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Original Report |
1 Department of Radiology, Nippon Medical School, 1-1-5 Sendagi, Bunkyo-ku,
Tokyo 113-8603, Japan.
2 First Department of Internal Medicine, Nippon Medical School, Tokyo 113-8603,
Japan.
Received February 25, 2003;
accepted after revision August 19, 2003.
Address correspondence to Y. Amano.
Abstract
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CONCLUSION. Black blood T2-weighted and contrast-enhanced fast inversion recovery gradient-echo images displayed ablated septal myocardium until 35 weeks after percutaneous transluminal septal myocardial ablation. Central hypointense areas were observed on MRI in patients until 4 weeks after ablation. Black blood and cine steady-state free precession MRI were used to assess the decreased septal wall thickness and diameter of the left atrium after ablation as well as the reduced motion of the ablated region. MRI may be useful for evaluation of cardiac structural, signal, and functional changes associated with percutaneous transluminal septal myocardial ablation.
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Conventional catheter angiography reveals the septal branches of the left anterior descending artery and allows the left ventricular outflow tract gradient to be directly monitored before and after ablation [2–4]. However, this technique is too invasive to be used for follow-up monitoring of the gradient and the left ventricular ejection fraction. Echocardiography can be used to measure the pressure gradient and ejection fraction of the left ventricle non-invasively and to visualize the motion of the myocardial wall [3]. The disadvantages of echocardiography include low reproducibility, a limited acoustic window, and an inability to identify ablated myocardium.
MRI is used to investigate various cardiac disorders, including hypertrophic obstructive cardiomyopathy, because it has high spatial and contrast resolution, a wide range of view, high reproducibility, and a variety of attendant techniques [5–8]. On MRI, heart morphology and myocardial damage can be identified, and estimates can be made of the left ventricular ejection fraction, the left ventricular outflow tract gradient, and the coronary flow reserve [6–10]. Cine steady-state free precession imaging provides excellent contrast between the myocardium and the chambers, greatly improving the visualization of myocardial wall motion [8]. However, only a few reports have been made on MR assessment of both structural and functional cardiac changes in patients with hypertrophic obstructive cardiomyopathy after percutaneous transluminal septal myocardial ablation [7, 11].
The aim of our study was to use MRI to evaluate the effects of percutaneous transluminal septal myocardial ablation on hypertrophic obstructive cardiomyopathy. To our knowledge, ours is the first report describing the structural and functional cardiac changes associated with this procedure in both the ablated and nonablated regions.
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MRI Protocol
MR examinations were performed using a 1.5-T MR imager (Signa CV/i, General
Electric Medical Systems, Milwaukee, WI). A cardiac phased array coil was used
for signal reception. Breath-hold transverse black blood T2-weighted fast
spin-echo imaging with cardiac gating and a double inversion recovery pulse
pair were performed with or without fat suppression
[11]. Typical imaging
parameters were: TR, 2 cardiac cycles; TE, 85 msec; echo-train length, 32;
receiver bandwidth, 100 kHz; imaging matrix, 160–320 x
224–256; field of view, 36–40 x 36–40 cm; and section
thickness, 8 mm with a gap of 2 mm. The time between the double inversion
recovery pulse pair and data acquisition was determined automatically to
suppress the flow signal in the selected section
[10]. Breath-hold
cardiac-gated segmented cine steady-state free precession or fast spoiled
gradient-echo (SPGR) imaging was performed in the long- and short-axis planes.
Steady-state free precession imaging (TR range/TE range,
3.9–4.7/1.6–2.0; view per segment, 16–24; flip angle,
45–60°; receiver bandwidth, 125 kHz; imaging matrix, 256 x
128–160; field of view, 32 x 32 cm; and section thickness, 8 mm,
with 2-mm gap) was mainly used because it provides good contrast between the
myocardium and the ventricular chamber
[8]. We used view-sharing to
double the cardiac phases to 16–24. The last nine MR examinations in
eight patients were contrast-enhanced inversion recovery fast gradient-echo
imaging sequences (5.5/1.4; flip angle, 20°; inversion time, 250 msec)
performed in the short-axis plane 10 min after a 0.15-mmol/L injection of
gadolinium. The in- and through-plane spatial resolutions were identical to
those of cine images.
Imaging Analysis
In nine MR examinations, we used black blood T2-weighted fast spin-echo
imaging to investigate the signal intensity of the septal myocardial wall
before percutaneous transluminal septal myocardial ablation. In 19
examinations, we used this sequence to investigate the signal change
associated with the ablation with regard to the interval between the ablation
and the MR study. In addition, we performed nine examinations to study the
delayed hyperenhancement of the septal myocardium associated with ablation.
Next, we estimated septal and posterior wall thicknesses at end diastole, the
ratio between septal wall and posterior wall thicknesses, the anteroposterior
diameter of the left atrium at diastole, and the left ventricular ejection
fraction in 11 patients. The border between the myocardium and the left
ventricular chamber was traced manually on the MRIs and the papillary muscles
were excluded from the endocardial border when the ejection fraction was
estimated using Simpson's rule.
Thirteen MRIs obtained after the first ablation provided cine MRIs of sufficient image quality to allow us to quantitatively evaluate the septal wall motion using the percentage of systolic wall thickening present. We determined the percentage of systolic wall thickening with the following formula: (end-systolic septal wall thickness – end-diastolic septal wall thickness) / end-diastolic septal wall thickness x 100. We then compared the percentage of systolic wall thickening of the ablated and nonablated regions of the septal wall. The ablated region was defined as the area of the myocardium fed by the ablated septal arteries and showing abnormal hyperintensity on black blood T2-weighted fast spin-echo images, delayed hyperenhancement, or focal atrophy. The nonablated region was defined as the apical septal wall that showed neither abnormal signals nor atrophy.
To compare the quantitative parameters before and after percutaneous transluminal septal myocardial ablation or between the ablated and nonablated regions, we used a paired Student's t test. The differences were considered statistically significant at a p value exceeding 0.05.
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The septal and posterior wall thicknesses, the ratio between the septal wall and posterior wall thicknesses, the diameter of the left atrium, and the left ventricular ejection fraction before and after percutaneous transluminal septal myocardial ablation are summarized in Table 1. We found significant decreases in septal wall thickness (p < 0.0012) and in the left atrium diameter at end diastole (p < 0.012) after ablation but found no significant changes in the posterior wall thickness, the ratio between the septal wall and posterior wall thicknesses, or the ejection fraction (p > 0.10).
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In all patients, the percentage of systolic wall thickening after ablation was lower in the ablated region of the septal wall (Figs. 2D and 2E) than in the nonablated region (Figs. 2F and 2G) (16.0 ± 8.0 mm in the ablated region vs 45.0 ± 18.0 mm in the nonablated region, p < 0.01).
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Black blood T2-weighted fast spin-echo and contrast-enhanced inversion recovery fast gradient-echo images depicted the ablated region of the septal wall as focal high signal intensity and hyperenhancement, which suggests iatrogenic myocardial infarction [9–12]. Central hypointense areas in the ablated region were observed on the T2-weighted fast spin-echo and contrast-enhanced MRIs within 4 weeks of ablation, which suggests microcirculatory obstruction and coagulation [9, 10]. On the other hand, MRI findings obtained more than 12 months after ablation showed no signal changes in the septal wall. These myocardial signal changes were similar to those of myocardial infarction [12]. In the two patients who underwent MRI within approximately 3 months of ablation, no signal changes were detected in the septal wall on black blood T2-weighted fast spin-echo imaging, but delayed hyperenhancement was identified. The reasons for this finding remain unknown, but contrast-enhanced inversion recovery fast gradient-echo imaging can reveal the dilated interstitial space and scarring associated with percutaneous transluminal septal myocardial ablation in addition to myocardial damages and interstitial edema that may be identified on T2-weighted MRI [9, 10, 12].
MRI showed decreases in septal wall thickness and left atrial diameter after percutaneous transluminal septal myocardial ablation. The decrease in septal wall thickness resulted from the direct effects of the ablation on the septal myocardium [8]. The decrease in left atrial diameter may have been caused by the reductions of mitral regurgitation and end-diastolic left ventricular pressure induced by the ablation. These reductions can contribute, to some extent, to the improvement of diastolic dysfunction [1]. However, we found no significant changes in posterior wall thickness or the ejection fraction. These results suggest that percutaneous transluminal septal myocardial ablation is an efficient procedure for rapidly decreasing septal wall thickness and end-diastolic pressure in the left ventricle [3, 4], but no effects on global heart morphology and function are induced. This finding may be explained by the fact that the myocardial infarction induced by ablation was limited to the septal wall, whereas myocardial disarray and fibrosis may diffusely underlie the myocardium in hypertrophic obstructive cardiomyopathy even before treatment [1, 2]. Cine MRI showed that the wall motion in the ablated region was significantly reduced compared with the motion in the nonablated region. Thus, during a single examination, cardiac MRI techniques provided high contrast resolution to show regional changes in both cardiac morphology and function associated with ablation.
This preliminary study had several limitations that should be addressed. First, the sample size was relatively small, and MRI examinations were performed serially in only four patients. Additionally, a comparative study of contrast-enhanced MRIs and T2-weighted MRIs obtained in a large population should be performed. We did not measure the left ventricular outflow tract gradient. In our study population, we needed a velocity-encoding value greater than 6 m/sec (based on the gradient value estimated by echocardiography), which was not possible to achieve with our MRI unit. Lastly, the myocardial tagging was not included in our imaging protocol partly because the tag diminished before end systole in the patients with bradycardia. However, a refined tagging technique may help delineate the degree and extent of the changes in septal wall motion after percutaneous transluminal septal myocardial ablation.
In conclusion, black blood T-2 weighted, contrast-enhanced inversion recovery fast gradient-echo, and cine steady-state free precession MRI were useful for evaluating the signal changes in, reduced thickness of, and decreased motion in the septal wall after percutaneous transluminal septal myocardial ablation. These regional myocardial changes and the decrease in left atrial diameter were revealed on cardiac MRI.
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