Biophysics Understanding Radiation Units Dr. Yousif Mohamed Y. Abdallah

Understanding radiation units Educational Objectives At the end of the lecture, the students should become familiar with the following: Why is it important to measure radiation dose ? How radiation dose can and should be expressed? Understand the radiation quantities and units used in diagnostic radiology. 2

Understanding radiation units 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. 3

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 Understanding radiation units

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 5

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 Understanding radiation units

Hot Coffee – Energy contained in a sip Excess Temperature = 60 º - 37 = 23 º 1 sip= 3ml 3x 23= 69 calories Understanding radiation units

Radiation Dose Lethal Dose= 4Gy LD 50/60 = 4 Gy For 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 Understanding radiation units X-rays

Dose of Radiation Radiation energy absorbed by a body per unit mass. Understanding radiation units 9

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. Understanding radiation units

Why so many quantities? Radiation dose is a complex topic Understanding radiation units 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

Understanding radiation units 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 12 Radiation at a specific point Photon fluence Absorbed dose Kerma Dose equivalent Total radiation Total photons Integral dose

Understanding radiation units 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 = C/kg

Understanding radiation units 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

Understanding radiation units 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

Understanding radiation units Mean absorbed dose in a tissue or organ The mean absorbed dose in a tissue or organ D T is the energy deposited in the organ divided by the mass of that organ.

Understanding radiation units Now things get a little more complicated ! 17

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 Protection in Paediatric Radiology L02. Understanding radiation units Understanding radiation units

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 Protection in Paediatric Radiology L02. Understanding radiation units Understanding radiation units

Radiation Weighting Factors, w R Radiation typeRadiation weighting factor, w R Photons1 Electrons and muons1 Protons and charged pions2 Alpha particle, fission fragments, heavy ions 20 NeutronsA continuous curve as a function of neutron energy (Source: ICRP 103) Radiation Protection in Paediatric Radiology L02. Understanding radiation units Understanding radiation units

Equivalent Dose : H T,R The absorbed dose in an organ or tissue multiplied by the relevant radiation weighting factor : where D T,R is the average absorbed dose in the organ or tissue T, and w R is the radiation weighting factor for radiation R.

Understanding radiation units 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. 22 Rolph Sievert

Understanding radiation units 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.

Understanding radiation units Effective Dose: E T Effective dose takes into account the organ specific radio-sensitivity to develop cancer and hereditary effects from radiation Unit = sievert, Sv 24

Understanding radiation units Effective Dose: E T A summation of the tissue equivalent doses, each multiplied by the appropriate tissue weighting factor: where H T is the equivalent dose in tissue T and w T is the tissue weighting factor for tissue T.

The organs have different weighting factors, w T. These factors are published in ICRP 103 (2007) and have been changed over the years due to increased knowledge. Understanding radiation units 26 Tissue Weighting Factors, w T

Radiation Protection in Paediatric Radiology L02. Understanding radiation units 27 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 Understanding radiation units

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 Tissue Weighting Factors

Understanding radiation units 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 Factors Tissue weighting factor w T * ∑ w T Bone-marrow (red), Colon, Lung, Stomach, Breast, Remainder Tissues**(nominal weighting factor applied to the average dose to 14 tissues) Gonads0.08 Bladder, Esophagus, Liver, Thyroid Bone surface, Brain, Salivary glands, Skin *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..

Understanding radiation units 30 Dose to lungs times their weighting factor; D L x w L + Dose (mean absorbed dose) to gastrointestinal tract times their weighting factor; D GI x w GI +....(summation over organ after organ) = Effective dose Effective Dose (E) where T stands for tissue

Understanding radiation units Effective Dose (E) We can compare different paediatric imaging procedures through their different effective doses, E. 31

Understanding radiation units 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 32

Understanding radiation units 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)

Understanding radiation units 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.

Understanding radiation units Entrance Surface Air Kerma (ESAK) 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)

Understanding radiation units Kerma in X-ray field can be measured using calibrated: Ionization chamber Semiconductor dosimeter Thermoluminescent dosimeter (TLD) Dose Measurement 36

Understanding radiation units 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

Understanding radiation units Kerma-Area Product: KAP The KAP (Gy · cm 2 ) 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.

Understanding radiation units KAP = K x Area the SI unit of KAP is the Gy·cm 2 Kerma-Area Product: KAP 39 Area = 1 Dose = 1 Area = 4 Dose = 1/4 d 1 =1 d 2 =2

Understanding radiation units Kerma-Area Product: KAP 40 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 = 1 Dose = 1 Area = 4 Dose = 1/4 d 1 =1 d 2 =2

Understanding radiation units This is a picture of a KAP meter which measures the kerma area product

Understanding radiation units 42 Example of a dose display during fluoroscopy or cine runs with dose rate as shown

Understanding radiation units 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

Understanding radiation units 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 Understanding radiation units KERMA (in phantom) CTDI (dose in phantom per slice) Length of scan and pitch DLP Effective dose Risk

Understanding radiation units 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. CTDI VOL should 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, P KA ) Understanding radiation units

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 Understanding radiation units

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 49 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 Understanding radiation units *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 Understanding radiation units 15 cm Isocenter IRP 15 cm Isocenter IRP (IEC )

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. Understanding radiation units

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 53

Understanding radiation units 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 54

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 Understanding radiation units

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 Understanding radiation units

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 Understanding radiation units

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...) Understanding radiation units

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 Understanding radiation units

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

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

Understanding radiation units Answer True or False 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. 62

Radiation Protection in Paediatric Radiology L02. Understanding radiation units 63 Answer True or False 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. Understanding radiation units

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

Understanding radiation units 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

Understanding radiation units 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

Understanding radiation units Physical Quantities

Understanding radiation units Physical quantities Fluence Exposure Kerma Absorbed dose

Understanding radiation units 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

Understanding radiation units 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 r ö ntgen (R). 1R = 2.58 x C kg -1

Understanding radiation units KERMA The KERMA (Kinetic Energy Released in a MAterial) where dE trans is the sum of the initial kinetic energies of all charged ionizing particles liberated by uncharged ionizing particles in a material of mass dm The SI unit of kerma is the joule per kilogram (J/kg), termed gray (Gy)..

Understanding radiation units 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

Understanding radiation units 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

Understanding radiation units 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.

Understanding radiation units Application Specific Quantities

Understanding radiation units Imaging modality Measurement subject Measured radiation quantity Remark RadiographyPhantom Patient Incident air kerma ESAK, KAPCalculated from X-ray tube output Fluoroscopy/ Interventional procedures Phantom Patient ESAK KAP/Peak skin dose CTPhantom Patient CT air kerma index CT air kerma- length product Measured in PMMA head and body phantom Mammogra- phy Phantom Patient Incident air kerma, ESAK Incident air kerma Calculation of mean glandular dose Dental radiography PatientIncident air kerma Air kerma-length product

Understanding radiation units 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 tube focal spot position Image receptor Schematic diagram showing some dosimetric and geometric quantities

Entrance Surface Air Kerma (ESAK) 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 Understanding radiation units

Backscatter Factors (Water) HVLField size (cm x cm) mmAl10 x 1015 x 1520 x 2025 x 2530 x

Understanding radiation units If the KAP is calculated by the system, you must know if the user added filtration you use is included or not ! 81 Kerma-Area Product: KAP

Understanding radiation units Kerma-Area Product: KAP 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

Understanding radiation units Dosimetric Quantities for CT Computed Tomography Dose Index (CTDI) CT air kerma index Dose-Length Product (DLP) Air kerma-length product

Understanding radiation units ICRU 74 / IAEA TRS 457 CT air kerma index – Free-in-air (C k ) – In phantom (C k,PMMA ) Air kerma length product (P KA )

Understanding radiation units Dosimetric Quantities for CT 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.

Understanding radiation units Dosimetric Quantities for CT Weighted CT air kerma index, C W, combines values of C PMMA,100 measured at the centre and periphery of a standard CT dosimetry phantoms

Understanding radiation units Dosimetric Quantities for CT 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 rotation Radiographic, 2002, 22:949-62

Understanding radiation units 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 Dosimetric Quantities for CT

Understanding radiation units Dosimetric Quantities for CT Kerma-length product (P KL ): where L is scan length is limited by outer margins of the exposed scan range (irrespective to pitch) P KL for different sequences are additive if refer to the same type of phantom (head/body)

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 Understanding radiation units

Method for MSD Evaluation: Radiochromic Large Area Detector Example: Radiochromic films type Gafchromic XR R 14 ” x17 ” useful dose range: Gy minimal photon energy dependence ( keV) acquisition with a flatbed scanner:b/w image, 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 Understanding radiation units 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 Understanding radiation units

DRLs for Complex Procedures 3rd level “ Patient risk ” 2nd level “ Clinical protocol ” 1st level “ Equipment performance ” 94 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)