Physics developments and protocol optimisation at GOSH

Great Ormond Street Hospital (GOSH) is the biggest paedi- atric hospital in the UK and the Cardiac Unit is the big- gest paediatric congenital heart disease unit in Europe. Our centre for cardiovascular imaging at GOSH performed 1000 clinical scans and 500 research scans in 2013-2014 with a projected 50% increase in numbers for 2014-2015. In the MR (Magneto Resonance) Physics Development Group we augment the work of the Imaging Service at GOSH by significantly reducing scan times for routine imaging and increasing the numbers of children who can be scanned without anaesthesia. The Cardioproof protocol is extremely long with significant additional scanning required over and above routine clinical imaging. This may prolong scans times to over 1.5 hours, which is unacceptable to most patients in our population. Therefore, we have made a significant attempt to speed up the protocol by developing new accelerated sequences.

4D flow

e14D flow is one of the foundations of the Cardioproof project as it allows modelling of blood flow in the heart and aorta. Unfortunately, current scan times for this sequences can take up to 20 minutes and unless used in conjunction with a respiratory navigator are unsuitable for assessment of intracardiac and myocardial velocities. Therefore, we have put a significant amount of work into developing rapid methodologies for acquiring 4D flow data in the aorta and ventricles.

Vascular 4D flow

We have developed a 4D flow sequences that can acquire the whole aorta with a true isotropic spatial resolution of 2.0-2.5 mm and a true temporal resolution of 40-50ms in between 2-4 minutes (represent- ing a 5-10x acceleration). This has been achieved using a combination of a non-Cartesian stack of spiral k-space filling strategy and 3D parallel imaging. Furthermore, we have implemented fast online GPU reconstruction so that data is quickly accessible to the operator. An example in a repaired coarctation is shown below.

Myocardial 4D flow

Assessment of myocardial velocities is not possible without some form of respiratory compensation. Respiratory navigation is extremely time consuming and therefore we have vastly accelerated our imaging in order to acquire 4D myocardial and intracardiac velocity data in a single breath hold. This utilises the same technology as the vascular 4D flow acquisition but also add temporal encoding (UNFOLD) to further accelerate the acquisition. An example is shown below.

Rapid 2D flow acquisition

The Cardioproof protocol requires several 2D flow acquisitions that in congenital heart disease are usually acquired using time consuming free breathing acquisitions. We have developed a new breath hold 2D flow sequence that uses the same technologies as described above to acquire extremely high temporal resolution flow data within a short breath hold. This has significantly reduced up total acquisition time. The recent introduction of all these techniques (and other techniques previously developed) has now enabled us to perform the whole Cardioproof protocol in less than 1 hour, making it feasible within our clinical environment and with our specific patient population.

e2

Example data from a healthy volunteer. Top; peak S wave, bottom; peak E wave. A) 2D vector plots from the mid slice, b) Longitudinal velocity, c) Radial velocity, d) Tangential velocity. The colours represent the veolcities, with reds representing shortening/contraction/clockwise rotation, and blues representing lengthening/expansion/anti-clockwise rotation.

Author: Vivek Muthurangu

This article was originally published in Cardioproof Newsletter-Issue 1