Synthetic Magnetic Resonance Imaging is based on one or more MRI acquisitions that measure the patient’s physical properties. Instead of acquiring images for direct interpretation an absolute quantification of the T1 and T2 relaxation times and the proton density PD is performed. Using the T1, T2 and PD values it can be calculated what the expected signal intensity of an MR acquisition would have been at any given setting of echo time TE and repetition time TR. Even an inversion pulse can be added in the calculation, with an inversion delay time TI. Doing this for all pixels will recreate, or synthesize, ‘normal’ T1-weighted, T2-weighted, FLAIR or Inversion Recovery images [1,2]. The advantage of synthetic MRI is that it translates the MR quantification maps to more familiar images, which will ease image interpretation. The image contrast can even be changed after the patient has left. Having access to quantitative MRI will benefit (automatic) tissue recognition, supporting quantitative follow-up. The purpose of this lecture is to explain the technique of synthetic MRI, its clinical application and to explore its limits and pitfalls. Examples will be provided for application in multiple sclerosis, dementia, hydrocephalus, cancer, cartilage assessment and cardiac infarction.

The most challenging part of synthetic MRI is obtaining the T1 and T2 relaxation and proton density (PD) maps [3-7]. Once these are measured the calculation of signal strength of a T1-weighted or T2-weighted synthetic image is given by

$$$𝑆=𝑃𝐷·exp(−𝑇𝐸/T2 )·(1−exp(−𝑇𝑅/T1))$$$

where TE is the echo time and TR is the repetition time. If an inversion pulse is added, to generate a synthetic FLAIR, STIR or any other inversion recovery sequence, the signal strength is given by

$$$𝑆=𝑃𝐷·exp(−𝑇𝐸/T2 )·(1−2·𝑒𝑥𝑝(−𝑇𝐼/T1)+ 𝑒𝑥𝑝(−𝑇R/T1))$$$

where TI is the inversion delay time. Since the calculation of synthetic images is a post-processing step the TE, TR and TI can be changed after the patients has left.

Using synthetic MRI a radiologists has access to quantitative T1, T2 and PD values, conventional looking images and computer-aided tissue segmentation. Examples will be provided for clinical applications of synthetic MRI in multiple sclerosis and dementia using brain volume estimation and lesion characterization, in hydrocephalus for ventricle volume measurement, in cancer cases for tumor characterization, in cartilage assessment of tissue properties and in cardiac infarction where changes in physical properties provide insight in severity of damage and edema.The combination of improved technology and better clinical access currently provides synthetic MRI a window to enter the clinical arena.