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Endocardial to epicardial perfusion ratios at rest and stress determined by perfusion-CMR
Journal of Cardiovascular Magnetic Resonancevolume 11, Article number: P6 (2009)
Animal experiments using labelled microspheres have shown that at rest, blood flow to the subendocardial layer is higher than to the subepicardium. With increasing levels of stress this transmural gradient of myocardial blood flow is reduced, so that the endocardium has a lower perfusion reserve than the epicardium . The causes for this observation include higher compressive forces and higher resting metabolic activity in the endocardium. If microvascular function is impaired, endocardial perfusion reserve is reduced further .
Myocardial perfusion-CMR is usually performed with coverage of several myocardial sections to allow detection of ischemic perfusion defects. Consequently, compromises regarding image quality and motion artefact are made. For the study of global physiological phenomena and diffuse myocardial disease, optimised acquisition of a single section may be more useful.
1. To develop a first pass myocardial perfusion method optimised for acquisition of a single midventricular myocardial section at systole and diastole.
2. To compare rest and stress myocardial perfusion between the endocardium and epicardium.
3. To compare rest and stress myocardial perfusion at mid-diastole and mid-systole.
10 volunteers (7 male, mean age 38 years) were studied on a 1.5 T Philips Intera system during adenosine stress (140 mcg/kg/min for 3 minutes) and at rest. For each perfusion acquisition 0.05 mmol/kg Gd-DTPA was administered with a power injector followed by a 20 ml Saline flush (5 ml/sec).
A saturation recovery segmented gradient echo perfusion method with twofold SENSE was optimised for imaging of a single cardiac section by timing the acquisition to a phase with minimal cardiac motion and by optimising the preparation pulse delay. Pulse sequence parameters were as follows: TR/TE/flip 2.7 ms/1.0/15°, FOV 380 × 380 mm, matrix 160 × 160, slice thickness 10 mm, preparation pulse delay (to middle of k-space) 150 ms, shot duration 130 ms. With the use of a software patch trigger delay for the acquisition of one midventricular slice could be individually to mid-systole, as determined on a high temporal resolution cine scout. A mid-diastolic phase in the same plane (adjusted for through-plane cardiac motion) was acquired if heart rate permitted.
Endo and epicardial contours were drawn (MASS, Medis, Leiden, The Netherlands) and the slice segmented into 6 equidistant sectors. These were further subdivided into a subepicardial, mid-myocardial and subendocardial third. The maximal myocardial upslope of the signal-intensity time profiles for each sector and the three layers was calculated. Then the ratio of upslopes between the endocardial and epicardial layers ("endo-epi ratio") was computed.
In systole, the mean (+/- SD) of the endo-epi ratio of all segments was 1.13 (+/- 0.1) at rest and 0.94 (+/- 0.1) at stress. In all segments the endo-epi ratio was higher at rest than stress, albeit with regional differences (Fig 1). In diastole, the mean endo-epi ratio of all segments was similar to systole at rest (1.13 +/- 0.2, p > 0.05 versus systole), but at stress it was significantly higher than at systole (1.08 +/- 0.1, p = 0.005). The mean upslope for all three layers combined however was similar between systole and diastole at rest and stress (17.9 vs 17.2 at stress and 8.0 vs 7.9 at rest, p > 0.05).
1. Differences in endocardial and epicardial perfusion can be detected with CMR in vivo.
2. Perfusion to the endocardial layer is higher at rest, with a diminishing endo-epi ratio at stress, consistent with known physiology.
3. Perfusion to whole myocardial segments is similar in systole and diastole. In diastole differences between layers of the myocardium are less than in systole, probably because of partial volume effects.
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