Vascular Application Tips: Time Resolved MRA

Contrast enhanced magnetic resonance angiography (CE-MRA) imaging is becoming an important modality in the assessment of the vascular tree. This technique overcomes the limitations of classical time-of-flight (TOF) techniques by a combination of short acquisition times and the possibility of covering areas as large as the receiver coil. Reliance on T1 shortening caused by the presence of gadolinium chelates renders the technique almost independent to pulsatility and flow. When vessels are imaged using a short TR, short TE gradient echo sequence, the blood appears very bright and the stationary tissues appear dark.

CE-MRA of the chest or abdomen is typically accomplished by completing the scan in a single breath-hold to limit respiratory artifacts, and during the first pass of a contrast agent for maximum arterial enhancement. However, depending on circulation, venous overlay might impair visualization of arteries and result in reduced diagnostic accuracy. This happens when the acquisition time is longer than the passage time. As the arterial-to-venous delay decreases, timing the bolus arrival and rapid scanning become more important. Time-resolved MRA techniques have become increasingly utilized because they can mitigate the need for precise bolus timing and, in addition, provide important dynamic information.

Phased array coils are placed cover the anatomic region of interest. After scouts and routine pre-contrast T1 and T2 weighted images (if needed) are obtained, a 3D pre-contrast MRA sequence (3D FLASH, typical parameters 3.4/1.3/25; 1.2-1.7mm slice thickness with the acquisition time < 10 seconds) is prescribed over the vascular region of interest. Subsequently multiple consecutive 3D CE-MRA data sets are initiated as contrast medium is injected. The number of repetitive measurements depends on the acquisition time of each single MRA data set. Typically a total acquisition time of 60-120 seconds is required to cover circumstances such as slow arterial inflow, and to make sure venous outflow is visualized. No timing exam is necessary. 15-20 mL of gadolinium-DPTA is generally required for a single station. The injection of contrast is performed using an MR-compatible remote-controlled power injector followed by 20mL of saline. The injection rate for both contrast and saline is set to 2-3 mL/second. When the upper extremity is being imaged, the contrast should be administered into the contralateral arm vein. Automatic inline subtraction of the first (nonenhanced) 3D volume from the subsequent identical 3D volumes obtained during and after contrast administration should be used, if available. Subsequently a 3D fat suppressed gradient echo sequence using the volumetric interpolated breathhold examination (VIBE) can be performed with a larger FOV through the area of interest ( TR/TE, 3.5/1.5; flip angle, 12º; slice thickness, 2-3mm;) for general anatomic survey.