Paraspinal volumetric modulated arc therapy

Slice Encoding for Metal Artefact Correction in magnetic resonance imaging examinations for radiotherapy planning

BACKGROUND AND PURPOSE

Magnetic resonance (MR) and computed tomography (CT) images are degraded in the presence of metallic implants. We investigate whether SEMAC (Slice Encoding for Metal Artifact Correction) MR is advantageous for radiotherapy (RT) planning.

METHODS

Conventional and SEMAC MR protocols were compared (1.5T). A spine fixation device suspended in gelatine, two patients with spine fixation devices and six patients with bilateral hip replacements were scanned with both conventional and SEMAC protocols. In spine patients the visibility of the spinal canal and spinal cord was assessed; in prostate patients, the visibility of the prostate, pelvic structures and the pelvic girdle.

RESULTS

The signal loss volume surrounding the spine fixation device was reduced by approximately 20% when the SEMAC protocol was employed, and registration errors were reduced. For spine patients, the spinal canal was completely visible only using the SEMAC protocol. In hip replacement patients, metal artifacts were local; the signal loss extended to the internal surface of the acetabulum in eight implants with conventional protocols, but only in four using SEMAC.

CONCLUSIONS

SEMAC MR contributes towards correct co-registration of MR and CT images for RT planning, and is particularly relevant when the TV or OARs are close to implants.

Radiotherapy planning using MRI

The use of magnetic resonance imaging (MRI) in radiotherapy (RT) planning is rapidly expanding. We review the wide range of image contrast mechanisms available to MRI and the way they are exploited for RT planning. However a number of challenges are also considered: the requirements that MR images are acquired in the RT treatment position, that they are geometrically accurate, that effects of patient motion during the scan are minimized, that tissue markers are clearly demonstrated, that an estimate of electron density can be obtained. These issues are discussed in detail, prior to the consideration of a number of specific clinical applications. This is followed by a brief discussion on the development of real-time MRI-guided RT.