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Nuclear medicine for cardiothoracic surgeons
1. Nuclear Medicine in CT
Surgery
Dr. K. Srikanth
DNB (Cardiothoracic Surgery) Resident – 2nd year
NH, Bangalore
03.01.2019
2. Objectives of this presentation
• To understand the application of nuclear medicine in CT surgery
• To have a basic knowledge of the various available diagnostic options
and when to use them
• To understand and interpret the various nuclear studies at a
beginner’s level
3. Introduction to Nuclear medicine
• Nuclear medicine is a medical specialty involving the application
of radioactive substances in the diagnosis and treatment of disease.
• In simple words, it is "radiology done inside out" or "endoradiology"
because it records radiation emitted from within the body rather
than radiation that is generated by external sources like X-rays
• SPECT(Single Photon Emission CT) & PET – most common methods
4. Role of Nuclear medicine in CT diseases
Cardiac
• Myocardial Perfusion imaging using SPECT / PET
• PET to look for infiltrative disorders/ infective endocarditis / aortic
root abscess
Thoracic
• V-Q scan in Pulmonary embolism
• PET CT in pulmonary/ mediastinal tumour staging
11. Myocardial Perfusion Imaging (MPI)
• Also known as thallium scan/ viability scan/ Stress myocardial imaging
• Aim of this scan is to guide the surgeon/physician if revascularisation
(surgical/ PCI) will be useful in improving myocardial recovery in a
patient with known CAD and also post-revascularisation to assess
need for re-intervention if the patient is symptomatic
• Presence of viable myocardium – Revascularisation will benefit the pt
• Revascularising infarcted myocardium is of no benefit
12. Indications of MPI
In a patient with known CAD, MPI is useful when
• There are extensive hypokinetic/ akinetic areas on echo, especially in
the apical regions
• In patients with reduced EF (Ischemic cardiomyopathy), to assess the
need for revascularisation vs medical therapy
• Post-CABG / PCI, if patients are symptomatic, MPI helps to assess
need for re-revascularisation
13. Modalities of MPI
• Single Photon Emission CT – uses a gamma camera
• Positron Emission Tomography (PET)
14. Principle of SPECT MPI
• After injection of the chosen radiotracer (Tc99 sestaMIBI), the isotope
is extracted from the blood by viable myocytes and retained within
the myocyte for some time.
• Photons are emitted from the myocardium in proportion to the
magnitude of tracer uptake, in turn related to perfusion.
• The standard camera used in nuclear cardiology studies, a gamma
camera, captures the gamma ray photons and converts the
information into digital data representing the magnitude of uptake
and the location of the emission.
15. How is it done?
• Needs 4 hours fasting, pt. should skip the morning dose of Beta-
blockers/ CCBs (as it would blunt the stress response)
• Patient is subjected to physical stress (Treadmill - Bruce protocol) or
pharmacological stress (adenosine/ dobutamine)
• Atleast 80% of target heart rate is to be achieved (Target HR = 220
minus age in years)
• One achieved, the radiotracer dye is injected IV and scan is performed
16. • This scan takes about 5 minutes and will give us the stress images
• The patient is asked to rest for about 1 hour and then dye is re-
injected and the rest images are taken
• The entire imaging is ECG gated i.e. the images are recorded in
conjunction with the cardiac cycle based on the R-R interval
• This radiotracer dye has no contraindications such as renal / liver
failure as it spontaneously decays over time and gets excreted
18. Radiotracers used in SPECT MPI
• Thallium 201 – historical, not used now due to longer half life
(73hours) and low emitting energy (80keV)
• Technetium 99 – presently used, shorter half life (6 hours) and higher
emitting energy (140keV)
• Tc99 is tagged with sestamibi/ tetrofosmin
20. Fundamentals of interpretation
• Presence of perfusion defects
• Location of perfusion defects / coronary artery territory involved
• Whether defects on stress images are reversible on the rest images
(implying stress-induced ischemia) or whether stress perfusion
defects are irreversible or fixed (often implying myocardial infarction)
27. SSS / SRS / SDS
• Summed Stress Score (SSS) - The sum of the segmental scores from
the stress images, represents the extent and severity of stress
perfusion abnormality related to both ischemia and infarction
• Summed Rest Score (SRS) - The sum of the 17 segmental scores from
the rest images represents the extent of infarction
• Summed Difference Score (SDS) – SSS minus SRS, represents the
extent and severity of stress-induced ischemia
28. Example & its clinical implication
SDS Ischemic
burden
0 No ischemia
1-3 Mild ischemia
4-7 Moderate
ischemia
>7 Severe ischemia
29. Quantifying the size of the perfusion
defect (2018 ASNC SPECT Guidelines)
Number of segments
involved
Size of the defect Percentage of LV
myocardium affected
1-2 Small <10%
3-4 Medium 10-20%
>=5 Large >20%
30. Important signs beyond myocardial perfusion
• Lung uptake of tracer
Patients with lung uptake often have severe multi-vessel disease and
exhibit elevation of pulmonary capillary wedge pressure and decreases
in ejection fraction (EF) during exercise, all implying extensive
myocardial ischemia.
It is likely that ischemia-induced elevation in left atrial and pulmonary
pressures slows pulmonary transit of the tracer, allowing more time for
extraction or transudation into the interstitial spaces of the lung,
accounting for this imaging sign
31. Transient ischemic dilatation of LV
• Transient ischemic dilation refers to an imaging pattern in which the
left ventricle or left ventricular (LV) cavity appears larger on the stress
images than on those obtained with the patient at rest
• For patients in whom the entire left ventricle appears larger during
stress, the pathophysiology probably is related to extensive ischemia
and prolonged post-ischemic systolic dysfunction, resulting in a
dilated, dysfunctional left ventricle during the stress acquisition
relative to the rest acquisition
32. Lung uptake of tracer material Transient ischemic LV dilatation
33. Analysis of LV volumes and EF
• With ECG gating of SPECT, it is possible to accurately predict the LV
volumes (LV-ESV & EDV), thereby calculate the EF with accuracy
comparable to LV angiogram and cardiac MRI
• In ECG gating, the R-R interval is divided into 8 equal segments and
events of the cardiac cycle are tracked
• The first few frames represent systolic events, and the latter frames
represent diastolic events
38. Advantages of PET over SPECT
• Higher spatial resolution
• Improved attenuation and scatter correction
• Potential for quantifying regional blood flow in absolute terms
(mL/min/g tissue)
• Higher sensitivity and specificity of PET (>95%) over SPECT (Sensitivity
87%, specificity 73%) in detecting CAD
39. Interpretation of PET viability scan
• Regionally increased F-18 FDG uptake relative to regional myocardial
blood flow (perfusion-metabolism mismatch) suggests myocardial
viability and predicts reversibility of contractile dysfunction following
revascularization. Consideration should be given to the fact that such
a mismatch does not always predict complete recovery of contractile
function because scar and viable myocardium may coexist in such
regions.
• Regionally reduced F-18 deoxyglucose uptake in proportion to
regionally reduced myocardial perfusion (perfusion-metabolism
match) suggests irreversibility of contractile dysfunction (ie, absence
of myocardial viability)
40. CAD-TVD on PET
Reversible ischemic changes in anteroseptal (LAD), lateral wall (LCX) and inferior
walls (RCA)
43. Newer imaging modalities
• Indium-111 (111In)–labeled antimyosin antibody, which specifically
targets myosin heavy chain, has been used for the detection of
necrosis associated with myocarditis and heart transplant rejection
• 99mTc-pyrophosphate imaging – used in cardiac amyloidosis
• 123I-MIBG Neurocardiac and 18F-FDG Metabolism - Cardiac Imaging
to Guide Ventricular Tachycardia Ablation in Heart Failure
46. Cardiac Prosthetic Valve/ LVAD related infection
• Almost half of prosthetic valve endocarditis cases are complicated by
peri-annular extensions and require urgent surgical intervention
• Trans-esophageal echocardiography (TEE) may fail to recognize this
potentially fatal complication. Although ECG-gated CT angiography
can improve the diagnostic accuracy in some patients, it is also a
purely anatomic technique.
• FDG PET-CT is not advocated as a first-line or confirmatory imaging
study for detecting prosthetic valve endocarditis. Rather, it should be
reserved for patients with clinical and microbiologic suspicion of
endocarditis but indeterminate or negative TEE
50. What is a V-Q scan?
V/Q (ventilation/perfusion) scan is a scintigraphic
examination of the lung that evaluates pulmonary
vasculature perfusion and segmental broncho-alveolar
tree ventilation
51. Indications
• Diagnosis of suspected pulmonary embolism (PE) – acute /chronic
• Monitor pulmonary function following lung transplant
• Provide preoperative estimates of lung function in lung
cancer patients, where pneumonectomy is planned
52. How is it done?
• Prior CXR is needed to rule out atelectasis/ other causes which may
produce perfusion defects
• Ventilation scan - performed using radio-isotope labelled aerosols
like technetium-99m DTPA or Xenon delivered to the patient through
a non-rebreathing mask, with the patient supine.
• The micro-aerosol particles are small enough to reach the distal
tracheobronchial tree and reflect regional ventilation.
• The patient is then imaged in the upright position in three phases:
initial breath, equilibrium and washout.
53. • The perfusion lung scan involves injecting Tc-99m MAA (macro-
aggregated albumin) intravenously in the supine position.
• The MAA particles are just small enough to get lodged in the pre-
capillary arterioles.
• A high resolution, large field of view gamma camera is used to image
the lungs
54.
55. Mismatched V-Q defects – Pulm emboli
Isolated Right Pulmonary embolus
Absent perfusion in Left middle and
lower, right lower lobes (posterior view)
57. Acute Vs Chronic PE
• Cannot differentiate acute and chronic PE with a single scan
• We need to repeat a V-Q scan after 3 months,
If the perfusion defect resolves, it is an acute PE
If the perfusion defect persists, it is a chronic PE
58. To summarise
• Myocardial viability assessment is crucial in the management of CAD,
especially for those in heart failure
• Recollect various methods of MPI – SPECT/ PET
• Important to know the physiological basis of stress scans
• MPI is an important predictor of long term mortality and outcome in
patients with CAD
• Non-CAD uses of nuclear cardiac scans (amyloidosis/ arrhythmias/
infections)
• V-Q scan and its interpretation – useful when you come across CTEPH
cases
59. Resources for further reading
• Braunwald’s Heart Disease 11th edition, Chapter 16
• American Society of Nuclear Cardiology (ASNC website)
• Read about PET in lung/ mediastinal malignancies – Shield’s General
Thoracic Surgery, 8th edition (not covered in this presentation)
Thank you!
60. To clarify
• Could MPI become an alternative to CAG, especially to anatomically
delineate the stenosis?
• What is the radiation risk from MPI studies (SPECT vs PET)?
• If cost was not a limiting factor, could MPI be a routine follow up tool
after revascularisation in asymptomatic patients?
