[Rationale and limits of echo planar magnetic resonance imaging]

The use of Echo Planar Imaging (EPI) techniques has extensively increased in the last few years. The technical improvements in new Magnetic Resonance Imaging (MRI) systems satisfy the instrumental requirements for these complex high-speed methods. The latest commercial systems are provided with standard EPI sequences. The increased data acquisition speed which characterizes EPI opens new and interesting perspectives, although the resolution is lower than the one typically achieved with conventional imaging.

The description of the data sampling process in terms of the k-space is a powerful tool for planning or understanding complex sequence diagrams like EPI. The basic concepts of the method are reported and discussed, pointing out the most noteworthy feature of the k-space, i.e., its Fourier inverse yield to the image. A generic pulse sequence has been considered to explain the sampling of k-space trajectories. Examples of ultrafast imaging techniques are subsequently considered and discussed.

EPI improves the resolution time of MRI, which is an intrinsically slow imaging procedure. Images can be obtained in few tenth of a second so that MRI suits the short temporal range of many interesting physiological processes. Recently the main goal of functional Magnetic Resonance Imaging (fMRI) has been the study of brain activity related to local changes in brain hemodynamics. These changes cause MR signal intensity variations. In the last years, several brain activity studies have been performed by means of Positron Emission Tomography (PET) which involves the administration of radioactive substances; fMRI has the advantage that it is a noninvasive technique, although peripheral nerve stimulation effects should be considered.