1. Part No...., Module No....Lesson No Image Quality and Patient Dose
Module title
RTC on RADIATION PROTECTION OF PATIENTS FOR RADIOGRAPHERS Accra, Ghana, July 2011
Image Quality and Patient Dose
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

2. Part No...., Module No....Lesson No
Module title
Overview
To become familiar with the factors that determine the image clarity and the way the image quality can be improved
Lecture notes: ( about 100 words)
Instructions for the lecturer/trainer
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

3. Imaging quality Efficient diagnosis requires These factors are linked
acceptable noise
good image contrast
sufficient spatial resolution
These factors are linked
“Objective” measurement of quality is difficult

4. Factors affecting image quality
Blur or
Unsharpness
Contrast
Image quality
Distortion
& artifact
Noise

5. Image contrast
High Contrast
Low Contrast
Medium Contrast
Image contrast refers to the fractional difference in optical density of brightness between two regions of an image

6. Some factors influencing contrast
Radiographic or subject contrast
Tissue thickness
Tissue density
Tissue electron density
Effective atomic number Z
X Ray energy in kVp
X Ray spectrum (filtration)
Scatter rejection
Collimator
Grid …
Image contrast
The radiographic contrast plus :
Film characteristics
Screen characteristics
Windowing level of CT and DSA

7. Technique factors (1)
Peak voltage value has an influence on the beam hardness (beam quality)
It has to be related to medical question
What is the anatomical structure to be investigated?
What is the contrast level needed?
For a thorax examination : kV is suitable to visualize the lung structure
However only 65 kV is necessary to see bone structure

8. Technique factors (2)
The higher the energy, the greater the penetrating power of X Rays
At very high energy levels, the difference between bone and soft tissue decreases and both become equally transparent
Image contrast can be enhanced by choosing a lower kVp so that photoelectric interactions are increased
Higher kVp is required when the contrast is high (chest)

9. Technique factors (3)
The mAs controls the quantity of X Rays (intensity or number of X Rays)
X Ray intensity is directly proportional to the mAs
Over or under-exposure can be controlled by adjusting the mAs
If the film is too “white”, increasing the mAs will bring up the intensity and optical density

10. Receptor contrast
The film as receptor has a major role to play in altering the image contrast
There are high contrast and high sensitivity films
The characteristic curve of the film describes the intrinsic properties of the receptor (base + fog, sensitivity, mean gradient, maximum optical density)
N.B.: Film processing has a pronounced effect on fog and contrast

11. Image Contrast Difference in signal – pixel value, film density High
Low

12. Video monitor
The video monitor is commonly used in fluoroscopy and digital imaging
The display on the monitor adds flexibility in the choice of image contrast
The dynamic range of the monitor is limited (limitation in displaying wide range of exposures)
Increased flexibility in displaying image contrast is achieved by adjustment of the window level or grey levels of a digital image

13. Blur or lack of sharpness
The boundaries of an organ or lesion may be very sharp but the image shows a lack of sharpness
Different factors may be responsible for such a degree of “fuzziness” or blurring
The radiologist viewing the image might express an opinion that the image lacks “detail” or “resolution” (subjective reaction of the viewer to the degree of sharpness present in the image)

14. Resolution
Smallest distance that two objects can be separated and still appear distinct
Example of limits
Film/screen: 0.01 mm
CT: 0.5 mm
Other definition: “Point-spread” function
Characteristic of a “point” object
Point object expected to be point in image
Blurring due to imperfections of imaging system

15. Factors affecting image sharpness
Geometric
Unsharpness
Object
Unsharpness
Image
Unsharpness
Motion
Unsharpness
Subject
Unsharpness

16. Resolution and Focal Spot Size
Part No...., Module No....Lesson No
Module title
Resolution and Focal Spot Size
Penumbra
More blur
Less blur
Appearance of image
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

17. Measuring Resolution
Line pair test object
One line pair {

18. Geometric blur
If the focal spot is infinitesimally small, the blur is minimized because of minimal geometric bluntness
As the focal spot increases, the blur in the image increases
Small focal spot
Large focal spot

19. Geometric blur
Another cause of lack of geometric sharpness is the distance of the receptor from the object
Moving the receptor away from the object results in an increased lack of sharpness
N.B.: The smaller the focal size and closer the contact between the object and the film (or receptor), the better the image quality as a result of a reduction in the geometric sharpness

20. Lack of sharpness in the subject
Not all structures in the body have well-defined and separate boundaries (superimposition essentially present in most situations)
The organs do not have square or rectangular boundaries
The fidelity with which details in the object are required to be imaged is an essential requirement of any imaging system
The absence of sharpness, in the subject/object is reflected in the image

21. Lack of sharpness due to motion (1)
Common and understandable blur in medical imaging
Patient movement :
uncooperative child
organ contraction or relaxation
heart beating, breathing etc.
Voluntary motion can be controlled by keeping examination time short and asking the patient to remain still during the examination

22. Lack of sharpness due to motion (2)
Shorter exposure times are achieved by the use of fast intensifying screens
N.B.: Faster screens result in loss of details (receptor sharpness)
Further, the use of shorter exposure time has to be compensated with increased mA to achieve a good image
This often implies use of large focal spot (geometric sharpness)

23. Distortion and artifacts
Unequal magnification of various anatomical structures
Inability to give an accurate impression of the real size, shape and relative positions
Grid artifact (grid visualized on the film)
Light spot simulating microcalcifications (dust on the screen)
Bad film screen contact, bad patient positioning (breast)

24. Noise
Defined as uncertainty or imprecision of the recording of a signal
Impressionist painting: precision of object increases with number of dots
X Ray imaging: when recorded with small number of X- photons has high degree of uncertainty,more photons give less noise
Other sources of noise:
Grains in radiographic film
Large grains in intensifying screens
Electronic noise of detector or amplifier

25. Part No...., Module No....Lesson No
Module title
Noise
Decreasing radiation intensity
Increasing noise
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

26. Part No...., Module No....Lesson No
Module title
Contrast & Noise
 Contrast
Noise 
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

27. Radiography Issues Correct positioning PRE-exposure collimation
Improves diagnosis and reduces retakes
PRE-exposure collimation
Minimises unnecessary tissue dose
With CR/DR, there is a temptation to post-exposure (electronically) collimate – RESIST THIS!!

28. Part No...., Module No....Lesson No
Module title
Summary
Different technical and physical factors may influence the image quality by impairing the detection capability of the anatomical structures useful for diagnosis (increasing the image unsharpness)
Some factors depend on the receptor, some others are more related to the radiographic technique
Let’s summarize the main subjects we did cover in this session. (List the main subjects covered and stress again the important features of the session)
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

29. Part No...., Module No....Lesson No Patient dose assessment
Module title
Patient dose assessment
Part …: (Add part number and title)
Module…: (Add module number and title)
Lesson …: (Add session number and title)
Learning objectives: Upon completion of this lesson, the students will be able to: …
. (Add a list of what the students are expected to learn or be able to do upon completion of the session)
Activity: (Add the method used for presenting or conducting the lesson – lecture, demonstration, exercise, laboratory exercise, case study, simulation, etc.)
Duration: (Add presentation time or duration of the session – hrs)
Materials and equipment needed: (List materials and equipment needed to conduct the session, if appropriate)
References: (List the references for the session)
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

30. Part No...., Module No....Lesson No
Module title
Overview
To become familiar with the patient dose assessment and dosimetry instrument characteristics.
Lecture notes: ( about 100 words)
Instructions for the lecturer/trainer
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

31. Part No...., Module No....Lesson No
Module title
Parameters influencing patient exposure
Part …: (Add part number and title)
Module…: (Add module number and title)
Lesson …: (Add session number and title)
Learning objectives: Upon completion of this lesson, the students will be able to: …
. (Add a list of what the students are expected to learn or be able to do upon completion of the session)
Activity: (Add the method used for presenting or conducting the lesson – lecture, demonstration, exercise, laboratory exercise, case study, simulation, etc.)
Duration: (Add presentation time or duration of the session – hrs)
Materials and equipment needed: (List materials and equipment needed to conduct the session, if appropriate)
References: (List the references for the session)
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

32. Essential parameters influencing patient exposure
Part No...., Module No....Lesson No
Module title
Essential parameters influencing patient exposure }
Tube voltage
Tube current
Effective filtration
Kerma rate
[mGy/min] }
Kerma
[Gy]
Exposure time
[min] }
Area exposure
product
[Gy m2 ]
Field size
[m2]
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

33. Factors in conventional radiography: beam, collimation
Beam energy
Depending on peak kV and filtration
Regulations require minimum total filtration to absorb lower energy photons
Added filtration reduces dose
Goal should be use of highest kV resulting in acceptable image contrast
Collimation
Area exposed should be limited to area of CLINICAL interest to lower dose
Additional benefit is less scatter, better contrast

34. Factors in conventional radiography: grid,patient size
Grids
Reduce the amount of scatter reaching image receptor
But at the cost of increased patient dose
Typically 2-5 times: “Bucky factor” or grid ratio
Patient size
Thickness, volume irradiated…and dose increases with patient size
Except for breast (compression), no control
Technique charts with suggested exposure factor for various examinations and patient thickness helpful to avoid retakes
Use of AEC exposure

35. Factors affecting dose in fluoroscopy
Beam energy and filtration
Collimation
Source-to-skin distance
Inverse square law: maintain max distance from patient
Patient-to-image intensifier
Minimizing patient-to- I I will lower dose
But slightly decrease image quality by increased scatter
Image magnification
Geometric and electronic magnification increase dose
Grid
If small sized patient (les scatter) perhaps without grid
Beam-on time!

36. Factors affecting dose in CT
Beam energy and filtration
kV; shaped filters
Collimation or section thickness
Post-patient collimator will reduce slice thickness imaged but not the irradiated thickness
Number and spacing of adjacent sections
Image quality and noise
Like all modalities: dose increase=>noise decreases

37. Factors affecting dose in spiral CT
Factors for conventional CT also valid
Scan pitch
Ratio of couch travel in one rotation divided by slice thickness
If pitch = 1, doses are comparable to conventional CT
Dose proportional to 1/pitch

38. Patient dosimetry methods

39. Patient dosimetry Radiography: entrance surface dose ESD
Output factors
Dose – area product (DAP)
Fluoroscopy: Dose - Area Product (DAP)
CT:
Computed Tomography Dose Index (CTDI)
Dose – length product (DLP)

40. From ESD to organ and effective dose
Except for invasive methods, no organ doses can be measured
The only way in radiography: measure the Entrance Surface Dose (ESD)
Use mathematical models to estimate internal dose.
Mathematical models based on Monte Carlo simulations
Dose to the organ tabulated as a fraction of the entrance dose for different projections
Since filtration, field size and orientation play a role: long lists of tables (See NRPB R262 and NRPB SR262)

41. From DAP to organ and effective dose
In fluoroscopy: moving field, measurement of Dose-Area Product (DAP)
In similar way organ doses calculated by Monte Carlo modelling
Based on mathematical model
Conversion coefficients estimated as organ doses per unit dose-area product
Again numerous factors are to be taken into account such as projection, filtration, …
Once organ doses are obtained, effective dose is calculated following ICRP103

42. Dose measurements: how to measure dose indicators ESD, DAP,CTDI…

43. Measurements of Radiation Output
X Ray tube
Filter
SDD
Ion. chamber
Lead slab
Table top
Phantom (PEP)

44. Measurements of Radiation Output
Operating conditions
Consistency check
The output as a function of kVp
The output as a function of mA
The output as a function of exposure time

45. Measurement of entrance surface dose
Part No...., Module No....Lesson No
Module title
Measurement of entrance surface dose
Includes backscatter (~30%)
TLD, solid state
dosimeter or
ion chamber
The entrance surface dose measured under these conditions includes the backscatter
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

46. Dose Area Product (DAP)
Part No...., Module No....Lesson No
Module title
Dose Area Product (DAP)
Transmission ionization chamber
DAP meter can be put anywhere between the tube and the patient since its reading is independent on the position
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

47. Dose Area Product (DAP)
Part No...., Module No....Lesson No
Module title
Dose Area Product (DAP)
0.5 m
1 m
2 m
Air Kerma:
Area:
Area exposure product
40*103 Gy
2.5*10-3 m2
100 Gy m2
10*103 Gy
10*10-3 m2
100 Gy m2
2.5*103 Gy
40*10-3 m2
100 Gy m2
As can be seen the measured DAP quantity is constant since the Air Kerma decreases according to the inverse square law and the area increases as the square of the distance
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

48. Calibration of a Dose Area Product (DAP)
Film cassette
10 cm
Ionization
chamber

49. Part No...., Module No....Lesson No
Module title
Levels of Dosimetry
Level 1 - published tables
Level 2 - Monte Carlo tables using known data
Level 3 - direct measurement of skin dose
Level 4 - humanoid phantom measurements with TLD
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

50. Level 1 - Published Dose Tables
Part No...., Module No....Lesson No
Module title
Level 1 - Published Dose Tables
ICRP, NCRP and various books have tables of “typical” doses for various x-rays
Tables show organ doses and sometimes effective dose
The data is usually old, from x-rays made with slower film/screen systems than in current use
Still useful as a guide
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

51. Level 2 - Monte Carlo Systems
Part No...., Module No....Lesson No
Module title
Level 2 - Monte Carlo Systems
Provide organ doses, and effective dose
Use calculated data but with user entry of various parameters :
HVL, kVp
skin dose or free-in-air exposure at skin distance
FSD, field size and position
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

52. Part No...., Module No....Lesson No
Module title
Monte Carlo Systems
Various computer programs and lookup tables, eg. TISSDOSE, XDOSE, PCXMC
Most users do not know the actual values for input variables, so often must use assumed values
HVL, kVp, FSD, field size easy to assume – kVp/mAs and field size not always easy to assume
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

53. Typical Radiology Doses (Melbourne Data)
Part No...., Module No....Lesson No
Module title
Typical Radiology Doses (Melbourne Data)
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

54. XDose

56. Part No...., Module No....Lesson No
Module title
PCXMC
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

57. Part No...., Module No....Lesson No
Module title
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

58. Part No...., Module No....Lesson No
Module title
ImPACT CT Dose
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

59. Part No...., Module No....Lesson No
Module title
IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

60. Summary
In this lesson we learned the factors influencing patient dose, and how to have access to an estimation of the detriment through measurement of entrance dose, dose area product or specific CT dosimetry methods.