Feasibility study of myocardial perfusion and oxygenation by noncontrast MRI: comparison with PET study in a canine model

Abstract

The purpose of this study was to examine the feasibility of quantifying myocardial blood flow (MBF) and rate of myocardial oxygen consumption (MVO(2)) during pharmacologically induced stress without using a contrast agent. The former was measured by the arterial spin labeling (ASL) method and the latter was obtained by measuring the oxygen extraction fraction (OEF) with the magnetic resonance imaging (MRI) blood oxygenation level-dependent effect and Fick's law. The MRI results were compared with the established positron emission tomography (PET) methods. Six mongrel dogs with induced acute moderate left coronary artery stenosis were scanned using a clinical PET and a 1.5-T MRI system, in the same day. Regional MBF, myocardial OEF and MVO(2) were measured with both imaging modalities. Correlation coefficients (R(2)) of the three myocardial indexes (MBF, OEF and MVO(2)) between MRI and PET methods ranged from 0.70 to 0.93. Bland-Altman statistics demonstrated that the estimated precision of the limits of agreement between MRI and PET measurements varied from 18% (OEF) to 37% (MBF) and 45% (MVO(2)). The detected changes in these indexes, at rest and during dobutamine stress, were similar between two image modalities. The proposed noncontrast MRI technique is a promising method to quantitatively assess myocardial perfusion and oxygenation.

Figure 1

The PET protocol for all dog studies. Dashed line represents time at rest and solid line represents dobutamine-induced stress.

Figure 2

Examples of ROI locations of MR and PET image. Top row are images obtained at rest and bottom row are images acquired during dobutamine stress. (a) and (e) are T₂ maps for measuring the OEF; (b) and (f) are MBF maps created by the ASL method; (c) and (g) are PET source images to calculate MBF; (d) and (h) are PET images to calculate MVO₂. The scalar bar indicates absolute MRI MBF magnitude in ml/g/min. The anterior (ANT) ROI is in the stenotic vessel-perfused bed and the inferior (INF) ROI is in the remote normal vessel-perfusion bed. Lateral (LAT) regions were also indicated. In all PET images, the signal intensities in each source image were normalized to its own maximal intensity of the image. Therefore, they are not comparable to each other.

Figure 3

(a) Correlation of MBF measured by MR ASL and PET methods; (b) Bland-Altman plot of the percent-differences in these MBF values versus the averaged MBF values. No significant difference was found between MRI and PET methods.

Figure 4

The averaged (a) myocardial MBF, (b) OEF, and (c) MVO₂ measured with MRI and PET methods, at rest and during the dobutamine stress (OEF only during dobutamine). Both methods show similar changes in these myocardial indexes. There were no significant differences between MRI and PET on any of the three parameters (MBF, OEF, or MVO₂).

Figure 5

Bland-Altman plots of the percent-differences of myocardial OEF measured by MRI and PET methods verses their averaged values during the dobutamine stress, (a) when the myocardial OEF was assumed to be 0.6 at rest for MRI stress OEF calculation; and (c) when the myocardial rest OEF was taken from PET measurement values. MRI values during stress were slightly underestimated in (a) (bias = 1.1%) and more drastically underestimated in (b) (bias = 6.5%). There are no significant differences between PET and either MRI methods. The second MRI method demonstrates slightly increased correlations from respective regression lines in (d) than (b). There is no significant difference between the two MRI OEF calculations.

Figure 6

Bland-Altman plots of the percent-differences of MVO₂ measured by MRI and PET verses their averaged values, at rest and during the dobutamine stress, (a) when the myocardial OEF was assumed to be 0.6 at rest for MRI calculation; and (c) when the myocardial OEF at rest was taken from PET measurement values. MRI values were underestimated in (a) (bias = -16.4%) and less underestimated in (b) (bias = -4.7%). The correlation was slightly improved when myocardial OEF at rest was estimated from PET data in (d) than in (b) when myocardial OEF at rest was fixed at 0.6. There is no significant difference between two sets of MRI MVO₂ data.