Abstract
The bi-exponential intravoxel-incoherent-motion (IVIM) model for diffusion-weighted MRI (DWI) fails to account for differential T2s in the model compartments, resulting in overestimation of pseudodiffusion fraction f. An extended model, T2-IVIM, allows removal of the confounding echo-time (TE) dependence of f, and provides direct compartment T2 estimates. Two consented healthy volunteer cohorts (n = 5, 6) underwent DWI comprising multiple TE/b-value combinations (Protocol 1: TE = 62–102 ms, b = 0–250 mm−2s, 30 combinations. Protocol 2: 8 b-values 0–800 mm−2s at TE = 62 ms, with 3 additional b-values 0–50 mm−2s at TE = 80, 100 ms; scanned twice). Data from liver ROIs were fitted with IVIM at individual TEs, and with the T2-IVIM model using all data. Repeat-measures coefficients of variation were assessed for Protocol 2. Conventional IVIM modelling at individual TEs (Protocol 1) demonstrated apparent f increasing with longer TE: 22.4 ± 7% (TE = 62 ms) to 30.7 ± 11% (TE = 102 ms); T2-IVIM model fitting accounted for all data variation. Fitting of Protocol 2 data using T2-IVIM yielded reduced f estimates (IVIM: 27.9 ± 6%, T2-IVIM: 18.3 ± 7%), as well as T2 = 42.1 ± 7 ms, 77.6 ± 30 ms for true and pseudodiffusion compartments, respectively. A reduced Protocol 2 dataset yielded comparable results in a clinical time frame (11 min). The confounding dependence of IVIM f on TE can be accounted for using additional b/TE images and the extended T2-IVIM model.
Highlights
Diffusion-weighted imaging (DWI) is an important functional imaging technique in oncology, where signal intensity modulated by the motion of water molecules can be used to inform on tumour cellularity, tortuosity of extracellular space, and microstructural organisation
The ranges of b-values and TEs were selected to explore the b-TE space while retaining adequate signal-to-noise ratio (SNR); the range of 0–250 mm−2s means the data are not optimised for robust quantitative intra-voxel incoherent motion (IVIM) analysis, but in the healthy liver are adequate for assessing the pseudo-diffusion components of the proposed model (Jerome et al 2013)
The limit of this curve at TE = 0 ms gives the TE-independent pseudo-diffusion volume fraction, and the figure shows that this is significantly lower than that reported by the standard IVIM model at all echo times
Summary
Diffusion-weighted imaging (DWI) is an important functional imaging technique in oncology, where signal intensity modulated by the (diffusive) motion of water molecules can be used to inform on tumour cellularity, tortuosity of extracellular space, and microstructural organisation. While the apparent diffusion coefficient (ADC), conventionally derived by a two-point measurement with application of diffusion-sensitising magnetic field gradients of varying strengths (b-values), has shown utility in oncology for disease localisation, diagnosis, staging and assessing therapy response (Yamada et al 1999, Taouli et al 2009, Rosenkrantz et al 2010, Lee et al 2011, Pope et al 2012, Song et al 2013), the diffusion decay curve in tissues is often observed to deviate from the single exponential behaviour expected by simple Gaussian diffusion (Lemke et al 2009, Koh et al 2011, Rosenkrantz et al 2015, Winfield et al 2015, Jerome et al 2016). The two-compartment intra-voxel incoherent motion (IVIM) model proposed by Le Bihan et al (1988) is a popular choice for diffusion studies in the body, with the associated pseudo-diffusion volume parameter f being a potentially useful biomarker in oncology for lesion characterisation or response. In the two-compartment model framework, components are commonly taken to represent pseudo-diffusion and true diffusion, which may in turn represent vascular and tissue compartments, giving signal dependence on b-value according to:. The TE and T2 dependency is typically absorbed into the signal scaling term S0
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call