Radiation Protection in Paediatric Radiology

Radiation Protection in Paediatric Radiation Protection in Paediatric RadiologyRadiology

Understanding Radiation Understanding Radiation UnitsUnitsL02L02

Radiation Protection in Paediatric Radiology L02. Understanding radiation units 22

Educational Objectives

At the end of the programme, the participantsshould become familiar with the following: • Why is it important to measure radiation

dose in children?• How radiation dose can and should be

expressed?• Understand the radiation quantities and

units used in diagnostic radiology.


Answer True or False

1. The same amount of radiation falling on the person at level of breast, head or gonads will have the same biological effects.

2. Effective dose can be easily measured.3. Diagnostic reference levels are not

applicable to paediatric radiology.


Contents

• Dose descriptors outside the patient’s body.

• Dose descriptors for effects that have threshold (deterministic effects)

• Dose descriptors to estimate stochastic risks

• Diagnostic reference levels

• Dose descriptors and units for staff dose assessment


Introduction

• Several quantities and units are used in the field of diagnostic radiology to measure and describe radiation dose

• Some can be measured directly while others can only be mathematically estimated

6

Two types of radiation effects

Stochastic effects• Where the severity of the result is the same but

the probability of occurrence increases with radiation dose, e.g., development of cancer

• There is no threshold for stochastic effects• Examples: cancer, hereditary effects

Deterministic effects• Where the severity depends upon the radiation

dose, e.g., skin burns• The higher the dose, the greater the effect• There is a threshold for deterministic effects• Examples: skin burns, cataract

Radiation Protection in Paediatric Radiology L02. Understanding radiation units


Hot Coffee – Energy contained in a sip

Excess Temperature = 60º - 37 = 23º1 sip = 3ml3x 23 = 69

calories


Radiation Dose

Lethal Dose= 4GyLD 50/60 = 4 GyFor man of 70 kg

Energy absorbed = 4 x 70 = 280 J= 280/418= 67 calories= 1 sip

Energy content of a sip of coffee if derived in the form of X-rays can be lethal

X-

rays


Dose of Radiation

• Radiation energy absorbed by a body per unit mass.


Dose Quantities and Radiation units

- Dose quantities external to the patient’s body.

- Dose quantities to estimate risks of skin injuries and effects that have threshold.

- Dose quantities to estimate stochastic risks.


Why so many quantities?Radiation dose is a complex topic

• 1000 Watt heater giving off heat (IR radiation) - unit is of power which is related with emission intensity

• Heat perceived by the person will vary with so many factors: distance, clothing, room temperature

• As can be seen with the example of heat, the energy transformation is a highly complicated issue

• This is the case with X-rays - radiation can’t be perceived


Basic Radiation Quantities

• Used to quantify a beam of X or γ-rays

• There are:• Quantities to

express total amount of radiation.

• Quantities to express radiation at a specific point

Radiation at a specific point

•Photon fluence

•Absorbed dose

•Kerma

•Dose equivalent

Total radiation

•Total photons

•Integral dose


Exposure: X

• Exposure is a dosimetric quantity for measuring ionizing electromagnetic radiation (X-rays & Ɣ-rays), based on the ability of the radiation to produce ionization in air.

Units: coulomb/kg (C/kg)

or roentgen (R)

1 R = 0.000258 C/kg

http://images.google.se/imgres?imgurl=http://www.hko.gov.hk/education/dbcp/radiation/eng/image/ionization.jpg&imgrefurl=http://www.hko.gov.hk/education/dbcp/radiation/eng/r7.htm&usg=__TnBtdWPl6MyaIL834IcXFfdTZMM=&h=166&w=353&sz=17&hl=sv&start=9&tbnid=S6KWNr4QQNIiVM:&tbnh=57&tbnw=121&prev=/images?q=ionization&gbv=2&hl=sv


KERMA

KERMA (Kinetic Energy Released in a Material):• Is the sum of the initial kinetic energies of all

charged ionizing particles liberated by uncharged ionizing particles in a material of unit mass

• For medical imaging use, KERMA is usually expressed in air

SI unit = joule per kilogram (J/kg) or gray (Gy)

1 J/kg = 1 Gy


Absorbed dose, D, is the mean energy imparted by ionizing radiation to matter per unit mass.

SI unit = joule per kg (J/kg) or gray (Gy).

In diagnostic radiology, KERMA and D are equal.

Absorbed dose: D

Harold Gray

http://www.spock.com/picture_frame/78c81wb451a51421


Mean absorbed dose in a tissue or organ

The mean absorbed dose in a tissue or organ DT is the energy deposited in the

organ divided by the mass of that organ.


Now things get a little more complicated !

Radiation Dose Quantities

• Primary physical quantities are not used directly for dose limitation

• The International Council on Radiation Protection (ICRP) has defined values for dose limits in occupational exposure


Radiation Dose Quantities

Equivalent Dose:

• Accounts for the type of radiation• Different radiation types have different level

of biologic damage per unit absorbed dose


Radiation Weighting Factors, wR

Radiation type Radiation weighting factor, wR

Photons 1

Electrons and muons 1

Protons and charged pions 2

Alpha particle, fission fragments, heavy ions

20

Neutrons A continuous curveas a function ofneutron energy

(Source: ICRP 103) Radiation Protection in Paediatric Radiology L02. Understanding radiation units


Equivalent Dose : HT,R

The absorbed dose in an organ or tissue multiplied by the relevant radiation weighting factor :

where DT,R is the average absorbed dose in the organ or tissue T, and wR is the radiation weighting factor for radiation R.

RTRRT DwH ,,


Radiation Quantities and Units

Equivalent dose (Unit = sievert, Sv )•Compares the biological effects

for different types of radiation, X-rays, Ɣ-rays, electrons, neutrons, protons, α-particles etc.

•For X-rays, Ɣ-rays, electrons : absorbed dose and equivalent dose have the same value Gy = Sv.

Rolph Sievert


Detriment

• Radiation exposure to different organs and tissues in the body results in different probabilities of harm and different levels of severity.

• The combination of probability and severity of harm is called “detriment”.

• Effective dose reflects the combined detriment from stochastic effects due to the equivalent doses in all the organs and tissues of the body.


Effective Dose: ET

• Effective dose takes into account the organ specific radio-sensitivity to develop cancer and hereditary effects from radiation

• Unit = sievert, Sv


Effective Dose: ET

A summation of the tissue equivalent doses, each multiplied by the appropriate tissue weighting factor:

where HT is the equivalent dose in tissue T and wT is the tissue weighting factor for tissue T.

T

TTHwE


• The organs have different weighting factors, wT.

• These factors are published in ICRP 103 (2007) and have been changed over the years due to increased knowledge.

Tissue Weighting Factors, wT


• The weighting factors sum up to 1.0.• They are relative and compares one organ

with the other.• They are the same for children and adults!

Tissue Weighting Factors


• Data is primarily taken from knowledge derived from studying the Japanese population exposed to atomic bombs in Hiroshima and Nagasaki

• On going research has changed the weighting factors from 1990 (ICRP 60) to 2007 (ICRP 103).

28


http://images.google.com/imgres?imgurl=http://www.century-of-flight.net/Aviation%20history/WW2/images2/atomic2.jpg&imgrefurl=http://www.century-of-flight.net/Aviation%20history/WW2/atom%20bomb.htm&usg=__QKEWaMjTTVKgsjlSHmb3A2c6hPI=&h=283&w=380&sz=28&hl=sv&start=7&tbnid=1uQKCUpEt7sgEM:&tbnh=92&tbnw=123&prev=/images?q=atombomb+nagasaki&gbv=2&hl=sv


Multipliers of the equivalent dose to an organ or tissue to account for the different sensitivities to the induction of stochastic effects of radiation.


Tissue weighting

factor

wT*

∑ wT

Bone-marrow (red), Colon, Lung, Stomach, Breast, Remainder Tissues**(nominal weighting factor applied to the average dose to 14 tissues)

0.12 0.72

Gonads 0.08 0.08Bladder, Esophagus, Liver, Thyroid 0.04 0.16Bone surface, Brain, Salivary glands, Skin 0.01 0.04*ICRP 103

**Remainder Tissues (14 in total): Adrenals, Extrathoracic (ET) region, Gall bladder,Heart, Kidneys, Lymphatic nodes, Muscle, Oral mucosa, Pancreas, Prostate, Small intestine, Spleen, Thymus, Uterus/cervix..


Dose to lungs times their weighting factor; DL x wL

+Dose (mean absorbed dose)

to gastrointestinal tract times their weighting factor;

DGI x wGI

+....(summation over organ

after organ)=

Effective dose

Effective Dose (E)

where T stands for tissuewhere T stands for tissue

T

TTHwE


Effective Dose (E)

We can compare different paediatric imaging procedures through their different effective

doses, E.


Radiation Quantities and Units used in Diagnostic Radiology

• Incident air kerma•Entrance surface air kerma•Air kerma-area product•Air kerma-length product•Dosimetric quantities for CT •Dosimetric quantities for interventional

radiology


Incident Air Kerma

Measured Free in Air on the central beam axis at the focal spot to surface distance.

Only primary beam is considered, that is, no scatter contribution.

Unit: joule/kg or gray (Gy)


Entrance Surface Air Kerma (ESAK)

• ESAK measured on the surface of the patient or phantom where X-ray beam enters the patient or phantom.

• Includes a contribution from photons scattered back from deeper tissues, which is not included in free in air measurements.



• If measurements are made at other distances than the true focus - to - skin distance, doses must be corrected by the inverse square law and backscatter factor incorporated into the calculation.

References:

• Dosimetry in Diagnostic Radiology: An International code of practice, TRS 457, IAEA, 2007

• Phys. Med. Biol. 43 (1998) 2237-2250.


Kerma in X-ray field can be measured using calibrated: • Ionization chamber

• Semiconductor dosimeter

• Thermoluminescent dosimeter (TLD)

Dose Measurement


Kerma-Area Product: KAP

• The kerma - area product (KAP) is defined as the kerma in air in a plane perpendicular to the incident beam axis, integrated over the area of interest.

• This is the dose related quantity measured and displayed on all modern X-ray equipment excluding CT.

KAP meter



• The KAP (Gy·cm2) is constant with distance since the cross section of the beam is a quadratic function which cancels the inverse quadratic dependence on dose .

• KAP remains constant along the beam axis as long as it is not measured close to the patient/phantom surface which introduces backscatter.


KAP = K x Area

the SI unit of KAP is the Gy·cm2

Area = 1Dose = 1

Area = 4Dose = 1/4

d1=1

d2=2



KAP is independent of distance from the X-ray source, as:

Air Kerma decreases with the inverse square law.

Area increase with the square distance

KAP is usually measured at the level of the tube diaphragms

Area = 1Dose = 1

Area = 4Dose = 1/4

d1=1

d2=2



KAPKAP (kerma(kerma--area product)area product)

UnitUnit: : GyGy··cmcm22This is a picture of a KAP meter which measures the kerma area product


Example of a dose display during fluoroscopy or cine

runs with dose rate as shown


In paediatric radiology KAP may be used for:

•Diagnostic reference levels (DRLs)•By use of conversion factors, it can be

converted to skin dose and/or effective dose

43

Kerma-Area Product


Dosimetric Quantities for CT

• Computed Tomography Dose Index (CTDI)

- determined using scan protocol parameters.-useful for comparison of different scanners.

• Dose-Length Product (DLP)- measure of dose to patient- used to estimate effective dose


CT and Risk KERMA (in phantom)

CTDI (dose in phantom per slice)

Length of scan and pitch

DLP

Effective dose

Risk


Measurement of Dosimetric Quantities in CT

• Pencil ionisation chamber with active length of 100 mm.

• • Measurements free-in-air or in

standard dosimetry phantom.

• Alternatives: TLD, solid state detectors.

• CTDIVOLshould be displayed on the console, reflecting the conditions of operation selected (IEC, 2003)


Dose Indicators in Interventional Radiology

• For quality assurance purposes• To estimate the probability of

occurrence of stochastic effects use:

Kerma-air product rate (KAP, PKA)


Dose Indicators in Interventional Radiology

• For quantifying the threshold and severity of deterministic effects use:• Maximum skin dose (MSD)• Cumulative dose (CD) to Interventional

Reference Point (IRP)

• In a complex procedure skin dose is highly variable


• In some procedures, patient skin doses approach those used in radiotherapy fractions

• Maximum skin dose (MSD) or peak skin dose is the maximum dose received by a portion of the exposed skin.

Interventional Procedures: Skin Dose

Radiodermatitis in the right arm. 7 year-old patient. Photograph taken 4 months after radiofrequency ablation. Surce: ICRP 85


Cumulative Dose to Interventional Reference Point*

• IRP is located 15 cm from the isocentre towards the focal spot

• The air kerma accumulated at a specific point in space relative to the fluoroscopic gantry (IRP) during a procedure

• Cumulative dose does not include tissue backscatter and is measured in Gy.

• Cumulative dose is sometimes referred to as cumulative air kerma

*IRP


Cumulative dose to Interventional Reference Point

Cumulative dose to IRP is measured with a flat ion chamber or calculated by the system and displayed in the angiography room

15 cm

Isocenter

IRP

15 cm

Isocenter

IRP

(IEC-60601-2-43)


MSD vs. Cumulative dose

• In some procedures, cumulative dose to IRP is well correlated with MSD

• Cumulative dose to IRP can be a good indicator of doses higher than the thresholds for skin injures

• A “trigger value” for cumulative dose can be adopted to alert interventionalists the threshold for skin erythema could be reached.

• A follow-up protocol can be adopted.


Other related dose parameters

Fluoroscopy time:•Has a weak correlation with KAP•But, in a quality assurance programme

it can be adopted as a starting unit for• comparison between operators,

centres, procedures• for the evaluation of protocol

optimization, and• to evaluate operator skill


Other related dose parameters

Number of acquired images and number of series:

• Patient dose is a function of total acquired images

• But dose/image can have big variations

• There is an evidence of large variation in protocols adopted in different centres


Diagnostic reference levels (DRLs)

• ICRP, IAEA, EC: introduced the concept of diagnostic reference levels (DRLs) for patients

• DRLs are a form of investigation level, apply to an easily measured quantity at the surface of a simple standard phantom or a representative patient.

• An optimisation tool, not dose limits


Diagnostic Reference Levels (DRLs)

• DRLs calls for local investigation (often very simple) if constantly exceeded

• DRLs: Management of patient doses must be consistent with the required clinical imaging information


Quantities for Establishment of DRLs

• Incident air kerma and entrance-surface air kerma

• Incident air kerma rate and entrance-surface air kerma rate

• Air kerma–area product• CT Dose index, CT Dose–length

product


Quantities and Units for Staff Dose Assessment

• Personal dosimetry services typically provide monthly estimates of Hp(10) (mSv), the dose equivalent in soft tissue at 10 mm depth. This is in most of the cases used to estimate the effective dose.

• Sometimes, Hp(0.07) (mSv) is also reported: the dose equivalent in soft tissue at 0.07 mm depth)

• Personal dosememters (film, thermoluminescent...)


Personal Dosimetry Methods

• Single dosimeter worn• above the apron at neck

level (recommended) or under the apron at waist level

• Two dosimeters worn (recommended in intrevational procedures)• one above the apron at

neck level • another under the lead

apron at waist level

Lens dose, optional

Finger dose, optional

Second dosemeter

outside and above the apron

at the neck, optional

Personal dose

dosemeter behind the lead apron

X-ray

tube

Image

intensifier

Patient

Radiation

protection

measures

Dose limits

of occupational exposure (ICRP 60)

Effective dose 20 mSv in a year

averaged over a period of 5 years

Anual equivalent dose in the

lens of the eye 150 mSv

skin 500 mSv

hands and feet 500 mSv


Dose due to scatter radiation at a point occupied by the operator can be measured with a portable ionization chamber

Dose Measurement


Summary

• Dosimetric quantities are useful to know the potential hazard from radiation and to determine radiation protection measures to be taken

• Physical quantities - Directly measurable• Protection quantities - Defined for dose

limitation purposes, but not directly measurable.

• Application specific quantities - Measurable in medical imaging.

• Diagnostic Refernce Levels



1. The same amount of radiation falling on the person at level of breast, head or gonads will have same biological effects.

2. Effective dose can be easily measured. 3. Diagnostic reference levels are not

applicable to paediatric radiology.



1. False -Different organs have different radio-sensitivity and tissue weighting factors as given by ICRP.

2. False -It can be only calculated using different methods.

3. False - DRLs apply for paediatric radiology, but these are age-specific.


References

• INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Patient Dosimetry for X Rays Used in Medical Imaging, ICRU, Rep. 74, ICRU, Bethesda, MD (2006).

• INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Radiological Protection in Medicine, Publication 105, Elsevier, Oxford (2008)

• INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Recommendations of the ICRP, Publication 103, Elsevier, Oxford (2008)

• EUROPEAN COMMISSION, Guidance on Diagnostic Reference Levels (DRLs) for Medical Exposure, Radiation Protection 109, Office for Official Publications of the European Communities, Luxembourg (1999)

• INTERNATIONAL ATOMIC ENERGY AGENCY, Dosimetry in Diagnostic Radiology: an International Code of Practice, Technical Report Series No 457, IAEA, Vienna (2007)

Additional information


Quantities for radiation measurement

• Physical quantities - Directly measurable

• Protection quantities - Defined for dose limitation purposes, but not directly measurable

• Application specific quantities - Measurable in medical imaging


Radiation quantities and units

• Fundamental dosimetric quantities• Protection quantities

•Equivalent dose•Effective dose

• Application specific dosimetric quantities used in DR• Incident air kerma•Entrance surface air kerma•Air kerma area product•Air kerma length product•Dosimetric quantities in CT and mammography


Physical Quantities


Physical quantities

• Fluence

• Exposure

• Kerma

• Absorbed dose


Fluence : f

The fluence, f , is the quotient of dN by da, where dN is the number of particles incident on a sphere of cross section da, thus

f = dN/da

The unit of fluence is m-2


Exposure: X

where dQ is the absolute value of the total charge of ions produced in air when all the electrons liberated in air of mass dm are completely stopped in air.

The SI unit of exposure is the coulomb per kilogram (C/kg)

The special unit of exposure is the röntgen (R).1R = 2.58 x 10-4 C kg-1

dm

dQX


KERMA

The KERMA (Kinetic Energy Released in a MAterial)

where dEtrans is the sum of the initial kinetic

energies of all charged ionizing particles liberatedby uncharged ionizing particles in a material ofmass dm The SI unit of kerma is the joule per kilogram (J/kg),termed gray (Gy).

.

dm

dEK trans


Exposure and KERMA

Exposure, X, in units of C kg-1, is related to air kerma as follows:

where W is the average energy spent by an electron to produce an ion pair, g is the fraction of secondary charged particles that is lost to bremsstrahlung radiation production and e is the electronic charge

W

egKX a

1


The fundamental dosimetric quantity absorbed dose, D, is defined as:

where is the mean energy imparted by ionizing radiation to matter in a volume element and dm is the mass of matter in the volume element.

The SI unit of absorbed dose is the joule per kilogram (J/kg), termed the gray (Gy)

In diagnostic radiology, KERMA and D are equal

Absorbed Dose: D

dm

dD

d


Exposure and Absorbed Dose or KERMA

• Exposure can be linked to air dose or kerma by suitable conversion coefficients.

• For example, 100 kV X-rays that produce an exposure of 1 R at a point will also give an air kerma of about 8.7 mGy and a tissue kerma of about 9.5 mGy at that point.


Application Specific Quantities


Imaging modality

Measurement subject

Measured radiation quantity

Remark

Radiography PhantomPatient

Incident air kermaESAK, KAP Calculated from X-ray

tube output

Fluoroscopy/ Interventional procedures

PhantomPatient

ESAKKAP/Peak skin dose

CT PhantomPatient

CT air kerma indexCT air kerma- length product

Measured in PMMA head and body phantom

Mammogra-phy

PhantomPatient

Incident air kerma, ESAKIncident air kerma

Calculation of mean glandular dose

Dental radiography

Patient Incident air kermaAir kerma-length product


Application Specific Quantities

Patient thickness

Focal-spot to image receptor distance (FFD)

Focal-spot to patient skin distance (FSD)

Incident air kerma (no backscatter)

Entrance surface air kerma (including backscatter)

X-ray tubefocal spot position

Image receptor

Schematic diagram showing some dosimetric and geometric quantities



where Y(kVp, FFD) is tube output for actual kVp used during examination, mAs is actual tube current-time product used during examination and FFD is focus-to-film distance. BSF is the backscatter factor that depends on kVp and total filtration of X-rays

BSFtFFD

FDDmAsFDDkVpYESAK

p

2

),(


Backscatter Factors (Water)

HVL Field size (cm x cm)

mmAl 10 x 10 15 x 15 20 x 20 25 x 25 30 x 30

2.0 1.26 1.28 1.29 1.30 1.30

2.5 1.28 1.31 1.32 1.33 1.34

3.0 1.30 1.33 1.35 1.36 1.37

4.0 1.32 1.37 1.39 1.40 1.41


• If the KAP is calculated by the system, you must know if the user added filtration you use is included or not !




• It is always necessary to calibrate and to check the transmission chamber for the X-ray installation in use

• In some European countries, it is compulsory that new equipment is equipped with an integrated ionization transmission chamber or with automatic calculation methods



• Computed Tomography Dose Index (CTDI)

• CT air kerma index

• Dose-Length Product (DLP)

• Air kerma-length product


ICRU 74 / IAEA TRS 457

• CT air kerma index•Free-in-air (Ck)• In phantom

(Ck,PMMA)

• Air kerma length product (PKA)



Principal dosimetric quantity in CT is CT air kerma index:

where K(z) is air kerma along a line parallel to the axis of rotation of the scanner over a length of 100 mm.

N = Number of detectors in multi-slice CT T = Individual detector dimension along z-dimension

The product NT defines the nominal scan beam width/collimation for a given protocol.

50

50

100, )(1

dzzKNT

Ca



Weighted CT air kerma index, CW, combines values of CPMMA,100 measured at the centre and periphery of a standard CT dosimetry phantoms

pPMMAcPMMAw CCC ,100,,100, 23

1



Pitch (IEC, 2003):

T= Single detector dimension along z-axis in mm.

N=Number of detectors used in a given scan protocol (N>1 for MDCT), N x T is total detector acquisition width or collimation

I=table travel per rotationRadiographic, 2002, 22:949-62

NT

Ip


• Volume CTDI describes the average dose over the total volume scanned in sequential or helical sequence, taking into account gaps and overlaps of dose profiles (IEC, 2003):

• Average dose over x, y and z direction• Protocol-specific information


l

NTCC WVOL



• Kerma-length product (PKL):

where L is scan length is limited by outer margins of the exposed scan range (irrespective to pitch)

• PKL for different sequences are additive if refer to the same type of phantom (head/body)

LCP VOLKL


Maximum Skin Dose (MSD)

• Measurement/evaluation of MSD•Point or area detectors •Cumulative dose at IRP (interventional

radiology point)•Calculation from technical data

• Off line methods•Area detectors: TLD array, slow films,

radiochromic films•From KAP and Cumulative dose measurement


Method for MSD Evaluation: Radiochromic Large Area Detector

Example: Radiochromic films type Gafchromic XR R 14”x17”• useful dose range: 0.1-15 Gy • minimal photon energy dependence (60 - 120 keV)• acquisition with a flatbed scanner:b/w image, 12-16

bit/pixel or, measure of OD measurement with a reflection densitometer


Benefits of Radiochromic Films

• The radiochromic film:• displays the maximum dose and its location• shows how the total dose is distributed• provides a quantitative record for patient files• provides physician with guidance to enable safe

planning of future fluoroscopically guided procedures

• improves fluoroscopic technique and patient safety

• possible rapid semi-quantitative evaluation Example of an exposed radiochromic film in a cardiac interventional procedure


Rapid Semi-Quantitative Evaluation: Example

• For each batch number (lot #) of gafchromic film a Comparison Tablet is provided

• In the reported example we easily can recognise that the darkness area of the film, corresponding to the skin area that has received the maximum local dose, has an Optical Density that correspond at about 4 Gy


DRLs for Complex Procedures

3rd level “Patient risk”

2nd level “Clinical protocol”

1st level“Equipment

performance”

Dose rate and dose/image(BSS, CDRH, AAPM)

Level 1 + No. images + fluoroscopy

time

Level 2 + DAP + Peak Skin Dose (MSD)

Reference levels (indicative of the state of the practice): to help operators to conduct optimized procedures with reference to patient exposure

For complex procedures reference levels should include:

• more parameters

• and, must take into account the complexity of the procedures.

(European Dimond Consortium recommendations)

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