Diffusion-weighted Magnetic Resonance Imaging: What Makes Water Run Fast or Slow?

Abstract

Diffusion-Weighted Magnetic Resonance Imaging (DWI) obtains information useful in diagnosing several diseases through the measurement of random, Brownian diffusion of water molecules in tissues. This pictorial essay illustrates the main factors, i.e., ratio between the volume occupied by cells and the extracellular space, composition of the extracellular space, and temperature, that determine the rate of the water diffusion. The mechanism through which these influencing factors affect water diffusion is explained. Clinical and experimental examples, derived both from physiology and from non-human models, are described.

Keywords: Diffusion weighted imaging; MR imaging; MRI Physics; water.

Figure 1

The sources of the signal measured with the Diffusion-Weighted Magnetic Resonance Imaging protocols routinely adopted. The overwhelming portion of the signal derives from the extracellular diffusion of water

Figure 2

The factors determining the speed of the extracellular diffusion of water: the ratio between the volume occupied by cells and the extracellular space (the most important), the composition of the extracellular milieu, and the temperature.

Figure 3

The dependence of the speed of the extracellular diffusion on the ratio between the volume occupied by cells and the extracellular space. The higher the percentage of tissue occupied by cells, the slower the extracellular motion of water molecules

Figure 4

The factors that lead to changes in the ratio between the volume occupied by cells and the extracellular space.

Figure 5

The inverse relationship of the speed of diffusion to the number of cells. A slower diffusion can be forecast in malignant than in benign proliferative lesions, and in benign lesions than in normal tissue

Figure 6

The relationship of diffusion to the number of cells. ADC values are lower, because of the higher cellularity, in proliferative lesions than in normal breast tissue; in an invasive ductal carcinoma the diffusion is slower than in a fibroadenoma

Figure 7

The inverse relationship of the speed of diffusion to the volume of cells

Figure 8

The relationship of diffusion to the volume of cells. ADC values are lower, because of cellular swelling, in areas of cerebral ischemia (light blue circle) than in normal brain

Figure 9

The direct relationship of the speed of diffusion to the volume of extracellular space.

Figure 10

The relationship of diffusion to the volume of extra-cellular space. ADC values are higher, because of interstitial edema, in an acute demyelinating plaque than in a normal area of white matter in a patient suffering from multiple sclerosis.

Figure 11

The dependence of the speed of the extracellular diffusion on the composition of the extracellular space. The higher the interstitial viscosity, slower is the extracellular motion of water molecules.

Figure 12

The relationship of diffusion to the composition of the extracellular space. ADC values are lower, because of collagen deposition, in a uterine leiomyoma than in the normal myometrium

Figure 13

The relationship of diffusion to the composition of the extracellular space. Endometrial ADC values are lower (we hypothesize because of glycogen extrusion), during the periovulatory than during the menstrual phase.

Figure 14

The relationship of diffusion to the composition of the extracellular space. ADC values are very low (we hypothesize because of the high amount of extracellular hydrophobic lipid molecules), mimicking cancer, in breast fat necrosis.

Figure 15

The relationship of diffusion to the composition of the extracellular space. ADC values are lower (we hypothesize because of the presence of hydrophobic lipid molecules) within an emulsion of butter in water than in pure water.

Figure 16

The relationship of diffusion to the composition of the extracellular space. ADC values are lower (we hypothesize because of the presence of proteins extruded from muscle tissue) within meat broth than in pure water

Figure 17

The relationship of diffusion to the composition of the extracellular space. ADC values are lower (we hypothesize because of the increased viscosity subsequent to denaturation of albumin molecules) in the white of a hard-boiled than of a raw egg.

Figure 18

The relationship of diffusion to the composition of the extracellular space. ADC values are lower (we hypothesize because of the increased viscosity subsequent to degradation of starch to numerous small saccharide molecules) in a boiled than in a raw potato

Figure 19

The direct relationship of the speed of diffusion to the temperature

Figure 20

The biochemical basis of heat production in muscles during exercise

Figure 21

The relationship of diffusion to the temperature. ADC values are higher, because of heat production, in the arm muscle of a healthy volunteer after exercise than at rest

Figure 22

The relationship of diffusion to the temperature. ADC values are higher in warm than in room temperature pure water

Figure 23

The relationship of diffusion to the temperature. ADC values are lower in a frozen than in a room temperature beefsteak