Patient Dose in CT

Radiation Safety Radiation safety refers to the methods and tools used to protect patients and personnel from ionizing radiation exposure. ALARA

Radiation safety refers to the methods and tools used to protect patients and personnel from ionizing radiation exposure.

ALARA

Radiation Damage Direct interaction with DNA bonds Indirect effect caused by ionization of chemicals in the body Damage may be short and/or long term effects

Direct interaction with DNA bonds

Indirect effect caused by ionization of chemicals in the body

Damage may be short and/or long term effects

Radiation Effects

Radiation During Pregnancy As a result of radiation exposure during pregnancy there is an increased incidence of prenatal deaths, abnormalities, mental retardation and neonatal deaths due to complications Which Trimester is the most sensitive????

As a result of radiation exposure during pregnancy there is an increased incidence of prenatal deaths, abnormalities, mental retardation and neonatal deaths due to complications

Radiation doses form CT are among the highest in diagnostic radiology despite being a low percentage of total radiology exams

Technical Factors Affecting Patient Dose kVp mAs Pitch Collimation Bed Index Beam Geometry Detector Setup

kVp

mAs

Pitch

Collimation

Bed Index

Beam Geometry

Detector Setup

Measuring Radiation Dose UNITS OF MEASUREMENT Rad Rem Gray Sievert Takes the form of either Absorbed dose or Equivalent dose

UNITS OF MEASUREMENT

Rad

Rem

Gray

Sievert

Absorbed Dose Measured with ionization chamber Quantifies the absolute energy deposited from photons per unit of irradiated tissue Cutie Pie

Measured with ionization chamber

Quantifies the absolute energy deposited from photons per unit of irradiated tissue

Equivalent Dose Equivalent dose is calculated by taking the absorbed dose multiplied by the specific tissue weighting factor (based on known tissue sensitivity to radiation)

Equivalent dose is calculated by taking the absorbed dose multiplied by the specific tissue weighting factor (based on known tissue sensitivity to radiation)

R adiation A bsorbed D ose Measures the absorbed dose Amount of radiation in 1 rad is equal to 0.01 joules of energy/Kg 100 rads = 1 gray (Gy)

Measures the absorbed dose

Amount of radiation in 1 rad is equal to 0.01 joules of energy/Kg

100 rads = 1 gray (Gy)

R adiation E quivalent M an Measures effective dose which factors in sensitivity to different organs. Sievert (Sv) also used: 100 rem = 1 Sv When measuring radiation dose only from x-rays 1 rad equals 1 rem People receive 2 rem/year from radon which is a bit less than one typical CT exam 0.05-0.1 rads Localizer 2-6 rads Body Scan 1-5 rads Head Scan DOSE STUDY

Measures effective dose which factors in sensitivity to different organs.

Sievert (Sv) also used: 100 rem = 1 Sv

When measuring radiation dose only from x-rays 1 rad equals 1 rem

Suggested Technique Higher kV & Low mA technique allows for More efficient x-ray beam Fewer low energy photons deposited in skin More photons available to the detector However low contrast delectability goes down

Higher kV & Low mA technique allows for

More efficient x-ray beam

Fewer low energy photons deposited in skin

More photons available to the detector

However low contrast delectability goes down

kVp Exponential Relationship by a factor of 2 (x 2 ) KV Dose

Exponential Relationship by a factor of 2 (x 2 )

mAs The relationship of radiation dose and mA is linear mAs Dose

The relationship of radiation dose and mA is linear

mAs

mAs Utilizing a faster table Pitch

Single slice scanners: Collimation

SHORT & LONG GEOMETRY SCANNING What is the effect on Dose? Pneumbra

Detector Setup Increased Detectors mean more dose because the collimation must be opened. However the pneumbra effect is a bit less.

Increased Detectors mean more dose because the collimation must be opened. However the pneumbra effect is a bit less.

Factors Affecting Dose in MSCT Related to Beam Geometry

Collimation Width Single Vs. Multislice DOSE GOES UP AS COLLIMATOR MUST BE OPENED WIDER TO ALLOW X-RAYS TO FALL ON MULTI-DETECTORS

Beam Pitch vs. Dose

X-ray Tube Vs. Dose

Historical Methods of Calculating Dose Thermo luminescent dosimeter (TLD) placed end to end and then measured Special ionization chambers capable of measuring dose at several points across the width of the beam

Thermo luminescent dosimeter (TLD) placed end to end and then measured

Special ionization chambers capable of measuring dose at several points across the width of the beam

CT Dose Index Know as CTDI based on pencil ionization chamber. 1981 Center for Devices and Radiological Health Utilization of both CTDI and Multiple Scan Average Dose ( MSAD )

Know as CTDI based on pencil ionization chamber.

1981 Center for Devices and Radiological Health

Utilization of both CTDI and Multiple Scan Average Dose ( MSAD )

CTDI Uses a single ionization chamber measurement and calculation to determine the average dose delivered to a patient who has received a series of scans with specified bed index AKA – Pencil ionization chamber method 1981

Uses a single ionization chamber measurement and calculation to determine the average dose delivered to a patient who has received a series of scans with specified bed index

CTDI (Single Slice) Note: The radiation intensity (DOSE) is actually always wider than the nominal slice width. Thus CTDI includes both the radiation in the intended slice and that of the penumbra outside

Increasing CTDI Increasing the area under the curve (the sw) This is done by increasing the radiation (raising the curve) or widening the curve by opening up the collimators. DOSE

Increasing the area under the curve (the sw) This is done by increasing the radiation (raising the curve) or widening the curve by opening up the collimators.

Multiple Scan Average Dose After a series of scans between each the patient moves per bed index. Each slice delivers a bell shaped dose. If the doses from all scans are summed the total dose resembles the oscillating curve. Where the curves overlap the dose is higher

After a series of scans between each the patient moves per bed index. Each slice delivers a bell shaped dose. If the doses from all scans are summed the total dose resembles the oscillating curve. Where the curves overlap the dose is higher

MSAD IN the regions where the bell curves overlap, the resultant dose is higher than from just one scan. BI MSAD I.e.. If the slices are spaced farther apart the radiation is spread over a greater distance and the average dose becomes smaller What's the trade off???

IN the regions where the bell curves overlap, the resultant dose is higher than from just one scan.

Ionization Chambers Instrument allowing radiation to pass through and collide with air molecules, some of which will become ionized. Free electrons collect on a conducting wire/plate. The collected charge is proportional to the ionization which is proportional to the amount of radiation

Instrument allowing radiation to pass through and collide with air molecules, some of which will become ionized.

Free electrons collect on a conducting wire/plate.

The collected charge is proportional to the ionization which is proportional to the amount of radiation

Electrometer Amount of collected charge is proportional to the amount of radiation that passes through the chamber. Charge is then measured by the electrometer.

Amount of collected charge is proportional to the amount of radiation that passes through the chamber. Charge is then measured by the electrometer.

Effect of Pitch on Dose Pitch of 1.0 has dose  axial scan Pitch of 0.75 has 33% more dose Pitch of 1.5 has 66% less dose

Methods to Reduce Dose Increase the Pitch Increase the BI (w/o changing SW) Reduce the area under the dose curve Width reduced by collimating (the detectors may not count as many photons and will cause noise) Height reduced by reducing the mAs (also creates more noise)

Increase the Pitch

Increase the BI (w/o changing SW)

Reduce the area under the dose curve

Width reduced by collimating (the detectors may not count as many photons and will cause noise)

Height reduced by reducing the mAs (also creates more noise)

Reduce mA and or Time Shield Good communication Methods to Reduce Dose

Reduce mA and or Time

Shield

Good communication

ALARA As Low As Reasonable Achievable

As Low As Reasonable Achievable