14-07 Contrast-Enhanced MR Angiography

Both TOF-MRA and PC-MRA have limitations. Enhancement of blood can be er­ra­tic, mostly due to the influence of flow irregularities. In some body regions, mo­tion of the surrounding organs by breathing, peristalsis or pulsation affects an­gio­gra­phic depiction of vessels negatively, and saturation effects influence sig­nal intensity and contrast of blood vessels.

There are numerous other inherent MR properties which can easily de­te­rio­ra­te both TOF or PC images (Table 14-02). Thus, the dream of finally having a com­ple­tely noninvasive imaging method was shattered once again. If MR an­gio­gra­phy was to compete with x-ray angiographic methods, higher spatial and tem­po­ral resolution and more reliable enhancement would be necessary — with the application of contrast agents.

There are four different categories of possible angiographic agents. Figure 14-18 classifies them. Their categories are based on their global ability to cross the en­do­the­lium and to filter through the renal glomeruli. Low-diffusion agents have an in­ter­me­dia­te position between ECF- space and blood-pool agents, and their interstitial diffusion occurs at a lower rate than that of ECF-space agents. Rapid-clearance blood-pool agents are mainly confined to the vas­cu­lar space, but are freely ex­cre­t­ed by the kidneys, whereas the re­nal excretion of slow clearance blood-pool agents is very re­strict­ed. Figure 14-18: Pharmacokinetic classification of various categories of angiographic contrast agents. (a) ECF-space agents; (b) low-diffusion agents; (c) rapid-clearance blood-pool agents; (d) slow-clearance blood-pool agents. [Modified from ⇒ Port].

For commercial and historical reasons, at present mainly ECF-space agents are used for contrast-enhanced MR angiography (CE-MRA) which can provide ex­cel­lent angiograms when combined with rapid T1-weighted GRE-imaging [⇒ Mar­chal 1991 + 1992].

Blood-pool agents remain in the blood for a significantly longer time and their tissue uptake is limited. However, their imaging window is wider; examination can even be repeated if necessary. The ideal contrast agent for bright blood MRA would have a high r1 relaxivity to make T1 as short as possible and, to avoid spin dephasing effects, a low r2 re­la­xi­vi­ty to keep: T2* > 2 × TE.

Besides gadolinium-based agents, ultrasmall superparamagnetic iron-oxide par­tic­les also seem to be well suited for MR angiography, with efficient and long lasting positive intravascular signal enhancement. These compounds remain al­most exclusively in the intravascular space and selectively display the blood ves­sels. Due to their prolonged plasma half-life, these compounds could also be used to enhance areas with vessels of varying permeability, and thereby reveal a certain tumor affinity. They also can help to define ischemia and reperfusion af­ter treatment, for example of cerebral or myocardial infarction. With appropriate calculative algorithms, such agents can also be used to estimate tissue blood flow in myocardial and cerebral ischemia, pulmonary embolism, the vas­cu­la­ri­za­tion of transplants, and perfusion of tumors.

14-07-01 CE-MRA: Application

Contrast-enhanced MR imaging depends mainly on T1 effects, less on TOF- or PC-imaging techniques.

If (during and immediately after injection) blood has the shortest T1 of all tis­sues, it will show the brightest signal and thus the blood vessels will be visible in the MIP image. Furthermore, even in periods of slow flow (diastole for most ves­sels), there still is good signal from the blood which reduces ghosting and/or eli­mi­na­tes the need for cardiac synchronization. This makes CE-MRA much easier to perform.

After the slow injection of an ECF-space agent, its concentration in the blood will rapidly decrease. Depending on the type, only 50% of the dose remains in the blood after 5-10 minutes. However, with bolus injections (injection time <60 seconds), the initial first-pass concentration is high; it decreases rapidly im­me­dia­te­ly after the end of the injection (cf. Figures 16-06 and 16-07).

The contrast agent is diluted with the total blood-pool volume, it leaks from the capillaries into the extracellular space in many tissues (e.g., muscle), and it is excreted by the kidneys. Thus, for vascular imaging, these contrast agents can best be used for imaging the first pass of the applied bolus.

However, even with contrast agents, the scan time can still be relatively long (20 seconds to 2 minutes). Therefore, it is necessary to keep the ar­te­rial con­cen­tra­tion high continuously by injecting during the entire scan. As a rule of thumb, the duration of the injection is equal to or slightly shorter than the scan time (Fi­gu­re 14-19).

Figure 14-19: Schematic drawing of a bolus injection for CE-MRA.

The delay between the start of the injection and the start of the scan depends on the delay between the start of the intravenous injection and arrival of the bo­lus in the arteries of interest. This delay depends on the distance of the arteries of interest to the heart, the cardiac output, and the quality of the veins in which the agent is injected (Figures 14-20 and 14-21).

The injected dose volume depends on the maximum allowed dose and the contrast agent available (cf. ad­ver­se events). Usually sin­gle or at the most double doses are in­ject­ed (single dose: 0.1 mmol/kg body weight). Figure 14-20: CE-MRA of the abdominal aorta.

Figure 14-21: Contrast-enhanced TOF angiography of a hand.

14-07-02 CE-MRA: Techniques

In order to make sure that the arterial bolus is at its peak during imaging, several techniques can be used.

The delay between the start of the intravenous injection and arterial arrival of the bolus can be determined by a small test injection of one or two milliliters of the contrast agent. The slice orientation of the test injection scan can be chosen in any direction, but if it is chosen perpendicular to the flow, presaturation slabs on both sides of the slice have to be used to suppress inflow effects so that only T1 effects will be visible.

Another approach is prospective bolus detection, where the acquisition is trig­ger­ed by the arrival of the arterial bolus. Because of the time needed for breath hold instructions and the unknown delay between injection and arterial bolus ar­ri­val, both prospective and retrospective bolus detection can be problematic when combined with breath hold.

The protocol used for strong T1- weight­ing is relatively simple and is similar to that used in 3D inflow. The main difference is the flip angle and the freedom of slice ori­en­ta­tion. Usually, a 3D gradient- echo sequence is applied. To suppress back­ground tissues, a short TR (ty­pi­cal­ly between 5 and 15 ms, de­pend­ing on gradient system and sequence) and a large flip angle (between 40° and 70°) are used. Such a large flip angle cannot be used for a normal 3D inflow pro­to­col (without contrast agent) be­cause blood will become saturated too fast. In combination with mechanical de­vi­ces the entire peripheral vascular system can thus be examined after a single contrast agent injection (Fi­gu­re 14-22). The combination of ra­pid automatic table movement and automatic injection and follow- up of the bolus allows mul­ti­ple suc­ces­si­ve acquisitions. Figure 14-22: Moving-bed CE-MRA of the pelvis and legs.

The differentiation between arteries and veins is still problematic. The easiest differentiation is by morphology or, if a contrast agent is injected, by following the first pass.

Different approaches have been applied to distinguish between arteries and veins, both during image acquisition and by postprocessing image data. None of these approaches has been found to be sufficiently reliable. Among them is the presaturation method described in Figure 17-10. However, if presaturation slabs are used for selective demonstration of veins, the venous signal intensity in re­tro­gra­de pathways may inadvertently be suppressed.

The use of gadolinium-based contrast agents obviates the dependence on in­flow and allows imaging with large fields-of-view in the coronal or sagittal plane, despite substantial in-plane venous flow. Subtraction techniques offer a se­lec­ti­ve demonstration of veins, but a vein-free arterial study must be obtained first [⇒ Shinde].