Principles of magnetic resonance imaging.

This article aims to provide an educational document of magnetic resonance imaging principles for applied biomedical users of the technology. Basic principles are illustrated using simple experimental models on a preclinical imaging system.

MRI relaxation fluctuations in acute reperfused hemorrhagic infarction.

MRI evaluations of intramyocardial hemorrhage in acute infarction have relied on T(2) and T(2)(*) shortening only. We propose a more comprehensive evaluation of hemorrhagic infarction based on the concept that fluctuations in T(2) and T(1) relaxation in acute reperfused infarction will reflect transient edema and hemoglobin oxidative denaturation to uncompartmentalized methemoglobin. Anteroapical infarction was created via percutaneous balloon in young swine (22-25 kg, N = 12). T(2), T(1), diastolic wall thickness (DWT), and the Gd-DTPA partition coefficient (lambda) were measured on days 0, 2, and 7. DWT was elevated at 1 hr postreperfusion (128% +/- 53%, P = 0.0001), and alleviated on days 2 and 7 (48% +/- 10%, P = 0.008; 53% +/- 24%, P = 0.003). T(2) and T(1) elevations were coincident with early edema (DeltaT(2) = 55% +/- 24%, P < 0.0001; DeltaT(1) = 27% +/- 18%, P < 0.04). T(2) and T(1) were nearly normal on day 2 (DeltaT(2) = 8% +/- 8%, P = 0.27; DeltaT(1) = 0% +/- 1%, P = 0.65). On day 7, T(2) increased while T(1) decreased (DeltaT(2) = 27% +/- 16%, P = 0.005; DeltaT(1) = -14% +/- 10%, P = 0.02). Lambda was elevated by >150% at all time points (P < or = 0.002). Histology verified hemorrhagic injury. T(1) and T(2) fluctuations are consistent with transient edema, as well as hemoglobin oxidative denaturation to decompartmentalized methemoglobin. This methodological development may broaden our understanding of hemorrhagic microvascular injury and improve its detection in clinical populations.

T2 fluctuations in ischemic and post-ischemic viable porcine myocardium in vivo.

T2 relaxation can augment delayed-enhancement viability imaging because it is sensitive to tissue edema and microcirculatory oxygen state. We demonstrate the T2 'signatures' of sub-lethal ischemia and stunning in porcine myocardium perfused by the distal left anterior descending artery, by imaging during percutaneous balloon occlusion for 25 minutes and subsequent reperfusion (n = 9). Muscle displayed ischemic dysfunction and partial post-ischemic functional recovery (p < or = 0.0004), concommitant with an elevated post-ischemic T2 (deltaT2 = 27 +/- 18%, p = 0.005). TTC staining verified muscle viability. The T2 fluctuations may reflect hyperemia and tissue cellular edema in accord with the known pathophysiology of ischemic and post-ischemic yet viable muscle.

Coronary venous oximetry using MRI.

Based on the Fick law, coronary venous blood oxygen measurements have value for assessing functional parameters such as the coronary flow reserve. At present, the application of this measure is restricted by its invasive nature. This report describes the design and testing of a noninvasive coronary venous blood oxygen measurement using MRI, with a preliminary focus on the coronary sinus. After design optimization including a four-coil phased array and an optimal set of data acquisition parameters, quality tests indicate measurement precision on the order of the gold standard optical measurement (3%O(2)). Comparative studies using catheter sampling suggest reasonable accuracy (3 subjects), with variability dominated by sampling location uncertainty ( approximately 7%O(2)). Intravenous dipyridamole (5 subjects) induces significant changes in sinus blood oxygenation (22 +/- 9% O(2)), corresponding to flow reserves of 1.8 +/- 0.4, suggesting the potential for clinical utility. Underestimation of flow reserve is dominated by right atrial mixing and the systemic effects of dipyridamole. Magn Reson Med 42:837-848, 1999.

T2 accuracy on a whole-body imager.

MR oximetry requires a T2 measurement that is accurate within 5% in vivo. Simple methods are susceptible to signal loss and tend to underestimate T2. Current methods utilize RF pulses or RF cycling patterns that prevent signal loss at each data acquisition. However, using these methods with imperfect pulses, T2 tends to be overestimated due to temporary storage of the magnetization along the longitudinal axis where it decays more slowly with a time constant T1 > T2. To reduce the T1 dependence while preventing signal loss, we utilize simple 90x180y90x composite pulses and good RF cycling patterns. These trains are critical for T2 accuracy over typical ranges of RF and static field inhomogeneities and refocusing intervals. T1 signal decay during each 90x180y90x pulse must be accounted for to yield accuracy within 5% when the pulse-width is 10% or more of the refocusing interval. A simple correction scheme compensates for this T1-related error effectively.