[Show abstract][Hide abstract] ABSTRACT: The Single Point Ramped Imaging with T1 Enhancement (SPRITE) sequence is well suited for the acquisition of magnetic resonance signals from fast relaxing nuclei and from heterogeneous materials. However, it is time inefficient compared to sequences that are based on frequency encoding because only one single point is acquired per excitation. Multiple-point SPRITE (mSPRITE) mitigates this problem with the acquisition of multiple FID points. mSPRITE images reconstructed from early FID samples suffer from reduced spatial resolution due to the limited extent of its corresponding k-space. In this work we present a new reconstruction algorithm for spatial resolution enhancement that solves this problem without changes to the mSPRITE sequence. The method, called Multi-Frame mSPRITE, substitutes high spatial frequencies from late FID points into k-spaces of limited extent constructed from early FID points. In this way, images of high quality and resolution can be obtained despite a large range of zoom factors used to reconstruct images with the same FOV and resolution.
Full-text · Article · Feb 2013 · Journal of Magnetic Resonance
[Show abstract][Hide abstract] ABSTRACT: Residual magnetisation is one of the major sources of artefacts in single point imaging sequences with short repetition times. The unwanted signal is caused by non-dephased transverse magnetisation excited in preceding acquisition cycles. Therefore, the problem emerges mainly around the centre of k-space and has been solved in the past by additional spoiling gradients. In this work, unwanted residual magnetisation acquired with the SPRITE sequence was investigated and a new method for the suppression of residual magnetisation is presented. It is shown that residual magnetisation experiences a different phase encoding leading to residual images with a different FOV. A phase cycling filter is able to eliminate the unwanted signal. Furthermore, a description of all signal components that occur is presented using an operator notation. The notation is new in this field with respect to its completeness. That is, the signal description is based on an understanding of single point imaging sequences, such as SPRITE, by the use of an extended phase encode graph. A prominent in vivo example is that of sodium imaging in biological tissue where transverse relaxation times are such that unwanted coherences can occur and therefore residual magnetisation becomes a significant problem. For instance, sodium in biological tissue has two transverse relaxation times of approximately 3ms and 15ms at 4T and this can result in significant artefacts if the encoding time is short and TR<3ms.
No preview · Article · Aug 2009 · Journal of Magnetic Resonance
[Show abstract][Hide abstract] ABSTRACT: A new algorithm is proposed for computing the discrete Fourier Transform (DFT) of purely phase encoded data acquired during Magnetic Resonance Imaging (MRI) experiments. These experiments use the SPRITE (Single Point Ramped Imaging with T1 Enhancement) method and multiple-point acquisition, sampling data in a nonuniform manner that prohibits reconstruction by fast Fourier transform. The chirp z-transform algorithm of Rabiner, Schafer, and Rader can be combined with phase corrections to compute the DFT of this data to extremely high accuracy. This algorithm outperforms the interpolation methods that are traditionally used to process nonuniform data, both in terms of execution time and in terms of accuracy as compared to the DFT.
No preview · Article · Aug 2007 · ACM Transactions on Mathematical Software
[Show abstract][Hide abstract] ABSTRACT: A version of the chirp z-transform (CZT) enabling signal intensity and phase-preserving field-of-view scaling has been programmed. The algorithm is important for all single-point imaging sequences such as SPRITE when used with multiple data acquisition for T2* mapping or signal averaging. CZT has particular utility for SPRITE imaging of nuclei with short relaxation times such as sodium at high field. Here, a complete theory of the properties of CZT is given. This method operates entirely in k-space. It is compared with a conventional interpolation approach that works in image space after the application of a fast Fourier transformation.
Full-text · Article · Feb 2006 · Journal of Magnetic Resonance
[Show abstract][Hide abstract] ABSTRACT: We present two methods for the mapping of the T1 and T2* relaxation times. A recently published sequence, T1 mApping with Partial Inversion Recovery (TAPIR), is used for T1 mapping in a cohort of patients suffering from hepatic encephalopathy (HE). TAPIR enables the fast and precise measurement of T1 with whole brain coverage in approximately 5 min, which makes it a favourable tool for clinical applications. A clear correlation between HE severity and T1 measured with TAPIR is demonstrated for several brain regions. Patients and control subjects could be classified solely based on their T1 pattern by making use of unsupervised pattern recognition tools. In addition, a newly developed sequence termed QUanTitativE-EPI (QUTE-EPI) is presented for the fast and precise mapping of T2* without significant image distortions. The information provided by TAPIR and QUTE-EPI can be used to extract an absolute measure of water content in vivo by MRI. The procedure of H2O content mapping is described and results are presented. We demonstrate that in vivo water measurement is possible in clinically relevant measurement times with a statistical and systematic measurement error
No preview · Article · Aug 2004 · International Congress Series
[Show abstract][Hide abstract] ABSTRACT: In this work, we present a modified, EPI-based sequence referred to as quantitative echo-planar imaging (“Qute-EPI”). Some applications of this highly flexible sequence, such as genuine T2* mapping, phase evolution mapping, and distortion correction in EPI, are discussed.
No preview · Article · Aug 2004 · International Congress Series
[Show abstract][Hide abstract] ABSTRACT: A version of a chirp z-transform (1) was programmed enabling phase-preserving FOV scaling for data sets with the zero of k-space in the middle. The method is important for all single-point imaging (SPI) sequences (2,3) such as SPRITE when used with multiple data acquisition for T2