Case 11 cont'd
Image findings: - This PET myocardial perfusion imaging study was performed using N-13 ammonia rest perfusion images and F-18 fluorodeoxyglucose (FDG) myocardial metabolic images. A large, fixed perfusion defect with akinesis on gated images is seen in the mid to basal anterior and septal segments of the LAD territory. No FDG uptake is seen in this territory suggesting no cardiac viability in this region.
As an analog of glucose, F-18 FDG only accumulates in viable cells and is therefore an indicator of cells performing glucose metabolism. Myocardium necrosis due to MI causes the myocytes in infarcted myocardium to be replaced by fibrocytes. Fibrotic tissue does not have glucose metabolism, and therefore appears as an area of decreased activity on F-18 FDG PET scan. Rb-82 and N-13 are radiotracers used in PET myocardial perfusion imaging. Tc-99m sestamibi is a radiotracer that may be used in either SPECT or planar image acquisition in the context of myocardial perfusion assessment.
Q1. Answer: A.
Coronary CT calcium scan is mostly used to assess severity of CAD by detecting calcium deposits in coronary arteries.
MRI is most appropriate to visualize heart shape, size, function and tissue. Gadolinium enhancement aids in visualizing scar tissue and heart infiltration (e.g. amyloidosis).
Nuclear medicine imaging using F-18 FDG PET is the best imaging modality to assess myocardium viability. F-18 FDG is an analog of glucose. Using this radiotracer allows visualization of metabolically active cells and is therefore a great method to assess cardiac tissue viability.
Ultrasound is most adequate to visualize the heart and the blood flow through the heart using Doppler.
Q2. Answer: C.
Hibernating myocardium is viable, yet due to chronic ischemia, presents with abnormal contractility and appears as a fixed perfusion defect on MPI i.e. unchanged cold defects present on rest and stress image sets. However, myocytes in such myocardium remain viable and appear normal on F-18 FDG PET scan as they metabolize glucose. This is known as a perfusion-metabolism mismatch and is a hallmark of hibernating myocardium (see Figure 1).
Infarcted myocardium is cardiac tissue that has been replaced by fibrocytes after an ischemic episodes where blood flow was restricted due to artery occlusion and oxygen supply did not meet demand. This tissue is non-viable and therefore not metabolically active.
Stunned myocardium is metabolically active with persistent abnormal contractility despite adequate reperfusion post-ischemic episode. Appears either as slightly reduced photon uptake or normal on MPI scintigraphy and normal on viability imaging.
Ischemic myocardium cannot keep up with increasing oxygen demand (stress) due to CAD. Blood flow is reduced and therefore appearance on MPI scan is photon deficiency (i.e. less radiotracer activity on post-stress set of images when compared to rest images). However, on viability imaging, tissue appears normal.
Q3. Answer: B.
Discussion:
PET viability imaging is used to assess how much heart muscle has been damaged by a myocardial infarction or other heart diseases. This imaging modality is used to determine the best next steps in patient management such as angiography, CABG, heart transplant, etc.
Viability imaging is done in two parts. The first portion is myocardial perfusion imaging (either SPECT or PET), which was described in the previous case. The second part, which is specific to viability imaging, is the use of a different radiotracer – F-18 FDG – to assess glucose metabolism by myocytes. Normally, FDG uptake should be uniform and matched. Diminished F-18 FDG uptake is a marker of non-viable tissue. This added step is especially important in patient management and outcome, as it helps physicians distinguish whether the areas of fixed cold defects on perfusion imaging (i.e. photon deficient at both rest and stress) are really due to scar tissue or due to severely ischemic areas which could benefit from reperfusion therapies and greatly improve patient survival. Misinterpreting these fixed defects as being scar tissue instead of viable myocardium leads to overestimation of actual scar tissue and delays potentially life-saving interventions such as surgical re-vascularization. Another important application of perfusion-viability studies is in heart transplantation candidates. Subjects with poor heart function who are shown to have multiple areas of hibernating myocardium are better managed with CABG rather than transplantation. On the other hand, poor global heart function and demonstration of truly non-viable myocardium through FDG metabolic imaging are better indicators for cardiac transplantation eligibility.
Regarding protocol, patient blood sugar level is measured. Diabetic patients are given a small amount of insulin and glucose to enhance heart tissue glucose absorption. Radioactive tracer is injected intravenously and image acquisition is initiated 45 minutes later, lasting 25 to 30 minutes.
Figure 1 – Possible findings in viability imaging
https://jnm.snmjournals.org/content/48/7/1135/tab-figures-data
Figure 2 – Perfusion-metabolism mismatch on PET viability imaging
MPI with PET using N-13 and viability study using F18-FDG. Regional myocardial F18-FDG uptake is disproportionately enhanced compared with regional myocardial blood flow. This pattern, known as perfusion–metabolism mismatch, is characteristic of hibernating myocardium.