X-ray Imaging. PHYS Lecture. Carlos Vinhais. Departamento de Física Instituto Superior de Engenharia do Porto

Transcription

1 X-ray Imaging PHYS Lecture Carlos Vinhais Departamento de Física Instituto Superior de Engenharia do Porto

2 Overview Projection Radiography Anode Angle Focal Spot Magnification Blurring Image Receptors Screen-Film Cassette Intensifying Screens Quantum Noise Optical Density HD Curve Contrast Sensitivity or Speed Latitude Screen-Film System Contrast and Dose Effect of kv on OD Effect of mas on OD Scattered Radiation Anti-scatter Grid Grid Artifacts Air Gap Carlos Vinhais 2

3 Projection Radiography X-ray PRODUCTION Interaction of X-rays with patient Image Receptor IMAGE Carlos Vinhais 3

4 Projection Radiography Projection of 3D anatomy into 2D space SID SOD OID SID SID Source to Image Distance SOD Source to Object Distance OID Object to Image Distance Carlos Vinhais 4

5 Anode Angle D Schueler98 Carlos Vinhais 5

6 Focal Spot Schueler98 Carlos Vinhais 6

7 Magnification x-ray beam diverges from focal spot to image plane SID source to image distance SOD source to object distance OID object to image distance For a point source: Carlos Vinhais 7

8 Blurring For focal spot (F) of x-ray tube, edges of projection indistinct Exaggerated by magnification Geometric blur: f (penumbra) Carlos Vinhais 8

9 Projection Radiography (main concepts) Projection Radiography is a transmission imaging procedure Magnification due to x-ray beam diverging Magnification largest when patient close to x-ray tube Clinically relevant in mammography Geometric blur due to large focal spot Blur increases with focal spot size and magnification Carlos Vinhais 9

10 Question D15. The penumbra associated with the image of the edge of an object placed 50 cm above the film plane, for an SID of 100 cm, and a focal spot size of 1.0 mm is mm. A B. 0.1 C. 1.0 D. 10 f/f = OID/SOD => f = F (OID/SOD) = 1 (50/50) = 1.0 Carlos Vinhais 10

11 Image Receptors General Radiography: Analog: Screen/Film Digital: Computed Radiography Digital Radiography Carlos Vinhais 11

12 Screen-Film Cassette Cassette 1 or 2 Intensifying Screens Convert x-rays to visible light Sheet of Film Register the x-ray distribution Chemically processed Storage and display Carlos Vinhais 12

13 Intensifying Screen Film relatively insensitive to x-rays, even though best spatial resolution Screens used with film and made of scintillating material: Phosphor Gd2O2S:Tb (gadolinium oxysulfide: terbium) common General radiography: two screens Mammography: single screen Screen absorbs x-rays and converts to visible or UV light, which exposes the film emulsion Light emitted darkens the film Screen-film detectors are considered an indirect detector Carlos Vinhais 13

14 Intensifying Screen Conversion efficiency (CE) Fraction of absorbed x-rays converted to light Gd2O2S:Tb 15% intrinsic conversion efficiency Quantum Detective Efficiency (QDE) how efficiently the screen detects x-ray photons that are incident on it QDE increases with screen thickness Overall efficiency Ability to convert the absorbed x-ray into film darkening Intensification factor is times greater than film alone Radiation dose reduced by a factor of 50! Carlos Vinhais 14

15 Intensifying Screen Using film-screen versus film only reduces radiation dose to patient by a factor of 50! Shorter exposure times, less blur due to patient motion Compromise between screen sensitivity and spatial resolution depending on clinical application Faster screens more efficient more image noise Slower screens less efficient more detail in image Carlos Vinhais 15

16 Quantum Noise Noise: local variations in film OD, not representing variations of attenuation in patient The visual perception of noise is reduced (better image quality) when the number of detected x-ray photons increases Carlos Vinhais 16

17 Film Film used to capture, display and store clinical images Film emulsion made of silver halide (AgBr and AgI) grains Latent image rendered visible through film processing by chemical reduction of silver halide into metallic silver grains Absorption of x-rays causes latent image to be formed in emulsion Latent image rendered visible through film processing by chemical reduction of silver halide into metallic silver grains which appears black on clinical image Static electricity can cause artifacts on film so should be stored carefully Carlos Vinhais 17

18 Optical Density Film is a negative recorder increased x-ray exposure, increased darkness on film Transmittance (T) is the fraction of incident light passing through the film Film darkening is measured using optical density (OD) As OD increases, T decreases: I 0 = light incident on film I t = light transmitted through film Carlos Vinhais 18

19 Optical Density The OD of superimposed films is additive Useful range of densities is from Carlos Vinhais 19

20 Hurter and Driffield (H&D) Curve H&D characteristic curve describes how film responds to x- ray exposure OD vs. log exposure Non-linear, sigmoidal shape Base Fog Toe low exposure region (ex. Chest mediastinum) Linear region: ideal region Shoulder: high exposure region Carlos Vinhais 20

21 Contrast Contrast of film is related to the slope of the H&D curve Overall contrast given by Average gradient (rise/run) Higher slope (steep curve) have higher contrast Reduced slope have lower contrast Carlos Vinhais 21

22 Sensitivity or Speed Absolute speed = 1 / R required to achieve Fast films (more sensitive) requires less exposure to achieve a given OD; slow films require more exposure Faster SF systems result in lower patient doses but have more quantum noise Carlos Vinhais 22

23 Latitude Dynamic range of x-ray exposures that deliver ODs in the usable range System A has higher contrast but reduced latitude System B has lower contrast but wider latitude Clinical application dictates the type - Chest requires wide latitude to cover lungs and mediastinum Carlos Vinhais 23

24 Question (D ) Consider the three characteristic curves in the diagram. Which statement is false? A. System B has the highest contrast. B. System C has the widest latitude. C. System A has the highest maximum density. D. System B has the highest basefog density. E. System C is the fastest. Carlos Vinhais 24

25 Screen-Film System Film emulsion should be sensitive to light emitted by screen The SF system governs the overall detector contrast The kvp (quality) and mas (quantity) are adjusted by the technologist to adjust the subject contrast Double mas, double OD 15% increase in kvp 2 mas 15% decrease in kvp ½ mas Increase in kvp will decrease radiation dose but also decrease contrast Carlos Vinhais 25

26 Contrast and Dose The contrast of a specific radiographic study depends on: Study requirements total exposure time radiation dose size of patient The kvp (quality) and mas (quantity) are adjusted by the technologist to adjust the subject contrast Classic compromise between image contrast and patient dose Carlos Vinhais 26

27 Effect of kv on OD Carlos Vinhais 27

28 Effect of mas on OD Carlos Vinhais 28

29 Scattered Radiation Scattered radiation: energy range in radiography 15 to 120 kev (above 26 kev, mainly Compton interaction) Scattered photons violation of the basic principle of projection imaging: mis-information reducing contrast Does not provide useful information on the image Carlos Vinhais 29

30 Scattered Radiation S/P ratio scatter photons vs primary photons S/P depends on: field of view (FOV) object thickness energy of x-ray beam (Compton) As FOV is reduced (collimate to area of interest), scatter is reduced Larger patients create more scatter As S/P contrast Carlos Vinhais 30

31 Anti-scatter Grid Grid cleans up scatter Increases contrast Increases dose Grid ratio = height / interspace width Typically: 5:1 (mammo) 8:1-12:1 (radiography) Carlos Vinhais 31

32 Anti-scatter Grid Carlos Vinhais 32

33 Anti-scatter Grid GRID (12:1) NO GRID Carlos Vinhais 33

34 Grid Artifacts Carlos Vinhais 34

35 Air Gap Air gaps used instead of grid to clean up scatter Magnification increases, FOV decreases Used in mammography Carlos Vinhais 35

36 Air Gap AIR GAP (12.5 cm) NO AIR GAP Carlos Vinhais 36

37 Question D12. L-4 is radiographed at a source-to-image distance (SID) of 100 cm, 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 The magnification is M = I/O = SID/SOD = SID / (SID - OID), or 100 / (100-20) = M = I/O so the true size is 35 / 1.25 = 28. Carlos Vinhais 37

38 Question Object on fluoro table is 4 inches and projects as 7 inches on the image receptor which is 12 inches above the fluoro table. What is the distance from the x-ray tube (source) to the table? A. 8 in B. 12 in C. 16 in D. 20 in Using similar triangles, (SOD+12)/SOD = I/O, SOD= 16 Carlos Vinhais 38

39 Question D15. If the absorption efficiency of each intensifying screen in a dual screen system is 30%, what percentage of x-rays is stopped by the screens together? A. 9% B. 30% C. 51% D. 60% E. 70% 30% is absorbed in the first screen, 70% passes through. The second screen absorbs 30% of that 70% (or 21%). Total stopped is 30% + 21%. Carlos Vinhais 39

40 Question D19. Changing to a higher speed film will: A. Decrease patient exposure and increase noise. B. Decrease patient exposure and decrease noise. C. Not change exposure or noise, but decrease contrast. D. Increase patient exposure and increase noise. E. Increase patient exposure and decrease noise. Carlos Vinhais 40

41 Question G70. A radiograph has little contrast in density from one region to the next. Which of the following would improve contrast in a retake film? 1. Change to higher ratio grid. 2. Move the film closer to the patient. 3. Collimate the beam to as small a field as possible. 4. Raise the kvp to lower the exposure time. A. 1, 3 B. 1, 4 C. 2, 3 D. 1, 2 and 4 E. 1, 2, 3, and 4 Carlos Vinhais 41

42 Question G77. The purpose of a screen is to: 1. Convert x-rays to light photons. 2. Reduce scatter reaching the film. 3. Reduce patient's exposure. 4. Increase radiographic resolution. A. 1, 2, 3 and 4 B. 2 only C. 2, 4 D. 1, 3 E. 4 only Carlos Vinhais 42

43 Question G72. Which of the following does not reduce patient dose (for the same optical density on the film)? A. Use of screens B. Using a high kvp C. Using a high ratio grid D. Collimation Carlos Vinhais 43

44 Question G72. A radiograph transmits 10% of the light from a viewbox with an illumination level of 400 lux. The optical density of the radiograph is: A. 10 B. 2 C. 1 D. 0.1 E OD = -log10(t) = log10(1/t) = log10(1/0.1) = log10(10) = 1 Carlos Vinhais 44

45 Question D20. Optical density (OD) regions on film of 1.0, 1.3, and 2.0 will transmit of the light from a viewbox: A. 10%, 5%, 1% B. 10%, 13%, 20% C. 1%, 5%, 10% D. 90%, 87%, 20% E. 50%, 33%, 25% T = 10-OD Carlos Vinhais 45

46 Question D24. In some situations, e.g., a chest exam, it is important to see radiographic anatomy in both highand low-density regions. To aid in this, one could choose a film with a. A. High gradient B. High gamma C. Slow speed D. Long latitude E. Low fog Carlos Vinhais 46

47 Question D19. In order to decrease the optical density of an overexposed radiograph from 2.0 to 1.2, the mas should be reduced by approximately %. (Assume a slope of the characteristic curve of 3.0): A B C D E. Greater than 95 OD2 - OD1 = Average Gradient x log10 (E2/E1) where E is exposure, proportional to mas = 3.0 log10 (E2/E1), E2/E1 = 0.54 or 46% reduction. Carlos Vinhais 47

48 End of Lecture!