|Factors affecting the appearance|
A myriad of factors affect the appearance of the final MRI image, including tissue composition, machine parameters such as the timing with which the radiofrequency (RF) pulse is sent in and when the machine "listens" for the returning MRI signal, the use of intravenous contrast, and whether the structure being imaged is static or dynamic (i.e. flowing blood).
A complete discussion of this is beyond the goals of this introductory module. It is not necessary to memorize the physics or the appearance of certain tissues on MRI, however you should be familiar with the concepts and realize that the same structures might look different on different types of MRI images.