The doctor prescribed acetylcysteine (uh-see-til-siss-teen) to take 24 hrs, 12 hrs & immediately before the scan and then 12 hrs after the scan. It is supposed to protect the kidneys from the adverse effects of the CT contrast agent.
Here is some information on it from wikipedia:
Oral acetylcysteine is used for the prevention of radiocontrast-induced nephropathy (a form of acute renal failure). Some studies show that prior administration of acetylcysteine markedly decreases (90%) radiocontrast nephropathy (Tepel et al 2000), while others appear to cast doubt on its efficacy (Hoffman et al., 2004; Miner et al., 2004). Worth considering is the newest data published in two papers in the New England Journal of Medicine and the Journal of the American Medical Association. The authors' conclusions in those papers were: 1) "Intravenous and oral N-acetylcysteine may prevent contrast-medium–induced nephropathy with a dose-dependent effect in patients treated with primary angioplasty and may improve hospital outcome."
(NEJM 2006; 354:2773-82) 2) "Acetylcysteine protects patients with moderate chronic renal insufficiency from contrast-induced deterioration in renal function after coronary angiographic procedures, with minimal adverse effects and at a low cost" (JAMA Vol. 289, No.5, pg. 553-558).
Acetylcysteine continues to be commonly used in individuals with renal impairment to prevent the precipitation of acute renal failure."
For more on this topic see CT Scans section, intravenous contrast subpage.