Introduction

Late Gadolinium enhancement (LGE) MRI has become the gold standard for evaluating myocardial infarction (MI) ¹. However, contrast injection is not permitted in patients with renal insufficiency, contrast agent alley or other reasons. Diffusion-weighted imaging (DWI) has been used to detect acute stroke ², and has also shown promising results for diagnosing ischemic or non-ischemic cardiac diseases^3-7. So far, various DWI models have been introduced to extract diffusion parameters. The conventional single-exponential apparent diffusion coefficient (ADC) model assumes Gaussian diffusion motion, while the more recent stretched exponential model requires no assumption of Gaussian distribution or tissue compartmentalization and may better reflect tissue characteristics⁸. However, applicability of these DWI models in the assessment of acute MI remains uninvestigated. The purpose of this study is to investigate retrospectively the diagnostic performance of the ADC and stretched exponential DWI models in the evaluation of acute myocardial infarction.

Methods

MR imaging: Ten patients (9 Males, mean age: 56) presenting with recent MI and without any contradictions to LGE MRI were included in this study. The patients were scanned on 3.0T Philips MR scanner with a 32-channel body coil. Cardiac DWI was performed in the short-axis view with a single shot EPI sequence under ECG gating and respiratory navigator. Eight signal averages of images with three diffusion encoding directions were obtained. Other parameters included: TR=3RR intervals, TE=61ms; FA=90°; voxel size=3.5x3.5x8.0mm; bandwidth=2371Hz/pixel; monopolar preparation with b-values of 0, 10, 20, 50, 100, 150, 200, 250 and 500 s/mm². LGE MRI was also performed to detect the infarcted region. The study was approved by the hospital’s Institutional Review Board and all subjects provided written informed consent.
Image analysis: The DWI images were analyzed to obtain myocardial diffusion parameters using the mono-exponential and stretched exponential models. The ADC was calculated using all b values by fitting the mono-exponential model: $$$S(b)/S(0) =exp⁡(-b·ADC)$$$, where $$$S(b)$$$ is the signal intensity at a given b value and $$$S(0)$$$ represents the signal without any diffusion weighting, $$$b$$$ is the b-factor. Using a stretched exponential model (SEM): $$$S(b)/S(0) =exp[(-b·DDC)]^α$$$, the distributed diffusion coefficient (DDC) representing the mean intravoxel diffusion rate and the water molecular diffusion heterogeneity index (α) were calculated using non-linear regression method. After obtaining the diffusion parametric maps, ROIs were drawn for the infarcted region and the remote healthy myocardium in the middle short-axis slice according to the LGE images. Care was taken to not include regions with motion artifacts. The mean and standard variation were calculated for each ROI.
Statistical analysis: The mean DWI parameters obtained using the two DWI models were compared between the infarcted and healthy myocardium using nonparametric Mann-Whitney test because of the small number of patients. A p-value less than 0.05 was considered statistically significant.

Results

Example LGE image and DWI image with b value of 50 s/mm² as well as the DWI parametric maps for a typical patient with acute MI are shown in Fig. 1. The DWI image shows high signal in the septal left ventricular wall which is the infarcted region as evident by the LGE image. Obviously lower values were observed in the ADC and DDC maps for the MI region, while shows higher values. The comparison of the DWI model parameters for all 10 patients are shown Fig. 2. ADC (1.946±0.452 ×10^-3mm²/s vs. 2.452±0.331 ×10^-3mm²/s, p=0.014) and DDC (1.395±0.514 ×10^-3mm²/s vs. 2.327±0.384 ×10^-3mm²/s, p<0.001) are significantly lower in the MI region than in the remote normal myocardium, while tends to be higher in the MI but the difference was not significant (0.6674±0.1206 ×10^-3mm²/s vs. 0.5681±0.1902 ×10^-3mm²/s, p=0.1801).

Discussion

This study showed that the DWI parameters from the conventional ADC model and the stretched exponential model both had capability of diagnosing acute myocardial infarction without contrast administration. The conventional ADC model assumes Gaussian diffusion motion, while the stretched exponential model adds an additional parameter to consider the heterogenous diffusion in each voxel⁸, which reflects more realistic physiological characteristics and should perform better in differentiating the infarcted and healthy myocardium. Therefore, more patients with acute MI will be recruited to compare the diagnostic performance of the ADC model and the stretched exponential model.

Acknowledgements

No acknowledgement found.

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