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Assessing the effects of water exchange on quantitative dynamic contrast enhanced MRI
[Thesis]. Manchester, UK: The University of Manchester; 2011.
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Abstract
Applying mathematical models to dynamic contrast enhanced MRI (DCE MRI) datato perform quantitative tracer kinetic analysis enables the estimation of tissue characteristics such as vascular permeability and the fractional volume of plasma in atissue. However, it is unclear to what extent modeling assumptions, particularly regarding water exchange between tissue compartments, impacts parameter estimatesderived from clinical DCE MRI data. In this work, a new model is developed whichincludes water exchange effects, termed the water exchange modified two compartment exchange model (WX-2CXM). Two boundaries of this model (the fast and noexchange limits) were used to analyse a clinical DCE MRI bladder cancer dataset.Comparisons with DCE CT, which is not affected by water exchange, suggested thatwater exchange may have affected estimates of vp , the fractional volume of plasma.Further investigation and simulations led to the development of a DCE MRI protocol which was sensitised to water exchange, in order to further evaluate the waterexchange effects found in the bladder cancer dataset. This protocol was tested byimaging the parotid glands in eight healthy volunteers, and confirmed evidence ofwater exchange effects on vp , as well as flow Fp and the fractional volume of extravascular extracellular space ve . This protocol also enabled preliminary estimatesof the water residence times in parotid tissue, however, these estimates had a largevariability and require further validation.The work presented in this thesis suggests that, although water exchange effects donot have a large effect on clinical data, the effect is measurable, and may lead tothe ability to estimate of tissue water residence times. Results do not support achange in the current practise of neglecting water exchange effects in clinical DCEMRI acquisitions.
Layman's Abstract
Mathematical models can be applied to dynamic contrast enhanced MRI (DCE MRI) data to investigate of tissue characteristics such as vascular permeability and the fractional volume of plasma in a tissue. However, it is not clear how modeling assumptions, particularly assumptions regarding water exchange between tissue compartments, affects the results of analysis of clinical DCE MRI data. In this thesis, a new model is developed which includes water exchange effects, called the water exchange modified two compartment exchange model (WX-2CXM). Two boundaries of this model (the fast and no exchange limits) were used to analyse a clinical DCE MRI bladder cancer dataset. Comparisons with DCE CT, which is not affected by water exchange, suggested that water exchange may have affected estimates of the fractional volume of plasma. In order to further evaluate the water exchange effects found in the bladder cancer dataset, computer simulations were performed to optimize the development of a DCE MRI protocol which was sensitive to water exchange. This protocol was tested by imaging the parotid glands in eight healthy volunteers, and provided evidence of water exchange effects on DCE MRI. This protocol also gave preliminary estimates of the water residence times in parotid tissue, but these estimates had a large variability and require validation.The work presented in this thesis suggests that, although water exchange effects do not have a large effect on clinical data, the effect is measurable, and may lead to the ability to estimate of tissue water residence times. Results do not suggest a change in the current practice of neglecting water exchange effects in clinical DCE-MRI acquisitions is required.
Keyword(s)
T1 weighted MRI; WX-2CXM; arterial input function; bladder cancer; cardiac output; computed tomography; dynamic contrast enhanced; magnetic resonance imaging; parotid; perfusion CT; tracer kinetic modeling; two compartment exchange model; water exchange; water residence time