Abstract: For in vivo diffusion weighted imaging , apparent diffusion coefficient (ADC) has been considered to reflect tissue diffusion. The ADC is widely used in clinical practice, including in the imaging of infectious lesions. It is generally assumed that the diffusion of tumors is restricted, while the diffusion of tissues with more water will be freer. However, the ADC values obtained in actual clinical work often deviate from the expected tissue diffusion. For example, although the abscess contains liquid components, the ADC measured for the abscess fluid is generally very low; while the ADC of some hard tissues such as cartilage and intervertebral discs is very high. Although the abscess fluid may have certain viscosity, myxomas often have high ADC. HCC ADC has been noted to have restricted diffusion relative to the liver. However, as HCCs are mostly associated with increased blood supply and increased proportion of arterial blood supply and higher water content, it is unlikely that HCC has a true lower diffusion. In fact, with the slow diffusion coefficient (SDC) method recently proposed by us, we measured faster diffusion for HCC than for the adjacent liver parenchyma. We proposed that in vivo ADC measure is strongly associated with T₂ relaxation time (T₂). T₂ can be divided into short T₂ band (<60ms), intermediate T₂ band (60-80ms), and long T₂ band (>80 ms, all 3T values). For the short T₂ time band, there is a negative correlation between T₂ and ADC. For the long T₂ time band, there is a positive correlation between T₂ and ADC. A tissue likely measures a low ADC if its T₂ is close to 70 ms. In this article, we attempt to explain the T₂ dependency of ADC with the concept of the intravoxel incoherent motion model. We suggest that we do not routinely use the term of "diffusion restriction" when interpreting clinical high b-value DW images and ADC maps.

Key words: Apparent diffusion coefficient, Slow diffusion coefficient, Intravoxel incoherent motion model, Diffusion restriction