Imaging & Oncology - Society of Radiographers

are several areas of research potential, including the early detection of
microvascular dysfunction, (which helps in the early detection of CAD) and
the assessment of vascular dysfunction in cardiomyopathies 23,24,25 .
Figures 5a and b (opposite page) show globally reduced perfusion in the
left ventricular myocardium unmasked more easily with globally reduced
MBF reserve.
Figure 6a shows significant reversibility and ischaemia in the right
coronary artery territory involving the inferior and lateral walls, however,
the MBF as shown in figure 6b is globally reduced therefore also implying
multivessel disease.
Figures 7a (above left) and 7b (above right): Image 7a shows severe reduction perfusion to the
septum and inferior wall in a SPECT study. Image 7b shows metabolic activity in the septum and
most of the inferior wall, suggesting hibernating/viable myocardium on an FDG PET study.
Figures 6a (above left) and 6b (above right): Images showing significant ischaemia in the RCA
territory on the relative perfusion assessment. However there is globally reduced MBF in all
three vascular territories, unmasking multivessel ischaemia.
Viability and perfusion
In patients with poor LV function, it is important to determine whether this
is secondary to ischaemia, scar tissue or non-ischaemic causes. In cases of
chronic ischaemia, the myocardium enters a state of hibernation, which is
difficult to distinguish from scar tissue by perfusion imaging alone as both
of these show reduced resting tracer uptake. Hibernating myocardium will
retain metabolic activity, whereas scar tissue does not. 18F-FDG imaging is
used to detect metabolic activity. Matching of normal myocardial perfusion
with normal regional FDG uptake is considered a marker of normal viable
myocardium. A decrease in both perfusion and metabolism is indicative of
irreversible tissue injury (ie scar tissue). A mismatch between perfusion and
metabolism identifies viable (ie normal FDG scan) but hibernating (abnormal
perfusion) myocardium. The latter patient would more likely benefit from
revascularisation. An example of this is shown in figures 7a and 7b 26 .
Hybrid PET-CT and SPECT-CT scanning
There is increasing evidence documenting the value of a hybrid imaging
approach, either using high-end PET-CT or SPECT-CT machines 27,28,29 . The
diagnosis of coronary artery disease requires, ideally, a combination of
state-of-the-art functional and anatomical imaging of the coronary arteries. A
hybrid PET-CT scanner combines the benefits of a PET scanner for functional
imaging of perfusion and metabolism as described and high end spiral CT
scanning for anatomical CT angiography. It should however be noted that
this can done in one of two ways including stand-alone hybrid machines
or separate PET and CT or SPECT and CT machines with side-by-side or
software fusion. The latter method does make more economic sense in most
departments, but does involve two separate patient visits.
Namdar et al 30 using 13N-Ammonia in 2005 were the first to document the
clinical robustness of PET-CT hybrid imaging using a four slice spiral CT
scanner and showed a sensitivity, specificity, PPV and NPV of 90%, 98%, 82%
and 99% respectively. Further studies using both SPECT-CT and PET-CT (with
minimum 64 slice CT) have supported this early study 31,32,33 .
Having both PET and CT information avoids unnecessary invasive coronary
artery catheterisations, which expose patients to increased morbidity and
mortality; also there is potential for important cost savings. The extra
information obtained from the hybrid system over other methods could help
create a unique ‘one-stop shop’ for patients, obviating the need to make
multiple visits to the hospital for different tests 34 .
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