Postoperative lumbar spine: contrast-enhanced chemical shift MR imaging.

A modified fat-suppression pulse sequence (consisting of combined frequency-selective fat presaturation followed by a spin-echo acquisition when fat and water magnetization vectors have opposite phase) was used to optimize the conspicuity of intravenous enhancement by gadopentetate dimeglumine on magnetic resonance images in 10 patients previously operated on for lumbar discogenic disease as well as in two patients with herniated disks who had not previously undergone surgery. This technique produced the greatest degree of fat suppression in the phantom study. In six of the patients who had previously undergone surgery, epidural enhancement was more obvious on the fat-suppressed images than on conventional spin-echo images, while in four patients, enhancement was equivalent. The herniated disks in two patients not previously operated on were not enhanced with either technique. Contrast enhancement was universally distinguishable from fat signal and from nonenhancing water-containing tissue on the fat-suppressed images obtained after contrast material administration. This technique may reduce the need for precontrast imaging. Furthermore, postoperative enhancement of nerve roots was more obvious on fat-suppressed images in seven of eight patients. This finding might represent previously undiagnosed degrees of arachnoidal inflammation, which may be a factor in the failed back syndrome.