IMAGING

Question 1

Parameters that reflect cardiac function

Answer 1

LVEF, end-diastolic volume, end-diastolic pressure, exercise capacity

Question 2

common causes of ischaemic LV dysfunction. what are the difficulties?

Answer 2

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

Question 3

viable myocardium - how does it relate to contractility?

Answer 3

viable myocardium may contract normally or it may be dysfunctional.

Question 4

signals of viability in different modalities

Answer 4

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

Question 5

what are the subgroups of viable myocardium

Answer 5

normal,
reversible ischaemic,
partial thickness MI (+/-ischaemia),
Hibernating, 
stunned,
myopathic

Question 6

what is stunning?

Answer 6

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

Question 7

what is hibernation>

Answer 7

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

Question 8

in PET, what suggests that myocardium is hibernating:

Answer 8

PET mismatch: decreased ammonia uptake with NORMAL or INCREASED FDG uptake

Question 9

animal models for MH?

Answer 9

rare, so difficult to determine the underlying pathological mechanisms.

Question 10

myocardium samples from humans receiving CABG show what<

Answer 10

severe changes of the sarcomeres, intracellular space and organelles, the cardiomyocytes themselves and extracellular matrix

Question 11

the strengths of PET:

Answer 11

1. versatility of positron emitting radionuclides that can be incorporated into important biochemical molecular
2. the distribution of these moleculas can be imaged
3. uptake can be quantified
4. can assess: myocardial perfusion, glucose utilisation, fatty acid uptake and oxidation, oxygen consumption, contractile function

Question 12

properties of FDG

Answer 12

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

Question 13

features of 13N-ammonia

Answer 13

it is a perfusion tracer. avidly extracted and retained in viable myocytes by incorporation into glutamine.

Question 14

ammonia and FDG uptakes in stunned, hybernating and infarcted myocardiums

Answer 14

stunned: normal uptake of both;
hibernating: decreased uptake of ammonia, normal or increased uptake of FDG;
infarcted: concordat decrease of uptake of both

Question 15

what is the simplification with ammonia >?

Answer 15

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

Question 16

what is the simplification with FDG?

Answer 16

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

Question 17

why is perfusiom-metabolism mismatch characteristic of hibernating myocardium?

Answer 17

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.

Question 18

ECG-gating of PET images:

Answer 18

imformation of regional myocardial thickening and motion is essential to the asessment of hibernation

Question 19

Thallium 201
what is it
how is it produced
excretion
redistribution properties
advantages and disadvantages

Answer 19

• 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

Question 20

Technetium-99m-labelled agents
what is it
how is it produced
excretion
redistribution properties
advantages and disadvantages

Answer 20

• 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

Question 21

What is a radionuclide?

Answer 21

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.

Question 22

What kinds of radioactivity are used for diagnostic purposes in the nuclear lab?
Two types:

Answer 22

1. 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)
   
   2. 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)

Question 23

what does the delivery of tracers in the circulation indicate? what is the term for abnormalities?

Answer 23

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.

Question 24

What techniques can be used for gamma camera radionuclide imaging?

Answer 24

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.

Question 25

During the acquisition of myocardial perfusion scans, there may be spatial overlap of adjacent walls and temporal blurring of images due to movement of the heart during contraction. What is the significance of these features?

Answer 25

Spatial overlap is associated with planar perfusion imaging: radioactivity originating from different depths in one of the heart walls overlaps with adjacent segments on that particular view. SPECT overcomes this limitation by allowing imaging of myocardial radioactivity at discrete levels of the heart.
The motion of the heart causes temporal blurring during acquisition of perfusion images. Studies may be acquired during hundreds to thousands individual heartbeats, and the resulting borders of endocardium on the final image are a composite average of the moving radioactivity. However, in perfusion studies the interest is not in the endocardial border motion but rather in the patterns of radionuclide uptake by the myocardium/

Question 26

How can EF assessment can be achieved?

Answer 26

For assessment of the ejection fraction, the precise distinction between the position of the endocardium at end-systole and end-diastole is important and is achieved by means of ECG-gating. ECG-gating allows 8 or 16 individual images or timeframes to be acquired for each heart beat, and corresponding frames from each beat are summed, so that the temporal motion of the heart can be captured. Such assessment is very accurate.

Question 27

How does a gamma camera work?

Answer 27

The main components of the gamma camera are:
1. The collimator is equivalent to a focus instrument that lets past only radiation perpendicular to the plane of the scintillation detector. It consists of lead-lined holes perpendicular to the face of the gamma camera, similar to a sieve. Radioactive decay occurs in all directions (360degrees), and imaging without a collimator produces a ‘blob’ of radioctivity without any spatial resolution. The o=collimator eliminates the radiation scattered from the patient’s body at different angles and only accepts photons that are completely parallel to the lead walls. This is essentail,, because scattered radiation can provide inaccurate indormation about the location of the emitting tracer.
2. Scintillation detector is a sspecial crystal that converts and amplifies the gamma radiation emitted by the radiotracer in the patient’s body to light photons.
3. Photocathode converts the photons to electrons.
4. Photomultiplier tubes amplify the electrons into detectable electrical current
5. Electronic processor generates images based on the intensity and spatial distribution of electrical signals.
Nuclear cameras have one to three heads, the acquisition time decreases proportionally with the number of heads.

Question 28

What are the main causes of artifact in nuclear cardio?

Answer 28

Attenuation, patient motion, image processing

Question 29

What is attenuation?

Answer 29

Attentuation represents energy loss as the particles emitted by the heart interact with body tissue on their way to the external detecting device. In the case of radionuclide imaging, attenuation results from absorption of photon energy by the tissue between the heart and the camera.

Question 30

Clinical data re myocardial ischaemia that can be assessed by nuclear studies:

Answer 30

1. The presence of ischaemia
   
   2. The extent of ischaemia
   3. The reversibility of ischaemia
   4. The severity and distribution of regional myocardial dysfunction caused by ischaemia

Question 31

Compared with SPECT, PET presents advantages with respect to

Answer 31

PET presents advantages with respect to spatial and temporal resolution, leading to higher sensitivity for the detection of perfusion defects.10

Question 32

what;s special about PET perfusion tracers?

Answer 32

ET perfusion tracers, such as 13N-ammonia or 15O-water, have a higher extraction from the blood pool and enable the absolute quantification of myocardial blood flow (MBF) based on methods developed for studies of cerebral perfusion.

Question 33

Thallium-201
what is it
half life 
energy
implications for imaging

Answer 33

• Thallium-201 (Tl-201) is a potassium analog, which is taken up by viable myocytes in proportion to blood flow.
  
  • However, the low energy of the Tl-201 photon (80 keV) and the long half-life (≈73 hours) are suboptimal for perfusion imaging.

Tl-201 has significant delayed redistribution so that stress images should be obtained less than 10 to 20 minutes after stress injection. Imaging of the 3- to 4-hour delayed redistribution permits distinguishing ischemia from scar

Question 34

Tc-99m based tracers
?redistribution
energy
half life 
implications

Answer 34

• Tc-99m sestamibi and Tc-99m tetrofosmin bind mitochondrial membranes, show virtually no redistribution after initial uptake, have a 140-keV photon peak that is near the optimal for γ cameras, and have a 6-hour half-life, permitting injection of higher activity of tracer.

The Tc-99m perfusion agents take longer to clear from the hepatobiliary system and gut, which may cause scattering of activity into the inferior wall of the heart. All of the agents have properties that allow the stress component and the imaging component to be separated in time and location, which is a significant advantage over other modalities that require imaging during the first pass of the contrast agent.

Question 35

first pass extraction of SPECT radiotracers: at resting flows and during stress (high flow)

Answer 35

The first-pass extraction of Tl-201, Tc-99m sestamibi, and Tc-99m tetrofosmin are 86%, 64%, and 54%, respectively, at resting flows

However, at high flows when extraction is diffusion-limited, the extraction is considerably lower causing the well-known-roll-off phenomenon

Question 36

uptake of all SPECT tracers is dependent on ….

Answer 36

uptake of all SPECT tracers is dependent on myocardial cellular integrity in addition to blood flow.