Table 1.
Conventional imaging biomarkers in glioblastoma (12).
| Imaging biomarker | Details |
|---|---|
| Apparent diffusion coefficient (ADC) | Measurement of inferred (‘apparent’ rather than actual) water diffusion with DWI. It is a measure of the relative decrease in the transverse magnetization induced by additional dephasing and rephasing magnetic field gradients. Net dephasing and therefore signal loss is greater in freely diffusive tissue. Quantitatively, the ADC is the slop of a line plotting the natural logarithm of the MRI signal (y-axis) per unit of applied magnetic field strength (b-value plotted on x-axis; units mm2/s). |
| Mean diffusivity (MD) | This is the magnitude of mean diffusion in a given voxel obtained with diffusion tensor imaging (DTI). ADC may not be uniform at all orientations. MD is the average diffusivity from the three eigenvalues of the diffusion tensor. It is often regarded as an approximation of the overall ADC (units mm2/s). |
| Fractional anisotropy (FA) | DTI provides FA values which indicate the overall directionality of water diffusion within a voxel. FA is a scalar value between 0-1 that describes the degree of anisotropy of the diffusion process. A value of zero means that diffusion is isotropic (i.e. equal in all directions, and the diffusion ellipsoid is a sphere). A value of one means that diffusion is totally anisotropic (i.e. diffusion occurs only along one axis and is fully restricted along all other directions). |
| Cerebral blood volume (CBV) | CBV is the volume of blood in a given amount of brain tissue, most commonly millilitres of blood per 100 g of brain tissue. CBV can be calculated by assessing the area under the concentration-time curve, which in turn can be generated from signal intensity-time curves generated using Dynamic Contrast Enhanced (DCE) MRI (measuring T1 signal recovery) or Dynamic Susceptibility Contrast (DSC) MRI (measuring T2 signal loss), respectively (units ml/100g). |
| Cerebral blood flow (CBF) | Cerebral blood flow is the volume of blood passing through a given amount of brain tissue per unit of time, most commonly millilitres of blood per minute per 100 g of brain tissue. Alternatively, one may express CBF in terms of flow per unit volume of brain tissue, thus in ml blood/min/100 ml tissue. |
| Mean transit time (MTT) | Mean transit time is the average period of time that blood spends within the blood vessels in a particular part of the brain (units seconds). |
| Volume transfer constant (K trans) | K trans is the volume transfer constant for contrast agent between blood plasma and the tissue extravascular extracellular space (EES). K trans is derived from a pharmacokinetic model and represents a mix of flow and permeability. It most commonly serves as a measure of permeability/vascular leak under permeability-limited conditions (units min-1). |
| Rate constant (k ep) | k ep determines the washout rate of contrast agent from the extravascular extracellular space back into the blood plasma (kep = Ktrans/ve; units min-1). |
| Extravascular extracellular space fractional volume (ve ) | v e is defined as the volume of the extravascular extracellular space (EES) per unit volume of tissue, and thus is a dimensionless number between 0 and 1. The parameter v e reflects the amount of “room” available within the tissue interstitium for accumulating contrast agent. Note that v e is different from V e, which represents the total volume of extravascular extracellular space in ml. |
| Fractional plasma volume (vp ) | Represents the volume of blood plasma per unit volume of tissue (therefore unitless). It is derived from a pharmacokinetic model. |
| Native longitudinal relaxation rate (R1N) | R1 is the longitudinal relaxation rate of the protons of tissue water (R1 = 1/T1). R1N is the baseline tissue R1 in the absence of the contrast agent. The R1N measurement inversely reflects the free water content of tissue (units s-1). |
This table provides an overview of the most commonly cited imaging biomarkers used in glioblastoma patients. Note that the prefix of ‘r’ before these imaging biomarkers represents comparison to a reference region, that is usually the contralateral normal appearing brain parenchyma, but defined differently from study to study.