Modifying MRA Contrast Administration For the Better
Multiple “switches” may tune up an already useful MR angiography. Harald Kramer, Munich, Germany, explained how radiologists can push an MRA scan from good to better.
“We find an awful lot of contrast injection schemes, rates and volumes in the literature –why is there no fixed protocol like there is for CT?”, asked Kramer.
He made the reason look simple: While there is a linear relation between iodine and signal intensity, this is different for MR: There is no linear relation between gadolinium and signal intensity. “If the gadolinium concentration is too high, there is even a negative effect on you images – so your gadolinium concentration should be high, but not too high,” explained Kramer.
“You also need to know that you use a relatively small volume”, he added. While the volume of iodinated contrast media is relatively high (≥1mL per kg body weight), gadolinium-based agents only take a fraction of that (0.1-0.2mL per kg body weight).
Kramer explained how contrast and blood undergo significant tumbling from the contrast administration site throughout the vessels. “Gadolinium concentration decreases by the factor ten between the superior vena cava to the ascending aorta,” said Kramer. This decreases contrast peak intensity and increases bolus length.
Modifying the injection rate has only a small influence on contrast distribution. Kramer cited a study that investigated the peak intensity with 1mL contrast per second versus up to 5mL/sec. It found a later and smaller peak enhancement for 1mL/sec, however, the other injection rates did not make any difference. “The influence of the pulmonary and heart passage is much higher then your injection rate,” concluded Kramer.
Small contrast volume may theoretically lead to a blurry depiction of vessel walls. However, one of Kramer’s own studies, which compared 0.5- to 1-molar agents has shown smaller volumes to not affect vessel sharpness (Kramer 2013).
Center of K-Space Synchronization
The k-space center is responsible for vessel contrast and its acquisition only takes a couple of seconds, where as the k-space periphery gives details, but takes minutes to acquire. To achieve high contrast, it is thus essential to synchronize the arterial first pass peak and the acquisition of k-space center. “This means you have to know the read-out time of your sequence”, underlined Kramer. The time to center of k-space is the essential parameter.
The synchronization can be done:
- automatically with bolus triggering. “But be aware that there is a three to five second switching time, if you change from 2D to 3D sequences”, said Kramer. In abdominal imaging this means that the switching button should be pressed, when the contrast medium is in the heart.
- Manually with a test bolus. The peak needs to be identified and the delay time then needs to be calculated.
“While contrast administration is fairly easy, contrast agent timing is really worth investing your time”, underlined Kramer.
Recently, feasible non-contrast techniques like KISS have been published. “However, these techniques do not work all the time,” commented Kramer. He considers gadolinium-based contrast enhanced MRA is still superior.
Kramer JH et al.Dynamic and static magnetic resonance angiography of the supra-aortic vessels at 3.0 T: intraindividual comparison of gadobutrol, gadobenate dimeglumine, and gadoterate meglumine at equimolar dose. Invest Radiol. 2013;48(3):121-8
Presentation Title: Essentials of comprehensive MRA
Speaker: Harald Kramer, Radiology Oberbayern Munich, Germany
Date: Thursday, 28th February 2019
Session Code: SY 14