CTA and MRA: visualization without catheterization.

The ideal modality for vascular imaging would be noninvasive and inexpensive. A volumetric acquisition would permit visualization of vessels from arbitrary angles. High contrast between the vessel lumen and background tissue would be coupled with excellent spatial resolution allowing accurate depiction of small vessels. Characterization of the constituent components of the vessel wall would be possible. High temporal resolution would both freeze the motion of fast moving vessels and show the direction and speed of blood flow. Finally, the modality would expose the patient to a minimal amount of ionizing radiation or potentially toxic contrast agents. Diagnostic conventional catheter angiography offers unsurpassed spatial and temporal resolution. However, catheter angiography is an interventional procedure, exposes the patient to both ionizing radiation and iodinated contrast, and does not depict the vessel wall. Additionally, view angles are chosen before the administration of contrast and may not demonstrate certain lesions. These limitations have driven the development of both computed tomography angiography (CTA) and magnetic resonance angiography (MRA). Both of these modalities rapidly acquire volumetric data sets, which can then be evaluated slice by slice or by more advanced volumetric rendering techniques. CTA and MRA are minimally invasive and less costly than angiography. While CTA and MRA cannot compete with the spatial or temporal resolution of conventional angiography, present technology has proven clinical efficacy in a wide range of applications. The principles behind CTA and MRA and their comparative strengths and weaknesses will be discussed. The different volumetric rendering techniques will be reviewed. Finally, recent advances that will likely further improve these modalities will be summarized.