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HYBRID IMAGING: SPECT / CT
Associate Professor Barry S Elison Director Dept Nuclear Medicine, Wollongong Hospital
Senior Staff Specialist, Bankstown Hospital Clinical Executive Director Clinical Director of Imaging
FRACP, FSCCT
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Hybrid Imaging in Cardiac Nuclear Medicine
1. Attenuation Correction 2. May derive some information from CT such as, a) Non-cardiac findings b) Pericardial abnormalities 3. Calcium score. 4. Combined CTCA and SPECT
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Why Attenuation Correction
• Anatomical regions usually uneven or heterogeneous and therefore photon attenuation is unavoidable.
• Unavoidable problems do affect quality. • Attenuation correction therefore desirable.
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1. PHOTON ATTENUATION • Gamma photons emitted by radiopharmaceuticals
interact with tissues as they pass through the body – Absorbed (attenuated) – Scattered (Compton) – Attenuation is energy-dependent
25
% T
RA
NS
MIT
TED
PH
OTO
NS
0 5 10 15 20 SOURCE DEPTH (cm)
50
75
100
LUNG
WATER
BONE 140 keV
0
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1. PHOTON ATTENUATION (CONT)
• Most anatomical regions are heterogeneous – radionuclide data contains errors due to photon attenuation (artifacts)
• Photon attenuation is unavoidable • Photon attenuation seriously affects quality of diagnostic
information
BREAST ATTENUATION IN MPS
DEPTH ATTENUATION b >> a
a b
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1. PHOTON ATTENUATION (CONT)
• Exponential relationship for photon attenuation:
xeII µ−= 0
where I0 = incident photon energy I = transmitted photon energy x = tissue thickness (cm) µ = linear attenuation coefficient (cm-1)
e-µa
e-µb
a
b
REF: ZAIDI, 2006
Attenuation Correction is vital to the production of artifact-free data
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2. DETERMINATION OF µ-MAP
• Need to generate an attenuation map – Spatial distribution of µ values (µ-map) – Incorporated into reconstruction algorithm to
correct for photon attenuation and scatter
• Patient-specific µ-map can be generated from CT data
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2. DETERMINATION OF µ-MAP (CONT)
• But raw CT data is normalised for variations in x-ray spectrum
• E.g., Air = -1000 HU, fat = -100 HU, water = 0 HU, bone = 1000 HU
• Need to transform CT numbers back into µ values at correct energy
( ) 1000, xyxCTW
WCT
µµµ −
= where CT = CT number (Hounsfield Units) µCT = LAC of irradiated tissue (cm-1) µW= LAC of water (0.1504 cm-1)
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2. DETERMINATION OF µ-MAP (CONT)
• Acquire CT scans of known µ values using phantom • Generate a lookup table using bilinear scaling:
0.05
µ (c
m-1
)
-1000 -500 0 500 CT NUMBER (HU)
0.10
0.15
0.20
140 keV
0
0.25
1000
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Radiation Protection Primer
Linear-No-Threshold Model (Any radiation = harmful even at low doses) : controversial ? inaccurate
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Radiation Protection Primer Most radiation workers adopt the ALARA principle = As Low As Reasonably Achievable Conventional diagnostic Cardiac Imaging with NM & CT: generally already low radiation exposure & can be justified if clinically indicated (CAD = potentially life-threatening) with CZT SPECT CT: radiation dosage even lower comparable to natural exposure in magnitude With NO compromise on image & diagnostic quality
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Calcium Score
• In Hybrid model using SPECT /CT there are different ways to score calcium
• Visual • Qualitative • Quantitative
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Coronary Calcium Score -Routinely performed in patients with no known CAD (unless specifically requested not to) -Low dose (<1mSv), non-contrast CT breath-hold, prospective ECG gating, software detection of HU >130 in coronary distribution -CT also used for attenuation correction,
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SPECT and calcium scoring – complimentary data Chang et al JACC 11/09
n = 1126 F/U over 6.9 years
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JACC 2009; 54: 1872-82
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Coronary Calcium Score: Incremental Dx Value over SPECT
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49-year-old female -Chest pain, positive EST -Normal stress /rest MIBI -Normal CACS = 0 confirming that she did not have significant CAD.
J Nucl Cardiol 2005;12: 392-400
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68-year-old female without known CAD -chest pain -risk factors: age & positive family history. -Normal stress / rest MIBI -CACS =1753, marked calcification in LAD, LCx, LM -reclassified from no significant CAD to significant disease, and aggressive medical therapy was started.
J Nucl Cardiol 2005;12: 392-400
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– Simple, fast, no contraindications, no contrast – Screen patients with intermediate risk: Positive CAC increased risk alter therapeutic goals (LDL, BP etc)
– Further stratify higher risk patients with normal stress-rest MIBI
– Identify patients who do not need further cardiac evaluation (CACS = 0, low clinical risk)
– Monitor progression of atherosclerosis – Improve compliance
COMMENTS on CACS
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GE Hybrid CZT SPECT-CT Discovery 570C
Better image quality In less time less radiation to patients
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Zurich University Hospital
Together with ASIR software (Adaptive Statistical Iterative Reconstruction) Markedly reduced radiation dose in CTCA and also in low dose CT for AC & coronary calcium score
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Validation of CT attenuation correction for CZT camera
Normal stress/rest MPI with inferior attenuation artefact corrected with CT AC for CZT Camera
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Method: Compare conventional state-of-the-art (Philips Precedence) SPECT-CT hybrid camera with the Discovery 570c SPECT-CT hybrid camera in 25 randomly selected patients from each camera (single day scan protocol). Conclusion: -CZT SPECT-CT with ASIR : major reductions in both radiation dose & scanning time while maintaining high image quality. -An ultralow dose stress only (100MBq) SPECT-CT protocol including calcium score with total effective radiation dose of 1.6mSv is achievable.
Bankstown Hospital Experience
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Method: Compare conventional state-of-the-art (Philips Precedence) SPECT-CT hybrid camera with the Discovery 570c SPECT-CT hybrid camera in 25 randomly selected patients from each camera (single day scan protocol). Conclusion: -CZT SPECT-CT with ASIR : major reductions in both radiation dose & scanning time while maintaining high image quality. -An ultralow dose stress only (100MBq) SPECT-CT protocol including calcium score with total effective radiation dose of 1.6mSv is achievable.
Bankstown Hospital Experience
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Method: Compare conventional state-of-the-art (Philips Precedence) SPECT-CT hybrid camera with the Discovery 570c SPECT-CT hybrid camera in 25 randomly selected patients from each camera (single day scan protocol). Conclusion: -CZT SPECT-CT with ASIR : major reductions in both radiation dose & scanning time while maintaining high image quality. -An ultralow dose stress only (100MBq) SPECT-CT protocol including calcium score with total effective radiation dose of 1.6mSv is achievable.
Bankstown Hospital Experience
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• Her recent chest pains were most likely not due to myocardial ischaemia. Given her age and dyslipidaemia she is however at risk for coronary artery disease. I arranged for her to have an ultralow radiation stress only MIBI scan and a Coronary Artery Calcium Score at Bankstown Hospital. If she has significant coronary artery calcifications I would have a low threshold in commencing her on lipid lowering medication. Obviously if she has evidence of ischaemia I would perform coronary angiography
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GE Hybrid CZT SPECT-CT Discovery 570C
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The best (or the prettiest) for last
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Incremental value of CTCA & software SPECT/CT fusion Assess Stent patency
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Incremental value of CTCA & software SPECT/CT fusion over CA Anatomical Anomaly
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Incremental value of CTCA & software SPECT/CT fusion over CA
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Where will this technology take us
• Clinical relevance ? • Will this technology change or influence
management • Outcomes : will combining technologies- CTCA
and MPS change or improve outcomes • Radiation burden • Cost
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1. Attenuation Correction- use it but cant always rely on it. Never ignore the NC data. Caveat: Quantification with commercially available software
2. Calcium score-Is the evidence trending towards doing it routinely with the hybrid model
CONCLUSION
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1. Combined anatomical and functional assessment of the heart is a reality with Novel CZT SPECT CT with major reduction in radiation while maintaining high image quality.
2. Ultralow dose stress only SPECT-CT with CAC with total dose <2mSv feasible.
3. Hybrid SPECT / CTCA : Not sure maybe for the low SDS to prevent ICA and provide one stop shop
CONCLUSION
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(may miss soft plaque)
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Thank You
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50 Copyright ©2003 American Heart Association
Hachamovitch, R. et al. Circulation 2003;107:2900-2907
Log hazard ratio for revascularization (Revasc) vs medical therapy (Medical Rx) as a function of % myocardium
ischemic based on final Cox proportional hazards model
•(Shaw et al JACC;1999 33:661-9) •(Shaw et al JACC;1999 33:661-9)