Thursday, October 18, 2012

MR + C (Static contrast-enhanced MRA)

In contrast to the "Time-of-Flight" techniques is Contrast-enhanced MR angiography.  I separate them out since they rely on different principles for their vascular contrast, and since practically the clinician usually orders one or the other... but in truth contrast-enchanced MRA incorporates principles of 3D Time-of-Flight acquisition, they're just not the dominant mode of vascular contrast.

MR angiogram MIP using Multihance gadolinium contrast agent

One of the main benefits of contrast enhanced MRA is that it eliminates some problematic time-of-flight artifacts, in particular, saturation from slow flow in aneurysms or vascular malformations, or saturation from flow not perpendicular to the plane of imaging.

In the dramatic example below, a large basilar tip aneurysm is only partially-visible on the 3D time-of-flight sequence where the flow is fast enough to remain unsaturated. This artifact is not an issue on contrast-enhanced MRA, where all the intravascular space is enhanced.

 Top two images from a contrast-enhanced MRA show a large basilar tip aneurysm. On the bottom two images, the same aneurysm is only barely seen (arrows)  because of turbulent flow within the aneurysm (Ref 3).

So how does gadolinium effect intravascular contrast?  The principle is different that that of iodinated CTA contrast -- in CTA the contrast agent itself is what's imaged, the dense iodine in the contrast attenuates the x-ray beam more than the surrounding tissues.  In MRA, the contrast agent (gadolinium) affects the surrounding water molecules, and the signal from those is what's picked up on MR imaging.

The gadolinium contrast agent decreases the spin-lattice (T1) relaxation time of the surrounding water hydrogen protons, causing them to give off increased signal in a T1-weighted sequence.  The water molecules can both transiently bind to the gadolinium (inner-sphere relaxation) or be affected by gadolinium's magnetic field (outer-sphere relaxation)

How does gadolinium (a gadolinium chelate) decrease the proton's T1 relaxation time (and also decreases the T2 relaxation time)? As a paramagnetic atom with seven unpaired electrons, it generates its own significant local magnetic field when placed in a larger magnetic field (i.e. the MRI B0 magnet).  At low concentrations, the T1 relaxivity effect dominates.  At higher concentrations, the T2 relaxivity effect dominates.

Similar to the TOF technique, vascular contrast is improved by saturating out the background with a bombardment of RF pulses, but the gadolinium keeps the blood pool T1 relaxation time short, so saturation of the vessels is less of an issue.

But although contrast-enhanced MR angiography has distinct advantages over Time-of-Flight techniques in certain settings, it has its own limitations. For one thing, it nonselectively images the blood pool, so timing of the sequence is important to limit venous contamination (or arterial contamination if you want to look at the veins). The "leakage" of contrast into the tissues also decreases contrast and necessitates good sequence timing. The location of k-space is also important for optimal visualization of the target area (higher sginal intensity at center, higher spatial details at the periphery). If the center of k-space is filled before the contrast bolus peaks, it can result in ring artifacts.

The other problem with contrast-enhanced MRA is that the contrast agent itself (gadolinium) is relatively expensive, so the technqiue may not always be the best clinical choice.  Even more importantly, gadolinium constrast agents can't or shouldn't be administered to some patients.  Gadolinium contrast has been linked with nephrogenic systemic fibrosis (NSF) in patients with poor renal function, and TOF techniques may be necessary in these patients. The use of gadolinium contrast in pregnant patients is also considered less than optimal.

1. Prince MR, Yucel EK, Kaufman JA, et al. "Dynamic Gadolinium-enhanced Three-dimensional Abdominal MR Arteriography" JMlU 1993; 3:877-881
2. Tatli, Lipton, et al. “MR Imaging of Aortic and Peripheral Vascular Disease” Radiographics 2003; 23:S59–S78
3. Nael K, Villablanca JP, Saleh R, et al. Contrastenhanced MR angiography at 3T in the evaluation of intracranial aneurysms: A comparison with time-of-flight MR angiography. AJNR Am J Neuroradiol. 2006;27:2118-2121.
4. "MRI Principles" Mitchell DG, Rosen MS. 2nd ed (2004)
5. Duke Review of MRI Principles: Case Review Series" Mangrum, et al. (2012)