CT Radiation Dose & Safety

1. ‫تعلم‬ ‫تكن‬ ‫مالم‬ ‫وعلمك‬
‫هللا‬ ‫فضل‬ ‫وكان‬
‫عظيما‬ ‫عليك‬

2. Basics Of Computed
Tomography (CT)
Lecture Ten
Gamal Fathalla M.
Mahdaly
gamal_mahdaly@hotmail.com

3. Radiation Safety and Protection
The potentially harmful effects of ionizing
radiation are either:
 Stochastic effects
Or
 Deterministic effects

4. Stochastic effects
 Effects where the probability of the occurrence
increases with radiation exposure.
e.g. carcinogenesis and genetic effects.
i.e.
((The probability, but not severity,
of the end point condition, is
dose-dependent))

5. Deterministic effects
 Effects related to a threshold dose, below
which the effect is not detected but above this
threshold dose , the probability that the effect
will occur is virtually 100%.
 The severity increases with increased dose.
e.g.
Erythema, epilation, desquamation, cataract,
fibrosis and hematopoietic damage.

6. CT Dosimetry
 To evaluate the potential risk/benefit of CT
scanning, dose should be estimated.
 2 main questions are to be answered:
1. How much radiation dose is my CT scanner
delivering to the patient?
2. How to compare my scanner dose to that of
other CT scanners?

7. Dose Measurements
 Erythema dose
 Slight film fog
 Roentgen (R)
 Exposure dose
 Absorbed dose
 Equivalent dose

8. Classic and SI units of radiation dose
Dose type Unit Abbr Unit Abbr
Exposure (air kerma)
For x-ray or gamma ray
ionization in air only
Roentgen R Coulomb
per
kilogram
C/kg
Absorption
Energy deposited by any type
of radiation in any material
Radiation
absorbed
dose
Rad Gray Gy
Equivalent
Biologic effect caused by any
radiation in a living organism
Roentgen
equivalent
mammal
Rem Sievert Sv
Classic units SI units

9. Exposure
 Ionizing radiation ionizes the gas molecules into
electrically charged ions . This has traditionally
been measured in terms of exposure E
Where Q is total electric charge produced and M is
the mass of air.
 The old exposure unit of 1 Roentgen is
equivalent to 2.58x10-4 C.kg-1.
Since 1 Gy =3x10-2 C.kg-1 then 1R = 8.6 Gy.
E= Q/M (C.Kg-1 )

10. Absorbed dose (Gy)
Is the energy absorbed by exposed material
(air or tissue) in joules/kg
E absorbed by a mass M of tissue: E/M is the
the gray (Gy)
1 Gy = 1x10-3 J.g-1
Absorbed Dose = E/M (Gy)

11. Dose equivalent (Sievert)
 Tissue damage due to different type of
ionizing radiation ( Gamma, X, Beta, and
Alfa) varies considerably.
 Dose equivalent allows for this by
multiplying the absorbed dose (grays) by
a weighting factor which depends on the
type of radiation.
Grays x Q = Sieverts (Sv)

12. CT Radiation Dose
CT Radiation Dose has 3 unique features:
Axial CT image is very much collimated with minimal
scattered radiation.
Dose is evenly distributed ( in modern CTs) due to
rotational acquistion.
To achieve high contrast resolution, CT needs high SNR
which necessiates high Dose/ Volume i.e. high technique.
PA Chest XR needs only 120 kVp, 2 mAs
Chest CT may need up to120 kVp, 200 mAs

13. X-ray beam geometry
Most modern scanners employ a fan-Shaped X-ray beam
Factors affecting Radiation Dose in CT

14. The width of x-ray beam is viewed form side
Collimator
determines the width:
An ideal dose distribution
along z axis is shown (B).
Actual bell-shaped dose
distribution curve (C).
A
B
C
A
Radiation Dose in CT

16. 16

17. 17
Radiation Dose
The main X-ray interaction
mechanism in CT is
Compton scattering.
CT slice acquisition delivers
a considerable dose from
scatter to adjacent tissues
outside the primary beam
path.
As slice number increases,
scattering also increases.

18. Methods of measuring patient dose
Of the many dose measurement methods, we
are going to consider only:
 The pencil ionization chamber method.
 CT dose index (CTDI) method.
 Multiple scan average dose (MSAD) method.

19. Radiation Dose Measurement - CTDI
CTDI is the dose to any point in
the patient including scatter from
7 CT slices in both directions (H/F
& F/H) ( a total of 14 slices).
The multiple scan average dose
MSAD can be estimated using a
single scan by measuring the CT
Dose Index (CTDI).
CTDI can be measured using a
pencil ionization chamber in
phantoms simulating head (16 cm
diameter acrylic) & bodies (32 cm
diameter acrylic.
Doses at the patient surface may
be higher than in the patient
center.

20. In head scans, the surface-to-center ratio is approximately 1:1.
In body scans, the surface-to-center ratio is approximately 2:1.
CTDI measurements are done at both the surface CTDI (Periphery) & center
CTDI (Center) of the phantom & then combined to give CTDIw
CTDIw = (2/3 CTDIperipheral + 1/3 CTDIcenter)

21. 21
Dose Measurement - CTDI

22. Dose index
 The CT dose index (CTDI) is mathematically defined as :
 Where n is the number of distinct planes of the data
collected during one revolution, sw is the slice width (in
mm), D(z) is the dose distribution, z is the dimension
along the patient’s axis. For axial CT scanners and spiral
CT scanners with single array of detectors, n = 1. For
multi slices CT scanner n is the number of active
detector rows.
*The integral sign merely instructs the user to determine the area single curve (D(z).
*

23. Measuring the CTDI
 CTDI is measured using long cylindrical
ionization chamber and radiation dose
from single slice.
 The ionization chamber receives radiation
from all parts of dose distribution D(z)
because its length is bigger than the width
of the X-ray beam.

24.  The total charge from the ionization chamber is
proportional to the integral in the CTDI definition.
•Where Q is the total charge collected during single scan and Cf is calibration
factor of ionization chamber.
•Because the ionization chamber measures the exposure and not the dose we, convert
Roentgen to cGy.

25. The integral in Equation is
numerically equal
to the area ( shaded region )
of the dose distribution
Note that the CTDI can be
increased by increasing the
area under the curve.
The area can be increased by
either :
1. increasing the intensity of
radiation, which
Raises top the of the curve.
Or
2. widening the beam ,
usually by opening the x-ray
collimator.

27. Multiple Scan Average Dose
 To measure radiation dose received by
patient from a series of CT scans, between
each two scans the patient is moved by a
bed index distance, each slice delivers its
characteristic bell-shaped dose.
 Finally, we have multiple consecutive bell-
shaped doses.

28. A series of 7spaced scans at a
constant bed index along the Z-
axis is acquired to produce 7 bell-
shaped dose distribution curves
(TOP).
Summation of these doses results
in the (BOTTOM) curve.
The total dose ( BOTTOM) curve
has peaks where the bell-shaped
curves overlap.
The dotted line through the total
dose curve is the multiple scan
average dose.
The MSAD is defined as the
average dose (at a particular depth
from the surface) resulting from a
large number of successive slices.
The MSAD refers to all the dose
delivered to the tissues including
dose due to scatter from all the
successive slices.

29. Multi-scan average dose (MSAD)
CTDI can be related to the MSAD by this equation
Where BI is the bed index or slice spacing (in mm), Sw is slice width (in
mm), and the number of active arrays of detectors.

30. CTDI-Dose Measurement

 Decreasing kVp reduces dose while other factors are constant
CTDI values for body scans are lower than those for head scans due to
greater attenuation of X-rays in the body.
These values DO NOT quantify patient risk because it DO NOT
consider the number of slices NOR the radiosensitivity of the irradiated
organs.
CTDI values increase with kVp, so decreasing kVp while others factors
remain constant reduces the CTDI values.

31. Effect o kVp On Radiation Dose
kVp not only
controls the
image
contrast but
also controls
the amount
of
penetration
that the x-
ray beam will
have as it
traverses the
patient
Parameter 80 kV 120 kV 140 kV
Image
Contrast
Best Intermediate Poor
Noise Most Average Least
Penetration Least Average Most
Patient Dose
per mAs
Lowest Intermediate Highest

32. Any Question???.
Take your Tiiime!!! .
Again Any Question???.
Otherwise, I’m going to ask!!!.
Should I Ask???.

33. Have a nice day