October--Lecture Notes 1

Image Density and Contrast
Based on previous discussions, we know that different tissues in the body will interact with radiofrequency (RF) energy in different ways. For example, fat will give up the RF energy it received during the 90 degree RF pulse very quickly (short T1 and an even shorter T2 time). Conversely, water takes longer to give up the absorbed RF energy (relax), and therefore has longer T1 and T2 times.

The T1 curves shown above compare the recovery times for fat, white matter, grey matter and CSF. The horizontal axis on the graph represents repetition time (TR) in seconds. Signal intensity can be measured (read by the machine) at any time.

Based on the TI curves in the graph shown above, answer the following questions:-
If signal intensity is measured at the time labeled T1 CSF:

• What is the contrast between grey and white matter on the resulting image?
• Which two tissues will have the highest contrast?

The density of each tissue varies based on when the signal intensity is measured. For the TE shown in the diagram below (slightly less than 0.2 seconds or 200ms), CSF will appear darker than all the other tissues because it has the lowest signal intensity.

Spatial Localization in MRI

We know how a MR signal is generated and how appearance of different tissues on the image is determined (image weighting), but how does the machine determine where the signal is coming from (spatial localization) ?

The machine needs this spatial information in order to form the image. Each voxel in the patient provides information for the corresponding pixel in the image.

The anatomy is broken down into slices of a particular thickness, and then further divided into voxels based on the scan matrix.

So the computer builds up the image one pixel at a time based on location and signal strength information that it receives from the corresponding voxel in the patient?

To localize the voxels, spatial information needs to be encoded into the MR signal.

Three steps are required :

• First of all, the desired slice must be selected
• Then, spatial information is encoded along the rows
• Finally, spatial information is encoded along the columns

All three functions are performed by the gradients. There are three gradients and they are named according to the axis in which they exert their influence.