Imaging of the vasculature has evolved greatly in the past 20 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 CT has replaced x-ray angiography as the imaging modality of choice in the assessment of patients with suspected vascular disease, because of the ever-increasing image quality, the noninvasive application, the ease and comfort for patients, and the clinical versatility of both CT and MRI. In addition to evaluating the degree of luminal stenosis, MRI and CT can now noninvasively detect the presence and composition of atherosclerotic plaques in various 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, focusing on both angiographic and plaque-based assessments. It also introduces emerging molecular imaging approaches for the assessment of disease activity.
Magnetic Resonance Angiography
Magnetic resonance angiography (MRA) can be divided into two categories: nonenhanced MRA and contrast-enhanced MRA (CE-MRA). 1 Nonenhanced MRA can be obtained by detecting the effect of blood flow on either the signal amplitude (time of flight [TOF]) or on the phase of moving protons (phase contrast [PC]). TOF angiography relies on differences in signal amplitude between in-slice stationary protons and protons in blood flowing into the slice. In-slice stationary protons become relatively saturated with repeated excitation pulses and produce low signal intensity, whereas inflowing blood protons have not experienced these excitation pulses, are not saturated, and generate high signal intensity. Two-dimensional (2D) or three-dimensional (3D) datasets can be acquired. Limitations of TOF imaging are long acquisition times and the need to position sections orthogonal to the direction of flow. In addition, slow flow or turbulent flow can lead to signal loss. PC angiography derives image contrast from differences in phases accumulated by stationary and moving spins in a magnetic field gradient. Phase data can be used to reconstruct velocity-encoded flow-quantification images or MRA images. With velocity-encoded imaging, phase amplitude is directly proportional to flow velocity, allowing for quantitative assessment of flow velocity and direction. This can assess flow and pressure gradients across stenoses in the carotid arteries, peripheral arteries, and renal arteries, as well as coarctation of the aorta. This technology also permits visualization of thoracic aortic dissection. However, clinical applications are hampered by the long acquisition times required for velocity-encoded imaging.
Other nonenhanced MRA sequences include electrocardiogram (ECG)-gated fast spin echo (FSE or turbo spin echo) and sequences based on steady-state free precession (SSFP). ECG-gated FSE has a shorter imaging time than TOF imaging and is sensitive to slow flow. It has, therefore, been used to assess peripheral and collateral vessels. 2 Image contrast with SSFP sequences stems from the T2/T1 ratio, which is ...