Standard cardiac protocols involve multiple localization scans which require consecutive breath-holds (the de facto standard), alternatively system vendors have developed automated prescription techniques to improve time efficiency and repeatability.^[1] Both of these to some extent assume normal cardiac chamber morphology. The application of both of these techniques is challenging when the anatomy is abnormal, for example, in congenital anomalies. The aim of this work is to develop a novel approach to prescribing cardiac scan plane locations using an initial 3D "localizer" dataset of the heart which is then retrospectively reformatted to define the desired acquisition planes using a constrained surface controller (CSC).^[2] The geometry for these planes is then sent to the MRI system to facilitate high-quality CINE acquisitions at the desired locations.

A desktop sized planar constrained surface controller (CSC) was utilized (figure 1). To prevent disorientation the motion of the CSC is analogous to an ultrasound transducer where the operator is constrained to a reference planar surface mimicking the skin surface whilst limiting the total number of degrees of freedom to four (two translations and two rotations). Purpose built software was developed to enable 2D planar reformatting of the 3D localizer image and to transfer scan plane geometry. The software was developed in C++ incorporating the following open-source libraries: ITK (Insight Toolkit, www.itk.org), VTK (Visualization Toolkit, www.vtk.org) and QT (www.qt.io) for image processing, image visualization and the graphical user interface respectively on a 64-bit Linux operating system. A 3D sagittal, ECG triggered, navigated, balanced steady-state-free-precession (bSSFP) acquisition was acquired for localization (TE/TR= 1.4/3.2ms, FA=15°, FOV=34, matrix size=256×256×60, ASSET factor=2 (phase)). T₂ preparation was included to improve contrast between the blood and myocardium. The volumetric localizer images were imported into the CSC software. Homogenous transfer matrices (HTM) were evaluated to reformat the localizer image and to prospectively define the scan plane positions for subsequent CINE acquisitions. The HTM were derived separately for the image reformats and the scanner acquisitions to match geometrical prescriptions after accounting for the fact that both systems have different origins and thus centers of rotation. A 2D ECG-triggered CINE bSSFP sequence was acquired (TE/TR= 1.1/3.2, FA=35°, matrix size=192×128, phases=20, scan time=20s) after modifying it to enable interactive scan plane prescription updates from the CSC. The sequence was modified to support data transfer using the User Datagram Protocol (UDP) which enables the sequence to pause and listen for incoming geometry packets from the CSC software. Upon navigating the CSC to the desired cardiac view (figure 2), a key-press event transfers the geometry to the scanner. Following the breath-held acquisition, the scanner pauses again until the next geometry packet is transmitted. To assist with the scan plan prescription relative sagittal and coronal transforms, with superimposed guidelines, are defined to represent the current imaging plane (figure 3). The completed system was initially tested on standard geometric phantoms using a 3T whole body scanner (MR750, GE Healthcare, Waukesha, WI) and an 8-channel receiver coil.The 2D planar images reconstructed at 4Hz (using a 6-core Intel i7 3.2GHz). Testing on geometrical phantoms established the required systematic correction to reformat the image plane and scan plane at matched geometrical locations.The results suggest a reformat rate correspondent with requirements to achieve interactive feedback which has been achieved by utilizing C++ (i.e. a compiled program language). We found it was possible to reformat standard cardiac views using the proposed paradigm.This work demonstrates the development of a novel interactive approach to facilitate cardiac scan plane prescription based on using a CSC. This is potentially useful in complex cardiac anatomy cases where the conventional scan plane prescription approach may be either inappropriate or excessively time consuming.. This interactive approach allows for the combination of high resolution reformats of 3D bSSFP and targeted 2D CINE acquisitions to assess anatomy and function in complex cardiac cases. Future work will compare this CSC based method against the current techniques initially in normal volunteers and then in patients with complex cardiac anatomy.