Introduction
(Doses & risks)
Adult & pediatric
doses in CT
CT doses
(CTDI/E)
Walter Huda PhD
Head CT doses
SUNY Upstate Medical
University, Syracuse NY
Reducing CT doses
Cataracts
Deterministic effect
x-ray skin dose
2 mGy/0.2 rad
CT skin dose
40 mGy/4 rad
Carcinogenesis
Stochastic effect
1
Dose distributions in CT
measured in phantoms
Computed Tomography
Dose Index (CTDI)
Contiguous CT imaging
CTDI
=
1/h ∫ D(z) dz
16 cm and
32 cm
diameter
Acrylic
phantoms
ρ= 1.19
Z = 6.56
2
Helical scanning & dose
Pitch of 1.0 has dose ≈ axial scan
Pitch of 0.75 has 33% more dose
Pitch of 1.5 has 66% less dose
3
Dose (risk) in CT
is best measured by
effective dose (E)
CTDI is independent of
section thickness T
& number of sections N
Patient risk does depend on section
thickness T & number of sections N
E = Σi wi x Di
127 patients undergoing head
CT examinations
Computing adult and pediatric
doses in head CT examinations
Measurements
made at
level of the
basal ganglia
Dimensions
& average HU
100
Heads modeled
as water
equivalent
cylinders
with a radius r
90
Radius (mm)
300
250
80
70
60
200
0
5
10
15
20
Patient age (Yr)
150
120
Infants ~60 mm
110
100
100
Radius (mm)
Mean HU
50
50
Adults ~90 mm
90
80
70
0
0
20
40
60
80
100
60
20
Patient age (Yr)
40
60
80
100
Patient age (Yr)
1
CTDI is equal to:
Energy imparted/Section mass
“CTDI” for any sized radius
(Energy imparted/Section mass)
Monte Carlo
modeling
Radiology
(1997)
203:417-422
Energy imparted
70
0 - 6 months
60
340 mAs
“CTDI”
x
Directly irradiated mass
(i.e., π⋅r
π⋅ 2⋅T⋅⋅N)
CTDI (mGy)
50
40
30
20
Adults
10
0
70
80
90
100
110
120
130
140
150
kVp
For an anthropomorphic phantom
& head CT scan, compute :
180
Adults
Energy imparted (mJ)
160
340 mAs
140
120
100
Effective dose E
Energy imparted ε
E/ε ratio
80
60
40
0 - 6 months
20
0
70
80
90
100
110
120
130
140
150
kVp
2
Effective dose per unit energy imparted
for head x-ray examinations vs size
Effective dose per unit energy
-1
imparted (E/ε mSv J )
Compute E/ε ratio for
different sized patients
(i.e., newborn to adults)
10
1
10
Patient age (year)
9
Chest CT
5
0 - 6 months
340 mAs
4
7
Effective dose (mSv)
Effective dose (mSv)
8
3
2
Adults
6
5
4
1
3
0
r ² = 0.465
70
80
90
100
110
120
130
140
150
kVp
2
40
50
60
70
80
90
Weight (kg)
Patient dose:
is proportional to mAs
Doses from CT
are a major concern
increases by factor of 5 when
going from 80 to 140 kV
(@ constant mAs)
3
Frequency of CT examinations
Germany 1994
CT contribution to collective medical dose
Germany 1994
4
Dose reduction strategies
Tube current modulation for non-circular
cross-sections of the body
Technology (AEC)
Dose reduction (ALARA)
Optimization
GE Smart mA (LightSpeed Ultra)
High dose scan
Low dose scan
120 kVp
5 mm
120 kVp
43mAs
5 mm
206 mAs
A Metastasis in vertebral body
B Subcutaneous gluteal metastatis
C Small gas inclusion
D ureter opacified c contrast
1
Increasing the x-ray photon
energy (kVp) reduces
image contrast
Optimizing CT
with respect to dose
Muscle
Relative HU
1.0
0.5
Iodine
0.0
50
55
60
65
70
75
80
Photon Energy (keV)
The mAs used to make a CT image
is very important
Contrast to noise ratio (CNR)
defines CT image quality.
Low
CNR
High
CNR
2
First CT
image
(120 kVp)
100
Noise (HU)
32 cm
CT CNR can be
changed via mAs;
CT is quantum
noise limited
10
16 cm
1
40
60
80
100
200
Relative energy imparted
Constant CNR at each kVp
Iodine
4
3
2
1
Soft tissue
80
100
120
140
X-ray tube voltage (kVp)
400
mAs
Stochastic risks are
important in CT
Deterministic risk
should not occur
Effective doses in CT
1 – 2 mSv for head
5 – 10 mSv for body
CT doses “dominate”
medical exposures
ALARA principle
should be used to
minimize patient doses
3