Longitudinal assessment of quality assurance measurements in a 1.5 T MR‐linac: Part II—Magnetic resonance imaging

Abstract

To describe and report longitudinal quality assurance (QA) measurements for the magnetic resonance imaging (MRI) component of the Elekta Unity MR-linac during the first year of clinical use in our institution. Materials and methods:The performance of the MRI component of Unity was evaluated with daily, weekly, monthly, and annual QA testing. The measurements monitor image uniformity, signal-to-noise ratio (SNR), resolution/detectability, slice position/thickness, linearity, central frequency, and geometric accuracy. In anticipation of routine use of quantitative imaging (qMRI), we characterize B0/B1 uniformity and the bias/reproducibility of longitudinal/transverse relaxation times (T1/T2) and apparent diffusion coefficient (ADC). Tolerance levels for QA measurements of qMRI biomarkers are derived from weekly monitoring of T1, T2, and ADC. Results: The 1-year assessment of QA measurements shows that daily variations in each MR quality metric are well below the threshold for failure. Routine testing procedures can reproducibly identify machine issues. The longitudinal three-dimensional (3D) geometric analysis reveals that the maximum distortion in a diameter of spherical volume (DSV) of 20, 30, 40, and 50 cm is 0.4, 0.6, 1.0, and 3.1 mm, respectively. The main source of distortion is gradient nonlinearity. Maximum peak-to-peak B0 inhomogeneity is 3.05 ppm, with gantry induced B0 inhomogeneities an order of magnitude smaller. The average deviation from the nominal B1 is within 2%, with minimal dependence on gantry angle. Mean ADC, T1, and T2 values are measured with high reproducibility. The median coefficient of variation for ADC, T1, and T2 is 1.3%, 1.1%, and 0.5%, respectively. The median bias for ADC, T1, and T2 is −0.8%, −0.1%, and 3.9%, respectively. Conclusion:The MRI component of Unity operates within the guidelines and recommendations for scanner performance and stability. Our findings support the recently published guidance in establishing clinically acceptable tolerance levels for image quality. Highly reproducible qMRI measurements are feasible in Unity.

Comisionado de un MR-linac Elekta Unity

“…Estas dos secuencias son las más comúnmente utilizadas en calidad de imagen de RM y el sistema utiliza automáticamente una u otra según los datos que se quieran obtener. 18,32 Todas las pruebas se llevan a cabo utilizando un maniquí de cabeza de 200 mm situado en el isocentro del sistema (fig. 4).…”

“…La resolución espacial mide la separación entre los dos primeros ceros de la función de difusión de línea en las direcciones horizontal y vertical. 18…”

“…[12][13][14][15] En estos últimos años, ante la falta de una guía con carácter oficial para la aceptación y el comisionado del MR-linac Unity (Elekta AB, Estocolmo, Suecia), esta tarea se ha venido realizando siguiendo las indicaciones de la empresa fabricante Elekta, y de diferentes trabajos publicados. [16][17][18][19][20][21][22] Una vez que concluye la instalación del equipo en el búnker, el personal de Elekta realiza las pruebas de aceptación (DAT), durante las que miembros del Servicio de Radiofísica están presentes para verificar que el equipo cumple especificaciones y solicitar modificaciones si no es así. El comisionado, las medidas del modelado del haz y su posterior validación se realizan conjuntamente por el Servicio de Radiofísica del centro, el personal de Elekta y el de Philips para las pruebas de calidad de imagen de la RM.…”

A model for gastrointestinal tract motility in a 4D imaging phantom of human anatomy

“…Broadly, the sources of uncertainty in MRI‐guided radiotherapy can be categorized into delivery‐ and patient‐specific factors. Recent work on the longitudinal assessment of quality assurance measurements has identified delivery‐specific factors related to performance, stability, safety, and machine tolerance levels 10,11 …”

“…Recent work on the longitudinal assessment of quality assurance measurements has identified deliveryspecific factors related to performance, stability, safety, and machine tolerance levels. 10,11 The uncertainties that arise from variations in patient anatomy and physiology remain the most challenging to quantify and mitigate. This is particularly challenging in the abdomen where respiratory, cardiac, and GI motion are often simultaneously present during delivery.…”