Enhancing the precision of quantitative water content mapping by optimizing sequence parameters

Abstract

Quantitative measurement of water content using MRI, e.g., in the human brain, is important for the study and diagnosis of pathologies associated with an altered hydration state, such as stroke, multiple sclerosis, and cerebral tumors and metastases (1). However, protocols for measuring water content have not yet found widespread use in clinical routine or scientific studies, either because of the long measurement times required or because of the low precision or accuracy inherent to the faster methods described thus far (for an overview of quantitative water content mapping see, e.g., Ref. 2).A new flexible method for quantitatively mapping water content that is based on the accurate and precise acquisition of T 2 * and T 1 relaxation time curves with the quantitative T 2 * image (QUTE) (3,4) and T 1 mapping with partial inversion recovery (TAPIR) (5-8) sequences was recently introduced (4). By placing a reference probe of 100% H 2 O in the field-of-view (FOV) and correcting for transmitter and receiver imperfections, temperature differences between the reference probe and the object to be imaged, as well as saturation and decay effects, one can quantify water content with a precision of Ͼ98% (4). The method was successfully applied to the study of age-and gender-specific variations of cerebral water content in a cohort of 44 healthy subjects (9). In addition, the presence of a low-grade edema was demonstrated for the first time unambiguously and quantitatively in patients with hepatic encephalopathy (10).The flexibility and complexity of the new approach requires a careful optimization of both T 2 * and T 1 acquisition schemes. However, even though both measurements are independent and therefore apparently are not correlated, this simple view does not hold for the new method, in which the effective flip angle is determined based on the information acquired from both acquisitions (4). In addition, the required precision of T 1 mapping to achieve a given precision of the water content measurement depends on the actual parameters chosen for T 2 * mapping and vice versa. Therefore, the two apparently independent experiments become correlated when their influence on the precision of the quantitative water content measurement is evaluated. This necessitates the use of a combined optimization scheme for both acquisitions.The general methodology for maximizing the precision (i.e., minimizing the random error) of quantitative water content maps with regard to the selection of sequence parameters is presented in this article together with an experimental validation of the theoretical results obtained. THEORY Quantitative Water Content W MRThe estimation of the quantitative water content, W MR , is extensively described in Ref. 4. However, for the sake of completeness, the basic principles and all relevant equations in the context of the current work are briefly described below. Water content mapping is based on extraction of the proton density (PD) from T 2 * relaxation time mapping with the multiecho gradient-echo sequ...

A new algebraic method for quantitative proton density mapping using multi-channel coil data

“…Potential clinical applications involve quantitative water content mapping in multiple sclerosis to determine demyelination tissue properties of white and gray matter lesions (Laule et al, 2004, 2006; Mezer et al, 2013). Other applications include quantitative imaging of edema that frequently arise in head trauma, stroke, brain tumors and other brain diseases such as hepatic encephalopathy (Ajata and Robber, 2002, Andersen, 1997; Neeb and Shah, 2006; Neeb et al, 2006; Oros-Peusquens et al, 2014; Shah et al, 2003, 2008). …”

Quantitative water content mapping at clinically relevant field strengths: A comparative study at 1.5T and 3T

“…Quantitative water content mapping is possible using MRI by estimating the MR-visible proton density, which is known to provide a very reliable measurement of the free water pool (Tofts, 2003). In the healthy brain, the water content is very highly regulated (Agre et al, 2004;Amiry-Moghaddam and Ottersen, 2003;Kimelberg, 2004;Neeb et al, 2006a), but in pathologies, such as ischaemia, brain tumours, neuroinflammation and neurodegeneration, it is known to change (Andersen, 1997;Laule et al, 2004;Neeb and Shah, 2006;Neeb et al, 2006b;Oros-Peusquens et al, 2014;Shah et al, 2003Shah et al, , 2008Volz et al, 2012b). Depending on the pathology, water content changes manifest locally (Abbas et al, 2014b) or globally in the form of ubiquitous cerebral oedema (Shah et al, 2008).…”

Is the magnetization transfer ratio a marker for myelin in multiple sclerosis?

“…Two approaches have been reported for measuring brain water content in vivo by MRI. By correcting the MR signal for T 1 and T 2 relaxation, accounting for radiofrequency inhomogeneity if required, and comparing to a water standard, brain WC can be measured directly in g/mL (18–22). Other studies have made use of the well‐known linear relationship between 1/T 1 and 1/WC (23–26) to use 1/T 1 for an indirect measure of WC.…”