Magnetic resonance imaging with zero echo time and slice selection
A novel radio frequency sequence, suitable for performing Magnetic Resonance Imaging (MRI) of 2-dimensional (2D) slices of samples exhibiting short magnetization coherence times (i.e., hard tissues). The SS-ZTE pulse sequence contains the following steps: a) magnetizing all spins in the sample from a longitudinal direction to the transverse plane; b) exciting the 2D slice of interest, which comprises the selective locking of said 2D sample slice magnetization while spoiling the magnetization in the rest of sample volume; c) making the magnetization of the selected 2D slice impervious to reconfigurations of the magnetic field gradients from slice selection to encoding and readout; d) reading out of the free induction decay signal of the sample; e) repeating steps (a-d) with different readout directions, so as to gather a corresponding number of radial spokes of the plane defined by the 2D sample slice.
1. A magnetic resonance imaging, MRI, method comprising a sample set in a magnetic field, B0, along a longitudinal direction so that the magnetization of the sample is initially parallel to said direction of the magnetic field, wherein said method comprises performing an imaging scan of a sample based on a ZTE sequence,
the imaging scan comprising the following steps:
a) applying a slice selection gradient pulse, G SS , followed by at least one radio frequency excitation pulse generating a transverse magnetization of the sample with respect to the direction of B0;
b) applying a slice selection pulse sequence comprising at least one rf pulse for selective locking of sample magnetization corresponding to a single 2D slice of the sample;
c) applying a preservation pulse sequence comprising at least one rf pulse, for keeping magnetization coherence of the selected 2D slice, and additionally comprising the termination of G SS and the onset of a readout gradient pulse, G ro ;
d) performing the encoding, readout, and acquisition of the FID signal of a sample radial spoke defined by G ro ;
e) repeating the steps (a-d) along a plurality of readout directions in order to gather a corresponding number of radial spokes in the k-space plane, corresponding to the 2D slice of the sample;
f) processing the FID signals gathered in steps a)-e) and reconstructing an image of the 2D slice of the sample.
2. The MRI method according to claim 1 , wherein the slice selection pulse sequence comprises a standard spin locking pulse.
3. The MRI method according to claim 1 , wherein the slice selection pulse sequence comprises at least one rotary echo SL pulse.
4. The MRI method according to claim 1 , wherein both the excitation and slice selection pulse sequences are merged into a homonuclear dipolar-decoupled version of a DANTE selective-excitation sequence.
5. The MRI method according to claim 1 , wherein the preservation pulse sequence starts with a hard rf pulse which rotates the transverse component of the magnetization to the longitudinal axis and, once the magnetization is longitudinal, the G SS gradient is ramped to zero and the readout gradient G ro is ramped in a perpendicular image-encoding direction; followed by an excitation rf pulse which rotates the sample magnetization towards the transverse plane.
6. The MRI method according to claim 5 , wherein the preservation pulse sequence comprises an additional spoiler gradient pulse, interleaved between the termination of G SS and the onset of G ro ; being this additional spoiler gradient ramped up and then down, to further spoil coherence of magnetization outside the selected 2D slice.
7. The MRI method according to claim 1 , wherein the preservation pulse sequence comprises a CHASE sequence.
8. The MRI method according to claim 7 , wherein the CHASE sequence is a single CHASE-5 pulse train; and additionally, the G SS is ramped down linearly before the first 90° pulse and blipped after the last one, while G ro is first blipped and then ramped up.
9. The MRI method according to claim 5 comprising a dead time, td, before acquisition; along with an acquisition step following a ZTE scheme.
10. The MRI method according to claim 7 , wherein a dead time, td, is incorporated into the waiting time between pulses of the preservation pulse sequence; along with an acquisition step following a ZTE scheme.
11. The MRI method according to claim 1 , wherein the sample comprises a hard tissue with T2 below 1 ms.
12. The MRI method according to claim 11 , wherein the hard tissue comprises bone tissue, and/or tendon tissue.
13. The MRI method according to claim 11 , wherein the hard tissue comprises teeth bone tissue.
14. An MRI apparatus comprising:
a magnet operable to provide a magnetic field;
a rf transmitter configured to transmit a rf field to a sample placed into the magnetic field;
a rf receiver arranged to receive a magnetic resonance signal;
a data acquisition unit to record the magnetic resonance signals; and
a processor for processing the information provided by the data acquisition unit;
wherein the rf transmitter is configured to yield ZTE sequences following a method according to claim 1 .
15. An MRI apparatus according to claim 14 , wherein the rf transmitter and rf receiver are combined in the same physical element.
16. Use of an MRI apparatus according to claim 14 in hard tissue imaging, imaging of archaeological objects, imaging analysis in mineralogy, imaging for gemology certification, soil prospection imaging and imaging analysis of chemical composition of solids.
17. Use of an MRI apparatus according to claim 14 in teeth bone imaging.