An MR-linac is a machine which combines the detailed imaging capabilities of a magnetic resonance (MR) scanner with the radiation delivery capabilities of a medical linear accelerator, or linac.

The two commercial MR-linacs in wide use in recent years: the Elekta Unity (left)
and ViewRay MRIdian (right). Some new options, such as the MagnetTx Aurora,
are now entering the market. (Images courtesy of the manufacturers)

What is magnetic resonance imaging?

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.

What is a linac?

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 that penetrate into a patient’s body and kill tumors.

The primary challenge in building an MR-linac is finding a way to make a linac function correctly in an MRI scanner’s extremely strong magnetic fields. To overcome this issue researchers had to find innovative solutions to numerous technical difficulties.

How are MRI scanners usually used in radiation therapy?

In traditional radiation therapy workflows, imaging scans are performed days or weeks before treatment because the images are needed to prepare a computerized plan for linac radiation delivery. MR images may be used to aid in identifying tumors and healthy tissues when creating this treatment plan.

Why was the MR-linac developed?

The human body is not a rigid object, and internal organs can shift in position from minute to minute and from day to day – in some cases by up to a few centimeters! As a result, a patient’s 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.

For this reason, a patient’s position is always carefully adjusted before linac treatment to ensure radiation is delivered accurately. This positioning can be accomplished in various ways, and with traditional linacs it is typically assisted by x-ray imagers attached to the machine. An MR-linac instead utilizes MR imaging to optimize treatment delivery.

(Left) A patient being set up for treatment on a Unity MR-linac.
(Right) MR image from a Unity machine, overlaid with dose from an
adapted treatment plan. (Images courtesy of Elekta)

An MR-linac’s unique combination of technologies makes it possible to better visualize a patient’s soft-tissue anatomy before and during radiation treatment. The MR images may be used to adjust the patient’s position, to “gate” the delivery of a treatment plan (that is, the linac will only deliver radiation when the images show the patient is in the same position as in the treatment plan, such as during respiration), or to “adapt” the treatment plan to match the current arrangement of the patient’s anatomy. These capabilities are 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.

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