1. CT physics and instrumentation
Lecture (10)
Radiation Dosimtery
RSSI 471
Prepared by
Mr. Essam Mohammed Alkhybari

2. Staff contact information:
Mr. Essam Mohammed Alkhybari
Radiological science and Medical Imaging Department
Lecturer in Nuclear Medicine stream
Prince salman university

3. Objectives: Radiation quantity and their units Biological effects
CT dosimtery measurement
CT and radiation risk
Estimation Effective dose
Diagnostic reference levels

4. Radiation quantities and their units
Radiation dose quantities are described in numerous ways:
Exposure
Absorbed dose
Equivalent dose
Cumulative Equivalent dose
Effective dose

5. 1- Exposure: Definition:
The amount of energy ionized in air per unit of mass
Number of ions produced in air by photon
Conventional unit Rontjen (R)
SI coulomb per kilogram (C/kg)

6. 2- Absorbed dose: Definition:
The amount of energy deposited per unit of material (e.g., human tissue), is called the absorb dose.
SI unit gray (Gy)
Conventional unit rad

7. 3- Equivalent dose: Definition:
It is used to assess how much biological damage is expected from the absorbed dose from specific type of radiation based on radiation quality factor (x-ray. Gamma ray, alpha …etc)
Different types of radiation have different damaging properties.
The unit for the quantity equivalent dose is the Sievert (Sv)
Equivalent dose (in Sv) = absorbed dose (in Gy) x radiation weighting factor(Quality factor, RBE)

8. Radiation Weighting factors

9. 4- Cumulative radiation dose:
Definition:
Total energy absorbed by patient’s body per examination or procedure.
SI unit mGy× cm
Conventional unit:
mrad× cm

10. 5- Effective Dose Definition:
Probability of a harmful effect from radiation exposure depends on what part or parts of the body are exposed, and
A method is required to permit comparison of the risks when different organs are irradiated
A tissue weighting factor is used to take into account that some organs are more sensitive to radiation than others.
When an equivalent dose to an organ is multiplied by the tissue weighting factor for that organ the result is the effective dose to that organ.
The unit of effective dose is the sievert (Sv).
convetional unit: rem
The effective dose is obtained by multiplying the equivalent dose of each tissue or organ by an appropriate tissue weighting factor and summing the products
When an equivalent dose to an organ is multiplied by the tissue weighting factor for that organ the result is the effective dose to that organ.

11. Biological effect: hazard
One of the reasons why technologists and radiologists should have a clear understanding of the dose in CT relates to radiation biological effect.
Can be classified as:
Stochastic
Deterministic (non-stochastic)

12. What makes ionizing Radiation dangerous?
Biological effect – hrs, days, years following exposure, next generation, not at all, depends on bonds broken.
It has the potential to break chemical bonds
The biological effects of radiation are principally related to
damage to DNA.
Direct- radiation reacts with DNA, leads to biological change e.g. alpha, neutrons
Indirect- radiation reacts with abundant water in cells, free radicals produced, free radicals damage DNA.e.g.
H20  H20+ + e-
H20+ + H20  H30+ + OH
OH – highly reactive! Hydroxyl radical that may interact with DNA
Biological effect – hrs, days, years following exposure, next
generation, not at all, depends on bonds broken.

13. Why measurement dose is important ?
Compare system
Compare protocol
Estimate Patient risk

14. Tools for measurement:
Pencil ionisation chamber mm length (Measurements free-in-air)
Standard dosimetry phantom (with pencil ionization chamber) using head and body phantom
Alternatives: TLD, solid state detectors.

15. CT radiation measurement:
Measurement or estimation the radiation dose quantity in CT can be performed by using:
CT Dose Index (CTDI) or Multiple Scan average Dose (MSAD)
Dose Length Product (DLP)

16. MSAD: Definition: average of absorbed dose from series of slices
Because the early CT examinations consist of a series of “ stop-and-go” scan (slices), MSAD was the dose descriptor for use in a clinical situation at that time.
The MSAD concept will be highlighted for historical reasons only.

17. CTDI:
The CTDI is the special quantity used to express radiation dose in CT.
When the appropriate factors are applied to convert the measured phantom CTDI to an actual patient scan, the CTDI is a reasonable estimation of the actual absorbed dose to the patient.
One reason it is used is because it is not easy to measure the actual dose delivered to the internal body regions.
When the appropriate factors are applied to convert the measured phantom CTDI to an actual patient scan, the CTDI is a reasonable estimation of the actual dose to the patient.

18. CTDI :
CT Dose Index (CTDI) is the measure of ionizing radiation exposure per slice of data acquisition.
CTDI is the total energy absorbed within a dose profile deposited within one nominal collimation
CTDI= area( mGy)/ T (slice thickness mm)

19. Dose at a single point: CTDI100 :
100 (CTDI100) is used to denote the measurement length.
Allowed calculation of the index for 100 mm along the length of entire pencil ionization chamber
Don’t normally want to measure dose at a point
We want dose in area or volume

20. Weighted CTDIw:
CTDIw is a weighted average of the CTDI at the center and periphery of the phantom
CTDIw represents the average radiation dose over the x and y direction
CTDIW = Weighted avg. of center (1/3)+ peripheral (2/3) contributions of dose.
This can be done using a phantom where a pencil ionization chamber is postioned in the center (CTDIcenter) and at the periphery (CTDIperiphery) of the phantom

21. Volume CTDIvol : CTDIVOL= CTDIW/pitch
CTDIvol (or CTDI volume) represents the dose for a specific scan protocol which takes into account gaps and overlaps between the radiation dose profile from consecutive rotations of the x-ray source.
CTDIvol is the approximate average radiation dose over x, y, and z axis of the patient
CTDIvol is similar to CTDIw but also includes the effect of pitch on the radiation dose.
CTDIVOL= CTDIW/pitch
CTDIvol (or CTDI volume) represents the dose for a specific scan protocol which takes into account gaps and overlaps between the radiation dose profile from consecutive rotations of the x-ray source.
CTDIvol is similar to CTDIw but also includes the effect of pitch on the radiation dose.

22. CT radiation measurement: dose length product (DLP):
Definition (DLP):
The DLP is a Practical Quantity for Expressing the Total Radiation Energy Deposited in the Body and to estimate radiation risk
DLP= CTDIVOL x scan length
(mGy.cm)
A way to relate scan to risk
Estimate stochastic radiation risk

23. CT Dose Descriptors

24. Effective Dose from DLP on console of CT scanner

25. (dose in phantom per slice) Length of scan and pitch
CT and Risk:
Exposure
(in phantom)
CTDI
(dose in phantom per slice)
Length of scan and pitch
DLP
Effective dose
Risk

27. Diagnostic Reference Level(DRL):
Definition of DRLs:
Dose levels in medical radiodiagnostic practices or, in the case of radio-pharmaceuticals, levels of activity, for typical examinations for groups of standard-sized patients or standard phantoms for broadly defined types of equipment. These levels are expected not to be exceeded for standard procedures when good and normal practice regarding diagnostic and technical performance is applied."

28. DRLs

29. DRLs:
The main purposes of establishing DRLs in CT are to limit the number of unjustifiably high-dose examinations by promoting good practice and implementing the ALARA principle
The levels are set at approximately the 75th percentile of these measured data, meaning that the procedures are performed at most institutions with doses at or below the reference level.
If such doses are found to exceed the corresponding reference dose(75%), possible causes should be investigated and corrective action taken accordingly, unless the unusually high doses could be clinically justified.

30. DRLs: DRLs can be established at different levels:
DRLs are established by a country at a national level.
Local DRLs are established by a hospital or group of hospitals to monitor local practice.

31. DRLs:
The impact of this new technology on established DRLs needs to be investigated , especially if changes in protocols, procedures, or equipment are affected