RAD309

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RAD309

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RAD309 Patient Dose * The contrast of a lesion relative to the surrounding background generally increases as kV is reduced, but the noise (mottle) level also ... – PowerPoint PPT presentation

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Title: RAD309

1
RAD309

Patient Dose

2
CT

Patients are exposed to higher radiation levels
from the use of computed tomography compared to
most imaging techniques

3
Radiation Dose in CT

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)

4
CT Beam Geometry

Most modern CT
Emit fan shaped beam
Narrow cross section
Thin beam slice across patient

5
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6
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 109 e

7
Multiple Scan Average Dose

Ave dose delivered to patient when a series of
scans are performed
Graphical

8
Multiple Scan Average Dose

MSAD is dose from many slices

9
BED Index

distance bed is moved between adjacent scans (mm)
each scans the patient is moved a bed index
distance

10
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

11
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

12
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

13
MSAD and BI

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

14
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

15
to use pencil chamber methods
16
CT DOSIMETRY
dosetools
17
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

18
Reduction of Dose

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

19
Factors Affecting Patient Dose

KVp
mAs
Pitch
Collimation
Bed Index
Beam Geometry
Detector Setup
Other repeats, shielding, alignment, patient
size...etc..

20
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

21
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 lt1 Higher Radiation Dose

Pitch is 0.75
Image shows 25 overlap

Shield thyroid, breast, eye lens, gonads
to reduce organ dose by 3060

24
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

25
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