Image contrast questions

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1. Define image contrast.

1. The difference in density/IR exposure between two adjacent areas on an image. One of the two properties that allow detail to be visualized on an image.

 

2. How do low contrast images differ from high contrast images?

2. Low contrast images have many shades of gray (more information on the image). Also referred to as long scale contrast or decreased contrast. Low contrast images generally are a result of higher kV settings; the outcome is a beam that is more penetrating, more energy, higher frequency and shorter wavelength.

 

High contrast images have very few shades of gray between black and white, also referred to as short scale contrast. High contrast images generally are a result of lower KV settings, resulting in a less penetrating beam with lower energy.

 

3. What is the difference between physical and visible contrast?

3. Physical contrast is the total range of density values/IR exposures recorded on the image; also known as subject contrast. This contrast can be objectively measured by using a densitometer.

Visual contrast is the total range of density values that can be perceived by the human eye. When viewing a radiograph this contrast depends on the individual’s eyes, the illumination from the view box, the distance at which it is viewed, the ambient room lighting, the experience of the viewer, if the radiograph is masked. Also known as subjective contrast as it is “subjective” to the viewer. This contrast cannot be measured.

4. What factors affect image receptor (film) contrast and subject contrast?

4. Image receptor (film) contrast

Intensifying screens – comparing a film exposed without intensifying screens to a film that was exposed using intensifying screens the latter will create a higher radiographic contrast

Film density/IR exposure – exposure factors will determine the film density/IR exposure. If the density is in the toe or the shoulder there is a decrease in contrast

D log E curve – inherent characteristics by the manufacturer

Processing – increase in developer temperature, immersion time, replenishment rate or contaminated developer all increase fog which decreases contrast

Subject contrast

Radiation quality (kV), filtration, subject composition, part thickness, atomic number of tissue, contrast media (positive and/or negative), and scatter radiation.

5. How does film density/IR exposure affect image receptor contrast?

5. If the radiographic density/IR exposure is too high (dark or overexposed) or too low (light or underexposed) image contrast will be low; there will be very little density differences across the image. Radiographic density/IR exposure needs to be optimal to best evaluate contrast.

 

6. What is the controlling factor of subject contrast and how does the factor affect the contrast?

6. KV controls subject contrast e.g. high kV technique for chest radiography produces a wide range of densities/IR exposures versus a lower kV technique for rib radiography required to show high density/IR exposure differences between bone and air filled lungs.

7. How do variations in the anatomical part affect contrast?

7. Variations in the anatomical part (part thickness, tissue density (kg/m3) atomic number, presence of contrast media) result in varying attenuation of the primary beam. When there are great differences i.e. tissue thickness, between adjacent structures the contrast will be high versus tissues with similar densities (kg/m3) will result in a low contrast image.

 

8. What effect does a grid have on contrast?

8. The purpose of a grid is to remove unwanted scatter radiation from reaching the image. Scatter radiation adds an unwanted density/IR exposure to the image reducing the image contrast. The result of using a grid, when imaging thicker body parts, will be an image with higher contrast.

9. How does film processing affect contrast?

9. An increase in developer immersion time, temperature, or replenishment rate above the manufactures recommended levels will increase chemical fog. Chemical fog causes a decrease in the slope of the characteristic curve resulting in decreased contrast. A decrease in developer immersion time, temperature, or replenishment rate below the manufacturers recommended levels will decrease density which will result in decreased contrast.

 

10. The difference in density/IR exposure between two adjacent areas of the radiograph defines __________________.

10. Contrast

11. List the two scales of radiographic contrast, and identify which is high and which is low contrast:

A. __________________________________

B. __________________________________

11. A. Long scale contrast is also known as low contrast

B. Short scale contrast is also known as high contrast

 

12. Which scale of contrast is produced with a 110 kVp technique? _______________________

12. Long scale contrast = many shades of grey

 

13. True/False: A low contrast image demonstrates more shades of grey on an image.

13. True

 

14. True/False: A 50 kVp technique generally produces a high contrast image

 

14. True

 

PIR: Part 2: Unit 3 and 4 Worksheet Compiled by L. Gendre RRC: November 20, 2013

 

15. What is the primary controlling factor for radiographic contrast? ___________________________

15. kV

16. Which one of the following set of exposure factors will result in the least patient exposure and produce long scale contrast for a PA chest radiographic image?

a. 50 kVp, 80 mAs

b. 70 kVp, 20 mAs

c. 80 kVp, 10 mAs

d. 110 kVp, 3 mAs

16. d. 110 kV, 3 mAs

 

17. Define recorded detail.

17. The sharpness of structural lines recorded on the radiographic image.

 

18. List the names also related to recorded detail.

18. Definition, sharpness, resolution or simply detail List the names also related to recorded detail.

 

19. Describe and define resolution and how it is measured.

19. The ability of a system to demonstrate closely spaced structures as separate entities on the radiographic image; measured in line pairs per millimetre (lp/mm).

 

20. How do the SID and the OID affect recorded detail?

20. As SID increases; penumbra is reduced and resolution increases. Alternately, as OID is decreased; penumbra is reduced and resolution increases.

 

21. What is the relationship between focal spot size and recorded detail?

21. As FSS decreases the geometry of the beam reduces penumbra and increases resolution.

22. What is the difference between umbra and penumbra?

22. Umbra is the image of the anatomy of interest versus the penumbra is the unsharpness around the periphery of the image. (Carlton & Adler P. 434; Figure 28-14 ALSO Carlton & Adler P. 435; Figure 28-15)

23. List all factors affecting recorded detail; including line spread function (LSF), modulation transfer function (MTF), and quantum mottle.

23. The OID, SID, focal spot size, motion, intensifying screens/digital capture element.

Line Spread Function (LSF) measures the ability of an image receptor (film/screen combination or digital) to accurately measure the image boundaries and may be calculated using a densitometer or electronic display device performance. (Checked using a test pattern such as the SMPTE or TG18; see Papp P. 129 or SC35 P. 55)

Modulation transfer function (MTF) is the best measurement of resolution for an imaging system. It measures the last information between subject and image receptor. There are no units for MTF and is controlled by the intensifying screens/ photostimulable phosphor size, thickness of phosphor layer, presence or absence of an absorptive layer, and by the contact between film and screen.

MTF = Recorded Detail (Resolution)

Available Detail (Resolution)

(This number is never greater than 1)

Quantum mottle will appear grainy on a radiograph or digital image and is caused by insufficient quantity of photons reaching the intensifying screens/digital capture element.

 

24. What factors can be minimized to improve resolution?

24. Reduce object to image distance, select a small focal spot (if appropriate), choose the shortest exposure time possible, and use detailed (slow) speed imaging system (if applicable).

 

25. List the three geometric factors that control or influence image resolution:

A. _______________ B. ________________ C. _______________

25. A. Focal spot size B. Source to image receptor distance C. Object to image receptor distance

 

26. The lack of visible sharpness is called _______________.

26. Blur or unsharpness

 

27. The term that describes the unsharp edges of the projected image is _______________.

27. Penumbra

 

28. True / False: The use of a small focal spot will entirely eliminate the problem identified in the previous question.

28. False

 

29. The greatest contributor to image unsharpness, as related to positioning, is

_______________.

29. Motion

 

30. List and describe the two types of motion.

 

30. Voluntary – is motion under the patient’s control. Communication is essential for all patients; instructions must be given in a professional and precise manner.

Involuntary – is motion not under the patient’s control but controlled by the involuntary nervous system. E.g. heartbeat, peristalsis

31. Describe how motion can be minimized.

31. To minimize motion use immobilization devices; the patient must be comfortable, reduce exposure time, maintain x-ray equipment to eliminate motion of x-ray tube, Bucky, etc. and communication must be understandable to the patient.

 

32. What is the best mechanism to control involuntary motion during exposure?

a. Use a small focal spot

b. Provide the patient with clear instructions

c. Shorten the exposure time

d. Increase the grid ratio

32. c.

 

33. Which of the following changes will improve image resolution?

a. Decreases OID

b. Decreased SID

c. Use a large focal spot

d. Use a higher kVp setting

33. a.

 

34. True/False: Every radiographic image illustrates some degree of penumbra or unsharpness, even when the smallest focal spot size is used.

34. True

35. Situation: The technologist is performing an elbow series on a pediatric patient. Because of the nature of the injury, the technologist has been asked to produce radiographs that have the highest degree of recorded resolution possible. Which of the following sets of factors will produce that level of detail?

a. 0.3 mm focal spot and 70 cm SID

b. 1 mm focal spot size and 80 cm SID

c. 0.5 mm focal spot size and 90 cm SID

d. 0.3 mm focal spot size and 100 cm SID

35. d.

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