https://youtu.be/oORA83_2JFw Mammography QA

2. CT Image Quality Parameters
Spatial
Resolution
Image
Noise
Contrast
Resolution
Artifacts

3. Spatial Resolution
• Quantifies image blurring
• “Ability to discriminate objects of varying
density a small distance apart against a
uniform background”
• Minimum separation required between
two high contrast objects for them to be
resolved as two objects

4. Resolvable Object Size &
Limiting Resolution
• Smallest resolvable high contrast object
• Often expressed as line pairs / cm
• “Pair” is one object + one space
One
Pair

5. Geometric Factors affecting
Spatial Resolution
• Focal spot size
• detector aperture width
• slice thickness or collimation
–Less variation likely for
thinner slices
–attenuation variations within
a voxel are averaged
• partial volume effect

6. Geometric Unsharpness & CT
• Object details
distributed (blurred)
over several detectors
decrease spatial
resolution
• detector aperture size
–must be smaller than
object for object to be
resolved
Focal Spot
Detectors
Small Object
to be Imaged

7. Contrast Resolution
• Ability of an imaging system to
demonstrate small changes in tissue
contrast
• The difference in contrast necessary to
resolve 2 large areas in image as separate
structures

8. CT Image Noise
• Standard deviation of pixel values
S(xi - xmean)2
Noise (s) = -------------------
(n-1)
Xi = individual pixel value
Xmean = average of all pixel values in ROI
n =total # pixels in ROI

9. Noise Level
• Units
– CT numbers (HU’s)
or
– % contrast
• Example
– CT # range: 1000 HU’s
– Standard deviation: 3 HU’s
– Noise level is 3 or 3 / 1000 X 100 = 0.3%

10. CT Image Noise
• Fluctuation of CT
#’s in an image of
uniform material
(water)
• Usually described
as standard
deviation of pixel
values

11. Noise Measurement in CT
• Scan water phantom
• Select regions of interest
• Take mean & standard
deviation in each region
• Standard deviation is
noise in ROI

12. CT Spatial vs. Contrast Resolution
• Spatial & contrast resolution interact
– High contrast objects are easier to resolve
• One can improve one at the expense of the
other
• Can only improve both by increasing dose
Increasing
object size
Increasing contrast

13. Contrast & Detail
• Larger objects easy to see even at low contrast
Increasing
object size
Increasing contrast

14. Contrast & Detail
• Small objects only visible at high contrast
Increasing
object size
Increasing contrast

15. CT Number
• Calculated from reconstructed pixel
attenuation coefficient
(mt - mW)
CT # = 1000 X ------------
mW
Where:
ut = linear attenuation coefficient for tissue in pixel
uW = linear attenuation coefficient for water

16. Linearity
• Linear relationship of CT #’s to linear
attenuation coefficients of objects
• Checked with phantom of several
known materials
– average CT # of each material obtained
from ROI analysis
– Compare CT #’s with known coefficients

17. 17
Acceptance Test:
• Performed upon newly installed equipment
to verify purchase specification and to
establish performance base line.
 Frequency:
• To be performed once
only when new
machine is installed

18. ACCEPTANCE TEST IN CT
• spatial
resolution
• contrast
resolution
• noise
• slice width
• kVp waveform
• average &
standard
deviation of
water phantom
CT #
• scatter & leakage

19. CT test Equipment
- water cylinder phantom
- System evaluation phantom
• high contrast resolution ,slice thickness ,laser
accuracy
• low contrast detectibility
• noise and field uniformity
- Head and Body phantom

20. Jun-20
CRHC 2013 Jaipur
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Testing of CT number for water
Procedure
• water filled plastic cylinder ( 20 cm diameter)
• scan and reconstruct the image ,place ROI
200-300 pixels in centre of field
Tolerance limits
• +/- 3HU at the centre
• +/- 5 HU at peripheral location
• outside the image -1000HU +/- 5 HU

21. Testing of CT number accuracy for
water

22. Measurement of field uniformity

23. Testing of image Noise

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CRHC 2013 Jaipur
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At 100% contrast difference the size
of the bar/hole pattern that could be
resolvable should be 1.6 mm ( 3.12
lp/cm).
High contrast sensitivity

25. Testing of High contrast sensitivity

26. High contrast (spatial ) Resolution

27. High contrast (spatial ) Resolution

28. Low contrast resolution/Detectability

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CRHC 2013 Jaipur
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Low contrast sensitivity
5.0 mm at 1% contrast
difference (minimum)

30. Slice Thickness

31. Alignment of Laser light

32. CT Dose Measurement
(CTDI)
 20% of the
quoted value
(Expected)
 40% of the
quoted value
(maximum)


T
T
sngle dzzDnTCTDI
7
7
)()/1(

33. CTDI100
• Computed tomography dose index 100
(CTDI100)


cm
cm
sngle dzzDNTCTDI
5
5
100 )()/1(
where N is the number of sections per scan, T is the
nominal width of each acquired section, f is the air
kerma to dose conversion factor, E is measured air kerma
and L is the pencil chamber length (100 mm)
LEfNTCTDI  )/1(100

34. CTDIvol
pitch/wvol CTDICTDI 
where pitch is the table distance traveled in a full
rotation / total collimated width of the beam

35. Dose-length Product
• Dose-length product (DLP)
• Units of DLP generally mGy-cm
• Analogous to dose-area-product (DAP) for
projection imaging
lengthscan volCTDIDLP

36. Jun-20 CRHC 2013 Jaipur 36

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CRHC 2013 Jaipur
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Periodic QA tests for Computed
Tomography (CT) unit
Daily
- indicators, mechanical and safety checks
Weekly
-Image noise , CT number of water
Monthly
CT Number uniformity
Contrast resolution
Spatial resolution

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CRHC 2013 Jaipur
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QA tests for Computed Tomography
(CT) unit Contd---
Semi-annual
• laser alignment
• slice thickness
• position accuracy
Annual
• patient dose measurement
• Beam HVL for 140 kVp and 120 kVp
• Area survey

40. ACR CT Accreditation Phantom
High contrast
resolution
4
Uniformity
& noise
Distance
Accuracy
& SSP
3
Low
contrast
resolution
Alignment
CT #
Slice width
2 1
Head Foot

41. Module 1
• Alignment, CT number accuracy, slice thickness

42. Module 1
• CT number accuracy
– Polyethylene: between -107 and -87 HU
– Water: between -7 and +7 HU (-5 and +5 HU
preferred)
– Acrylic: between +110 and +130 HU
– Bone: between +850 and +970 HU
– Air: between -1005 and -970 HU
• Slice thickness
– Measure at 7, 5, 3 and 1 mm (HRC)
– Must be accurate to within 1.5 mm

43. Slice Thickness
• Each wire is 0.5 mm slice thickness
• Count number of wires >50% as bright as the
center wire divide by 2

44. Module 2
• Low contrast resolution
0.6% difference
from background

45. Module 2
• All four 6-mm rods must be visible
• Record contrast
(no control limit)

46. Module 3
• Uniformity,
noise, distance,
section
sensitivity profile

47. Uniformity Control Limit
• Mean in all
edge ROIs
within 5 HU of
mean in center
ROI
• Center ROI
between -7 HU
and +7 HU (-5
HU and +5 HU
preferred)
• No artifacts

48. Module 4
• High contrast resolution (bar patterns)
– 4-12 lp/cm

49. High Contrast Resolution
• Control limit:
– Adult abdomen
protocol
• at least 5 lp/mm
– High resolution
chest protocol
• at least 6 lp/mm

50. Catphan
The Phantom Laboratory, http://www.phantomlab.com/catphan.html

51. Catphan 600 Modules
• CTP404 Slice width, CT #, pixel size
• CTP591 Bead geometry
• CTP528 Line pair resolution, point source
• CTP515 Low contrast resolution
• CTP486 Uniformity

52. CPT404
• Slice width, CT number accuracy, pixel size

53. Slice Width
• Slice width = FWHM(wire ramp) x tan (23)

54. CT Number Accuracy
Material CT Number
Air -1000
PMP -200
LDPE -100
Water 0
Polystyrene -35
Acrylic 120
Delrin 340
Teflon 990

55. CPT591
• Bead geometry
– Calculation of slice width and MTF

56. CPT528
• High contrast spatial resolution
– 1-21 lp/cm bar patterns
• 2 point sources
– 0.28 mm (subpixel)
– MTF measurement

57. CPT515
• Low contrast resolution targets 3 mm
5 mm
7 mm
Subslice
Supra-slice

58. CPT486
• Uniformity
– Profiles in horizontal and vertical directions
indicate
possible
cupping
or capping

59. Uniformity

60. Mammography Machine

61. Jun-20 61
Acceptance tests for
Mammography Unit
Focal Spot test
for f < 0.8 mm + 0.5 f
Accuracy of tube potential (kVp) ± 1 Kv
Accuracy of exposure timer ± 10 %
Linearity of mA loading station
Coefficient of mA linearity < 0.1
Linearity of timer
Coefficient of timer linearity < 0.1

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CRHC 2013 Jaipur
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Recommended values of HVL for 28 kVp
0.3 mm Al  HVL  0.37 mm Al
( for Mo target & Mo filter )
Recommended values of HVL for 49 kVp
0.8 mm Al  HVL  1.0 mm Al
(for Mo target & Rh filter )
1.1 mm Al  HVL  1.6 mm Al
(for Mo target & Al filter)
Al equivalence of the  0.1 mm of Al
compression device  2.0 mm of
plexi glass

63. Imaging Setup
Compression Paddle
Compressed Breast
Table
Grid
Image Detector
Chest Wall
X-ray Source
X-ray Beam

64. ACR Accreditation Phantom
(RMI 156 )

65. RMI 156 Phantom
ACR minimum score is 4 visible fibers, 3 speck groups and 3 masses

66. SETUP For Entrance Skin Exposure
Measurement
Ionization
Chamber Breast
Equivalent
Phantom
X-Ray Beam
Table Film
Film
Compression
Paddle

67. Entrance Skin Exposure Measurement
Dosimeter
Mammography
Phantom