Translational Applications of Photon Counting Detectors: Medical CT Imaging - Indico
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7/12/2021
Translational Applications of Photon Counting
Detectors: Medical CT Imaging
Cynthia H. McCollough, PhD, DABR, FAAPM, FACR, FAIMBE
Brooks-Hollern Professor of Research, Professor of Medical Physics and Biomedical Engineering
Director, CT Clinical Innovation Center and X-ray Imaging Research Core
Department of Radiology, Mayo Clinic, Rochester, MN
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Disclosures
▸ Research support
– NIH (EB017095, EB028590, EB028591)
– Mayo Clinic Discovery Translation Grant
– Siemens Healthcare
▸ Board membership
– ISCT, Vice-president
▸ Use of off-label medical devices
– The research photon-counting-detector CT scanners described in this
work are not commercially available
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Energy integrating scintillating detectors (EID)
X-rays
TiO2 based reflectors
Gd2O2S
Photodiodes
Electronic signals
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X-ray detectors and medical CT
Typical Parameters for High End Medical CT Systems
X-ray Spectra 70 kV to 150 kV
X-ray On Time ~0.2 s up to 100 s
Typical Flux ~ 108 photons / (s.mm²)
Peak Flux > 109 photons / (s.mm²)
Detector Type Indirect conversion (scintillating EIDs)
Active Area up to 32 cm x 100 cm
Pixel Pitch ~1 mm x 1 mm
Frame Rate Up to 7x103 fps
Dynamic Range 18 bit to 22 bit
Signal Stability ~0.1%
Centrifugal Force Up to 70 g
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Photon-counting-detector (PCD) CT
X-rays
CdTe or CdZT
Cathode
+ High voltage
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Pixelated anodes
Electronic signals 5
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Energy thresholding
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Energy bin
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2
1
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Challenges for PCDs in medical CT
▸ Pulse pile-up
– Reduces dose efficiency and signal accuracy: pixels can’t be too large
▸ k-escape, charge sharing, and Swank noise
– Reduces spectral resolution: pixels can’t be too small
▸ Advanced ASIC designs
– Fast pulse shaping < 10-20 ns FWHM
– Two or more counters per pixel
▸ Bandwidth
– Pixel pitch ~ 0.275 mm x 0.322 mm with 2 counters per pixel
– > 22 times more data to move
▸ Large scale manufacturing
– High purity, high yield, long term stability, affordable cost 7
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Clinical impact of photon counting detectors
Ability to exclude electronic noise
Increased iodine CNR at high kV
125 micron resolution spectral imaging
Simultaneous, single kV, multi-energy CT
Ability to distinguish multiple k-edge contrast agents
New applications
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Energy thresholding
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Reduced streaking artifacts in low-dose regime
Energy Integrating Detector Photon Counting Detector
(low electronic noise system)
EID-CT PCD-CT
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Yu et al. J Med Imaging 2016
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Clinical impact of photon counting detectors
Ability to exclude electronic noise
Increased iodine CNR at high kV
125 micron resolution spectral imaging
Simultaneous, single kV, multi-energy CT
Ability to distinguish multiple k-edge contrast agents
New applications
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Count weighting instead of energy weighting
Iodine K-edge &
x-ray absorption 100
Counting
Detector response
D(E) ~ E
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Higher iodine SNR = Higher dose efficiency 12
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Increased iodine signal at high kV
80 kV 140kV
EID-CT
317 HU 134 HU
T Low Bin 1
25 – 140 keV 25 – 65 keV
PCD-CT
441 HU 549 HU
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© Mayo Clinic
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Clinical benefit
▸ Increased conspicuity of subtle iodine-enhancing
structures or pathology
▸ Increased iodine CNR could be traded for dose reduction
– Radiation dose
• Beneficial for children and patients with chronic disease
– Iodinated contrast material dose
• Beneficial for patients with decreased kidney function
▸ Increased iodine CNR at high kV
– Obese patients
– Patients with metal implants
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Clinical impact of photon counting detectors
Ability to exclude electronic noise
Increased iodine CNR at high kV
125-micron resolution spectral imaging
Simultaneous, single kV, multi-energy CT
Ability to distinguish multiple k-edge contrast agents
New applications
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At isocenter:
0.300 mm 0.150 mm
0.176 mm
0.350 mm
Sub-pixel
Readout pixel
Collimator blades
z z
phi phi
Multi-energy mode High resolution mode
144 x 0.4 mm (57.6 mm z-coverage) 120 x 0.2 mm (24 mm z-coverage)
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Improved bronchial wall delineation
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© Mayo Clinic
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Improved trabecular visualization
© Mayo Clinic
EID-CT PCD-CT 18
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Temporal bone: Improved resolution and lower noise/dose
σ=59
σ= 42
EID-CT with Comb Filter PCD-CT
40% noise reduction or 50% dose reduction using PCD-CT 19
Leng et al, Dose-efficient ultrahigh-resolution scan mode using a photon counting detector computed tomography system. JMI 2016
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Stent imaging
Zhou et al. ARRS 2018 EID-CT PCD-CT 20
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Simultaneous High Resolution & Multi-Energy
▸ Higher spatial resolution better delineates coronary artery and stent
▸ Multi-energy enables material decomposition, e.g., iodine map
EID-CT PCD-CT PCD Iodine Map
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© Mayo Clinic
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Clinical benefit
▸ 125-micron resolution for fine anatomic features
PLUS
▸ data perfectly registered – temporally and spatially
▸ ability to diagnose gout or bone marrow edema,
perform bone removal, or any other spectral application
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Clinical impact of photon counting detectors
Ability to exclude electronic noise
Increased iodine CNR at high kV
125 micron resolution spectral imaging
Simultaneous, single kV, multi-energy CT
Ability to distinguish multiple k-edge contrast agents
New applications
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Use of multiple k-edge contrast agents
▸ Multi-contrast imaging
– Simultaneous imaging of multiple contrast agents
• I, Gd, Bi
– Might enable novel molecular imaging applications
• Au nanoparticles
▸ Challenges
– Noise amplification
– Lack of FDA-cleared high Z contrast agents
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Animal Study (Material Decomposition)
6 cm MIP
After denoising
© Mayo Clinic
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Clinical impact of photon counting detectors
Ability to exclude electronic noise
Increased iodine CNR at high kV
125 micron resolution spectral imaging
Simultaneous, single kV, multi-energy CT
Ability to distinguish multiple k-edge contrast agents
New applications
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Metal artifact avoidance with use of Sn filter
25-40 keV 75-140 keV 75-140 keV
Spine Hardware
w/o Tin w/o Tin with Tin
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Zhou et al., Invest Radiol. 2019
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Imaging of torso with 125-micron resolution
EID-CT PCD-CT 28
© Mayo Clinic
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147/12/2021
First cardiac PCD-CT study (April 30, 2021)
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© Mayo Clinic
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© Mayo Clinic
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157/12/2021
Clinical impact of photon counting detector CT
▸ Photon counting detectors (PCDs) record individual photons and their associated energy
▸ Offers multiple benefits over conventional CT with energy integrating detectors (EIDs)
Ability to exclude electronic noise
Increased iodine CNR at high kV
125-micron resolution spectral imaging
Simultaneous, single kV, multi-energy CT
Ability to distinguish multiple k-edge contrast agents
New applications
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© Mayo Clinic
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Acknowledgements
– J. Fletcher, MD – T. Drees – A. Henning, PhD
– S. Leng, PhD – B. Andrist – S. Kappler, PhD
– L. Yu, PhD
– N. Weber – A. Halaweish, PhD
– K. Rajendran, PhD
– H. Kasten – S. Ulzheimer, PhD
– S. Tao, PhD
– W. Zhou, PhD – K. Nunez, MS – B. Schmidt, PhD
– H. Gong, PhD – A. Inoue, MD – T. Flohr, PhD
– D. Bartlett, MD – F. Diehn, MD – K. Stierstorfer, PhD
– P. Mohammadinejad , MD – T. Johnson, MD – M. Baer-Beck, PhD
– Y. Lee, PhD – E. Shanblatt, PhD
– K. Glazebrook, MD
– J. Weaver, BA – A. Halweish, PhD
– J. Thorne, BS – F. Baffour, MD
– B. McCollough, BS
– J. Marsh, BS
Thank you!
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