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Basics Of Computed Tomography (CT)

Lecture Ten

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Radiation Safety and Radiation Safety and ProtectionProtection

The potentially harmful effects of The potentially harmful effects of ionizing radiation are either:ionizing radiation are either:

Stochastic effectsStochastic effects

OrOr

Deterministic effectsDeterministic effects

Stochastic effectsStochastic effects Effects where the Effects where the probabilityprobability of the of the

occurrence increases with radiation exposure.occurrence increases with radiation exposure.e.g. e.g. carcinogenesis and genetic effects.carcinogenesis and genetic effects.

i.e.i.e.((The ((The probabilityprobability, but , but not not

severityseverity, of the end point , of the end point condition, is condition, is dose-dependentdose-dependent))))

Deterministic Deterministic effectseffects

Effects related to a Effects related to a threshold dosethreshold dose,, below below which the effect is not detected but which the effect is not detected but aboveabove this threshold dose , the probability that the this threshold dose , the probability that the effect will occur is virtually effect will occur is virtually 100%.100%.

The The severityseverity increases with increased dose. increases with increased dose.

e.g.e.g. Erythema, epilation, desquamation, cataract, Erythema, epilation, desquamation, cataract,

fibrosis and hematopoietic damage.fibrosis and hematopoietic damage.

CT DosimetryCT Dosimetry To evaluate the To evaluate the potential risk/benefitpotential risk/benefit of CT of CT

scanning, dose should be estimated.scanning, dose should be estimated.

2 main questions are to be answered:2 main questions are to be answered:

1.1. How muchHow much radiation dose is my CT scanner radiation dose is my CT scanner delivering to the patient?delivering to the patient?

2.2. How to compareHow to compare my scanner dose to that my scanner dose to that of other CT scanners?of other CT scanners?

Dose MeasurementsDose Measurements Erythema doseErythema dose Slight film fogSlight film fog Roentgen (R)Roentgen (R) Exposure doseExposure dose Absorbed dose Absorbed dose Equivalent doseEquivalent dose

Classic and SI units of radiation Classic and SI units of radiation dosedose

Dose type Unit Abbr Unit Abbr

Exposure (air kerma) For x-ray or gamma ray ionization in air only

Roentgen R Coulomb per kilogram

C/kg

Absorption Energy deposited by any type of radiation in any material

Radiation absorbed dose

Rad Gray Gy

Equivalent Biologic effect caused by any radiation in a living organism

Roentgen equivalent mammal

Rem Sievert Sv

Classic units SI units

Exposure Exposure Ionizing radiation ionizes the gas molecules Ionizing radiation ionizes the gas molecules

into electrically charged ions . This has into electrically charged ions . This has traditionally been measured in terms of traditionally been measured in terms of exposure Eexposure E

Where Q is total electric charge produced and M Where Q is total electric charge produced and M is the mass of air. is the mass of air.

The old exposure unit of 1 Roentgen is The old exposure unit of 1 Roentgen is equivalent to 2.58x10equivalent to 2.58x10-4-4 C.kg C.kg-1-1..

Since 1 Gy =3x10Since 1 Gy =3x10-2-2 C.kg C.kg-1-1 then 1R = 8.6 Gy. then 1R = 8.6 Gy.

E= Q/M (C.Kg-1 )

Absorbed dose Absorbed dose (Gy)(Gy)

Is the energy absorbed by exposed material (air or tissue) in joules/kg E absorbed by a mass M of tissue: E/M is the the gray (Gy)

1 Gy = 1x10-3 J.g-1

Absorbed Dose = E/M (Gy)

Dose equivalent Dose equivalent (Sievert)(Sievert)

Tissue damage due to different type Tissue damage due to different type of ionizing radiation ( Gamma, X, of ionizing radiation ( Gamma, X, Beta, and Alfa) varies considerably.Beta, and Alfa) varies considerably.

Dose equivalent allows for this by Dose equivalent allows for this by multiplying the absorbed dose multiplying the absorbed dose (grays) by a weighting factor which (grays) by a weighting factor which depends on the type of radiation.depends on the type of radiation.

Grays x Q = Sieverts (Sv)

CT Radiation DoseCT Radiation Dose has 3 unique features: Axial CT image is very much collimated with minimal scattered radiation.

Dose is evenly distributed ( in modern CTs) due to rotational acquistion.

To achieve high contrast resolution, CT needs high SNR which necessiates high Dose/ Volume i.e. high technique.

PA Chest XR needs only 120 kVp, 2 mAsChest CT may need up to120 kVp, 200 mAs

X-ray beam X-ray beam geometrygeometry

Most modern scanners employ a fan-Shaped X-ray beamMost modern scanners employ a fan-Shaped X-ray beam

Factors affecting Radiation Dose in CT

The width of x-ray beam is viewed form sideThe width of x-ray beam is viewed form sideCollimatorCollimatordetermines the width:determines the width:

An ideal dose An ideal dose distribution distribution

along z axis is shown (B).along z axis is shown (B).

Actual bell-shaped dose Actual bell-shaped dose distribution curve (C).distribution curve (C).

A

B

C

ARadiation Dose in CT

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1717

Radiation DoseThe main X-ray interaction mechanism in CT is Compton scattering.

CT slice acquisition delivers a considerable dose from scatter to adjacent tissues outside the primary beam path.

As slice number increases, scattering also increases.

Methods of measuring patient Methods of measuring patient dosedose

Of the many dose measurement methods, Of the many dose measurement methods, we are going to consider only:we are going to consider only:

The The pencil ionizationpencil ionization chamberchamber method. method.

CT dose indexCT dose index (CTDI) method. (CTDI) method.

Multiple scan average doseMultiple scan average dose (MSAD) (MSAD) method. method.

Radiation Dose Measurement - CTDI

CTDI is the dose to any point in the patient including scatter from 7 CT slices in both directions (H/F & F/H) ( a total of 14 slices).The multiple scan average dose MSAD can be estimated using a single scan by measuring the CT Dose Index (CTDI).CTDI can be measured using a pencil ionization chamber in phantoms simulating head (16 cm diameter acrylic) & bodies (32 cm diameter acrylic.Doses at the patient surface may be higher than in the patient center.

In head scans, the surface-to-center ratio is approximately 1:1.In body scans, the surface-to-center ratio is approximately 2:1.

CTDI measurements are done at both the surface CTDI (Periphery) & center CTDI (Center) of the phantom & then combined to give CTDIw

CTDIw = (2/3 CTDIperipheral + 1/3 CTDIcenter)

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Dose Measurement - CTDI

Dose indexDose index The CT dose index (CTDI) is mathematically defined The CT dose index (CTDI) is mathematically defined

as :as :

Where n is the number of distinct planes of the data Where n is the number of distinct planes of the data collected during one revolution, collected during one revolution, sw is the slice width sw is the slice width (in mm), D(z) is the dose distribution, z is the (in mm), D(z) is the dose distribution, z is the dimension along the patient’s axis. For axial CT dimension along the patient’s axis. For axial CT scanners and spiral CT scanners with single array of scanners and spiral CT scanners with single array of detectors, n = 1. For multi slices CT scanner n is the detectors, n = 1. For multi slices CT scanner n is the number of active detector rows.number of active detector rows.

*The integral sign merely instructs the user to determine the area single curve (D(z).

*

Measuring the CTDIMeasuring the CTDI

CTDI is measured using long cylindrical CTDI is measured using long cylindrical ionization chamber and radiation dose ionization chamber and radiation dose from single slice.from single slice.

The ionization chamber receives The ionization chamber receives radiation from all parts of dose radiation from all parts of dose distribution D(z) because its length is distribution D(z) because its length is bigger than the width of the X-ray bigger than the width of the X-ray beam.beam.

The total charge from the ionization chamber The total charge from the ionization chamber is proportional to the integral in the CTDI is proportional to the integral in the CTDI definition.definition.

•Where Q is the total charge collected during single scan and Cf is calibration factor of ionization chamber.

•Because the ionization chamber measures the exposure and not the dose we, convert Roentgen to cGy.

The integral in Equation is numerically equalto the area ( shaded region ) of the dose distribution

Note that the CTDI can be increased by increasing the area under the curve.The area can be increased by either :1.increasing the intensity of radiation, which Raises top the of the curve.Or2. widening the beam , usually by opening the x-ray collimator.

Multiple Scan Average DoseMultiple Scan Average Dose To measure radiation dose received by To measure radiation dose received by

patient from a series of CT scans, patient from a series of CT scans, between each two scans the patient is between each two scans the patient is moved by a bed index distance, each moved by a bed index distance, each slice delivers its characteristic bell-slice delivers its characteristic bell-shaped dose. shaped dose.

Finally, we have multiple consecutive Finally, we have multiple consecutive bell- shaped doses.bell- shaped doses.

A series of 7spaced scans at a constant bed index along the Z- axis is acquired to produce 7 bell- shaped dose distribution curves (TOP).Summation of these doses results in the (BOTTOM) curve.The total dose ( BOTTOM) curve has peaks where the bell-shaped curves overlap.The dotted line through the total dose curve is the multiple scan average dose.The MSAD is defined as the average dose (at a particular depth from the surface) resulting from a large number of successive slices. The MSAD refers to all the dose delivered to the tissues including dose due to scatter from all the successive slices.

Multi-scan average dose Multi-scan average dose (MSAD)(MSAD)

CTDI can be related to the MSAD by this equation

Where BI is the bed index or slice spacing (in mm), Sw is slice width (in mm), and the number of active arrays of detectors.

CTDI-Dose Measurement

Decreasing kVp reduces dose while other factors are constantDecreasing kVp reduces dose while other factors are constant

CTDI values for body scans are lower than those for head scans due to greater attenuation of X-rays in the body.These values DO NOT quantify patient risk because they DO NOT consider the number of slices NOR the radiosensitivity of the irradiated organs.CTDI values increase with kVp, so decreasing kVp while other factors remain constant reduces the CTDI values.

Effect o kVp On Radiation Dose

kVp not only controls the image contrast but also controls the amount of penetration that the x-ray beam will have as it traverses the patient

Parameter 80 kV 120 kV 140 kV

Image Contrast

Best Intermediate Poor

Noise Most Average Least

Penetration Least Average Most

Patient Dose per mAs

Lowest Intermediate Highest

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