Theory
What is a parametric map
In clinical MRI we can acquire series of images with different acquisition settings or in different physiological states. The changes observed on the images correspond to change of certain physical or physiological parameters. If the model of this change is known, a mathematical procedure called fitting can be performed to extract the parameters from the data. If this procedure is performed in every pixel, a parametric map is generated.
The most common physical parameters used in MRI are relaxation times: T1, T2, T2star. The relation between relaxation times and different acquisition settings allow modelling of the changes observed on the images and generation of relaxation time maps.
When a contrast agent is injected, its propagation in the organ can be modelled. This way physiological parameters of the organ are reflected in generated perfusion maps.
Relaxation time maps and perfusion maps are often called parametric maps. Parametric maps are a promising research tool in cardiac diagnosis and therapy management.
In other words:
To generate a parametric map of perfusion or relaxation times, multiple images of the same region of the myocardium are acquired with different sensitivity to the parameter of interest, and the signal intensities of these images are fit to a model that describes the underlying physiology or relaxation parameters. The parametric map is an image of the fitted perfusion parameters or relaxation times.
According to https://doi.org/10.1016/j.jcmg.2013.05.005
Below we present a set of online resources helpful in understanding parametric maps.
T1
What is T1?
T1 (longitudinal relaxation time) is a native magnetic property of an organ or a material. T1 is usually measured by acquiring a series of images after an inversion pulse, so called inversion times, as presented on the graph below. The recovery after an inversion pulse can be modelled with the following equation:
Alternatively a saturation pulse can be used.
For more information, see the following online resources.
- http://mriquestions.com/what-is-t1.html
- https://qmrlab.org/jekyll/2018/10/23/T1-mapping-inversion-recovery.html
Why T1 matters in cardiac MRI?
T2
What is T2?
T2 (transversal relaxation time) is a native magnetic property of an organ or a material. T2 is usually measured with Spin Echo MRI sequences. A series of images with different echo times is acquired. The T2 recovery can be modelled with the following equation:
For more information, see the following online resources.
Why T2 matters in cardiac MRI?
T2star
What is T2star?
T2star (transversal relaxation time) is a native magnetic property of an organ or a material. Unlike T2, T2star is usually measured with Gradient Echo MRI sequences. This method is sensitive to the magnetic field inhomogeneities and the observed relaxation is faster that of T2. A series of images with different echo times is acquired. The T2 recovery can be modelled with the following equation:
For more information, see the following online resources.
- https://pmj.bmj.com/content/89/1050/209#F7
- http://mriquestions.com/t2-vs-t2.html
- https://youtu.be/r0dYgcN3HcU