The invention relates to a method for magnetic resonance imaging. In magnetic resonance imaging (MRI), pulse sequences including RF and magnetic field gradient pulses are applied to an object (a patient) to generate magnetic resonance signals, which are scanned in order to obtain information therefrom and to reconstruct images of the object. Since its initial development, the number of clinical relevant fields of application of MRI has grown enormously. MRI can be applied to almost every part of the body, and it can be used to obtain information about a number of important functions of the human body. The pulse sequence which is applied during an MRI scan determines characteristics of the reconstructed images, such as location and orientation in the object, dimensions, resolution, signal-to-noise ratio, contrast, sensitivity for movements, etcetera. An operator of a MRI device chooses the appropriate sequence and adjusts and optimizes its parameters for the respective application.
T2-contrast enhanced spin echo sequences are the most widely used scans in clinical application because they provide exquisite soft tissue contrast. However, their utility is limited by their long acquisition time. This is why fast T2-contrast enhanced sequences have been the object of recent developments. Known techniques, which are capable of producing T2-weighted images within a few seconds, are spin echo sequences like the so-called RARE (rapid acquisition by repeated echoes) and TSE (turbo spin echo) sequences. The known spin echo sequences usually consist of an initial contrast preparation period and a subsequent data acquisition period. During the first period the longitudinal magnetization is prepared according to the desired contrast while many phase-encoded echoes are generated and acquired during the second period to form the image.
A T2-contrast enhanced MRI method which is particularly useful for imaging of tissue with a high T2/T1 ratio is known from the U.S. Pat. No. 6,219,571 B1. This technique uses a so-called driven equilibrium Fourier transform (DEFT) sequence, which enhances signal strength without waiting for full T1 recovery. DEFT imaging provides particularly high contrast between tissues with different values of T2 while maintaining a high signal to noise ratio. The typical DEFT pulse sequence begins with a slice-selective RF excitation pulse (αX) followed by a plurality of 180°Y refocusing pulses resulting in phase encoded spin echoes, which are used for imaging. At the last spin echo, a “driven equilibrium” (or simply “drive”) pulse (−αX) transforms the remaining transverse magnetization into positive longitudinal magnetization. An optional gradient pulse at the end of the sequence might help to spoil residual transverse magnetization. This pulse sequence is then repeated after a recovery period which is shorter than the T1 of the examined tissue.
It is well known that also the generation of high quality T1-contrast enhanced images is essential for many MRI applications. This is particularly valid for the examination of the human brain. For example, the significant difference between the longitudinal relaxation times of white matter tissue and cerebra spinal fluid can be utilized to produce images with high contrast between these two materials. The main drawback of the well-known inversion recovery (IR) techniques, which are routinely employed for T1-weighted imaging, is again their comparatively long acquisition time. Adequate and efficient T1-weighted imaging is especially challenging at high magnetic fields of 3 Tesla or more, as a significant increase of T1 is then observed.
Therefore it is readily appreciated that there is a need for MRI methods which enable a fast T1-contrast enhanced imaging. It is consequently the primary object of the present invention to provide a fast and reliable method for T1-weighted imaging, which gives a high T1 contrast and also a sufficient signal-to-noise ratio.