Rapid Estimation of IVIM Pseudo-Diffusion Fraction with Correction of TE Dependence

Neil Peter Jerome^{1}, Matthew R Orton^{1}, Thorsten Feiweier^{2}, Dow-Mu Koh^{3}, Martin O Leach^{1}, and David J Collins^{1}

The
two-compartment IVIM diffusion model proposed by Le Bihan (1) is commonly used
for DWI studies in the body. In this model, compartments are taken to represent
*pseudo*-diffusion and *true* diffusion, which may in turn
represent vascular and tissue fractions. Standard
diffusion-weighted imaging (DWI) protocols are acquired at a single (usually
minimum) TE and incorrectly assume a single (apparent) T_{2}, thus causing the
observed pseudo-diffusion fraction f to be dependent on the TE chosen (2).
Distinct transverse relaxation constants for these components (T_{2p}
and T_{2t}
for pseudo- and true diffusion compartments, respectively) modify the standard
IVIM model (Eq. 1) for echo time (TE) dependency (Eq. 2), where f is pseudo-diffusion
fraction, D and D* are true and pseudodiffusion coefficients, T_{2p} and T_{2t} are the transverse
relaxations constants for pseudo- and true diffusion compartments, and $$${S_{eff}} ={S_0}.\exp\left(\frac{-TE}{T_{2apparent}}\right)$$$:

$${S_{b}} ={S_{eff}}.\left[ f.\exp\left(-b.D^*\right) + \left(1-f\right).\exp\left(-b.D\right)\right]$$ Eq 1 (standard IVIM model)

$${S_{b,TE}} ={S_0}.\left[ f.\exp\left(\frac{-TE}{T_2p}\right).\exp\left(-b.D^*\right) + \left(1-f\right).\exp\left(\frac{-TE}{T_2t}\right).\exp\left(-b.D\right)\right]$$ Eq 2 (extended T2-IVIM model)

We
present additional measurements at low b-values with increased TE as a method
of deriving an estimate of f that is independent of TE, a parameter not commonly
fixed in clinical MR studies. A b-value of 50mm^{-2}s is sufficient to
remove the pseudo-diffusion component in the liver (3), with the assumption
that associated signal decay due to true diffusion is small (<5% for D of
1x10^{-3}mm^{2}s^{-1} in the liver). Full sampling of
the b-value/TE space is challenging (4), however a clinical timeframe
acquisition is able to give a TE-independent estimation of f that may be more accurate,
and thus clinically useful and sensitive to modulation of pseudo-diffusion
fraction, as well as providing native estimations of T_{2}.

**1.** Le Bihan D, et al. *Radiology*; 1998;168:497 **2.** Lemke
A, et al. *Magn.
Reson. Med* 64:1580–1585 (2010) **3.** Jerome
NP, et al. *Proc.
Intl. Soc. Mag. Reson. Med*. 21 (2013), #2201 **4. **Orton
MR, et al. *Submitted
to ISMRM 2016* **5**. Stansiz GJ, et al. *Magn. Reson. Med* 54:507-512
(2005) **6.** Miyazaki K, et al. *Eur.
Rad* 25:2641-50 (2015) **7.** Cheng L, et al. *Proc. Intl. Soc. Mag.
Reson. Med*. 23 (2015), #2878

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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