## 2 Pulp Suspensions Imaging Overview  
Pulp suspensions are characterised as being opaque and intricate mixtures that exhibit a quite heterogeneous distribution of fibres under flow, presenting areas in which flocs are present (the term flocs refers to a high concentration of fibres). In addition, along the entire pulp suspension and above certain concentrations, as a function of the observed shear rates, a continuous fibre network forms. For the purposes of process control nowadays, knowledge regarding flow characteristics is essential, once the suspensions have been subjected to high shear forces. The numbers regarding turnover in the pulp and paper industry confirm the importance of this knowledge, as in some countries they exceed the return from other industrial areas, as is the case in Sweden and Canada. Understanding and controlling the manufacturing process enables the development of newer products with a high innovation content. Additionally, obtaining precise values for the velocity profiles associated with a certain shear rate in a pulp suspension is also of high importance for optimising the flow and, consequently, the energy consumption; for the later, however, there is scarce information reported in the literature. Diverse flow imaging methods, along with particle following velocimetry approaches, have been described in the literature; however, most of these need to be experimentally trained, with data acquisition procedures thus usually lasting quite a long time, and they can typically only be used in non-opaque suspensions. Consequently, it is not possible to use them in pulp suspension flows not belonging to the dilute range. For data collection in pulp suspensions, among the more promising techniques can be found Laser Doppler Anemometry (LDA) (Optomet GmbH, Darmstadt, Germany) [4], Magnetic Resonance Imaging (MRI) (National Institute of Biomedical Imaging and Bioengineering, Bethesda, MD, USA) [5], Ultrasonic Velocity Profiling (UVP) (MET-FLOW S.A., Lausanne, Switzerland) [6], Optical Coherence Tomography (OCT) (PHOENIX TECHNOLOGY GROUP, Pleasanton, CA, USA) [7], Particle Image Velocimetry (PIV) (DANTEC DYNAMICS, Skovlunde, Denmark) [8], X-ray tomography (Department of Physics P.O., University of Jyvaskyla, Jyvaskylan yliopisto, Finland) [9], Gamma ray densitometry [10], and Electrical Tomography (ET) (PASI SRL, Torino, Italy) [11]. Magnetic Resonance Imaging is a tomographic technology with a high spatial resolution that can supply information regarding the behaviour of the protons present in a system, which are typically contained in the 1H nuclei of water. MRI can be used, among other applications, to image the motion of water, e.g., expressed in the form of velocity profiles. The major limitations of MRI are the types of particles and the size of the system that can be studied, together with the size, weight and cost of the instrumentation. Only particles containing MR-sensitive nuclei, such as 1H, can be detected [12; 13]. The maximum diameter of the system is bounded by the inner diameter of the coil in the magnet. Thus, MRI experiments are typically limited to laboratory-scale fluidised beds or pipes with small diameters [14]. In summary, the strength of MRI is in studying centimetre-scale systems at spatial resolutions of ~100 \(\upmu\)m. Time-averaged velocity profiles of diluted pulp suspension have been obtained using MRI with fibre concentrations of up to 0.86% (w/w) [15; 16; 17; 18; 19; 20]. However, when compared with LDA and UPV, MRI presents some disadvantages, such as longer observation times and equipment cost. [PERSON] et al. [20] were able, to gather information about the suspension velocity profiles of 0.5% fibres diluted in water (w/w) using MRI in a matter of milliseconds. Also using MRI, under steady flow conditions, and for pulp suspensions with concentrations above 3% (w/w), [PERSON] et al. [21] were able to measure time-averaged velocity profiles. Despite these lab trials that have taken place around the world, wide MRI usage in industry is not expected in the coming years.