Download 03 Fluoroscopy Dosimetry KAP CM

Doses in Fluoroscopy
factors influencing patient doses
Colin Martin and David Sutton
Fluoroscopy
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Used to visualize motion of internal fluid,
structures
Image intensifier or digital image plate
gives a live image
Coupled to display monitor
Radiologist can watch the images “live” on
TV-monitor; images can be recorded
Fluoroscopy used to observe digestive tract
• Upper GI series, Barium Swallow
• Lower GI series Barium Enema
Definitions
 Fluoroscopy (radioscopy)
Examination that allows real time continuous
imaging using x-rays
 Can be used to demonstrate function or guide
an interventional procedure where still images
do not convey the required information
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 Fluorography (acquisition or digital spot
imaging)
 Method of acquiring a single still image using
fluoroscopic equipment
Fluoroscopic and Fluorographic images
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Fluorographic image is of higher quality
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Two techniques are used together
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Fluorographic acquisition uses sufficient exposure to
produce a clinically acceptable image s(higher dose)
Fluoroscopic exposure uses the radiation available in
one frame
Fluoroscopy for patient positioning and the
manipulation of instruments
Fluorography only where image quality is essential to
diagnosis or a decision as to the progression of the
procedure
Features of the fluoroscopic and fluorographic images
Still frame of
fluoroscopic exposure
Fluorographic image
Patient entrance dose rates in
fluoroscopy
Example GE-CGR Advantix LCV (Fluoroscopy)
LOW DOSE 10 mGy/min
MEDIUM DOSE 20 mGy/min
HIGH DOSE 40 mGy/min
Automatic Brightness Control
Exposure factors varied according to
image receptor signal level
Signal feedback
Image intensifier: Photon
output brightness gauge
Flat panel: Signal to noise
ratio assessment
Automatic
brightness
control
Signal used to
control exposure
factors: mA and kV
Generator
Other factors influencing
patient dose
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Pulsing of X-ray beam
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Additional filtration
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Use of magnification with image
intensifiers
Pulsed Fluoroscopy
 Continuous
fluoroscopy:
Any movements of the object during the
frame (40 ms) will be superimposed
 Pulsed
fluoroscopy:
Short pulses (e.g. 5 ms) with higher mA
gives less degradation of image due to
movement
Additional Filters
Equipment can place additional filtration in
x-ray beam
 This cuts out lower energy x-rays that do
not contribute towards image generation
 Reduces ESAK by 30-50%
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Patient dosimetry
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Irradiation geometry can vary throughout
exam & between patients
Patient exposure measured by air kermaarea product
Measured with flat transmission ionization
chamber (KAP meter)
Kerma-Area Product (KAP)
or Dose-Area Product (DAP)
Equal to: Dose in beam x Area of beam
 KAP is easily measured
 KAP can be assessed for
multiple projections, field sizes
 KAP is a measure of the total
amount of radiation incident on
a patient
Kerma-area product
Area of
beam = A
cm2
X-ray focus
KAP meter
Ave dose in
beam = D Gy
KAP = D x A Gycm2
Dose x area is independent of distance from X-ray
tube focus, because of inverse square law.
KAP is measured with large area
ionisation chamber which intercepts
entire X-ray beam
Patient selection for dose survey
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Select examination of interest
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Define weight range (typically 60-80 kg)
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Make measurements / collect data for at least
20 patients – or more
(more variability in dose data than for
radiography)
Patient data to collect
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Tube voltage
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Tube current
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Screening time
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ABC mode
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Dose rate setting
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Image Intensifier field size used
Summary
Uses both fluoroscopy and fluorography
 Different dose options
 Factors such as pulsing, filters, etc. affect
dose
 Use KAP to record all the radiation
incident on the patient
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