Georgios Krokos1, Neil Thacker1, Ibrahim Djoukhadar1, David Morris1, Alan Jackson1, and Marie-Claude Asselin1
1Medical and Human Sciences, University of Manchester, Manchester, United Kingdom
Synopsis
Variability
in the underlying assumptions and mathematical formulations of the commonly
used models for pharmacokinetic analysis of dynamic contrast-enhanced (DCE) MRI
data make model selection not straightforward[1]. The variable noise in DCE-MR images affects
the precision of parameters such that correcting the chi-square for the non-uniform
noise is required for model comparison[2]. After noise correction, the two-compartmental
exchange model fitted the data better, particularly in grade IV glioma,
compared to the extended Tofts and adiabatic approximation to the tissue
homogeneity models. Lengthening the acquisition duration not only provided more
precise parameter estimates but also reduced the mathematical correlations
between parameters.Purpose
This
work aims to incorporate a non-uniform noise correction into the pharmacokinetic
analysis of DCE-MR
images in patients with glioma in order (i) to compare the fit quality of the three
commonly used models, (ii) to evaluate
the impact of the acquisition duration on the parameter values and precision and
(iii) to estimate the parameter correlations for each model.
Methods
Seven patients with glioma (three grade II, three
grade III and one grade IV) underwent a pre-surgical DCE-MRI examination on a
1.5T Philips Achieva MR scanner with a SENSE coil. A low dose acquisition (pre-bolus,
0.025mmol/kg) was performed in the sagittal orientation for 3min with 1.7s
temporal resolution followed by an axial acquisition covering the whole brain for
15min with 3.4s temporal resolution after injection of the main bolus (0.075mmol/kg).
The images were acquired into a 128x128x20 matrix (pre-bolus) or 128x128x40
(main bolus), using slice oversampling with voxel sizes 1.9x1.9x2.5mm
3.
The peak of the arterial input function was generated from the internal carotid
artery using the pre-bolus acquisition and combined with the tail from the
superior sagittal sinus using the main bolus acquisition[3]. Kinetic analysis
was performed at the voxel level using the extended Tofts (ETM), the two-compartmental
exchange (2CXM)[4] and the adiabatic approximation to the tissue homogeneity (AATHM)
models with the bolus arrival time included as an additional fitted parameter
on 6 and 15min of data from the main bolus acquisition. All parameters in all
models were constrained to be non-negative; additionally, the extraction
fraction was constrained below 1 and the mean transit time below 1.5min in the
AATHM[5]. Parametric maps were generated and corrected for the non-uniform
noise distribution[2]. The tumour region was manually delineated by a
radiologist using the post contrast T
1-weighted and FLAIR images while the grey (GM) and white matter
(WM) regions were automatically segmented using FSL/FAST after masking the
tumour and oedema.
Results
Before
correction for non-uniform noise, all tissues had much broader chi-square (χ2) per degree of freedom (DOF) distributions than
theoretically predicted for all model used (figure 1). Applying the noise
correction narrowed down and aligned the distributions of the three tissues,
except for the grade IV tumour when fitted to the ETM and AATHM, revealing poorer
fits. Some parameters are not common to all models and v
p is also
defined differently between models (figure 3). Nonetheless comparing the
parameters estimated from the 15min data (table 1), v
e estimates were
similar between all models in all tissues whereas v
p estimates from
the AATHM were twice higher compared to the other models except in grade IV glioma
where they agreed with the 2CXM estimates F
E estimates from the 2CXM
were comparable to K
trans, and both parameters were an order of
magnitude lower than F
p estimated from either the 2CXM or AATHM. Except
in grade IV glioma, SEs on all parameters other than ve were twice
higher for the 2CXM than the other two models despite having similar χ
2/DOF values. When 6min of data were used (table 2), Ktrans
and FE increased by a factor of two while ve decreased by
~30%, F
p increased by ~20%, and v
p remained relatively
unaffected. Strong inverse correlations were observed in all tissues between K
trans
and v
p (ETM), F
E and v
p (2CXM) and E
with F
p and v
p (AATHM) which were weaker when using 15min
instead of 6min of data (figure 2). The other parameters were weakly correlated.
Discussion
The
distinct broad non-Gaussian χ
2/DOF
distributions arising from the non-uniform noise make it impossible to appraise
model performance. After noise correction, only the 2CXM was able to fit the fast
and slow kinetics, more clearly measured in grade IV glioma. In GM, WM and grades
II and III glioma where the contrast agent leakage is limited and thus time-activity
curves noisier, all models fitted the data similarly well. The slow kinetics
are partially captured in the short acquisitions, explaining the higher exchange
rate constants returned by all models compared to longer acquisitions. Furthermore, fewer frames are available with
short acquisitions for the estimation of the noise variance, making the noise correction,
χ
2 and SE estimates less accurate.
Conclusion
Non-uniform noise correction allowed assessment of
the performance of pharmacokinetic models and their comparison. The three
models provided comparable fits to GM/WM but only the 2CXM fitted the data from
the grade IV glioma equally well. More precise estimates of the pharmacokinetic
parameters were obtained by lengthening the acquisition duration. Strong
inverse correlations were found in all tissues between v
p and K
trans
(ETM), F
E (2CXM) and E (AATHM) which were weakened with longer
acquisition durations.
Acknowledgements
The
research leading to these results was supported by the Cancer Research UK and
EPSRC Cancer Imaging Centre in Cambridge and Manchester (grant C8742/A18097).References
[1]
Sourbron and Buckley (2012) Phys. Med. Biol. 57:R1-R33
[2]
Krokos et al ISMRM 2016 abstract in submission
[3] Li et al (2012) Magn. Reson. Imaging 68:452-62
[4] Sourbron et al
(2009) Magn. Reson. Med. 62:672-81
[5]
Kersaw et al (2010) Magn. Reson. Med.
64:1772-80