Magnetic resonance imaging (MRI) refers to an imaging technique for visualization of primarily tissue structure and function. The strength of the magnetic resonance (MR) signal depends on the relative density of excited nuclei, on relaxation properties (longitudinal T1 and transversal T2 relaxation), on the signal upon excitation and other parameters such as, but not limited to, flow and chemical shifts. As a result, the contrast in an MR image is per se a mixture of all these effects and would, if used as such, lack diagnostic specificity. However, dedicated imaging sequences can emphasize one specific effect, such as the transverse relaxation time (T2), yielding a so called T2-weighted image, a term that already insinuates that the contrast in the image is dominated by T2. It is, however, also evident from the term that in such an image only the contrast in the image is weighted by the specific tissue property but that it does not represent an actual measure of this specific tissue property. As a result, MRI often provides only a qualitative imaging technique that is focused on the analysis of either hyper- or hypo-intense appearing blobs, which represent tissue abnormalities. In contrast, a real quantification of biochemical or biophysical tissue parameters offers many advantages, in particular, it allows quantifying alterations in biological tissue resulting from disease or a particular treatment regime. Quantitative rather than qualitative imaging also provides the advantage that the measurements are independent of the MRI system used, non-biased, reproducible and/or offer much less room for interpretation. Although quantification is in many ways superior to qualitative MRI, methods and apparatuses available require much more MRI scanner time and data analysis can be very time consuming. As a result, fast, reliable and easy methods for quantitative MRI are highly desirable.
The publications and other materials, including patents, used herein to illustrate the invention and, in particular, to provide additional details respecting the practice are incorporated herein by reference. For convenience, the non-patent publications are referenced in the following text by author and date and are listed in the appended bibliography.
The image contrast as achieved with available MR imaging sequences generally depends heavily on either the longitudinal (T1) or the transverse relaxation (T2) time. Since most disease related tissue alterations, such as, but not limited to, inflammations, result in a modification of the transverse relaxation time, T2 is one of the most important tissue parameter and thus fast and accurate methods for measuring T2 are clinically of great importance (Boulby & Rugg-Gunn, 2003). State-of-the-art T2 quantification techniques are usually based on two dimensional (2D) single or multi spin-echo (SE) sequences (Boulby & Rugg-Gunn, 2003). Although the SE-based methods yield accurate estimates of T2 in combination with good anatomical information, they suffer from long acquisition times, making it clinically unattractive. Other quantitative T2 imaging techniques make use of fast imaging sequences, such as steady-state free precession (SSFP) techniques, where the steady-state signal is a weighted combination of T1 and T2 (Deoni et al., 2003). SSFP-based quantification of T2 is thus mainly interesting in combination with a measurement of T1, since this information is practically needed. Furthermore, only recently, systematic deviations in the assessed T2 and T1 values from the true T1 and T2 tissue parameters were observed for SSFP-based methods due to magnetization transfer effects arising from the methodologically required variation in the excitation angle (Crooijmans, et al, 2010; in press). As a result, a T1 and SSFP imaging sequence parameter related independent quantitative T2 imaging technique is highly desirable.
In view of the above, it is apparent that there exists a need in the art for imaging methods and/or apparatus which solve or at least ameliorate one or more of the above drawbacks of the prior art. It is a purpose of this invention to fulfill this need in the art as well as other needs which will become more apparent to the skilled artisan once given the following disclosure.