Sandro Romanzetti

Advances in multimodal neuroimaging: hybrid MR-PET and MR-PET-EEG at 3 T and 9.4 T.

Multi-modal MR-PET-EEG data acquisition in simultaneous mode confers a number of advantages at 3 T and 9.4 T. The three modalities complement each other well; structural-functional imaging being the domain of MRI, molecular imaging with specific tracers is the strength of PET, and EEG provides a temporal dimension where the other two modalities are weak. The utility of hybrid MR-PET at 3 T in a clinical setting is presented and critically discussed. The potential problems and the putative gains to be accrued from hybrid imaging at 9.4 T, with examples from the human brain, are outlined. Steps on the road to 9.4 T multi-modal MR-PET-EEG are also illustrated. From an MR perspective, the potential for ultra-high resolution structural imaging is discussed and example images of the cerebellum with an isotropic resolution of 320 μm are presented, setting the stage for hybrid imaging at ultra-high field. Further, metabolic imaging is discussed and high-resolution images of the sodium distribution are presented. Examples of tumour imaging on a 3 T MR-PET system are presented and discussed. Finally, the perspectives for multi-modal imaging are discussed based on two on-going studies, the first comparing MR and PET methods for the measurement of perfusion and the second which looks at tumour delineation based on MRI contrasts but the knowledge of tumour extent is based on simultaneously acquired PET data.

Optimised in vivo visualisation of cortical structures in the human brain at 3 T using IR-TSE.

The primary visual cortex in humans can be identified using magnetic resonance imaging (MRI) in vivo by detection of the stria of Gennari. To fully characterize this area, high spatial resolution is essential, including the use of very thin image slices to avoid loss of definition due to partial volume effects. A three-dimensional magnetization-prepared turbo spin-echo sequence, with appropriate parameter optimization, provided high-resolution imaging (0.4 x 0.4 x 0.5 mm3) on a clinical 3-T scanner with adequate contrast to noise ratio. These images allowed visualisation of the stria of Gennari in every slice of a volume covering most of the occipital cortex, in each of six healthy volunteers. The effective longitudinal relaxation time was measured with the isotropic resolution turbo spin echo sequence and found to be substantially shorter than values measured with a dedicated relaxometric sequence. The shortening was attributed to magnetization transfer effects, as supported by the investigation of its slab and turbo-factor dependence.

Simultaneous Single-Quantum and Triple-Quantum- Filtered MRI of 23Na (SISTINA)

The low MR sensitivity of the sodium nucleus and its low concentration in the human body constrain acquisition time. The use of both single‐quantum and triple‐quantum sodium imaging is, therefore, restricted. In this work, we present a novel MRI sequence that interleaves an ultra‐short echo time radial projection readout into the three‐pulse triple‐quantum preparation. This allows for simultaneous acquisition of tissue sodium concentration weighted as well as triple‐quantum filtered images. Performance of the sequence is shown on phantoms. The method is demonstrated on six healthy informed volunteers and is applied to three cases of brain tumors. A comparison with images from tumor specific O‐(2‐[18F]fluoroethyl)‐L‐tyrosine positron emission tomography and standard MR images is presented. The combined information of the triple‐quantum‐filtered images with single‐quantum images may enable a better understanding of tissue viability. Future studies can benefit from the evaluation of both contrasts with shortened acquisition times. Magn Reson Med, 2013.

Mapping tissue sodium concentration in the human brain: A comparison of MR sequences at 9.4 Tesla

Sodium is the second most abundant MR-active nucleus in the human body and is of fundamental importance for the function of cells. Previous studies have shown that many pathophysiological conditions induce an increase of the average tissue sodium concentration. To date, several MR sequences have been used to measure sodium. The aim of this study was to evaluate the performance and suitability of five different MR sequences for quantitative sodium imaging on a whole-body 9.4Tesla MR scanner. Numerical simulations, phantom experiments and in vivo imaging on healthy subjects were carried out. The results demonstrate that, of these five sequences, the Twisted Projection Imaging sequence is optimal for quantitative sodium imaging, as it combines a number of features which are particularly relevant in order to obtain high quality quantitative images of sodium. These include: ultra-short echo times, efficient k-space sampling, and robustness against off-resonance effects. Mapping of sodium in the human brain is a technique not yet fully explored in neuroscience. Ultra-high field sodium MRI may provide new insights into the pathogenesis of neurological disorders, and may help to develop new and disease-specific biomarkers for the early diagnosis and therapeutic intervention before irreversible brain damage has taken place.

Studying variability in human brain aging in a population-based German cohort—rationale and design of 1000BRAINS

The ongoing 1000 brains study (1000BRAINS) is an epidemiological and neuroscientific investigation of structural and functional variability in the human brain during aging. The two recruitment sources are the 10-year follow-up cohort of the German Heinz Nixdorf Recall (HNR) Study, and the HNR MultiGeneration Study cohort, which comprises spouses and offspring of HNR subjects. The HNR is a longitudinal epidemiological investigation of cardiovascular risk factors, with a comprehensive collection of clinical, laboratory, socioeconomic, and environmental data from population-based subjects aged 45–75 years on inclusion. HNR subjects underwent detailed assessments in 2000, 2006, and 2011, and completed annual postal questionnaires on health status. 1000BRAINS accesses these HNR data and applies a separate protocol comprising: neuropsychological tests of attention, memory, executive functions and language; examination of motor skills; ratings of personality, life quality, mood and daily activities; analysis of laboratory and genetic data; and state-of-the-art magnetic resonance imaging (MRI, 3 Tesla) of the brain. The latter includes (i) 3D-T1- and 3D-T2-weighted scans for structural analyses and myelin mapping; (ii) three diffusion imaging sequences optimized for diffusion tensor imaging, high-angular resolution diffusion imaging for detailed fiber tracking and for diffusion kurtosis imaging; (iii) resting-state and task-based functional MRI; and (iv) fluid-attenuated inversion recovery and MR angiography for the detection of vascular lesions and the mapping of white matter lesions. The unique design of 1000BRAINS allows: (i) comprehensive investigation of various influences including genetics, environment and health status on variability in brain structure and function during aging; and (ii) identification of the impact of selected influencing factors on specific cognitive subsystems and their anatomical correlates.

B0 insensitive multiple-quantum resolved sodium imaging using a phase-rotation scheme.

Triple-quantum filtering has been suggested as a mechanism to differentiate signals from different physiological compartments. However, the filtering method is sensitive to static field inhomogeneities because different coherence pathways may interfere destructively. Previously suggested methods employed additional phase-cycles to separately acquire pathways. Whilst this removes the signal dropouts, it reduces the signal-to-noise per unit time. In this work we suggest the use of a phase-rotation scheme to simultaneously acquire all coherence pathways and then separate them via Fourier transform. Hence the method yields single-, double- and triple-quantum filtered images. The phase-rotation requires a minimum of 36 instead of six cycling steps. However, destructive interference is circumvented whilst maintaining full signal-to-noise efficiency for all coherences.

Multi-Frame SPRITE: a method for resolution enhancement of multiple-point SPRITE data.

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.

An accurate nonuniform fourier transform for SPRITE magnetic resonance imaging data

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.

Phase-cycled averaging for the suppression of residual magnetisation in SPI sequences

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.

Repetition time and flip angle variation in SPRITE imaging for acquisition time and SAR reduction

Single point imaging methods such as SPRITE are often the technique of choice for imaging fast-relaxing nuclei in solids. Single point imaging sequences based on SPRITE in their conventional form are ill-suited for in vivo applications since the acquisition time is long and the SAR is high. A new sequence design is presented employing variable repetition times and variable flip angles in order to improve the characteristics of SPRITE for in vivo applications. The achievable acquisition time savings as well as SAR reductions and/or SNR increases afforded by this approach were investigated using a resolution phantom as well as PSF simulations. Imaging results in phantoms indicate that acquisition times may be reduced by up to 70% and the SAR may be reduced by 40% without an appreciable loss of image quality.