How does an angiogram work? An angiogram works similar to an x-ray. The body casts a "shadow" on film when it is exposed to the x-ray, much like when you hold a flashlight up to your hand and cast a shadow on a wall. Normally your blood vessels cannot be seen in an x-ray, but adding a dye (contrast agent) into the blood stream makes your arteries and veins visible (Fig. 1). Contrast agent contains iodine, a substance that x-rays cannot pass through.  Figure 1. Side view of the head shows a lateral angiogram of the internal carotid artery. The dye passes through the arteries, into the capillaries, and finally the veins. To deliver the contrast agent, a catheter is advanced from the femoral artery in the leg to one of four arteries in the neck that lead to the brain. The doctor steers the catheter through the blood vessels while watching a monitor. A fluoroscope machine, called a C-arm, is an arc shaped piece of equipment that generates x-rays from one side and photographs them on the other side (Fig. 2). Contrast is injected into the bloodstream to make the blood vessels visible on the monitor. The result is a kind of roadmap of the arteries. Figure 2. A fluoroscopic C-arm is an arc-shaped piece of equipment that generates x-rays from one side and photographs them on the other side. Today many catheter angiographic studies have been replaced by less invasive methods such as computed tomography angiography (CTA) and magnetic resonance angiography (MRA) that do not require a catheter be inserted. Catheter angiography has the benefit of combining diagnosis and treatment in patients who may undergo surgery or other endovascular procedure such as angioplasty, aneurysm coiling, or stent placement. What does an angiogram show? Angiograms are very good at detecting problems with the blood vessels such as an aneurysm, arteriovenous malformation (AVM), arterial stenosis from plaque build-up, tumors, and clots (Fig. 3). 
