|Factors affecting the appearance|
Unlike CT, not all MRI images are obtained the same way, and as a result, the images can look quite different. MRI machine parameters, such as the strength of the RF pulse, the timing of repetitive RF pulses, the time the machine "listens" for the returning signal from the patient can all be selected and manipulated to create specific types of images and to optimize visualization of certain structures.
Two common "sequences" or types of MRI images which you should be familiar with are T1 and T2 weighted images. The images are "weighted" to bring out various characteristics of tissues. Some tissues look the same on both T1 and T2 images and some look different.
The sagittal lumbar spine MRI image on the left is T1 weighted and that on the right is T2 weighted. Note that subcutaneous fat is bright on both images. The cerebrospinal fluid is dark on T1 and bright on T2. Note the difference in the appearance of the intervertebral discs on the two images. The central portion of a disc contains fluid and so appears brighter on T2 than T1.
It is not necessary to understand the physics or memorize the appearances of tissues on T1 or T2 weighted images. The goal of this section is for you to become familiar with the concepts and realize why the same structures might look so different on different MRI images or "sequences".