Diffusion Imaging  

• Diffusion imaging is currently the most sensitive way to image an acute infarct.
• An acute infarct has a DECREASED apparent diffusion coefficient.

• On a diffusion weighted image, an acute infarct is hyperintense.  CSF, which has a higher diffusion coefficient, is hypointense.

MRA - left middle cerebral branches are without flow	T2-weighted MR shows 12 hour old left MCA infarct	Diffusion weighted image show the infarct with greater conspicuity

• Unfortunately, on a diffusion weighted image, some lesions which have normal diffusion coefficients are hyperintense due to "T2 Shine Through".
• We can eliminate the "T2 Shine Through" so that only lesions with abnormal apparent diffusion coefficients are hyperintense using an "ADC Map".
• The map generated has hyperintense CSF due to the higher ADC.  An infarct will appear HYPOintense due to the lower ADC.
• To make the map look like the original Diffusion Weighted Images, we invert it.  Now the CSF is dark and the acute infarct is BRIGHT.

• You can recognize the ADC map images, because they are the only ones that alternate inverted images and uninverted images, and also they are the images labeled B or TE = 100 with only 2 images per location.  The source images have 3 images per location.

T2 Shine Through

This is an example of T2 shine through.  On image 1 (T2-weighted axial) there is an area of hyperintensity in the left centrum semiovale.  On image 2 (Axial diffusion weighted image) this is hyperintense, suggesting it is an acute infarct.  On image 3 (ADC map, inverted) the area is hypointense, indicating that it is not acute.  It was bright on the diffusion weighted image due to T2 shine though.  If the lesion was truly an acute infarct, it should have remained bright on the ADC map.
 
 

 
Duration

The decreased diffusion coefficient will persist for about 7 days.
   

Produced by Ed Escott and Jack Simon, 3/11/98