T2 Mapping Using ZTE Combined with Burst Encoding (BURZTE)
Rolf F Schulte1 and Ana Beatriz Solana1

1GE Healthcare, Munich, Germany


ZTE acquisition is combined with spin-echo burst encoding for quiet T2 mapping. An initial ZTE excitation train encodes multiple 3D radial spokes, which get refocused by reversing the gradients. A double spin-echo leads to T2 decay, from which T2 maps are extracted by exponential fitting. Accuracy is validated in the Eurospin TO5 relaxation phantom, while in vivo feasibility is demonstrated by T2 mapping in healthy brains.


Burst imaging enables relatively rapid and silent encoding in MRI via encoding multiple k-space lines with a train of short and evenly-spaced RF block pulses [1,2]. Another silent acquisition method is ZTE [3,4,5], where the object is encoded with 3D radial trajectories, ramping the gradients already before the RF pulse and updating them with slow, hence silent switching from one spoke to the next one. In this work, ZTE is combined with spin-echo burst encoding for quiet T2 mapping.


Burst encoding was implemented into a ZTE sequence by reversing the gradients, switching off the RF pulses during the burst acquisition part and repeating the same gradient trajectories. A double spin-echo module was included for T2 mapping using optimal adiabatic pulses [6]. The pulse sequence, named T2-BURZTE (Fig.1), was implemented on a GE MR750w scanner. Data is reconstructed automatically on the scanner using standard 3D gridding and FFT. T2 maps were extracted by fitting the images of three echo times to an exponential decay. Accuracy was investigated in a reference relaxation-phantom, consisting of 12 different vials with different T1 and T2 relaxation times [7]. Each vial was segmented using region growing, leading to 12 3D masks each containing approximately 2500 voxels. In vivo feasibility was demonstrated by imaging the brain of healthy volunteers.

Results and Discussion

Accurate T2 mapping is possible with T2-BURZTE, as demonstrated in the reference relaxation phantom (Figs.2 and 3). The impact of different T1 times is relatively small, because of fitting the three echo times, hence reducing the impact of initial magnetisation. The main impact of long T1 times on the T2 estimation is a decrease in SNR, hence reduced accuracy.

Brain scans with two different resolutions (2mm)3 and (1.7mm)3 are shown in Figs.4 and 5, respectively. A main limitation of T2-BURZTE in vivo is SNR, hence a relatively large flip angle of 5° was chosen with recovery times of 250ms and 200ms, respectively. The resulting acquisition times increase rapidly with 3D isotropic encoding, because the number of spokes is proportional to the squared matrix size and because of the required waiting times for double spin-echo and recovery at the end.

The power deposition (SAR) is limited because of choosing an optimal adiabatic pulse obeying both constant adiabaticity and offset independent adiabaticity with a small sweep width (500Hz) and flip angle (integral of 400° under magnitude), hence making this sequence a good candidate for T2 mapping also at higher fields (7T).

Burst encoding never made it into the clinic [2], mainly because of SNR limitations and calibration requirements. The SNR is improved considerably by a full 3D-encoding, while the fully compensated gradient trajectory and the double spin-echo removes unknown phases, hence obviating the need for any calibration. This novel T2 mapping method might enable more quantitative and comparable imaging approaches in MRI, as compared to standard T2 weighted imaging.


No acknowledgement found.


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Fig. 1: Sequence diagram for spin-echo T2-BURZTE. Multiple spokes in k-space are encoded during the initial ZTE RF-excitation train (1&2). Reversing the gradients in train 2 refocuses all the different spokes, such that the next train (3) recalls all these excitations one after the other. Including a double-spin echo in between 2 and 3 leads to T2 decay. By repeating the double-spin echo and the burst trains 3 and 4, multiple echoes can be acquired.

Fig. 2: Relaxation phantom with numbering starting at top left (vial #1) in horizontal and then vertical order. (FOV=(18cm)3, resolution=(2mm)3, matrix size=903, flip angle=3°, BW=±31.25kHz, TE=n∙102ms, acquisition duration (for recovery time) 3:59min (40ms), 4:51min (100ms), 10:31min(500ms), 24:43min(1500ms)).

Fig. 3: Validation results of T2-BURZTE in the reference relaxation phantom [6] (all times in [ms]; mean ± std over all voxel in mask of each vial). Most measured T2 are close to the actual published, calibrated T2 times, with larger deviations for vials with very long T1 times. The large standard deviation in vial 12 suggests inaccurate T2 determination. Increasing recovery time helps to improve T2 determination, mainly by improving SNR through T1 recovery.

Fig. 4: T2 mapping in healthy brain with (2mm)3 isotropic resolution. ZTE images (TE=0ms) are shown in the top row, first echo image (TE=95.5ms) below and second echo image (TE=191ms) in the third row. First and second echo are scaled up in intensity by a factor of two and three, respectively. Fitting those three sets of images voxel-by-voxel (linear LSQ of log(signal)) yields the T2-map in the bottom row (maximum of colour bar T2=200ms). The quality of the initial ZTE image is already somewhat degraded due to the relatively large flip angle. (FOV=(20cm)3, resolution=(2mm)3, matrix size=1003, flip angle=5°, BW=±31.25kHz, acquisition duration=10:03min, recovery time=250ms).

Fig. 5: T2 mapping in healthy brain with (1.7mm)3 isotropic resolution with TE=0ms (ZTE), 89.7ms (first echo), 179ms (second echo). (FOV=(20cm)3, resolution=(1.7mm)3, matrix size=1183, flip angle=5°, BW=±31.25kHz, acquisition duration=11:27min, recovery time=200ms).

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)