EVALUATION OF MYOCARDIAL FAT DEPOSITION

EVALUATION OF MYOCARDIAL FAT DEPOSITION IN CORONARY ARTERY DISEASE BY MULTI-SLICE COMPUTED TOMOGRAPHY ABSTRACT OBJECTİVE To investigate correlations between myocardial fat (MF) deposition with unknown etiology and myocardial infarction (MI), coronary artery stenosis and level of stenosis, time after the infarct, aneurysmatic dilatation of arteries, dominancy, stent or by-pass operation. MATERIALS AND METHODS We evaluated coronary CT angiographies (CTA) of 266 patients presenting with a history of MI or having risk factors for coronary artery disease (CAD), analyzed the characteristics of MF and compared the clinical parameters of the patients with and without MF. RESULTS A total of 34 out of 266 had MF. Of 47 patients with a history of MI, 34% had MF. Of 34 patients with MF, 47% had a history of MI. MF occurrence after MI was statistically significant. MF was more frequently associated with greater infarct age, less coronary arteries involved and myocardial bridging but without statistically significant relation. Relation between by-pass operation and MF deposition, correlation between aneurysmatic dilatation of coronary artery and MF deposition within the area supplied by this artery were statistically significant. CONCLUSION MF was detected in 34% of patients with MI with occurrence of MF was significant after MI. MF was significantly more frequent in patients who underwent by-pass operation and, in patients who had aneurysmatic dilatation of coronary arteries. Frequency of MF increased with the involvement of less coronary arteries and longer duration after infarction. Keywords: cardiac imaging. coronary arteries, CT, myocardial fat, myocardial infarction.

INTRODUCTION MF was first defined by a French physicist, Senac (1), in pathological series. At 19th century, Quain (2) investigated the correlation between MF and obesity, alcoholism, sudden death and heart failure. When the relation between MI and coronary artery disease (CAD) was defined, interest in MF infiltration diminished. With increasing use of multi-slice computed tomography (MSCT), the frequency of MF detection increased, and investigations about existence and etiology of MF started. Although there are many studies regarding MF formation after MI, pathogenesis of MF formation was not completely clarified. There were 3 different hypotheses proposed by previous studies. First is MF may be deposited as myocardial tissue cannot metabolise free fatty acids –main source of energy for heart, second is due to ischemic injury, cells at the transition zone of infarct might sustain fatty degeneration in order to survive and third is mild coronary artery stenosis might cause fat deposition within the infarcted myocardial region by well formed collateral veins (3, 4, 5, 6, 7). In this study, we aimed to investigate the correlations between MF deposition with unknown etiology, and myocardial infarction, CAD and level of stenosis, time after the infarct, aneurysmatic dilatation, dominancy of coronary arteries, presence of stent or by-pass operation and other clinical parameters. MATERIAL-METHOD Patient Population Two hundred and seventy eight coronary CTA views evaluated retrospectively. Twelve cases were excluded from the study because of their views could not be achieved from the archive or could not be evaluated due to movement artifact. A total of 266 patients (170 men,

96 women, mean age 55.5, range between 30-80 ages) who underwent coronary CTA between May 2007 and January 2011 for evaluation of coronary arteries in patients who had myocardial infarction, evaluation of CAD in patients who are suspected to have the disease or evaluation of progression in patients with CAD, for evaluation of coronary arteries in patients with no complaints but risks and for evaluation of by-pass grafts and stent patency on followup patients, were included. The MI was diagnosed with electrocardiography (ECG) results (pathological Q wave, ST elevation), elevation of cardiac enzymes (CKMB>5, troponin T>0.1 ng/mL) and concomitant clinical features. Coronary CT Angiography Coronary CTA was performed with 64-Slice CT (Toshiba, Aquilion 64 Otawara, Japan). Scanning parameters for cardiac CT were, maximum tube voltage/current 135/89 kVp/mAs, collimation 64 x 0.5 mm, slice thickness 0.5, rotation time 400 msn, reconstruction thickness 0.5 mm. Initially risk for CAD was calculated on precontrast views by using Agatston score . Later contrast agent was introduced for CTA. Contrast-enhanced imaging was performed when attenuation measurement reached 200 HU within ascending aorta by automatic injection and trigger system. Images were reconstructed retrospectively (ECG gating). Before imaging, beta-blockers were given to patients with a heart rate> 65/min and heart rate was kept at 65/min or lower. A total of 80-100 ml contrast agent (370-400 mg/ml) and followingly 30 cc physiological saline was administered intravenously with a speed of 4.5-5 cc/second through 18-20 IV cannula. The area from arcus aorta to the apex of the heart was scanned. Analysis of Myocardial Fat Tissue Three experienced radiologists on coronary CTA evaluated MF involvement. Consensus was reached in all cases. MF was diagnosed with negative attenuation values on density measurement, fat density and hypodense appearance compared to healthy myocardium on

contrast enhanced CT. MF deposition was observed as hypodense area with low attenuation values and no contrast enhancement in all patients. Mean attenuation value of MF was negative hounsfield units (HU) (range, -106 HU to -16 HU) and significantly low compared to healthy myocardial tissue. Distribution and transmural extension of MF was evaluated. Density measurements were performed from the area with lowest attenuation on contrast enhanced CT (mean area width: 27±10 mm2) and from healthy myocardial area away from that site. Transmural extension of MF was calculated by proportioning the MF thickness to the myocardial wall thickness (Figure 1) and graded between 1 and 4. The cases with a rate of 025% were grouped as grade 1, 25-50% as grade 2, 50-75% as grade 3, more than 75% as grade 4. Correlation of MF distribution to the areas supplied by left anterior descending artery (LAD), right coronary artery (RCA) , left circumflex artery (LCX) was investigated (Figure 2). Stenosis of coronary artery responsible of MF formation and number of involved or diseased coronary arteries were investigated. Measurement of stenosis of coronary artery was calculated by proportioning the diameter of the stenotic segment to that of the healthy segment on its distal part. Cases were evaluated for dominancy. If posterior descending artery (PDA) originates from RCA with posterior left ventricular branches, it’s called right dominance and if it originates from LCX, it’s called left-dominance. Besides if PDA originates from RCA and posterior left ventricular branches originate from LCX, it’s accepted as co-dominance. All cases were evaluated for numbers of diagonal (D) and obtuse marginal (OM) branches and a correlation with MF was investigated. Patients were evaluated for coronary artery aneurysm and association with MF was analysed. Coronary artery aneurysm diagnosis was

conducted in patients who have enlarged segment more than 1.5 times in comparison to adjacent segment (8, 9). Statistical Analysis Comparisons between the two groups were analyzed using independent z Categoric variables were compared using the chi-square test or Fisher's exact test. A p value of less than 0.05 was considered statistically significant. RESULTS A total of 169 cases out of 266 cases had no serious stenosis. A threshold value for serious stenosis was 50%. Ninety seven patients had a stenosis of at least 50% or more. Thirty eight cases had 50% or more stenosis of 1 coronary artery, 28 had 2 coronary arteries, 18 had 3 coronary arteries and 13 had 4 coronary arteries. A total of 34 out of 266 cases (24 men, 10 women) had MF. All patients had this MF within the subendocardial layer. Of 47 patients with a history of MI, 16 (34%) had MF. Of 34 patients with MF, 16 (47%) had a history of MI and 18 (53%) didn’t have such a history (Table 1). Occurrence of MF after MI was found statistically significant (P0.05). But it was noticed that as the number of %50 or more stenotic coronary arteries decreases, frequency of MF increases (Table 2). Table 2: Relation of myocardial fat to the coronary artery stenosis among the cases with a previous history of MI. MI (47)

MF(+)16

MF(-)31

Total

1 coronary artery stenosis

4

8

12

2 coronary arteries stenosis

3

5

8


4

7

11


2

6

8

No stenosis above 50%

3

5

8

1.87

2.03

1.95

Averaging

Of all enrolled patients, 16 had by-pass operation, 9 had stents and by-pass operation, 20 had stents within one or two coronary arteries. Nine out of 25 patients with by-pass operation had MF (36%). Of 34 patients with MF, 7 had by-pass operation, 2 had stent and by-pass operation, 4 had only stents (Table 3). Relation between by-pass operation and MF was found statistically significant (P0.05). Table 4: Timing of occurrence of myocardial fat after MI. Elapsed time after MI

MF (-)

MF (+)

Within the 1. year after MI

4

1

Between 1. to 2. years after MI

5

2


5

2


8

3


3

4

5 year or longer after MI

6

4

TOTAL

31

16

Patients were evaluated for age, numbers of diagonal, OM branches. Mean age of MF (+) group was slightly higher than that of MF (-) group. Additionally, mean numbers of diagonal and OM branches were higher in MF (+) group. Of 266 enrolled patients, 42 (15.7%) had myocardial bridging. Myocardial bridging was over LAD in 28 patients (4 of the patients with MF (+)), over LAD and D1 in 5 patients (2 of the patients with MF (+)), over LAD and ramus intermedius (RI) in 1 patient, over LCX in 2 patients (1 of the patients with MF (+)), over OM in 3 patients, over acute marginal in 1

patient, over RI in 1 patient, over LCX and RI in 1 patient. Although myocardial bridging was more frequent at MF (+) (20%) group, there was no statistically significant relation inbetween (P>0.05). Seventy eight of 266 patients had RI. Out of 34 cases with MF, 6 had RI. Among enrolled patients, 14 patients have segmental aneurysmatic dilatation of at least one of coronary arteries. Out of these 14 cases, 8 (57.1 %) had MF. Among cases with MF, segmental aneurysmatic dilatations were observed on LAD and RCA in 3 cases, on RCA in 3 cases and on LCX in 2 cases, where 6 of these 8 patients (75%) had MF within the area supplied by coronary artery with aneurysm. There was no MF in 6 patients with aneurysmatic dilatation of coronary arteries (1 patient had it on all coronary arteries, 1 had it on LAD and LCX, 1 had it on RCA and LCX, and 3 had it on RCA). MF was present within the areas supplied by aneurysmatic coronary arteries in 42.8% of patients with aneurysmatic dilatation of coronary arteries. Correlation between aneurysmatic dilatation of coronary artery and MF deposition within the area supplied by this artery was found statistically significant (P