Figure 4b. Contrast-enhanced MR angiography of the aorta. Source (a), subtracted (b), and MIP (c) coronal images show the aortic arch. The subtracted image (b) was obtained by subtracting the mask image from the source image (a) and demonstrates better suppression of the background signal from the body in comparison with the source image. On the MIP image (c), note the significant focal stenosis of the left subclavian artery (arrowhead). Also note the focal signal loss in the midportion of the left subclavian artery (arrows in c), which was due to concentrated gadolinium contrast material in the adjacent vein.