  Considering liver as the reference, that both fast diffusion (PF) and slow
diffusion (Dslow) of the spleen are much underestimated is likely due to the
MRI properties of the spleen such as the much longer T2 relaxation time. It is
possible that longer T2 relaxation time partially mitigates the signal decay
effect of various gradients on diffusion weighted image. This phenomenon will
not be limited to the spleen. Most liver tumors have a longer T2 relaxation
time than their native normal tissue and this is considered to be associated
with oedema. On the other hand, most tumors are measured with lower MRI
diffusion (despite being oedematous). The reason why malignant tumors have
lower diffusion value [apparent diffusion coefficient (ADC) and Dslow] are
poorly understood but has been proposed to be related to a combination of
higher cellularity, tissue disorganization, and increased extracellular space
tortuosity. These explanations may be true, but it is also possible to that
many tumors have MRI properties similar to the spleen such as longer T2
(relative to the liver) and these MRI properties may also contribute to the
lower MRI measured ADC and Dslow . In other words, if we could hypothetically
plant a piece of spleen tissue in the liver, MRI would recognize this planted
spleen tissue as being similar to a tumor and measure it to have lower
diffusion than the liver.
