MRI at 3.0 T can improve resolution and speed in musculoskeletal imaging; however, interactions between field strength and relaxation times need to be considered for optimal image contrast and signal-to-noise ratio. Scanning can be performed in shorter times at 3.0 T using single-average acquisitions. Efficient higher-resolution imaging at 3.0 T can be done by increasing the TR to account for increased T1 relaxation times and acquiring thinner slices than at 1.5 T.

Background and Purpose-Blood oxygen level-dependent MRI (BOLD MRI) of hypercapnia-induced changes in cerebral blood flow is an emerging technique for mapping cerebrovascular reactivity (CVR). BOLD MRI signal reflects cerebral blood flow, but also depends on cerebral blood volume, cerebral metabolic rate, arterial oxygenation, and hematocrit. The purpose of this study was to determine whether, in patients with stenoocclusive disease, the BOLD MRI signal response to hypercapnia is directly related to changes in cerebral blood flow. Methods-Thirty-eight patients with steno-occlusive disease underwent mapping of CVR by both BOLD MRI and arterial spin labeling MRI. The latter technique was used as a reference standard for measurement of cerebral blood flow changes. Results-Hemispheric

Microcirculation oxygen levels and blood volumes should be reflected in measurements of myocardial T 2 relaxation. This work describes the optimization of a spiral imaging strategy for robust myocardial T 2 measurement to minimize the standard deviation of T 2 measurement ( T 2 ). Theoretical and experimental studies of blurring at muscle/blood interfaces enabled the derivation of parameter sets which reduce T 2 to the level of 5%. Differences in myocardial T 2 relaxation should reflect differences in the underlying tissue characteristics. Prolongation of T 2 relaxation is characteristic of infarction, likely reflecting structural degradation and increases in local water content (1). The contributing tissue characteristics within viable myocardium include microcirculation blood volume and oxygen levels. Modulations in these parameters during vasodilation are the basis for myocardial BOLD-MRI (2-5).Myocardium which exhibits prolonged T 2 relaxation may be detected with confidence if the elevation in T 2 relaxation is greater than twice the standard deviation in the relaxation measurement ( T 2 ). Maximizing sensitivity to changes in the relaxation behavior, and thus to the underlying tissue characteristics, requires minimization of T 2 . T* 2 has been proposed as an alternative means for BOLD-MRI. However, the standard deviation in T* 2 ( T* 2 ) across measurements at different spatial locations at the same time point is about 40% (6). Regions of T* 2 nonuniformity correlated with regions of off-resonance. We believe variability in myocardial T 2 relaxation is more controllable, and will therefore yield improved sensitivity to underlying physiology.A magnetization preparation sequence with spiral imaging can provide robust T 2 relaxation measurement in vivo, with considerable insensitivity to off-resonance (4). However, spiral images are prone to blurring, which should lead to partial volume effects across the endocardial border in cardiac applications. Blurring within a spiral imaging strategy will depend on the resolution, the spiral readout duration, the local resonance frequency offset, and residual uncertainty in heart position following motion compensation.There are three goals to this work: 1) the spiral imaging method is optimized to minimize T 2 within a total scan time less than 5 min, while maintaining adequate pervoxel signal-to-noise ratio (SNR); 2) the residual T 2 using this method is characterized in vivo; and 3) the implications of the technical performance are examined using a two-site exchange relaxation model which relates the residual T 2 to a standard deviation in microcirculation oxygen levels ( %O 2 ) (7). MATERIALS AND METHODSAll measurements were performed using a 1.5 T GE Signa CV/i system (4 G/cm peak gradient amplitude, 150 mT/m/s peak slew rate). T 2 relaxation data were acquired using a T 2 -weighted magnetization-preparation spiral imaging protocol which has been validated for coronary sinus oximetry (4). Aspects of the pulse sequence and measurement protocol minimize errors associ...