IMAGING Flashcards
Parameters that reflect cardiac function
LVEF, end-diastolic volume, end-diastolic pressure, exercise capacity
common causes of ischaemic LV dysfunction. what are the difficulties?
full-thickness MI, partial thickness infarction, myocardial stunning or hubernation. difficult to differentiate. differnet states can co-exist in the same patient or even the same myocardial region
viable myocardium - how does it relate to contractility?
viable myocardium may contract normally or it may be dysfunctional.
signals of viability in different modalities
NUCLEAR: 1. thallium - reflects membrane function 2// technetium-99m - membrane and/or mitochondrial function 3. FDG - glucose metabolism 4. fatty acids ECHO or CINE MRI 1. myocardial thickness/motion, thickening (also recruitment and deterioration) CONTRAST MRI 1. absence of late enhancement
what are the subgroups of viable myocardium
normal, reversible ischaemic, partial thickness MI (+/-ischaemia), Hibernating, stunned, myopathic
what is stunning?
is a form of contractile dyfunction of VIABLE myocardium that is caused by a BRIEF period of ischaemia followed by RESTORATION of PERFUSION. may be the result of reperfusion injury whereby restoration of blood flow leads to generation of free radicals and transient Ca overload and thus temporary damage to the contractile mechanism
what is hibernation>
a state of contractile dysfunction in VIABLE myocardium but in the setting of CHRONIC ischaemic heart disease. Requires an intervention such as revascularisation for recovery. Strictly, it is a RETROSPECTIVE diagnosis (by definition you need to demonstrate improved contraction after revasc). there must be inducible ischaemia, thus a surrogate definition: Hibernation is viable and dysfunctional myocardium in ehich IMPAIRED PERFUSION RESERVE LEADS TO INDUCIBLE ISCHAEMIA
in PET, what suggests that myocardium is hibernating:
PET mismatch: decreased ammonia uptake with NORMAL or INCREASED FDG uptake
animal models for MH?
rare, so difficult to determine the underlying pathological mechanisms.
myocardium samples from humans receiving CABG show what<
severe changes of the sarcomeres, intracellular space and organelles, the cardiomyocytes themselves and extracellular matrix
the strengths of PET:
- versatility of positron emitting radionuclides that can be incorporated into important biochemical molecular
- the distribution of these moleculas can be imaged
- uptake can be quantified
- can assess: myocardial perfusion, glucose utilisation, fatty acid uptake and oxidation, oxygen consumption, contractile function
properties of FDG
18F-fluoro-2-deoxy-D-glycose is a glucose analogue taken up by VIABLE myocytes in the same way as glucose but its subsequent metabolism is blocked and it remains within the myocyte.
It’s a tracer of exogenous glucose uptake and thus myocardial VIABILITY
features of 13N-ammonia
it is a perfusion tracer. avidly extracted and retained in viable myocytes by incorporation into glutamine.
ammonia and FDG uptakes in stunned, hybernating and infarcted myocardiums
stunned: normal uptake of both;
hibernating: decreased uptake of ammonia, normal or increased uptake of FDG;
infarcted: concordat decrease of uptake of both
what is the simplification with ammonia >?
ammonia is not a pure perfusion tracer, its similar to technetium based tracers: decresed uptake is seen when PERFUSION is DECREASED, When VIABILITY is DECREASED or with combinations of both. not possible to distinguish from the two by doing a resting ammonia study alone
what is the simplification with FDG?
FDG is only a good viability tracer if given under conditions that encourage glucose metabolism instead of the preferred fatty acid metabolism of normal mmyocardium: thus studies performed after an oral glusoce load or during insuling and glucose infusion
why is perfusiom-metabolism mismatch characteristic of hibernating myocardium?
it is not clear, also uncertain whether the domminant feature is reduction of ammonia uptake or increase in FDG uptake or a mixture of both.
ECG-gating of PET images:
imformation of regional myocardial thickening and motion is essential to the asessment of hibernation
Thallium 201 what is it how is it produced excretion redistribution properties advantages and disadvantages
• Thallous chloride • Potassium analogue • Cyclotron produced • First pass extraction of 88% - peak uptake in heart by10 min • Redistributes soon after injection • Excreted by the kidney Advantages • Lower cost • Large body of evidence • Low liver/bowel uptake • No need for routine resting injection • Redistribution into myocardium with reduced resting blood supply Disadvantages • Low energy 80keV à low resolution, vulnerable to attenuation • Long physical half-life of 72 hours • Moderate radiation burden • Stress imaging within 10 min of tracer injection
Technetium-99m-labelled agents what is it how is it produced excretion redistribution properties advantages and disadvantages
- Produced on site (99Mo generator)
- Half-life 6 hours
- 140 keV gamma emission
- Bound to other compounds
- MIBI
- tetrofosmin
- Excreted by the bowel
- Distribution proportional to viability and perfusion
Advantages • 140 keV à higher resolution • Physical half life 6h à lower radiation burden • Stable retention within myocardium • Stress injection at remote site • First pass imaging and ECG-gating for LV function Disadvantages • Poorer tracer of perfusion • Lower first pass extraction (40-60%) • Resting injection required • High hepato-biliary activity
What is a radionuclide?
In all physical systems, there is a tendency toward stability (expressed in higher degree of entropy). In the case of radionuclide, the stability is reached by getting rid of part of the excess energy of the compound. A radionuclide is an unstable atom that emits radioactivity in the form of atomic particles, photons, or both. These emitted photons are immaged by a gamma camera, a special radiation detector. The original, unstable radionuclide is called the parent, whereas the more stable one (resulting from a particle or photon emission) is the daughter.
What kinds of radioactivity are used for diagnostic purposes in the nuclear lab?
Two types:
- The direct emission of single photons by the injected radionuclide, which is then detected by the gamma camera. In this case, the photons are emitted one at a time by the radionuclide during decay and are detected by the camera (single-proton techniques)
- The emission of special atomic particles, positrons, which have to first interact with an electron in order to emit photons (indirect emission of photons). The collision of a positron and an electron results in the emission of two high energy photons with an energy of 511keV. These two photons travel along the same axis but in opposite directions at 180degrees and are simultaneously detected by gamma camera elements that face each other. This is PET (positron emission tomography)
what does the delivery of tracers in the circulation indicate? what is the term for abnormalities?
The delivery or radiotracers to the capillary microcirculation shows the integrity of the capillaries, whereas cellular uptake of the tracer is dependent on the structural and functional integrity of the myocardial cellls.
The areas of absent or reduced uptake of the tracer are perfusion defects.
What techniques can be used for gamma camera radionuclide imaging?
Three basic techniques, classified based on the type of radionuclides they are able to image.
Planar and single photon emission computed tomography (SPECT) imaging are used for detecting conventional single photon radionuclides such as thallium-201 (Tl-201) and technetium 99m (Tc-99m) perfusion agents, whereas PET is used for detecting dual-photon radionuclides such as rubidium 82 or fluorine 18 fluorodeoxyglucose.
1. Planar imaging is a 2D technique in which the heart is imaged in two planes by a gamma camera, and all the structures contained within that plane are superimposed on each other. In order to see all the walls of the heart, multiple planes must be imaged. Not widely used today
2. SPECT technique uses a rotating gamma camera with one or more heads to acquire 32-64 individual planar images of the heart these images are used to reconstruct the heart in a 3D or tomographic format. This is the currently preferred method of imaging.
3. PET is less widely available but can be used with cyclotron-produced radioisotopes for imaging perfusion and myocardial viability.
