Basic MRI I Dr. Mark D. Herbst Chapter 8 Basic MRI I Dr. Mark D. Herbst

Selective Signal Suppression Reasons; Increase contrast between tissues Between normal and abnormal in one tissue Between two different normal tissues Usually applied to the whole image Based on T1 values, resonant frequencies, or molecular binding properties Reduce artifacts Usually applied to part of image or outside the image –”specific anatomic regions”

Fat and Fluid Suppression T1 based methods Fat T1 = about 260 ms Fluid T1 = about 2000-5000 ms STIR –use TI about 120-150 ms to catch fat at its null point for 0.5-1.5T. FLAIR—use TI about 1500-2000 to catch fluid at its null point SPIR inversion pulse applied at fat frequency only, then 90 degree pulse applied (to fat and water) to start sequence when fat is at its null point. This gives a water-only image.

Fat and Fluid Suppression Frequency based methods SPIR above is partly based on frequency, see above. FAT SAT—saturation pulse at fat frequency is applied to whole image so fat gives no signal, only water is seen. WATER SAT—saturation pulse at water frequency is applied to whole image so water gives no signal, only fat is seen.

Fat and Fluid Suppression More frequency based methods Water excitation Fat excitation MTC = magnetization transfer contrast Sat pulse applied to the wider peak of the bound water, and it becomes saturated. But the unbound water that exchanges with it also gets saturated. Only the separate water, not exchanging with bound water, escapes the effect of this saturation pulse. That is why this works well in MRA, since water molecules in the blood vessels are separate from the bound and unbound water in the brain tissue. This sat pulse does not affect fat or separate bulk water.