CT physics and instrumentation

Lecture (10)

Radiation Dosimtery

RSSI 471

Prepared by

Mr. Essam Mohammed Alkhybari

Mr. Essam Mohammed AlkhybariRadiological science and Medical

Imaging DepartmentLecturer in Nuclear Medicine streamPrince salman universityE-mail: e.alkhybari@sau.edu.sa

Staff contact information:

Radiation quantity and their unitsBiological effectsCT dosimtery measurementCT and radiation riskEstimation Effective doseDiagnostic reference levels

Objectives:

Radiation dose quantities are described in numerous ways:

1. Exposure 2. Absorbed dose3. Equivalent dose 4. Cumulative Equivalent dose 5. Effective dose

Radiation quantities and their units

Definition:

The amount of energy ionized in air per unit of mass

Number of ions produced in air by photonConventional unit Rontjen (R) SI coulomb per kilogram (C/kg)

1- Exposure:

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

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)

3- Equivalent dose:

Radiation Weighting factors

Definition:Total energy absorbed by patient’s body per examination or procedure.SI unit mGy× cmConventional unit:mrad× cm

4- Cumulative radiation 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

5- Effective Dose

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:1. Stochastic2. Deterministic (non-

stochastic)

What makes ionizing Radiation dangerous?

Biological effect – hrs, days, years following exposure, next generation, not at all, depends on bonds broken.

1.Compare system2.Compare protocol3.Estimate Patient risk

Why measurement dose is important ?

Tools for measurement:

Pencil ionisation chamber- 100 mm length (Measurements free-in-air)

Standard dosimetry phantom (with pencil ionization chamber) using head and body phantom

Alternatives: TLD, solid state detectors.

CT radiation measurement:

Measurement or estimation the radiation dose quantity in CT can be performed by using:

1. CT Dose Index (CTDI) or Multiple Scan average Dose (MSAD)

2. Dose Length Product (DLP)

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.

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.

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)

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

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.

Volume CTDIvol :

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

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

CT Dose Descriptors

Effective Dose from DLP on console of CT scanner

CT and Risk:

25

Exposure (in phantom)

CTDI (dose in phantom per slice)

Length of scan and pitch

DLP

Effective dose

Risk

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."

Diagnostic Reference Level(DRL):

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.

DRLs:

DRLs can be established at different levels:

1) DRLs are established by a country at a national level.

2) Local DRLs are established by a hospital or group of hospitals to monitor local practice.

DRLs:

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

DRLs: