Radiological imaging questions Flashcards
What are the principles of positron emission tomography (PET)
Decay of radio-isotope causes emission of positron which interacts with surrounding electrons to emit gamma rays that are detected by the gamma camera of the PET scanner.
PET is better at detecting changes in physiology than anatomy depicted by greater update of the radioactive tracer. Often combined withe CT. Best test
18 flurodeoxyglucose (FDG) is a radio-isotope marker used as surrogate for cellular activity. With myocardium metabolism, during ischemia, myocytes switch from fatty acid to glucose as the predominant source of energy.
So relative uptake of FDG would be index of viability
What are indications of cardiac PET
Myocardial perfusion–risk stratify pts with critical coronary anatomy stenosis (N ammonia, O water or Rb rubidum as the radio-isotope
Myocardial viability–distinguish hibernating viable myocardium from infarcted non-viable tissue using FDG (metabolism) and N ammonia (perfusion) as the radio-isotopes.
a) hibernating myocardium= normal metabolism and reduced perfusion (metabolism-perfusion mismatch)
b) Infarcted tissue= matched reduction in metabolism and perfusion
What are principles of nuclear medicine scanning
Myocardial perfusion scanning uses the radioisotope Thallium-201 at rest and under stress to assess the functional importance of coronary artery disease.
As a potassium analogue, which is actively taken up into cells via the Na-K-ATPase pump the uptake of Thallium into the cells is directly proportional to regional myocardial blood flow.
The initial injection of Thallium represents myocardial blood flow whereas re-injection allows assessment of redistribution, a phenomenon associated with the deranged cellular metaolism of hibernating myocardium.
What are principles of stress nuclear scanning
normal myocardium —represented by normal uptake both in rest and stress images
reversible ischemia— which is represented by a defect in the stress image that normalises after rest
a chronic tear–(infarcted myocardium) which is represented by a fixed defect that remains unchanged in both rest and stress images
hibernating viable myocardium, which is represented by a fixed defect (in both rest and stress image) that improves on delayed imaging
What are the principles of multi-gated acquistiong (MUGA) scanning
uses the prinicples of cardiac nuclear medicine to provide a more accurate and reproducible cine assessement of left ventricular contractility than echocardiography
Following intravenous injection of radioactive material (technetium-99) which radio-labels red blood cells gamma images are acquired “gated” to the patients cardiac cycle.
Regional wall motion abnormalities (hypokinesia, dyskinesia or akinesia) can be detected at rest and under stress (using adenosine or exercise)
Reduced emission of gamma rays at rest usually indicates a ventricular scar following myocardial infarction, whereas defects observed during stress are indicative of areas of ischemia
Describe LV contractile and/or flow reserve
Patients with no LV flow reserve are defined by a percent increase in stroke volume of < 20% during dobutamine stress echo or cath.
Have a higher mortality (22-33%) compared to those with flow reserve (5 to 8%) if planning for AVR
Distinguising between true severe and pseudo-severe AS
Changes in the EOA and gradient during dobutamine infusion are important
Pseudosevere AS = increase in EOA and relatively no increase in gradient in response to increasing flow. Peak gradients stay at < 30 or 40mmHg and EOA of > 1.0 or an increase in EOA of > 0.3.
Severe AS = little or no increase in EOA WITH an increase in gradient
What are tests for viable myocardium
PET
Cardiac MRI
SPECT (thallmium scan)
Dobutamine stress echo
How does PET viability work
PET imaging using metabolic tracing pf PDG enables detection of metabolic changes at the cellular level associated with ischemia.
Preserved or increased glucose utilization and FDG updated in hypoperfused and dysfunction myocardium (flow-metabolism mismatch) is regarded as metabolic marker of cell viability.
Reduction in flow and FDG update is indicative of scar
Thallium 201 myocardial perfusion scan
Thallium is used as tracer because of its high first pass myocardial extraction.
The up take of thallium is dependent on Na/K ATPas depent active process requiring cell membrane intetegrtity.
you do a second test because of late redistribution that may accurate marker of regional viability.
quantitative “cut off” of 50 to 60% uptake is good thing and anything below that is unlikely to receive benefit from revascularization
Describe MIBI (Myocardial perfusion imaging)
Nuclear test used to demonstrate the presence of coronary heart disease in a patient that if cannot tolerate an exercise stress test (such as a treadmill)
Utilizes a vasodilator (dipyridamole/dobutamine/adenosine/theophylline) to induce coronary vasodilation and generate perfusion defects.
Allows for precise quantification of LVEF
Signs of viability on echo
Preserved end diastolic wall thickness.
If wall thickness is < 0.6 cm then functional recovery is very unlikley with revascularization
What are features of Dobutamine stress echo
examines inotropic reserve.
Viable myocardium shows improved contractile function (inotropic reserve)
about 75% sensitive for myocyctes.
What are important features on MRI to assess when it comes to viability
wall thickness
regionals that fail to hyperenhance with gadolinium are viable
What is stress echo protocol
Dobutamine infusion of 8-min with increments of 2.5 to 5 ug/kg/min to a maximum of 20 uug/kg/min
