MRI can be used to measure total body and regional tissue and organ volumes. The mass of each tissue or organ is then calculated as the product of volume and corresponding density (i.e. adipose tissue 0.92 kg/l, skeletal muscle 1.04 kg/l) [18
]. Adipose tissue can be further divided into subcutaneous, visceral, and intramuscular adipose tissue [19
]. Organ volumes that are usually quantified by MRI include liver, kidney, spleen, and brain [20
]. Cardiac MRI can be used to separately study heart mass and function. The mass of these metabolically active organs accurately predicts resting energy expenditure in adults [20
], but the energy cost of growth requires additional model development for estimation of resting energy expenditure in children [21
Usually whole body magnetic resonance T1-weighted images are acquired as axial images with 5–10 mm thickness and 40 mm or less between-slice intervals. Due to both the long analysis time and the incurred high cost, contiguous whole body MRI is rarely recommended for use, except in infants [22
], who require much fewer slices. The subject is usually positioned either prone or supine with their arms stretched above their head. Due to the limited magnet bore length, the upper and lower body are scanned separately in adults with repositioning using the L4–L5 intervertebral disc as the point of origin. In young children this repositioning may not be necessary. Some technician training will facilitate acquisition of the whole body protocol, which is different from most clinical diagnostic scans. It is also very important to keep the body within the field of view, especially when scanning obese subjects.
Once acquired, MRI and CT scans must be analyzed by the process referred to as image segmentation. There are various segmentation programs available, both commercial and developed in-house. The segmentation procedure for MRI is partially automated, although expert analyst input is still required.
Generally, the segmentation tools can be divided into three groups: manual delineation; low-level segmentation such as thresholding and region growing; and model-based segmentation methods [24
]. Specific technical considerations are required for analyzing each tissue and organ. For example, analyzing small adipose tissue depots requires consideration of the regional threshold, particularly when inhomogeneity of the magnetic field is present. On the other hand, organ analysis is carried out by manual tracing in most cases and knowledge of anatomy is required.
Segmentation of a ‘whole-body’ into subcutaneous, visceral, and intramuscular adipose tissue, skeletal muscle, and ‘residual’ usually requires about 4–8 h with a slice gap of 5 cm (i.e. 40 slices) and three to five working days for contiguous MRI scans (i.e. 200 slices), depending on the number of acquired slices and the expertise of the analyst. MRI segmentation solely for adipose tissue requires less time than for segmenting a scan for all compartments. The analyst’s training includes use of segmentation software and knowledge of cross-sectional anatomy. Moreover, it is important for the analyst to recognize some commonly observed artifacts that could influence the differentiation of tissues. These include motion artifacts caused by breathing or bowel peristaltic movements, blood flow artifacts, and other sources of image distortion. Some scanners also have magnetic field inhomogeneity. Although artifacts can be reduced or eliminated by setting specific scan parameters and breath holding, analysts should be trained to recognize the presence of image artifacts.
Most image analysis software now offers three-dimensional reconstruction based on image segmentation data (). Three-dimensional reconstructions are very useful for demonstration purposes. Investigators can visually sense not only the size and distribution of tissue compartments, but also longitudinal changes in tissue/organ size and distribution.
Magnetic resonance scan and three-dimensional reconstruction of the different tissue compartments of a healthy 16-year-old girl