Image-guided radiotherapy (IGRT) is the state-of-the-art in radiation treatment, and the recent introduction of integrated MRI-linac systems adds the potential for real-time tumor tracking during beam delivery.

However, achieving this requires the ability to quickly and accurately determine the position of the target volume and the critical structures from the magnetic resonance images.

When MRI is used to track structures for real-time radiotherapy guidance, these two coordinate geometries do not match and tracking errors can result, especially for thick image slices.

Divergent ray-tracing of  MR images is technically possible, but not suitable for real-time guidance due to the lengthy 3D acquisition and ray-tracing reconstruction times.

Now, Keith Wachowicz, Brad Murray and B. Gino Fallone from the University of Alberta have developed a theoretical framework that allows – for the first time – direct acquisition of BEV projection images in MRI. They also describe how their concept can be applied to various types of MRI-linac configurations.

Warping fields

To use MRI to track anatomy in real time, the researchers propose the use of non-conventional gradient field patterns, implemented through hardware additions to a standard scanner architecture.

They developed nonlinear encoding gradient fields that allow MR images to be generated in a divergent beam geometry. For MRI-linac systems where the radiation source is fixed relative to the magnet, adding two warping coils to the linear X and Y coils can produce these encoding fields.

For MRI-linac architectures in which the beam source is not fixed to the imaging magnet, the identified warping field pattern will only be appropriate for one source position.

In such cases, the researchers showed that a basic set of second-order spherical harmonic functions, together with linear gradients, provides a good approximation of the BEV gradient patterns at any angle.

They propose the use of a set of second-order warping coils fixed to the magnet, employed in various combinations to generate the conditions for divergent imaging as the source rotates. This would require four additional warping coils.

Proof of principle

To test their proposed theory, the researchers used a 3T scanner to image a phantom with nonlinear encoding-gradient field patterns. The phantom comprised gel-filled rods oriented to converge at a single point 100 cm away.

As the derived encoding gradients are not readily available, they approximated the ideal warping field to a second-order field gradient and created this using second-order shim coils. Such coils, however, are not currently designed for rapid switching in tandem with the linear encoding gradients.

“To test the feasibility of this approach in an environment without rapid-switching capability, we had first to find a sequence that maintained as much as possible a constant encoding gradient amplitude during image encoding,” Wachowicz explains.

"The closest match we could find was a short-echo radial acquisition,” Wachowicz added. “Secondly, we had to manually alter the shim coil currents according to our calculations for each of the 102 radial spokes that we acquired.”