This work compares two methods of MRA: non-triggered and triggered MRA. Non-triggered MRA benefits from suspended respiration, permits first-pass selective angiography, suffers from significant cardiac ghosting artifacts, and can be performed with most MR contrast agents. Triggered MRA benefits from a stable breathing pattern, avoids a contrast test bolus for precise acquisition timing, provides equilibrium phase MRA only, may allow lighter general anesthesia requirements, takes longer to acquire, and thus requires a blood pool contrast agent. This work shows significantly improved delineation of most anatomic structures in pediatric cardiovascular MRA when cardiac and respiratory motions are minimized through combined triggering.
Previously, intracardiac structures have been known to be relatively poorly delineated on contrast-enhanced MRA. In this study, we show there is marked improvement in the definition of intracardiac structures when using combined cardiac and respiratory triggered MRA. However, although the coronary arteries were better seen on triggered versus non-triggered scans, there were still only a few cases in which the coronary visualization was considered excellent or outstanding. The greater anatomic detail afforded by a triggered MR angiographic study is likely to be especially important in small children with complex heart disease where high quality MRA images with 2D and 3D reconstructions might obviate the need for other imaging studies with ionizing radiation exposure such as cardiac catheterization and gated CTA. Although triggered scans took longer to acquire, the additional time expended was small relative to the typical overall exam time of a cardiac MRI.
A potential alternative approach to MRA is unenhanced navigated cardiac triggered balanced steady state imaging (bSSFP), which has routinely been used for coronary artery imaging. Similar to the method presented in this work, bSSFP delineates intra-cardiac anatomy well. However, it suffers inhomogeneity related artifacts, so delineating peripheral pulmonary vessels is challenging. Further, our clinical population often has a history prior cardiac surgery or catheter interventions, which results in severe metallic artifacts on bSSFP images.
Similar to bSSFP, with the triggered MRA technique described here, first pass selective angiograms are not obtained. Thus, deciphering arteries from veins may be more challenging and require detailed knowledge of cardiothoracic anatomy and complex congenital heart disease. Further, this may limit the degree to which some structures can be volume-rendered.
With current clinically approved gradients, acquisition of thoracic contrast enhanced MRA with cardiac gating at a resolution high enough to depict small structures such as coronary artery origins requires a lengthy scan time beyond a practical breath-hold. Thus, respiratory compensation with either navigation or triggering is required, further lengthening scan time. Thus, to gain the benefit of non-breath-hold MRA study, an intravascular contrast such as gadofosveset is needed to maintain a constant blood pool signal. However, by combining respiratory and cardiac triggering with a blood pool contrast agent, the typical constraints of MRA are removed. Bolus duration and breath-hold capacity are no longer factors limiting resolution and anatomic coverage. Rather, scan time and SNR become larger concerns. Though our triggered scans were limited to six-minute acquisitions, longer acquisitions could be employed to improve spatial resolution, further diminish motion artifacts, or improve SNR. These longer acquisitions may potentially replace other sequences of the cardiac MR exam that are targeted to anatomic delineation, thereby compensating for the increased scan time.
Though intracardiac anatomy may be delineated to greater extent with triggered MRA, various clinical applications may benefit from selective first-pass angiography. These potentially include coarctation, where 3D volume rendering may be improved by high SNR. Other applications, such as aortic root aneurysm in the context of Marfans may benefit from triggering, as may other cases requiring surgical planning of outlet reconstruction. Additionally, cases in which timing acquisition of imaging after contrast administration, such as Fontan circuits, may benefit from a blood-pool contrast agent. Decision-making for the use of blood-pool angiography may then best involve consideration of the clinical imaging need to assess moving structures, the ease of avoiding suspended respiration, and the increased cost of blood pool agent. At the moment, diagnosis of aberrant coronary artery is probably not reliable with either approach, though better optimization of trigger windows may improve this application.
The study has several limitations. First, scan parameters are not uniform in all subjects although their adjustments reflect “best clinical practice.” They were optimized for clinical indication, patient size, and heart rate. The resulting variation in scan parameters diminishes clean comparison of two techniques, but on the other hand, results in a study assessing how the two techniques would be used in a practical clinical setting. Second, image analysis was performed in orthogonal planes at fixed slice thickness only. Effects on additional imaging post-processing methods, such as oblique reformation, maximal intensity projection, and volume rendering, were not evaluated in this study. Third, in the triggered study, the cardiac window of data acquisition has not been optimized for specific anatomic structures. For example, with further adjustment of the cardiac triggering parameters, it may be possible to visualize the coronary arteries reliably. Finally, a somewhat arbitrary scan time limitation of six minutes for triggered MRA was imposed for this study, which may not be an optimal choice for various clinical situations. However, the promising results of this initial study motivate further evaluation of triggered MRA using a blood pool contrast agent.
Combined cardiac/respiratory triggering, enabled by a blood-pool contrast agent, improves delineation of anatomic structures in pediatric cardiovascular MRA.