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Cardiovascular Magnetic Resonance Angiography : Carotids, Aorta, and Peripheral Vessels
Magnetic resonance angiography (MRA) is an important imaging modality for the diagnosis, clinical workup, and treatment planning in patients suspected of a wide range of vascular pathology. The aim of MRA is to visualize the arterial and/or venous system by creating high contrast between the blood flow and its surrounding stationary tissue. To fulfill this aim, several techniques may be used in clinical practice. Depending on its demands and the conditions required to optimally visualize specific vessels of interest, imaging techniques are chosen. In the first part of this chapter, techniques will be addressed that are currently available and applied for MRA. Contrast-enhanced MRA (CE-MRA) is probably the most widely used MRA technique. CE-MRA is applied either by fast imaging of the first pass of an intravenously injected bolus of a gadolinium (Gd) chelate agent or by a prolonged imaging approach with a blood-pool contrast agent. The latter approach is applied when imaging requires a time window that surpasses the first arterial pass. Because of the reported adverse events, such as nephrogenic systemic fibrosis (NSF) (see Chapter 3 ), accumulation of Gd in the brain, kidneys, and bone tissue associated with Gd-based contrast agents, an increasing interest in non–CE-MRA techniques is currently observed. Black-blood and bright-blood non–CE-MRA techniques will be discussed. Finally, flow imaging by time-resolved three-dimensional (3D) phase contrast (i.e., four-dimensional [4D] flow magnetic resonance imaging [MRI]), with the aid of newly developed visualization tools, is a relatively new technique that may be used to visualize complex flow structures and add quantitative hemodynamic information.
In the second part of this chapter, we will address the anatomical regions imaged by MRA and discuss the state of the art. Special focus will be on the carotid arteries, thoracic and abdominal aorta, renal arteries, mesenteric artery, and the peripheral arteries.
Contrast-Enhanced Magnetic Resonance Angiography: Technical Approach
In CE-MRA, vessels in the field of view of interest are imaged during the arterial first pass of intravenously injected paramagnetic contrast material. These contrast agents have a short T1 relaxation time and will produce high signal on T1-weighted images. In clinical practice, Gd-bound chelates, for example diethylenetriamine pentaacetic acid (DTPA), are used as the paramagnetic contrast agents. The Gd ion is a rare-earth element, and toxic to humans when unbound. Therefore in MRA, Gd contrast is based on chelates to control the distribution of Gd within the body and to overcome toxicity while maintaining their contrast enhancement capacity. A very important property of these chelates is their chemical stability, which depends on the chemical structure and ionicity of the complex. A stable chelate has less tendency to release free Gd ions in the human body. Gd chelates can be distinguished in two structural categories: macrocyclic and linear Gd chelates. In macrocyclic structures, the Gd chelate is more stable and therefore the chance of free Gd ions in the body is reduced. Breaking of the bond between Gd and the chelate (transmetallation) is more likely to occur with linear agents than with macrocyclic agents. However, even though the macrocyclic agents are more resistant to transmetallation, development of NSF is described in patients with advanced renal failure (effective glomerular infiltration rate <20 mL/min/1.74 m 2 , acute deterioration of renal function or on dialysis) before Gd contrast administrations. Most reported cases of NSF fibrosis are linked with linear Gd-chelates. Awareness of this association has given rise to effective screening of patient with possible renal failure using the Choyke questionnaire, use of more stable Gd-chelates, and international recommendations for the use of Gd-based contrast agents in daily practice. Somewhat concerning are anecdotal reports of NSF-like symptoms following Gd administration but in the absence of renal dysfunction.
Gd-chelates create intravascular signal by shortening the T1 relaxation time of blood in proportion to the concentration of contrast material. At 1.5 T, T1 of blood is 1200 ms. The T1 relaxation time is shortened by Gd according to Eq. 44.1 :
where R 1 = T1 relaxivity of gadolinium, and [Gd] = gadolinium concentration in blood.
Gd CE-MRA is insensitive to blood flow, in contrast to non-CE-MRA techniques as time-of-flight (TOF) and phase-contrast (PCA) MRA. Consequently, in CE-MRA, image quality is not degraded by flow disturbances.
Gd contrast agents can be classified depending on their distribution in tissue after intravenous administration, that is, extracellular or intravascular. Most of the Gd chelates used in clinical routine are extracellular agents. After intravascular administration they diffuse rapidly through the capillary walls into the extravascular space. In CE-MRA, arteries are preferably imaged during the initial passage of contrast. Timing of starting the image acquisition is essential to reduce signal from Gd diffused to the extracellular space and to overcome venous enhancement, which can lead to artifacts and image degradation.
The application of CE-MRA was improved by the introduction of Gd-chelates that have the capability to bind to larger molecules in the blood such as albumin and thus remain relatively intravascular. These contrast agents (e.g., gadobenate dimeglumine) provide a prolonged imaging time window because of a decreased decay time of contrast in blood. The use of such agents enables imaging beyond the initial passage of contrast bolus, which may be a benefit when acquisition needs to be gated to cardiac or respiratory motion, or when high resolution is required in small vessels such as the coronary arteries. When compared with an extravascular Gd-chelate, blood pool agents show an improved conspicuity of small vessels. Further development of macromolecular blood pool agents in the near future may play an important role in CE-MRA.
Three-Dimensional Contrast-Enhanced Magnetic Resonance Angiography Pulse Sequences
The CMR sequence used in CE-MRA needs to fulfill the following: T1-weighting is required, as well as the obtaining of a large 3D volume with sufficient spatial resolution, preferably fast within the first pass of contrast and within a breath-hold to suppress respiratory motion. Therefore fast T1-weighted 3D spoiled gradient-echo (GRE) sequences are used for image acquisition. In clinical routine, 3D CE-MRA preferentially is performed on a 1.5 T or 3 T system with strong gradient systems to reduce scan time, such that 3D datasets can be acquired during breath-holding. Typically, a short repetition time (TR) and echo time (TE) is used, 3 to 8 ms and 1 to 3 ms, respectively. Flip angle is 20 to 40 degrees to achieve optimized T1 weighting.
Furthermore, subsampling imaging techniques such as parallel imaging are implemented to reduce total scan time. Advanced k- space sampling techniques (spiral, keyhole, echo planar imaging [EPI]) may also be used to speed up acquisition.
At postprocessing, the 3D nature of the dataset allows viewing of the data from any desired angle, that is by creating multiplanar reformats (MPR) or rotational maximum intensity projection (MIP). This results in multiple images in various anatomic orientations. Thin (i.e., <3 mm thick) slices are required to provide a useful evaluation of these images. In some situations, it may be useful to use thicker slices (e.g., 10 mm) to reduce the number of slices and allow faster scanning. Although the ability to rotate the MIP is lost with such thick slices, scan times can be as short as 1 second.
Bolus Timing
Precise bolus timing is essential for first-pass CE-MRA. Arterial imaging is performed in the time window between arterial and venous enhancement and this time window depends on the rate of contrast agent injection, and will be patient specific. The transit time of contrast from the injection site (usually in the veins in the arm) to the field of view with the vessels of interest depends on several factors, for example, injection rate, injection site, heart rate, and stroke volume.
The Gd contrast concentration in the arterial blood is proportional to the injection rate and inversely proportional to the cardiac output:
T1 for blood is related to the injection rate and cardiac output by combining Eq. 44.1 and Eq. 44.2 .
Maximum arterial signal intensity is achieved when the start of the acquisition and the contrast infusion are synchronized such that peak arterial Gd concentration coincides with the acquisition of the central portion of k- space. The center of k- space contributes most to overall image contrast, whereas the periphery of k- space provides image information of details and contributes to spatial resolution. Therefore for improved arterial/venous differentiation, central k- lines have to be sampled before venous return. In CE-MRA, different types of k- space filling can be used in the available 3D pulse sequences. It is important to adjust bolus timing to the order of k- space filling used.
In linear ordering, lines of k- space can be acquired in any order (high k- space lines first, low k- space lines first, or at random). Linear ordering can be used in a CE-MRA protocol when precise timing of the contrast bolus is difficult or arrival time may be prolonged.
For most CE-MRA examinations, elliptical-centric ordering is the preferred k- space sampling method used. Typically, the center of k- space is acquired first and the periphery later (e.g., elliptic-centric Contrast-ENhanced Timing Robust Angiography [CENTRA], Differential Rate K- space Sampling [DRKS], or PEak Arterial K- Space filling [PEAKS]).
To determine the delay between the start of the venous contrast injection and the start of the acquisition, either a small test bolus or fluoroscopic real-time imaging can be used. The timing method by using a test bolus (1 to 2 mL) is robust and easy to perform. However, it will lengthen the procedure by a few minutes and requires an additional administration of contrast. In fluoroscopic real-time imaging, the inflow of contrast is imaged in the field of view with the vessels of interest. At the moment of contrast arrival, the acquisition is started automatically; however, the short delay of a few seconds, which is needed to switch from the fluoroscopic real-time imaging to the actual start of CE-MRA acquisition, can be a disadvantage.
Contrast-Enhanced Versus Non–Contrast-Enhanced Magnetic Resonance Angiography
The association between CE-MRA and NSF in patients with severe renal dysfunction and linear Gd-chelates has been appreciated for over a decade.
Moreover, Gd accumulation in tissue in patients without renal impairment has been reported in several studies. Kanda et al. reported residual Gd concentrations in the brain, particularly in the dentate nucleus and globus pallidus, of patients without severe renal dysfunction and foci of hyperintensity on unenhanced T1-weighted magnetic resonance (MR) images associated with previous administration of linear Gd-chelates, and which may be associated with the total number of previous Gd-based contrast material administrations. Deposition of Gd has also been demonstrated in bone, skin, and liver. Currently, previous administration of macrocyclic Gd-chelates shows no association with Gd accumulation in tissue ; however, these macrocyclic contrast chelates have been in use in clinical practice for a shorter time than the linear chelates. Nevertheless, the application of CE-MRA and the amount of administered Gd contrast is of clinical importance, especially in patients with impaired renal function, and issues of long-term retention may be of particular concern in children/young adults. Awareness of NSF-related incidents associated with CE-MRA and reports on accumulation of Gd in various tissues have also led to a renewed interest in non–CE-MRA. Improvements in CMR hardware and software, including the widespread availability of parallel imaging have helped to reduce acquisition times and have made some non–CE-MRA methods clinically practical. Non–CE-MRA methods may be classified into three categories: black-blood, bright-blood, and 4D flow visualization.
Black-Blood Imaging
Black-blood imaging of blood vessels uses double-inversion recovery to null the signal of flowing blood. This technique is flow-sensitive. Two 180 degree inversion recovery prepulses are applied to null the signal of flowing blood: the first prepulse is nonslice selective and inverts the longitudinal magnetization vector in the entire body, whereas the second inversion prepulse is slice selective and inverts the magnetization in the imaging slice back to its original orientation. Signal of blood entering the imaging slice after the second prepulse has undergone reinversion and will appear dark in the images. Electrocardiogram (ECG)-gated partial Fourier fast spin echo (FSE) is the sequence of choice, using a slice thickness of 6 to 8 mm stacked in the transverse plane of the chest or in a double-oblique sagittal view of the aorta (i.e., the candy-cane view). Single-shot FSE (SSFSE) sequences are much faster than basic FSE sequences and allow for acquisition of the entire imaging stack in a single breath-hold. Fat saturation is recommended to increase the conspicuity of aortic wall hematoma. Despite being an established technique, black-blood imaging is prone to artifacts because inadequate nulling of blood signal may occur by slow flow, or when in-plane blood flow is present.
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