1) CT dose index (CTDI) measures radiation output of CT scanners. Modern measures include CTDI100, CTDIvol, and dose length product (DLP). 2) Automatic exposure control (AEC) modulates tube current based on patient attenuation to maintain consistent image quality while reducing dose. 3) Dose reduction techniques in CT include AEC, bowtie filters, iterative reconstruction, prospective gating, and dynamic collimation.

1. Radiation dose modulation and dose
reduction techniques

2. Introduction to presentation
CT Dose Index
CTDI100
CTDIvol
DLP
Effective dose
CT dose reduction techniques
AEC
Filter
Reconstruction

3. CT Dose Index
CT dose index (CTDI) is the standardized
measure of the radiation output of a CT system,
Currently used measures are the CTDI100 and
CTDIw, modified to CTDIvol for modern helical
scanners.

4. CTDI phantoms(PMMA)

6. Measurement of CTDI
• Ionisation pencil chamber
– Air-filled chamber
– X-rays cause ionisations in
chamber, causing current to
flow, proportional to exposure
– Instant readings, good
accuracy, easy to use
100 mm

7. Measurement of CTDI
• Single slice measurement
(z-axis)
Dose
T
Nominal
beam width
( )dzzD
T
1
CTDI
-
∫
+∞
∞
=
( )dzzD
T
1
CTDI
50
50-
100
∫
+
=
-50 +50
∫
+
−
=
50
50
D(z)dz
n.T
1
CTDI100
n = no. slices imaged simultaneously
T = nominal imaged width
Davge
T
100Davge
CTDI100
×
=

8. CTDI100
• CTDI100 is a measure of radiation dose on a 100mm
long pencil ionization chamber.
• Centre
• Periphery - 1 cm depth
(mean of 4 positions)
C
P1
P2
P3
P4
320 mm
‘Body’
‘Head’
160 mm
140
mm

9. Measurement of MSAD(Multiple Scan
Average Dose)
CTDI ≈ ‘Series’ dose
Multi-scan
‘Series’ dose
CTDI×
×
=
I
TN
MSAD
T
T is the nominal scan width (mm)
I is the distance between scans (mm)
I
N is the number of scans
N × T is the total nominal scan width

10. CTDI100
• Typical CTDI100 values (mGy)
40
Head
Body
40
4040
40
20
2020
20
10
Periphery : centre ≅ 1:1 Periphery : centre ≅ 2:1

11. Derivatives of CTDI100
• CTDIW
• Weighted average CTDI: represents the average
dose for contiguous irradiation
• CTDIw = 1/3 CTDI100,C + 2/3 CTDI100,P
CTDIC CTDIP

12. Derivatives of CTDI100
• CTDI100 & CTDIw refer to a pitch of 1
• Average absorbed dose dependent on pitch
Contiguous Extended Overlapping

13. CTDIvol
• Volume CTDI: Considers contiguous scanning
• CTDIvol = CTDIw
Pitch
Pitch = 1
CTDIvol = CTDIw
Pitch = 2
CTDIvol = CTDIw
2
Pitch = 0.5
CTDIvol = 2 x CTDIw

14. Radiation risk: DLP
• CTDIvol is a measure of absorbed dose, energy
absorbed per unit mass
• To measure radiation risk from stochastic effects
the total energy absorbed must be considered
• In CT this can be estimated using the Dose Length
Product, DLP

15. DLP = CTDIvol . L (mGy.cm)
DLP1 ≅ DLP2
where L = scan length
Dose length product
L1
T
Pitch 1
8 rotations
Pitch 2
8 rotations
L2
T
as CTDIvol2 = CTDIvol1
2

16. Estimates of effective dose (E)
• Can be obtained from DLP
• Effective dose = DLP. CF (mSv)
•
•
•
•
•
•
Conversion factors not scanner specific or location
specific
Region of body Conversion factor,EDLP
(mSv mGy-1
cm-1
)
Head & neck 0.0031
Head 0.0021
Neck 0.0059
Chest 0.014
Abdomen& pelvis 0.015
Trunk 0.015

17. CT dose reduction techniques
AEC systems have a number of
potential advantages, including better
control of patient radiation dose,
avoidance of photon starvation
artifacts, reduced load on the x-ray
tube, and the maintenance of image
quality in spite of different attenuation
values on CT scans

18. Types of AEC
Patient-Size AEC
Z-axis AEC
Rotational or Angular AEC
Operation of AEC Systems on
Different Multidetector CT Scanners
Exposure 3D
CARE Dose 4D

19. Patient-Size AECPatient size AEC: the tube current is adjusted
based on the overall size of the patient to reduce the
variation in image quality between small patients and
large patients. For a given patient size, the
appropriate milliamperage is selected and is used for
the entire examination or scan series
Hi-mA
Lo-mA

20. Z-axis AEC: tube current is modulated
according to patient attenuation along the z-
axis.The goal is to reduce the variation in image
quality of images from the same series.
attenuation

21. Patient-Size AECRotational AEC: The tube current is decreased
and increased rapidly (modulated) during the course
of each rotation to compensate for differences in
attenuation between lateral (left-right) and A-P
(anterior-posterior) projections .
highattenuationlow attenuation

22. Combination of AEC functions
• Tube current is adjusted during scanning to compensate for
attenuation differences – dose applied to patient only where
needed, avoiding dose where it isn’t
mA
position

23. On line’ modulation–uses attenuation data from previous rotation–
adapts tube current to patient attenuation ‘on the fly’

25. Scan projection radiographs (SPRs, known as scout,
scanogram or topogram views) are the main way that
AEC systems assess the attenuation of the patient in
order to set the tube current

26. To obtain correct attenuation data from SPR
always centre the patient carefully
Patient is positioned in the
isocenter –
optimal dose and image quality
Patient is positioned too high -
increased mAsPatient is positioned too low -
reduced mAs and increased noise

27. Toshiba:defining image quality
requirements
Specify s.d. level (or ‘image quality level)–patient
mAcalculated to achieve this noise level at any
scan parameter settings
Set min & max mA

28. Benefits of CT scanner
AEC
Consistent image quality
Potential for dose reduction
through
exposure optimisation
Reduced tube loading
Extended scan runs(OLP)
Reduction in photon starvation
artifact

30. 30
• Images along length of phantom (no AEC)
•
•
•
•
•
•
•
•
Constant mA
Testing the AEC

31. 31
Testing the AEC
• Measure noise with AEC off and on
• Monitor mA, CTDIvol
0
4
8
12
16
20
24
28
-150 -100 -50 0 50 100 150
Z-position (mm)
Noise(%)
automA off
Noise Index 12
Increased
mA
Decreased
mA
Constant mA
Same
mA
Increased
mA
Decreased
mA

32. 32
Coronal view Sagittal view
z-axis
AEC off
z-axis
AEC on
Noise
increases
Constant
noise
Testing the AEC – Viewing with MPR

34. photon starvation artifact

35. without angular mA modulation with angular mA modulation

36. Bowtie Filter
Longer bowtie path lines up with shorter
patient path

37. Reduce x-ray scatter (noise and artifact)
Maintain uniform x-ray at detector
Reduce surface dose by 50%

38. • Retrospectively gated CCTA(dose modulation )
~15 mSv100%
20% ~30% reduction

39. Image obtained in middiastole
(75% of R-R interval)
reconstructed using 1.5-mm slice
thickness at high tube current
shows low-noise
Image obtained in midsystole
(30% of R-R interval)
reconstructed using 1.5-mm slice
thickness shows higher-noise
image obtained at low radiation
dose

40. • Prospectively gated CCTA
~ 1 mSv
Prospective Gating of CTA(snapshot plus)
40mm
5mm overlap

41. Scan range
Conventional technology
without Dose Shield
SOMATOM Definition AS+ with
Adaptive Dose Shield
Scan range
Dynamic Collimation
• In helical scanning extra rotations are needed at end of imaged
volume
– Significant extra dose: wide beam widths and short scans
• Dynamic collimation - collimator blades open and close
asymetrically at start and end of scan

42. (a) conventional and (b) adaptive section collimation CT scanning protocols. For
adaptive section collimation, shape of x-ray cone beam at beginning and end of spiral
acquisition is controlled by two collimators made of absorbent material.

43. Image reconstruction(FBP)
attenuation
detectorposition
90°2 projections4 projections8 projections16 projections
attenuation detector position
0°

44. 0(3) 0(9)
0(7) 0(5)
6 6
6 6
10 14
12
12
12 12
5 7
5 7
3 9
7 5
3
5
9
7
816
12 12
+1-1
-2+2
Iterative reconstruction

46. Iterative reconstruction

47. 120 kVp and 300 mA FBP
120 kVp and 300 mA ASiR 100%
120 kVp and 300 mA ASiR 50%