SPRITE Imaging
Although imaging of sodium in vivo is very demanding due to its low MR sensitivity and the fast, biexponential signal decay of this nucleus, it is desirable because changes in sodium concentrations in the brain can be indicative of a number of diseases, including Alzheimer’s disease.
A number of studies have shown that in order to detect the total sodium signal of tissues, sequences with ultra-short echo times are essential (TE <0.5 ms). This is the case in twisted projection imaging (TPI) and radial imaging techniques, which make use of ultra-short echo times and have been used to measure sodium concentrations in musculoskeletal pathologies, the heart and in the brain.
A novel approach for the in vivo measurement of sodium is the SPRITE (Single Point Ramped Imaging with T1 Enhancement) sequence: a pure phase-encoding MRI technique first developed to obtain MR images of systems with ultra-short signal decays (<100us).
In SPRITE, a short, broadband, RF pulse is applied in the presence of phase encode gradients and a single k-space point is sampled after an ultra-short encoding time. The k-space is then scanned point-by-point at predefined Cartesian locations by changing the magnitude of the gradients for each repetition time. The final image is simply obtained by the application of a standard fast fourier transform (FFT) of the acquired k-space data.
Images obtained with SPRITE are free of artefacts originating from B0 inhomogeneities and chemical shifts, and do not suffer from blurring due to the convolution with signal decays. Furthermore, the gradient step-by-step switching scheme makes this imaging method practically inaudible, hence it is particularly comfortable for volunteers and patients. SPRITE imaging, however, requires long acquisition times, high gradient duty cycles (especially in the case of sodium), and may lead to high specific absorption rates (SAR).
For the in vivo application of the original SPRITE sequence a number of novel methods, such as the use of centric trajectories and the variation of the repetition time, TR, as well as the RF flip angle, α, as a function of the k-space sampling location, and the acquisition of multiple samples at each k-space location have been developed in our lab. These features lead to a reduction of the acquisition time as well as SAR.
Furthermore, the acquisition of multiple points at each k-space position enables the measurement of consecutive images containing time information. The different timing of the k-space pointsets leads to a decreasing k-space density of consecutive pointsets, resulting in a different FOV for each image. Thus, special reconstruction methods are necessary to correct the images to a common FOV combined with an intensity scaling of each voxel. For this purpose, a Chirp-Z transformation (CZT) is used as an extension of the FFT, employing frequency cutoffs to achieve the scaling. Another method, called the multi-frame method, adds missing k-space information to datasets aquired early in the process by adding information from the outer k-space regions acquired later, thus affording bigger, k-spaces. The latter method offers an increased resolution for images with higher scaling factors.
Due to the multi-point acquisition, the time information gained can be used to analyse the decay of different voxels and thus, to segment faster relaxing tissues, such as bone, out of 1H in-vivo images.
