Real-time radiation beam imaging on an MR linear accelerator using quantitative T₁ mapping

Area of Science:

• Medical Physics
• Radiotherapy
• Magnetic Resonance Imaging

Background:

• Direct 3D imaging of radiation beams can improve radiation dosimetry accuracy.
• T1-weighted MRI intensity changes during radiation, linked to radiochemical oxygen depletion, have been observed.
• Quantitative T1 mapping offers enhanced sensitivity for dosimetry applications.

Purpose of the Study:

• To visualize radiation delivery in real-time using an MR-Linac by observing dose-induced changes in water's spin-lattice relaxation time (T1).
• To quantify the relationships between radiation dose, spin-lattice relaxation rates (R1), and dissolved oxygen concentration.
• To investigate the underlying mechanisms of T1 changes during radiation exposure.

Main Methods:

• Irradiation and imaging of ultrapure water and 1% agarose gel phantoms using a 1.5 T Elekta Unity MR-Linac.
• Acquisition of images before, during, and after radiation using a dual-echo Look-Locker sequence for simultaneous dynamic T1/B0 mapping.
• Measurement of R1 change with dose (ΔR1/ΔDose), radiochemical oxygen depletion (ROD), and oxygen relaxivity (r1,O2).

Main Results:

• R1 changes correlated spatiotemporally with delivered radiation dose and persisted for over an hour post-irradiation.
• In water, ΔR1/ΔDose was -1.0 × 10⁻⁴ s⁻¹/Gy, r1,O2 was 5.4 × 10⁻³ s⁻¹/(mg/L), and ROD was -0.010 (mg/L)/Gy.
• Radiochemical oxygen depletion alone under-predicted the observed MRI effect; minimum detectable dose was 11.1 Gy.

Conclusions:

• Quantitative T1 mapping successfully imaged radiation dose patterns in real-time in water and agarose gel.
• Radiochemical oxygen depletion accounts for only part of the measured T1 changes.
• Agarose gel can serve as a phantom for 3D patient-specific quality assurance; future applications may include in vivo dosimetry for FLASH radiotherapy.