We see that the T1 contrast is essentially controlled by TR and a short TR is necessary to achieve maximum T1 contrast. Conversely, T2 contrast is controlled by TE and for maximum results, a long TE is desirable. If we negate the T1 contrast by using a long TR and then use a short TE to negate the T2 contrast, we will be left with an image that has a contrast only different in the proton density. This image is called the proton density, or spin density, image. Certain tissues, such as bone, fibrous tissue, and calcification have little or no free water.
Blood has a variable appearance, depending on whether it is hyperacute, acute, subacute, or chronic.
In some cases it is a combination. There are three basic types of images. Those with a short TR and a short TE are heavily T1 contrast weighted. Those with a long TR and long TE are heavily T2 contrast weighted. What we have just reviewed is called the Spin Echo or SE pulse sequence.