In recent years, magnetic resonance angiography (MRA) of the peripheral vascular tree has evolved into a widely available and valuable technique in the diagnostic and pre-interventional work-up of patients with peripheral arterial disease. Numerous studies have demonstrated the diagnostic accuracy of MRA for the assessment of abdominal and peripheral arteries, and in many hospitals the technique is now solidly integrated into the clinical workflow.^1–3 The recent introduction of the first high-relaxivity blood-pool agent, Vasovist, offers the opportunity for further improvements in image quality and expansion of the range of clinical indications for which MRA can be used. This article provides considerations on practical aspects of bloodpool- enhanced-MRA and current clinical indications, as well as examples of the added value of blood-pool imaging over conventional first-pass imaging.

Technical Principles and Practical Considerations

Background

Over the past few years, contrast-enhanced MRA (CE-MRA) has evolved as the preferred technique for evaluation of patients with different forms of peripheral arterial disease. The basic idea is to acquire an arterial luminogram during initial arterial passage (the ‘first pass’) of contrast material. First-pass CE-MRA essentially necessitates a compromise between the desire for high spatial resolution and large volumetric coverage (i.e. long acquisition duration), the desire to avoid disturbing venous enhancement (i.e. short acquisition duration) and high vessel-to-background contrast. Because of the higher relaxivity and prolonged intravascular residence time of Vasovist, the aforementioned trade-off is governed by much less stringent conditions.

Blood-pool Contrast Media

Blood-pool contrast agents can be used in exactly the same way as extracellular agents with regard to first-pass imaging. The advantage of using these agents for first-pass imaging lies in their much higher relaxivity (see Table 1).4 This means that a higher signal-to-noise ratio can be obtained when parameters are kept identical or, conversely, that spatial resolution can be increased while maintaining the same signal-to-noise ratio. The truly interesting property of blood-pool agents, however, is their much longer intravascular residence time. Equilibrium imaging is possible because – despite the fact that dilution of the injected contrast medium after first arterial passage leads to a T1 increase of the blood pool compared with the first pass – the value is still much lower than that of fat. Hartmann et al. estimate that T1 of blood in the equilibrium phase, 3–5 minutes after injection of 0.03mmol/kg Vasovist, is about 130ms, increasing to about 150ms after 10–15 minutes.⁵ This prolonged T1 reduction offers the opportunity to obtain images of the peripheral vascular tree up to about 45–60 minutes after injection. The extended imaging window can be used to acquire images with much higher spatial resolution without a significant loss of vessel-to-background contrast. In clinical practice this means that scan duration is no longer determined by the transient T1 shortening, but by the capacity of the patient to sustain a breathhold or to remain motionless. The apparent drawback of using a blood-pool agent is the simultaneous enhancement of venous structures close to arteries. This phenomenon is a well-known problem at first-pass imaging, often resulting in images that cannot be used for clinical decision-making. However, because equilibriumphase images can be acquired at much higher spatial resolution – often with a 5–15-fold decrease in voxel size compared with first-pass protocols – arteries can be readily separated from accompanying veins.

Practical Aspects of Contrast-enhanced Magnetic Resonance Angiography with Blood-pool Agents

The use of a blood-pool contrast agent has reduced the deleterious consequences of missing the bolus in the first pass. If, for whatever reason, acquisition in the first pass fails, images can always be obtained in the equilibrium phase because of the prolonged intravascular retention. Although prolonged intravascular retention is highly advantageous, it is not recommended to perform a test bolus when using a blood-pool agent because of this property. If possible, it is better to acquire a dynamic series of acquisitions using a time-resolved MRA technique and to evaluate the data set with the best selective arterial opacification. The most commonly used format to display 3-D MR angiographic data is the maximum intensity projection (MIP). Although MIP is an elegant way to collapse a 3-D volumetric data set into a 2-D projection, review of cross-sectional images remains an integral part of the evaluation, especially for data acquired in the equilibrium phase. The MIP algorithm works best when using thin-slab or curved subvolume selections. In whole-volume MIPs, contrast-enhancing organs or other vascular structures may superimpose over smaller arteries when they have higher signal intensities along a particular viewing path. When working with equilibrium-phase images, the use of thin-slab sub-volume MIPs can be particularly useful. Another helpful technique for the precise evaluation of vessel morphology, especially when evaluating equilibrium phase data, is curved multiplanar reformation (cMPR) along the axis of the arterial segment of interest. Most post-processing workstations offer the ability to interactively generate a cMPR while scrolling through source images. This technique is particularly useful to obtain views of eccentric stenoses, and as a basis to generate views perpendicular to the central axis of the vessel to measure cross-sectional area reduction in stenoses.