Download Dose Reduction Strategies for SPECT/CT and PET/CT

Dose Reduction Strategies for
SPECT/CT and PET/CT !
Adam Alessio, PhD, DABSNM
[email protected]
Department of Radiology
University of Washington
http://faculty.washington.edu/aalessio/
DISCLOSURE:
Dr. Alessio has received grant/research support from GE Healthcare
© Adam Alessio 2015, [email protected]
Alessio, 1
Image Quality Tradeoffs in NM
Technology
Dose Savings
20
Image Quality
Diagnostic Utility
Information Density
10
Technique/Scanner 1
Technique/Scanner 2
New Technology ?
0
0
10
20
Radiation Dose (a.u.)
30
Scan Duration (minutes)
Alessio, 2
Image Quality Tradeoffs in NM
20
Image Quality
Diagnostic Utility
Information Density
10
Operational
Dose Savings
Technique/Scanner 1
Technique/Scanner 2
New Technology ?
0
0
10
20
Radiation Dose (a.u.)
30
Scan Duration (minutes)
Alessio, 3
Goal of Dose Optimization?
A.  Make prettiest image possible
B.  Minimize radiation dose
C.  Maximize physician’s happiness
D.  Maximize technologist’s happiness (i.e., shortest
acquisition time)
E.  Acquire with maximum image quality at minimum of dose
F.  Define a task and sufficient image quality to achieve task
Alessio, 4
Dose Optimization in Nuclear Medicine
•  Is all about
–  Injected Activity?
OR
–  Defining the desired task and the necessary
image quality to achieve that task
–  Dose Optimization = Rational Protocol Selection
§  !! More than just a question of injected activity!!
§  Appropriate protocol for the appropriate scanner, clinical resources,
study, and patient
§  We need better approaches for rational protocol selection…
Alessio, 5
Diagnostic Reference Levels
“Diagnostic reference levels (DRLs), which are a form of
investigation levels, represent an important tool to optimize
image quality and the radiation dose delivered to patients.”
DRL’s help promote (not dictate) good practice for a more
specific medical imaging task; and
•  Proposed 20 years ago. Used extensively in Europe for
Quality Assurance
• 
• 
ICRP 73 (1996)
NCRP, Report 172: Reference Levels and Achievable Doses in Medical and Dental Imaging:
Recommendations for the United States (National Council on Radiation Protection and
Measurements, 2012).
Alessio, 6
Diagnostic Reference Levels
•  DRLs are set at approximately the 75th
percentile of similar studies for similar patients
•  Achievable doses, AD, represent the median
(50th percentile) of doses
• 
• 
ICRP 73 (1996)
NCRP, Report 172: Reference Levels and Achievable Doses in Medical and Dental Imaging:
Recommendations for the United States (National Council on Radiation Protection and
Measurements, 2012).
Alessio, 7
Diagnostic Reference Levels & Achievable Doses
# Exams
75th %
50th %
Dose
AD
DRL
•  75% of doses below Diagnostic Reference Level
•  50% of doses below Achievable Dose (encourage
dose optimization for sites below 75% level)
Alessio, 8
Diagnostic Reference Levels, NCRP 172
Nuclear Medicine Reference Levels
NM Doses from a 2010 Survey of 9 Academic Centers
Sample of Suggested Reference Levels
Examination
DRL (mCi)
AD (mCi)
Tc99m-Tetrofosmin (Stress)
39.0
25.0
Tc99m-Tetrofosmin (Rest)
29.0
18.0
99mTc-MAG3
10.0
7.5
Tc99m-MDP
32.0
23.0
F18-FDG
19.0
15.0
“For nuclear medicine, the 75th percentile maximum RLs should be used as
guidelines to limit unnecessary radiation dose as long as diagnostic-quality
nuclear medicine studies are obtained, but not as absolute limits.”
NCRP, Report 172: Reference Levels and Achievable Doses in Medical and Dental Imaging: Recommendations for the United
States (National Council on Radiation Protection and Measurements, 2012).
Alessio, 9
Diagnostic Reference Levels, NCRP 172
CT Reference Levels
Many pages of DRL’s for CT based primarily on
ACR CT Accreditation Materials
Potential
Diagnostic Ref Level
NCRP, Report 172: Reference Levels and Achievable Doses in Medical and Dental Imaging: Recommendations for the United
States (National Council on Radiation Protection and Measurements, 2012).
Alessio, 10
SAM Question:
Diagnostic Reference Levels (DRLs)
can be used in clinical practice to:
A.  Provide legal justification in event of malpractice
law suit
B.  Set standards to identify normal, average doses
C.  Set standards to identify unusually low doses
D.  Compare local practice with peer institutions and
national levels
E.  Provide required protocol settings for local practice
Alessio, 11
Technology Dose Savings
Current technologies providing genuine
improvements:
•  Improved collimators (SPECT)
•  Improved solid-angle coverage (SPECT, PET)
•  Improved detectors and electronics (SPECT, PET,
and CT)
•  Improved data processing (SPECT, PET, and CT)
–  Iterative image reconstruction
•  Improved review software/workstations
Alessio, 13
Collimator Efficiency
•  Collimators typically absorb well over 99.95% of
all incident photons.
•  Trade-off between spatial resolution and
detection efficiency (sensitivity).
•  Collimator choices: LEGP, LEHR, MEGP, High
Energy
Ø  balance the trade-off
Ø  used for different isotopes
Alessio, 14
Collimator Sensitivity
Point Source Geometric Efficiency in Air!
From: Physics in Nuclear Medicine (Cherry, Sorenson and Phelps)
Alessio, 15
Collimator Design
Conventional
Siemens AUTOFORM
NEMA planar LEHR system
sensitivity
NEMA planar LEHR system
sensitivity
(cpm/µCi @ 10cm)
(cpm/µCi @ 10cm)
202
195
195
185
Regular detectors
-20%
175
165
185
-26%
175
168
` 165
160
155
155
Competitor 1
Symbia
Competitor 2
§  Thicker septa lead to more
attenuation
§  Low sensitivity
§  Unique septa design enables
industry-leading NEMA
sensitivity* (up to 26% higher)
*Vendor Statement: Slide provided from Siemens Healthcare
Alessio, 16
A Benefit of Application Specific Geometries:
Solid-Angle Coverage
Parallel Collimator:
Same detection efficiency,
Different resolution
x % of sphere (counts)
detected
Focused geometries can
provide significantly better
solid angle coverage
à Many more counts
detected at a time
x % of sphere (counts)
detected
Alessio, 17
Detectors: NaI vs Cadmium Zinc Tellurium (CZT)
• 
• 
• 
Inexpensive
Energy Resolution ~9%
Spatial Resolution ~4mm
• 
• 
• 
• 
Relatively expensive
Energy Resolution ~5%
Spatial Resolution ~2mm
Compact
Figure from GE Healthcare, Alcyone Technology White Paper
Alessio, 18
The Reconstruction Problem:
An Inverse Problem
Unknown image
y = Px + n
Observed data
system matrix
Error in observations
(noise, scatter, etc)
−1
x = P ( y − n)
DIFFICULT:
Requires Iterative Solution
1.  Each Vendor can have unique
representation for y, P, x, n
2.  And, how they solve P-1
Main Point: Not all “OSEM” algorithms the same
Not all Vendors Recon algorithms are the same…
Alessio, 19
Iterative Image Reconstruction in SPECT
Faster, Better Images:
1.  Garcia et al, Cardiac Dedicated Ultrafast SPECT Cameras: New Designs and Clinical
Implications. J Nucl Med, 2011; 52.
2.  Borges-Neto et al. Clinical results of a novel wide beam reconstruction method for shortening
scan time of Tc-99m cardiac SPECT perfusion studies. J Nucl Cardiol. 2007.
Increasing Applications For Quantitative SPECT:
1.  Bailey, An Evidence-Based Review of Quantitative SPECT Imaging and Potential Clinical
Applications, JNM 2013
2.  Beauregard et al, Quantitative 177Lu SPECT (QSPECT) imaging using a commercially
available SPECT/CT system, Cancer Imaging 2011.
3.  Dewaraja et al, Accurate Dosimetry in 131I Radionuclide Therapy Using Patient-Specific, 3Dimensional Methods for SPECT Reconstruction and Absorbed Dose Calculation, JNM 2005.
Alessio, 20
Quantitative SPECT Reconstruction
PMTs
Key Components:
1.  Attenuation correction
2.  Intra-Patient Scatter correction
3.  Accurate System Model (includes
collimator-resolution modeling)
4.  Intra-Collimator Scatter correction
5.  View-dependent decay correction
Scintillator
collimator!
Ø  ALL incorporated into
reconstruction algorithm
Alessio, 21
Collimator Resolution
Dependent on source-collimator distance
From: The Essential Physics of Medical Imaging (Bushberg, et al)!
Alessio, 22
Collimator Resolution
Dependent on source-collimator distance
Ø  All collimators suffer from
depth dependent
resolution response
Ø  Iterative reconstruction
methods can model, and
therefore somewhat
compensate for, the
resolution response of the
collimator
From: The Essential Physics of Medical Imaging (Bushberg, et al)!
Alessio, 23
Example of Iterative Recon Trial
Stress/rest Tc-99m tetrofosmin single-isotope study
Full Duration, Filtered Backprojection
Half Duration, Wide Beam Reconstruction from UltraSPECT
Conclusion from this study: cardiac SPECT perfusion studies may be performed with the WBR algorithm using
half of the scan time without compromising qualitative or quantitative imaging results.
Borges-Neto et al, Clinical results of a novel wide beam reconstruction method for shortening scan time of Tc-99m
cardiac SPECT perfusion studies. J Nucl Card, 2007.
Alessio, 24
SAM Question
Technology dose savings can be achieved in
SPECT imaging through all the following except:
A.  Improved collimator designs
B.  Higher resolution detectors made from materials
such as CzT
C.  Improved data processing and reconstruction
algorithms
D.  Faster rotation of detector heads
E.  Greater solid angle coverage
Alessio, 25
Trends in PET Technology
•  Larger Bore Sizes (70cm towards 78cm and
more…)
•  More Reproducible Quantitation
–  Better Calibration (ex: Siemen’s Quanti-QC)
–  Respiratory Compensation
–  Better image reconstruction (ex: GE’s Q.CLEAR)
•  Better Signal to Noise through:
–  Better Time-of-Flight (ex. Philip’s Digital PET)
–  Larger axial sampling
–  Better image reconstruction
Alessio, 27
Digital Photon Counting PET
Converts scintillation light directly
to a digital signal, with zero analog
noise.
Allows for Faster Timing Resolution
Vendor Statements from Philips Healthcare
Alessio, 28
Variations in resolution loss vs. size and smoothing
OSEM
Increasing
smoothing
Max
Mean
FBP
Alessio, 29
How to reduce partial volume effect in PET?
PSF-Based Iterative Reconstruction
Detectors
Scanner bore
Improve System Model
Locations of point sources
PPSF (sv ,s)
Events (a.u.)
Each radial location has blur in radial direction
sv = 2mm
s
sv = 348mm
s
Alessio, 30
Measured Spatially Variant System Modeling (PSF) in Iterative Reconstruction
Images with “clinical” reconstruction parameters: 2.7mm/pixel, 7mm post-filter, 28 subsets
Contrast Recovery vs. Size
Contrast Recovery vs. “True” Noise across 50 scans
4 it
8 it
Proposed
Method
Prior “best”
method
Prior Method
OSEM+LOR
Proposed Method
OSEM+LOR+PSF
Observations: Addition of PSF…
•  Leads to roughly 7% bias improvement at
matched true noise levels across all sphere sizes
FDG PET Exam, 109kg patient
Alessio et al, “Application and Evaluation of a Measured Spatially Variant
System Model for PET Image Reconstruction,” IEEE Trans Med Imaging,
2010.
Alessio, 31
Time-Of-Flight PET
•  Measures time difference of detection of photons
–  If time difference =0, annihilation at center of field of view
•  Timing resolution 500 ps = 7.5 cm
Conventional
backprojection
TOF
backprojection
Alessio, 32
Time-of-Flight PET
Contrast recovery coefficient versus noise
for 27 cm diameter cylinder
TOF
Contrast recovery coefficient versus noise
for 35cm diameter cylinder
non-TOF
Karp et al, Benefit of Time-of-Flight in PET: Experimental and Clinical Results, JNM
49:3, 2008.
Alessio, 33
TOF gain as a function of patient mass
TOF gain for matched noise levels, averaged over 6–9 lesions (1- to 2-cm
diameter) for each patient, is plotted as function of patient mass. Error bars
reflect the range of TOF gains seen for this patient.
Karp et al, Benefit of Time-of-Flight in PET: Experimental and Clinical Results, JNM
49:3, 2008.
Alessio, 34
Question:
Time-of-flight PET is especially
beneficial for:
A.  High resolution brain imaging
B.  Smaller pediatric patients
C.  Depth of interaction detectors
D.  Obese patients
Answer: D. Larger patients will have more
signal to noise gains than smaller objects.
Alessio, 35
CT: Instrumentation/Processing
•  CT Detectors
•  Improved data/image processing
–  Discussion of CT image enhancement
–  Iterative image reconstruction
Alessio, 36
X-ray
CT Detectors
Modern Systems use Solid State Scintillation
Detectors
–  Scintillation Materials: CdW04, Gd2O2S,
HiLight™, GEMSStone™, CsI
–  Coupled to photodiodes
• 
Visible light
scintillator
photodiode
Electrical signal
Conventional 3rd Generation CT Detector:
2-4 cm axial, ~55° arc
Flat-panel detectors usually use CesiumIodide (CsI) coupled to amorphous silicon
photodiodes
–  Originally developed for angiography
–  Used in
§  C-Arm Conebeam CT Systems
§  Philips BrightviewXCT SPECT/CT
–  Cons: Low contrast resolution and Slow
acquisition
–  Pros: High spatial resolution and Large area
Flat panel detector:
35 cm x 45 cm
Alessio, 37
CT Iterative Recon Summary
Raw CT Data
(projection
data)
• 
• 
• 
• 
Images
Each vendor is offering “iterative” methods to reduce image
noise, effectively allowing for reduced dose acquisitions at
matched image quality
“Iterative” data enhancement methods can be applied at any
step in imaging chain
Image-Based Iterative Methods:
– 
– 
– 
– 
all essentially
Image-Based
Iterative
Methods
Tomographic Image
Reconstruction
Philips: iDose
Siemens: IRIS (iterative reconstruction in image space)
Toshiba: AIDR (adaptive iterative dose reduction)
Third Party Solutions: Clarity™ CT from Sapheneia (Sweden);
ContextVision Inc. (Sweden)
“More Fully” iterative reconstruction methods
– 
– 
– 
– 
Toshiba: AIDR3D (adaptive iterative dose reduction)
GE Healthcare: ASIR (Adaptive Statistical Iterative Recon), ASIR-V, Veo
Philips: iDose4
Siemens: SAFIRE (Sinogram Affirmed Iterative Recon)
Alessio, 38
Image-Based Enhancement
Marketing Brochure for Clarity CT Solutions,
www.claritysolutions.org
Alessio, 39
So# Tissue Conspicuity Increased conspicuity
Increased noise
FBP
GE:ASIR
GE:VEO
120 kVp, variable mAs (NI=36), 1.375 pitch. 0.625/0.8 mm slice: Width = 400, Level = 40 HU
65 YO female, 83.7 kg, 160 cm, BMI = 32.7
40
Alessio, 40
Coronary CT angiography
FBP
Siemens: SAFIRE
Reconstruction of half dose data
Moscariello et al, Coronary CT angiography: image quality, diagnostic accuracy,
and potential for radiation dose reduction using a novel iterative image
reconstruction technique—comparison with traditional filtered back projection.
Eur Radiol, 2011.
Alessio, 41
Dose Reduction Techniques
CT: Operational
•  Factors that affect radiation dose with CT
•  Appropriate protocols
–  Diagnostic CT vs Localization CT vs
Attenuation Correction CT
Alessio, 42
Factors Affecting CT Dose
Direct Influence on Dose
♦ X-ray beam energy (kVp)
♦ X-ray tube current (mA)
♦ Rotation or exposure time
♦ Slice thickness
♦ Object thickness
♦ Pitch or spacing
♦ Dose-reduction techniques
♦ X-ray source to isocenter distance
Indirect Influence on Dose
♦ Reconstructed slice thickness
image statistics require higher kVp and/or mAs in thinner slices to achieve equivalent
level of noise as in thicker slices.
♦ Reconstructed image resolution
algorithms enhancing spatial resolution also increase image noise- higher kVp and/or mAs
may be used to compensate.
Alessio, 43
Factors Affecting CT Dose
CTDIw measured in 16cm head & 32 cm body phantoms
Dose reduces
more than linearly
with tube voltage
60
60
50
50
120kVp, 10mm
Head
40
CTDI_w(mGy)
CTDI_w(mGy)
300mAs, 10mm
Dose varies linearly
with tube current
30
Body
20
10
Head
40
30
Body
20
10
0
0
As you
decrease
dose, you increase
noise
tube voltage
(kVp)
tube current (mAs)
(usually decrease image quality) – No Free Lunch
80
100
120
140
100
90
300
400
70
120kVp, 300mAs
80
120kVp, 300mAs
60
Head
70
Head
50
60
CTDI_w(mGy)
CTDI_vol(mGy)
200
50
Body
40
30
40
Body
30
20
20
10
10
0
0
0
0.5
1
1.5
helical pitch
2
2.5
4
Adaped from McNitt-Gray, “Radiation Dose in CT”, Radiographics, 2002, 22:1541-1553.
8
12
16
multislice collimation (mm)
20
Alessio, 44
RadioGraphic
Table 3
Longitudinal Tube Current Modulation Systems
Characteristic
Product name
Requires CT projection
radiograph
Modulation algorithm
Online feedback
GE Healthcare
Auto mA
Philips
Siemens
...
...
Toshiba
Real E.C.
Automatic Exposure Control
Yes
Attenuation based
No
...
...
...
...
...
...
Yes
Attenuation based
No
variesTube
between Current
anatomic regions
(Fig 3). Details
•  Modulate
based
regarding implementation by several manufacturon patient
ers arespecific
given in Tableinformation
3.
in the Angular-Longitudinal
Tube Current Modulation
–  Longitudinal
(z-axis)
The simultaneous combination of angular and
– 
for example:
AutoMA
Z-DOM
longitudinal
(x-, (GE),
y-, and
z-axis)(Philips),
tube current
CareDose (Siemens)
modulation involves variation of the tube current
both during gantry rotation and along the z-axis
Angular
Direction
of the patient
(ie, from the anteroposterior direction to the lateral direction, and from the shoulders to the abdomen). The operator must still
indicate the desired level of image quality by one
of the methods described earlier. This is the most
comprehensive approach to CT dose reduction
because the x-ray dose is adjusted according to
the patient-specific attenuation in all three planes.
Details regarding the implementation of this dose
modulation technique by several manufacturers
are given in Table 4. A graphic illustration of this
approach is shown in Figure 4.
•  On average, can achieve
~20% (3-45%) dose reductions
at matched quality*
Figure 3.
Graph of tube current (in milliamperes)
Graph
of tube
superimposed
superimposed
on a current
CT projection
radiograph illustrates
concept of longitudinal dose modulation, with varionthe
CT
projection radiograph showing
ation of the tube current along the z-axis. The curve is
longitudinal
modulation*
determined by using
attenuation data from the CT projection radiograph and the manufacturer-specific algorithm.
Automatic Exposure Control
AEC is analogous to acquisition timing in general
z-axis, or during movement in all three dimen-
to-noise ratio), and the CT system determines the
right tube current–time product. In practice, it is
relatively straightforward for the system to deliver
the desired image quality, once that has been de-
differentiate between the modulation of the tube
Alessio, 45
current to achieve a defined image quality, and
the prescription of the desired image quality by
the user. Together these tasks are referred to as
*McCollough
CH, et
al.user
CTdetermines
dose reduction
and dose
management
overview
of and
sions,
according to thetools:
patient’s
body habitus
radiography.
The
the image qualthe user’s image quality requirements. Thus, we
requirements
(as regards noise 2006;26:503-512.
or the contrastavailableityoptions.
Radiographics.
Comparison of Typical Doses Hybrid CT Acquisitions
Study Technique Effective Dose (mSv) 20 mCi Tc99m MDP
(740 MBq)
4.2
Stress:Rest Tc99m trofosmin
(40:10 mCi)
13
CT for diagnostic purposes
[110-200] mAs1
CTDIvol = [8-14] mGy
11-20
CT for anatomic localization
[30-60] mAs3
CTDIvol = [2-4] mGy
3-6
CT for attenuation correction
only [5-10] mAs4 CTDIvol = [0.3-1.0] mGy 0.5-1.0 NM Bone Scan
High-low Myocardial Perfusion
For ease of comparison, all CT studies performed with 120kVp, pitch 1.375, 40mm collimation,
900 mm scan range, average tube current-time product is presented
NM Dosimetry: ICRP. Radiation dose to patients from radiopharmaceuticals: (Addendum 2 to ICRP Publication 53) ICRP
Publication 80 Approved by the Commission in September 1997. 1998.
CT Dosimetry: CT dosimetry tool. London: ImPACT, St. George's Healthcare NHS Trust; 2007.
Alessio, 46
Summary
Dose Reduction is possible through
•  Operational Dose Savings:
–  Rational Protocol Selection
–  Potentially use reference levels to align with peer institutions
•  Technology Dose Savings:
– 
– 
– 
– 
Application-specific geometries (SPECT, PET)
Improved collimators (SPECT)
Improved detectors (SPECT, PET, and CT)
Improved image reconstruction (SPECT, PET, and CT)
•  Review software is critical part of realizing potential of hybrid
devices
Thank You
Adam Alessio
[email protected]
Alessio, 47