Lifetime Attributable Risk
10 mGy in 100,000 exposed persons
(BEIR VII 2006)
SPECT/CT and PET/CT Dosimetry
All Solid Tumors
Frederic H. Fahey DSc
Children’s Hospital Boston
Harvard Medical School
frederic.fahey@childrens.harvard.edu
Leukemia
Male
Female
Male
Female
Excess Cases
80
130
10
7
Excess Deaths
41
61
7
5
Note: About 45% will contract cancer and 22% will die.
Excess Attributable Risk (Deaths)
from All Solid Tumors
per 10,000 Person-Year-Sv by 60Y
(BEIR VII 2006)
Age at Exposure (Y)
EAR (Mortality)
Relative to >30Y
1
35.1
2.92
5
30.3
2.52
10
25.2
2.10
20
17.4
1.45
>30
12.0
1.00
Thus, if 1,000,000 10 YOs receive 10 mSv,
25 will die from solid tumors by age 60 due to this exposure.
Outline
• SPECT
– Cardiac
• PET
– FDG
• Application of CT
– Dosimetry
– Attenuation Correction
– Anatomical Correlation
– Diagnosis
1
MIRD Equation
D=Ã
/m
Where
D is radiation dose in Gy
à is the cumulated activity in MBq-h
is mean energy per disintegration
in g-Gy/MBq-h
m is mass of the target organ in g
Factors Affecting Dose in NM
• Administered activity
• Biology
– Uptake and clearance rates of activity in various
source organs (biology)
• Physics
– Patient geometry (organ size and orientation)
– Radioactive decay characteristics
• Half-life
• Number, type and energy of emissions
Patient Effective Dose (mSv)
Factors Affecting Dose in SPECT
Summary
• Injected activity
– Total counts and imaging time
• Choice of camera
– Crystal thickness (1/4, 3/8, 1/2, 1”)
– Number of heads
• Choice of collimator
– Hi Sens, Gen Purpose, Hi Res, Ultra-High Res
• Image processing?
Newborn 1 Year 5 Year
10 Year 15 Year Adult
Mass (kg)
3.4
9.7
19.8
33.2
56.8
70
Tc-MDP (20 mCi*)
4.0
4.3
4.4
4.9
5.3
5.3
Tc-ECD (20 mCi*)
2.9
3.8
4.8
5.3
8.4
8.2
I-123 MIBG (10 mCi*)
3.1
3.9
4.7
5.3
7.5
7.0
Tc-MIBI (20 mCi*)
4.7
6.2
9.4
10.1
11.4
11.1
Ga-67 (6 mCi*)
19.5
21.6
23.8
25.3
27.3
26.4
*max admin activ
Courtesy of Rad Internal Dose Information Center
2
SPECT has Established Value for Risk Stratification
Rates of Cardiac Death and Myocardial
Infarction by Scan Result
SPECT: Laboratory Throughput
99mTc
8-11 mCi
99mTc
2727-30 mCi
Ado/Dipy
60 min Gated Rest
35 min
60 min
4-6 min
99mTc
5.0
SPECT
4.0
Gated Stress
Event Rate 3.0
per Year
2.0
(%)
1.0
30 min
0.0
* p<0.001
**p<0.01
• Choice of scanner
– Crystal material and thickness
– 2D vs 3D
– Axial field of view
• Image processing?
2.7
0.3 0.5
2946
Normal
**
2.9
*
2.9
2.3
0.8
884
455
898
Mildly
Moderately Severely
Abnormal Abnormal Abnormal
Hachamovitch et al, Circulation 1998
Factors Affecting Dose in PET
– Total counts and imaging time
*
Scan Result
Courtesy of Sharmila Dorbala,
Dorbala, MD
Brigham and Women’
Women’s Hospital
• Injected activity
MI
4.2
3-4 hr
Total admin act = 35-41 mCi
CD
PET Imaging Agents
•
18F
Fluoro-deoxy-glucose (FDG)
– Practically all clinical PET imaging
– Oncology
– Brain and cardiac
• 82Ru for cardiac imaging
• 18F NaF for bone imaging
• Other agents (FLT, F-DOPA, F-Miso…)
3
Patient Dose from 18F NaF and 82Rb (mSv)
Patient Dose from FDG (mSv)
Summary
Newborn
1 Year
5 Year
10 Year
15 Year
Adult
Summary
1 Year
5 Year
10 Year
15 Year
Adult
Mass (kg)
3.4
9.7
19.8
33.2
56.8
70
Mass (kg)
9.7
19.8
33.2
56.8
70
18F
NaF
Act (mCi)
0.68
1.94
3.96
6.64
11.36
14.0
Act (mCi)
0.56
1.13
1.90
3.24
4.00
Bladder*
86
98
106
116
133
128
Bladder*
27.9
30.2
33.0
37.9
36.5
Eff Dose*
10
11
11
11
12
12
Eff Dose*
3.0
3.1
3.2
3.5
3.4
82Rb-1.3m
Act (mCi)
0.51
1.46
2.97
4.98
8.52
10.5
Act (mCi)
6.8
13.9
23.2
39.8
49.0
Bladder*
64
73
79
87
100
96
Thyroid*
108
112
82.5
91.2
68.9
Eff Dose*
7.0
8.0
8.0
8.0
9.0
9.0
Eff Dose*
8.3
9.2
8.6
9.9
8.7
Courtesy of Rad Internal Dose Information Center
Pediatric Administered Dose Survey
• Surveyed 15 dedicated pediatric hospitals in
North America
• Requested information on 16 studies
commonly performed in pediatric NM
– Administered dose per kg
– Maximum administered dose
– Minimum administered dose
*ICRP 80
Pediatric Administered Dose Survey
Radiopharmaceu tical
Parameter
Number of
Respondents
Minimum
Maximum
Median
Mean
Tc-99m DMSA
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
8
1.11 (0.030)
3.70 (0.100)
2.22 (0.060)
2.35 (0.064)
11
5.55 (0.150)
74.00 (2.000)
18.50 (0.500)
26.40 (0.714)
11
74.00 (2.00)
222.00 (6.000)
185.00 (5.000)
151.36 (4.091)
Tc-99m MAG3
Tc-99m MDP
Tc-99m DISIDA
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
8
1.85 (0.050)
10.36 (0.280)
5.55 (0.150)
5.69 (0.154)
12
18.50 (0.500)
148.00 (4.000)
37.00 (1.000)
53.96 (1.458)
13
111.00 (3.000)
370.00 (10.000)
370.00 (10.000)
278.92 (7.538)
8
7.40 (0.200)
13.32 (0.360)
11.10 (0.300)
10.87 (0.294)
13
22.20 (0.600)
185.00 (5.000)
92.50 (2.500)
99.90 (2.700)
13
666.00 (18.000)
925.00 (25.000)
740.00 (20.000)
819.69 (22.154)
7
1.85 (0.050)
3.70 (0.100)
2.78 (0.075)
2.97 (0.080)
13
14.80 (0.400)
74.00 (2.000)
37.00 (1.000)
36.17 (0.978)
13
92.50 (2.500)
370.00 (10.000)
185.00 (5.000)
200.65 (5.423)
Treves ST, Davis RT, Fahey FH. J Nucl Med, 2008;49:1024-1027.
4
Pediatric Administered Dose Survey
Pediatric Administered Dose Survey
Radiopharamceutical
Parameter
Number of
Respondents
Minimum
Maximum
Median
Mean
Radiopharamceutical
Parameter
Number of
Respondents
Minimum
Maximum
Median
Mean
I-123 MIBG
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
7
5.18 (0.140)
7.40 (0.200)
5.55 (0.150)
5.45 (0.147)
Tc-99m SestaMIBI
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
7
5.70 (0.154)
18.50 (0.500)
12.95 (0.350)
11.97 (0.323)
Tc-99m NaTcO4
(Meckels)
I-123 (NaI)
(Thyroid)
Tc-99m ECD
Tc-99m HMPAO
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
11
37.00 (1.000)
185.00 (5.000)
37.00 (1.000)
75.68 (2.045)
13
296.00 (8.000)
370.00 (10.000)
370.00 (10.000)
360.04 (9.731)
7
1.63 (0.044)
5.92 (0.160)
5.18 (0.140)
4.46 (0.121)
12
7.40 (0.200)
148.00 (4.000)
37.00 (1.000)
49.27 (1.332)
12
111.00 (3.000)
555.00 (15.000)
370.00 (10.000)
348.42 (9.417)
4
0.06 (0.0015)
0.22 (0.0060)
0.10 (0.0028)
0.12 (0.0033)
10
0.56 (0.0150)
11.10 (0.3000)
3.70 (0.1000)
3.76 (0.1015)
11
3.70 (0.1000)
19.98 (0.5400)
8.14 (0.2200)
9.45 (0.2555)
7
1.85 (0.050)
15.73 (0.425)
10.55 (0.285)
10.81 (0.292)
11
18.50 (0.500)
370.00 (10.000)
185.00 (5.000)
174.91 (4.727)
12
370.00 (10.000)
1110.00 (30.000)
740.00 (20.000)
810.92 (21.917)
Pediatric Administered Dose Survey
Radiopharamceutical
Parameter
Number of
Respondents
Minimum
Maximum
Median
Mean
Tc-99m Ultratag
(MUGA)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
5
7.40 (0.200)
14.80 (0.400)
8.14 (0.220)
9.69 (0.262)
Ga-67
(for Inflammatory
Disease)
Ga-67
(for Tumor Imaging)
F-18 FDG
10
44.40 (1.200)
370.00 (10.000)
101.75 (2.750)
132.09 (3.570)
11
555.00 (15.000)
925.00 (25.000)
740.00 (20.000)
733.27 (19.818)
Activity/Mass MBq/kg
(mCi/kg)
5
1.48 (0.040)
2.59 (0.070)
1.85 (0.050)
1.90 (0.051)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
4
9.25 (0.250)
74.00 (2.000)
27.75 (0.750)
34.69 (0.938)
6
111.00 (3.000)
185.00 (5.000)
166.50 (4.500)
154.17 (4.167)
5
2.96 (0.080)
5.25 (0.142)
4.07 (0.110)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
3.90 (0.105)
10
9.25 (0.250)
111.00 (3.000)
37.00 (1.000)
41.07 (1.110)
12
222.00 (6.000)
370.00 (10.000)
333.00 (9.000)
312.96 (8.458)
6
5.18 (0.140)
7.40 (0.200)
5.37 (0.145)
Tc-99m MAA
Tc-99m Ultratag
(GI Bleed)
Tc-99m Denatured RBC
(Spleen)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
Activity/Mass MBq/kg
(mCi/kg)
Minimum Activity
MBq (mCi)
Maximum Activity
MBq (mCi)
10
37.00 (1.000)
518.00 (14.000)
148.00 (4.000)
210.90 (5.700)
12
370.00 (10.000)
1110.00 (30.000)
777.00 (21.000)
792.42 (21.417)
7
1.11 (0.030)
4.88 (0.132)
1.85 (0.050)
2.31 (0.062)
12
7.40 (0.200)
37.00 (1.000)
18.50 (0.500)
21.40 (0.578)
12
111.00 (3.000)
222.00 (6.000)
138.75 (3.750)
143.99 (3.892)
6
3.70 (0.100)
11.10 (0.300)
8.33 (0.225)
7.92 (0.214)
9
37.00 (1.000)
148.00 (4.000)
74.00 (2.000)
78.11 (2.111)
11
185.00 (5.000)
740.00 (20.000)
740.00 (20.000)
615.55 (16.636)
2
1.85 (0.050)
2.59 (0.070)
2.22 (0.060)
6
18.50 (0.500)
92.50 (2.500)
37.00 (1.000)
40.08 (1.083)
6
74.00 (2.000)
740.00 (20.000)
111.00 (3.000)
215.83 (5.833)
2.22 (0.060)
Variability in Administered Doses
in Pediatrics
• Admin dose/kg and maximum dose varied, on
average, by a range of a factor of 3, but by as
much as a factor of 10
• Minimum dose varied, on average, by a factor
of 10 and as much as a factor of 20
5.67 (0.153)
6
18.50 (0.500)
74.00 (2.000)
37.00 (1.000)
46.25 (1.250)
8
370.00 (10.000)
555.00 (15.000)
407.00 (11.000)
430.13 (11.625)
5
Minimum Administered Dose
in Pediatrics
Use of Image Processing and
Reconstruction
• Based on clinical task, subjective appearance,
tradition and instrumentation
• May not be based on current state-of-the-art
• Establishment of universally accepted values
• Filtered backprojection routinely used
• Newer image processing and reconstruction
algorithms may provide adequate diagnostic
image quality with fewer counts
• Lower administered dose or faster imaging times
• Subjective grading of studies acquired with
varying numbers of counts
– Consensus among experts
– Acquisition of clinical data at variety of dose levels
– Simulation of clinical studies with noise
Use of OSEM-3D Reconstruction
CT in PET/CT and SPECT/CT
• Risk
• Dosimetry
• Application
FBP Full Cts
– CT-Based attenuation correction
– Anatomical correlation
– Diagnostic
OSEM Half Cts
FBP Full Cts
OSEM Half Cts
6
CIRS Tissue Equivalent Phantoms
Radiation Risk for CT
For 10 & 1 YO, ~ 4X & 7X
higher lifetime risk per CT
scan, respectively
•Dosimetric CT phantoms
•Simulated spine
•Five 1.3 cm holes
Brenner, Hall, NEJM,
2007;357:2277-2282
Phantom
AP x Lat
(cm)
Circum
(cm)
Newborn
9 x 10.5
32
1 Year
Old
11.5 x 14
42
5 Year
Old
10 Year
Old
Med
Adult
14 x 18
53
16 x 20.5
61
25 x 32.5
96
•Five different sizes
Fahey et al. Radiology 2007;243:96-104
Dosimetry of PET-CT
and SPECT-CT
Dose from CT of PET-CT
GE Discovery LS (4-slice)
CTDIvol (160 m A, 0.8 s , 1.5:1 pitch)
• PET/CT
• SPECT/CT
–Philips Precedent
25.00
CTADIvol (mGy)
–GE Discovery LS
30.00
New Born
20.00
1 Y ear Old
15.00
5 Y ear Old
10 Y ear Old
10.00
Med Adult
5.00
0.00
70
90
110
130
150
ED from
14 mCi of FDG
11 mSv
Tube V oltage (k V p)
7
Dose from CT of PET-CT
GE Discovery LS (4-slice)
Dose from CT of SPECT-CT
Philips Precedence 6-slice
CTDIvol (10 year old, 0.8 s, 1.5:1 pitch)
CTADIw for 10 YO, pitch =1:1
21 mGy
25.00
35.00
30.00
80 kVp
15.00
100 kVp
120 kVp
10.00
140 kVp
5.00
120 kVp
15.00
140 kVp
10.00
0.00
0
50
0.3 mGy
100
150
0
200
CT ADIw for 140 kVp, Pitch = 1:1
45.00
40.00
35.00
Infant Body
30.00
1 YO Body
25.00
5 YO Body
20.00
10 YO Body
15.00
Med Adult Body
10.00
5.00
0.00
50
100
150
50
100
150
200
mAs
Tube Current (m A)
Dose from CT of SPECT-CT
Philips Precedence 6-slice
CTADIw (mGy)
90 kVp
20.00
5.00
0.00
0
25.00
CTADI (m Gy)
CTADIvol (mGy)
20.00
Typical Effective Dose Values
Low Dose Chest CT
0.7 mSv (70 mrem)
Head CT
1 - 2 mSv (100 - 200 mrem)
Chest CT
5 - 7 mSv (500 - 700 mrem)
Abdomen CT
5 - 7 mSv (500 - 700 mrem)
Pelvis CT
3 - 4 mSv (300 - 400 mrem)
Abd & pelvis CT
8 - 11 mSv (800 - 1100 mrem)
Average U.S. background radiation
≈ 3 mSv (300 mrem)
200
mAs
8
Estimated Cumulative Radiation Dose from
PET/CT in Pediatric Patients with Malignancies
A 5 Year Retrospective Review,
Soni C. Chawla, MD, UCLA
PET-CT Attenuation Correction
• 248 PET-CT studies on 78 patients
– 50 males, 28 females, 1.3-18 YO
• CT effective dose estimated by ImPACT - 20.3 mSv/scan
• PET effective dose estimated by OLINDA - 4.6 mSv/scan
• PET-CT effective dose - 24.8 mSv/scan
• Average number of scans/patient - 3.2 (range 1-14)
• Cumulative effective dose per patient – 78.9 mSv (range 6.2 – 399)
• 27% of patients received > 100 mSv
Poster PO71at SPR Meeting 2008 - Pediat Radiol 2008;38(Suppl 2):S339
PET-CT Attenuation Correction
•
•
•
•
•
Acquire CT Scan and reconstruct
Apply energy transformation
Reproject to generate correction matrix
Smooth to resolution of PET
Apply during reconstruction
Applications for very low-dose CT
for anatomical correlation
• Pediatrics
• MR is correlative imagine modality of
choice (e.g. brain imaging)
• Multiple acquisitions in single imaging
session (stress-rest 82Rb cardiac imaging)
9
Quality of CTAC
Quality of CTAC
80 kVp
10 mA
0.5 s/rot
1.5:1
140 kVp
160 mA
0.8 s/rot
1.5:1
80 kVp
10 mA
0.5 s/rot
1.5:1
140 kVp
160 mA
0.8 s/rot
1.5:1
Effect of Patient Size
80 kVp, 10 mA, 0.5 s/rotation
SPECT-CT CTAC
Accuracy of Atte nuation Correction w ith Patie nt Size
Hounsfield to Linaer Attenuation Coefficient
0.3
-1
)
0.0920
New born
0.0880
1 YO
5 YO
0.0860
10 Y O
15 Y O
0.0840
Small A dult
Med Adult
0.0820
Large A dult
Linear Attenuation Coefficient (cm
Linear Attenuation Coefficient (cm-1)
0.0900
0.25
0.2
140 kVp
0.1
0.05
0
-1000
0.0800
100 kVp
0.15
-500
0
500
1000
Hounsfield Units
0.0780
80/10/.5
10
SPECT-CT CTAC
•
•
•
•
SPECT-CT CTAC
CT image reoriented into SPECT coordinates
CT image resampled to match SPECT pixel size
CT image smoothed to SPECT resolution
Conversion from HU to linear attenuation
coefficient (piecewise linear)
CT Scan (140 kVp, 225 mAs)
SPECT-CT CTAC
90 kVp/22 mAs
SPECT-CT CTAC
140 kVp/225 mAs
Jaszczak
CTAC Map
90 kVp/22 mAs
140 kVp/225 mAs
DS Torso Phantom
11
CT-Based Attenuation Correction
• For PET/CT, adequate attenuation correction can
be obtain for pediatric patients with acquisition
parameters as low as 80 kVp, 10 mA and 0.5 s per
rotation. In adults, 120 kVp, 10 mA, 0.8 s. For
pediatric patients, if one is only using the CT for
attenuation correction, the CT radiation dose can
be substantially reduced, by on the order of a
factor of 50.
• For SPECT/CT, adequate attenuation correction
can be obtained with parameters as low as 90 kVp,
15 mAs for pediatric and adult patients.
Low Dose CT for PET-CT
• Previous conventional imaging at
standard pediatric CT technique
• 80 kVp 40-60 mAs
• No missed diagnoses if PET +ve or in
follow-up
• PET abnormality more important
finding than CT localization
Courtesy of Helen Nadal, British Columbia Children’s
CT for Anatomical Correlation
• In some cases, CT only needed for
anatomical correlation
• Potential for large reduction in dose in these
cases
• How much less?
– Clinical cases acquired with less dose
– Noise simulation
Conventional CT
kVp 90 mAs 175
PET CT
kVp 80 mAs 35
Dose reduction by factor of 6!
12
Non avid
Pulmonary
nodule
PET/CT
Dedicated
CT
Axial Extent of CT
• “Whole Body” PET typically acquired “Eyes to
Thighs”
• Potential for SPECT acquisitions to all be
extended, particularly with more efficient
reconstruction
• Thus CT component can be combination of head
& neck, thoracic, abdominal and pelvic CT
• One size fits all
CT for Diagnosis
• Use of contrast material
• Respiration
– Breath hold
– Gating
• Axial extent
Different Paradigm for PET/CT and
SPECT/CT
• Suggested by George Segall (Stanford)
• Limit high-quality CT to area of interest, i.e.
area traditionally imaged in CT
• Very low-dose CT for remainder of
PET/SPECT axial field of view
• If necessary, acquire additional high-quality
CT in areas indicated on PET/SPECT
13
Summary
• SPECT and PET
– Increasing use, particularly cardiac SPECT and FDG
tumor imaging
– Evaluation of pediatric dosimetry
– Use of image processing
• CT
– Large reduction in dose if only used for attenaution
correction
– Possible reduction in dose for anatomical correlation
– For diagnosis, consider new acquisition paradigms
14