MRI diffusion phantoms: ADC and relaxometric measurement comparisons between polyacrylamide and agarose gels.

PURPOSE: The aim of this study is to compare polyacrylamide and agarose gels, as components of a simple MRI phantom, for the measurements of Apparent Diffusion Coefficient (ADC), T1 and T2 relaxation times. MATERIALS AND METHODS: Five (5) test tubes with polyacrylamide gels of different monomer concentrations and six (6) test tubes of different agarose gel concentrations were used as a phantom for ADC, T1 and T2 measurements, which were expressed as 2D color parametric maps, on a 1.5 T clinical MRI system. ADC and T2 maps were calculated utilizing a Weighted Linear (WL) regression fitting algorithm. T1 maps were calculated utilizing a standard non-linear fitting algorithm. RESULTS: In agarose gels, ADC measurements are independent of the agarose concentration, whereas the T1 and T2 relaxation times decrease with increasing agarose concentration. On the contrary, in polyacrylamide gels, ADC measurements decrease quadratically while increasing the monomer concentration, whereas the T1 and T2 relaxation times reveal a linear decrease with increasing monomer concentration. CONCLUSION: Polyacrylamide gels can serve as a better means for simulating ADC values, as compared with the agarose gels used in this study.

Dosimetric performance of the Elekta Unity MR-linac system: 2D and 3D dosimetry in anthropomorphic inhomogeneous geometry.

Following the clinical introduction of the Elekta Unity MR-linac, there is an urgent need for development of dosimetry protocols and tools, not affected by the presence of a magnetic field. This work presents a benchmarking methodology comprising 2D/3D passive dosimetry and involving on-couch adaptive treatment planning, a unique step in MR-linac workflows. Two identical commercially available 3D-printed head phantoms (featuring realistic bone anatomy and MR/CT contrast) were employed. One phantom incorporated a film dosimetry insert, while the second was filled with polymer gel. Gel dose-response characteristics were evaluated under the Unity irradiation and read-out conditions, using vials and a cubic container filled with gel from the same batch. Treatment plan for the head phantoms involved a hypothetical large C-shape brain lesion, partly surrounding the brainstem. An IMRT step-and-shoot 7-beam plan was employed. Pre-treatment on-couch MR-images were acquired in order for the treatment planning system to calculate the virtual couch shifts and perform adaptive planning. Absolute 2D and relative 3D measurements were compared against calculations related to both adapted and original plans. Real-time dose accumulation monitoring in the gel-filled phantom was also performed. Results from the vials and cubic container suggest that gel dose-response is linear in the dose range investigated and signal integrity is mature at the read-out timings considered. Head phantom 2D and 3D measurements agreed well with calculations with 3D gamma index passing rates above 90% in all cases, even with the most stringent criteria used (2 mm/2%). By exploiting the 3D information provided by the gel, comparison also involved DVHs, dose-volume and plan quality metrics, which also reflected the agreement between adapted and delivered plans within ±4%. No considerable discrepancies were detected between adapted and original plans. A novel methodology was developed and implemented, suitable for QA procedures in Unity. TPS calculations were validated within the experimental uncertainties involved.

The impact of spin coupling signal loss on fat content characterization in multi-echo acquisitions with different echo spacing.

PURPOSE: This study aimed to assess the effect of echo spacing in transverse magnetization (T2) signal decay of gel and fat (oil) samples. Additionally, we assess the feasibility of using spin coupling as a determinant of fat content. METHODS: Phantoms of known T2 values, as well as vegetable oil phantoms, were scanned at 1.5T scanner with a multi echo FSE sequence of variable echo spacing above and below the empirical threshold of 20ms for echo train signal modulation (6.7, 13.6, 26.8, and 40ms). T2 values were calculated from monoexponential fitting of the data. Relative signal loss between the four acquisitions of different echo spacing was calculated. RESULTS: Agreement in the T2 values of water gel phantom was observed in all acquisitions as opposed to fat phantom (oil) samples. Relative differences in signal intensity between two successive sequences of different echo spacing on composite fat/water regions of interest was found to be linearly correlated to fat fraction of the ROI. CONCLUSION: The sample specific degree of signal loss that was observed between different fat samples (vegetable oils) can be attributed to the composition of each sample in J coupled fat components. Hence, spin coupling may be used as a determinant of fat content.