1)Department of Radiology, Ehime Prefectural
Central Hospital, Ehime, Japan;and1)Department of Radiology,
Ehime Prefectural Central Hospital, Ehime, Japan;and 2)Department
of Radiology, The University of Tokushima School of Medicine,
Tokushima, Japan
Department of Radiology, The University of Tokushima School
of Medicine, Tokushima, Japan
Abstract: Myocardial cells obtain 60-90% of their energy
from free fatty acids under aerobic conditions. 123I-BMIPP
can demonstrate fatty acid metabolism in the myocardium
and is used to evaluate cardiac diseases. Forty-three patients
underwent BMIPP imaging in the early (15min) and delayed
(4hr) phase, and the washout rate was calculated. We evaluate
the washout rate by two methods, the polar map method and
the planar image method. The two methods showed close correlation
(r=0.473). J. Med. Invest. 50:176-179, 2003
Keywords:123I-BMIPP, myocardium, fatty acid, washout rate,
nuclear medicine
INTRODUCTION
The myocardium requires fatty acids, glucose and lactic
acid as energy sources to maintain pumping function. About
60-90% of myocardial energy metabolism is supplied by fatty
acid under aerobic conditions. Fatty acids metabolism requires
a large amount of oxygen, so under the hypoxic or ischemic
state, fatty acid metabolism is suppressed and replaced
by glucose metabolism, which uses less oxygen than fatty
acid metabolism (1).
123I-BMIPP (β-methyl-iodophenyl pentadecanoic acid)
can demonstrate fatty acid metabolism in the myocardium
and is used to evaluate hypertrophic cardiomyopathy, myocardial
infarction, angina pectoris and prognosis (1-4). Dynamic
changes and the mechanism of fatty acid metabolism in myocardial
cells have not been elucidated. Some recent papers have
reported that the washout rate (WR) of 123I-BMIPP provided
useful information under some clinical conditions (1, 5,
6). Both polar map images (Bull's eye) and multiplane SPECT
images can demonstrate BMIPP uptake.
This study compared the polar map method with the planar
image method to evaluate WR.
PATIENTS AND METHODS
Patients
The study population consisted of 43 patients (34 males
and 9 females, aged between 37 and 89 years old, mean 66.6
years old). The diagnosis of myocardial infarction (thirty-four
patients), angina (eight patients) or PSVT (Paroxysmal Supraventricular
Tachycardia, one patient) was based on all available clinical
information and made by the cardiologists in our institute.
Informed consent was obtained from all patients before BMIPP
examination.
123I-BMIPP study
SPECT (single photon emission computed tomography) was performed
15 min (early) and 4 h (delayed) after intravenous injection
of 148MBq 123I-BMIPP with a three-head gamma camera (GCA-9300A/DI,
TOSHIBA). The system was equipped with a low energy, high-resolution
collimator, and interfaced with a computer. Data were obtained
in a 128×128 matrix with 3 min/rot, 8 deg over
360°, 4 times continuous mode SPECT. The vertical
long, horizontal long and short axis images were reconstructed
using backprojection with a Shepp and Logan filter. The
threshold level was set at 20% for attenuation, and absorption
was corrected by the Chang method.
With the polar map presentation, WR between early and delayed
acquisition was calculated by the following equation:
(count at 15 minutes - count at 4 hours) / (count at 15
minutes) ×100 (%).
With planar image presentation, we selected one view at
60° in all images. The region of interest was set manually
by the radiological technician, specially trained for nuclear
medicine. The WR was calculated by the same equation.
The results were evaluated by two radiology-board doctors,
including a specialist in nuclear medicine.
Statistical analysis
The correlation coefficient was calculated
between the WR of polar map presentation and that of planar
image presentation. r=0.0-0.2:no correlation, r=0.2-0.4:slight
correlation, r=0.4-0.7:close correlation, r=0.7-1.0:very
close correlation.
RESULTS
The results are shown in Figures 1, 2. Fig. 1 demonstrates
polar map images of acute myocardial infarction (#2, 100%
occlusion). The accumulation in the inferior wall (territory
of the right coronary artery, RCA) was impaired in both
the early and delayed phase. The washout of BMIPP was delayed.
The two methods showed close correlation (r=0.473) in Fig.
2. When linear fitting was applied, the result was Y=0.975X
- 14.2. The outlier represented the WR of a 74-year-old
female with acute myocardial infarction (#6). The value
was -81.4 in the polar map image and 22.1 in the planar
image.
DISCUSSION
Fatty acids are introduced into myocardial cells via CD36-positive
fatty acid binding protein on the myocardial cell membrane
and are acylated with ATP and β-oxidized in mitochondria.
About 70% of the incorporated fatty acids are accumulated
in the lipid pool and the others are metabolized to PIPA
(p-iodophenyl acetic acid) in mitochondria and diffuse back
from the lipid pool into the blood. 123I-BMIPP is considered
an indicator of fatty acid metabolism and is used for the
evaluation of some kind of heart disease (7-10). The washout
of BMIPP increased during exercise in normal myocardium
but not in ischemic myocardium. The decreased BMIPP washout
rate after angioplasty indicates improved fatty acid utilization
in chronic coronary artery disease. Myocardial fatty acid
uptake increased and its washout decreased in diabetes mellitus.
In HCM (hypertrophic cardiomyopathy), the severity of the
BMIPP defect could indicate cardiac function and predict
the outcome. The peripheral region of a myocardial infarction
sometimes presents impaired BMIPP uptake, demonstrating
perfusion-BMIPP SPECT `mismatch'. Such a mismatch region
may become infarcted in the future. A larger mismatch area
indicates a higher risk of another cardiac event after the
first event (2). After reperfusion of severe transient ischemia,
the recovery of fatty acid metabolism is sometimes delayed.
The stunned myocardium shows impaired BMIPP uptake and BMIPP
study is used as a memory image of previous ischemic lesions
due to coronary stenosis or spasm.
Either multiplane tomographic images or polar map display
can be used to evaluate fatty acid metabolism. The objectivity
of polar maps is superior. Some reports have described about
the advantages of polar display (11, 12). This method assisted
observers to more consistently identify and quantify myocardial
perfusion defects or impaired fatty acid metabolism. However,
polar maps sometime overestimate the extent of lesions at
the cardiac base, because the cardiac base is larger than
the cardiac apex on polar maps due to the concentric circle
arrangement. Accurate evaluation may not be possible if
polar maps are constructed with inappropriate settings for
the area of cardiac muscles, such as settings that miss
part of the cardiac muscles while including non cardiac
areas (13). Garcia et al described that selecting which
method to use depends largely on the application and on
the instrumentation and technical expertise available (11).
In this study, the WR obtained by planar images in one direction
closely correlated with that by polar mapping, indicating
the importance of confirmation of SPECT source images.
ACKNOWLEDGEMENTS
We gratefully acknowledge all the staff of the Departments
of Radiology and Cardiology, Ehime Central Prefectural Hospital.
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