1. RADIATION DOSE ISSUES

2. Exposure
The term exposure describes the ability of x rays to ionize air.
It is measured in roentgens (R); this unit is defined as the quantity of x rays that produces × 10−4 C of charge collected per unit mass (kilograms) of air at standard temperature and pressure (STP):
1 R = C/kg air. It describes how much ionization is present in the volume, but it does not tell how much energy is absorbed by the tissues being irradiated.
November 2002 RadioGraphics, 22,

3. Absorbed Radiation Dose
describes the amount of energy absorbed per unit mass at a specific point.
It is measured in grays (1 Gy = 1 J/kg) or
rads (1 rad = 100 erg/g).
100 rad = 1 Gy.
Absorbed dose essentially describes how much energy from ionizing radiation has been absorbed in a small volume centered at a point
it does not describe where that radiation dose is absorbed or reflect the relative radiosensitivity or risk of detriment to those tissues being irradiated.

4. Effective Dose
takes into account where the radiation dose is being absorbed (eg, which tissue has absorbed that radiation dose)
It is a weighted average of organ doses, as described in Publication 60 of the International Commission on Radiological Protection (ICRP)
Unit is the Sievert (Sv) or rem.
100 rem = 1 Sv.
E = effective dose, T-tissue, wT- tissue weighting factor, wR - radiation weighting factor, DT,R – average absorbed dose

5. CT dose Due to CT geometry dose estimation needs specialized methods
In CT the dose is continuous and around the patient, not projectional
There are multiple repeated exposures of an area during the scan
There are multiple scans

6. Dose differences within a section
In projectional radiography there is entry and exit side of the patient. The dose is higher to the skin at the entry and less at the exit.
In CT the tube rotates 360° around the patient and dose is symmetrically distributed from the periphery to the centre
There is a dose gradient from the periphery to the centre

7. The slope of the gradient depends on
The size of the object
The radiation spectrum
The degree of attenuation by tissues

8. Large part of anatomy 32-cm phantom one detector row
2:1 radiation dose difference
120 kVp,
280 mA,
1-sec rotation time (280 mAs),
10-mm slice thickness

9. Small part of anatomy 16-cm phantom one detector row
No dose difference
120 kVp,
300 mA,
1-sec rotation time
5mm slice thickness

10. Z-axis
Apart from dose variations within a section there are differences along the Z- axis
They describe the dose outside a section due to single tube rotation
Radiation affects tissues at the section neibourghood because
There is no ideal beam collimation and
There is scatter generated within the patient
A series of sections add up to make the total dose

11. Dose histogram from a single tube rotation at the scanner isocenter Z-axis

12. Isocenter, where the central lines of the beam dissect
In CT the isocenter is in the center of the gantry opening

13. The stray dose depends on:
Therefore
The dose in a section is the sum of the dose that produces the section image and the dose the section absorbs when the surrounding sections are taken
The stray dose depends on:
Distance between sections
Section thickness
Shape of the beam

14. Ion chamber Measures radiation >10 keV It contains air
Closed cylinder with a positive (anode) an a negative (cathode) electrode. Potential difference applied in between them
The radiation causes ionization and the amount measured by the instrument

15. Dose measurements CT specific
Multiple Scan Average Dose (MSAD)
The average dose from a series of scans over an interval in length
Computed Tomography Dose Index
CTDI100- dose over 100mm length of ion chamber
CTDIw – weighted average between peripheral and central distribution
CTDIvol – takes into account protocol parameters like pitch for spiral scans or acquisition spacing for axial scans.

16. CTDIvol
One final CTDI descriptor takes into account the parameters that are related to a specific imaging protocol, the helical pitch or axial scan spacing
where N and T are as defined earlier and represent the total collimated width of the x- ray beam and I is the table travel per rotation for a helical scan or the spacing between acquisitions for axial scans.

17. Phantoms only partly reflect reality
They produce estimations and not exact measurements
They don’t take into account specific tissue sensitivity

18. Dose-length product (DLP)

19. Factors that affect CT dose
Direct effect
kilovolt peak
Milliamperes
Rotation time
Section thickness
Subject thickness
Pitch
Dose modulation techniques
Tube – isocenter distance
Indirect effect
Affect the image quality and indirectly the dose, i.e. reconstruction algorithms

20. kVp Radiographer's choice Affected by tube filtration
120 to 140kVp ≈ 35% increase in dose

21. mAs mA rotation time Linear dose increase
mAs=effective mAs = mA Χ time/pitch
When using effmAs changing one of the parameters automatically affects the others to keep effmAs constant.

22. Pitch Table movement in 360° / section thickness
Increasing the pitch reduces dose
CTDIvol the only marker taking into account pitch

23. Section Thickness – Single detector scanner
Section thickness affects dose
For thicker section more photons will pass through the patient because of wider collimation all other parameters kept stable. BUT …..
Exposure and adsorbed dose depend on the irradiated mass and in thicker sections more mass is irradiated so doses may be almost equivalent
There is however more dose overlap for thin sections.
Dose in single section scanner with thickness change only

24. Dose Efficiency vs Slice Thickness
What fraction of the x-ray beam is “lost” by discarding the penumbra region depends both on colllimator design and total beam width. The size of the penumbra region depends on collimation and focal spot size, and is approximately constant for all usefull beam widths. Thus the penumbra will represent a larger percent loss (perhaps ~30%) for a 4x1.25 mm scan (5 mm total width) than for a 4x2.5 mm acquisition (10 mm total beam width). For a 4x5 mm scan (total width of 20 mm) the loss is even smaller (<10%). As more simultaneous slices are scanned and total beam widths enlarge with future cone beam scanners, the lost penumbra will be of minimal concern.

25. Section thickness – Multislice CT
Dose depends on beam thickness and not on the thickness of the reconstructed image. All other factors kept stable this may mean an increase up to 50% for smaller beam width.
As seen on table below a 5mm section can be reconstructed from
4 × 1.25 mm
4 × 2.5 mm
4 × 5 mm but CTDI is stable

26. Patient size
All parameters stable the smaller object absorbs more radiation.
This is extremely important for the paediatric patient
It is due to the fact that the body is exposed to entry and exit dose as the tube is rotating and
the entrance dose is not much attenuated so the smaller obbject is homogenously and highly irradiated all over
Radiation dose to children is therefore a big issue

27. Options for reducing Dose
Adjust mA to patient size and absorption
Usually a maximum mA value is selected which is reduced whenever absorption is lower in a certain area or subject
A 0° and a 90° scanogram is required. On these the mA value is calculated along the scanning length and the value of mA is changing during acquisition
It is even possible to adjust mA in real time mode

28. Indirect effects on dose
Slice thickness
Thin sections reduce SNR. This is seen as wide standard deviation of HU numbers
Noise = 1/√T, T = slice thickness
Noise at 10-mm = noise at 1-mm / 3.2
This implies higher dose for satisfactory thin section SNR with increased mA or kV.
Reconstruction filters
High resolution filters iincrease noise and reduce SNR
This may mean increased dose for better SNR when such algorithms are used.

29. Ways of dose reduction

30. Reduce Milliamper-Seconds
Lowering mA by 50% reduces dose by 50% but increases noise by 41% (1/√ 2 = 1.41).
Depending on the clinical needs the above changes may be acceptable or not
i.e.
In the lung there is high inherent contrast between lung parenchyma and solid lesions and high noise is easily acceptable
In the liver there is low inherent contrast between parenchyma and lesions therefore a higher dose is necessary

31. Increase Pitch
Dose reduction when all other parameters are kept stable
BUT ………. it increases the effective section thickness which increase partial volume averaging and therefore reduces contrast between lesion and surrounding tissue.

32. Milliamper-Seconds and patient size
Image quality increases with higher dose and the is no image deterioration by overexposure
Doses to patients are high
Several methods are in use which attempt to adjust dose to patient size and weight
Exposure tables for CT are available as in plain radiography

33. kVp reduction Dose reduction and noise increase
Also changes in contrast
CTDI100 (and not CTDIw) shows that the dose gradient from the periphery to the center of the object is grater in lower kV
The dose to the skin is also higher at low kVp

34. Effective dose calculation
effective dose = dose that takes into account the differences in sensitivity to radiation of the several body tissues
It is calculated with several ways in phantoms

35. Noise lines
They are oriented along the maximum absorption of radiation because only a small number of photons reach the detector in this direction
They are lines either in the laterolateral or the anteroposterior direction (shoulders, pelvis)
When a stable mA is used patient taking into account the lateral absorption i.e. in the shoulders the chest will be irradiated more than necessary in the AP direction
The same holds true for changes in the Ζ-axis

36. Noise lines in the shoulders (photon starvation artifact)
European Journal of Radiology Volume 76, Issue 1, October 2010

37. Automatic Exposure Control - AEC
AEC systems adjust mA depending on body thickness along the Z-axis and within a section
in order to achieve satisfactory SNR with lower dose
There are 3 ways to apply AEC
Patient size
rotational
3-D

38. Patient size - AEC
In CT patient is exposed in entry and exit radiation
AEC adjusts mAs taking into account the patient size and this is very efficient in children
Radiation dose to children is more important
The disadvantage the scan is performed with stable mAs regardless of the changes in patient size along the z-axis or within the section

39. Z-axis AEC Beam attenuation changes along the patient
High in the shoulder area
Low in the chest
Hogh in the abdomen
This leads to different noise levels along the images when stable mAs is used.
Satisfactory noise level in the chest means higher noise in shoulders and abdomen, not acceptable.
Satisfactory noise level in the shoulders means higher dose in the chest
Z-axis AEC adjusts the current along the scan depending on the scanogram in order to achieve stable noise in all images.
Dose reduction up to 30%

40. Beam attenuation in Z-axis in the 2 scoutviews

41. Renal images
α) stable mAs
b) Z-axis AEC
Different examinations same patient.
Satisfactory images

42. Rotational AEC
mAs changes as the tube rotates around the patient. mAs increases automatically at the tube positions where beam attenuation is higher
Adjustment is calculated by
Measurements in the previous section
Measurements on the 2 scout views
Dose reduction 20-30%

43. Images of the pelvis (a) without and (b) with rotational AEC.
Dose in β = 27% < α.
Image satisfactory

44. 3D-AEC
Combination of Z-axis and rotational AEC for even better results.
Available on all modern CT scanners.

46. Previous slide explanation
3D-AEC in chest and
(a) coronal reconstruction from CT performed with stable mAs (green line) and resultant noise (red line)
(b) coronal reconstruction from CT with 3D-AEC
The thick green line sows the average mAs value in the Z-axis and the thin green line changes in mAs due to patient thickness in the axial plane
Image noise is relatively stable in all images along the Z-axis.
(c) radiation dose distribution shows overexposure of the lungs with stable mAs when compared with (d) 3D-AEC

47. Dose reduction in cardiac CT
Is used in patients with average risk for coronary disease and in patients with acute onset chest pain.
Dose up to 20 mSv when no reduction techniques are utilized (CT abdomen 14 mSv, Lung CTA 15 mSv)
It requires thin sections 0.5–0.625 mm to show the thin coronary branches and low pitch
Fast tube rotation to achieve a “stationary heart” (0.3–0.4s)
Data are acquired during the whole cardiac cycle
only data from the diastole are used for Ct coronary angiography.
the complete set of data offer functional information
ejection fraction
movement of the heart wall.

48. Different ways of ECG gating
Retrospective gating
Continuous data collection
Reconstruction window is selected retrospectively at the middle of the diastolic phase of the ECG
Prospective gating with current modulation (PECMTC).
The mA is adjusted based on the ECG with a high value at the middle of the diastole and only at 30% during the remainder of the cardiac cycle.
Reconstruction window in the middle of diastole
Prospective gating (PEGAS).
Current is applied only during the middle of the diastole on ECG.
Depending on the heart rate the table moves either before the next R-wave on the ECG or with the next cardiac pulsation
“step and shoot”

50. Prospective electrocardiogram correlated modulation of tube current (PECMTC)
At the end of diastole the vessels are less affected by cardiac motion
By reducing current at the rest of the cardiac cycle the dose to the patient is reduced.
Current at 100% in middle diastole and at 30% in between
The data at reduced current are not sufficient for vessel study but they allow study of cardiac function.
More recently mA is reduced to 4% with a dose reduction by 25%

51. Prospective electrocardiogram gated axial scanning (PEGAS)
Scanners with very short rotation time for patients with heart rates >60 per minute
A very small pitch is necessary which results in radiation dose increase
PEGAS
Prospective ECG gating with axial sections (pitch=0), no dose from overlapping tube rotations
Prospective ECG gating with current applied in middle diastole only
PECMTC dose reduction by 25%,
PEGAS dose reduction by 68–78%
PEGAS requires stable heart rate
Only diastole information – no functional information

52. Dual source CT (DSCT)
A way to reduce scan time using 2 sets of tube-detectors at 90 rotating simultaneously
Scan time reduced by 50% SOMATOM Definition, Siemens Medical Solutions
? Radiation Dose reduction
Dual energy scanning possible at one acquisition

53. 256- & 320- rows of detectors
This large series of detectors can cover a long Z distance and area of the subject eliminating the need for table translation when applied to cardiac CT.
64-slice (4cm coverage) with 3 rotation per cardiac cycle may produce reconstruction steps
256 (Brilliance iCT, Philips Healthcare) και 320 (AquilionOne Dynamic Volume CT, Toshiba Medical Systems) approximatelly 16cm coverage per rotation = no need for table movement
Dose reduction by 50–65% compared with traditional Coronary CTA
Need for further studies

54. Use of low kVp There are 2 advantages: Lower dose Increased contrast
But
Increased noise
When IV contrast is administered the noise increase is balanced by the increase of anatomy contrast
100kVp instead of 120kVp good vessel resolution with reduced patient dose by %
The same holds true for children and small size adults

55. Dose reduction with noise reduction algorithms
adaptive statistical iterative reconstruction (ASIR)
model based iterative reconstruction (MBIR
iterative reconstruction in image space (IRIS)
adaptive iterative dose reduction (IARD)
The image is reconstructed with different methods than the traditional backprojection
Dose reduction by 30-60%

56. Comparison of images (a) backprojection and (b) ASIR in different CT scanners
dose length product (DLP) in ASIR study (b) 35% < (α) filtered back projection FBP
ASIR image less noisy

57. Overscanning - Overbeaming

58. Overranging/overscanning
Data from a further ½ rotation are needed in order to reconstruct the first and the last image of a plan.

59. Overbeaming Z-Axis Dose Profile for 10 mm slice
The beam width must be wider than the detector coverage in order to achieve homogenous beam density over all rows of detectors.
This is due to the diverging rays of the beam
A second factor may be understood by referring back to the dose profile for the single slice scanner, for which the slice thickness was taken as the profile’s full width at half maximum (FWHM) size. If we were to divide this 10 mm slice into 4 slices scanned by a 4-slice MSCT with 4 x 2.5 mm detector configuration, it is clear the the “outer” two slices would receive considerably less radiation and would thus be considerably noisier.
The only way to avoid this is to widen the x-ray beam so that all four slices are uniformly irradiated. Now, however, we waste the entire penumbra (i.e., edge region) of the dose profile, which can no longer contribute to image formation. Depending on what fraction of the total x-ray beam this represents, this represents another 10-30%

61. AAPM/RSNA Physics Tutorial for Residents: Topics in CT Radiation Dose in CT1. Michael F. McNitt-Gray, PhD. November 2002 RadioGraphics, 22,
Impact of new technologies on dose reduction in CT. Ting-Yim Leea, , and Rethy K. Chhem. European Journal of Radiology Volume 76, Issue 1, October 2010, 28-35
Radiation Dose Associated With Common Computed Tomography Examinations and the Associated Lifetime Attributable Risk of Cancer. Rebecca Smith-Bindman, MD; Jafi Lipson, MD; Ralph Marcus, BA; Kwang-Pyo Kim, PhD; Mahadevappa Mahesh, MS, PhD; Robert Gould, ScD; Amy Berrington de Gonza´ lez, DPhil; Diana L. Miglioretti, PhD. ARCH INTERN MED/VOL 169 (NO. 22), DEC 14/28, 2009