Imaging of the vasculature has greatly evolved in the past 15 years. Although the current gold standard remains invasive x-ray angiography, noninvasive modalities such as magnetic resonance imaging (MRI) and computed tomography (CT) are becoming routine for the evaluation of patients with vascular diseases. In many instances, either MRI or multidetector computed tomography (MDCT) has replaced x-ray angiography as the imaging modality of choice in the assessment of patients with suspected vascular disease due to the ever-increasing quality of the images, the noninvasive application of these modalities, the ease and comfort of the patients, and the clinical versatility of both CT and MRI. In addition to evaluating the degree of luminal stenosis, MRI and CT can now detect noninvasively the presence and composition of atherosclerotic plaques in the different arterial beds. This chapter provides a comprehensive and state-of-the-art overview of the clinical indications for MRI and CT in the evaluation of vascular diseases.
Magnetic Resonance Angiography
Magnetic resonance angiography (MRA) can be divided into two categories: nonenhanced and contrast-enhanced angiography.1 Nonenhanced MRA can be obtained either by detecting the effect of blood flow on signal amplitude (time of flight [TOF]) or on phase (phase contrast [PC]) of moving protons. TOF angiography relies on the differences in signal amplitude between in-slice stationary protons and blood protons flowing into the slice. In-slice stationary protons become relatively saturated with repeated excitation pulses and produce low signal intensity, whereas inflow blood protons in arteries and veins have not experienced the excitation pulses, are not saturated, and therefore generate high signal intensity. Limitations of TOF imaging are long acquisition times, the need to position sections orthogonal to the direction of flow, and saturation of protons, particularly in case of three-dimensional (3D) acquisition. Therefore, clinical applications of TOF imaging are restricted to the evaluation of extra- and intracranial arteries. PC angiography derives image contrast from the differences in the phases accumulated by stationary and moving spins in a magnetic field gradient. PC-MRA uses pairs of bipolar or flow-compensated and uncompensated gradient pulses to generate flow-sensitive phase images. Phase data can be used either to reconstruct velocity-encoded flow-quantification images or MRA images. With velocity-encoded imaging, amplitude of the phase is directly proportional to the flow velocity, allowing for quantitative measurements of flow velocities and the identification of flow direction. In addition to morphologic MRA, velocity-encoded imaging can help for the evaluation of flow and pressure gradients across stenoses in the carotid arteries, peripheral arteries, and renal arteries, as well as for thoracic dissection and coarctation. However, clinical applications are hampered by long acquisition times required for velocity-encoded imaging.
In contrast to nonenhanced MRA, first-pass MRA with gadolinium-based contrast agents has gained widespread acceptance owing to shorter acquisition times. The strong increase of luminal signal intensity after intravenous injection of T1-shortening contrast agents such as gadolinium chelates allows for fast angiographic acquisitions with 3D gradient echo sequences.2...