Method and system for simultaneous mapping of quantitative MRI parameters using a T2 prepared inversion
A qMRI system and method map qMRI parameters of a biological object. The method includes performing, by the qMRI system, N scans wherein each scan, includes: performing T2-prepared inversion pulse series, each followed by readout blocks, each magnetization preparation RF pulse series is a T2-prepared inversion pulse series containing multiple pulses and an inter-pulse duration, for varying to obtain different T2 weightings; and acquiring, by the MRI system and during each readout block of an MRI, a recovery signal generated by a part of the biological object, wherein for each readout block, an MRI signal is acquired by the MRI system at different inversion times. An image of the part is reconstructed for and from each MRI signal. A voxel-wise signal is created by concatenating intensity values for a same voxel for the reconstructed images. A physical model is fitted to the concatenated intensity values to obtain a qMRI map.
1. A quantitative magnetic resonance imaging (qMRI) method for mapping at least one qMRI parameter of a biological object with an MRI system, the method comprises the following steps of:
performing, by the MRI system, N scans C_1, . . . , C_N, wherein each scan C_j, with j=1, . . . , N, includes:
performing at least one magnetization preparation radio frequency (RF) pulse series, each followed by M_j readout blocks, wherein two successive said magnetization preparation RF pulse series are separated by a repetition time interval TR_j, wherein each said magnetization preparation RF pulse series is a T2-prepared inversion pulse series containing multiple pulses and characterized by an echo time, being an inter-pulse duration, TE p _j, wherein the inter-pulse duration is a time between first and last pulses, and is varied by the MRI system in order to obtain different T2 weightings, the at least one magnetization preparation RF pulse series being configured for inverting a longitudinal magnetization while encoding T2 contrast in a subsequent T1 recover;
acquiring, by the MRI system and during each readout block, an MRI recovery signal generated by at least a part of the biological object, wherein for each said readout block R_i,j, an MRI signal S_i,j is acquired by the MRI system at a different inversion time TI_i,j, with i=1, . . . , M_j, and M_j≥2;
reconstructing, for and from each said MRI signal S_i,j, an image I_i,j of the part;
creating a voxel-wise signal by concatenating intensity values obtained for a same voxel for reconstructed images; and
fitting a physical model to concatenated intensity values to obtain at least one qMRI map of the at least one qMRI parameter.
2. The qMRI method according to claim 1 , wherein the echo time TE p _j takes different values for each of the magnetization preparation RF pulse series of a same scan, or an identical value for all the magnetization preparation RF pulse series of the same scan, the identical value being different for each said scan.
3. The qMRI method according to claim 1 , wherein the fitting is a dictionary fitting which comprises the further steps of:
performing a numerical simulation of each said scan C_j, wherein the numerical simulation is configured for generating, for each voxel, a set of simulated voxel-wise signals and, for providing, for each simulated voxel-wise signal, a value for the at least one qMRI parameter; and
reconstructing, for the at least one gMRI parameter, a quantitative map representing an imaged biological part by matching, for each voxel of the quantitative map, a created voxel-wise signal of a corresponding voxel in reconstructed images and all simulated voxel-wise signals generated for the voxel, and assigning to a considered voxel in the quantitative map the value of the at least one qMRI parameter for which a best match has been obtained.
4. The qMRI method according to claim 3 , which further comprises using a physics-based model for creating the simulated voxel-wise signal.
5. The qMRI method according to claim 3 , which further comprises acquiring an additional map for selecting the set of simulated voxel-wise signals to be matched with an acquired voxel-wise signal.
6. The qMRI method according to claim 5 , wherein the additional map is selected from the group consisting of:
a B1+ map;
a B1− map;
a B0 frequency offset map; and
a diffusion map.
7. The qMRI method according to claim 1 , wherein the T2-prepared inversion pulse series is adiabatic or non-adiabatic.
8. The qMRI method according to claim 7 , wherein the T2-prepared inversion pulse series is an adiabatic T2-prepared inversion pulse sequence containing two tip-down RF pulses temporally framing two refocusing adiabatic RF pulses, wherein a time interval between a middle of a first tip-down RF pulse of the two tip-down RF pulses and a middle of a last tip-down RF pulse of the two tip-down RF pulses is the inter-pulse duration TE p _j.
9. The qMRI method according to claim 8 , wherein the adiabatic T2-prepared inversion pulse sequence contains a following series of temporally successive pulses including:
a first pulse that is a rectangular tip-down RF pulse, followed successively by a second pulse, a third pulse, and finally a fourth pulse, wherein the second and third pulses are both hyperbolic-secant-refocusing adiabatic RF pulses, and the fourth pulse is a rectangular tip-down RF pulse.
10. The qMRI method according to claim 1 , wherein the voxel-wise signal is obtained by plotting for each voxel an intensity value of the voxel obtained in function of each couple (TI_i,j; TE p _j).
11. The qMRI method according to claim 1 , which further comprises selecting the at least one qMRI parameter from the group consisting of:
a T1 relaxation time;
a T2 relaxation time;
a T2* and/or T2′ relaxation time;
a B1+ RF field;
a proton density;
a T1p and/or T2p relaxation time;
a magnetization transfer parameter;
a diffusion parameter;
a perfusion parameter; and
a fat and/or water fraction parameter.
12. The qMRI method according to claim 1 , wherein the physical model uses extended-phase-graph, or Bloch-Equations, or analytical solutions.
13. A magnetic resonance imaging system configured for carrying out the qMRI method according to claim 1 .