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RAD309
Patient Dose
CT
• Patients are exposed to higher radiation levels
from the use of computed tomography
compared to most imaging techniques
Radiation Dose in CT
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How much dose is scanner delivering?
Inter-scanner comparison of dose
Estimate patient potential risk
Weigh risk against benefit
Patient exposure tables/ a requirement by
regulatory agencies (JCAHO)
CT Beam Geometry
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Most modern CT
Emit fan shaped beam
Narrow cross section
Thin beam slice across patient
How do we measure dose?
• Ionization chamber
Quantifies radiation exposure
Air filled container
Amount of collected charge is proportional to
amount of radiation passing through it
Charge is removed and measured with
electrometer
Total Q measured in coulombs
1 C = 1.6 x 10^9 e
Multiple Scan Average Dose
• Ave dose delivered to patient when a series of
scans are performed
• Graphical
Multiple Scan Average Dose
• MSAD is dose from “many” slices
BED Index
distance bed is moved between adjacent scans (mm)
each scans the patient is moved a bed index distance
Computed Tomography Dose Index
• (CTDI) special quantity expresses radiation dose in CT
• Measured with a dosimeter inserted into a phantom
that represents a patient
• Dose is measured by scanning one slice
• factors are applied to convert the measured
phantom CTDI to an actual patient scan
• a reasonable estimation of the actual dose to the
patient
CTDI and MSAD
• Slice dose versus procedure dose
• CTDI: ionization chamber used for measurement
– Area of dose cure for single slice dived by slice width
• MSAD: use CTDI to calculate an ave dose in
middle of series of scans
– Ratio of slice width to slice spacing multiplied by CTDI
SW
MSAD 
CTDI
BI
CTDI and MSAD Affect
• To increase CTDI
area under curve
intensity ( raises height of curve) OR
Widening the curve ( open collimator)
CTDI
Patient Dose
CTDI is the MSAD at canter of a series of 14
contiguous slices
MSAD and BI
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BI
MSAD
Large gap b/w slices (slices further apart)
Radiation spread over a large area (ave. Dose)
BUT relevant tissue can be missed out
Limit to BI increase
BRH Recommendation
• A measurement based on 2 concepts CTDI and
MSAD (using pencil ion chamber)
• Dose from CT is commonly specified in terms
of CTDI
• CTDI is derived from measurement dose from
single slice
…to use pencil chamber methods
CT DOSIMETRY
CTDI/MSAD method
• a pencil ionization chamber
• 2 sized phantoms (16cm and 32cm) made of acrylic to
standardize CT dose measurements
• Both phantoms have holes drilled at specific locations to
accommodate the ionization chamber during dose
measurement
• The chamber is positioned in 1 hole at a time while the other
holes are filled with acrylic plugs
• An exposure is made and recorded. This is done for all holes
so that dose measurements can be obtained for a number of
positions in the phantom
• It is critical to note at this point that the ionization chamber is
measuring the exposure and not dose
• a factor is used to convert exposure to dose
Reduction of Dose
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You ,the operator, must know dose
You, the operator, how to keep it at minimum
What can be done to reduce MSAD
How does this affect image
Factors Affecting Patient Dose
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KVp
mAs
Pitch
Collimation
Bed Index
Beam Geometry
Detector Setup
• Other: repeats, shielding, alignment, patient size...etc..
How does KV affects Dose
Reducing x-ray tube voltage (kV) while keeping
mAs constant, the patient dose is decreased
Dropping KV from 120 to 80 kV at a
constant current (mAs) typically reduces
the patient dose by about 60%
Pitch and Dose
Pitch = The distance the patient couch travels during
one 360 degree turn
As pitch increases, the time spent in any one point in
space is decreased
Pitch <1 = Higher Radiation Dose
Pitch is 0.75
Image shows 25% overlap
• Shield thyroid, breast, eye lens, gonads
• to reduce organ dose by 30—60%
Patient AlignmentPatient Alignment
• Correctly centering patient on CT gantry can
reduce radiation dose by as much as 56%
• When the patient is in the incorrect position the
patient must be moved and the CT scan repeated
Perspective
• When a CT scan is justified by medical need,
the associated risk is small relative to the
diagnostic information obtained
• CT scans save thousands of lives daily
• CT scans greatly reduce exploratory surgeries
Justification : is the scan necessary for ongoing patient care? Can
the examination be replaced by a low- or no-dose examination
Equipment maintenance: Having to repeat a slice or an entire
examination due to equipment failure increases dose with no
benefit to the patient. QA program
Limit the scan boundaries to area of interest. careful positioning
of the scan volume, source to object distance, limiting the scan
coverage to the area of interest, or angling the gantry away
from sensitive structures can be very effective
Decrease the exposure: The correct balance between dose and
image quality .The guidelines also give reference values for
patient dose for particular examinations.
Customize the exposure to patient size: adjust the exposure
factors to compensate for changes in patient size (mAs for
children examinations)
Customize scan parameters to examination type : High
resolution (fine detail) scans such as sinuses, inner ear and
skeletal structures can be performed with lower exposure
factors as spatial resolution is relatively independent of dose
levels. for example, report dose levels as low as 20mAs
Education and research: Radiographers review CT protocols at
their institution and compare image quality and dose to the
ICRP guidelines. education on dose reduction techniques in
departmental CT training Programs
Detector sensitivity: When purchasing CT scanners ask
questions about scanner performance and radiation dose.
Don't assume that all manufacturers scanners will have similar
detector sensitivity and performance