The potential for an enhanced role for MRI in radiation-therapy treatment planning

Abstract

The exquisite soft-tissue contrast of magnetic resonance imaging (MRI) has meant that the technique is having an increasing role in contouring the gross tumor volume (GTV) and organs at risk (OAR) in radiation therapy treatment planning systems (TPS). MRI-planning scans from diagnostic MRI scanners are currently incorporated into the planning process by being registered to CT data. The soft-tissue data from the MRI provides target outline guidance and the CT provides a solid geometric and electron density map for accurate dose calculation on the TPS computer. There is increasing interest in MRI machine placement in radiotherapy clinics as an adjunct to CT simulators. Most vendors now offer 70 cm bores with flat couch inserts and specialised RF coil designs. We would refer to these devices as MR-simulators. There is also research into the future application of MR-simulators independent of CT and as in-room image-guidance devices. It is within the background of this increased interest in the utility of MRI in radiotherapy treatment planning that this paper is couched. The paper outlines publications that deal with standard MRI sequences used in current clinical practice. It then discusses the potential for using processed functional diffusion maps (fDM) derived from diffusion weighted image sequences in tracking tumor activity and tumor recurrence. Next, this paper reviews publications that describe the use of MRI in patient-management applications that may, in turn, be relevant to radiotherapy treatment planning. The review briefly discusses the concepts behind functional techniques such as dynamic contrast enhanced (DCE), diffusion-weighted (DW) MRI sequences and magnetic resonance spectroscopic imaging (MRSI). Significant applications of MR are discussed in terms of the following treatment sites: brain, head and neck, breast, lung, prostate and cervix. While not yet routine, the use of apparent diffusion coefficient (ADC) map analysis indicates an exciting future application for functional MRI. Although DW-MRI has not yet been routinely used in boost adaptive techniques, it is being assessed in cohort studies for sub-volume boosting in prostate tumors.

Figure 1:

³He polarization image showing polarized MR helium 3 diffusion scan (right) fused on a proton-density-weighted MR scan (left). These active zones would be the regions of conformal avoidance, defined as low-dose objectives during the radiotherapy treatment planning process.

Figure 2:

CT (left) co-registered with a T1-3D-SPGR MRI image set (right). The difference in tumor visualization can be clearly appreciated.

Figure 3:

Visualization of the right trigeminal nerve root using FIESTA MR imaging. The arrow points to the right trigeminal nerve root.

Figure 4:

MR images at various stages of radiotherapy: (A) before radiotherapy, (B) mid-treatment and (C) post-treatment. The graphs in B and C show the voxel-by-voxel ADC values changing as determined during ADC image analysis on MRI. Green points on the graph indicate no change in ADC for a voxel from the baseline to the assessment time point, while red points indicate that the ADC in a voxel has increased compared to the baseline, and blue points indicate that it has decreased compared to the baseline.

Figure 5:

Photograph showing the set-up for head & neck planning on a wide-bore MRI scanner. A flat tabletop is used in conjunction with the standard thermoplastic immobilization shell. Two dedicated RF surface-coil arrays are placed laterally around the shell to enable high-quality imaging in the treatment position.

Figure 6:

MR image of breast showing irregularities (anatomical right breast). Most breast images are obtained using breast surface coils to reduce the signal-to-noise ratio, and the patient is usually positioned prone in the coils. This image is courtesy of the Aroura web site (www.Aroura.com).

Figure 7:

Axial, coronal and sagittal plane of a T2-weighted single-shot fast-spin echo image of a small-cell lung cancer for a 64-year-old male with T4 N2 M0 disease. There is excellent differentiation between the mediastinum and tumor mass, although the boundary between the tumor and the consolidation is unclear. (Images done on 1.5T GE scanner, with inspiration breath-hold.)