Joint Program in Nuclear Medicine
The Scintigraphic Evaluation of Myocardial Viability with Tc-99m Sestamibi
and FDG
Jac D. Scheiner, MD
J. Anthony Parker, MD, PhD
November 12, 1996
Presentation
A 75 year old male with progressive congestive heart failure and left ventricle
ejection fraction of 25% had a history of a 95% mid right coronary artery
stenosis. A resting SPECT Tc-99m sestamibi exam demonstrated a severe defect
in the inferior and posterior walls.
Imaging Findings
Resting SPECT FDG exam demonstrated moderately decreased
uptake in the inferior and posterior walls (shown
by arrowheads), although the uptake was slightly improved compared
to the SPECT Tc-99m sestamibi exam. (Alternate rows show FDG and perfusion.
The top 4 rows show the short axis; the next two rows show the vertical
long axis; and the bottom two rows show the horizontal long axis. The defect
is also shown on the "bullseye" display, arrows.) There was good perfusion
and FDG uptake elsewhere in the myocardium. In light of these findings,
the patient was scheduled for angioplasty of the right coronary artery,
in hopes of improving overall cardiac function.
Discussion
A study of over 9676 patients with heart disease, managed medically over
5 years, demonstrated a decline in patient survival with a decline in left
ventricle ejection fraction (1). As a result of this study and others,
efforts have been made to detect viable myocardium for revascularization
procedures (mainly angioplasty and coronary artery bypass grafting). The
theory has been that if sufficient viable myocardium is revascularized,
the overall cardiac function should improve.
FDG and Viability:
Several studies have shown the correlation between increased FDG uptake
and the recovery of myocardial function status post revascularization.
A prospective study was performed on 43 non-diabetic patients with a history
of coronary artery disease and resting wall motion abnormalities on echocardiography
(ECHO) (2). All patients had a PET FDG study and echocardiography pre-revascularization,
and another echocardiography after revascularization. 8 segments were assigned
per left ventricle (384 total). Myocardium was considered viable if the
FDG uptake was greater than the mean uptake in normal segments - 2 standard
deviations. Initially, there were 53 dysfunctional segments on echocardiography.
The results are summarized below.
SPECT
Pefusion
Defect |
# |
ECHO
Functional
Recovery? |
# |
FDG
Mean
Uptake |
p value |
| Moderate |
17 |
Yes |
6 |
128+/-28 |
0.003 |
| No |
11 |
69+/-12 |
| Severe |
36 |
Yes |
5 |
116+/-13 |
0.0006 |
| No |
31 |
51+/-20 |
There was a statistically significant difference in the increased FDG uptake
in myocardial segments that recovered function status post revascularization,
as compared to the decreased FDG uptake in those that did not. In addition,
all revascularized segments with moderate or severe perfusion defects,
that were shown to be viable by FDG, recovered function.
Another study looked at 43 patients with resting wall motion abnormalities
(RWMA) on contrast ventriculography (3). All patients had contrast ventriculography
and PET FDG studies pre- revascularization, and contrast ventriculography
post-revascularization. The left ventricle was divided into 5 segments
(215 total). Myocardium was considered viable if the FDG uptake was more
than 2 standard deviations above the mean normal value for that segment.
The results are summarized below.
|
FDG Viable |
|
+ |
- |
RWMAs s/p
revasculariztion |
Improved |
45 |
6 |
| No Change |
14 |
65 |
The concordance of FDG viability with resting wall motion abnormalities
improvement after revascularization was 85%. 76% of the revascularized
segments shown viable by FDG had improved resting wall motion. 92% of the
revascularized segments shown not to be viable by FDG did not have improved
resting wall motion.
Tc-99m MIBI vs. FDG:
Studies have been performed which compare viability detection with Tc-99m
MIBI to FDG. In general, between 25% and 64% of the “severe defects” detected
on Tc-99m MIBI scans have been shown to be viable by PET FDG studies (4,5,6,7).
A study was performed in which 37 patients with known coronary artery disease
had SPECT Tc-99m MIBI studies and PET FDG studies within 2 weeks of each
other (5). 5 scan segments (185 total) were assigned per left ventricle.
Myocardial tissue was considered viable if the FDG uptake was > 50% of
the activity of normalized myocardium. 57% (39/68) of the severe defects
on SPECT Tc-99m MIBI scans were shown to be viable by PET FDG scans. Of
these 39 segments, 22 were in the inferior wall. This suggests attenuation
as being one of the main factors that cause SPECT Tc-99m MIBI scans to
underestimate viable myocardial viability.
Another study looked at 111 patients with coronary artery disease (7).
All patients had SPECT Tc-99m MIBI studies and PET FDG studies within 3
days of eachother. 13 scan segments (1443 total) were assigned per left
ventricle. Myocardial tissue was considered viable if the uptake was >
70% of the activity of normalized myocardium. The results are summarized
below.
| Tc-99m MIBI uptake |
#segments |
%viable |
| 0-20% |
20 |
5 |
| 21-30% |
37 |
11 |
| 31-40% |
71 |
13 |
| 41-50% |
167 |
24 |
| 51-60% |
195 |
48 |
| 61-70% |
223 |
61 |
| 71-100% |
730 |
88 |
As expected, the extremely severe defects (0-20%) were only rarely viable.
However, the 41% - 60% defects, which many readers may also consider severe,
were viable by FDG imaging 24% - 48% of the time. According to this study,
‘severe’ resting SPECT Tc-99m MIBI defects that have the greatest chance
of being FDG viable are those in this 41% - 60% range.
FDG PET vs. FDG SPECT:
As stated above, SPECT Tc-99m MIBI studies appear to underestimate viable
myocardium when compared to PET FDG studies. Due to the expense and limited
access to PET cameras, interest has developed in performing FDG myocardial
viability studies using SPECT with 511 keV collimators. A prospective study
was performed in which 20 patients with possible coronary artery disease
and a severe defect (<50% of normalized myocardium) on resting Tl-201
studies had both PET and SPECT FDG studies (8). The left ventricle was
divided into 7 segments (140 total). Viability was defined as FDG uptake
>50% of normalized myocardium. Looking only at the severe defects (60 segments),
the FDG results are summarized below.
FDG Viability
|
|
SPECT |
|
+ |
- |
| PET |
+ |
11 |
3 |
| - |
1 |
45 |
The concordance between PET and SPECT regarding myocardial viability was
92%.
Summary:
FDG uptake in myocardium is associated with a higher likelihood of wall
motion recovery upon revascularization. The literature has shown that certain
Tc-99m MIBI defects underestimate viable myocardium when compared to FDG
exams. The patients with “severe” defects on SPECT Tc- 99m MIBI exams that
would have the highest yield of viable myocardium detection with FDG seem
to be those with defects which are between 40% and 60% of normalized myocardium,
as well as those with defects in the inferior wall (presumedly due to attenuation
by overlying soft tissue). One study has shown comparable viability detection
results with FDG, using both PET and SPECT. Larger studies need to be performed
to substantiate these findings.
References
1. Circulation 1985;72:23-30.
2. American Heart Journal 1994;127:785-96.
3. Circulation 1995;91:1697-1705.
4. European Journal of Nuclear Medicine 1995;22:1306-12.
5. American Journal of Cardiology 1995;75:1214-19.
6. Journal of American College of Cardiology 1994;23:92-98.
7. Journal of Nuclear Medicine 1994;35:569-74.
8. Journal of Nuclear Medicine 1995;36:176-79.
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J. Anthony Parker, MD PhD, Tony_Parker@bidmc.harvard.edu