MR-Linacs

Magnetic resonance imaging (MRI) is a technology that uses powerful, time-varying magnetic fields and computers to obtain detailed information about the internal structure of certain types of objects. MRI scanners are widely used in modern medicine because they provide excellent 3D images of patients’ internal anatomy. In particular, MRI scanners are better than CT scanners at imaging soft tissues and organs, such as in the abdomen or pelvis, and they do not deliver any radiation dose, unlike CT and other x-ray-based imaging methods.

A medical linear accelerator, or linac, is a machine commonly used to deliver therapeutic doses of radiation to patients, usually for the treatment of cancer. Linacs are capable of delivering beams of high-energy x-rays or electrons that penetrate into a patient’s body and kill tumors.

In standard radiation therapy workflows, medical imaging is performed in preparation for radiation therapy and used for planning treatment. MRI scans may be used to help identify the precise location of tumors and healthy tissues as part of the process of creating a radiation treatment plan for treatment on a linac.

The human body is not a rigid object, and bones and internal organs can shift in their positions from minute to minute and from day to day – in some cases by up to a few centimeters! As a result, patient anatomy at the time of linac treatment may not be exactly the same as it was at the time of MRI or CT imaging a few days earlier. In all radiation therapy workflows, adjustments to patient position are made before treatment is delivered to ensure that radiation is delivered accurately. This can be done with various imaging methods, including x-ray imaging. An MR-linac, which combines an MRI scanner and a medical linac into a single machine, offers the capability of performing high-quality soft-tissue imaging and radiation delivery at the same time.

The primary challenge in building an MR-linac is to find a way to make a linac function correctly in an MRI scanner’s extremely strong magnetic fields. There are currently two MR-linacs commercially available. For both products, researchers had to find innovative ways to overcome a myriad of technical challenges.

An MR-linac’s unique combination of technologies makes it possible to better visualize a patient’s soft-tissue anatomy immediately before and during radiation treatment. The MR images can be used either to “gate” the delivery of an existing treatment plan – that is, the linac will only deliver radiation when the images show that the patient is in the same position as in the treatment plan – or to “adapt,” or adjust, the treatment plan slightly to match the current arrangement of the patient’s anatomy. This capability is especially valuable when treating areas of the body such as the abdomen and pelvis where many soft tissues are packed together and can only be clearly visualized using MRI. The scanner and linac thus work together in an MR-linac to better achieve the ultimate goal of radiation therapy, which is to deliver radiation to a target inside the body with high precision while avoiding surrounding healthy tissues and organs.