Multiplane Review Or Multiplanar Reformating (MPR)
Real Time Three Dimensional Echocardiography (RT3DE) has remarkably enhanced the way we understand cardiac morphology, especially in congenital heart disease. Performing RT3DE has three important components: image acquisition, storage and analysis. Stored full volume loops of biometric data sets of RT3DE can be considered as a biological event immortalized and frozen in time which can be analyzed at any point in time. The main advantage of this form of data collection is that the stored information can be interrogated at anatomically appropriate planes and analyzed. The method of cutting a previously acquired full volume data set (FVD) of 3D images at infinite planes is known as Multi Planar Review (MPR). Specific software developed for the analysis of these data sets like Qlab (Philips Medical systems.) or TomTec Image Arens has now become a valuable tool in understanding the morphology of the congenitallymalformed heart and has revolutionalised the way we analyze the functional morphology of congenital heart defects.FVD with the co-ordinates preserved can be independently retrieved and modified for improved resolution and detailed interrogation with various biophysical analysis systems.
The most important aspect of 3dimensional echocardiography is the potential to slice the dynamic cardiac structures (while beating), in infinite planes through all the three dimensions. I improvised this technique so that the planes are moved systematically thorugh multiple planes throughout the cardiac cycle, in anatomically appropriate planes and in an attitudinally appropriate manner. MPR can be used even when the resolution of the images are poor and traditional 3D imaging has inadequate resolution to display underlying anatomy.
MPR could be considered as a transitional phase of cardiac imaging between 2D and 3D. This could be due to various reasons. The echocardiographic window may be inadequate or there could be anatomic tissue interposition like bone or non-echogenic structural interposition. The 3D images obtained may not have the desired definition for 3D dysplay but still have adequate anatomic information for analysis. Even when the image quality is excellent for reproduction of 3D images, the acquired images need not be in an anatomically appropriate plane for dissection. Cutting 3D volumes in a fixed plane could also introduce artifacts like defects that are created by cutting a septum that is out of plane or viewing structures distant from the region-of-interest due to lack of clarity in depth. MPR has the potential for reconstructing the images in attitudinally appropriate planes for accurate morphological display. MPR allows precise, focused, reproducible, quantification and reference points.
I will discuss the methodology of analyzing FVD in attitudinally appropriate anatomic planes and its clinical relevance below. I believe that this technique has not yet been exploited to its full potential.
In the vedio above, various echocardiographic method of understanding the cardiac anatomy is displayed. The first few frames show the traditional 2D cross-sectional imaging where the operator moves the probe in different planes to reconstruct a 3D image in their mind. Next you see the 2 plane visualization of the anatomy which is possible in most current echo equipments. Here the anatomy is displayed in two orthogonal planes simultaneously. The next part of the vedio displays aspects of real time 3D. This while giving real time 3D visualization of the heart, masks the anatomic detailsas only part of the anatomy is visualized in an anatomically inappropriate plane. This is followed by 3D MPR. Please study the alignment of the planes in anatomcally appropriate manner and carefully study the video.
The Three Steps to MPR
Here the focus is clearly on the anatomy of interest. For example, if you question is the morphology of the aortic valve is that you want to understand, then the planes are aligned to focus on the aortic valve. The heart is stopped in systole (as the anatomy of the aortic valve is best demonstrated in systole) Then one of the dissecting plane is moved to bring the aortic valve in to view. Each plane is now moved and aligned in such a way that the planes are oriented in anatomic plane appropriate to visualize the aortic valve. Most cardiac structures being symmetric, this often mean bringing the planes in perpendicular to each other. (Flash images to be inserted)
Once the anatomy of interest is aligned in anatomic planes, The structure is studied first in the frozen mode and then in dynamic mode. To optimise this one plane is moved at a time, and the pannel in which the plane is dysplayed is carefully observed to understand the anatomic details. This is repeated in other planes. Once the anatomic details are understood, the next step is to disply it in a way to explain the anatomic details so that others can appreciate it (3D visualization)
This is the final step of MPR. Once you have analized and understood the anatomy, the final step enable others to visualize the anatomic details in 3D. 3D display of cardiac anatomy is often attempted without the prior steps, resulting in poor understanding of the anatomy and its display, bringing 3D to its current state of misunderstanding, under utilization.
1. How to use MPR for determining septatability of complex congenital heart defects
2D TOE and colour Doppler images from a 6 year old girl with Double Inlet LV and TGA. Undervent atrial septostomy on day 1 floowed by PA band and atrial septectomy ( 6 weeks) at another centre. Subsequent bidirectional superior cavo-pulmonary anastomosis at 19 months at a major tirtiary care centre. The image above confirms the right atrio-ventricular valve over-riding the ventricular septum and the RV appears small. The image below shows classic double inlet LV.
Her saturations were low (72%) and has decreasing effort tolerence. Parents are keen on further intervention. Cardiac catheterization showed low PA pressures (PA mean 11) small forward flow through the PA band. No intra-pulmonary AVMs.
Double Inlet Left Ventricle with straddling right AV valve
Determining adequacy of ventricular volume and degree of commitment of the atriovetricular valve to corresponding ventricles along with connection of the great vessels to appropriate ventricular mass can be difficult in complex heart defects as that of this patient. The 2D images are rather convincing that this defect is unsuitable for septation. Please take a few minutes and go through the steps of MPR in this patient and see the difference MPR makes in understanding the morphology.
The transthoracic images are displayed below to illustrate the anatomic details as understood by standard cross sectional 2D echocardiography
The RV appears small. The septal leaflet of the right AV valve opens fully into the Lv cavity. On colour imaging, there is a wide open, un-restricted atrial communication with right to left shunt. The flow from the right AV valve is seen going into the LV cavity. There is an un-restricted ventricular septal defect.
The subcostal para coronal view shows the full profile of the right AV valve. There is cordal attachement of the septal leaflet attachinto the posterio-medial papillary muscle of the left AV valve. The right AV valve appears to be straddling the VSD. On the colour image at the right the para-sternal long axis view demonstrates the ventriculo-arterial relationship. The PA band is seen well in position with a turbulent flow pattern.
3D analysis and MPR
1. Alignment Video image in the right lower corner is a full volume 3D loop from the 3DTOE. It is sliced into 3 images by aligning the dissecting planes in sagital and coronal planes to illustrate the anatomy ( the upper 2 images). The cross sectional short-axis sliced image is displayed in the left lower corner. The left upper image is the standard 2D view one gets during initial the TOE. By aligning the cutting planes appropriately through the heart, the inlet and outlet aspects of the right heart is brought into view and displayed in the right upper corner. The shows an adequate size right heart. By further aligning the short-axis plane the appropriate right ventricular volume is brought out and the RV now appears slightly larger than the LV.
With further movement of the cutting planes in anatomically appropriate planes the crest of the ventricular septum is brought in line with the crux of the heart. The primum septum and the hinge points of the medial leaf lets of the two AV valve is brought in to view. The arrows illustrates the plane of the patch that may used to close the VSD
The magenda coloured dotted line goes through the the palne where VSD patch would be going through. Here the over riding of the right AV valve is less striking as it is prominent only in diastole. It was envisages that re-attaching the cordal attachments in the posterio-medial papillary muscle to the RV side would result in appropriate septation and a competent right AV valve.
This step of the MPR c(3D visualization or dysplay) is not an important aspect in this patient as all the necessary information is gained from the first two steps.
The post-operative 2D images shows successful septation of the ventricles with no significant intra cardiac shunts or AV valve regurgitation. The child now has normal oxygen saturations and good effort tolerence.