**2.4. DTI limitations**

To date, DT remains the only noninvasive method for visualizing human brain and spinal cord connections. DT suffers from both fundamental and practical limitations that limit its use for modelling brain connections. Unlike many invasive modalities, DT is incapable of determining the direction of information flow, nor can it distinguish single- and multineuron connections. DT may also have difficulty in resolving complex intravoxel fiber crossings or nondominant fiber populations due to limitations in scan time, hardware, or processing methods. Despite its many limitations, DT has been successfully used to model human neuronal connections for over two decades, including several pathways that are putatively deep brain stimulation targets. DT generation can be divided into three separate steps: data acquisition, data processing, and tracking. Each of these steps has several variables that must be considered in order to ensure accurate DT [41].

#### **2.5. New research on DTI**

Different methods for the acquisition and analysis of DTI have been developed and have improved the precision of diffusion tensor measurements in recent years, so, new innovations can be expected. New pulse sequences and diffusion tensor encoding schemes are being developed to improve the spatial resolution, accuracy and to decrease artifacts in diffusion tensor measurements [3].

Even though DTI provides quantitative parameters of clinical relevance, it is limited in representing complex diffusion schemes. Methods based on high angular resolution diffusion imaging (HARDI) provide a more precise diffusion profile visualization [36].

Q-ball imaging (QBI) allows the detection of subtle anatomical features of the spinal cord that were not seen with DTI.

QBI has also been applied to the injured spinal cord, demonstrating its ability to detect directional abnormalities. Metrics derived from QBI may therefore provide useful markers of diffusion characteristics in the healthy and injured SC [36, 37].
