The purpose of these studies is to develop methods for non-invasive evaluation of perfusion and blood oxygenation, and to perfect real-time imaging of rapidly moving organs such as the heart. The basic experimental tool is the use of Echo-Planar Imaging (EPI), a type of magnetic resonance imaging which acquires images in 0.1 s or less, much faster than conven-tional MRI. The images can be made sensitive, through the use of injected intravascular NMR contrast agents, to spatial variations in blood flow. As a further benefit the images, without contrast agent, reflect changes in blood oxygenation, since deoxyhemoglobin is highly paramagnetic. Images can be obtained at a rate of up to 1 per second. In the past year we have mainly been engaged on studies of cerebral perfusion and blood oxygenation. We have used a monkey visual stimulation model to assess the sensitivity of MRI dynamic contrast agent studies to changes in brain activation, and a cat respiratory challenge model to observe the time course of imbalances of oxygen supply and utilization in brain tissue. The following findings have been made. 1) Qualitative blood volume maps of single slices of monkey brain show good contrast between grey and white matter arising from the differential blood supply to these tissues. 2) Such maps show variability associated with the level of anesthesia of the animal, and blood volume is clearly increased when the animal is hypercapnic. 3) Short periods (60 s or less) of respiratory anoxia cause a loss of signal of up to 10% in gradient-echo images of cat brain, the changes being largest in more highly perfused tissue regions. 4) Temporary apnea also causes changes in brain tissue signal, the largest changes (an increase in signal) occurring after normal breathing has recommenced.
