ESTRO 2024 - Abstract Book

S4964

Physics - Radiomics, functional and biological imaging and outcome prediction

ESTRO 2024

Figure 1: Schematic illustration of tissue microstructure with intra- and extracellular compartments. D in and D ex are the intra- and extracellular diffusivities, and k is the exchange rate between the compartments. Water diffusion in the intracellular space (orange trajectory) is restricted, i.e. cannot exceed the limits of the intracellular space and is thus time-dependent. Diffusion in the extracellular space (blue trajectory) is only mildly hindered by tissue microstructure and is less time dependent. Exchange can mix water between the compartments (green trajectory), which also makes the diffusivity time dependent. Figure adapted from [6].

Material/Methods:

Four patients (2 male, 2 female, mean age 63 ± 9 yrs) with brain lesions (2 grade IV gliomas, 2 brain metastases) were imaged with TDD-MRI on a 1.5 T MR sim (Ingenia, Philips HealthCare) with a 45 mT/m gradient system. TDD-MRI was implemented as two (standard) DW-MRI acquisitions (b-values = 0, 1250, 2500 s/mm 2 , spin-echo EPI read-out), which had different diffusion times ( t d, short = 26 ms and t d, long = 44 ms) to assess the time-dependency of the diffusivity, estimated by the ADC. The ADC maps were calculated separately for the t d, short and t d, long acquisition by fitting a log linear model to the MR signal voxel-wise. The time-dependency of the ADC (ΔADC) was investigated by subtracting the ADC map acquired with t d, short from the map acquired with t d, long .

Results:

The ΔADC showed clear regional signal variation within the gross tumor volumes (GTVs) (Figure 2). The two grade IV glioma cases are largely defined by positive signal in the ΔADC map. This indicates that water exchange is the dominant interaction between water molecules and cell membranes, indicating substantial membrane permeability. The two brain metastasis cases have clear areas of negative signal close to the edges of the GTV in the ΔADC map. This indicates that restricted diffusion is the dominant interaction, indicating restriction sizes, e.g. cell sizes and cell density, within the sensitivity range of the measurement. The structure visible in the ΔADC maps partly corresponds to structures visible in the conventional ADC map. However, the ΔADC gives more specific information about the tissue microstructure than the conventional ADC map alone.