The syngo InSpace 3D/3D fusion functionality was designed to enable accurate registration of previously acquired MRI data to 3D syngo rotational angiography data sets. However, this requires a significant amount of radiation and contrast, making it impractical for routine use in children. We report on a method that we have developed to perform the registration process using the same commercially available software tools but with minimal radiation and without the need for contrast injection, making it feasible to perform XMRF on a routine clinical basis. Furthermore, we have shown that the registration in a phantom and in patients is accurate, with error values similar to those that have been previously reported.14
The current registration software requires that we perform a blank spin to acquire the table coordinates and the dimensions of the x-ray space. However, in practice this step could be easily eliminated because the registration parameters are derived exclusively from geometric consideration, such as the geometry of the gantry and the relative table position, which are fixed once the patient is placed in isocenter. Furthermore, the registration parameters for all possible camera angles can be derived from similar geometric considerations, which probably will increase the accuracy of the registration process in off-axis (steep) camera angles. At more extreme camera angles, other factors affecting the x-ray beam, such as x-ray beam distortion, also must be considered.
Rigid body registration algorithms result in an MRI volume whose shape is fixed in time. In reality, there are nonperiodic changes in the geometry of the heart and vessels caused by physiological factors, such as change in preload, and caused by distortion of the vessel conformation by stiff wires and catheters (). In addition, there are periodic changes from respiratory and cardiac motion that produce a time-varying registration error. The magnitude of the periodic errors in most cases does not exceed the error tolerance for most of the interventions that we encounter in congenital heart disease. In contrast, the nonperiodic errors, especially those caused by anatomy distortion by stiff wires and catheters, can be significant limiting the utility of this modality in certain situations. In these cases, nonrigid registration algorithms would need to be used.
Fiducial marker–based registration has been shown to be accurate to high spatial resolution under certain conditions, but this method has several drawbacks. First, it requires that the MRI scan immediately precede catheterization because the markers must remain attached during both procedures. Second, skin mobility and movement of internal organs relative to the skin can make this method inaccurate. Third, the need to include the markers in the MRI images requires a larger scanned volume, increasing imaging time, and could result in reduced imaging resolution. Fourth, fiducial markers can interfere with the catheterizer’s view and obstruct important structures during contrast angiograms. Last, fiducial marker–based registration requires propriety software that is not currently commercially available.
Internal marker–based registration offers several advantages over fiducial marker–based registration. Internal markers are inherently more stable. They allow for easy correction of patient motion and they do not interfere with contrast angiograms. Furthermore, internal marker–based registration alleviates the constraint imposed by fiducial marker–based registration, namely, the need to perform the MRI and the catheterization successively. In the present study, the heart and vessel borders were the most frequently used markers. In addition, we report on registration to other possible internal markers as a proof of concept and to show the generalizability of our methodology. In 2 cases, in which the heart and vessel borders were not clearly visible because of significant pulmonary edema, registration was done to the airway. Registration to the imaging artifact, stent, and calcification were easy to implement and subjectively accurate, but although more common in pediatric patients, these markers are not routinely present. In addition, the flexibility of our methodology allows for switching between MRI volumes and markers quickly as well as for using multiple markers simultaneously.
MRI is able to provide diagnostic-quality 3D images in most cases, even in the presence of metallic artifact. In contrast to rotational angiography or CT, MRI does not require ionizing radiation and it provides hemodynamic data. Furthermore, the 3D anatomy of the entire circulatory system is obtained with a single contrast injection, allowing us to roadmap the entire procedure and select camera angles for all vessels before the start of the procedure. However, there are certain conditions, such as in the presence of epicardial pacemaker leads or stainless steel coils, in which MRI is either contraindicated or nondiagnostic. In these cases, other modalities such as CT or rotational angiography can be used if 3D data are needed.16
The ability to roadmap without the need for contrast offers a distinct advantage in certain cases because contrast is known to significantly affect hemodynamics. Furthermore, the ability to roadmap the entire procedure and store the roadmaps as bookmarks has the potential to save time and reduce radiation exposure and contrast load. However, there are 2 limitations to this roadmapping modality that must be considered. First, the spatial resolution of MRI is on the order of 1 mm, making contrast injections necessary for roadmapping in very small vessels. Second, as previously mentioned, the presence of stiff wires or catheters can significantly alter the geometry of the vessels, making roadmapping based on previously acquired images inaccurate.
In all cases in which the nonfused 3D MRI anatomic data were used for initial approximate sizing of devices, the actual final device sizing was obtained from measurements performed on contrast angiograms. Care must be taken in trying to size devices on the basis of MRI data. The diameter of vessels in volume-rendered data sets highly depends on thresholding levels and is consequently not reliable.17
Sizing from noncontrast images, such as cine or bright-blood images, is potentially more accurate; however, one must take into account that the MRI images are averaged over multiple heart cycles and not obtained instantaneously, as is the case in angiography. In contrast to device diameter, sizing of device lengths can be more reliably determined from volume-rendered MRI data because this dimension is less dependent on rendering levels. Further studies must be conducted to determine if accurate sizing of devices based on MRI image can be reliably done.
The total dose-area product that we calculated in this study is lower than the results reported in the literature by Bacher et al18
of 4.6 mSv (range, 0.6 to 23.2) for all types of congenital heart disease diagnostic procedures. However, certain limitations in the study make it impossible to say with certainty if these results are significant. First, in the present study, we only had a small number of patients with each type of disease, and, of those, only a small number had purely diagnostic procedures. Second, because we only studied patients who needed both a catheterization and MRI, there is a significant selection bias in the cases that we evaluated. Last, in the present study, there was a large variation in patient size and age. Further studies must be conducted to determine the true effect of the XMRF modality on radiation exposure.
In the present study, we describe the new XMRF method that we have developed and show how the XMRF modality can be used during cardiac catheterization. In addition, the substantial hemodynamic and 3D anatomic information obtained from the MRI scans could allow us to plan the catheterization and may allow us to perform a more targeted catheterization aimed at only obtaining missing information. Further studies are currently underway to determine the effect of this modality on catheterization outcomes.
Limitations of the Current Study
In the current study, we describe the XMRF registration process and retrospectively discuss our early experience with this new modality in a relatively small number of patients and as such has several inherit limitations. Because of the way patients were selected, there may be significant selection bias. The clinical portion of the study is descriptive in nature; as such, we cannot draw definitive conclusions about the effect of this modality on catheterization outcomes. In particular, our assessments of the advantage that we derived from XMRF in specific cases were inherently subjective.
In patients, the registration error was calculated only in the AP projection during 1 phase of the cardiac cycle. Although we did perform angiograms in other angles and have subjectively seen accurate registration in off-axis angles, the small number of overlaid angiograms at each angle does not allow us to comment on the accuracy of the registration in all angles. However, it is likely that the registration error will increase with steeper camera angles. Also, we cannot comment on the dependence of the error on the temporal proximity of the MRI and catheterization studies because most of the MRI and catheterization cases were performed on the same day, and, although we demonstrated the feasibility of using internal markers other than the heart and vessel borders, the small number of cases that were performed using these markers make it impossible to comment on the accuracy of the registration to each of the different markers. In addition, in this work we did not account for periodic changes from respiratory and heart motion in the error calculation. In reality, these factors produce a time-varying error ε(t) that must be calculated using more elaborate methods. Further studies must be conducted to determine how these different factors affect the registration process and utility.