kVps.

24. Geo metric 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 imag ing sys tem.
    C. The overall spatial resolution characteristics of the system.
    D. The full width at half maximu m of the line spread function.
    E. The temporal resolution of the system.

26. Methods of assessing spatial resolution of an imaging system include all o f the follo wing 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. Dig ital Angiography.
    D. Screen-film radiography
    E. CT

28. Regard ing 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 imperfect ions 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 to mography, 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 th in sections.
    E. It yields very high contrast images for thin sections.
Image Quality Geometry – ans wers

1.       D     At relatively h igh kVp , the photoelectric attenuation is minimal and the Co mpton 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 s ince 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 (h igher quality, less photoelectric effect), s maller
               tissue thickness, lower tissue density, and lower ato mic 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 fro m 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 rad iographs.

6.       E     The heel effect causes a lowered film density toward the anode side of the film that can be
               dramat ic near the edge of large films at shorter SID if the anode angle is small enough. High rat io
               parallel grids produce film density which decreases away fro m 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 t he 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 determin ing resolving power.

8.       B     Geo metric unsharpness = (focal spot size) x (magnificat ion – 1). If the C-arm x-ray tube is closer
               to the patient, the magnificat ion 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 magnificat ion), screen determines image sharpness. Single thin screen is sharper.

9.       D     Frame averag ing in DSA decreases noise by a factor of N1/2 , 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 p rocessor temperature means faster film speed, fewer x-rays needed;
               hence more noise. A bone algorith m 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 allo w a reduction in exposure time.

11.      B     The heart is situated 10” fro m 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 Z 3 whereas Co mpton 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 rat io grid to reduce scatter components
               reaching the film. Similarly, reducing the primary beam size to a minimu m 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 fro m 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 limit ing 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 hard ly 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 p inhole camera, slit camera, o r 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 co mponent. The mass attenuation coefficient of soft tissue does not change
          dramat ically over this range.

24.   A   Penumbra, caused by a finite focal spot, increases with magnificat ion. 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 magnificat ion increas es.

25.   C   In nuclear med icine 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 (ext rinsic 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   Geo metric blur is reduced by minimizing magnificat ion 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 bec ause 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 effect ively thicker.

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