B P3 C18 Nuclear Cardiology Flashcards
Advantages of PET
High spatial and contrast resolution
Capability for tomographic dynamic imaging with high temporal resolution
Accurate and depth-independent attenuation correction
High count sensitivity, making possible rapid protocols
Low radiation dose protocols (due to short half-life of the PET tracers)
Quantitation of absolute radiotracer concentration in tissue, including myocardial blood flow
CT hybrid imaging for quantification of atherosclerotic burden and localizing hot spot imaging tracers
Availability of a wide array of molecularly targeted clinical and research radiotracers that can image molecular processes in the pico and nano molar concentrations
Indications for Cardiac Positron Emission Tomography Myocardial Perfusion Imaging and Myocardial Blood Flow Measurements
Rest-stress myocardial perfusion PET is a first-line preferred test for patients with known or suspected CAD who meet appropriate criteria for a stress imaging test and are unable to complete a diagnostic-level exercise stress imaging study
Indications for Cardiac Positron Emission Tomography Myocardial Perfusion Imaging and Myocardial Blood Flow Measurements
Rest-stress myocardial perfusion PET is recommended for patients with suspected active CAD, who meet appropriate use criteria for a stress imaging test, and who also meet one or more of the following criteria:
• Prior stress imaging study that is of poor quality, equivocal or inconclusive, affected by attenuation artifact, or discordant with clinical impressions or other diagnostic test results including findings at coronary angiography
• Body characteristics that commonly affect image quality such as large breasts, breast implants, obesity, etc.
• High-risk patients in whom diagnostic error carries even greater clinical implications, such as chronic kidney disease stages 3, 4, or 5; diabetes mellitus; and high-risk CAD
• Young patients with established CAD who are expected to need repeated exposures to radiation associated cardiac procedures
• Patients in whom myocardial blood flow quantitation is needed
• Patients without known CAD who present with symptoms suspicious for myocardial ischemia
• Increased suspicion of multivessel CAD
• Suspected heart transplant vasculopathy
• Patients with known CAD in whom more specific physiologic assessment is desired
A low-dose, noncontrast, ungated free tidal b ing scan of the chest.
Attenuation correction CT
A noncontrast, prospectively gated CT scan for dose reduction, acquired during an inspiratory breath-hold
Calcium score CT
Plays a major diagnostic role, particularly in the evaluation of patients without prior known coronary artery disease (CAD) and with normal MPI
Calcium score
Radionuclide imaging has limited anatomic resolution and hybrid CT imaging (without or with iodinated CT contrast) provides localization of the tracer uptake, which is helpful in hot spot imaging:
________________ for amyloidosis
________________ for sarcoidosis or infection imaging
99m Tc-pyrophosphate
2-deoxy-2-[18 F] fluoro-D-glucose [18 F-FDG]
There are four common modes of image acquisition with SPECT or PET:
List mode, static, ECG gated, or dynamic
____________ allow assessment of regional wall motion and quantification of left ventricular (LV) volumes and EF
ECG-gated images
___________ allows tracking of radiotracer transit through the blood vessels and the heart starting with the time of radiotracer injection
Myocardial blood flow estimates can be derived by this approach
Dynamic imaging
This can be performed at rest and during peak pharmacologic stress to compute rest and stress myocardial blood flow, respectively.
The ratio of stress to rest myocardial blood flow is termed ________________
Myocardial flow reserve (MFR)
Dynamic imaging for myocardial blood flow quantitation requires pharmacologic stress testing with _____________ (preferred) or dobutamine.
Vasodilators
An ideal radiotracer for MPI should be extracted by the myocardium at a rate that is ____________
Linearly related to myocardial blood flow
Most SPECT and PET perfusion radiotracers demonstrate ____________, as in the resting state, or when there is significant obstructive CAD with a reduction in stress myocardial blood flow.
Linear extraction at relatively low blood flow rates
As myocardial blood flow increases with exercise or pharmacologic stress, radiotracer extraction falls off and, consequently, myocardial blood flow is underestimated.
Accuracy can be increased by using radiotracers with greater extraction at high flow rates; this is particularly important for the evaluation of nonobstructive CAD, diffuse CAD, or microvascular dysfunction.
FDA-approved SPECT myocardial perfusion tracers
99mTc-sestamibi
99mTc-tetrofosmin
201thalium
99mTc is produced by a 99mmolybdenum generator and then compounded into 99mTc-sestamibi or 99mTc-tetrofosmin; have a half-life of ___________
6 hours
99mTc perfusion tracers passively diffuse into cardiomyocytes at rates proportional to blood flow and bind to the mitochondria within the first _________ after injection.
60 to 90 seconds
Mechanism of uptake of
99mTc-sestamibi
99mTc-tetrofosmin
Mitochondrial uptake
201thallium is produced by a cyclotron, emits lower energy photons (80 keV), and has a half-life of ___________.
73 hours
201Thallium circulates to the heart at a rate proportional to blood flow and enters the cardiomyocytes via the ____________
Na+/K +ATPase pump
Early perfusion defects on 201thallium images represent _____________ from ischemia or scar.
Perfusion defects may resolve over time because of redistribution of 201 thallium in ischemic and hibernating regions; therefore poststress 201thallium images are obtained ______________ after injection
Reduced blood flow
Within 10 to 15 minutes
Because of its long half-life and relatively low photon energy, 201 thallium imaging is associated with a ________________
For this reason, it is currently not recommended for _____________; instead, it is used for ________________ at sites without access to other viability tests
Higher radiation dose
Perfusion imaging
Viability assessment
Highest extraction ratio among spect tracers
201Thallium - 85% (lowest photon energey 69-81 keV)
Sestamubi - 65% (140 keV)
Tetrofosmin - 60% (140 keV)
FDA-approved PET perfusion tracers
82Rubidium
13N-ammonia
Monovalent cation that enters the cardiomyocyte via the Na+/K +ATPase pump
82Rubidium
Half life of Rubidium
76 seconds
What is not feasible because of the short half life of Rubidium?
Exercise stress imaging is not feasible
Other considerations due to short half life:
(1) it is produced from a 82 strontium/82 rubidium generator housed in an infusion cart next to the PET scanner
(3) rapid sequential imaging is possible
Enters the cardiomyocytes passively where it is c verted into 13N-glutamine and trapped in the glutamate pool.
13N-ammonia
Half life of N-ammonia
9.96 minutes
Advantage of N-ammonia’s its short (9.96 minutes) half-life:
Exercise PET is feasible
Other considerations:
(1) an on-site cyclotron is required
(3) lower injected doses are administered
Exercise PET does not allow for quantitation of ________________, which is an important advantage of PET MPI.
Myocardial blood flow
During treadmill exercise stress, radiotracer injection occurs outside the PET gantry. Postexercise myocardial blood flow quantification is not feasible because of the lack of an arterial input function.
The superior extraction characteristics of PET tracers, compared with SPECT perfusion tracers, makes them more suitable for ___________
Quantifying myocardial blood flow
Highest extraction fraction among PET MPI tracers
15Oxygen-water - 100% (not approved by FDA in the US)
13N-ammonia - 75%
82 Rubidium(82Rb) - 55% (only generator as source, others cyclotron)
The only clinically available FDA approved tracer to image myocardial metabolism is ______
18F-FDG
Glucose analog used to image myocardial glucose metabolism.
18F-FDG
The primary clinical applications of cardiac 18F-FDG PET are for
Imaging myocardial viability
Myocardial and vascular inflammation
Infective endocarditis
18F-FDG enters the cardiomyocytes through glucose transporters ( ______ and ______ ), where it is phosphorylated by the enzyme hexokinase and trapped as 18F-FDG-6-phosphate.
GLUT 1 and 4
Unlike glucose-6-phosphate, 18 F-FDG-6-phosphate cannot be metabolized.
Insulin, ischemia, and hypoxia induce translocation of glucose transporters to the plasma membrane and increase myocyte glucose uptake
Cells which preferentially uses glucose for their metabolic needs independent of insulin
Ischemic and hypoxic cells
Malignant and inflammatory cells
Dietary preparation to switch myocardial metabolism to glucose or fatty acids forms the basis for the use18 F-FDG to image _____________ (glucose load with IV insulin) and cardiovascular inflammatory conditions (low-carbohydrate, high-fat diet followed by prolonged fasting) such as ________, _________, __________
Myocardial viability
Sarcoidosis, infective endocarditis, and vasculitis
The heart extracts oxygen nearly maximally at rest (__________)
60% to 80%
With exercise stress (or dobutamine infusion) there is a severalfold increase in oxygen demand from high heart rate, contractility, and ventricular work that is met physiologically by increased blood supply from _____________
Metabolic vasodilation
Normal coronary arteries have a coronary blood flow at rest of _________, which can increase three- to fivefold during maximal vasodilation.
0.7 to 1 mL/min
Coronary blood flow remains constant over a wide range of coronary perfusion pressures through dynamic changes in tone in arterioles and other resistance vessels, and it only falls in the presence of very severe upstream coronary stenosis ( _______ luminal narrowing)
> 90%
Augmentation of myocardial blood flow in response to exercise/vasodilator stress is __________ and forms the basis for the use of stress radionuclide MPI for detection of obstructive CAD.
Progressively blunted with increasing severity of upstream coronary stenosis
Earliest event in the ischemic cascade
Reduced regional myocardial perfusion
In contrast to exercise stress, vasodilator stress does not ____________
Increase oxygen demand
The diseased and nondiseased territories manifest differential hyperemic responses due to ___________.
Differences in resting microvascular dilation
In myocardial territories supplied by coronary arteries with critical stenosis (>90%), where the microvasculature is maximally vasodilated at rest, vasodilation of the epicardial coronaries by vasodilator stress agents can redistribute flow away from the subendocardium causing _________, which can often manifest as ischemic ST depression during vasodilator stress testing.
Coronary steal
If there is severe multivessel obstructive CAD and coronary blood flow is reduced in all vascular territories, this can result in an apparently normal appearing relative myocardial perfusion image with no perfusion defects, also known as _________
Balanced ischemia
____________ safe and is the preferred mode of stress in conjunction with radionuclide MPI.
Stress testing using exercise stress with treadmill or bicycle
Preferred as it is physiologic, providing information on symptoms, functional capacity, and hemodynamic and ECG changes with stress.
The most widely used protocol in exercise testing
Standard Bruce treadmill exercise
______________ provides an excellent alternative if exercise stress is not feasible (orthopedic or other limitations), contraindicated (recent acute coronary syndrome [ACS], or recent deep vein thrombosis, very large aortic aneurysm, etc.), or if patients are unable to exercise maximally.
Pharmacologic stress testing
To evaluate anginal symptoms on maximal medical therapy in patients with known prior CAD, a ___________ irrespective of heart rate is often adequate if a reasonable workload of at least 5 metabolic equivalents (METS) is achieved.
Symptom-limited stress test
To evaluate anginal symptoms in patients without documented prior CAD, _____________ is desirable with exercise stress.
If not, the radiotracer is not administered, and stress test is converted to a vasodilator stress.
In those instances, ____________, a non–weight-based fixed dose stress agent is well suited for administration on the treadmill or soon after termination of exercise.
Maximal heart rate response
Regadenoson
____________ preferred stress modality for radionuclide MPI (SPECT and PET) in patients who are unable to exercise adequately, and for evaluation of residual ischemia in patients with recent ACS/myocardial infarction (MI)
Pharmacologic stress testing
The three most commonly used vasodilator stress agents
Adenosine, dipyridamole, and regadenoson
Binds to four types of adenosine receptors.
Effects after binding to the receptors
A2A
A1
A2B
A3
Adenosine
A2A - coronary vasodilation
A1- heart block
A2B - wheezing
A3 - peripheral vasodilation
Causes coronary vasodilation by increasing endogenous adenosine levels.
Dipyridamole
A specific A 2Areceptor agonist
Regadenoson
Contraindications to vasodilator agents
Active wheezing
High-grade atrioventricular (AV) block without a functioning pacemaker
Systolic blood pressure (BP) <90 mm Hg
Any contraindications for stress testing (acute MI, unstable angina, aortic dissection, acute pulmonary embolism).
Regadenoson stress testing is associated with __________
Seizures
Contraindicated in patients with a history of seizures that are not well controlled or in those with structural brain injury
Doses of vasodilator agents
Adenosine
Dipyridamole
Regadenoson
Adenosine - 140 mcg/kg/min x 4mins
Dipyridamole - 0.56 mg/kg x 4 minutes
Regadenoson - 0.4 mg/5 mL prefilled solution administered as a rapid bolus over 10 seconds (fixed dose)
Vasodilator stress agents often cause symptoms of hyperemia in about 50% of patients including
Urge to breathe deeply
Chest tightness
Headache, flushing
10 to 20 beat increase in heart rate
10-mm Hg decrease in systolic BP.
____________ provide the lowest possible radiation dose and optimal image quality because there is no shine through of rest radiotracer activity.
Two-day protocols
F-FDG Metabolic Imaging Protocols
Viability
Cardiac sarcoidosis, infection or vasculitis
For viability imaging, glucose/insulin preparation is necessary. Then, 18 F-FDG (5 to 10 mCi) is administered intravenously and cardiac PET/CT images are acquired 60 minutes later.
For cardiac sarcoidosis, infection, or vasculitis imaging, patients undergo the high-fat/low-carbohydrate dietary preparation. Cardiac and partial or full-body PET/CT images are acquired 90 minutes after IV injection of 18F-FDG.
The perfusion images are reviewed visually and semiquantitatively. Interpretation is performed in a segmental fashion using a 17-segment heart model and a 0 to 4 scale (where each of the 17 segments are scored using 0 = normal, 1 = mild, 2 = moderate, 3 = severe, and 4 = absent tracer uptake).
Identify the pattern of myocardial perfusion
Ischemia
Perfusion - reversible defect
Wall motion - normal or abnormal
Identify the myocardial perfusion images
Fixed defect
Abnormal wall motion
Scar
Pattern of myocardial perfusion and viability
Defect with normal/abnormal wall motion and partial FDG uptake
Nontransmural scar
High risk features in MPI
Large single or multiterritorial fixed and/or reversible myocardial perfusion defects involving >15% of the LV mass
Transient ischemic dilation of the left ventricle
Stress-induced myocardial stunning with a drop in LVEF poststress
Transient RV tracer uptake
Increased pulmonary tracer uptake
Hot-spot images (sarcoidosis, amyloidosis, and infective endocarditis) are quantified using target to background ratio (myocardium to rib uptake in _______ and myocardium to blood activity in ________),
Amyloidosis
Sarcoidosis
Give the clinical diagnosis
Myocardial ischemia from nonobstructive atherosclerosis and coronary microvascular dysfunction
Cardiac PET/CT images of a 76-year-old woman with dyslipidemia, hypertension, and nonobstructive angiographic coronary artery disease who presented with atypical chest pain and dyspnea. Vasodilator-stress and rest13 N ammonia PET images demonstrate visually normal myocardial perfusion. The ECG gated images demonstrated a rest left ventricular (LV) ejection fraction of 71% that increased to 73% during stress with normal LV volumes (not shown). The CT transmission scan showed severe coronary artery calcification (lower left). Quantitative stress myocardial blood flow and myocardial flow reserve was moderately reduced in all coronary territories and globally.
Identify the pattern of MPI
Myocardial scar
Mildly dilated left ventricle and a large perfusion defect of severe intensity throughout the inferior and inferolateral and basal anterolateral walls, which was irreversible.
64-year-old man with known chronic total occlusion (CTO) of the LAD coronary artery
Interpret the MPI
Transient left ventricular (LV) dilatation with stress and a large perfusion defect of severe intensity throughout the anteroseptal and anterior walls and LV apex, showing complete reversibility
There is a small but severe perfusion defect involving the basal inferolateral wall, also showing complete reversibility
Consistent with postischemic stunning
The study shows complete viability of the LAD territory with evidence of severe stressischemia, and a small area of moderate stress-induced ischemia in the inferolateral wall.
A 74-year-old woman with hypertension, diabetes, and chronic kidney dysfunction who presented with new-onset exertional dyspnea. ECG showed left bundle branch block (LBBB). EF 15%.
Interpret the MPI
Nonischemic cardiomyopathy
PET MPI shows a severely dilated left ventricle and a small septal perfusion defect showing apparent reversibility, likely secondary to LBBB. The ECG-gated images demonstrated an ejection fraction of 15% that rose to 20% during peak stress, and evidence of septal dyssynchrony. Myocardial flow reserve was preserved in all coronary territories.
68/M with prior MI presenting with worsening HF symptoms. EF 30%
Interpret the MPI
Ischemic cardiomyopathy
Myocardial perfusion images MPI demonstrate a large and severe perfusion defect throughout the inferior and inferolateral walls, which was irreversible and consistent with prior MI. However, the quantitative myocardial blood flow and flow reserve (MFR) data show blunted augmentation of flow during stress, resulting in a severe reduction in MFR in all three vascular territories.
70-year-old man with progressive dyspnea, hypotension, and new severe biventricular systolic dysfunction with elevated natriuretic peptides
Interpret the MPI
Ischemic cardiomyopathy with hibernating myocardium (LCx and RCA - mismatch) and flow limiting stenosis in LAD
99mTc-PYP/DPD/HMDP images are typically interpreted visually comparing radiotracer uptake in myocardium in relation to that in the ________
Ribs
Bone-avid scintigraphy can detect early amyloid deposition in the heart before increase in myocardial wall thickening,
______ has a low yield because of frequent sampling errors given the focal nature of the cardiac sarcoidosis most frequently affecting the left ventricle
RV endomyocardial biopsy
________ is necessary, when CMR unavailable, contraindicated, or inconclusive; and even when CMR is positive,to guide the potential need of anti-inflammatory therapy
F-FDG PET
18 F-FDG PET uptake in the heart, however, is not specific for sarcoidosis and can represent normal myocardium, inflamed myocardium, malignancy, or hibernating myocardium. Hence the 18 F-FDG PET scan is performed with a special diet of ________ for 24 hours before the test followed by prolonged fasting of at least 12 hours.
Low-carbohydrate and high-fat meals
