Download Lecture Outline 16: Special Topics in Protection

RAD 114 RADIATION
PROTECTION
SPECIAL TOPICS IN PROTECTION:
PATIENT DOSE IN COMPUTED TOMOGRAPHY
COMPUTED TOMOGRAPHY
An x-ray modality developed
in the early ’70’s by Sir
Godfrey Hounsfield. Originally
designed to produce crosssectional images of the brain.
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
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CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
SPIRAL (HELICAL) CT
A refinement that allows continuous
patient scanning. The x-ray tube
within the gantry revolves around
the patient at the same time that
the table is moving into the bore of
the gantry.
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
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HELICAL (SPIRAL) CT
To convert the helical scan data
into slices that can be individually
viewed,
i
d th
the computer
t performs
f
what is called interpolation. This
process fills in the gaps created
by the helical scan.
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
ADVANTAGES
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•
•
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RAPID DATA ACQUISITION
SINGLE BREATH-HOLD IMAGING
LARGE VOLUME COVERAGE
3D AND MULTIPLANAR
RECONSTRUCTION
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CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
RADIATION DOSE DEPENDS ON
•
•
•
•
•
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ENERGY OF THE BEAM
COLLIMATION
MAS
FILTRATION
SCAN PITCH
LEAD SHIELDING?
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ENERGY OF THE BEAM
As with radiography, the energy
of the beam controls penetrability
and scatter production
production. In CT
CT,
the kVp is usually kept in the
range of 120 to 130.
COLLIMATION
Two sets of collimators are used.
One is positioned in front of the
primary
i
b
beam ((pre-patient).
ti t) The
Th
other is positioned in front of the
detector array (post-patient).
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
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MAS
The mAs is varied in order to perform each
scan. More mAs is used to scan larger
volumes of tissue or compensate for thinner
scan slices
li
(ti
(tight
ht collimation).
lli ti ) B
Because CT
is a computer assisted technology, the
effects of changes in mAs do not appear the
same way as with film-screen radiography.
Instead, optimization in signal-to-noise (s/n)
is sought.
FILTRATION
Filtration serves two purposes: skin
dose reduction and reduction in
certain kinds of artifacts that result
from the effects of “beam
hardening.”
PITCH
The pitch is the way in which the
continuous spiral of the scan is
stretched
t t h d outt or compressed.
d It is
i
defined as the ratio of the scan
table increment (BI) per 360
degrees of x-ray tube rotation to
the slice thickness (SW):
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PITCH
P
BI
SW
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
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CT DOSE INDEX (CTDI)
For a single slice, an ion chamber
can be used to measure dose
dose. That
dose becomes what is called the CT
dose index (CTDI) and can be used
to perform calibration checks for
quality control purposes.
MULTIPLE SLICE AVERAGE
DOSE (MSAD)
When dose calculations are taken
from multiple scans (slices) then the
dose is an average based on
measurements taken from the center
of those slices.
CTDI and MSAD
As long as the slices are
contiguous (one next to the
other) the CTDI and MSAD are
the same.
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CTDI and MSAD
With changes in pitch, MSAD
increases or decreases and is equal
to:
CTDI x INVERSE OF PITCH
PROBLEM
Assume a CTDI of 1000 mrem. If
spiral CT images are obtained
using a table increment (BI) of
10mm per tube rotation and a slice
thickness (SW) of 2.5 mm, what is
the MSAD?
ANSWER
•
•
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CTDI = 1000 MREM
PITCH = 10 mm ÷ 2.5 mm = 4
MSAD = CTDI x INVERSE OF PITCH
MSAD = 1000 x (1/4) = 250 mrem
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REPRESENTATIVE DOSES
(EFFECTIVE DOSE EQUIVALENTS)
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CT HEAD ≈ 200 MREM (7 MREM)
CT CHEST ≈ 700 MREM (5 MREM)
CT PELVIS ≈ 900 MREM (160 MREM)
CT ABDOMEN ≈ 1000 MREM (130 MREM)
RISK vs. BENEFIT: QUESTIONS
PRACTIONERS SHOULD ASK
• CAN A LOWER DOSE ALTERNATIVE
PROVIDE AN EQUAL BENEFIT TO CT?
• WILL CT PROVIDE A BENEFIT THAT
EXCEEDS THAT OF LOWER DOSE
ALTERNATIVES?
• CAN A NON-IONIZING DIAGNOSTIC
ALTERNATIVE PROVIDE AND EQUAL
OR INCREASED BENEFIT OVER CT?
WHAT TECHNOLOGISTS CAN
DO TO LOWER DOSE
• USE ONLY OPTIMAL MAS VALUES
• CHILDREN REQUIRE LESS MAS THAN
ADULTS
• USE SLICE THICKNESSES THAT ARE
OPTIMAL (THIN SLICES REQUIRE HIGHER
MAS)
• SET PITCH VALUES THAT ARE CONSISTENT
WITH GOOD QUALITY (DOSE IS HIGHER
WITH LOWER PITCH
• PROVIDE WRAP-AROUND SHIELDING
ESPECIALLY WHEN SCANNING CLOSE TO
REPRODUCTIVE ORGANS(?)
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